WO2024062686A1 - Feuille d'acier laminée à chaud, tube d'acier carré, procédés de fabrication associés et structure de bâtiment - Google Patents

Feuille d'acier laminée à chaud, tube d'acier carré, procédés de fabrication associés et structure de bâtiment Download PDF

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WO2024062686A1
WO2024062686A1 PCT/JP2023/020167 JP2023020167W WO2024062686A1 WO 2024062686 A1 WO2024062686 A1 WO 2024062686A1 JP 2023020167 W JP2023020167 W JP 2023020167W WO 2024062686 A1 WO2024062686 A1 WO 2024062686A1
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hot
temperature
rolled steel
steel
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PCT/JP2023/020167
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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 JP2023552143A priority Critical patent/JP7396552B1/ja
Publication of WO2024062686A1 publication Critical patent/WO2024062686A1/fr

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    • 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 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
    • 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
    • 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
    • C22C38/60—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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints

Definitions

  • the present invention relates to a hot-rolled steel plate with a low yield ratio used for square steel pipes, a square steel pipe (square column) manufactured from the hot-rolled steel plate by cold roll forming and having a low yield ratio and low-temperature toughness, and Regarding their manufacturing method.
  • the present invention relates to a rectangular steel pipe suitably used as a structural member of a large building.
  • the present invention also relates to a building structure using such a square steel pipe.
  • buildings architectural structural members used in large buildings such as factories, warehouses, and commercial facilities (hereinafter referred to as "buildings") have become stronger in order to reduce construction costs by making them lighter.
  • square steel pipes square columns having a flat plate portion and a corner portion and used as pillar materials for buildings are required to have high strength in the flat plate portion, and at the same time, they are required to have excellent toughness from the standpoint of earthquake resistance.
  • Square steel pipes are generally made from hot-rolled steel plates (hot-rolled steel strips) or thick steel plates, and are produced by cold-forming this material.
  • Examples of the cold forming method include a cold press bending method and a cold roll forming method.
  • Square steel pipes (hereinafter sometimes referred to as "press-formed square steel pipes") are manufactured by press-bending a material into a rectangular (square-shaped) cross-sectional shape by cold press-bending a thick steel plate. ) or U-shaped, and these are manufactured by joining them by submerged arc welding.
  • roller-formed square steel pipes manufactured by roll-forming the raw material (hereinafter sometimes referred to as “roll-formed square steel pipes”) are cold-roll-formed hot-rolled steel plates to form cylindrical open pipes, and then A round steel pipe is manufactured by electric resistance welding the parts. Thereafter, the cylindrical round steel tube is compressed by several percent in the axial direction using rolls placed on the top, bottom, left and right of the round steel tube, and then formed into a square shape to produce a square steel tube.
  • the manufacturing method of roll-formed square steel pipes has the advantage of being more productive and capable of manufacturing in a short period of time compared to the manufacturing method of press-formed square steel pipes.
  • press-formed rectangular steel pipes the flat plate portion is not cold-formed, but the corners are cold-formed, so only the corners undergo work hardening.
  • large processing strain is introduced in the axial direction over the entire circumference of the steel pipe, especially when cold-forming into a cylindrical shape, which is the first step in forming the rectangular steel pipe.
  • the roll-formed rectangular steel pipe has a high yield ratio in the axial direction not only in the corners but also in the flat plate portion, resulting in low toughness.
  • Patent Document 1 discloses that the carbon content is 0.20% or less by weight, Mn: 0.40 to 0.90%, and Nb: 0.005 to 0.040%. and Ti: 0.005 to 0.050%.
  • a steel material containing one or two of Ti: 0.005 to 0.050% is prepared at a rolling reduction rate of 55% or more in the non-recrystallization temperature range, a rolling end temperature of 730 to 830°C, and a coiling temperature of 550°C. After forming the hot-rolled coil obtained by the following hot rolling process into a coil and welding it into an ERW steel pipe, when making it into a square steel pipe by cold working, the outer circumference long drawing in the steel pipe forming process is determined by the plate thickness.
  • a square steel pipe has been proposed that has a yield ratio of 90% or less and a Charpy absorbed energy of 27 J or more at a test temperature of 0° C. by setting the ratio to 3 times or less.
  • Patent Document 2 after heating steel containing C: 0.07 to 0.18% and Mn: 0.3 to 1.5% in mass % to a heating temperature of 1100 to 1300°C, rough rolling is completed. Temperature: 1,150 to 950°C Rough rolling and finish rolling start temperature: 1,100 to 850°C, finish rolling end temperature: 900 to 750°C After finishing rolling, the cooling stop temperature is 550°C or higher at the surface temperature. 650 with a cooling rate of 4 to 15 °C/s for the average cooling rate in the temperature range of 750 to 650 °C at the center of the plate thickness.
  • a thick hot-rolled steel plate is obtained by performing tertiary cooling to a temperature below °C and setting the frequency of the second phase included in the steel structure to 0.20 to 0.42.
  • a rectangular steel pipe has been proposed that exhibits a low yield ratio of 80% or less by cold forming, and has mechanical properties such that the absorbed energy in the Charpy impact test is 150 J or more at a test temperature of 0°C.
  • Patent Document 3 steel containing C: 0.07 to 0.18% and Mn: 0.3 to 1.5% in mass % is heated to a heating temperature of 1100 to 1300°C, and then rough rolled. Finishing temperature: 1150 to 950°C Rough rolling and finish rolling start temperature: 1100 to 850°C, finish rolling finishing temperature: 900 to 750°C After finish rolling, the surface temperature is in the temperature range of 750 to 650°C.
  • the average cooling rate of 20°C/s or less, the time until the temperature at the center of the plate thickness reaches 650°C is within 35 seconds, and the average cooling rate in the temperature range of 750 to 650°C at the center of the plate thickness is 4 to 15 °C/s, the thick hot rolled steel sheet obtained by cooling to the coiling temperature of 500 to 650°C is used as the material and cold forming shows a low yield ratio of 80% or less.
  • a rectangular steel pipe has been proposed that has mechanical properties such that the absorbed energy in the Charpy impact test is 150 J or more at a temperature of 0°C.
  • Patent Document 4 in mass %, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0 .01 to 0.06%, N: 0.006% or less, and the balance consists of Fe and unavoidable impurities.
  • the rough rolling end temperature is : Rough rolling at 1150 to 950°C and finish rolling at 1100 to 850°C, finish rolling at 900 to 750°C.
  • Cooling is performed at a cooling rate with an average cooling rate of 4 to 25 °C/s to a cooling stop temperature of 580 °C or less, and a cooling step of 0.2 seconds or more and less than 3.0 seconds in the initial cooling step that is 10 seconds from the start of cooling.
  • the steel structure at the center of the sheet thickness is divided into a main phase consisting of ferrite, pearlite, pseudo pearlite and It has a second phase consisting of one or more selected from upper bainite and has an area ratio of 8 to 20%, and the average grain size of the steel structure including the main phase and the second phase is 7 to 7.
  • the steel structure on the front and back surfaces of the plate is single phase ferrite or single phase bainitic ferrite, the average grain size is 2 to 20 ⁇ m, exhibits a low yield ratio of 90% or less, and the test temperature: Square steel pipes have been proposed that have mechanical properties such that the absorbed energy in the Charpy impact test at 0° C. is 27 J or more.
  • the hot-rolled steel sheet used as the raw material is required to have a steel structure that suppresses the increase in yield ratio during forming, and excellent low-temperature toughness that can withstand deterioration of toughness due to large working strain.
  • the present invention was made in view of the above-mentioned problems, and provides a square steel pipe with high yield strength and tensile strength, low yield ratio, and excellent low-temperature toughness and work hardenability in the pipe axial direction and circumferential direction.
  • the object of the present invention is to provide a hot-rolled steel plate that can be used, together with a square steel pipe using the hot-rolled steel plate, a method for manufacturing the same, and a building structure using the square steel pipe.
  • (1) low yield ratio, (2) high yield strength, and (3) high tensile strength all mean that the tensile direction is parallel to the rolling direction.
  • results of a tensile test in accordance with the provisions of JIS Z 2241 (2011) were as follows: (1) yield ratio was 0.75 or less; (2) yield ratio was 0.75 or less; (3) tensile strength is 400 MPa or more.
  • work hardening is an index for evaluating uniform elongation (plastic elongation at the highest load point in a tensile test), and the work hardening index n 3-7 specified by JIS Z 2253 (2011) is 0.20 or more. It refers to being. In other words, if the work hardening index n3-7 of the hot rolled steel sheet is less than 0.20, when manufactured into a square steel pipe, the uniform elongation of the flat plate portion of the square steel pipe will decrease, the earthquake resistance will decrease, and the The yield ratio of the flat plate portion may exceed 0.90.
  • having excellent low-temperature toughness means that, in accordance with the regulations of JIS Z 2242 (2018), the specimen is sampled at the t/2 position of the plate thickness t (the center of the plate thickness) so that the longitudinal direction of the test piece is parallel to the rolling direction.
  • a Charpy impact test was conducted using a V-notch standard test piece at test temperatures of -80°C, -60°C, -40°C, -20°C, and 0°C, and the Charpy absorbed energy at -20°C was 100 J or more. It means that the ductile-brittle transition temperature is -20°C or lower.
  • low yield ratio rectangular steel pipes are tensile test specimens taken under JIS No.
  • the test results show that the yield strength in the flat plate part is 295 MPa or more, the tensile strength in the flat plate part is 400 MPa or more, the yield ratio in the flat plate part is 0.90 or less, and the uniform elongation in the flat plate part is 5. 0% or more, and at a position of 1/4t of the wall thickness t from the outside surface of the pipe, using a V-notch standard test piece taken from a flat plate part of a square steel pipe so that the longitudinal direction of the test piece is parallel to the pipe axis direction.
  • a Charpy impact test was conducted in accordance with the provisions of JIS Z 2242 (2016) at test temperatures of -60°C, -40°C, -20°C, and 0°C, and Charpy absorption at -20°C in the tube axis direction in the flat plate part was measured. It refers to a square steel pipe with an energy of 60 J or more and a ductile-brittle transition temperature of the flat plate part of -10°C or less.
  • a rectangular steel pipe with a superior low yield ratio is defined as a square steel pipe with a superior low yield ratio, in accordance with the provisions of JIS Z 2242 (2018), in which the longitudinal direction of the test piece is in the circumferential direction at the t/4 position of the wall thickness t from the outer surface of the pipe.
  • Charpy absorbed energy at -20°C in the tube axis direction and tube circumferential direction in the flat plate part was measured using a V-notch standard test piece taken parallel to the tube axis, and Refers to square steel pipes with a Charpy absorbed energy ratio P of 0.5 to 1.2.
  • P (Charpy absorbed energy at -20°C in the pipe circumferential direction) / (Charpy absorbed energy at -20°C in the pipe axial direction)
  • the present inventors conducted extensive studies to solve the above problems. As a result, the following findings (1) to (3) were obtained.
  • the content of C must be 0.07% by mass or more, and the center of the thickness of the hot-rolled steel sheet must be It is necessary that the main phase in is ferrite.
  • the component composition In order to obtain the steel structure described in (1) and (2) above, the component composition must be adjusted to an appropriate range, and the contents of Mn and Si in particular must be controlled within a specific range; Furthermore, in addition to starting finish rolling after a predetermined time has elapsed after the completion of rough rolling in the hot rolling process, it is necessary to maintain a predetermined temperature range for a predetermined time after winding.
  • the present invention was completed based on this knowledge and further studies. That is, the gist of the present invention is as follows. 1. A hot rolled steel plate, In mass%, C: 0.07% or more and 0.20% or less, Si: 0.40% or less, Mn: 0.20% or more and 1.00% or less, P: 0.100% or less, S: 0.050% or less, Al: 0.005% or more and 0.100% or less and N: 0.0100% or less, the balance is Fe and unavoidable impurities, and the content of Mn and Si satisfies the following formula (1).
  • the steel structure at the center of the plate thickness has a main phase of ferrite and a second phase in which the total area ratio of pearlite and pseudo-pearlite is 6 to 25% and the area ratio of upper bainite is 5% or less. death,
  • the average crystal grain size of such crystal grain is 10.0 to 30.0 ⁇ m.
  • the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m is 35% or more, and further, among the crystal grains, the major axis with respect to the minor axis
  • %Mn and %Si are the contents (% by mass) of each element in the steel sheet.
  • the component composition further includes, in mass%, Nb: 0.005% or more and 0.020% or less, Ti: 0.005% or more and 0.020% or less, V: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 0.30% or less, Ni: 0.01% or more and 0.30% or less,
  • Nb 0.005% or more and 0.020% or less
  • Ti 0.005% or more and 0.020% or less
  • V 0.01% or more and 0.10% or less
  • Cr 0.01% or more and 0.50% or less
  • Mo 0.01% or more and 0.50% or less
  • Cu 0.01% or more and 0.30% or less
  • Ni 0.01% or more and 0.30% or less
  • the hot-rolled steel sheet according to 1 above containing one or more selected from Ca: 0.0005% to 0.0100% and B: 0.0003% to 0.0100%.
  • the base steel plate obtained by the hot rolling is cooled such that the average cooling rate Vc (°C/s) at the center of the plate thickness satisfies the following formula (2) and the cooling stop temperature at the center of the plate thickness is 550°C or more and 680°C or less,
  • the base steel plate is coiled at a plate thickness center temperature of 550°C or more and 680°C or less
  • the coiled steel sheet obtained by the coiling is subjected to a second cooling step in which the coiled steel sheet is retained in a temperature range of 400°C to 300°C for 1.0 h to 10.0 h. 4 ⁇ Vc ⁇ 20 (2)
  • a square steel pipe made from the hot rolled steel plate according to any one of items 1 to 3 above.
  • a method for manufacturing a square steel pipe which comprises cold roll forming a hot rolled steel sheet obtained by the method for manufacturing a hot rolled steel sheet according to item 4 above.
  • a technique for obtaining a hot-rolled steel sheet that can be used for a low-yield-ratio rectangular steel pipe, has high yield strength and tensile strength, has a low yield ratio, and has excellent low-temperature toughness and work hardenability. Furthermore, a building structure using the square steel pipe of the present invention as a column material can obtain better seismic performance compared to a building structure using a square steel pipe manufactured by conventional cold forming. .
  • 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 positions at which test specimens were taken for the flat tensile test carried out in the examples.
  • FIG. 2 is a schematic diagram showing the positions at which test specimens were taken for the Charpy impact test carried out in the examples.
  • the present invention will be explained below.
  • the low yield ratio hot rolled steel sheet (hereinafter also simply referred to as "hot rolled steel sheet”) used for the low yield ratio square steel pipe (hereinafter also simply referred to as "square steel pipe”) of the present invention has a C content of 0.07% by mass. 0.20% or less, Si: 0.40% or less, Mn: 0.20% or more and 1.00% or less, P: 0.100% or less, S: 0.050% or less, Al: 0.005% N: 0.100% or less, N: 0.0100% or less, the remainder being Fe and unavoidable impurities, and the content of Mn and Si satisfies the following formula (1). .
  • the hot rolled steel sheet has a steel structure at the center of the sheet thickness in which the total area ratio of the main phase of ferrite, pearlite and pseudo pearlite is 6 to 25%, and the area ratio of upper bainite is 5% or less. It has a second phase.
  • the average crystal grain size of such a crystal grain is 10.0 to 30.0 ⁇ m, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m is 35% or more;
  • the number density of crystal grains having a ratio of the major axis to the minor axis (major axis)/(minor axis) of 3.0 or more is 30 pieces/mm 2 or less.
  • the "hot-rolled steel sheet” of the present invention includes hot-rolled steel sheets and hot-rolled steel strips.
  • %Mn/%Si ⁇ 3.5 %Mn and %Si are the contents (% by mass) of each element in the steel sheet.
  • C 0.07% or more and 0.20% or less
  • C is an element that increases the strength of steel through solid solution strengthening. Further, C is an element that contributes to the formation of pearlite and pseudo-pearlite, which are one of the second phases. In order to ensure the strength and yield ratio targeted by the present invention, it is necessary to contain 0.07% or more of C.
  • the C content exceeds 0.20%, not only does the proportion of the hard phase increase and the toughness decreases, but also the yield ratio exceeds 0.90, making it impossible to obtain the desired yield ratio. Moreover, weldability also deteriorates. Therefore, the C content is set to 0.07% or more and 0.20% or less.
  • the C content is preferably 0.08% or more. Further, the C content is preferably 0.18% or less, more preferably 0.17% or less.
  • Si 0.40% or less
  • Si is an element that increases the strength of steel through solid solution strengthening.
  • the lower limit of Si is not particularly specified (usually it is more than 0%), in order to obtain such an effect, it is desirable to contain 0.01% or more of Si.
  • the Si content is more preferably 0.05% or more.
  • the Si content is set to 0.40% or less. Preferably it is 0.37% or less, more preferably 0.35% or less.
  • Mn 0.20% or more and 1.00% or less
  • Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to ensure the strength and structure targeted by the present invention, it is necessary to contain 0.20% or more of Mn.
  • the Mn content exceeds 1.00%, the yield ratio exceeds 0.90 because the amount of bainite produced becomes too large, making it impossible to obtain the desired yield ratio.
  • the Mn content is set to 0.20% or more and 1.00% or less.
  • the Mn content is preferably 0.25% or more, more preferably 0.30% or more. Further, the Mn content is preferably 0.95% or less, and even more preferably 0.90% or less.
  • P 0.100% or less Since P segregates at grain boundaries and causes material inhomogeneity, it is preferable to reduce it as much as possible, but a content of up to 0.100% is acceptable. Therefore, the P content is set to 0.100% or less.
  • the P content is preferably 0.030% or less, more preferably 0.020% or less. There is no particular lower limit for P (usually it is over 0%), but excessive reduction will lead to a rise in smelting costs, so P is preferably 0.002% or more.
  • S 0.050% or less S usually exists as MnS in steel, but MnS is stretched thin during the hot rolling process and has a negative effect on ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of up to 0.050% is permissible. Therefore, the S content is set to 0.050% or less.
  • the S content is preferably 0.015% or less, more preferably 0.010% or less, even more preferably 0.008% or less. Although there is no particular lower limit for S (usually it is over 0%), it is preferable that S be 0.0002% or more, since excessive reduction will lead to a rise in smelting costs.
  • Al 0.005% or more and 0.100% or less
  • Al is an element that acts as a strong deoxidizing agent. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al.
  • the Al content exceeds 0.100%, weldability deteriorates, alumina-based inclusions increase, and the surface quality deteriorates. Moreover, the toughness of the welded part also decreases. 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. Further, the Al content is preferably 0.070% or less, more preferably 0.050% or less.
  • N is an element that has the effect of reducing toughness by firmly fixing the movement of dislocations.
  • the N content is set to 0.0100% or less.
  • the N content is preferably 0.0080% or less, more preferably 0.0040% or less, even more preferably 0.0035% or less.
  • the lower limit of N is not particularly stipulated (usually over 0%), but excessive reduction will lead to a rise in smelting costs, so the N content is preferably 0.0010% or more, and 0.001% or more. More preferably, it is 0.0015% or more.
  • %Mn and %Si in the above formula are the contents (mass%) of each element in the steel sheet.
  • Mn and Si contents are within the above-mentioned ranges and further that the relationship 1.0 ⁇ %Mn/%Si ⁇ 3.5 is satisfied.
  • the value of %Mn/%Si is preferably 1.2 or more, and more preferably 1.4 or more.
  • the value of %Mn/%Si is preferably 3.2 or less, and more preferably 3.0 or less.
  • the remainder is Fe and inevitable impurities.
  • this does not preclude O from being contained in an amount of 0.005% or less as long as the effects of the present invention are not impaired.
  • O refers to total oxygen including O as an oxide.
  • Nb less than 0.005%
  • Ti less than 0.005%
  • V 0.
  • 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 0.0003 Any range below % is considered to be included in unavoidable impurities.
  • the above components are the basic composition of the hot rolled steel sheet in the present invention. Although the characteristics aimed at in the present invention can be obtained with the above-mentioned component composition, the component composition can contain the following elements as necessary.
  • Nb 0.005% to 0.020%
  • Ti 0.005% to 0.020%
  • V 0.01% to 0.10%
  • Cr 0.01%.
  • Mo 0.01% or more and 0.50% or less
  • Cu 0.01% or more and 0.30% or less
  • Ni 0.01% or more and 0.30% or less
  • Ca 0. B: 0.0005% or more and 0.0100% or less and B: 0.0003% or more and 0.0100% or less.
  • Nb 0.005% or more and 0.020% or less
  • Ti 0.005% or more and 0.020% or less
  • Nb and Ti form fine carbides and nitrides in steel and improve the strength of steel through precipitation strengthening. It is an element that contributes to In order to obtain such an effect, when Nb is contained, it is preferably 0.005% or more. Furthermore, when Ti is contained, it is preferably 0.005% or more. On the other hand, if the content of each of Nb and Ti exceeds 0.020%, coarse carbides and nitrides are formed, which may lead to a decrease in toughness. Therefore, when Nb is contained, the range is 0.020% or less, and when Ti is contained, the range is 0.020% or less. The content of each of Nb and Ti is preferably 0.007% or more, and still more preferably 0.009% or more. Further, each content of Nb and Ti is preferably 0.018% or less, and preferably 0.016% or less.
  • V 0.01% or more and 0.10% or less
  • Cr 0.01% or more and 0.50% or less
  • Mo 0.01% or more and 0.50% or less
  • V, Cr, and Mo improve the hardenability of steel. It is an element that increases the strength of steel, and can be included as necessary.
  • V 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, respectively. More preferably, V: 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more.
  • excessive content may lead to a decrease in toughness and deterioration in weldability.
  • V 0.10% or less
  • Cr 0.50% or less
  • Mo 0.50% or less
  • V 0.08% or less
  • Cr 0.40% or less
  • Mo 0.40% or less
  • Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained as necessary.
  • 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.
  • excessive inclusion may cause a decrease in toughness and a deterioration in weldability. Therefore, when Cu and Ni are contained, Cu: 0.30% or less and Ni: 0.30% or less, respectively.
  • Ca 0.0005% or more and 0.0100% or less
  • Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process. It can contain. In order to obtain such an effect, when Ca is contained, it is preferably 0.0005% or more. More preferably, the Ca content is 0.0010% or more. On the other hand, when the Ca content exceeds 0.0100%, Ca oxide clusters are formed in the steel, which may deteriorate the toughness. Therefore, when Ca is contained, the content should be 0.0100% or less. Preferably, the Ca content is 0.0050% or less.
  • B 0.0003% or more and 0.0100% or less
  • B is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature.
  • B when B is contained, it is preferably 0.0003% or more. More preferably, the B content is 0.0005% or more.
  • the B content when the B content exceeds 0.0100%, the yield ratio may increase. Therefore, if B is contained, it should be 0.0100% or less.
  • the B content is 0.0050% or less.
  • the hot-rolled steel sheet of the present invention has a steel structure at the center of the sheet thickness in which the total area ratio of the main phase of ferrite, pearlite and pseudo-pearlite is 6 to 25%, and the area ratio of upper bainite is 5% or less.
  • the average crystal grain size of such crystal grain is 10.0 to 30.0 ⁇ m, and among the crystal grains, the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m is 35% or more;
  • the number density of crystal grains in which the ratio of the major axis to the minor axis (major axis)/(minor axis) is 3.0 or more is 30 pieces/mm 2 or less.
  • the crystal grain size is defined as the diameter of a circle having the same area as the target crystal grain (circle equivalent diameter).
  • Ferrite (main phase) Ferrite is a soft structure and is used as the main phase in the present invention in order to obtain the desired yield strength and low yield ratio.
  • the term "main phase” refers to an area ratio of 50% or more. If the area ratio of ferrite is less than 50%, the yield stress becomes excessively large, the work hardening index becomes small, and the desired yield ratio may not be obtained. Further, from the viewpoint of the yield stress and yield ratio mentioned above, the area ratio of ferrite is preferably 70% or more, more preferably 72% or more. On the other hand, when the area ratio of ferrite exceeds 94%, the strength decreases, and desired yield strength and tensile strength may not be obtained. Therefore, the area ratio of ferrite is 94% or less, and preferably, the area ratio of ferrite is 92% or less.
  • Total area ratio of pearlite and pseudo pearlite 6 to 25%, area ratio of upper bainite: 5% or less (second phase)
  • Pearlite and pseudo-pearlite are hard structures, and are the most important steel structures for increasing the strength of steel and obtaining a low yield ratio.
  • the total area ratio of pearlite and pseudo-pearlite be 6% or more.
  • it is 7% or more, more preferably 9% or more.
  • the total area ratio of pearlite and pseudo-pearlite exceeds 25%, toughness may deteriorate. Therefore, the total area ratio of pearlite and pseudo-pearlite needs to be 25% or less.
  • the area ratio of the pseudo pearlite is preferably 5% or more.
  • the area ratio of the pseudo pearlite is preferably 15% or less.
  • upper bainite is a structure with hardness intermediate between ferrite and pearlite, and increases the strength of steel.
  • the area ratio of upper bainite exceeds 5%, the low yield ratio targeted by the present invention cannot be obtained. Therefore, it is necessary that the area ratio of upper bainite is 5% or less. Preferably it is 4% or less.
  • Upper bainite may be 0%.
  • the structures other than the main phase and the second phase are austenite and martensite.
  • the area ratio of ferrite, pearlite, pseudo pearlite, and upper bainite can be measured by the method described below.
  • the average crystal grain size of such crystal grain is 10.0 to 30.0 ⁇ m.
  • the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m is 35% or more;
  • the steel structure of the present invention has a number density of crystal grains having a ratio (major axis)/(minor axis) of 3.0 or more of 30 pieces/ mm2 or less.
  • the steel structure of the present invention has a low yield ratio, yield strength, And in order to obtain tensile strength, the steel is a mixture of soft and hard structures (hereinafter referred to as "composite structure steel").
  • such composite structure steel has poor toughness compared to single structure steel. Therefore, in the present invention, in order to achieve both the above-mentioned mechanical properties and excellent toughness, when a region surrounded by a boundary with a crystal orientation difference of 15° or more is defined as a crystal grain, at the center of the thickness of the steel sheet,
  • the grain size of the steel structure including the main phase and the second phase, the area ratio of coarse grains, and the number density of elongated grains are defined.
  • the average crystal grain size (average equivalent circular diameter) of the crystal grains is less than 10.0 ⁇ m, the yield ratio increases and the yield ratio targeted by the present invention cannot be obtained.
  • the average crystal grain size exceeds 30.0 ⁇ m, toughness deteriorates. Therefore, the average grain size of the steel structure including the main phase and the second phase needs to be in the range of 10.0 to 30.0 ⁇ m.
  • the average crystal grain size is preferably 11.0 ⁇ m or more, more preferably 12.5 ⁇ m or more. Further, the average crystal grain size is preferably 28.0 ⁇ m or less, more preferably 26.0 ⁇ m or less.
  • the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m of the average crystal grain size is It was also found that the number density of elongated grains is extremely important.
  • the crystal orientation difference, average crystal grain size, and the area ratio of crystal grains having a grain size within the average crystal grain size ⁇ 5.0 ⁇ m can all be measured by the SEM/EBSD method. In the present invention, they can be measured by the method described below.
  • the hot rolled steel sheet of the present invention has a yield strength of 250 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.75 or less, a work hardening index of 0.20 or more at a plastic strain of 3 to 7%, and a temperature of -20°C.
  • the Charpy absorbed energy can be set to 100 J or more, and the ductile-brittle transition temperature can be set to -20°C or lower.
  • the hot rolled steel sheet of the present invention preferably has a thickness of 12 mm or more, and more preferably has a thickness in the range of 12 to 32 mm.
  • a method for manufacturing a hot rolled steel sheet of the present invention a method for manufacturing a hot rolled steel sheet according to an embodiment of the present invention will be described.
  • a steel material having the above-described composition is heated to a heating temperature of 1100° C. or higher and 1300° C. or lower (heating step).
  • hot rolling rough rolling is performed at a rough rolling end temperature of 850 ° C. or more and 1150 ° C. or less, and finish rolling is started after 15 seconds or more have passed after the end of such rough rolling, and the finish rolling end temperature is 750 ° C.
  • Hot rolling is performed at a temperature of 850° C.
  • the average cooling rate Vc (°C/s) at the center of the plate thickness satisfies the following formula (2), and the cooling stop temperature at the center of the plate thickness is 550°C. Cooling is performed to a temperature of 680° C. or lower (cooling step). Next, the raw steel plate is wound at a thickness center temperature of 550°C or more and 680°C or less (winding step).
  • the rolled steel plate obtained in the above-mentioned winding process is subjected to second cooling in which it is allowed to stay in a temperature range of 400° C. to 300° C. for 1.0 h or more and 10.0 h or less (second cooling step).
  • second cooling step the hot rolled steel sheet of the present invention is obtained. 4 ⁇ Vc ⁇ 20...(2)
  • °C in relation to temperature refers to the surface temperature of the steel material or steel plate (hot-rolled plate, raw steel plate, hot-rolled steel plate) (hereinafter also referred to as steel plate, etc.) shall be. These surface temperatures can be measured with a radiation thermometer or the like. Further, the temperature at the center of the thickness of a steel plate or the like can be determined by calculating the temperature distribution in the cross section of the steel plate or the like by heat transfer analysis, and correcting the result based on the surface temperature of the steel plate or the like.
  • the method for melting the steel material is not particularly limited, and any known melting method such as a converter, electric furnace, vacuum melting furnace, etc. is suitable.
  • the casting method is not particularly limited, it is manufactured to desired dimensions by a known casting method such as a continuous casting method. It should be noted that there is no problem in applying an ingot-blowing rolling method instead of the continuous casting method.
  • the molten steel may further be subjected to secondary refining such as ladle refining.
  • the obtained steel material (steel slab) is heated to a heating temperature of 1100°C or higher and 1300°C or lower.
  • a hot rolling step rough rolling is performed at a rough rolling end temperature of 850 ° C. or more and 1150 ° C. or less, and finish rolling is started after 15 seconds or more have passed after the end of the rough rolling, and the finish rolling end temperature is 750 ° C.
  • Hot rolling is performed at a temperature of .degree. C. or higher and 850.degree. C. or lower, and a total reduction rate of 40% or higher and 59% or lower throughout the hot rolling process at 930.degree. C. or lower, to obtain a raw steel sheet for a hot-rolled steel sheet.
  • Heating temperature 1100° C. or more and 1300° C. or less
  • the heating temperature is less than 1100° C.
  • the deformation resistance of the material to be rolled increases and rolling becomes difficult.
  • the heating temperature exceeds 1300°C
  • the austenite grains will become coarse and fine austenite grains will not be obtained in subsequent rolling (rough rolling, finish rolling), and the steel structure of the hot rolled steel sheet aimed at in the present invention will not be obtained. It becomes difficult to ensure an average crystal grain size.
  • it becomes difficult to suppress the formation of coarse bainite and it is difficult to control the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m within the range targeted by the present invention. .
  • the heating temperature in the heating step is set to 1100°C or more and 1300°C or less. More preferably, the temperature is 1120°C or higher. Moreover, the heating temperature in the heating step is more preferably 1280° C. or lower.
  • the present invention In addition to the conventional method of manufacturing a steel slab, cooling it to room temperature and then heating it again, the present invention also uses a direct delivery method in which the steel slab is charged into a heating furnace as a hot piece without being cooled to room temperature.
  • the energy-saving process of rolling can also be applied without problems.
  • Rough rolling end temperature 850°C or more and 1150°C or less
  • the steel plate surface temperature will fall below the ferrite transformation start temperature during the subsequent finish rolling, resulting in a large amount of ferrite. is generated and the area ratio of pearlite and pseudo-pearlite decreases, making it difficult to obtain the low yield ratio rectangular steel pipe targeted by the present invention.
  • the rough rolling end temperature exceeds 1150°C, the reduction amount in the austenite non-recrystallization temperature range will be insufficient, and fine austenite grains will not be obtained.
  • the steel structure of the hot-rolled steel sheet that is the objective of the present invention could not be obtained, and the region surrounded by boundaries where the orientation difference between adjacent crystals is 15° or more at the center of the thickness of the steel sheet is classified as crystal grains.
  • the average crystal grain size of such crystal grains is 10.0 to 30.0 ⁇ m, and the ratio of the major axis to the minor axis (major axis)/(minor axis) of such crystal grains is 3.0 or more
  • the density is 30 particles/mm 2 or less, it becomes difficult to obtain a steel structure in which the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m is 35% or more. Moreover, it becomes difficult to suppress the generation of coarse bainite.
  • the rough rolling end temperature is set to 850°C or more and 1150°C or less. Preferably it is 860°C or higher, more preferably 870°C or higher. Further, the rough rolling end temperature is preferably 1100°C or lower, more preferably 1050°C or lower.
  • Time elapsed from the end of rough rolling to the start of finish rolling 15 s or more
  • the grain size of austenite will vary greatly, and it will be difficult to achieve the area ratio of grains having a grain size within the average grain size ⁇ 5.0 ⁇ m in the center of the thickness of the steel plate, which is the objective of the present invention, of 35% or more.
  • the above time is preferably 18 s or more, more preferably 20 s or more.
  • Finish rolling end temperature 750°C or more and 850°C or less
  • the finish rolling end temperature is less than 750°C, the steel plate surface temperature becomes below the ferrite transformation start temperature during finish rolling and elongates in the rolling direction. There is a possibility that ferrite is formed and the workability is reduced.
  • the finish rolling end temperature exceeds 850° C., the reduction amount in the austenite non-recrystallization temperature range will be insufficient, and fine austenite grains will not be obtained. As a result, the crystal grains become coarse, making it difficult to secure the strength targeted by the present invention. Moreover, it becomes difficult to suppress the generation of coarse bainite.
  • the finish rolling end temperature is set to 750°C or more and 850°C or less.
  • the finish rolling end temperature is preferably 770°C or higher, more preferably 780°C or higher.
  • the finish rolling end temperature is preferably 830°C or lower, more preferably 820°C or lower.
  • Total rolling reduction ratio of 930°C or less 40% or more and 59% or less
  • subgrains in the austenite are refined, so that the subsequent cooling process and rolling Ferrite and bainite generated in the rolling process are refined to obtain a steel structure of a hot rolled steel sheet having the strength and toughness targeted by the present invention.
  • the total rolling reduction exceeds 59%, crystal grains with a large ratio of major axis to minor axis are likely to be formed, resulting in a decrease in toughness.
  • the total rolling reduction rate at 930° C. or lower is set to 59% or lower. Preferably it is 57% or less, more preferably 55% or less.
  • the total rolling reduction rate below 930° C. was set at 40% or more. Preferably it is 42% or more, more preferably 45% or more.
  • the reason why the total rolling reduction ratio was specified to be 930°C or less is because in the rolling process, if the temperature exceeds 930°C, austenite will recrystallize and the dislocations introduced by rolling will disappear, making it impossible to obtain fine austenite. It's for a reason.
  • the above-mentioned total rolling reduction refers to the sum of the rolling reductions of each rolling pass in a temperature range of 930° C. or lower.
  • the total rolling reduction at 930°C or less is 40% or more and 59% or less across both the rough rolling and finishing rolling described above. good.
  • the total rolling reduction at 930° C. or lower may be set to 40% or more and 59% or less by only finish rolling.
  • the steel material (slab) may be cooled to a temperature of 930°C or less during rough rolling. After that, rolling may be performed such that the total rolling reduction ratio at 930° C. or lower in both rough rolling and finish rolling is 40% or more and 59% or less.
  • the raw steel plate for hot rolled steel plate (hereinafter also simply referred to as raw steel plate, also referred to as hot rolled plate) is cooled.
  • cooling is carried out under the conditions that the average cooling rate Vc (° C./s) at the center of the plate thickness satisfies the following formula (2), and the cooling stop temperature at the center of the plate thickness is 550° C. or more and 680° C. or less. 4 ⁇ Vc ⁇ 20...(2)
  • Average cooling rate Vc at the center of the plate thickness 4°C/s or more and 20°C/s or less
  • the average cooling rate Vc is preferably 6°C/s or more, more preferably 8°C/s or more. Moreover, it is preferably 18°C/s or less, more preferably 16°C/s or less. In the present invention, it is preferable to start cooling immediately after finish rolling from the viewpoint of suppressing grain size from becoming coarse.
  • Cooling stop temperature at the center of the plate thickness 550°C or more and 680°C or less
  • the cooling stop temperature at the center of the plate thickness is less than 550°C, temperature unevenness will occur in the length direction and/or width direction of the raw steel sheet during cooling. There is a possibility that variations in mechanical properties may occur.
  • the cooling stop temperature at the center of the plate thickness exceeds 680°C, the ferrite grains become coarse and the desired crystal grain size cannot be obtained.
  • the cooling stop temperature at the center of the plate thickness is preferably 560°C or higher, more preferably 580°C or higher. Further, the temperature is preferably 660°C or lower, more preferably 650°C or lower.
  • the average cooling rate Vc (° C./s) at the center of the plate thickness is the average cooling rate in the temperature range from the start of cooling to the stop of cooling at the center of the plate thickness.
  • the above average cooling rate is a value obtained by ((temperature of the raw steel plate before cooling - temperature of the raw steel plate after cooling)/cooling time), and is the temperature in the cross section of the raw steel plate obtained by heat transfer analysis. It can be calculated from the distribution.
  • the cooling method include water cooling such as water injection from a nozzle, cooling by cooling gas injection, and the like.
  • cooling process it is preferable to perform a cooling operation (treatment) on both sides of the raw steel plate (hot rolled sheet) so that both sides of the raw steel plate (hot rolled sheet) are cooled under the same conditions.
  • a cooling operation treatment
  • the amount and pressure of cooling water or cooling gas, the injection time and angle, the conveyance speed of the raw steel plate, etc. can be adjusted.
  • heat transfer analysis is performed in advance to determine the cooling treatment conditions for the steel sheet material, and then these conditions can be reflected in the manufacturing conditions.
  • the raw steel plate is wound up in the winding process.
  • winding temperature a temperature at the center of the thickness of the raw steel sheet (winding temperature): 550° C. or more and 680° C. or less. If the coiling temperature is less than 550° C., a large amount of upper bainite is generated on the surface of the steel sheet, and the area ratio may exceed 5%. On the other hand, if the winding temperature exceeds 680° C., the ferrite grains become coarse and the desired crystal grain size cannot be obtained.
  • the winding temperature is preferably 570°C or higher, more preferably 580°C or higher. Further, the winding temperature is preferably 660°C or lower, more preferably 650°C or lower.
  • a second cooling process is performed to cool the rolled steel sheet obtained in the winding process.
  • the rolled steel sheet obtained in the above-mentioned winding step is allowed to stay in a temperature range of 400° C. to 300° C. for a period of 1.0 h or more and 10.0 h or less. If the residence time in the temperature range from 400° C. to 300° C. is less than 1.0 h, the desired work hardening index cannot be obtained, and the desired yield ratio and toughness cannot be obtained. If the residence time in the temperature range from 400°C to 300°C exceeds 10.0 hours, the desired yield strength and tensile strength may not be obtained.
  • the residence time in the temperature range of 400° C. to 300° C. is preferably 1.5 h or more, more preferably 2.0 h or more.
  • the residence time in the temperature range of 400° C. to 300° C. is preferably 9.0 h or less, more preferably 8.5 h or less.
  • the temperature of the steel plate (wound steel plate) after winding changes depending on the thickness, width, length, etc. of the raw steel plate. Therefore, in the second cooling process, it is preferable to perform a heat transfer analysis in advance to calculate the temperature change of the steel plate after winding, and to cool the steel plate using a heat retention treatment in which the steel plate is covered with a heat insulating material or the like, or using cooling water or cooling gas, etc., so that the residence time in the temperature range of 400°C to 300°C falls within the range of this invention. Since the steel structure, strength, and other properties hardly change in the temperature range below 300°C, any type of cooling can be used to cool the steel plate from 300°C to room temperature, such as rapid cooling or natural cooling.
  • the temperature of the steel plate after coiling varies depending on the location, so as shown in Figure 1, the temperature of the side surface of the steel plate after coiling (ultimately hot-rolled steel plate) is Measurements were made at locations (number 10; the outer surface, inner surface, and center of the steel sheet after winding), and the average value was calculated and used as the temperature of the rolled steel sheet.
  • the temperature of the rolled steel plate was measured every 20 minutes, and the residence time from 400°C to 300°C was calculated.
  • the temperature of the rolled steel sheet can be measured with a non-contact thermometer such as a radiation thermometer or a contact thermometer such as a thermocouple.
  • the low yield ratio rectangular steel pipe of the present invention is made from the above-mentioned low yield ratio hot rolled steel sheet.
  • the low yield ratio rectangular steel pipe not only exhibits a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, and a low yield ratio of 0.90 or less in the flat plate part in the tube axis direction, but also has a uniform elongation of 5. 0% or more, the absorbed energy in the Charpy impact test at -20°C is 60J or more, the energy ratio P is 0.5 to 1.2, and the low-temperature toughness is such that the ductile-brittle transition temperature is -10°C or less.
  • Low yield ratio rectangular steel pipes are manufactured by roll-forming a hot rolled steel plate into a cylindrical open pipe, and then welding the butt portions by electric resistance welding to produce a round steel pipe. Thereafter, a several percent reduction is applied in the axial direction of the round steel tube using rolls placed on the top, bottom, right and left sides of the tube, and the tube is formed into a square shape to obtain a square steel tube.
  • cylindrical shape refers to a shape obtained by forming a hot rolled steel plate into a circular shape by roll forming, and means a state in which the ends of the hot rolled steel plate are not electrically welded.
  • a coiled low yield ratio hot rolled steel plate is cold formed into a circular shape using a roll forming method using rolls to produce a round steel pipe, and then the round shape is formed into a square shape using a roll forming method using a roll.
  • the low yield ratio rectangular steel pipe of the present invention is made of the above-mentioned low yield ratio hot rolled steel sheet, an increase in the yield ratio is suppressed, and it is possible to reduce the yield ratio, for example, with a thick wall of 12 mm or more. Even if it is a material, it can have a low yield ratio.
  • the square steel pipe of the present invention has a Charpy absorbed energy ratio P of 0.5 to 1.2 at -20°C, and has excellent low-temperature toughness in the pipe axial direction and the pipe circumferential direction.
  • a hot-rolled steel plate with a small difference in steel structure between the width direction and longitudinal direction (rolling direction) can be obtained, and the hot-rolled steel plate can be rolled.
  • a low yield ratio rectangular steel pipe having the aforementioned Charpy absorbed energy ratio P of 0.5 to 1.2 at -20°C can be obtained.
  • the heat treatment temperature is more preferably 150°C or higher. Further, the heat treatment temperature is more preferably 500°C or lower.
  • the heat treatment time is preferably 30 seconds or more, more preferably 1 minute or more. The upper limit is not particularly specified, but from the viewpoint of reducing heat treatment costs, the time is preferably 10 minutes or less, more preferably 5 minutes or less.
  • the method of heat treating such a square steel pipe is not limited, but known heat treatment equipment (heating equipment) such as combustion of flammable gas, heating with an electric heater, heating with IH (induction heating), etc. can be used.
  • the square steel pipe in the present invention is not limited to a square steel pipe whose sides are all equal in length (the ratio of the length of the long side to the length of the short side (long side length/short side length) is 1.0).
  • rectangular steel pipes with such a ratio of more than 1.0 are also included.
  • this ratio is preferably in the range of 1.0 or more and 2.5 or less. This ratio is more preferably 1.0 or more and 2.0 or less.
  • the square steel pipe of the present invention is manufactured.
  • a square steel pipe having a flat plate portion with excellent mechanical properties can be obtained. More specifically, according to the present invention, the yield strength of the flat plate part is 295 MPa or more, the tensile strength of the flat plate part is 400 MPa or more, the yield ratio of the flat plate part is 0.90 or less, uniform elongation: 5.0% or more, and the flat plate
  • the square steel pipe has a Charpy absorbed energy of 60 J or more at -20°C, a ductile-brittle transition temperature of the flat plate part of -10°C or less, and a Charpy absorbed energy ratio P of 0.5 to 1.2 at -20°C. can get.
  • the square steel pipe of the present invention has a ductile-brittle transition temperature of less than 0°C and has excellent toughness not only in the axial direction but also in the circumferential direction, so it can be used in buildings in cold regions where the air temperature or room temperature is below freezing.
  • the building structure can be suitably used as a structural member, and even in the event of a huge earthquake, the building structure is unlikely to collapse and can ensure excellent earthquake resistance.
  • FIG. 2 is a schematic diagram showing an example of the architectural structure of the present invention.
  • the building structure of the present invention includes the above-described square steel pipe (low yield ratio square steel pipe) 1 of the present invention as a column material.
  • Reference numerals 4, 5, 6, and 7 respectively indicate a girder, a small beam, a diaphragm, and a stud.
  • the square steel pipe of the present invention has excellent mechanical properties in the flat plate portion. Therefore, the building structure of the present invention using this square steel pipe as a column material exhibits excellent seismic performance.
  • Molten steel having the composition shown in Table 1 was cast to obtain a slab.
  • the obtained slab was subjected to a heating process, a hot rolling process, a cooling process, a winding process, and a second cooling process after the winding process under the conditions shown in Table 2 to obtain a hot rolled steel plate.
  • a hyphen (-) means that the content is 0 (zero) or corresponds to an impurity.
  • the hot-rolled steel sheet was rolled into a cylindrical shape, and the butt joints were electric resistance welded to produce a round steel pipe.
  • the round steel pipe was rolled into a square shape (square in cross section perpendicular to the pipe axis direction) by rolls arranged above, below, left and right of the round steel pipe, to obtain a roll-formed square steel pipe having a corner portion and a flat portion and a side length (mm) and a wall thickness (mm) shown in Table 4 below.
  • Test pieces were taken from the above-mentioned hot-rolled steel sheets, and the following structure observation, tensile test, and Charpy impact test were carried out.
  • Test pieces for microstructural observation were taken from the center of the width of the hot rolled steel plate, including the 1/2t position (t: plate thickness) and covering a range of 5 mm in the thickness direction from the 1/2t position. did.
  • the observation surface was made to be a cross section in the rolling direction during hot rolling, and after polishing, nital corrosion was performed.
  • the structure was observed using an optical microscope (magnification: 1000x) or a scanning electron microscope (SEM, magnification: 1000x) to observe the structure within a range of ⁇ 1 mm in the thickness direction from the 1/2t position of the hot rolled steel sheet. Observed and imaged.
  • the area ratios of ferrite, pearlite, pseudo pearlite, and upper bainite were determined from the obtained optical microscope images and SEM images.
  • the area ratio of each tissue was observed in 5 visual fields and calculated as the average value of the values obtained in each visual field.
  • the area ratio obtained by tissue observation was defined as the area ratio of each tissue.
  • ferrite is a product of diffusion transformation, and has a low dislocation density and a nearly restored structure.
  • Pearlite is a structure in which cementite and ferrite are arranged in layers
  • pseudo-pearlite is a structure in which cementite is confirmed to be arranged in dotted rows in ferrite.
  • Upper bainite is a complex structure of lath-shaped ferrite and cementite with a high dislocation density. In addition to the above shapes, they are distinguished by their white color, pearlite is black, pseudo-pearlite is black or gray, and upper bainite is white or gray.
  • the crystal grain size was determined by determining the orientation difference between adjacent crystal grains, and measuring the boundary where the orientation difference was 15° or more as a grain boundary. The grain size (equivalent circle diameter) of each crystal grain was calculated from the obtained grain boundaries, and the arithmetic mean thereof was determined to be the average grain size (average equivalent circle diameter).
  • the average crystal grain size ⁇ 5.
  • the area ratio of crystal grains having a crystal grain size of 0 ⁇ m or less was calculated.
  • the major axis and minor axis of the crystal grains were measured by the method described in JIS R 1670 (2006), and the ratio of the major axis to the minor axis (major axis)/(minor axis) was calculated.
  • the major axis to the minor axis can be calculated.
  • the number of crystal grains (pieces/mm 2 ) having a ratio of (major axis)/(minor axis) of 3.0 or more was calculated.
  • crystal grains having a diameter of less than 2.0 ⁇ m were excluded from the analysis target as measurement noise.
  • test pieces were taken from the obtained square steel pipe (roll-formed square steel pipe) and subjected to the following tensile test and Charpy impact test.
  • FIG. 3 is a schematic diagram showing the sampling position of the tensile test piece of the flat plate portion.
  • a JIS No. 5 tensile test piece was taken from the flat plate portion of the square steel pipe 1 so that the tensile direction was parallel to the pipe axis direction, as shown by X in FIG.
  • a tensile test was conducted on the sampled tensile test piece in accordance with the provisions of JIS Z 2241 (2011), and the yield strength YS and tensile strength TS were measured. The ratio was calculated.
  • plastic elongation (uniform elongation) at the highest load point was also measured (A g of JIS Z 2241 (2011)).
  • the tensile test piece of the flat plate part was measured at the center of the width of the flat plate part (X in Figure 3) at the 3 o'clock side when the welded part of the square steel pipe (W in Figure 3) was taken as the 12 o'clock direction. Collected from. The number of test pieces was two each, and their average values were calculated to determine YS, TS, and yield ratio.
  • FIG. 4 is a schematic diagram showing the sampling positions of Charpy test pieces.
  • the square steel pipe 1 was tested at a position of 1/4t of the wall thickness t from the outer surface of the square steel pipe 1 so that the longitudinal direction of the test piece was parallel to the pipe axis direction, as shown in Y in Fig. 4.
  • a V-notch standard test piece taken from the flat plate portion and compliant with the provisions of JIS Z 2242 (2016) was used.
  • Charpy impact tests were conducted at test temperatures of -60°C, -40°C, -20°C, and 0°C in accordance with the provisions of JIS Z 2242 (2018).
  • the number of test pieces was three at each test temperature, and the average value (J) of the ductile-brittle transition temperature and absorbed energy was determined.
  • the longitudinal direction of the test piece is parallel to the circumferential direction at a position of 1/4t of the wall thickness t from the outer surface of the square steel tube 1, as shown in Z in FIG.
  • the ratio P of Charpy absorbed energy at ⁇ 20° C. in the tube circumferential direction to the tube axis direction was determined. The obtained results are also listed in Table 4.
  • steel plate No. 1 to 20 are examples of the present invention, and steel plate No. 1 to 20 are examples of the present invention.
  • Nos. 21 to 49 are comparative examples.
  • Steel plate No. in Table 4. means that it is a square steel pipe manufactured using the steel plate of Table 3 with the same number.
  • steel plate No. in Table 4. 1 is steel plate No. 1 in Table 3. This is a square steel pipe manufactured from 1.
  • the hot-rolled steel sheets of the examples of the present invention all have a steel structure at the center of the sheet thickness that includes a main phase of ferrite, pearlite and pseudo-pearlite with a total area ratio of 6 to 25%, and an area of
  • a grain is defined as a region that contains upper bainite (second phase) with a ratio of 5% or less and is surrounded by boundaries with a misorientation of 15° or more, the main phase and second phase are separated in the steel structure at the center of the plate thickness.
  • the average crystal grain size of the steel structure contained is 10.0 to 30.0 ⁇ m, the area ratio of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m of the average crystal grain size is 35% or more, and the minor axis
  • the number density of crystal grains having a ratio of (major axis)/(breadth axis) of 3.0 or more was 30 pieces/mm 2 or less.
  • the yield strength is 250 MPa or more
  • the tensile strength is 400 MPa or more
  • the yield ratio is 0.75 or less
  • the work hardening index n 3-7 is 0.20 or more at a plastic strain of 3 to 7%
  • the Charpy absorbed energy at -20°C is 100J or more
  • the ductile-brittle transition temperature was -20°C or less.
  • the square steel pipes manufactured using the hot-rolled steel sheets of the present invention all had a yield strength in the flat portion of 295 MPa or more, a tensile strength in the flat portion of 400 MPa or more, a yield ratio in the flat portion of 0.90 or less, a uniform elongation in the flat portion of 5.0% or more, a Charpy absorbed energy in the axial direction of the flat portion at -20°C of 60 J or more, an energy ratio P of 0.5 to 1.2, and a ductile-brittle transition temperature in the flat portion of -10°C or less.
  • the result was a hot-rolled steel sheet whose work hardening index did not reach the desired value.
  • the square steel pipes manufactured using such hot-rolled steel sheets had lower uniform elongation and yield ratios and the like did not reach desired values.
  • Comparative example No. 24 (Steel The result was a hot-rolled steel sheet that did not reach the specified value. Furthermore, in the square steel pipe manufactured using such a hot rolled steel sheet, the Charpy absorbed energy in the pipe axis direction at -20°C did not reach the desired value.
  • Comparative example No. 25 (Steel Y) has a Si content exceeding the range of the present invention, so the area ratio of pseudo pearlite increases excessively, and the Charpy absorbed energy at -20°C does not reach the desired value. It became a steel plate. Furthermore, in the square steel pipe manufactured using such a hot rolled steel sheet, the Charpy absorbed energy in the pipe axis direction at -20°C did not reach the desired value.
  • Comparative example No. 30 (Steel AD) had a Cr content exceeding the range of the present invention, so the amount of upper bainite was outside the range of the present invention.
  • the hot-rolled steel sheet did not reach the desired values for Charpy absorbed energy at -20°C.
  • the square steel pipe manufactured using this hot-rolled steel sheet did not reach the desired values for Charpy absorbed energy in the axial direction and ductile-brittle transition temperature at -20°C.
  • Comparative example No. 35 is a hot-rolled steel sheet in which the content of B exceeds the range of the present invention, so the amount of upper bainite is outside the range of the present invention, and the yield ratio of the flat plate portion does not reach the desired value. It became. In addition, the yield ratio of square steel pipes manufactured using such hot-rolled steel sheets did not reach desired values.
  • the slab heating temperature exceeds the range of the present invention, the crystal grains become coarse, and the area of the crystal grains having an average crystal grain size and a crystal grain size within ⁇ 5.0 ⁇ m rate was outside the scope of the present invention.
  • a hot rolled steel sheet was obtained whose yield strength, tensile strength, Charpy absorbed energy at -20°C, etc. did not reach the desired values.
  • the yield strength, tensile strength, etc. of square steel pipes manufactured using such hot-rolled steel sheets did not reach desired values.
  • Comparative example No. 41 has a cooling stop temperature and a coiling temperature below the range of the present invention, so the area ratio of upper bainite and the area of crystal grains having a crystal grain size within ⁇ 5.0 ⁇ m of the average crystal grain size ratio, etc. are outside the scope of the present invention.
  • the yield ratio of both the hot-rolled steel sheet and the square steel pipe manufactured using the hot-rolled steel sheet did not reach the desired value.
  • the uniform elongation of the square steel pipe was less than 5.0%.
  • Comparative example No. 42 (Steel T) had a low average cooling rate at the center of the plate thickness, and the cooling stop temperature and coiling temperature exceeded the range of the present invention, so the average grain size was outside the range of the present invention. .
  • a hot rolled steel sheet was obtained whose yield strength, tensile strength, Charpy absorbed energy at -20°C, etc. did not reach the desired values.
  • the yield strength, tensile strength, etc. of square steel pipes manufactured using such hot-rolled steel sheets did not reach desired values.
  • Comparative example No. 43 has a residence time in the temperature range of 400°C to 300°C that is below the range of the present invention, so the area ratio of upper bainite exceeds 5%, and the work hardening index at a plastic strain of 3% to 7%. , yield ratio and Charpy absorbed energy at -20°C did not reach desired values, resulting in a hot-rolled steel sheet. Furthermore, in the square steel pipe manufactured using such a hot rolled steel sheet, the Charpy absorbed energy in the pipe axis direction at -20°C did not reach the desired value.
  • Comparative example No. 44 has a residence time in the temperature range of 400°C to 300°C that exceeds the range of the present invention, so the average grain size is outside the range of the present invention, and the yield strength and tensile strength are below the desired values.
  • the result was a hot-rolled steel sheet that did not reach this level.
  • the yield strength and tensile strength of square steel pipes manufactured using such hot-rolled steel sheets also did not reach desired values.
  • Comparative example No. 45 has a rough rolling end temperature exceeding the range of the present invention, so the number density of crystal grains with a ratio of the major axis to the minor axis (major axis)/(minor axis) of 3.0 or more is within the scope of the present invention. is out of range.
  • a hot rolled steel sheet was obtained whose ductile-brittle transition temperature did not reach the desired value.
  • square steel pipes manufactured using such hot-rolled steel sheets had ductile-brittle transition temperatures that did not reach desired values.
  • Comparative example No. 46 (Steel T)
  • the rough rolling end temperature and finish rolling end temperature are below the range of the present invention, so a large amount of ferrite is generated and the total area ratio of pearlite and pseudo pearlite is less than 6%, resulting in poor plasticity.
  • the result was a hot-rolled steel sheet whose work hardening index, etc., at a strain of 3 to 7% did not reach the desired value.
  • the square steel pipe manufactured using such a hot rolled steel sheet had a uniform elongation of less than 5.0%, and furthermore, the tensile strength and yield ratio did not reach the desired values.
  • the slab heating temperature was below the range of the present invention, so the deformation resistance of the rolled material increased, making rolling difficult, and rolling was interrupted midway through rough rolling. As a result, it was not possible to produce hot-rolled steel plates and square steel pipes.

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Abstract

L'invention fournit une feuille d'acier laminée à chaud à faible rapport d'élasticité présentant une excellente résistance et une excellente ténacité à basse température. La feuille d'acier comprend une composition de composant prédéterminée. La structure en acier de la partie centrale d'épaisseur de feuille comprend une phase principale, qui est de la ferrite, et une seconde phase dans laquelle le rapport de surface totale de perlite et de pseudoperlite est de 6 à 25 % et le rapport de surface de bainite supérieure est de 5 % ou moins. Lorsqu'une région entourée par des limites où la différence d'orientation entre des cristaux adjacents est de 15° ou plus est prise en tant que grain cristallin, la granulométrie moyenne des grains cristallins de la structure en acier contenant la phase principale et la seconde phase dans la partie centrale d'épaisseur de feuille est de 10,0 à 30,0 µm, le rapport de surface de grains cristallins présentant une granulométrie de grains cristallins dans cette granulométrie moyenne de grains cristallins ± 5,0 µm est de 35 % ou plus, et le nombre de grains cristallins dans lesquels le rapport (axe majeur) / (axe mineur) de l'axe majeur à l'axe mineur est de 3,0 ou plus est de 30/mm2 ou moins.
PCT/JP2023/020167 2022-09-20 2023-05-30 Feuille d'acier laminée à chaud, tube d'acier carré, procédés de fabrication associés et structure de bâtiment WO2024062686A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2013153679A1 (fr) * 2012-04-12 2013-10-17 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud pour tube d'acier carré destiné à être utilisé comme élément structural de construction et procédé pour sa production
WO2018110152A1 (fr) * 2016-12-12 2018-06-21 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud à faible coefficient d'élasticité pour tuyau en acier à section carrée
JP2019196508A (ja) * 2018-05-08 2019-11-14 日本製鉄株式会社 熱延鋼板、角形鋼管、およびその製造方法
WO2020039980A1 (fr) * 2018-08-23 2020-02-27 Ｊｆｅスチール株式会社 Tuyau en acier carré, son procédé de fabrication, et structure de construction
JP2020037734A (ja) * 2018-08-30 2020-03-12 株式会社神戸製鋼所 母材と溶接熱影響部の靭性に優れかつ音響異方性の小さい高強度低降伏比厚鋼板およびその製造方法

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2013153679A1 (fr) * 2012-04-12 2013-10-17 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud pour tube d'acier carré destiné à être utilisé comme élément structural de construction et procédé pour sa production
WO2018110152A1 (fr) * 2016-12-12 2018-06-21 Ｊｆｅスチール株式会社 Tôle d'acier laminée à chaud à faible coefficient d'élasticité pour tuyau en acier à section carrée
JP2019196508A (ja) * 2018-05-08 2019-11-14 日本製鉄株式会社 熱延鋼板、角形鋼管、およびその製造方法
WO2020039980A1 (fr) * 2018-08-23 2020-02-27 Ｊｆｅスチール株式会社 Tuyau en acier carré, son procédé de fabrication, et structure de construction
JP2020037734A (ja) * 2018-08-30 2020-03-12 株式会社神戸製鋼所 母材と溶接熱影響部の靭性に優れかつ音響異方性の小さい高強度低降伏比厚鋼板およびその製造方法

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