Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
• • 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
  • B21B1/02—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 heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B1/024—Forging or pressing
• • 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
  • B21B1/02—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 heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B1/04—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 heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing in a continuous process
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
  • C21D1/28—Normalising
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
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  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
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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
• • 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
  • C21D9/40—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rings; for bearing races
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—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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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
  • B21B1/02—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 heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B2001/028—Slabs
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite
• • 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
  • C21D2261/00—Machining or cutting being involved

Definitions

• the present disclosure relates to steel that may be used for wind power generation towers, systems and the like, and a method of manufacturing the same, and more specifically, to an extremely thick steel material for a flange having excellent strength and low-temperature impact toughness, and a method of manufacturing the same.
• Wind power generators are gaining attention as an eco-friendly means of generating electricity, and include components such as tower flanges, bearings, main shafts and the like.
• tower flanges are joint components necessary for connecting towers, and usually 5 to 7 flanges are used for one tower, and are also installed in the sea or in extreme temperature regions, and thus, high durability is required.
• wind towers are also increasing in size, and accordingly, the steel used is also continuously required to be high-strength, high-toughness, and thick.
• the trend is to reduce the concentration of impurities such as non-metallic inclusions, segregation or the like, or to control cracks, pores and the like on the surface and inside the material to the extreme.
• Patent document 1 related thereto is a technology for applying high reduction ratio in a thick plate rough rolling process.
• a technology for determining a thickness-specific limit reduction ratio at which thickness-specific plate bite occurs from a pass-specific reduction ratio set to be close to the design allowance (load and torque) of a rolling mill a technology for distributing the reduction ratio by adjusting the index of a thickness ratio for each pass to secure the target thickness of a roughing mill, and a technology for modifying the reduction ratio so that plate bite does not occur based on the thickness-specific limit reduction ratio, thereby providing a manufacturing method in which an average reduction ratio of approximately 27.5% in the final three passes of roughing milling based on 80 mmt may be applied.
• the average reduction ratio of the entire thickness of a product is measured, and in the case of extremely thick materials with a maximum thickness of 200 mmt or more, it is technically difficult to apply high strain to the center where residual pores exist.
• Patent Document 2 provides a method of manufacturing a thick-walled, high-toughness and high-strength material, using a slab comprising, in mass%, C: 0.08 to 0.20%, Si: 0.40% or less, Mn: 0.5 to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 3.0% or less, Ni: 0.1 to 5.0%, Al: 0.010 to 0.080%, N: 0.0070% or less, and O: 0.0025% or less, and satisfying the relationships of Formulas (1) and (2), the remainder being Fe and unavoidable impurities, in which hot forging is performed with a cumulative reduction amount of 25% or more, heating is performed at a temperature of Ac3 point or higher and 1200°C or lower, hot rolling is performed with a cumulative reduction amount of 40% or more, and rapid cooling from a temperature of Ar3 point or higher to a low temperature
• the manufacturing method may cause surface defects due to localized strain concentration in the case in which the cumulative reduction amount is too high, and in particular, in the case in which a surface or subsurface defect exists in the cast state before forging, the defect may propagate during the forging process, further deteriorating the surface quality in the product state after rolling.
• the forging reduction amount per pass is insufficient, it is difficult to sufficiently pressurize the pores remaining in the center even if the cumulative reduction amount is high, and the rolling process is also not suitable for controlling the central pores and structure of extremely thick materials because the effective deformation amount in the center is small compared to the surface deformation.
• Patent Document 3 discloses that a thick-walled high-strength steel plate having 100 mmt or more and a yield strength of 620 MPa or more may be manufactured through a process of heating a material provided with a predetermined alloy composition to 1200-1350°C, performing hot forging with a cumulative reduction amount of 25% or more, heating to a temperature of Ac3 point or higher and 1200°C or lower, performing hot rolling with a cumulative reduction amount of 40% or more, reheating to a temperature of Ac3 point or higher and 1050°C or lower, rapidly cooling from a temperature of Ac3 point or higher to a low temperature of 350°C or lower or Ar3 point or lower, and performing tempering at a temperature of 450°C to 700°C.
• the carbon equivalent (Ceq) and hardenability index (DI) are high, and thus in addition to being vulnerable to surface cracks during casting, in the case of flange steel manufactured by normalizing heat treatment, the corresponding process conditions cannot be easily applied.
• the carbon equivalent (Ceq) and hardenability index (DI) are high, cracks easily occur on the surface layer of the cast due to the formation of surface hard tissue during the second cooling process of steelmaking, and the cracks may propagate during the forging process, which may deteriorate the surface quality of the final product.
• An aspect of the present disclosure is to provide an extremely thick steel material for a flange having excellent strength and low-temperature impact toughness and a method of manufacturing the same.
• an extremely thick steel material for a flange includes,
• [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent contents (in weight%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel, respectively, and 0 is substituted if these components are not added intentionally.
• the steel may have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorbed energy value of -50°C Charpy impact test of 50 J or more.
• a maximum surface crack depth of the steel may be 0.1 mm or less (including 0).
• a method of manufacturing an extremely thick steel material for a flange includes,
• [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent contents (weight%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in steel, respectively, and 0 is substituted if these components are not intentionally added.
• the slab may be manufactured using one of a continuous casting process, a semi-continuous casting process, and an ingot casting process.
• a prior austenite grain size of a surface layer of the slab before forging is 1000 ⁇ m or less, and a microstructure of the surface layer of the slab before forging is composed of a composite structure of polygonal ferrite of 15% or more and residual bainite.
• a size of a forging surface punched during the first upsetting may be 1000-1200 mm ⁇ 1800-2000 mm when being initially 700 mm ⁇ 1800 mm.
• a size of a forging surface may be 1450-1850mm x 2100-2500mm when being initially 1000-1200mm x 1800-2000mm.
• a size of a product may be 1450-18500 ⁇ 1300-1700mm.
• a size of a product may be 2300-2800 ⁇ ⁇ 400-800mm.
• the flange made of the steel may have a maximum thickness of 200 to 500 mm, an inner diameter of 4000 to 7000 mm, and an outer diameter of 5000 to 8000 mm.
• a heat treatment is performed such that an LMP defined by the following relational expression 2 satisfies 20 to 33.
• LMP T Logt + 20 ⁇ 1 / 1000
• T Kelvin reference temperature
• t time
• an exponent of log is 10.
• the method may further include an operation of performing a post-weld heat treatment, a stress relieving heat treatment, or a tempering heat treatment, when welding is performed on the steel after the normalizing heat treatment.
• the post-weld heat treatment is performed in a range where a value defined by the following relational expression 2 is LMP 19.3 or less.
• LMP T Logt + 20 ⁇ 1 / 1000
• T Kelvin reference temperature
• t time
• an exponent of log is 10.
• a method of manufacturing an extremely thick steel material for a flange includes,
• [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent contents (weight%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in steel, respectively, and 0 is substituted if these components are not intentionally added.
• a heat treatment is performed so that an LMP defined by the following relational expression 2 satisfies 20 to 33.
• LMP T Logt + 20 ⁇ 1 / 1000 ,
• T Kelvin reference temperature
• t time
• a exponent of log is 10.
• the present disclosure having the above-described configuration may effectively provide an extremely thick steel material that may be used for flanges, having excellent low-temperature impact toughness as well as strength, by compressing the central pore of the steel material by optimizing a forging process and thereby improving the internal soundness of a final product.
• the present disclosure relates to an extremely thick steel for flanges having excellent strength and low-temperature impact toughness and a method of manufacturing a product.
• preferred embodiments of the present disclosure will be described.
• the embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below.
• An extra heavy steel material for a flange of the present disclosure includes, in wt%, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 1.0%, Ca: 0.0005 to 0.004%, the remainder of Fe and other unavoidable impurities, the extremely thick steel material for a flange having a Ceq according to the relational expression 1 satisfying a range of 0.35 to 0.55, having a thickness of 200 to 500 mm, having a steel microstructure composed of a composite structure of ferrite and pearlite with an average grain size of 30 ⁇ m or less, having a maximum size of cementite existing in a ferrite-
• alloy composition of the present disclosure will be described in more detail, and unless otherwise specifically indicated, the % and ppm described in relation to the alloy composition are based on weight.
• Carbon (C) is the most important element for securing basic strength, and thus it needs to be contained in steel within an appropriate range, and to obtain this addition effect, 0.05% or more of carbon (C) may be added. Preferably, 0.10% or more of carbon (C) may be added.
• the present disclosure may limit the carbon (C) content to 0.20%, and the upper limit of the more desirable carbon (C) content may be 0.18%.
• Silicon (Si) is a substitutional element that improves the strength of steel through solid solution strengthening and has a strong deoxidation effect, and thus is an essential element for manufacturing clean steel. Therefore, silicon (Si) may be added at 0.05% or more, and more preferably, may be added at 0.20% or more. On the other hand, if silicon (Si) is added in a large amount, a Martensite-Austenite (MA) phase is generated and the strength of the ferrite matrix excessively increases, which may deteriorate the surface quality of the ultra-thick product, and thus the upper limit of the content may be limited to 0.50%. A more preferable upper limit of the silicon (Si) content may be 0.40%.
• Manganese (Mn) is a useful element that improves strength by solid solution strengthening and enhances hardenability to generate a low-temperature transformation phase. Therefore, to secure a tensile strength of 550 MPa or more, it is preferable to add 1.0% or more of manganese (Mn). A more preferable manganese (Mn) content may be 1.1% or more.
• manganese (Mn) forms MnS, a non-metallic inclusion that is elongated with sulfur (S), and reduces toughness and may act as an impact initiation point, and may thus be a factor that rapidly reduces the low-temperature impact toughness of the product. Therefore, it is preferable to manage the manganese (Mn) content to 2.0% or less, and a more desirable manganese (Mn) content may be 1.5% or less.
• Aluminum (Al) is one of the powerful deoxidizers in the steelmaking process along with silicon (Si), and it is preferable to add 0.005% or more to obtain this effect.
• the lower limit of the more desirable aluminum (Al) content may be 0.01%.
• the aluminum (Al) content is excessive, the fraction of Al 2 O 3 among the oxidizing inclusions generated as a result of deoxidation increases excessively, and the size thereof becomes coarse, causing a problem in which it is difficult to remove the inclusions during refining, which may be a factor that reduces the low-temperature impact toughness. Therefore, it is preferable to manage the aluminum (Al) content to 0.1% or less.
• a more desirable aluminum (Al) content may be 0.07% or less.
• Phosphorus (P) and sulfur (S) are elements that cause brittleness at grain boundaries or form coarse inclusions to cause brittleness. Therefore, to improve brittle crack propagation resistance, it is preferable to limit phosphorus (P) to 0.010% or less and sulfur (S) to 0.0015% or less.
• Niobium (Nb) is an element that improves the strength of the base material by precipitating in the form of NbC or NbCN.
• niobium (Nb) dissolved during high-temperature reheating is significantly finely precipitated in the form of NbC during rolling, which inhibits recrystallization of austenite, thus having the effect of refining the structure. Therefore, it is preferable that niobium (Nb) be added in an amount of 0.001% or more, and a more preferable niobium (Nb) content may be 0.005% or more.
• vanadium (V) Since vanadium (V) is almost completely reused during reheating, the strengthening effect by precipitation or solid solution during subsequent rolling is minimal, but in the case of extremely thick forged products, since the air cooling speed is very slow, it has the effect of improving the strength by precipitating as very fine carbonitrides during the cooling process or additional heat treatment process. To sufficiently obtain this effect, it is necessary to add vanadium (V) of 0.001% or more.
• the lower limit of the more desirable vanadium (V) content may be 0.01%.
• the vanadium (V) content may be limited to 0.3% or less.
• the more desirable vanadium (V) content may be 0.25% or less.
• Titanium (Ti) is a component that significantly improves low-temperature toughness by precipitating as TiN during reheating and inhibiting the growth of prior austenite grains at high temperatures. To obtain this effect, it is preferable to add 0.001% or more of titanium (Ti). On the other hand, if titanium (Ti) is added excessively, low-temperature toughness may decrease due to clogging of the casting nozzle may occur or low-temperature toughness may decrease due to central crystallization. In addition, titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates at the thickness center, which reduces the elongation of the product, thereby reducing the uniform elongation during the forging process and causing surface cracks. Therefore, the titanium (Ti) content may be 0.03% or less. The preferred titanium (Ti) content may be 0.025% or less, and the more preferred titanium (Ti) content may be 0.018% or less.
• Chromium (Cr) is a component that increases the yield strength and tensile strength by increasing the hardenability and forming a low-temperature transformation structure. It is also a component that has the effect of preventing the decrease in strength by slowing down the spheroidization rate of cementite. For this effect, 0.01% or more of chromium (Cr) may be added.
• the upper limit of the chromium (Cr) content in the present disclosure may be limited to 0.30%.
• the preferable upper limit of the chromium (Cr) content may be 0.25%.
• Molybdenum (Mo) is an element that increases grain boundary strength and has a large effect of solid solution strengthening in ferrite, and is an element that effectively contributes to increasing the strength and ductility of the product.
• molybdenum (Mo) has the effect of preventing the deterioration of toughness due to grain boundary segregation of impurity elements such as phosphorus (P) or the like. For this effect, 0.10% or more of molybdenum (Mo) may be added.
• Molybdenum (Mo) is an expensive element, and if added excessively, the manufacturing cost may increase significantly, and thus the upper limit of the molybdenum (Mo) content may be limited to 0.12%.
• Copper (Cu) may significantly improve the strength of the matrix phase by solid solution strengthening in ferrite, and also has the effect of suppressing corrosion in a wet hydrogen sulfide atmosphere, and thus is an advantageous element in the present disclosure.
• 0.01% or more of copper (Cu) may be included.
• a more preferable copper (Cu) content may be 0.03% or more.
• the present disclosure may limit the upper limit of the copper (Cu) content to 0.60%.
• the upper limit of the desirable copper (Cu) content may be 0.35%.
• Nickel (Ni) is an element that effectively contributes to improving impact toughness and improving strength by easily providing cross-slip of dislocations by increasing stacking faults at low temperatures and improving hardenability. For this effect, 0.05% or more of nickel (Ni) may be added.
• the desirable nickel (Ni) content may be 0.10% or more.
• the upper limit of the nickel (Ni) content may be limited to 1.00%.
• the upper limit of the desirable nickel (Ni) content may be 0.80%.
• calcium (Ca) When calcium (Ca) is added after deoxidation by aluminum (Al), it combines with sulfur (S) forming MnS inclusions to suppress the formation of MnS, and at the same time, it forms spherical CaS to suppress the occurrence of cracks due to hydrogen-induced cracking.
• sulfur (S) contained as an impurity into CaS it is preferable to add 0.0005% or more of calcium (Ca).
• the amount added is excessive, the calcium (Ca) remaining after forming CaS combines with oxygen (O) to generate coarse oxidized inclusions, which may be elongated and destroyed during rolling, thereby deteriorating the lamellar tearing characteristics. Therefore, the upper limit of the calcium (Ca) content may be limited to 0.0040%.
• [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (weight%) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel, respectively, and if these components are not intentionally added, 0 is substituted.
• An extremely thick steel material for a flange of the present disclosure having excellent strength and low-temperature impact toughness, and a product thereof may contain the remaining Fe and other unavoidable impurities in addition to the aforementioned components.
• unintended impurities may inevitably be mixed in during a normal manufacturing process from raw materials or the surrounding environment, they cannot be completely excluded. Since these impurities are known to anyone with ordinary knowledge in the art, not all of their contents are specifically mentioned in this specification. In addition, the addition of additional effective components other than the aforementioned components is not completely excluded.
• the extremely thick steel material of the present disclosure has a steel microstructure composed of a composite structure of pearlite and ferrite with an average grain size of 30 ⁇ m or less. If the average grain size of the ferrite exceeds 30 ⁇ m, the length of the crack path during impact fracture becomes shorter and the Ductile Brittle Transition Temperature (DBTT) increases, thereby deteriorating the low-temperature impact toughness. Therefore, it is appropriate that the grain size of the ferrite is 30 ⁇ m or less.
• DBTT Ductile Brittle Transition Temperature
• a maximum size of the cementite existing at the grain boundary of the microstructure is 5 ⁇ m or less. If the maximum size of the cementite exceeds 5 ⁇ m, the coarse cementite may act as an impact initiation point, so the impact toughness deteriorates and the grain boundary strength decreases, and accordingly, intergranular fracture easily occurs, thereby deteriorating the impact toughness. Therefore, it is preferable that the maximum size of cementite be 5 ⁇ m or less.
• the fraction of cementite existing in the grain boundary is controlled to 3 area% or less.
• the extremely thick steel material of the present disclosure has a porosity of 0.1 mm 3 /g or less in the central portion of the product, which is a region of 3/8t to 5/8t (where t means the steel thickness (mm)) in the thickness direction from the steel surface.
• the extremely thick steel material of the present disclosure has 5 or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm among the precipitates observed in the cross section of the steel, per 1 ⁇ m 2 . If the number of the fine precipitates is less than 5, the precipitation strengthening effect is weakened, and there may be a problem in securing the properties required in the present disclosure.
• the extremely thick heavy steel material of the present disclosure may have a thickness of 200 to 500 mm.
• the extremely thick steel material of the present disclosure may have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorption energy value of 50 J or more in -50°C Charpy impact test.
• a maximum surface crack depth of the steel material may be 0.1 mm or less (including 0).
• a method of manufacturing an extremely thick steel material of the present disclosure includes, in manufacturing a slab by using molten steel having the composition as described above, an operation of manufacturing a slab by second cooling the cast iron discharged from a mold to a temperature within a range of 800 to 850°C at a cooling rate of 0.01 to 3°C/s; an operation of heating the manufactured slab to a temperature within a range of 1100 to 1300°C, and then performing a first upsetting with a forging ratio of 1.3 to 2.4; an operation of bloom forging with a forging ratio of 1.5 to 2.0 after the first upsetting; an operation of reheating the bloom-forged material to a temperature within a range of 1100 to 1300°C, and then performing round forging with a forging ratio of 1.65 to 2.25, and then performing a second upsetting with a forging ratio of 1.3 to 2.3; an operation of performing a third upsetting of the second-upset material with a forging ratio of 2.0 to 2.8, and then performing hole processing; an operation of
• a slab is manufactured.
• the cast iron discharged from the mold is secondarily cooled at a cooling rate of 0.01 to 3°C/s to a temperature within a range of 800 to 850°C, thereby manufacturing the slab.
• the inventor of the present disclosure has conducted in-depth research on a method of manufacturing an extremely thick steel material having properties suitable for flanges while also having excellent strength, impact toughness, and surface quality, and in particular, to secure the strength, toughness, and surface quality of the final flange product in a slab manufactured with a thickness of 500 mm or more, it was recognized that it is necessary to control the carbon equivalent (Ceq) of the slab within a certain range, and the grain size and microstructure fraction of the prior austenite of the slab surface layer are also effective conditions, which led to the derivation of the present invention.
• Ceq carbon equivalent
• the casting speed of a large-section casting machine that manufactures slabs with a thickness of 650 mm or more is 0.06 to 0.1 m/min
• the casting process is performed at a significantly slower speed than that of a general casting machine (casting speed: 0.4 to 1.5 m/min) that manufactures slabs with a thickness of 250 to 400 mm. Therefore, when manufacturing slabs with a thickness of 500 mm or more, the time maintained in the mold is relatively long, and thus an environment is created where austenite may grow more coarsely.
• the manganese (Mn) segregation index of the austenite grain boundary increases, and the grain boundary strength decreases while the hardenability increases at the same time, so the fraction of hard bainite and martensite, rather than soft ferrite and pearlite, increases in the surface layer of the slab. Since the hard structure has low uniform elongation, intergranular cracking may easily occur when thermal deformation, external deformation, or stress is applied. Therefore, when the prior austenite grain size of the slab surface layer is large, intergranular cracking on the slab surface may occur more actively, and the depth of crack intrusion may further increase during subsequent high-strain processes such as forging, rolling and the like. Therefore, to suppress surface cracking of the final product, it is very important to control the prior austenite grain size to an appropriate level or less and secure the ratio of the soft intergranular polygonal ferrite to an appropriate level or more.
• the prior austenite grain size of the slab surface layer is 1000 ⁇ m or less, and its microstructure is composed of a composite structure of polygonal ferrite of 15 area% or more and the remainder bainite.
• [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (in weight%) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel, respectively, and 0 is substituted if these components are not intentionally added.
• the manufactured slab is heated to a temperature within a range of 1100-1300°C, and then first upsetting is performed at a forging ratio of 1.3-2.4.
• the manufactured slab may be heated in a temperature within a range of 1100-1300°C.
• the thickness of the slab may be 500mm or more, and the preferred thickness may be 700mm or more.
• the slab heating of the present disclosure is performed in a temperature within a range of 1100°C or higher.
• the slab heating of the present disclosure is performed in a range of 1300°C or lower.
• upsetting is a method of performing strong plastic deformation vertically with the longitudinal axis as the axis, and the forging ratio at the time of the first upsetting is appropriately 1.3 to 2.4, and preferably 1.5 to 2.0.
• the forging ratio refers to the ratio of the cross-sectional area changed by forging.
• the forging ratio is less than 1.3 during the first upsetting, it is difficult to sufficiently pressurize the porosity remaining in the center of the slab. Therefore, it is difficult to control the porosity required in the final product of the present disclosure to an appropriate level of 0.1 mm 3 /g or less, so it is not easy to secure the low-temperature impact toughness in the center.
• the forging ratio exceeds 2.4 during the first upsetting, buckling occurs during the forging process, so it is not easy to obtain the surface quality and appropriate shape control required in the flange product. Therefore, the forging ratio is appropriately 1.3 to 2.4 during the first upsetting.
• bloom forging is performed on the first upsetting material with a forging ratio of 1.5 to 2.0.
• Bloom forging is a method of further compressing the first upsetting material into a bloom shape, and is a method of expanding the area while processing both the upper and lower surfaces in a certain direction of width or length.
• the size of the forged surface when forging is completed may be 1450 to 1850 mm ⁇ 2100 to 2500 mm if it is initially 1000 to 1200 mm ⁇ 1800 to 2000 mm.
• the forging ratio is appropriately 1.5 to 2.0. If the forging ratio is less than 1.5, it is difficult to secure the appropriate pore quality required in the present disclosure, like in upsetting forging, and if it exceeds 2.0, surface cracks may occur.
• Forging may be performed in both the longitudinal and transverse directions, but in the longitudinal direction, since the casting structure is more densely structured, the elongation of the surface layer structure is high, which may lead to excellent workability. Therefore, longitudinal bloom forging may be more appropriate than transverse bloom forging in terms of surface cracks.
• the bloom-forged material is reheated to a temperature within a range of 1100 to 1300°C, then round forged at a forging ratio of 1.65 to 2.25, and then second upsetting is performed at a forging ratio of 1.3 to 2.3.
• the bloom surface layer temperature is 950°C or lower, and if processing continues, surface cracks or material fracture may occur. Therefore, the material may be heated again to a temperature within a range of 1100 to 1300°C after bloom forging. As mentioned above, it is preferable to heat to 1100°C or higher for reasons such as re-dissolution of the crystallized material, homogenization of the structure, prevention of surface cracks and the like, and it is preferable to control to 1300°C or lower due to problems such as excessive scale, grain coarsening, and the like.
• round forging is performed to process the flange edge into a circular shape, and then second upsetting is applied again.
• the size of the product may be 1450-1850 ⁇ ⁇ 1300-1700mm.
• the forging ratio for round forging and second upsetting may be 1.65-2.25 and 1.3-2.3, respectively.
• the forging ratio is lower than the level required in the present disclosure during round forging and second upsetting, it is difficult to control the center porosity in the final product to 0.1 mm 3 /g or less, so it is not easy to secure the center low-temperature impact toughness, and if the forging ratio standard is exceeded, the desired processed shape of the flange product may not be obtained due to problems such as buckling and surface cracks, shape defects and the like.
• round forging may be applied again for shape control, and then heating may be performed under the same conditions as the aforementioned reheating temperature.
• the material that has been second upset is upset 3rd with a forging ratio of 2.0 to 2.8, and then a hole is processed.
• the material processed into the cylindrical shape may be processed to an appropriate flange thickness through third upsetting before hole processing (piercing).
• the size of the product may be 2300-2800 ⁇ ⁇ 400-800mm.
• the forging ratio of the third upsetting may be 2.0-2.8, and if the forging ratio is insufficient or exceeded, problems such as residual gap pore control, surface cracks/shape control failure and the like as mentioned above may occur.
• a hole may be made in the center of the material using a 500-10000 punch.
• the material with the hole processed is reheated to a temperature within a range of 1100 to 1300°C, and then ring forged with a forging ratio of 1.0 to 1.6.
• the material with the hole processed is reheated to the temperature within a range of 1100 to 1300°C mentioned above, and may then be processed into a final flange ring shape.
• a maximum thickness of the flange made of the steel may be 200 to 500mm, the inner diameter may be 4000 to 7000mm, and the outer diameter may be 5000 to 8000mm. Since ring forging is a process in which final shape and dimension control are more important than pore compression, strong plastic processing is not applied. Therefore, the forging ratio may be 1.0 to 1.6, and more preferably, may be 1.2 to 1.4.
• the strain rate in all the forging processes presented in the present disclosure may be 1/s to 4/s. At a strain rate of less than 1/s, the temperature of the finishing forging may decrease, which may cause possibility of surface cracks. On the other hand, when a high strain rate exceeding 4/s is applied in the non-recrystallized region, surface cracks may be induced due to a decrease in elongation caused by excessive local work hardening.
• a normalizing heat treatment may be performed by heating the flange product, which has completed the forging, to a temperature within a range of 820 to 930°C based on the temperature measurement standard of the central portion of the product, maintaining the temperature for 5 to 600 minutes, and then air cooling to room temperature.
• the heating temperature is lower than 820°C or the maintaining time is lower than 5 minutes, the carbides generated during cooling after forging or the impurity elements segregated at the grain boundaries do not re-dissolve smoothly, so that the low-temperature toughness of the steel after the heat treatment may be significantly reduced.
• the heating temperature exceeds 930°C or the holding time exceeds 600 minutes during the normalizing heat treatment, the ferrite matrix phase grain size of the ferrite-pearlite composite structure may exceed 30 ⁇ m required in the present disclosure or the strength and low-temperature impact toughness may deteriorate due to the coarsening of precipitated phases such as Nb (C,N), V(C,N) and the like.
• the normalizing heat treatment and holding time may be expressed by the Larson-Miller Parameter Equation 2 (Literature: F.R. Larson and J. Miller: Trans. ASME, 1952, vol. 74, pp. 765-75 ) as follows, and the LMP for the normalizing temperature and time conditions may be 20 to 23 to satisfy the impact toughness required in the present disclosure by refining the size of pearlite colonies.
• LMP T Logt + 20 ⁇ 1 / 1000
• T is the normalizing heat treatment temperature in Kelvin
• t is the heat treatment time
• log exponent 10
• the LMP is less than 20, there is a disadvantage that the material may not be sufficiently heated to the austenite single-phase region or the diffusion of the solute does not occur uniformly, resulting in material deviation, and if the LMP exceeds 23, the ferrite and pearlite colonies are formed too coarsely, and thus it is difficult to secure the low-temperature impact toughness required by the present disclosure.
• post-weld heat treatment or stress-relieving heat treatment or tempering heat treatment may be performed.
• This post-weld heat treatment may be performed in a range where the value defined by the relational expression 2 is LMP 19.3 or less. If LMP exceeds 19.3, the grain boundary cementite size increases and exceeds 5 ⁇ m required in the present disclosure, and thus the impact toughness may deteriorate. Therefore, when welding is performed, it is preferable that the LMP of the subsequent heat treatment be 19.3 or less.
• a 700mm thick cast steel having the alloy composition of Table 1 above was manufactured.
• a slab was prepared by cooling according to the process conditions of Table 2 below, and then a final 320mmt flange was manufactured through a forging process (reheating and 1st upsetting, bloom forging, reheating-2nd upsetting, 3rd upsetting, reheating and ring forging) and normalizing heat treatment.
• Process conditions satisfying the range of the present disclosure were applied to all processes other than the processes described in Table 2.
• the ferrite grain size of the steel was also measured using an image auto-analyzer by collecting a specimen from the final steel structure.
• the product microstructure was a mixed structure of ferrite and pearlite.
• the yield/tensile strength was evaluated through a room temperature tensile test, and a 0.2% offset was applied for the yield strength.
• the impact toughness for each specimen used the average of the absorbed energy values measured three times at each temperature by the Charpy V-Notch Test was used.
• NbC precipitates in the cross section of the steel was measured using TEM.
• the NbC precipitates were confirmed through the diffraction pattern of NbC and EDX mapping, and the number of NbC precipitates located at 1 ⁇ m 2 was counted.
• the porosity at the central portion of the product was measured by measuring the density (g/mm 3 ) and taking the reciprocal (mm 3 /g).
• the crack is not limited to the surface layer portion, but penetrates deep into the interior, and the total length of the crack introduced was measured by cutting the cross-section.
• Comparative Examples 1-15 and 21-22 satisfy the alloy composition proposed by the present disclosure but do not satisfy the manufacturing conditions, and thus it can be seen that the strength and low-temperature impact toughness values are low because they do not satisfy the characteristics of the slab's prior austenite grain size, polygonal ferrite fraction, or center porosity, and ferrite grain size and the like in the flange product state proposed by the present disclosure.
• the forging ratio conditions are not satisfied at each stage of forging, poor surface quality characteristics may also be confirmed in the product state due to the occurrence of surface cracks or penetrating cracks.
• Comparative Examples 16-20 satisfy the manufacturing conditions proposed by the present disclosure, but do not satisfy the alloy composition, so it can be seen that the quality level is low, such as exceeding the strength (not meeting the impact toughness) or not meeting the strength.

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