US20220316019A1 - Section steel and method for manufacturing same - Google Patents

Section steel and method for manufacturing same Download PDF

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US20220316019A1
US20220316019A1 US17/609,965 US202017609965A US2022316019A1 US 20220316019 A1 US20220316019 A1 US 20220316019A1 US 202017609965 A US202017609965 A US 202017609965A US 2022316019 A1 US2022316019 A1 US 2022316019A1
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section steel
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Jun Ho Chung
Hong Ki Jang
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Hyundai Steel Co
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/63—Quenching devices for bath quenching
    • C—CHEMISTRY; METALLURGY
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21—METALLURGY OF IRON
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    • 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
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
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    • 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
    • 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

Definitions

  • Exemplary embodiments of the present invention relate to section steel and a method for manufacturing the same, and more particularly, to high-strength and high-performance section steel having fireproof/aseismic performance and a method for manufacturing the same.
  • a section steel generally refers to a steel material whose cross-sectional shape is variously changed. Recently, the section steel has been applied as a structural steel material such as pillars of large buildings, and has also been applied as temporary materials for civil works such as subways and bridges and piles for foundation.
  • the section steel may be manufactured by hot rolling a cast piece such as bloom, billet, beam blank, and the like. manufactured by continuous casting.
  • An object of the present invention is to provide high-strength and high-performance section steel having fireproof/aseismic performance and a method for manufacturing the same.
  • a section steel according to an exemplary embodiment of the present invention for achieving the above object is characterized in that it includes an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (
  • the section steel may have a shock absorption energy of 200 J or greater at 0° C.
  • a final microstructure of the section steel may include bainite.
  • a method for manufacturing section steel according to an exemplary embodiment of the present invention for achieving the above object comprises the steps of: (a) reheating a steel material including an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or greater of nickel (Ni),
  • the QST (Quenching & Self-Tempering) treatment step may have a water-cooling end temperature and a self-tempering temperature of 765 to 800° C.
  • the section steel which has been subjected to the step (c) may have a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
  • the step (b) of the method for manufacturing section steel may comprise a step of hot-rolling the steel material so that a rolling start temperature becomes 1,050 to 1,100° C.
  • FIG. 1 is a flowchart illustrating a method for manufacturing section steel according to an exemplary embodiment of the present invention.
  • fireproof thick plate materials have been developed in Korea, they are not commercially available, and there is no development and performance evaluation of fire-resistant steel materials for steel materials (H-steel, etc.) for building structures with shapes.
  • high-strength and high-performance section steel having stable fireproof/aseismic performance and a method for manufacturing the same will be described.
  • Section steel according to an exemplary embodiment of the present invention includes an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of
  • Carbon (C) is an element that is added to secure strength and has the greatest influence on weldability. Further, carbon reacts with Nb, Ti, etc. to promote the formation of fine carbides, thereby effectively contributing to strength improvement through precipitation strengthening, and also impedes dislocation movement at high temperatures to improve high-temperature strength, which makes it possible to effectively secure fireproof performance.
  • the carbon (C) may be added at a content ratio of 0.08 to 0.17% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of carbon is less than 0.08% by weight of the total weight, it may be difficult to secure sufficient strength.
  • Manganese (Mn) as a solid solution strengthening element, is an element which is effective in generating bainite structure by not only contributing to securing strength, but also improving the hardenability of steel.
  • Manganese may be added at a content ratio of 0.50 to 1.60% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of manganese is less than 0.50% by weight, the effect of solid solution strengthening cannot be sufficiently exhibited. Further, when the content of manganese exceeds 1.60% by weight, it may combine with S to form MnS inclusions or may cause central segregation to be generated in the ingot, thereby lowering ductility of the section steel and lowering the corrosion resistance.
  • Silicon (Si) is added together with aluminum as a deoxidizer for removing oxygen in steel in the steelmaking process. Further, silicon may also have a solid solution strengthening effect.
  • the silicon may be added at a content ratio of 0.10 to 0.50% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of silicon is less than 0.10% by weight of the total weight, the effect of adding silicon may not be properly exhibited. When the content of silicon is added in a large amount exceeding 0.50% by weight of the total weight, the weldability of steel may be lowered, a red scale may be formed during reheating and hot rolling, which causes a problem with the surface quality.
  • Chromium (Cr) is an element that contributes to securing bainite microstructure by improving the hardenability of steel, and when chromium (Cr) is added to C—Mn steel as a ferrite stabilizing element, it delays the diffusion of carbon due to the solute interference effects, thereby affecting particle size refinement.
  • the chromium may be added at a content ratio of 0.10 to 0.70% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of chromium is less than 0.10% by weight of the total weight, the effect of adding chromium may not be properly exhibited.
  • Copper (Cu) is an element that is solid-solutioned in ferrite to exhibit a solid solution strengthening effect. Further, in the bainite transformation, copper that is not precipitated and is supersaturated is solid-solutioned in the structure at room temperature, a copper phase is precipitated on dislocations introduced by bainite transformation when it is heated to a use temperature of refractory steel of 600° C., and the strength of the base material is increased by the precipitation hardening.
  • the copper may be added at a content ratio of greater than 0 and 0.5% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of copper is added in a large amount exceeding 0.5% by weight of the total weight, problems may arise in that hot working is difficult, precipitation strengthening is saturated, toughness is lowered, and red shortness occurs.
  • Molybdenum is an element that may contribute to securing the bainite microstructure by improving the hardenability of steel and is very effective in securing high-temperature strength, and is an element that is effective in securing the strength and high-temperature strength of the base material.
  • the molybdenum may be added at a content ratio of 0.30 to 0.70% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention.
  • Phosphorus (P) may allow solid solution strengthening to serve to increase the intensity of the strength and suppressing the formation of carbides.
  • the phosphorus may be added at a content ratio of greater than 0 and 0.020% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of phosphorus exceeds 0.020% by weight, problems may arise in that inclusions, etc. are formed as tramp elements so that the ductility of the steel is reduced, and the low-temperature impact value is lowered by the precipitation behavior.
  • S may improve processability by forming fine MnS precipitates.
  • the sulfur may be added at a content ratio of greater than 0 and 0.01% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention.
  • inclusions, etc. may be formed as tramp elements so that the ductility of the steel is reduced, toughness and weldability may be impaired, and the low-temperature impact value may be lowered.
  • Nitrogen (N) may contribute to crystal grain refinement and contribute to securing high-temperature strength by forming nitride-based precipitates such as AlN, etc.
  • the nitrogen may be added at a content ratio of greater than 0 and 0.012% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of nitrogen exceeds 0.012% by weight, the toughness of the weld may be lowered, and the impact value may be lowered.
  • Boron (B) contributes to improving the strength of steel as a strong hardenable element.
  • boron may be optionally added in an amount greater than 0 and 0.003% by weight or less. If the content of boron exceeds 0.003% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention, a problem may arise in that material deviation is generated due to grain boundary segregation.
  • Nickel (Ni) is an element that increases hardenability and improves toughness
  • vanadium (V) is an element that has an effect of increasing strength by forming precipitates during rolling, and in particular, is capable of controlling the precipitation amount depending on the amount of nitrogen added
  • niobium (Nb) is an element that is precipitated in the form of NbC or Nb(C, N) to improve strength of the base material and the weld
  • titanium (Ti) is an element that suppresses the formation of AlN by high-temperature TiN formation and has the effect of refining the size of crystal grains by forming Ti(C, N), etc.
  • the section steel according to an exemplary embodiment of the present invention contains at least one of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), but they may be added so that the sum of the contents thereof is 0.01 to 0.5% by weight of the total weight of the section steel.
  • the sum of the contents of at least one of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti) contained in the section steel according to an exemplary embodiment of the present invention is lower than 0.01% by weight, the above-described addition effects cannot be expected, and when it is higher than 0.5% by weight, problems may arise in that the manufacturing cost of parts is increased, brittle cracks occurs, and the carbon content in the matrix decreases so that the properties of steel are lowered.
  • the section steel according to an exemplary embodiment of the present invention having the above-described alloy element composition may have a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C. Further, it may have a shock absorption energy of 200 J or greater at temperature of 0° C.
  • the final microstructure may include bainite.
  • FIG. 1 is a flowchart schematically illustrating a method for manufacturing section steel according to an exemplary embodiment of the present invention.
  • a method for manufacturing section steel having excellent fire resistance properties comprises a reheating step (S 100 ), a hot rolling step (S 200 ), and a Quenching & Self-Tempering (QST) step (S 300 ).
  • the section steel rolling process is performed through a reheating process, a hot deformation process, and a cooling process.
  • reheating process a beam blank in a semi-finished product state is reheated to a temperature of 1,200 to 1,250° C.
  • the hot rolling process is characterized in that, after completing final finishing rolling at a temperature of 910 to 950° C. while passing the reheated beam blank through each of the rolling rolls (RM, IM, and FM), STT (Self Tempering Temperature) of 765 to 800° C. is secured through QST (Quenching and Self Tempering) equipment which is a surface accelerated cooling system.
  • STT Self Tempering Temperature
  • a steel material of the predetermined composition described above is reheated.
  • the steel material may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through the steelmaking process.
  • the steel material may be, for example, a billet or a beam blank.
  • the steel material may include an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other un
  • the steel material may be reheated at a temperature of 1,200 to 1,250° C.
  • a reheating temperature is lower than 1,200° C., problems may arise in that the solid solution of various carbides are not sufficient, and the components segregated during the continuous casting process are not sufficiently evenly distributed.
  • the reheating temperature exceeds 1,250° C., very coarse austenite grains are formed so that it may be difficult to secure strength. Further, when it exceeds 1,250° C., heating cost increases and process time is added, which may result in an increase in manufacturing cost and a decrease in productivity.
  • the reheated steel material is hot-rolled.
  • the hot rolling may be controlled so that the rolling end temperature becomes 910 to 950° C.
  • the rolling end temperature is lower than 910° C.
  • rolling is carried out in the non-recrystallization region so that the rolling addition may increase, and the yield ratio of the section steel as a rolling result may increase.
  • the hot rolling may be controlled so that the rolling start temperature becomes 1,050 to 1,100° C.
  • the hot-rolled section steel is cooled and self-tempered.
  • a quenching method of spraying cooling water to the section steel is applied to the cooling.
  • the QST step may be performed in a state where the water cooling end temperature and the self-tempering temperature are controlled to 765 to 800° C. by controlling the feed rate of the section steel or the amount of cooling water sprayed.
  • the steel material is manufactured through a reheating process, a hot deformation process, and a cooling process.
  • a reheating process a billet or a beam blank in a semi-finished product state is reheated at a temperature of 1,200 to 1,250° C.
  • the QST (Quenching & Self-Tempering) treatment may be performed in a state where the water cooling end temperature and the self-tempering temperature are controlled to 765 to 800° C.
  • steel grade design and process conditions with added chromium (Cr) and some alloying elements are applied so that strength and toughness can be improved while niobium (Nb) or titanium (Ti), which is an expensive precipitation hardening alloying element commonly used, is not used or is used in a small amount. Further, low-temperature toughness may be secured through self-tempering temperature control during the cooling.
  • the section steel may have a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
  • the final microstructure may include bainite in the section steel according to an exemplary embodiment of the present invention.
  • Table 1 shows main alloy element compositions (unit: % by weight) of the present experimental example
  • Table 2 shows process conditions for manufacturing specimens of the present experimental example and results of measuring mechanical properties of the specimens implemented accordingly. After manufacturing beam blanks having the compositions of Table 1 using an electric furnace, H-steel having a 15 mm-thick flange portion were manufactured through hot rolling.
  • components of the composition system 2 of the present invention satisfy a composition including an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (T
  • components of the composition system 1 of the present invention do not satisfy a composition comprising greater than 0 and 0.02% by weight or less of phosphorus (P), greater than 0 and 0.01% by weight or less of sulfur (S), and greater than 0 and 0.003% by weight or less of boron (B).
  • P phosphorus
  • S sulfur
  • B boron
  • the specimen according to Example 1 of the present experimental example satisfies a composition including an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb
  • Example 1 which satisfies such composition and process conditions, satisfies all of the requirements of a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
  • Comparative Example 1 of the present experimental example does not satisfy composition ranges including an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), and an amount greater than 0 and 0.003% by weight or less of boron (B).
  • the recuperation temperature that is a self-tempering temperature in the QST (Quenching & Self-Tempering) treatment does not satisfy the range of 765 to 800° C. Accordingly, Comparative Example 1 does not satisfy the range of 490 to 620 MPa of the tensile strength at room temperature, and does not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
  • the specimen according to Comparative Example 2 of the present experimental example does not satisfy the composition ranges including an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), and an amount greater than 0 and 0.003% by weight or less of boron (B).
  • the recuperation temperature that is the self-tempering temperature in the QST (Quenching & Self-Tempering) treatment does not satisfy the range of 765 to 800° C. Accordingly, Comparative Example 2 does not satisfy the range of 490 to 620 MPa of the tensile strength at room temperature, and does not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
  • the specimens according to Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6 of the present experimental example do not satisfy the range of 765 to 800° C. of the recuperation temperature that is the self-tempering temperature in the QST (Quenching & Self-Tempering) treatment. Accordingly, the specimens do not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
  • the specimen according to Comparative Example 7 of the present experimental example does not satisfy the range of 765 to 800° C. of the recuperation temperature that is the self-tempering temperature in the QST (Quenching & Self-Tempering) treatment. Accordingly, the specimen does not satisfy the yield strength of 355 MPa or greater at room temperature, and does not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

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