Classifications

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  • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
  • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
  • B21C37/08—Making tubes with welded or soldered seams
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  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/002—Resistance welding; Severing by resistance heating specially adapted for particular articles or work
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  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/02—Pressure butt welding
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  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/08—Seam welding not restricted to one of the preceding subgroups
  • B23K11/087—Seam welding not restricted to one of the preceding subgroups for rectilinear seams
  • B23K11/0873—Seam welding not restricted to one of the preceding subgroups for rectilinear seams of the longitudinal seam of tubes
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  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
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  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
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  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K13/00—Welding by high-frequency current heating
  • B23K13/01—Welding by high-frequency current heating by induction heating
  • B23K13/02—Seam welding
  • B23K13/025—Seam welding for tubes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K31/00—Processes relevant to this subclass, specially adapted for particular articles or purposes, but not covered by only one of the preceding main groups
  • B23K31/02—Processes relevant to this subclass, specially adapted for particular articles or purposes, but not covered by only one of the preceding main groups relating to soldering or welding
  • B23K31/027—Making tubes with soldering or welding
• • 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/26—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
  • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2101/00—Articles made by soldering, welding or cutting
  • B23K2101/04—Tubular or hollow articles
  • B23K2101/06—Tubes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2101/00—Articles made by soldering, welding or cutting
  • B23K2101/18—Sheet panels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2103/00—Materials to be soldered, welded or cut
  • B23K2103/02—Iron or ferrous alloys
  • B23K2103/04—Steel or steel 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • 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
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
  • F16L9/00—Rigid pipes
  • F16L9/17—Rigid pipes obtained by bending a sheet longitudinally and connecting the edges
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the present invention relates to an electric resistance welded steel pipe for line pipes through which petroleum and natural gas are transported.
• the present invention relates to a high-strength electric resistance welded steel pipe that is suitable for reel barge laying, has a high strength of Grade X60 (yield strength YS: 415 MPa or more) or more, and has excellent bendability, and a method for producing the high-strength electric resistance welded steel pipe.
• the reel barge method is a method in which the girth welding, inspection, coating, and the like of a pipe are performed on land in advance and the resulting long pipe is coiled around a reel of a barge, and the pipe is uncoiled from the reel at a target place at sea to lay a pipeline on the sea bottom.
• offshore pipelines can be very efficiently laid.
• tensile stress and compressive stress due to bending and unbending are applied to a part of the pipe when the pipe is coiled and laid. Consequently, local rupture and buckling occur in the pipe used, which may cause fracture of the pipe.
• steel pipes for pipelines laid by the reel barge method need to have excellent bendability, that is, high buckling resistance on the compressive side and high rupture resistance on the tensile side during bending deformation.
• the buckling resistance is highly dependent on the shape uniformity of a pipe.
• For the rupture resistance it is important to have high uniform elongation and to prevent ductile fracture.
• Electric resistance welded steel pipes have been used as line pipes from the economical viewpoint in recent years. Electric resistance welded steel pipes have better thickness deviation and circularity than seamless steel pipes. The buckling resistance strongly affected by shape factor is higher than that of seamless steel pipes.
• electric resistance welded steel pipes are obtained by continuously cold-rolling a hot-rolled steel sheet so as to have a substantially cylindrical shape, a considerable amount of plastic strain is introduced in the pipe axial direction, which deteriorates the uniform elongation in the pipe axial direction. Consequently, the uniform elongation of electric resistance welded steel pipes is normally lower than that of seamless steel pipes. Thus, even low strain easily causes rupture, and the rupture resistance deteriorates compared with seamless steel pipes.
• One of methods for improving the uniform elongation of steel materials is a method that uses a TRIP (transformation induced plasticity) phenomenon of retained austenite by increasing the Si content.
• TRIP transformation induced plasticity
• the Si content in a steel sheet is generally increased.
• a high-melting-point oxide formed during electric resistance welding is left in an electric resistance welded part, which deteriorates the quality of the electric resistance welded part.
• Patent Literature 1 proposes a method for producing a high-strength steel pipe having high buckling resistance.
• the resulting steel sheet is cooled at a cooling rate of 5° C./s or more from the temperature of Ar 3 transformation temperature [° C.] or higher, kept at (Ts ⁇ 50° C.) to (Ts+100° C.) for 30 to 300 seconds, then cooled to 350° C. to 450° C. at a cooling rate of 20° C./s or more, and then slowly cooled to obtain a steel sheet containing retained austenite left therein.
• Ts is represented by formula (1) below.
• Ts [° C.] 780 ⁇ 270 ⁇ C ⁇ 90 ⁇ Mn ⁇ 37 ⁇ Ni ⁇ 70 ⁇ Cr ⁇ 83 ⁇ Mo (1)
• Patent Literature 1 In the technique described in Patent Literature 1, however, the temperature needs to be kept constant in the middle of cooling. In a hot-rolling line in which a steel sheet is cooled while being conveyed in one direction in continuously arranged cooling zones, the length of the facility needs to be considerably increased. In the technique described in Patent Literature 1, the cooling stop temperature is 350° C. to 450° C. This increases deformation resistance, which makes it difficult to coil the steel sheet. Furthermore, Patent Literature 1 does not describe an improvement in uniform elongation of a steel pipe.
• a high-strength electric resistance welded steel pipe whose uniform elongation in a pipe axial direction is improved without performing heat treatment on the whole pipe and which has excellent bendability, and a method for producing the high-strength electric resistance welded steel pipe is provided.
• high-strength refers to a yield strength YS of 415 MPa or more in a pipe axial direction.
• excellent bendability herein particularly relates to rupture resistance and refers to a uniform elongation Elu of 8% or more in a pipe axial direction.
• a high-strength electric resistance welded steel pipe including a composition containing, on a mass % basis, C: 0.04% to 0.15%, Si: 0.10% to 0.50%, Mn: 1.0% to 2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2% to 1.0%, Ti: 0.005% to 0.030%, and Al: 0.010% to 0.050%, the balance being Fe and unavoidable impurities, and a microstructure including polygonal ferrite with a volume fraction of 70% or more and retained austenite with a volume fraction of 3% to 20%, the balance being at least one selected from martensite, bainite, and pearlite, wherein the polygonal ferrite has an average grain size of 5 m or more and an aspect ratio of 1.40 or less.
• the composition further contains, on a mass % basis, at least one selected from Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Co: 1.0% or less.
• the composition further contains, on a mass % basis, at least one selected from Nb: 0.10% or less and V: 0.10% or less.
• the composition further contains Ca: 0.0005% to 0.0050% on a mass % basis.
• a method for producing the high-strength electric resistance welded steel pipe according to any one of (1) to (4) includes a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip; a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner.
• the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.
• the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20 after 20 seconds from a time t 0 at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80 after 80 seconds from the time t 0 is lower than 650° C.
• the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.
• the hot-rolled steel strip is uncoiled and continuously formed using a plurality of rolls to obtain an open pipe having a substantially circular section, then end surfaces of the open pipe in a width direction are butted against each other and heated to a temperature higher than or equal to the melting point thereof and pressure welding is performed on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe.
• Embodiments according to the present invention produce the following industrially marked effect. That is, a high-strength electric resistance welded steel pipe that is suitable for line pipes of offshore pipelines laid by a laying method such as a reel barge method, an S-lay method, or a J-lay method and pipelines laid in tectonic areas such as seismic areas, that has a high strength of Grade X60 or more, and that has excellent bendability can be produced at a lower cost than seamless steel pipes without performing heat treatment on the whole pipe.
• Embodiments according to the present invention also effectively contribute to uses requiring high deformability, such as uses in civil engineering and construction, in addition to line pipes.
• An electric resistance welded steel pipe has a composition containing, on a mass % basis, C: 0.04% to 0.15%, Si: 0.10% to 0.50%, Mn: 1.0% to 2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2% to 1.0%, Ti: 0.005% to 0.030%, and Al: 0.010% to 0.050%, the balance being Fe and unavoidable impurities and has a microstructure including polygonal ferrite with a volume fraction of 70% or more and retained austenite with a volume fraction of 3% to 20%, the balance being at least one selected from martensite, bainite, and pearlite, wherein the polygonal ferrite has an average grain size of 5 m or more and an aspect ratio of 1.40 or less.
• the yield strength YS in a pipe axial direction is 415 MPa or more and the uniform elongation Elu in a pipe axial direction is 8% or more.
• C is an element that contributes to stabilizing an austenite phase.
• C is an important element for ensuring a desired amount of retained austenite.
• the C content needs to be 0.04% or more. If the C content exceeds 0.15%, the weldability deteriorates. Therefore, the C content is limited to the range of 0.04% to 0.15%.
• the C content is preferably 0.06% or more and is also preferably 0.12% or less.
• the C content is more preferably 0.08% to 0.12%.
• Si is an element that serves as a deoxidizer and considerably contributes to generation of retained austenite by suppressing the precipitation of cementite. Si also has an effect of decreasing the scale-off quantity during hot-rolling. To produce such effects, the Si content needs to be 0.10% or more. If the Si content exceeds 0.50%, the weldability of electric resistance welding deteriorates. Therefore, the Si content is limited to the range of 0.10% to 0.50%. The Si content is preferably 0.10% to 0.30%.
• Mn is an element that improves the stability of an austenite phase and suppresses the decomposition into pearlite and bainite. To produce such an effect, the Mn content needs to be 1.0% or more. At an excessive Mn content of more than 2.2%, the generation of high-temperature transformed ferrite is suppressed, which prevents the release and concentration of C into non-transformed austenite. Therefore, the Mn content is limited to the range of 1.0% to 2.2%.
• the Mn content is preferably 1.2% or more and is preferably 1.6% or less.
• Cr is an important element that contributes to generation of retained austenite by suppressing the precipitation of cementite in non-transformed austenite.
• the Cr content needs to be 0.2% or more.
• the Cr content is limited to the range of 0.2% to 1.0%.
• the Cr content is preferably 0.2% to 0.8% and more preferably 0.2% to 0.5%.
• Ti is an element that fixes N in the form of TiN to suppress deterioration of toughness of steel due to N. Such an effect is produced at a Ti content of 0.005% or more. If the Ti content exceeds 0.030%, the amount of titanium carbonitride that precipitates along the cleaved surface of Fe increases, which deteriorates the toughness of steel. Therefore, the Ti content is limited to the range of 0.005% to 0.030%. The Ti content is preferably 0.005% to 0.025%.
• the balance other than the above-described components is Fe and unavoidable impurities.
• N 0.005% or less
• O oxygen
• the above-described basic composition may further optionally contain at least one selected from Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Co: 1.0% or less, at least one selected from Nb: 0.10% or less and V: 0.10% or less, and/or Ca: 0.0005% to 0.0050%.
• All of Mo, Cu, Ni, and Co are elements that improve the stability of an austenite phase and contribute to generation of retained austenite.
• Mo: 0.05% or more, Cu: 0.05% or more, Ni: 0.05% or more, and Co: 0.05% or more are desirably satisfied. If the contents exceed Mo: 0.5%, Cu: 0.5%, Ni: 1.0%, and Co: 1.0%, the above effect is saturated and the weldability deteriorates.
• the contents are preferably limited to the ranges of Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Co: 1.0% or less and more preferably limited to the ranges of Mo: 0.4% or less, Cu: 0.4% or less, Ni: 0.4% or less, and Co: 0.4% or less.
• Nb 0.10% or Less
• V 0.10% or Less
• Nb and V are elements that form a carbonitride or a carbide and contribute to an improvement in the strength of a hot-rolled steel strip through precipitation strengthening.
• the contents are desirably Nb: 0.01% or more and V: 0.01% or more. If the contents exceed Nb: 0.10% and V: 0.10%, a coarse precipitate is formed, which deteriorates the toughness of a base material or deteriorates the weldability. Therefore, if these elements are contained, the contents are limited to the ranges of Nb: 0.10% or less and V: 0.10% or less.
• Ca is an element that contributes to effectively controlling the form of sulfide-based inclusions. Ca makes a sulfide such as MnS harmless and improves the toughness of a hot-rolled steel strip. To produce such an effect, the Ca content needs to be 0.0005% or more. If the Ca content exceeds 0.0050%, a Ca-based oxide cluster is formed, which deteriorates the toughness of a hot-rolled steel strip. Therefore, if Ca is contained, the Ca content is preferably limited to the range of 0.0005% to 0.0050%. The Ca content is more preferably 0.0010% or more and 0.0040% or less.
• the electric resistance welded steel pipe according to of the present invention has the above composition and also has a microstructure including polygonal ferrite (with a volume fraction of 70% or more) as a main microstructure and retained austenite with a volume fraction of 3% to 20%, the balance being at least one selected from martensite, bainite, and pearlite.
• the polygonal ferrite has an average grain size of 5 m or more and an aspect ratio of 1.40 or less.
• polygonal ferrite herein refers to high-temperature transformed ferrite, which is transformed with diffusion. In the high-temperature transformed ferrite, C is released into non-transformed austenite when the transformation proceeds. Thus, the non-transformed austenite is stabilized, which makes it easy to generate a desired amount of retained austenite. Therefore, in embodiments of the present invention that provides a high-strength hot-rolled steel strip having excellent uniform elongation by using a TRIP phenomenon of retained austenite, such polygonal ferrite is a main microstructure.
• the term “main microstructure” in embodiments of the present invention refers to a microstructure having a volume fraction of 70% or more.
• the main microstructure is bainitic ferrite or bainite
• the amount of C released during transformation is small or almost zero, resulting in insufficient concentration of C into the non-transformed austenite. Consequently, the non-transformed austenite is not stabilized and is transformed into pearlite or bainite after cooling, which makes it difficult to form a retained austenite phase with a volume fraction of 3% to 20%. Therefore, the main microstructure is polygonal ferrite.
• polygonal ferrite is defined as a microstructure having an aspect ratio of 1.40 or less, which is determined by (crystal grain diameter in rolling direction)/(crystal grain diameter in sheet thickness direction), and an average grain size of 5 m or more in embodiments of the present invention.
• Retained austenite contributes to an improvement in the uniform elongation of an electric resistance welded steel pipe through stress-induced transformation (TRIP phenomenon)
• TRIP phenomenon stress-induced transformation
• the retained austenite phase needs to have a volume fraction of 3% or more.
• the concentration of carbon contained in the retained austenite decreases and the retained austenite becomes unstable against deformation, resulting in deterioration of uniform elongation. Therefore, the volume fraction of the retained austenite is limited to the range of 3% to 20%.
• the volume fraction of the retained austenite is preferably 3% to 15% and more preferably 5% to 15%.
• the balance other than the polygonal ferrite serving as a main microstructure and the retained austenite is preferably at least one selected from martensite, bainite, and pearlite with a volume fraction of 10% or less (including 0%). If the volume fraction in total of the balance, that is, at least one selected from martensite, bainite, and pearlite exceeds 10%, the strength excessively increases and the uniform elongation deteriorates. Note that ferrite other than polygonal ferrite is classified as bainite.
• the above-described microstructure including polygonal ferrite with a volume fraction of 70% or more and retained austenite with a volume fraction of 3% to 20%, the balance being at least one selected from martensite, bainite, and pearlite, is measured as follows. First, a test specimen for observing a microstructure is sampled from an electric resistance welded steel pipe so that a section in the rolling direction (L section) serves as an observation surface. The sampled test specimen for observing a microstructure is polished and etched (etchant: nital).
• a microstructure at a position of 1 ⁇ 2t of the sheet thickness is observed with an optical microscope (magnification: 400 times) and a scanning electron microscope SEM (magnification: 2000 times), and two or more view areas are photographed in each of the specimens.
• the type of microstructure, the area fraction of each phase, and the aspect ratio of crystal grains of the polygonal ferrite are determined using an image analyzer.
• the average grain size of the polygonal ferrite can be determined by a cutting method in conformity with JIS G 0551. In the measurement of the microstructure, the calculation is performed using an arithmetic mean.
• an area fraction is determined for retained austenite by a SEM/EBSD (electron backscatter diffraction) method.
• SEM/EBSD electron backscatter diffraction
• a pipe material production step of heating, hot-rolling, and then cooling a steel material having the above composition to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip is performed.
• a steel material having the above composition is heated at a heating temperature of 1100° C. to 1250° C. and then hot-rolled to obtain a hot-rolled steel strip serving as a pipe material.
• Heating Temperature of Steel Material 1100° C. to 1250° C.
• the heating temperature of the steel material refers to a setting temperature in a heating furnace.
• the heated steel material is hot-rolled to obtain a hot-rolled steel strip having a predetermined size and shape.
• the hot-rolling conditions are not particularly limited as long as the hot-rolled steel strip has a predetermined size and shape.
• the temperature after the final pass of the hot-rolling that is, the finish rolling end temperature is preferably set to 750° C. or higher.
• the cooling after the hot-rolling is continuously performed on the hot-rolled steel strip to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, the temperature T 20 after 20 seconds from the time t 0 at which a final pass of the hot-rolling is finished is higher than 650° C. and the temperature T 80 after 80 seconds from the time t 0 is lower than 650° C.
• the “cooling” in embodiments of the present invention is preferably performed by spraying cooling water onto upper and lower surfaces of the hot-rolled steel strip from a water cooling zone continuously arranged in a run-out table disposed on the delivery side of the finish rolling mill.
• the arrangement intervals, water flow rate, and the like in the water cooling zone are not particularly limited.
• the temperature at the central position of the hot-rolled steel strip in a thickness direction is a temperature determined by heat transfer analysis on the basis of the temperature measured with a surface thermometer.
• Cooling after Hot-Rolling At a Central Position of the Steel Strip in a Thickness Direction, the Temperature T 20 after 20 Seconds from the Time t 0 at which a Final Pass of the Hot-Rolling is Finished is Higher than 650° C. and the Temperature T 80 after 80 Seconds from the Time t 0 is Lower than 650° C.
• polygonal ferrite transformation is caused by controlling the cooling so that, at a central position of the steel strip in a thickness direction, the temperature T 20 after 20 seconds from the time t 0 at which a final pass of the hot-rolling is finished is higher than 650° C. and the temperature T 80 after 80 seconds from the time t 0 is lower than 650° C.
• the steel strip microstructure can be controlled to a microstructure mainly containing polygonal ferrite.
• the steel strip microstructure can be controlled to a microstructure mainly containing polygonal ferrite.
• the cooling is performed so that the temperature T 20 at a central position of the steel strip in a thickness direction is 650° C. or lower, bainitic ferrite or bainite is mainly generated, and thus a microstructure mainly containing polygonal ferrite cannot be provided.
• the temperature T 80 at a central position of the steel strip in a thickness direction is 650° C. or higher, precipitation of carbonitride and cementite readily occurs together with ferrite transformation, which makes it difficult to cause the concentration of C into non-transformed austenite.
• the cooling after the hot-rolling is controlled so that, at a central position of the steel strip in a thickness direction, the temperature T 20 after 20 seconds from the time t 0 at which a final pass of the hot-rolling is finished is higher than 650° C. and the temperature T 80 after 80 seconds from the time t 0 is lower than 650° C.
• Cooling Stop Temperature 600° C. to 450° C.
• the cooling after the hot-rolling is limited to a cooling treatment in which the cooling is controlled so that, at a central position of the steel strip in a thickness direction, the temperature T 20 after 20 seconds from the time t 0 at which a final pass of the hot-rolling is finished is higher than 650° C. and the temperature T 80 after 80 seconds from the time t 0 is lower than 650° C., and the cooling is continuously performed to a cooling stop temperature of 600° C. to 450° C.
• the resulting coiled hot-rolled steel strip is used as a pipe material and a pipe making step is performed.
• the coiled hot-rolled steel strip serving as a pipe material is uncoiled and continuously cold-formed by using a plurality of rolls to obtain an open pipe having a substantially circular cross section.
• end surfaces of the open pipe in a width direction are butted against each other and heated to a temperature higher than or equal to the melting point thereof by high-frequency induction heating or high-frequency resistance heating, and pressure welding is performed on the butted and heated end surfaces of the open pipe in the width direction with a squeeze roll.
• an electric resistance welded steel pipe is obtained.
• the pipe making step in embodiments of the present invention is not particularly limited as long as an electric resistance welded steel pipe having a desired size and shape can be produced through the pipe making step. Any typical pipe making step that uses a general facility for producing an electric resistance welded steel pipe can be employed.
• the electric resistance welded steel pipe produced in the pipe making step is then subjected to an in-line heat treatment step in which an electric resistance welded part is heat-treated in an in-line manner.
• the resulting electric resistance welded part has a microstructure mainly containing martensite and/or upper bainite because of rapid heating and rapid cooling during the welding.
• These microstructures are microstructures having low toughness.
• such a microstructure is modified into a microstructure having high toughness by performing an in-line heat treatment step.
• the “high toughness” herein is expressed when the Charpy impact test absorbed energy vE 0 (J) in a circumferential direction is 150 J or more at a test temperature of 0° C.
• a series of common apparatuses are preferably used that include one or more induction heating apparatuses and cooling apparatuses that use water cooling or the like, which are capable of heating and cooling the electric resistance welded part, sequentially arranged in an in-line manner on the downstream side of a squeeze roll in a facility for producing electric resistance welded steel pipes.
• the in-line heat treatment includes heating the electric resistance welded part so that the minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and the maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that the maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.
• the term “in-line” refers to an arrangement in a straight line.
• the term “in-line heat treatment” refers to, for example, a heat treatment that uses heating apparatuses arranged in a straight line along the welded part.
• the heating apparatuses are not particularly limited and, for example, direct electrifying heating can be employed instead of induction heating.
• Heating Temperature in in-Line Heat Treatment 800° C. to 1150° C.
• the heating temperature in the minimum temperature portion is lower than 800° C.
• the microstructure in the electric resistance welded part cannot be controlled to bainitic ferrite and/or bainite having high toughness in the entire region in the sheet thickness direction.
• the heating temperature in the maximum heating portion is higher than 1150° C., austenite grains markedly coarsen and the hardenability increases, resulting in formation of martensite after cooling. Therefore, the heating temperature of the electric resistance welded part in the in-line heat treatment is limited to the range of 800° C. to 1150° C. between the minimum temperature portion and the maximum temperature portion.
• the heating temperature is preferably 850° C. to 1100° C.
• the cooling after the heating may be performed by allowed to cooling or water cooling in accordance with the required strength and toughness, but water cooling is preferably employed to achieve both strength and toughness.
• an in-line tempering treatment may be optionally performed at a heating temperature (tempering temperature) of 400° C. to 700° C.
• the in-line tempering treatment is preferably performed using a series of induction heating apparatuses and the like arranged on the downstream side of the in-line heat treatment apparatus.
• the in-line heat treatment time is preferably 5 seconds or more at 800° C. or higher.
• a molten steel having a composition listed in Table 1 was refined with a converter, and slab (steel material: thickness 220 mm) was obtained by a continuous casting method.
• the slab (steel material) was subjected to a pipe material production step under the conditions listed in Table 2 to obtain a hot-rolled steel strip having a sheet thickness listed in Table 2.
• the hot-rolled steel strip was coiled to obtain a pipe material.
• the coiled hot-rolled steel strip serving as a pipe material was uncoiled and continuously cold-formed using a plurality of rolls to obtain an open pipe having a substantially circular section.
• the average cooling rate of the pipe outer surface in the electric resistance welded part was about 2° C./s.
• test specimen was sampled from the obtained electric resistance welded steel pipe, and microstructure observation, a tensile test, and an impact test were conducted.
• the test methods are as follows.
• a test specimen for observing a microstructure was sampled from the obtained electric resistance welded steel pipe so that a section in the rolling direction (L section) served as an observation surface.
• the sampled test specimen for observing a microstructure was polished and etched (etchant: nital).
• a microstructure at a position of 1 ⁇ 2t of the sheet thickness was observed with an optical microscope (magnification: 400 times) and a scanning electron microscope SEM (magnification: 2000 times), and two or more view area were photographed in each of the specimens. From the obtained photographs of the microstructures, the type of microstructure, the area fraction of each phase, and the aspect ratio of crystal grains of the main phase were determined using an image analyzer.
• the average grain size of the main phase was determined by a cutting method in conformity with JIS G 0551.
• the arithmetic mean of the obtained values was used as a value of the steel pipe.
• an area fraction was determined for retained austenite by a SEM/EBSD (electron backscatter diffraction) method because it was difficult to visually distinguish retained austenite. On the assumption that the microstructure was three-dimensionally homogeneous, the determined area fraction was defined as a volume fraction.
• a tensile test specimen was sampled from the obtained electric resistance welded steel pipe at a 900 clockwise position in a circumferential direction from the electric resistance welded part when viewed from the leading end of the pipe.
• the tensile test specimen was sampled in conformity with ASTM A 370 so that the tensile direction was a pipe axial direction.
• a tensile test was conducted to determine tensile characteristics (yield strength YS, tensile strength TS, and uniform elongation Elu).
• a V-notch test specimen was sampled from the electric resistance welded part of the obtained electric resistance welded steel pipe at a position of 1 ⁇ 2 the thickness so that the circumferential direction was a longitudinal direction of the test specimen.
• a Charpy impact test was conducted in conformity with ASTM A 370 to determine a Charpy impact test absorbed energy vE 0 (J) at a test temperature of 0° C. Three test specimens were used for the test, and the arithmetic mean of the values was defined as an absorbed energy of the steel pipe.
• a high-strength electric resistance welded steel pipe includes a base material portion having high strength with a yield strength YS in a pipe axial direction of 415 MPa or more and “excellent bendability” with a uniform elongation Elu in a pipe axial direction of 8% or more, and an electric resistance welded part having excellent toughness with a Charpy impact test absorbed energy vE 0 of 150 J or more at 0° C.
• the uniform elongation Elu in a pipe axial direction is less than 8% and the bendability deteriorates.

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