EP4585705A1 - Warmgewalztes stahlblech, widerstandsgeschweisstes stahlrohr, rechteckiges stahlrohr, leitungsrohr und baustruktur - Google Patents

Warmgewalztes stahlblech, widerstandsgeschweisstes stahlrohr, rechteckiges stahlrohr, leitungsrohr und baustruktur

Info

Publication number
EP4585705A1
EP4585705A1 EP23888294.8A EP23888294A EP4585705A1 EP 4585705 A1 EP4585705 A1 EP 4585705A1 EP 23888294 A EP23888294 A EP 23888294A EP 4585705 A1 EP4585705 A1 EP 4585705A1
Authority
EP
European Patent Office
Prior art keywords
less
steel pipe
electric resistance
hot
resistance welded
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP23888294.8A
Other languages
English (en)
French (fr)
Other versions
EP4585705A4 (de
Inventor
Akihide MATSUMOTO
Naomichi IWATA
Shinsuke Ide
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
JFE Steel Corp
Original Assignee
JFE Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
Publication of EP4585705A1 publication Critical patent/EP4585705A1/de
Publication of EP4585705A4 publication Critical patent/EP4585705A4/de
Pending legal-status Critical Current

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Classifications

    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, rods, wire, tubes, profiles or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, rods, wire, tubes, profiles 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
    • B21C37/15—Making tubes of special shape; Making tube fittings
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • C21D8/0426—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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/04—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
    • C22C38/08—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
    • C22C38/22—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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, rods, wire, tubes, profiles or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, rods, wire, tubes, profiles 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
    • 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
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to an electric resistance welded steel pipe and a rectangular steel pipe, a hot-rolled steel sheet used as a material for these pipes, and a line pipe and a building structure using these.
  • An electric resistance welded steel pipe or a roll-formed rectangular steel pipe uses a hot-rolled steel sheet (hot-rolled steel strip) as a material. This is cold-rolled to form a cylindrical open pipe, and a butt portion is subjected to electric resistance welding (sometimes referred to as electrical resistance welding) to form a round steel pipe.
  • An electric resistance welded steel pipe is produced by adjusting the outer diameter and the roundness using a forming roller disposed outside the round steel pipe.
  • a rectangular steel pipe is produced by further roll-forming the round steel pipe into a rectangular shape using a roll with a target polygonal hole shape.
  • the method for producing a rectangular steel pipe by roll forming advantageously has higher productivity than a method for producing a steel pipe by press bend forming.
  • a high tensile strain is applied in the pipe axis direction during roll forming, and an electric resistance welded steel pipe or a roll-formed rectangular steel pipe therefore has problems of low ductility in the pipe axis direction and low buckling resistance.
  • a material to be subjected to roll forming is required to select an appropriate hot-rolled steel sheet (hot-rolled steel strip) in consideration of a decrease in ductility due to the roll forming.
  • Patent Literature 1 discloses a high-strength hot-rolled steel sheet with high uniform elongation after cold working, which is a steel containing, on a weight basis, C: 0.04% to 0.25%, N: 0.0050% to 0.0150%, and Ti: 0.003% to 0.050% and having an equivalent carbon content (Ceq.) in the range of 0.10% to 0.45% as determined using a predetermined formula and in which a pearlite phase ranges from 5% to 20% by area and TiN with an average particle size in the range of 1 to 30 ⁇ m is dispersed in the steel at a ratio of 0.0008% to 0.015% by weight.
  • C 0.04% to 0.25%
  • N 0.0050% to 0.0150%
  • Ti 0.003% to 0.050% and having an equivalent carbon content (Ceq.) in the range of 0.10% to 0.45% as determined using a predetermined formula and in which a pearlite phase ranges from 5% to 20% by area and TiN with an average particle size in the
  • Patent Literature 3 discloses an electric resistance welded steel pipe for a line pipe with a low yield ratio, characterized in that a dislocation introduced in a forming process is pinned by a carbon atom cluster, a fine carbide, and a Nb carbide during tempering after pipe production.
  • Patent Literature 4 discloses a low-yield-ratio rectangular steel pipe made of a hot-rolled steel sheet characterized in that ferrite is a main phase, a second phase frequency ranges from 0.05 to 0.15, a second phase area fraction ranges from 3% to 15%, and the average grain size of the main phase and the second phase at a quarter thickness of the steel sheet ranges from 10 to 25 ⁇ m.
  • Patent Literature 5 discloses a rectangular steel pipe that is produced by hot forming and has high deformability and toughness.
  • the present invention has been made in view of these circumstances and aims to provide an electric resistance welded steel pipe or a rectangular steel pipe with high buckling resistance and a hot-rolled steel sheet used as a material thereof.
  • the present invention also aims to provide a line pipe or a building structure using the electric resistance welded steel pipe or the rectangular steel pipe.
  • t denotes the wall thickness (mm) of the electric resistance welded steel pipe or the rectangular steel pipe
  • D denotes the outer diameter (mm) of the electric resistance welded steel pipe
  • B denotes the side length (mm) of the rectangular steel pipe
  • ⁇ max denotes the maximum stress intensity (N/mm 2 ) in the axial compression test.
  • the hot-rolled steel sheet as the material includes a hot-rolled steel strip.
  • Fig. 1 is a schematic view of a test piece for a tensile test used for measurement of an equivalent plastic strain distribution.
  • a hot-rolled steel sheet according to the present invention has a chemical composition containing, on a mass percent basis, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less, with the balance being Fe and incidental impurities.
  • a steel microstructure at the center of the sheet thickness of the hot-rolled steel sheet contains 70% or more and 98% or less by volume of ferrite and bainite in total, the remainder being one or two or more selected from pearlite, martensite, and austenite, has an average grain size of 15.0 ⁇ m or less, and has a CP value of 0.090 or less as determined using the following formula (1).
  • the tensile strength is 400 MPa or more, and the yield ratio is 90% or less.
  • the logarithmic standard deviation of the equivalent plastic strain distribution after application of an 8.0% tensile strain is preferably 0.70 or less.
  • CP total length of high-angle grain boundaries in a region excluding crystal grains with a grain size of less than 20 ⁇ m)/(total length of high-angle grain boundaries)
  • An electric resistance welded steel pipe includes a base metal portion and an electric resistance weld and has a chemical composition containing, on a mass percent basis, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less, with the balance being Fe and incidental impurities.
  • a steel microstructure at the center of the wall thickness of the elctric resistance welded steel pipe contains 70% or more and 98% or less by volume of ferrite and bainite in total, the remainder being one or two or more selected from pearlite, martensite, and austenite, has an average grain size of 15.0 ⁇ m or less, and has a CP value of 0.090 or less as determined using the formula (1).
  • the base metal portion has a tensile strength of 400 MPa or more, and the base metal portion has a yield ratio of 97% or less.
  • the logarithmic standard deviation of the equivalent plastic strain distribution after application of a 4.0% tensile strain in the base metal portion is preferably 0.60 or less.
  • the crystal grains have an average grain size of 15.0 ⁇ m or less.
  • the crystal grains preferably have an average grain size of 13.0 ⁇ m or less, more preferably 10.0 ⁇ m or less.
  • a small average grain size results in a high yield ratio, so that the average grain size is preferably 2.0 ⁇ m or more.
  • the average grain size is more preferably 3.0 ⁇ m or more.
  • the CP value is a numerical value representing the connectivity between coarse grains with a grain size of 20 ⁇ m or more and is determined using the following formula (1).
  • a high CP value results in a high proportion of grain boundaries between coarse crystal grains and more coarse grains connected to each other.
  • the CP value is more than 0.090, strains generated in coarse grains during deformation are connected to each other, and the strain distribution becomes more nonuniform as the deformation proceeds, so that an appropriate logarithmic standard deviation of the equivalent plastic strain distribution intended in the present invention cannot be achieved.
  • the CP value is 0.090 or less.
  • the CP value is preferably 0.080 or less, more preferably 0.070 or less.
  • CP (total length of high-angle grain boundaries in a region excluding crystal grains with a grain size of less than 20 ⁇ m)/(total length of high-angle grain boundaries)
  • the "total length of high-angle grain boundaries in a region excluding crystal grains with a grain size of less than 20 ⁇ m" in the formula (1) is the total length of high-angle grain boundaries in a portion where crystal grains with a grain size of 20 ⁇ m or more are adjacent to each other.
  • Tensile strength of hot-rolled steel sheet 400 MPa or more
  • Yield ratio of hot-rolled steel sheet 90% or less
  • the equivalent plastic strain distribution can be approximated by a logarithmic normal distribution with the horizontal axis representing the equivalent plastic strain (unit: none) and the vertical axis representing the fraction (area fraction) (unit: %).
  • the logarithm of a variable follows a normal distribution.
  • the horizontal axis represents the natural logarithm of the equivalent plastic strain (unit: none)
  • the vertical axis represents the fraction (area fraction) (unit: %).
  • the standard deviation at this time is defined as the "logarithmic standard deviation". As the logarithmic standard deviation decreases, a peak of the equivalent plastic strain distribution becomes sharper, and the plastic strain distribution becomes more uniform.
  • the logarithmic standard deviation of an equivalent plastic strain distribution after application of an 8.0% tensile strain to a hot-rolled steel sheet is 0.70 or less, it is easier to achieve an appropriate logarithmic standard deviation of an equivalent plastic strain distribution of an electric resistance welded steel pipe and an appropriate logarithmic standard deviation of an equivalent plastic strain distribution of a rectangular steel pipe intended in the present invention.
  • the logarithmic standard deviation after application of an 8.0% tensile strain to a hot-rolled steel sheet is preferably 0.70 or less.
  • the logarithmic standard deviation is more preferably 0.68 or less, still more preferably 0.65 or less.
  • the logarithmic standard deviation is preferably as small as possible and has no particular lower limit, an excessive decrease results in an increase in production costs and production load, so that the logarithmic standard deviation is preferably 0.050 or more.
  • the tensile strength of a base metal portion of an electric resistance welded steel pipe and the tensile strength of a flat portion of a rectangular steel pipe are less than 400 MPa, the buckling resistance decreases.
  • the tensile strength is 400 MPa or more.
  • the tensile strength is preferably 420 MPa or more, more preferably 450 MPa or more.
  • the upper limit of the tensile strength is not particularly limited, the tensile strength is, for example, 700 MPa or less.
  • the yield ratio of a base metal portion of an electric resistance welded steel pipe and the yield ratio of a flat portion of a rectangular steel pipe are more than 97%, the buckling resistance decreases.
  • the yield ratio is 97% or less.
  • the yield ratio is preferably 96% or less, more preferably 95% or less.
  • the lower limit of the yield ratio is not particularly limited, the yield ratio is, for example, 75% or more.
  • the logarithmic standard deviation of an equivalent plastic strain distribution after application of a 4.0% tensile strain in a base metal portion of an electric resistance welded steel pipe and a flat portion of a rectangular steel pipe is 0.60 or less, buckling resistance is more likely to be improved.
  • the logarithmic standard deviation is preferably 0.60 or less.
  • the logarithmic standard deviation is more preferably 0.58 or less, still more preferably 0.55 or less.
  • the logarithmic standard deviation is preferably as small as possible and has no particular lower limit, an excessive decrease results in an increase in production costs and production load, so that the logarithmic standard deviation is preferably 0.050 or more.
  • the tensile strength and the yield ratio can be measured in a tensile test described later in Examples. Furthermore, the logarithmic standard deviation of an equivalent plastic strain distribution can be measured by combining a tensile test and a SEM-DIC method described later in Examples. More specifically, the logarithmic standard deviation of an equivalent plastic strain distribution can be determined by a method described later in Examples.
  • a hot-rolled steel sheet according to the present invention can be produced, for example, by performing a heating step of heating a steel material with the above-described chemical composition to a heating temperature of 1100°C or more and 1300°C or less, then performing a hot rolling step of rolling the steel material at a finish rolling delivery temperature of 750°C or more and 850°C or less and at an average cooling rate of 1.0°C/s or more in the temperature range of 900°C or more in terms of the temperature at the center of the sheet thickness to produce a hot-rolled sheet, after the hot rolling step, performing a cooling step of cooling the hot-rolled sheet at an average cooling rate of 5°C/s or more and 50°C/s or less from the start of cooling to the stop of cooling in terms of the temperature at the center of the sheet thickness and at a cooling stop temperature of 400°C or more and 650°C or less, and after the cooling step performing a coiling step of coiling the hot-rolled sheet.
  • an electric resistance welded steel pipe according to the present invention is produced by forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming, butt-welding both circumferential end portions of the cylindrical shape by electric resistance welding, and then adjusting the outer diameter and the roundness by cold forming using a roll with a perfect circular hole shape.
  • a rectangular steel pipe according to the present invention is produced by forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming, butt-welding both circumferential end portions of the cylindrical shape by electric resistance welding, and then forming a flat portion and a corner portion by cold forming using a roll with a target polygonal hole shape.
  • a rectangular steel pipe according to the present invention includes a regular polygon (equilateral triangle, square, regular pentagon, or the like), an equilateral polygon with a combination of different interior angles (rhombus, star, or the like), and a polygon with a combination of different side lengths (isosceles triangle, rectangle, parallelogram, trapezoid, or the like).
  • the rectangular steel pipe preferably has a square or rectangular cross section.
  • cylindrical shape means that the cross section of the pipe has a "C” shape.
  • the temperature expressed in "°C” is the surface temperature of a steel material or a steel sheet (hot-rolled sheet).
  • the surface temperature can be measured with a radiation thermometer or the like.
  • the temperature at the center of the sheet thickness of a steel sheet can be determined by calculating the temperature distribution in a cross section of the steel sheet by heat transfer analysis and correcting the result by the surface temperature of the steel sheet.
  • a steel material (steel slab) can be melted by any method, for example, by a known melting method using a converter, an electric arc furnace, a vacuum melting furnace, or the like. Any casting method may be used, and a desired size is obtained by a known casting method, such as a continuous casting method. Instead of the continuous casting method, an ingot casting and blooming method may be applied without any problem. Molten steel may be further subjected to secondary refining, such as ladle refining.
  • a steel material (steel slab) thus produced is then heated to a heating temperature of 1100°C or more and 1300°C or less and is then subjected to a hot rolling step at a finish rolling delivery temperature of 750°C or more and 850°C or less and at an average cooling rate of 1.0°C/s or more in the temperature range of 900°C or more in terms of the temperature at the center of the sheet thickness to produce a hot-rolled sheet.
  • Heating temperature 1100°C or more and 1300°C or less
  • the heating temperature in a furnace before hot rolling is 1100°C or more and 1300°C or less.
  • the heating temperature is more preferably 1120°C or more.
  • the heating temperature is more preferably 1280°C or less.
  • the steel slab may be temporarily cooled to room temperature and then reheated by a known method.
  • a steel slab may be subjected without problems to an energy-saving process, such as hot direct rolling, in which a hot slab is charged directly into a furnace or is immediately rolled after slight heat retention.
  • Finish rolling delivery temperature 750°C or more and 850°C or less
  • the finish rolling delivery temperature is 750°C or more and 850°C or less.
  • the finish rolling delivery temperature is more preferably 760°C or more.
  • the finish rolling delivery temperature is more preferably 840°C or less.
  • Average cooling rate in the temperature range of 900°C or more in terms of the temperature at the center of the sheet thickness 1.0°C/s or more
  • the average cooling rate in the temperature range of 900°C or more in terms of the temperature at the center of the sheet thickness (hereinafter sometimes referred to as the average cooling rate in hot rolling) can be increased to suppress coarsening of austenite in an austenite recrystallization temperature range and achieve the average grain size and the CP value intended in the present invention.
  • a material to be rolled may be cooled using water cooling equipment during rolling.
  • austenite is coarsened in the austenite recrystallization temperature range, and it is difficult to ensure the average grain size intended in the present invention.
  • the average cooling rate is preferably 1.2°C/s or more, more preferably 1.5°C/s or more. Since the equipment load increases at an average cooling rate of more than 5.0°C/s, the average cooling rate is preferably 5.0°C/s or less.
  • the average cooling rate in the temperature range of 900°C or more in terms of the temperature at the center of the sheet thickness is determined as the average cooling rate at the center of the sheet thickness from when a steel material (steel slab) is extracted from a furnace to when the temperature at the center of the sheet thickness reaches 900°C. That is, the average cooling rate is determined by [(the temperature at the center of the sheet thickness (°C) when a steel material is extracted from a furnace - 900 (°C))/the time (s) from when the steel material is extracted from the furnace to when the temperature at the center of the sheet thickness of the steel material reaches 900°C].
  • the hot-rolled sheet is subjected to a cooling step.
  • cooling is performed at an average cooling rate of 5°C/s or more and 50°C/s or less from the start of cooling to the stop of cooling and at a cooling stop temperature of 400°C or more and 650°C or less.
  • the average cooling rate in the temperature range from the start of cooling to the stop of cooling described later in terms of the temperature at the center of the sheet thickness of the hot-rolled sheet (hereinafter sometimes referred to as the average cooling rate in the cooling step) is less than 5°C/s, the nucleation frequency of ferrite decreases, and ferrite grains are coarsened, so that it is difficult to ensure the average grain size intended in the present invention. Furthermore, it is difficult to suppress the formation of coarse grains, and it is difficult to control the CP value in the target range of the present invention. On the other hand, when the average cooling rate is more than 50°C/s, a large amount of martensite is formed, and the total volume fraction of ferrite and bainite intended in the present invention cannot be achieved.
  • the average cooling rate is preferably 7°C/s or more, more preferably 10°C/s or more.
  • the average cooling rate is preferably 45°C/s or less, more preferably 40°C/s or less.
  • an intentional cooling start time point of water cooling or the like is defined as the start of cooling, and natural cooling before that is not included in the cooling.
  • cooling is preferably started immediately after completion of finish rolling.
  • the cooling stop temperature is preferably 420°C or more, more preferably 450°C or more.
  • the cooling stop temperature is preferably 620°C or less, more preferably 600°C or less.
  • the average cooling rate in the cooling step is a value determined by ((the temperature at the center of the sheet thickness of the hot-rolled sheet before cooling - the temperature at the center of the sheet thickness of the hot-rolled sheet after cooling)/cooling time).
  • the cooling method may be water cooling, such as injection of water from a nozzle, cooling by injection of a coolant gas, or the like.
  • a cooling operation is preferably performed on both sides of the hot-rolled sheet so that both sides of the hot-rolled sheet are cooled under the same conditions.
  • the cooling step is followed by a coiling step of coiling the hot-rolled sheet and then naturally cooling the hot-rolled sheet.
  • the hot-rolled steel sheet thus produced was formed into a cylindrical open pipe (round steel pipe) by cold roll forming, and butt portions of the open pipe were electric-resistance-welded to produce a steel pipe material.
  • the steel pipe material was then formed by rolls arranged vertically and horizontally to produce an electric resistance welded steel pipe with an outer diameter D (mm) and a wall thickness t (mm) or a rectangular steel pipe with a side length B (mm) and a wall thickness t (mm) shown in Table 3.
  • the rectangular steel pipe has a square cross section.
  • test specimen was taken from the hot-rolled steel sheet, the electric resistance welded steel pipe, or the rectangular steel pipe thus produced, and microstructure observation, a tensile test, and measurement of an equivalent plastic strain distribution described below were performed.
  • Ferrite is a product of diffusional transformation and has a microstructure with a low dislocation density and almost recovered. This includes polygonal ferrite and quasi-polygonal ferrite. Bainite is a two-phase microstructure of cementite and lath-shaped ferrite with a high dislocation density. Pearlite is a microstructure with layered cementite and ferrite. Austenite has no cement compared with bainite. Martensite and austenite were distinguished from bainite due to high contrast in the SEM image.
  • the volume fraction of martensite was determined by measuring the area fraction of a microstructure observed as martensite or austenite from the SEM image and subtracting the volume fraction of austenite measured by a method described later from the area fraction.
  • a JIS No. 5 test piece for tensile test was taken such that the tensile direction was parallel to the rolling direction.
  • the test piece for tensile test was taken from a 1/4W (W: sheet width) position in the width direction from a widthwise end portion for a hot-rolled steel sheet, was taken from a position separated by 90 degrees in the circumferential direction from an electric resistance weld for an electric resistance welded steel pipe, and was taken from a flat portion adjacent to a flat portion including an electric resistance weld for a rectangular steel pipe.
  • the tensile test was performed in accordance with the provisions of JIS Z 2241 (2011), and the yield stress ⁇ y and the tensile strength were measured to calculate the yield ratio defined by (yield stress ⁇ y)/(tensile strength).
  • the equivalent plastic strain distribution was measured by the SEM-DIC method.
  • a test piece for tensile test illustrated in Fig. 1 was taken from the center of the sheet thickness of a hot-rolled steel sheet, the center of the wall thickness of an electric resistance welded steel pipe, or the center of the wall thickness of a rectangular steel pipe such that the tensile direction was parallel to the rolling direction.
  • sampling was performed at 1/4W (W: sheet width) in the width direction from a widthwise end portion.
  • W sheet width
  • sampling was performed at a position separated by 90 degrees in the circumferential direction from an electric resistance weld.
  • the DIC method is a method of measuring a displacement or a strain at each portion of an observation surface by comparing a random pattern of an object surface before and after deformation. More specifically, a rectangular region called a subset is defined in an image before deformation, and the subset is tracked before and after deformation based on a random pattern inside the subset to calculate the displacement of the subset center point.
  • This operation is exhaustively performed on the entire image to obtain a displacement distribution and a strain distribution.
  • a nital etching mark of a metallic microstructure was used as a random pattern, and for an image of 1910 pixels x 2560 pixels the subset size was 80 pixels x 80 pixels (3.6 ⁇ m x 3.6 ⁇ m), and the measurement interval was 10 pixels (0.45 ⁇ m).
  • the logarithmic standard deviation was determined by the following method.
  • the fraction (area fraction) (unit: %) of each class was determined at a class width of 0.02.
  • a class with an equivalent plastic strain of 0 or more and less than 0.02 was defined as a first class
  • a class with an equivalent plastic strain of 0.02 or more and less than 0.04 was defined as a second class
  • a class with an equivalent plastic strain of 0.18 or more and less than 0.20 was defined as a tenth class.
  • a pressure-resistant plate was attached to both ends of an electric resistance welded steel pipe and a rectangular steel pipe, and an axial compression test was performed with a large compression testing machine.
  • the stress at the maximum compressive load was defined as the maximum stress intensity ⁇ max (N/mm 2 ).
  • Table 4 shows the results for the hot-rolled steel sheets.
  • No Steel microstructure of hot-rolled steel sheet Mechanical properties of hot-rolled steel sheet Notes F fraction (%) B fraction (%) F+B fraction (%) Remaining microstructure Average grain size ( ⁇ m) CP
  • Tensile strength (MPa) Yield ratio (%) Logarithmic standard deviation 1 2 96 98 M 4.7 0.053 604 83.3 0.58 Inventive example 2 6 91 97 M 4.2 0.043 593 80.2 0.60 Inventive example 3 65 8 73 P,M,A 13.5 0.070 419 75.9 0.52 Inventive example 4 33 49 82 P 8.4 0.021 568 75.0 0.34 Inventive example 5 47 44 91 P 5.1 0.027 559 78.5 0.41 Inventive example 6 28 58 86 P 7.3 0.061 527 81.4 0.55 Inventive example 7 86 10 96 M 12.4 0.034 384 71.7 0.54 Comparative example 8 17 49 66 P,M 5.2 0.068 613
  • Table 5 shows the results for the electric resistance welded steel pipes and the rectangular steel pipes.
  • No Steel pipe type Steel microstructure Mechanical properties Axial compressibility of steel pipe Notes F fraction (%) B fraction (%) F+B fraction (%) Remaining microstructure Average grain size ( ⁇ m) CP Yield stress ⁇ y (MPa) Tensile strength (MPa) Yield ratio (%) Logarithmic standard deviation Maximum stress intensity ⁇ max (MPa) Proof strength increase rate ⁇ Necessary lower limit of ⁇ 1 Electric resistance welded steel pipe 3 93 96 M 4.7 0.050 577 630 91.6 0.46 640 1.11 1.02 Inventive example 2 Rectangular steel pipe 4 94 98 M 4.5 0.041 566 621 91.1 0.42 611 1.08 1.02 Inventive example 3 Electric resistance welded steel pipe 69 5 74 P,M,A 13.8 0.076 362 427 84.8 0.55 431 1.19 1.09 Inventive example 4 Rectangular steel pipe 36 44 80 P 6.5 0.020 512 585 8
  • Nos. 1 to 6 are examples of the present invention, and Nos. 7 to 12 are comparative examples.
  • a steel microstructure at the center of the sheet thickness contained 70% or more and 98% or less by volume of ferrite and bainite in total, the remainder being one or two or more selected from pearlite, martensite, and austenite, had an average grain size of 15.0 ⁇ m or less, and had a CP value of 0.090 or less as determined using the specified formula (1).
  • the tensile strength was 400 MPa or more, and the yield ratio was 90% or less.
  • the logarithmic standard deviation of an equivalent plastic strain distribution after application of an 8.0% tensile strain was 0.70 or less.
  • An electric resistance welded steel pipe or a rectangular steel pipe according to an example of the present invention was produced from a hot-rolled steel sheet of an example of the present invention, and a steel microstructure at the center of the wall thickness contained 70% or more and 98% or less by volume of ferrite and bainite in total, the remainder being one or two or more selected from pearlite, martensite, and austenite, had an average grain size of 15.0 ⁇ m or less, and had a CP value of 0.090 or less as determined using the specified formula (1). Furthermore, the base metal portion or the flat portion had a tensile strength of 400 MPa or more, and the base metal portion or the flat portion had a yield ratio of 97% or less.
  • the logarithmic standard deviation of an equivalent plastic strain distribution after application of a 4.0% tensile strain in the base metal portion or the flat portion was 0.60 or less.
  • Table 5 values obtained by substituting t and D of the electric resistance welded steel pipes and t and B of the rectangular steel pipes into the right sides of the formulae (2) and (3), respectively, are shown as the necessary lower limits of ⁇ .
  • Comparative Example No. 7 had a C content below the scope of the present invention and had a tensile strength outside the scope of the present invention.
  • Comparative Example No. 8 had a C content above the scope of the present invention and had a total volume fraction of ferrite and bainite below the scope of the present invention. Consequently, the yield ratio was outside the scope of the present invention, the logarithmic standard deviation was outside the preferred range, and the proof strength increase rate did not reach the desired value.
  • Comparative Example No . 10 had Si and Mn contents above the scope of the present invention and had a total volume fraction of ferrite and bainite below the scope of the present invention. Consequently, the yield ratio was outside the scope of the present invention, the logarithmic standard deviation was outside the preferred range, and the proof strength increase rate did not reach the desired value.
  • Comparative Example No. 12 had a cooling stop temperature in the hot rolling step above the suitable range of the production method and therefore had a CP value above the scope of the present invention. Consequently, the logarithmic standard deviation exceeded the preferred range of the present invention, and the proof strength increase rate did not reach the desired value.

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