WO2023053837A1 - 角形鋼管およびその製造方法、熱延鋼板およびその製造方法、並びに建築構造物 - Google Patents

角形鋼管およびその製造方法、熱延鋼板およびその製造方法、並びに建築構造物 Download PDF

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WO2023053837A1
WO2023053837A1 PCT/JP2022/032953 JP2022032953W WO2023053837A1 WO 2023053837 A1 WO2023053837 A1 WO 2023053837A1 JP 2022032953 W JP2022032953 W JP 2022032953W WO 2023053837 A1 WO2023053837 A1 WO 2023053837A1
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hot
steel pipe
bainite
present
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PCT/JP2022/032953
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English (en)
French (fr)
Japanese (ja)
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直道 岩田
晃英 松本
信介 井手
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Ｊｆｅスチール株式会社
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Priority to JP2022573502A priority Critical patent/JP7529049B2/ja
Priority to CN202280063910.7A priority patent/CN117980519A/zh
Priority to KR1020247009566A priority patent/KR20240053606A/ko
Publication of WO2023053837A1 publication Critical patent/WO2023053837A1/ja

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • B21C37/0803—Making tubes with welded or soldered seams the tubes having a special shape, e.g. polygonal tubes
    • 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
    • B21C37/15—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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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

Definitions

  • the present invention relates to a square steel pipe having high strength, a low yield ratio and excellent low-temperature toughness, which is particularly suitable for building structural members of large buildings, a method for producing the same, and a hot-rolled steel plate used as a raw material for the square steel pipe. and its manufacturing method, and a building structure using this square steel pipe.
  • building structural members used in large-scale buildings such as factories, warehouses, and commercial facilities have been increasing in strength in order to reduce construction costs by reducing weight.
  • buildings large-scale buildings
  • square steel pipe square column
  • corner portions which is used as a column material for buildings
  • high strength is required for the flat plate portion.
  • square steel pipes used for building structural members are also required to have both high plastic deformability and excellent low-temperature toughness. In order to meet these requirements, it is necessary to select an appropriate square steel pipe material.
  • Square steel pipes are generally made from hot-rolled steel sheets (hot-rolled steel strips) or thick steel sheets, and are manufactured by cold-forming this material.
  • Cold forming methods include a cold press bending method and a cold roll forming method.
  • a square steel pipe manufactured by roll-forming a raw material (hereinafter sometimes referred to as a roll-formed square steel pipe) is made by cold-roll-forming a hot-rolled steel plate into a cylindrical open pipe, and Sewing welding (sometimes called electric resistance welding) is performed. After that, the cylindrical round steel pipe is drawn by several percent in the axial direction by rolls arranged on the top, bottom, right and left of the round steel pipe, and then formed into a square shape to produce a square steel pipe.
  • square steel pipes manufactured by press-bending materials (hereinafter sometimes referred to as press-formed square steel pipes) are made by cold press-bending thick steel plates to form square cross-sectional shapes. shape) and the butt part is joined by submerged arc welding, or two members with a U-shaped cross section are butted and joined by submerged arc welding. may be manufactured.
  • the method of manufacturing roll-formed square steel pipes has the advantage of being highly productive and capable of being manufactured in a short period of time compared to the method of manufacturing press-formed square steel pipes.
  • press-formed square steel pipes only the corners are work-hardened without being cold-formed on the flat plate part
  • roll-formed square steel pipes when the pipe is cold-formed into a cylindrical shape in particular, the entire circumference of the pipe is hardened. A large working strain is introduced in the axial direction. Therefore, the roll-formed square steel pipe has a high yield ratio in the tube axial direction not only at the corners but also at the flat portions, and has a problem of low low-temperature toughness.
  • the thicker the roll-formed square steel pipe the greater the work hardening during roll forming, so the yield ratio becomes higher and the toughness decreases. Therefore, especially when a thick-walled roll-formed square steel pipe is manufactured, it is necessary to select an appropriate material in consideration of changes in mechanical properties such as an increase in yield ratio and a decrease in toughness due to roll-forming.
  • Patent Document 1 proposes a square steel pipe in which the area fraction of bainite structure is 40% or more in the microstructure of the flat plate portion.
  • Patent document 2 proposes a square steel pipe with excellent weldability and plastic deformation capability of cold-worked parts, with the steel composition and cleanliness within a predetermined range.
  • Patent Document 3 proposes a square steel pipe having a low yield ratio and high toughness by subjecting the whole pipe to strain relief annealing after pipemaking by cold forming.
  • Patent Document 4 when the steel composition is within a predetermined range and the crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals, the average circle equivalent diameter of the crystal grain is 7.0 ⁇ m.
  • a square steel pipe has been proposed in which the total volume fraction of the crystal grains having an equivalent circle diameter of 40.0 ⁇ m or more is 30% or less of the entire steel structure at the 1/4t position.
  • Patent Document 5 the steel composition is set to a predetermined range, and the steel structure at a position of 1/4 t of the plate thickness t from the outer surface of the steel pipe has a ratio of the total area ratio of bainite and pearlite to the area ratio of ferrite of 2.0.
  • a square steel pipe has been proposed in which the ratio of the area ratio of bainite to the area ratio of pearlite is 5.0 or more and 20.0 or less.
  • Patent Document 6 in terms of % by mass, C ⁇ 0.02%, Si ⁇ 1.0%, Mn: 0.05-2.0%, S ⁇ 0.02%, Al: 0.01-0. 1%, Nb: 0.08 to 0.25%, Ti ⁇ 0.2%, B ⁇ 0.0020%, and one or more of Ni, Cr, Sn, and Cu in a total amount of 0.00 02% or more and 0.3% or less, the balance being Fe and unavoidable impurities, and the Nb amount satisfying Nb ⁇ 0.05 + 7.75C - 1.98Ti + 6.64N + 0.000035 / (B + 0.0004),
  • the ferrite phase in the metal structure has a volume fraction of 70% or more, the ferrite grain size is 10.5 or more and 15 or less in grain size number, and the yield ratio at room temperature is 70% or less, so that toughness is excellent.
  • a hot-rolled steel sheet for fire resistance with a low yield ratio is disclosed.
  • Patent Document 7 in mass%, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, composition consisting of the balance Fe and inevitable impurities, ferrite as the main phase, and pearlite or pearlite and bainite as the second phase.
  • the second phase frequency defined by the predetermined formula is 0.20 to 0.42, and the average grain size including the main phase and the second phase is 7 to 15 ⁇ m.
  • a thick hot-rolled steel plate for square steel pipes for building structural members is disclosed.
  • Patent Document 8 C: 0.06 to 0.12% (meaning % by mass, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, Al: 0.01 to 0.06%, P: 0.025% or less (not including 0%), S: 0.01% or less (not including 0%), Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002 to 0.009%, and B: 0.0005 to 0.003%, and the carbon equivalent Ceq defined by the predetermined formula is 0 .40% or less, with the balance consisting of iron and unavoidable impurities, consisting of a structure mainly composed of the bainite phase, and adjacent When a region surrounded by large-angle grain boundaries with a crystal misorientation of 15° or more is defined as a crystal grain, the average equivalent circle diameter D A of the crystal grain measured by an electron backscatter diffraction pattern method is 10 ⁇ m or less, and The grain size of the crystal grains measured by the electron backsc
  • Patent Document 9 contains C: 0.04 to 0.25%, N: 0.0050 to 0.0150% and Ti: 0.003 to 0.050% by weight, and is calculated by a predetermined formula A steel with a carbon equivalent (Ceq.) of 0.10 to 0.45%, a pearlite phase in an area fraction of 5 to 20%, and an average grain size in the steel of 1 to 20%.
  • a high-strength hot-rolled steel sheet with excellent uniform elongation after cold working that is, a low yield ratio
  • Patent Document 10 discloses that the carbon equivalent Ceq calculated from the steel composition (% by mass) is 0.33% or more and 0.43% or less, the weld crack sensitivity composition PCM is 0.15% or more and 0.24% or less, and the welding A thick steel plate for cold press-forming square steel pipe is disclosed, which is made of steel having a composition in which the heat affected zone toughness index f HAZ is 0.30% or more and 0.47% or less.
  • the thick steel plate for cold press-formed square steel pipes of Patent Document 10 has a steel structure composed of ferrite and the balance of bainite or pearlite.
  • Patent Document 11 in mass%, C: 0.05 to 0.20%, Si: 0.10 to 0.40%, Mn: 1.20 to 1.50%, Al: 0.003 to 0 0.06%, Ti: 0.005 to 0.050%, the balance being Fe and impurities, and a steel material satisfying a Ceq defined by a predetermined formula of 0.34 or more is heated to 900 to 1200 ° C. After heating, rolling is started, and after rolling is completed at Ar 3 point or higher, water cooling is performed from Ar 3 point or lower to Ar 3 point ⁇ 400° C. or lower, and then tempering is performed at 500° C. or lower. is disclosed.
  • the steel plate for square steel pipes of Patent Document 11 has a steel structure composed of soft ferrite and hard bainite or martensite.
  • Patent Document 12 discloses that the steel composition is within a predetermined range, and the steel structure at a position of 1/2 t of the plate thickness t from the steel plate surface has a volume fraction of more than 30% ferrite and 10% or more bainite.
  • the sum of ferrite and bainite is 70% or more and 95% or less of the entire steel structure at the 1/2t position, and the balance is one or more selected from pearlite, martensite, and austenite, and is adjacent to each other
  • the crystal grain has an average equivalent circle diameter of less than 7.0 ⁇ m and an equivalent circle diameter of 40.0 ⁇ m or more.
  • a hot-rolled steel sheet has been proposed in which the total grain content is 30% or less by volume with respect to the entire steel structure at the 1/2t position.
  • Patent Documents 1 and 2 are based on the premise of manufacturing square steel pipes by press bending. Therefore, when applying the techniques described in Patent Documents 1 and 2 to roll-formed square steel pipes whose mechanical properties are significantly deteriorated during cold forming, there is a problem that the yield ratio and toughness cannot be achieved at the same time.
  • the techniques described in Patent Documents 1 and 2 only evaluate the Charpy absorbed energy (vE 0 ) at 0°C, and the results of evaluating toughness at low temperatures below 0°C are not described. No mention is made of whether it can be used in the environment.
  • Patent Document 3 With the technology described in Patent Document 3, in order to obtain a low yield ratio and high toughness, it is necessary to heat-treat square steel pipes after pipemaking. Therefore, the manufacturing cost is very high as compared with the square steel pipe as cold-worked.
  • Patent Document 5 in the production of a steel material, the number of times of resting for 30 seconds or more in a state where the plate thickness center temperature is 1000 ° C. or more until the rough rolling process of hot rolling is completed is 1 or more times. It was necessary to control to 5 times or less, and there was a problem regarding productivity.
  • the average crystal grain size including the main phase and the second phase is 7 to 15 ⁇ m. Within this average crystal grain size range, there is a problem that a tensile strength of 520 MPa or more cannot be obtained after roll forming.
  • the bainite phase is the main component (70 area% or more). Since the area ratio of hard bainite is high, there is a problem that the yield ratio of the steel sheet exceeds 0.75.
  • Patent Document 9 is a composite structure steel of soft ferrite and hard pearlite. For this reason, the yield ratio is low, but the toughness is poor, so there is a problem that the toughness required for square steel pipes cannot be secured.
  • Patent Document 11 The steel sheet manufactured by the above manufacturing method of Patent Document 11 requires tempering after hot rolling and subsequent cooling in order to make the yield ratio 80% or less. Therefore, it is disadvantageous in terms of manufacturing cost.
  • the present invention has been made in view of the above circumstances, and provides a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness suitable for building structural members, a method for producing the same, and a material for the square steel pipe.
  • An object of the present invention is to provide a hot-rolled steel sheet, a method for manufacturing the same, and a building structure using the square steel pipe.
  • the present invention is particularly suitable for application to thick square steel pipes and thick hot-rolled steel sheets used for thick square steel pipes.
  • high strength of the square steel pipe means that the yield strength of the flat plate portion of the square steel pipe manufactured by cold roll forming (hereinafter sometimes referred to as cold roll formed square steel pipe) is It refers to having a strength of 385 MPa or more and a tensile strength of the flat plate portion of 520 MPa or more.
  • excellent in low-temperature toughness of the square steel pipe means that the flat plate portion of the square steel pipe has a Charpy absorbed energy of 110 J or more at -20°C.
  • high strength refers to the hot-rolled steel sheet that is the raw material of the square steel pipe manufactured by cold roll forming (hereinafter sometimes referred to as cold roll-forming square steel pipe).
  • hot-rolled steel sheet for square steel pipe has a yield strength of 330 MPa or more and a tensile strength of 520 MPa or more.
  • the hot-rolled steel sheet of the present invention “excellent in low-temperature toughness” means that the material has a Charpy absorbed energy of 180 J or more at -20°C.
  • the term “thick” as used in the present invention means that the thickness and plate thickness are more than 5 mm and less than 26 mm.
  • the hot-rolled steel sheet as the raw material includes a hot-rolled steel strip.
  • the wall thickness referred to in the present invention refers to the thickness of the square steel pipe
  • the plate thickness refers to the thickness of the hot-rolled steel plate.
  • the rectangular steel pipe manufactured by cold roll forming should have a flat plate portion with a yield strength of 385 MPa or more, a flat plate portion with a tensile strength of 520 MPa or more, high plastic deformability, and excellent toughness.
  • the C content should be 0.04% by mass or more, and the main structure of the steel sheet should be a mixed structure of ferrite and bainite. It is necessary to
  • the residual structure of the steel sheet may be one or more selected from hard pearlite, martensite, and austenite. is necessary.
  • the C content must be 0.04% by mass or more.
  • the main structure at a depth of 1/4t (surface layer) of the wall thickness t from the outer surface of the square steel pipe must be a mixed structure of ferrite and bainite.
  • a square steel pipe having a flat plate portion and corner portions The component composition of the flat plate portion is % by mass, C: 0.04% or more and 0.45% or less, Si: 1.8% or less, Mn: 0.5% or more and 2.5% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.012% or more and 0.100% or less, with the remainder consisting of Fe and unavoidable impurities, The content of Nb and Ti satisfies the following formula (1), When the thickness of the flat plate portion is t, the steel structure of the flat plate portion at a depth of 1/4t of the wall thickness t from the outer surface of the pipe is The volume fraction is more than 30% ferrite and 10% or more bainite, The total of the ferrite and the
  • the number of crystal grains of .0 or more is 30 / mm 2 or less
  • 1.20 ⁇ %Nb ⁇ %Ti (1)
  • %Nb and %Ti are contents (% by mass) of each element.
  • the yield strength of the flat plate portion is 385 MPa or more
  • the tensile strength of the flat plate portion is 520 MPa or more
  • the yield ratio of the flat plate portion is 0.90 or less
  • the Charpy absorbed energy of the flat plate portion at -20 ° C. is 110 J or more.
  • the component composition is mass%, C: 0.04% or more and 0.45% or less, Si: 1.8% or less, Mn: 0.5% or more and 2.5% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, Nb: 0.005% or more and 0.050% or less, Ti: 0.012% or more and 0.100% or less, with the remainder consisting of Fe and unavoidable impurities,
  • the content of Nb and Ti satisfies the following formula (1)
  • the steel structure at the 1/4t position of the plate thickness t from the steel plate surface is The volume fraction is more than 30% ferrite and 10% or more bainite, The total of the ferrite and the bainite is 75% or more and 95% or less,
  • the balance consists of one or more selected from pearlite, martensite, and austenite, When a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between
  • the number of crystal grains of .0 or more is 30 / mm 2 or less, A hot-rolled steel sheet containing 20% or less by volume of crystal grains having an equivalent circle diameter of 40.0 ⁇ m or more. 1.20 ⁇ %Nb ⁇ %Ti (1) Here, %Nb and %Ti are contents (% by mass) of each element. [8] The hot-rolled steel sheet according to [7], which has a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more.
  • V 0.01% or more and 0.15% or less
  • Cr 0.01% or more and 1.0% or less
  • Mo 0.01% or more and 1.0% or less
  • Cu 0.01% or more and 0.5% or less
  • Ni 0.01% or more and 0.3% or less
  • Ca 0.0005% or more and 0.010% or less
  • B 0.0003% or more and 0.010% or less
  • the rough rolling finish temperature is 850° C. or higher and 1150° C. or lower
  • finish rolling is finished.
  • cooling stop temperature A method for manufacturing a hot-rolled steel sheet, comprising cooling at 450° C. or higher and 650° C. or lower and winding at 440° C. or higher and 650° C. or lower.
  • a hot-rolled steel sheet having high strength, a low yield ratio and excellent low temperature toughness and a method for producing the same and a steel sheet having high strength, a low yield ratio and excellent low temperature toughness.
  • a square steel pipe and a method for manufacturing the same can be provided.
  • FIG. 1 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention.
  • FIG. 2 is a schematic diagram showing the sampling positions of flat plate portion tensile test pieces of square steel pipes carried out in the present invention.
  • FIG. 3 is a schematic diagram showing the sampling positions of the Charpy test piece of the square steel pipe implemented in the present invention.
  • FIG. 4 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. of a square steel pipe and the number of crystal grains having a ratio of major axis to minor axis of the crystal grain of 4.0 or more.
  • FIG. 5 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. of a hot-rolled steel sheet and the number of crystal grains having a ratio of major axis to minor axis of the crystal grain of 4.0 or more.
  • the present invention relates to a square steel pipe having a flat portion and corner portions and a hot-rolled steel sheet used as a material for the square steel pipe, wherein the chemical composition of the flat portion and the hot-rolled steel sheet of the square steel pipe is 0.04% by mass and C: 0.04%.
  • Nb and Ti satisfies the formula (1), and the thickness t (meaning the wall thickness t and the plate thickness t; the same applies hereinafter) from the outer surface of the pipe and the surface of the steel plate 1/4t depth position
  • the steel structure in the volume fraction is more than 30% ferrite and 10% or more bainite, the total of the ferrite and the bainite is 75% or more and 95% or less, and the balance is pearlite, martensite, and austenite.
  • a crystal grain is a region surrounded by a boundary with an orientation difference of 15° or more between adjacent crystals
  • the major axis is 50 ⁇ m or more
  • the ratio of the major axis to the minor axis (major axis) / (minor axis)) is 4.0 or more
  • the number of crystal grains is 30/mm2 or less in the steel structure
  • the volume fraction of crystal grains with an equivalent circle diameter of 40.0 ⁇ m or more is 20%. It is below. 1.20 ⁇ %Nb ⁇ %Ti (1)
  • %Nb and %Ti are contents (% by mass) of each element.
  • % indicating steel composition is “% by mass” unless otherwise specified. Since the square steel pipe of the present invention is manufactured by cold roll forming a hot-rolled steel plate, the flat plate portion and the corner portions are made of the same hot-rolled steel plate, and the flat plate portion and the corner portions have the same chemical composition. is. On the other hand, since the welded portion is heated to a high temperature during welding, it reacts with oxygen in the atmosphere and is oxidized, so there is a possibility that the component composition is different from that of the flat portion and the corner portion. Since the volume of the weld zone in the total volume of the square steel pipe is small, the chemical composition of the weld zone has little effect on the characteristics of the square steel pipe. , either is fine.
  • C 0.04% to 0.45%
  • C is an element that increases the strength of steel by solid solution strengthening.
  • C is an element that promotes the formation of pearlite, improves hardenability, and contributes to the formation of bainite.
  • it is necessary to contain 0.04% or more of C.
  • the C content should be 0.04% or more and 0.45% or less.
  • the C content is preferably 0.08% or more, more preferably over 0.12%, and even more preferably 0.14% or more. Also, the C content is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.22% or less.
  • Si 1.8% or less
  • Si is an element that increases the strength of steel by solid-solution strengthening, and can be contained as necessary. In order to obtain such effects, it is desirable to contain 0.01% or more of Si. However, if the Si content exceeds 1.8%, oxides are likely to form in the electric resistance welded portion, resulting in deterioration of the welded portion properties. In addition, the toughness of the base metal portion other than the electric resistance welded portion is also lowered. Therefore, the Si content is set to 1.8% or less.
  • the Si content is preferably 0.01% or more, more preferably 0.10% or more. Also, the Si content is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.
  • Mn 0.5% to 2.5%
  • Mn is an element that increases the strength of steel through solid solution strengthening. Moreover, Mn is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to secure the strength and structure targeted in the present invention, it is necessary to contain 0.5% or more of Mn. However, when the Mn content exceeds 2.5%, the yield ratio of the flat plate portion of the square steel pipe exceeds 0.90 due to an excessive amount of bainite structure, and the desired yield ratio cannot be obtained. On the other hand, if the Mn content exceeds 2.5%, oxides are likely to form in the electric resistance welded portion, resulting in deterioration of the welded portion properties. Therefore, the Mn content should be 0.5% or more and 2.5% or less. The Mn content is preferably 0.7% or more, more preferably 0.9% or more, and even more preferably 1.0% or more. Also, the Mn content is preferably 2.0% or less.
  • P 0.10% or less P segregates at grain boundaries and causes nonhomogeneity of the material. Therefore, it is preferable to reduce P as an unavoidable impurity as much as possible, but a content of 0.10% or less is acceptable. Therefore, the P content should be within the range of 0.10% or less.
  • the P content is preferably 0.03% or less, more preferably 0.020% or less, and even more preferably 0.015% or less.
  • the lower limit of P is not specified, it is preferable to set P to 0.002% or more because excessive reduction leads to a rise in smelting costs.
  • S 0.05% or less S usually exists as MnS in steel, but MnS is thinly drawn in the hot rolling process and adversely affects ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.05% or less is permissible. Therefore, the S content should be 0.05% or less.
  • the S content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. Although the lower limit of S is not specified, excessive reduction leads to an increase in smelting costs, so S is preferably 0.0002% or more.
  • Al 0.005% to 0.100%
  • Al is an element that acts as a strong deoxidizing agent. In order to obtain such effects, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates and the amount of alumina-based inclusions increases, resulting in deterioration of the surface properties. Also, the toughness of the weld zone is reduced. Therefore, the Al content is set to 0.005% or more and 0.100% or less.
  • the Al content is preferably 0.010% or more, more preferably 0.015% or more. Also, the Al content is preferably 0.070% or less, more preferably 0.050% or less.
  • N 0.010% or less
  • N is an unavoidable impurity, and is an element that has the effect of lowering the toughness by firmly fixing the movement of dislocations.
  • the N content can be allowed up to 0.010%. Therefore, the N content is set to 0.010% or less.
  • the N content is preferably 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. Since an excessive reduction causes a rise in smelting costs, the N content is preferably 0.0010% or more, more preferably 0.0015% or more.
  • Nb 0.005% or more and 0.050% or less
  • Nb is an element that forms fine carbides and nitrides in steel and contributes to strength improvement of steel through precipitation strengthening. In order to obtain such effects, it is necessary to contain 0.005% or more. However, when the Nb content exceeds 0.050%, coarse carbides and nitrides are formed, and the formation of crystal grains with a large ratio of major axis to minor axis as described later is promoted, resulting in a decrease in toughness. may lead to Therefore, the Nb content should be 0.005% or more and 0.050% or less.
  • the Nb content is preferably 0.006% or more, more preferably 0.007% or more, and even more preferably 0.008% or more. Also, the Nb content is preferably 0.045% or less, more preferably 0.035% or less.
  • Ti 0.012% or more and 0.100% or less
  • Ti is an element that forms fine carbides and nitrides in steel and contributes to strength improvement of steel through precipitation strengthening. Moreover, when Ti is added in an appropriate amount, it is possible to improve the strength without promoting the formation of coarse crystal grains, and it is one of the most important elements in the present invention. In order to obtain such effects, it is necessary to contain 0.012% or more. However, when the Ti content exceeds 0.100%, coarse carbides and nitrides are formed, which may lead to a decrease in toughness. Therefore, the Ti content should be 0.012% or more and 0.100% or less. The Ti content is preferably 0.015% or more, more preferably 0.017% or more, and even more preferably 0.018% or more. Also, the Ti content is preferably 0.090% or less, more preferably 0.070% or less.
  • %Nb and %Ti are contents (% by mass) of each element. In the present invention, it is necessary that the contents of Nb and Ti be within the above ranges and that 1.20 ⁇ %Nb ⁇ %Ti is satisfied.
  • the low temperature toughness is lowered.
  • 1.50 ⁇ %Nb ⁇ %Ti more preferably 2.30 ⁇ %Nb ⁇ %Ti.
  • the balance is Fe and unavoidable impurities.
  • the O content may be 0.005% or less.
  • V less than 0.01%
  • Cr less than 0.01%
  • Mo less than 0.01%
  • Cu less than 0.01%
  • Ni less than 0.01%
  • Ca less than 0.0005%
  • B Less than 0.0003% can be included among the inevitable impurities.
  • the above ingredients are the basic ingredient composition of the square steel pipe in the present invention. Although the properties aimed at in the present invention can be obtained with the essential elements described above, the following elements can be contained as necessary.
  • V 0.01% to 0.15%
  • Cr 0.01% to 1.0%
  • Mo 0.01% to 1.0%
  • Cu 0.01% to 0.5%
  • Ni 0.01% or more and 0.3% or less
  • Ca 0.0005% or more and 0.010% or less
  • B 0.0003% or more and 0.010% or less
  • V 0.01% to 0.15%
  • Cr 0.01% to 1.0%
  • Mo 0.01% to 1.0%
  • V, Cr, and Mo are used for quenching steel It is an element that enhances the strength of steel and can be contained as necessary. In order to obtain the above effects, when V, Cr, and Mo are contained, it is preferable that V: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively.
  • V 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more.
  • an excessive content may lead to a decrease in toughness and deterioration of weldability. Therefore, when V, Cr, and Mo are contained, it is preferable that V: 0.15% or less, Cr: 1.0% or less, and Mo: 1.0% or less, respectively. More preferably, V: 0.10% or less, Cr: 0.50% or less, and Mo: 0.50% or less.
  • Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and are contained as necessary. be able to.
  • Cu when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferably, Cu: 0.10% or more and Ni: 0.10% or more.
  • an excessive content may lead to a decrease in toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.5% or less and Ni: 0.3% or less, respectively. More preferably, Cu: 0.40% or less and Ni: 0.20% or less.
  • Ca 0.0005% or more and 0.010% or less
  • Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process. can be contained.
  • the Ca content is preferably 0.010% or less. More preferably, the Ca content is 0.0050% or less.
  • B 0.0003% or more and 0.010% or less
  • B is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature.
  • the B content is 0.0005% or more.
  • the yield ratio may increase. Therefore, when B is contained, it is preferably 0.010% or less. More preferably, the B content is 0.0050% or less.
  • the steel structure at a depth position of 1/4t of the thickness t from the pipe outer surface of the steel pipe and the surface of the steel plate has a volume fraction of more than 30% ferrite and 10% or more bainite. and the sum of the ferrite and the bainite is 75% or more and 95% or less of the entire steel structure at a depth position of 1/4t of the thickness t from the outer surface of the pipe and the surface of the steel plate, and the balance is pearlite and marten It consists of one or more selected from site and austenite.
  • the number of crystal grains of 0.0 or more is 30/mm2 or less, and the crystal grains with an equivalent circle diameter (crystal grain size) of 40.0 ⁇ m or more are 1/4 t of the thickness t from the outer surface of the pipe and the surface of the steel plate. It is 20% or less in volume ratio with respect to the entire steel structure at the depth position.
  • the equivalent circle diameter is the diameter of a circle having the same area as the target crystal grain.
  • the steel structure of the square steel pipe is at a depth of 1/4t of the wall thickness t from the outer surface of the flat plate portion of the square steel pipe, excluding the electric resistance welded portion.
  • both the corner portion and the flat plate portion have the same steel structure at a depth of 1/4t of the wall thickness t from the outer surface of the pipe. Therefore, the steel structure of the flat plate portion is specified here.
  • the steel structure of the hot-rolled steel sheet is at a depth of 1/4t of the thickness t from the surface of the steel sheet.
  • volume fraction of ferrite more than 30%
  • volume fraction of bainite 10% or more
  • total volume fraction of ferrite and bainite to the steel structure 75% or more and 95% or less
  • the volume fraction of ferrite must exceed 30%.
  • the volume fraction of ferrite is preferably 40% or more, more preferably 43% or more, and even more preferably 45% or more.
  • the volume fraction of ferrite is preferably less than 75%, more preferably less than 70%, and still more preferably 60% or less in order to ensure a desired yield ratio. .
  • Bainite is a structure with intermediate hardness and increases the strength of steel. Since the yield strength and tensile strength targeted in the present invention cannot be obtained with the ferrite alone, the volume fraction of bainite must be 10% or more.
  • the volume fraction of bainite is preferably 15% or more, more preferably 20% or more, and still more preferably 25% or more. Although no particular upper limit is specified, the volume fraction of bainite is preferably 55% or less, more preferably 50% or less, and still more preferably 45% or less in order to ensure a desired yield ratio. , even more preferably less than 40%.
  • the total volume fraction of ferrite and bainite must be 75% or more and 95% or less. Preferably, it is 78% or more, preferably 93% or less. More preferably, it is 80% or more, and more preferably 90% or less.
  • the sum of the volume fractions of pearlite, martensite, and austenite should be 5% or more and 25% or less. Preferably, it is 7% or more, preferably 23% or less. More preferably, it is 10% or more, and more preferably 20% or less.
  • volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.
  • the steel structure is a steel in which a soft structure and a hard structure are mixed (hereinafter referred to as "composite structure steel") in order to obtain the low yield ratio, yield strength, and tensile strength aimed at in the present invention.
  • the ratio of the major axis to the minor axis of the crystal grains with the major axis of 50 ⁇ m or more exceeds 30 / mm2 , or the circle equivalent diameter is
  • crystal grains of 40.0 ⁇ m or more exceed 20% in volume ratio with respect to the entire steel structure at a depth position of 1/4t of the thickness t from the outer surface of the pipe and the surface of the hot-rolled steel sheet, the desired low temperature toughness cannot be obtained. .
  • the number of crystal grains with a major axis to minor axis ratio of 4.0 or more with a major axis of 50 ⁇ m or more is 30 / mm 2 or less, and a crystal grain with an equivalent circle diameter of 40.0 ⁇ m or more
  • the low-temperature toughness aimed at in the present invention can be ensured by setting the volume ratio to 20% or less with respect to the entire steel structure at a depth position of 1/4t of the thickness t from the surface of the hot-rolled steel sheet.
  • the number of crystal grains having a ratio of major axis to minor axis of 4.0 or more is preferably 28/mm 2 or less, more preferably 26/mm 2 or less.
  • the volume fraction of crystal grains having an equivalent circle diameter of 40.0 ⁇ m or more is preferably 18% or less, more preferably 16% or less.
  • Bainite does not grow across boundaries with large misorientation (austenite grain boundaries and sub-boundaries formed by accumulation of dislocations). Therefore, in order to suppress the formation of coarse bainite as described above, finish rolling in hot rolling is performed at a temperature as low as possible to introduce a large amount of dislocations into austenite to increase the area of sub-boundaries, resulting in a fine sub-grain structure. (hereinafter also referred to as “miniaturization”) is particularly effective.
  • the crystal misorientation, the average crystal grain size, and the volume fraction of crystal grains with a crystal grain size of 40.0 ⁇ m or more can be measured by the SEM/EBSD method. Here, it can be measured by the method described in Examples described later.
  • the “steel structure at a depth of 1/4t of the thickness t from the outer surface of the steel pipe and the surface of the steel plate” means a depth of 1/4t of the thickness t from the outer surface of the steel pipe and the surface of the steel plate. It means that the above steel structure exists in any of the range of ⁇ 1.0 mm in the thickness direction centered on the position.
  • the method for producing a square steel pipe of the present invention includes, for example, heating a steel material having the above-described chemical composition to a heating temperature of 1100° C. or higher and 1300° C. or lower, followed by finishing rolling at a rough rolling end temperature of 850° C. or higher and 1150° C. or lower. Finishing temperature: 750° C. or higher and 850° C. or lower and total rolling reduction: 40% or higher and 63% or lower at 930° C. or lower. Next, cooling is performed at the sheet thickness center temperature at an average cooling rate of 2 ° C./s or more and 27 ° C./s or less, cooling stop temperature: 450 ° C. or more and 650 ° C.
  • the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming, and the butted portions are electric resistance welded, and then formed into a square shape to obtain a square steel pipe.
  • °C indicates the surface temperature of the steel material or steel plate (hot-rolled steel plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. Further, the temperature at the center of the steel plate thickness can be obtained by calculating the temperature distribution in the steel plate cross section by heat transfer analysis and correcting the result with the surface temperature of the steel plate.
  • hot-rolled steel sheet includes hot-rolled steel sheet and hot-rolled steel strip.
  • the method of melting the steel material is not particularly limited, and any known melting method such as a converter, an electric furnace, or a vacuum melting furnace is suitable.
  • the casting method is also not particularly limited, but the desired dimensions are manufactured by a known casting method such as a continuous casting method. It should be noted that there is no problem even if an ingot casting-slabbing rolling method is applied instead of the continuous casting method.
  • the molten steel may be further subjected to secondary refining such as ladle refining.
  • the obtained steel material (steel slab) is heated to a heating temperature of 1100° C. or more and 1300° C. or less, and then subjected to rough rolling at a rough rolling end temperature of 850° C. or more and 1150° C. or less. Finish rolling is performed at 750° C. or higher and 850° C. or lower, and hot rolling is performed such that the total rolling reduction at 930° C. or lower is 40% or higher and 63% or lower to obtain a hot rolled steel sheet.
  • Heating temperature 1100° C. or higher and 1300° C. or lower If the heating temperature is lower than 1100° C., the deformation resistance of the material to be rolled increases and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling and finish rolling). It becomes difficult to ensure the crystal grain size. In addition, it becomes difficult to suppress the formation of coarse bainite, and it is difficult to control the volume fraction of crystal grains having a grain size of 40.0 ⁇ m or more within the target range of the present invention. Therefore, the heating temperature in the hot rolling process is set to 1100° C. or higher and 1300° C. or lower. It is preferably 1120° C. or higher and preferably 1280° C. or lower.
  • Rough rolling finish temperature 850° C. or more and 1150° C. or less
  • the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during the subsequent finish rolling, and a large amount of ferrite is generated, and bainite is formed.
  • the volume ratio becomes less than 10%.
  • the rough rolling finish temperature exceeds 1150° C., the rolling reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the steel structure of the square steel pipe, which is the object of the present invention, cannot be obtained.
  • the finish temperature of rough rolling is set to 850° C. or higher and 1150° C. or lower. It is preferably 860° C. or higher, more preferably 870° C. or higher. It is preferably 1000° C. or lower, more preferably 980° C. or lower.
  • Finish rolling finish temperature 750° C. or higher and 850° C. or lower
  • the finishing temperature of finish rolling is set to 750° C. or more and 850° C. or less. It is preferably 770° C. or higher, more preferably 780° C. or higher. It is preferably 830° C. or lower, more preferably 820° C. or lower.
  • Total rolling reduction at 930° C. or less 40% or more and 63% or less
  • subgrains in austenite are refined in the hot rolling process, so that ferrite, bainite and the remainder generated in the subsequent cooling process and coiling process
  • the steel structure of the square steel pipe having the strength and toughness targeted by the present invention can be obtained by refining the structure.
  • the total rolling reduction exceeds 63%, crystal grains with a large major axis to minor axis ratio tend to form, resulting in a decrease in toughness.
  • the total rolling reduction from 930° C. or lower to the finish rolling finish temperature is set to 63% or lower. It is preferably 61% or less, more preferably 60% or less. If the total rolling reduction is less than 40% up to the finish rolling finish temperature of 930° C. or lower, the grain size of ferrite and bainite increases, leading to a decrease in toughness. Therefore, the total rolling reduction from 930° C. or lower to the finish rolling finish temperature was set to 40% or higher. It is preferably 42% or more, more preferably 45% or more.
  • the reason why the temperature is set to 930°C or less is that if it exceeds 930°C, the austenite recrystallizes in the rolling process, dislocations introduced by rolling disappear, and refined austenite cannot be obtained.
  • the above-mentioned total rolling reduction refers to the total rolling reduction of each rolling pass in the temperature range from 930°C to the finish rolling end temperature.
  • hot rolling may be performed with a total reduction rate of 40% or more and 63% or less to the finish rolling end temperature of 930°C or less in both the rough rolling and the finish rolling described above.
  • only finish rolling may be performed by hot rolling with a total rolling reduction of 40% or more and 63% or less until the finish rolling finish temperature of 930° C. or less.
  • the slab is cooled during rough rolling to reduce the temperature to 930°C or less. After that, the total rolling reduction from 930° C. or lower to finish rolling finish temperature in both rough rolling and finish rolling is set to 40% or more and 63% or less.
  • the upper limit of the finished plate thickness (the thickness of the hot-rolled steel plate after finish rolling) is not particularly specified, but from the viewpoint of ensuring the required rolling reduction and steel plate temperature control, the finished plate thickness is more than 5 mm and less than 26 mm. is preferred.
  • the hot rolled steel sheet is subjected to a cooling process.
  • cooling is performed at an average cooling rate to the cooling stop temperature: 2°C/s or more and 27°C/s or less, and the cooling stop temperature: 450°C or more and 650°C or less.
  • the average cooling rate exceeds 27° C./s, a large amount of martensite or bainite is generated at a depth position of 1/4t of the wall thickness t from the outer surface of the steel structure of the obtained square steel pipe, and ferrite and bainite are generated. is less than 75%.
  • the average cooling rate is preferably 4°C/s or higher, more preferably 6°C/s or higher. It is preferably 25° C./s or less, more preferably 20° C./s or less.
  • Cooling stop temperature 450° C. or higher and 650° C. or lower
  • the cooling stop temperature is less than 450° C. at the thickness center temperature of the hot-rolled steel sheet
  • a large amount of martensite is generated at a depth position of 1/4t of the wall thickness t from the outer surface of the pipe of the steel structure, and the total volume fraction of ferrite and bainite may be less than 75%.
  • the volume fraction of ferrite may be 30% or less.
  • the cooling stop temperature is preferably 460°C or higher, more preferably 470°C or higher. It is preferably 620° C. or lower, more preferably 600° C. or lower.
  • the average cooling rate is a value obtained by ((thickness center temperature of hot-rolled steel sheet before cooling-thickness center temperature of hot-rolled steel sheet after cooling)/cooling time).
  • Cooling methods include, but are not limited to, water cooling such as water injection from nozzles, cooling by cooling gas injection, and the like.
  • both sides of the hot-rolled steel sheet are preferably cooled (treated) so that both sides of the hot-rolled steel sheet are cooled under the same conditions.
  • the hot-rolled steel sheet is coiled, and then subjected to a coiling process of standing to cool.
  • the steel sheet is coiled at a coiling temperature of 440° C. or higher and 650° C. or lower from the viewpoint of building the steel sheet structure. If the coiling temperature is less than 440°C, a large amount of martensite may be generated and the total volume fraction of ferrite and bainite may be less than 75%. Moreover, the volume fraction of ferrite may be 30% or less.
  • the winding temperature is preferably 450°C or higher, more preferably 460°C or higher. It is preferably 620° C. or lower, more preferably 590° C. or lower.
  • the hot-rolled steel sheet of the present invention is produced.
  • a hot-rolled steel sheet having a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -20°C of 180 J or more can be obtained.
  • a pipe-making process is applied.
  • a hot-rolled steel plate is roll-formed into a cylindrical open pipe (round steel pipe), and the butt portions are electric resistance welded.
  • the round steel pipe is drawn in the axial direction by several percent by rolls arranged vertically and horizontally with respect to the round steel pipe, and formed into a square shape to obtain a square steel pipe.
  • the square steel pipe in the present invention is not limited to a square steel pipe in which each side length is equal (the value of (long side length/short side length) is 1.0), (long side length/short side length) value of more than 1.0 is also included.
  • the value of (long side length/short side length) of the square steel pipe is preferably 1.0 or more and 2.5 or less.
  • the value of (long side length/short side length) is more preferably 1.0 or more and 2.0 or less.
  • the square steel pipe of the present invention is manufactured.
  • the yield strength of the flat plate portion is 385 MPa or more
  • the tensile strength of the flat plate portion is 520 MPa or more
  • the yield ratio of the flat plate portion is 0.90 or less
  • the Charpy absorbed energy of the flat plate portion at ⁇ 20° C. is 110 J or more.
  • the present invention can be suitably used particularly for thick square steel pipes.
  • the term "thickness” as used herein means that the thickness of the flat plate portion of the square steel pipe is more than 5 mm and less than 26 mm.
  • Fig. 1 schematically shows an example of a building structure using the square steel pipe of the present invention described above.
  • a plurality of rectangular steel pipes 1 of the present invention are erected and used as pillars.
  • a plurality of girders 4 made of steel such as H-shaped steel are installed between the adjacent square steel pipes 1 .
  • a plurality of small beams 5 made of steel such as H-shaped steel are installed between adjacent large beams 4 .
  • the square steel pipes 1 and the H-section steel forming the large girders 4 are welded together via through-diaphragms 6, so that the large beams 4 made of steel such as H-section steel are constructed between the adjacent square steel pipes 1.
  • studs 7 are provided as necessary for attachment to a wall or the like.
  • the square steel pipe of the present invention has excellent strength and low-temperature toughness, so even when used in large buildings, it is possible to sufficiently ensure the deformation performance of the entire structure. Therefore, the building structure of the present invention exhibits better earthquake resistance performance than building structures using conventional rectangular steel pipes. In addition, even when used in a building in a low-temperature environment such as a cold region, the above-mentioned excellent earthquake resistance performance can be exhibited.
  • a molten steel having the chemical composition shown in Table 1 was cast into a slab.
  • the obtained slabs were subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Table 2 to obtain hot rolled steel sheets for square steel pipes. After the winding process, the following pipe-making process was performed.
  • the obtained hot-rolled steel sheets for square steel pipes were formed into cylindrical round steel pipes by roll forming, and the butt portions were electric resistance welded.
  • the round steel pipe was formed into a square shape by rolls arranged on the top, bottom, left, and right sides of the round steel pipe to obtain a roll-formed square steel pipe having a side length (mm) and a wall thickness (mm) shown in Table 2.
  • Test pieces were taken from the obtained square steel pipes (roll-formed square steel pipes) and hot-rolled steel sheets, and the following structural observations, tensile tests, and Charpy impact tests were performed.
  • test piece for observing the structure of the square steel pipe was measured from the flat plate portion next to the side including the welded portion of the square steel pipe (the 3 o'clock side when the welded portion is in the 12 o'clock direction). The sample was taken from the tube axial direction section and the depth position of 1/4t of the wall thickness t from the tube outer surface, polished, and then nital corroded.
  • a test piece for observing the structure of the hot-rolled steel sheet was taken from the central portion in the width direction of the hot-rolled steel sheet and at a depth of 1/4t of the sheet thickness t. The observation surface was made to be a cross section in the rolling direction at the time of hot rolling, and after polishing, it was produced by nital corrosion.
  • Microstructural observation is performed using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) from the outer surface of the flat plate portion of the square steel pipe and the surface of the hot rolled steel plate to 1/ of the thickness t.
  • the tissue at the 4t depth position was observed and imaged.
  • the area ratios of ferrite, pearlite, bainite and residual structures were obtained from the obtained optical microscope images and SEM images.
  • the area ratio of each tissue was calculated as the average value of the values obtained in each field of view after observing five or more fields of view using a test piece taken from one representative flat plate.
  • the area ratio obtained by tissue observation was used as the volume ratio of each tissue.
  • ferrite is a product of diffusion transformation, and has a low dislocation density and exhibits a nearly recovered structure.
  • Pearlite is a structure in which cementite and ferrite are arranged in layers.
  • bainite is a lath-like multi-phase structure of ferrite and cementite with a high dislocation density.
  • the volume fraction of austenite was measured by X-ray diffraction.
  • the test piece for structural observation was ground so that the diffractive surface was at a depth of 1/4 t of the thickness t from the outer surface of the flat plate portion of the steel pipe and the surface of the hot-rolled steel plate, and then chemically polished to form a surface processed layer.
  • the K ⁇ ray of Mo was used for the measurement, and the volume fraction of austenite was obtained from the integrated intensities of the (200), (220) and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.
  • the average equivalent circle diameter (average crystal grain size) and the volume fraction of crystal grains having an equivalent circle diameter (crystal grain size) of 40.0 ⁇ m or more were measured using the SEM/EBSD method.
  • the measurement area was 500 ⁇ m ⁇ 1000 ⁇ m, and the measurement step size was 0.5 ⁇ m.
  • the crystal grain size was obtained by determining the orientation difference between adjacent crystal grains, and measuring the boundary with the orientation difference of 15° or more as the crystal grain boundary.
  • the average grain size was obtained by calculating the arithmetic mean of grain sizes from the obtained grain boundaries.
  • the number of crystal grains with a ratio of major axis to minor axis of 4.0 or more is measured, and divided by the area of the measurement area (0.5 mm 2 ).
  • the number (particles/mm 2 ) of crystal grains with a ratio of 4.0 or more was calculated.
  • crystal grain sizes of 2.0 ⁇ m or less are excluded from the analysis as measurement noise, and the area ratio obtained in the crystal grain size analysis is equal to the volume ratio.
  • FIG. 2 is a schematic diagram showing the sampling positions of the tensile test pieces of the flat plate portion of the square steel pipe.
  • a JIS No. 5 tensile test piece was taken from the flat plate portion of the square steel pipe so that the tensile direction was parallel to the pipe axial direction.
  • a JIS No. 5 tensile test piece was taken so that the tensile direction was parallel to the rolling direction.
  • a tensile test was performed on the collected tensile test pieces in accordance with the provisions of JIS Z 2241, the yield strength YS and tensile strength TS were measured, and the yield ratio defined as (yield strength) / (tensile strength) was calculated. bottom.
  • the tensile test piece of the flat plate portion of the square steel pipe was taken from the position of the width center of the flat plate portion (see FIG. 2) on the side of 3 o'clock when the welded portion of the square steel pipe is in the 12 o'clock direction. .
  • the number of test pieces was set to two for each, and the average values thereof were calculated to obtain YS, TS, and yield ratio.
  • FIG. 3 is a schematic diagram showing the sampling positions of Charpy test pieces of square steel pipes.
  • the longitudinal direction of the test piece was taken parallel to the pipe axis direction at a depth of 1/4t of the wall thickness t from the outer surface of the square steel pipe.
  • a V-notch standard test piece conforming to JIS Z 2242 was used.
  • the test piece was sampled from a depth of 1/4 t in the thickness of the obtained hot-rolled steel sheet so that the longitudinal direction of the test piece was parallel to the rolling direction.
  • a V-notch standard specimen was used.
  • a Charpy impact test was carried out at a test temperature of -20°C in accordance with JIS Z 2242 to determine absorbed energy (J). Incidentally, the number of test pieces was three, and the average value was calculated to obtain the absorbed energy (J).
  • Tables 3-1 and 3-2 show the results for the obtained square steel pipes, and Tables 4-1 and 4-2 show the results for the hot-rolled steel sheets.
  • Steel No. in Table 1 steel plate Nos. in Tables 2 and 4; and steel pipe No. in Table 3. correspond to each other, and the same No.
  • a hot-rolled steel plate is manufactured from the steel of 1998, and a square steel pipe is manufactured from the hot-rolled steel plate.
  • steel pipe No. 1 to 22 are examples of the present invention.
  • 23 to 46 are comparative examples.
  • the steel structure contains ferrite with a volume fraction of more than 30% and bainite with a volume fraction of 10% or more, the total volume fraction of ferrite and bainite is 75% or more and 95% or less, and the balance is A crystal grain having an equivalent circle diameter of 40.0 ⁇ m or more when a region surrounded by a boundary with a misorientation of 15° or more is made of one or more selected from pearlite, martensite, and austenite.
  • the flat plate portion had a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.90 or less, and a Charpy absorbed energy of -20°C of the flat plate portion of 110 J or more.
  • Comparative steel pipe No. 26 since the Si content exceeded the range of the present invention, the yield strength was excessively increased due to solid solution strengthening without refinement of the structure. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • Comparative steel pipe No. 32 Comparative steel pipe No. In No. 32, the content of Ti exceeded the range of the present invention, so it is considered that coarse carbides and nitrides were formed. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • Comparative steel pipe No. No. 39 is considered to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • Comparative steel pipe No. 41 Comparative steel pipe No. In No. 41, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the volume ratio of crystal grains with a grain size of 40.0 ⁇ m or more was outside the range of the present invention. As a result, the tensile strength of the flat plate portion and the Charpy absorbed energy at -20°C did not reach the desired values.
  • the volume fraction of crystal grains is outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C of the flat plate part did not reach the desired values.
  • Comparative steel pipe No. 43 Comparative steel pipe No. In No. 43, the total reduction ratio at 930 ° C. or lower exceeds the range of the present invention, the formation of coarse bainite cannot be suppressed, and the volume ratio of crystal grains with a grain size of 40.0 ⁇ m or more is outside the range of the present invention. became. As a result, the Charpy absorbed energy of the flat plate portion at -20°C did not reach the desired value.
  • FIG. 4 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. and the number of crystal grains having a major axis/minor axis ratio of 4.0 or more among crystal grains having a major axis of 50 ⁇ m or more.
  • the Charpy absorbed energy at ⁇ 20° C. is 110 J or more, and excellent low temperature toughness is exhibited.
  • the Charpy absorbed energy at -20°C was less than 110J.
  • steel plate No. Steel sheets Nos. 1 to 22 are examples of the present invention.
  • 23 to 46 are comparative examples.
  • the steel structure contains ferrite with a volume fraction of more than 30% and bainite with a volume fraction of 10% or more, the total volume fraction of ferrite and bainite is 75% or more and 95% or less, and the balance is is composed of one or more selected from pearlite, martensite, and austenite, and a crystal grain having an equivalent circle diameter of 40.0 ⁇ m or more when a region surrounded by a boundary with a misorientation of 15° or more is defined as a crystal grain.
  • the volume ratio of 20% or less and the major axis is 50 ⁇ m or more
  • these mechanical properties were such that the yield strength was 330 MPa or more, the tensile strength was 520 MPa or more, the yield ratio was 0.75 or less, and the Charpy absorbed energy at -20°C was 180 J or more.
  • Comparative example steel plate No. 28 the P content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 29 the S content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 32 the content of Ti exceeded the range of the present invention, so it is considered that coarse carbides and nitrides were formed. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 35 Comparative example steel plate No. In No. 35, the Cr content exceeded the range of the present invention, so the Charpy absorbed energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 38 the Ni content exceeded the range of the present invention, so the yield ratio and the Charpy absorbed energy at -20°C did not reach the desired values.
  • Comparative example steel plate No. No. 39 is considered to have formed Ca oxide clusters because the Ca content exceeded the range of the present invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
  • Comparative example steel plate No. 41 Comparative example steel plate No. In No. 41, the slab heating temperature exceeded the range of the present invention, the crystal grains became coarse, and the volume ratio of crystal grains with a grain size of 40.0 ⁇ m or more was outside the range of the present invention. As a result, the Charpy absorption energy at -20°C did not reach the desired value.
  • the volume fraction of crystal grains is outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C did not reach the desired values.
  • Comparative example steel plate No. 44 since the average cooling rate was below the range of the present invention, the volume fraction of bainite was less than 10%, and fell outside the scope of the present invention. As a result, the yield strength, tensile strength and Charpy absorbed energy at -20°C did not reach the desired values.
  • FIG. 5 is a graph showing the relationship between the Charpy absorbed energy at ⁇ 20° C. and the number of crystal grains having a major axis/minor axis ratio of 4.0 or more among crystal grains having a major axis of 50 ⁇ m or more.
  • the Charpy absorbed energy at ⁇ 20° C. is 180 J or more, and excellent low temperature toughness is exhibited.
  • the Charpy absorbed energy at -20°C was less than 180J.

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PCT/JP2022/032953 2021-09-29 2022-09-01 角形鋼管およびその製造方法、熱延鋼板およびその製造方法、並びに建築構造物 WO2023053837A1 (ja)

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JP7571918B1 (ja) 2023-06-29 2024-10-23 Ｊｆｅスチール株式会社 角形鋼管およびその製造方法ならびに建築構造物

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