Ultra-thick steel material for flange with excellent strength and low-temperature impact toughness and method for manufacturing the same

Abstract

Provided are an extra-thick steel material for a flange and a method for producing the same, which are excellent in strength and low-temperature impact toughness. The steel material of the present invention contains, by weight%, 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%, : 0.0005 to 0.004%, the balance being Fe and other inevitable impurities, and satisfies the range of Ceq of 0.35 to 0.55 according to the following relational expression 1, has a thickness of 200 to 500 mm, and has a steel material microstructure composed of a composite structure of ferrite and pearlite with an average grain size of 30 μm or less. The maximum size of the cementite present at the ferrite-ferrite and / or ferrite-pearlite grain boundaries is 5 μm or less, and the porosity at the center of the product in the region of 3 / 8t to 5 / 8t in the thickness direction from the steel material surface (where t means the steel material thickness (mm)) is 0.1 mm 3 / g or less, and among the precipitates observed in the cross-section of the steel material, there are 5 or more fine NbC or NbCN precipitates having a diameter of 5 to 15 nm per 1 μm 2 or more.

Description

Technical Field

[0001] The present invention relates to steel materials that can be used in wind power generation towers and systems, etc., and more particularly to an extra-thick steel material for flanges that is excellent in strength and low-temperature impact toughness, and a manufacturing method thereof.

Background Art

[0002] Wind turbines have attracted attention as an environmentally friendly means of electricity production and include components such as tower flanges, bearings, and main shafts. Among these, tower flanges are joint components necessary for connecting towers, and usually 5 to 7 flanges are used for one tower. Since they are installed even in the sea or extremely cold regions, high durability is required. In particular, in response to the requirements for large-capacity energy production and high efficiency, the scale of wind towers is also increasing, and accordingly, the steel materials used are continuously required to have higher strength, higher toughness, and greater thickness. As the thickness of the material increases, the total deformation amount decreases, so the microstructure becomes larger, and the material tends to deteriorate due to material defects such as inclusions and segregation. Therefore, in order to improve the soundness of the inside and outside of the steel material, there is a tendency to reduce the concentration of impurities such as non-metallic inclusions and segregation, and to limit cracks, voids, etc. on the surface and inside of the material to the extreme.

[0003] In particular, in the case of an extra-thick material with a thickness exceeding 200 mm, since the deformation amount at the center of the material is not large, if the unfrozen shrinkage holes generated during continuous casting or casting are not sufficiently crimped during the forging process, they remain in the form of residual voids at the center of the flange.

[0004] Such residual voids act as crack initiation points when the structure receives stress in the thickness axis direction, and may eventually cause damage to the entire equipment in the form of lamellar tearing. Therefore, before piercing (drilling forging) and ring forging (product forming) with a small deformation amount, it is necessary to have a process of sufficiently crimping the central void so that there are no residual voids.

[0005] Patent Document 1 related to this is a technique for applying rolling pressure reduction in the heavy plate rough rolling process. Specifically, it is a technique for determining the thickness-specific limit rolling reduction rate at which sheet biting occurs by thickness from the rolling reduction rate by pass set to be close to the design allowable values (load and torque) of the rolling mill, a technique for distributing the rolling reduction rate by adjusting the exponent of the thickness ratio by pass to ensure the target thickness of the rough rolling mill, and a technique for adjusting the rolling reduction rate so that sheet biting does not occur based on the thickness-specific limit rolling reduction rate. It provides a manufacturing method that can approve the average rolling reduction rate in the final three passes of the 80 mmt standard rough rolling at about 27.5%. However, in the case of the above rolling method, it measures the average rolling reduction rate of the entire product thickness, and in the case of extremely thick materials with a maximum thickness of 200 mmt or more, there are technical difficulties in applying high deformation to the central part where residual voids exist.

[0006] One of the other methods for manufacturing extremely thick materials is a method that utilizes a forging machine with a higher effective deformation amount per pass than a rolling mill. Patent Document 2 contains, in mass%, C: 0.08 - 0.20%, Si: 0.40% or less, Mn: 0.5 - 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 3.0% or less, Ni: 0.1 - 5.0%, Al: 0.010 - 0.080%, N: 0.0070% or less, O: 0.0025% or less, and performs hot forging with a slab composed of the balance of Fe and inevitable impurities having a cumulative rolling reduction of 25% or more, performs hot rolling by heating to 1200°C or less above the Ac3 point and having a cumulative rolling reduction of 40% or more, quenches rapidly to a temperature of 350°C or less at a temperature above the Ar3 point or to a low temperature below the Ar3 point, and performs temper heat treatment at a temperature of 450 - 700°C to produce a thick high-toughness high-strength material with a sheet thickness of 100 mmt or more, a yield strength of 620 MPa or more, and an absorbed energy of 70 J or more when evaluating the low-temperature impact toughness at -40°C.

[0007] However, in the above manufacturing method, when the cumulative reduction amount is too high, surface defects may occur due to local deformation concentration. In particular, when there are surface or subsurface defects in the as-cast state before forging, the defects may propagate during the forging process, resulting in further deterioration of the surface quality in the state of the rolled product. Also, when the reduction amount per pass in forging is insufficient, even if the cumulative reduction amount is high, it is difficult to sufficiently crimp the voids remaining in the center, and in the rolling process, the effective deformation amount of the center relative to the surface deformation is small, so it is not suitable for controlling the voids and microstructure in the center of extremely thick materials.

[0008] On the other hand, in Patent Document 3, it is disclosed that a material provided with a predetermined alloy composition is heated to 1200 - 1350°C, hot forging is performed with a cumulative reduction amount of 25% or more, heated to 1200°C or lower but above the Ac3 point, hot rolling is performed with a cumulative reduction amount of 40% or more, reheated to 1050°C or lower but above the Ac3 point, quenched rapidly to a temperature of 350°C or lower or the lower of the Ar3 point or lower at a temperature above the Ac3 point, and tempered at a temperature of 450 - 700°C, through which a thick high-strength steel plate with a yield strength of 620 MPa or more and a thickness of 100 mmt or more can be manufactured.

[0009] However, in the case of the above-mentioned ultra-high-strength steel plate, not only is it vulnerable to surface cracks during casting due to a high carbon equivalent (Ceq) and hardenability index (DI), but also in the case of steel materials for flanges manufactured by normalizing heat treatment, the above process conditions cannot be easily applied. Also, when the carbon equivalent (Ceq) and hardenability index (DI) are high, cracks are likely to occur on the surface layer of the slab due to the formation of a surface hard structure during the secondary cooling process of steelmaking, and the cracks may propagate during the forging process, deteriorating the surface quality of the final product.

[0010] Therefore, a plan has been proposed to perform forging to crimp the voids in the center and improve the internal soundness of the final product, but no substantial plan has been presented to ensure both the appropriate material quality and excellent surface quality of the steel materials for flanges.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0012] Therefore, an object of the present invention is to provide an extra-thick steel sheet for flanges having excellent strength and low-temperature impact toughness, and a method for manufacturing the same.

[0013] The problems of the present invention are not limited to the above-described content. An ordinary person skilled in the art will have no difficulty in understanding further problems of the present invention from the entire content of this specification.

Means for Solving the Problems

[0014] One aspect of the present invention is by weight, 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 balance being Fe and other unavoidable impurities, and Ceq satisfies the range of 0.35 to 0.55 according to the following relational expression 1, 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 μm or less, The maximum size of cementite present at ferrite-ferrite and / or ferrite-pearlite grain boundaries is 5 μm or less, In the central part of the product, which is a region of 3 / 8t to 5 / 8t in the thickness direction from the steel surface (where t means the thickness of the steel in mm), the porosity is 0.1 mm 3 / g or less, and Among the precipitates observed in the cross-section of the steel, there are 5 or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per 1 μm 2 This relates to a thick steel sheet for flanges. [Relational Expression 1] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the above Relational Expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively mean the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel. When these components are not intentionally added, 0 is substituted.

[0015] Also, the above steel can have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorbed energy value in the Charpy impact test at -50 °C of 50 J or more.

[0016] The depth of the maximum surface crack of the above steel can be 0.1 mm or less (including 0).

[0017] Also, other aspects of the present invention are In terms of weight percentage, C: 0.05 - 0.2%, Si: 0.05 - 0.5%, Mn: 1.0 - 2.0%, Al: 0.005 - 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 - 0.07%, V: 0.001 - 0.3%, Ti: 0.001 - 0.03%, Cr: 0.01 - 0.3%, Mo: 0.01 - 0.12%, Cu: 0.01 - 0.6%, Ni: 0.05 - 1.0%, Ca: 0.0005 - 0.004%, including the remaining Fe and other inevitable impurities, and satisfying the range of 0.35 - 0.55 for Ceq according to the following relational expression 1, and manufacturing a slab having a thickness of 500 mm or more; After heating the manufactured slab in the temperature range of 1100 - 1300 °C, performing a first upsetting with a forging ratio of 1.3 - 2.4; After the first upsetting, performing bloom forging with a forging ratio of 1.5 - 2.0; Reheating the bloom-forged material in the temperature range of 1100 - 1300 °C, then performing round forging with a forging ratio of 1.65 - 2.25, and then performing a second upsetting with a forging ratio of 1.3 - 2.3; Performing a third upsetting with a forging ratio of 2.0 - 2.8 on the material after the second upsetting, and then performing hole machining; Reheating the hole-machined material in the temperature range of 1100 - 1300 °C, and then performing ring forging with a forging ratio of 1.0 - 1.6; and Heating the ring-forged material in the temperature range of 820 - 930 °C based on the central part temperature measurement standard, maintaining it for 5 - 600 minutes, and then performing a normalizing heat treatment of air cooling to room temperature; The present invention relates to a method for manufacturing a thick steel material for a flange. [Relational Expression 1] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the above relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively mean the contents (weight percentage) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and when these components are not intentionally added, 0 is substituted.

[0018] The above slab can be manufactured using one of the processes of continuous casting, semi - continuous casting, and Ingot casting.

[0019] After manufacturing the above slab and before forging, the prior austenite grain size of the slab surface layer is 1000 μm or less, and the microstructure of the slab surface layer before forging is preferably composed of a composite structure of 15% or more polygonal ferrite and the balance bainite.

[0020] When the size of the forging surface punched at the first setting is initially 700 mm×1800 mm, it can be 1000 - 1200 mm×1800 - 2000 mm.

[0021] In the case of the above bloom forging, when the size of the forging surface at the end of forging is initially 1000 - 1200 mm×1800 - 2000 mm, it can be 1450 - 1850 mm×2100 - 2500 mm.

[0022] When finishing the above round forging and the second setting, the size of the product can be 1450 - 1850Φ×1300 - 1700 mm.

[0023] When finishing the above third setting, the size of the product can be 2300 - 2800Φ×400 - 800 mm.

[0024] The maximum thickness of the flange made of the above steel material can be 200 - 500 mm, the inner diameter can be 4000 - 7000 mm, and the outer diameter can be 5000 - 8000 mm.

[0025] During the above normalizing heat treatment, it is preferable to perform the heat treatment so that the LMP defined by the following relational expression 2 satisfies 20 - 33. [Relational expression 2] LMP = T(Logt + 20)×(1 / 1000) In the above relational expression 2, T is the temperature on the Kelvin scale, t is the time, and the exponent of log is 10.

[0026] After the above normalizing heat treatment, when welding is performed on the steel material, it may further include a step of performing post-weld heat treatment, stress relieving heat treatment, or tempering heat treatment.

[0027] The above post-weld heat treatment is preferably performed in a range where the value defined by the following relational expression 2 is LMP 19.3 or less. [Relational Expression 2] LMP = T(Logt + 20)×(1 / 1000) In the above relational expression 2, T is the reference temperature in Kelvin, t is the time, and the exponent of log is 10.

[0028] Also, the present invention In weight %, 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 balance being Fe and other inevitable impurities, when producing a slab using molten steel satisfying the range of Ceq of 0.35 to 0.55 according to the following relational expression 1, the step of producing a slab by performing secondary cooling on the slab discharged from the mold at a cooling rate of 0.01 to 3°C / s up to a temperature range of 800 to 850°C; The step of heating the above-produced slab to a temperature range of 1100 to 1300°C and then performing primary upsetting at a forging ratio of 1.3 to 2.4; After the above primary upsetting, the step of bloom forging at a forging ratio of 1.5 to 2.0; The step of reheating the above bloom-forged material to a temperature range of 1100 to 1300°C, subsequently performing round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3; The step of performing tertiary upsetting on the material on which the above secondary upsetting has been performed at a forging ratio of 2.0 to 2.8 and then performing hole machining; After reheating the drilled material in the temperature range of 1100 to 1300 °C, performing ring forging with a forging ratio of 1.0 to 1.6; and After heating the ring-forged material in the temperature range of 820 to 930 °C based on the central part temperature measurement and maintaining it for 5 to 600 minutes, performing a normalizing heat treatment of air-cooling to room temperature; A method for manufacturing an extra-thick steel material for a flange, comprising the steps of: [Equation 1] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the above Equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively represent the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and 0 is substituted when these components are not intentionally added.

[0029] During the above normalizing heat treatment, it is preferable to perform heat treatment so that the LMP defined by the following Equation 2 satisfies 20 to 33. [Equation 2] LMP = T(Logt + 20)×(1 / 1000) In the above Equation 2, T is the temperature in Kelvin, t is the time, and the exponent of log is 10.

Advantages of the Invention

[0030] The present invention having the above-described configuration can improve the internal soundness of the final product by crimping the voids in the center of the steel material by optimizing the forging process, and can effectively provide an extra-thick steel material that can be used for a flange and is excellent not only in strength but also in low-temperature impact toughness.

Embodiments for Carrying Out the Invention

[0031] The present invention relates to an extra-thick steel material for flanges that is excellent in strength and low-temperature impact toughness, and a product manufacturing method. Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be deformed into various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment further provides the present invention in more detail to those with ordinary knowledge in the technical field to which the invention belongs.

[0032] Hereinafter, the extra-thick steel material for flanges of the present invention will be described in more detail.

[0033] The extra-thick steel material for flanges of the present invention contains, by weight%, 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 balance being Fe and other inevitable impurities, and the Ceq according to relational expression 1 satisfies the range of 0.35 to 0.55, has a thickness of 200 to 500 mm, and has a steel microstructure composed of a composite structure of ferrite and pearlite with an average grain size of 30 μm or less. The maximum size of cementite present at the ferrite-ferrite and / or ferrite-pearlite grain boundaries is 5 μm or less, and the porosity in the central part of the product, which is a region from the steel surface in the thickness direction of 3 / 8t to 5 / 8t (where t means the steel thickness (mm)), is 0.1 mm 3 / g or less, and among the precipitates observed in the cross-section of the steel material, there are 5 or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per 1 μm 2 or more.

[0034] Hereinafter, the alloy composition of the present invention will be described in more detail. Unless otherwise specified, the % and ppm described regarding the alloy composition are based on weight.

[0035] · Carbon (C): 0.05 to 0.20% Carbon (C) is the most important element for ensuring basic strength, so it needs to be contained in steel within an appropriate range. In order to obtain such an additive effect, carbon (C) of 0.05% or more can be added. Preferably, carbon (C) of 0.10% or more can be added. On the other hand, when the carbon (C) content exceeds a certain level, the fraction of pearlite increases during annealing heat treatment, and the base metal strength and hardness may excessively exceed, which may cause surface cracks during forging and reduce the low-temperature impact toughness and lamellar tearing resistance properties in the final product. Therefore, the present invention can limit the carbon (C) content to 0.20%, and the upper limit of the more preferable carbon (C) content can be 0.18%.

[0036] · Silicon (Si): 0.05 - 0.50% Silicon (Si) is an essential element for the production of clean steel because it improves the strength of steel by solid solution strengthening as a substitutional element and has a strong deoxidizing effect. Therefore, silicon (Si) can be added in an amount of 0.05% or more, and more preferably 0.20% or more. On the other hand, when a large amount of silicon (Si) is added, it may generate the MA (Martensite - Austenite) phase and excessively increase the ferrite matrix strength, resulting in deterioration of the surface quality of extremely thick products. Therefore, the upper limit of its content can be limited to 0.50%. The upper limit of the more preferable silicon (Si) content can be 0.40%.

[0037] · Manganese (Mn): 1.0 - 2.0% Manganese (Mn) is a useful element that improves strength through solid solution strengthening and enhances hardenability to generate a low-temperature transformation phase. Therefore, to ensure 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 can be 1.1% or more. On the other hand, manganese (Mn) forms MnS, a non-metallic inclusion that extends with sulfur (S), reducing toughness and acting as a shock initiation point, which can be a factor in rapidly reducing the low-temperature impact toughness of the product. Therefore, it is preferable to control the manganese (Mn) content to 2.0% or less, and a more preferable manganese (Mn) content can be 1.5% or less.

[0038] · Aluminum (Al): 0.005 - 0.1% Aluminum (Al) is one of the powerful deoxidizers in the steelmaking process together with silicon (Si), and it is preferable to add 0.005% or more to obtain such an effect. The lower limit of the more preferable aluminum (Al) content can be 0.01%. On the other hand, when the aluminum (Al) content is excessive, the fraction of Al2O3 in the oxidizing inclusions generated as a result of deoxidation increases excessively and their size becomes coarse, making it difficult to remove the inclusions during refining, which can be a factor in reducing low-temperature impact toughness. Therefore, it is preferable to control the aluminum (Al) content to 0.1% or less. A more preferable aluminum (Al) content can be 0.07% or less.

[0039] · Phosphorus (P): 0.010% or less (including 0%), Sulfur (S): 0.0015% or less (including 0) Phosphorus (P) and sulfur (S) are elements that induce brittleness at grain boundaries or form coarse inclusions to induce brittleness. Therefore, to improve the resistance to brittle crack propagation, it is preferable to limit phosphorus (P) to 0.010% or less and sulfur (S) to 0.0015% or less.

[0040] · Niobium (Nb): 0.001 - 0.07% Niobium (Nb) is an element that precipitates in the form of NbC or NbCN to improve the strength of the base material. Also, niobium (Nb) dissolved during high-temperature reheating precipitates very finely in the form of NbC during rolling, suppressing the recrystallization of austenite, and thus has the effect of refining the microstructure. Therefore, it is preferable to add niobium (Nb) in an amount of 0.001% or more, and a more preferable niobium (Nb) content can be 0.005% or more. On the other hand, when niobium (Nb) is added excessively, undissolved niobium (Nb) is generated in the form of TiNb(C,N), which becomes a factor inhibiting low-temperature impact toughness. Therefore, it is preferable to limit the upper limit of the niobium (Nb) content to 0.07%. A more preferable niobium (Nb) content can be 0.065% or less.

[0041] · Vanadium (V): 0.001 - 0.3% Since almost all vanadium (V) redissolves during reheating, the strengthening effect due to precipitation and solid solution during subsequent rolling is slight. However, in the case of extremely thick forged materials, since the air-cooling rate is very slow, it precipitates as very fine carbonitrides during the cooling process or additional heat treatment process, improving the strength. To sufficiently obtain such an effect, it is necessary to add 0.001% or more of vanadium (V). The lower limit of the more preferable vanadium (V) content can be 0.01%. On the other hand, when its content is excessive, not only may it act as a factor such as excessive increase in slab surface hardness due to high hardenability and cause surface cracks during flange processing, but also the manufacturing cost may increase rapidly, making it not commercially meaningful. Therefore, the vanadium (V) content can be limited to 0.3% or less. A more preferable vanadium (V) content can be 0.25% or less.

[0042] · Titanium (Ti): 0.001 - 0.03% Titanium (Ti) is a component that precipitates as TiN during reheating, suppresses the growth of prior austenite grain at high temperature, and greatly improves low-temperature toughness. To obtain such an effect, it is preferable to add 0.001% or more of titanium (Ti). On the other hand, when titanium (Ti) is added excessively, clogging of the continuous casting nozzle or a decrease in low-temperature toughness due to crystallization at the center may occur. In addition, since titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates in the center of the thickness, reducing the elongation rate of the product, it may reduce the uniform elongation rate during the forging process and cause surface cracks. Therefore, the titanium (Ti) content can be 0.03% or less. The preferable titanium (Ti) content can be 0.025% or less, and the more preferable titanium (Ti) content can be 0.018% or less.

[0043] · Chromium (Cr): 0.01 - 0.30% Chromium (Cr) is a component that increases hardenability and forms a low-temperature transformation structure, thereby increasing the yield strength and tensile strength. It is also a component that has the effect of preventing a decrease in strength by slowing down the spheroidization rate of cementite. Due to such an effect, 0.01% or more of chromium (Cr) can be added. On the other hand, when the chromium (Cr) content is excessive, the size and fraction of Cr-Rich coarse carbides such as M 23 C6 increase, the impact toughness of the product decreases, and the solid solubility of niobium (Nb) and the fraction of fine precipitates such as NbC in the product decrease. Therefore, a decrease in the strength of the product may become a problem. Therefore, the present invention can limit the upper limit of the chromium (Cr) content to 0.30%. The preferable upper limit of the chromium (Cr) content can be 0.25%.

[0044] · Molybdenum (Mo): 0.01 - 0.12% Molybdenum (Mo) is an element that increases the grain boundary strength and has a large solid solution strengthening effect in ferrite, and effectively contributes to the increase in the strength and ductility of the product. In addition, molybdenum (Mo) has the effect of preventing the decrease in toughness due to the grain boundary segregation of impurity elements such as phosphorus (P). Due to such effects, molybdenum (Mo) of 0.10% or more can be added. However, when molybdenum (Mo) is added excessively as an expensive element, the manufacturing cost increases significantly. Therefore, the upper limit of the molybdenum (Mo) content can be limited to 0.12%.

[0045] · Copper (Cu): 0.01 - 0.60% Copper (Cu) can not only greatly improve the strength of the matrix phase by solid solution strengthening in ferrite, but also has the effect of suppressing corrosion in a wet hydrogen sulfide atmosphere, and is an advantageous element in the present invention. Due to such effects, copper (Cu) of 0.01% or more can be included. A more preferable copper (Cu) content can be 0.03% or more. However, when the content of copper (Cu) is excessive, there is a high possibility of inducing star cracks on the surface of the steel plate, and there is a problem that the manufacturing cost increases significantly as copper (Cu) is an expensive element. Therefore, the present invention can limit the upper limit of the copper (Cu) content to 0.60%. A preferable upper limit of the copper (Cu) content can be 0.35%.

[0046] · Nickel (Ni): 0.05 - 1.00% Nickel (Ni) is an element that effectively contributes to increasing the stacking defects at low temperatures, facilitating the cross slip of the potential to improve the impact toughness, and improving the hardening ability to improve the strength. Due to such effects, nickel (Ni) of 0.05% or more can be added. A preferable nickel (Ni) content can be 0.10% or more. On the other hand, when nickel (Ni) is added excessively, the manufacturing cost may increase due to the high cost. Therefore, the upper limit of the nickel (Ni) content can be limited to 1.00%. A preferable upper limit of the nickel (Ni) content can be 0.80%.

[0047] · Calcium (Ca): 0.0005 to 0.0040% When calcium (Ca) is added after deoxidation with aluminum (Al), it binds to sulfur (S) that forms MnS inclusions, suppressing the formation of MnS. At the same time, it forms spherical CaS, which has the effect of suppressing the occurrence of cracks due to hydrogen-induced cracking. To sufficiently form sulfur (S) contained as an impurity into CaS, it is preferable to add 0.0005% or more of calcium (Ca). However, if the addition amount becomes excessive, CaS is formed, and the remaining calcium (Ca) binds to oxygen (O) to form coarse oxidizing inclusions, which can be a factor in reducing the lamellar tearing resistance characteristics by being stretched and broken during rolling. Therefore, the upper limit of the calcium (Ca) content can be limited to 0.0040%.

[0048] · Relational expression 1 In the present invention, it is required that Ceq according to the following relational expression 1 satisfies the range of 0.35 to 0.55. When Ceq according to the following relational expression 1 is less than 0.35, the pearlite fraction decreases, so the tensile strength value of 510 to 690 MPa required in the present invention cannot be ensured. When it exceeds 0.55, the pearlite fraction exceeds 30%, so it is not easy to ensure the -50 °C low-temperature impact energy value. Therefore, in the present invention, it is preferable to limit Ceq to the range of 0.35 to 0.55. [Relational expression 1] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the above relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively represent the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and 0 is substituted when these components are not intentionally added.

[0049] The extremely thick steel material for flanges and its products, which are excellent in strength and low-temperature impact toughness of the present invention, may contain the remaining Fe and other inevitable impurities in addition to the components described above. However, in the normal manufacturing process, unintended impurities may inevitably mix in from raw materials or the surrounding environment, so it is impossible to completely exclude them. Since these impurities are understandable to anyone with ordinary knowledge in this technical field, all of their contents are not particularly mentioned in this specification. Furthermore, the additional addition of effective components is not completely excluded in addition to the components described above.

[0050] On the other hand, the extremely thick steel material of the present invention has a steel microstructure composed of a composite structure of ferrite and pearlite with an average grain size of 30 μm or less. If the average ferrite grain size exceeds 30 μm, the length of the Crack Path will be shortened during impact fracture, the DBTT (Ductile Brittle Transition Temperature) will increase, and the low-temperature impact toughness will deteriorate. Therefore, it is appropriate that the crystal grain size of ferrite is 30 μm or less.

[0051] Also, it is preferable that the maximum size of cementite existing at the grain boundaries of the above microstructure (the grain boundaries between ferrite-ferrite and / or ferrite-pearlite) is 5 μm or less. If the maximum size of the above cementite exceeds 5 μm, the coarse cementite may act as an impact initiation point, resulting in deterioration of impact toughness, lower grain boundary strength, and thus easier occurrence of Intergranular fracture and a decrease in impact toughness. Therefore, it is preferable that the maximum size of cementite is 5 μm or less.

[0052] More preferably, it is to control the fraction of cementite existing at the above grain boundaries to 3 area% or less.

[0053] And the extremely thick steel material of the present invention has a porosity of 0.1 mm 3 / g or less in the central part of the product, which is a region of 3 / 8t to 5 / 8t in the thickness direction from the steel surface (where t means the thickness of the steel material (mm)).

[0054] In addition, in the thick steel material of the present invention, among the precipitates observed in the cross-section of the steel material, the fine NbC or NbCN precipitates with a diameter of 5 to 15 nm are 1 μm 2 There are preferably 5 or more per hit. If the number of the above fine precipitates is less than 5, the precipitation strengthening effect becomes weak, and there may be a problem in ensuring the physical properties required in the present invention.

[0055] In addition, the thick steel material of the present invention can have a thickness of 200 to 500 mm.

[0056] In addition, the thick steel material of the present invention can have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorbed energy value of the Charpy impact test at -50°C of 50 J or more.

[0057] And the depth of the maximum surface crack of the above steel material can be 0.1 mm or less (including 0).

[0058] Next, the manufacturing method of the thick steel material for flanges, which is another aspect of the present invention, will be described in detail.

[0059] The method for manufacturing a thick steel material of the present invention, when manufacturing a slab using molten steel having the composition components as described above, includes the step of manufacturing a slab by performing secondary cooling on the slab discharged from the mold at a cooling rate of 0.01 to 3 °C / s until the temperature range of 800 to 850 °C; the step of performing primary upsetting at a forging ratio of 1.3 to 2.4 after heating the manufactured slab to the temperature range of 1100 to 1300 °C; the step of bloom forging at a forging ratio of 1.5 to 2.0 after the primary upsetting; the step of reheating the bloom-forged material to the temperature range of 1100 to 1300 °C, subsequently performing round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3; the step of performing tertiary upsetting on the material after the secondary upsetting at a forging ratio of 2.0 to 2.8 and then performing hole machining; the step of reheating the hole-machined material to the temperature range of 1100 to 1300 °C and then performing ring forging at a forging ratio of 1.0 to 1.6; and the step of performing a normalizing heat treatment by heating the ring-forged material to the temperature range of 820 to 930 °C, which is the measurement standard of its central temperature, maintaining it for 5 to 600 minutes, and then air-cooling it to room temperature.

[0060] "Slab Preparation" First, a slab is manufactured in the present invention. Preferably, when manufacturing a slab using molten steel having the composition components as described above, the slab can be manufactured by performing secondary cooling on the slab discharged from the mold at a cooling rate of 0.01 to 3 °C / s until the temperature range of 800 to 850 °C.

[0061] The inventor of the present invention has conducted in-depth research on a solution for manufacturing a thick steel material having excellent strength, impact toughness, and surface quality while having physical properties suitable for flanges. In particular, in a slab manufactured with a thickness of 500 mm or more, in order to ensure the strength, toughness, and surface quality of the final flange product, it is not only necessary to control the carbon equivalent (Ceq) of the slab within a certain range, but also to recognize that the prior austenite crystal grain size and fine tissue fraction on the slab surface layer are effective conditions, and thus the present invention has been derived.

[0062] Since the casting speed of a large cross-section casting machine for producing slabs with a thickness of 650 mm or more is 0.06 - 0.1 m / min, the casting operation is carried out at a significantly slower speed compared to a general casting machine (casting speed: 0.4 - 1.5 m / min) for producing slabs with a thickness of 250 - 400 mm. Therefore, when producing slabs with a thickness of 500 mm or more, the time maintained in the mold is relatively long, so the austenite is placed in an environment where it can grow coarser.

[0063] As the initial austenite crystal grain size increases, the manganese (Mn) segregation index at the austenite grain boundaries increases, the grain boundary strength decreases, and at the same time, the hardenability increases. As a result, the fraction of hard bainite and martensite increases in the surface layer of the slab instead of soft ferrite and pearlite. Since the hard tissue has a low uniform elongation rate, intergranular cracking may easily occur when thermal deformation, external deformation, or stress is applied. Therefore, when the prior austenite crystal grain size on the slab surface is large, intergranular cracks on the slab surface are more likely to occur, and the depth of crack penetration may further increase during subsequent high-deformation processes such as forging and rolling. Therefore, in order to suppress surface cracks in the final product, it is very important to control the prior austenite crystal grain size below an appropriate level and ensure that the ratio of soft-phase grain boundary polygonal ferrite is above an appropriate level.

[0064] That is, in the present invention, it is preferable that the prior austenite crystal grain size on the slab surface is 1000 μm or less, and its microstructure is composed of a composite structure of 15 area% or more of polygonal ferrite and the balance bainite.

[0065] In order to reduce the original austenite crystal grain size and ensure a polygonal ferrite fraction of 15% or more by area, there is a solution of designing high contents of components of carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) that have a solute dragging effect or a pinning effect. However, when the contents of these carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) components increase, the carbon equivalent (Ceq) also increases, and a low-temperature transformation structure may be generated during the cooling process of the slab. Therefore, the present invention can limit the carbon equivalent (Ceq) of the steel slab to 0.55 or less according to the following relational expression 1. The preferable carbon equivalent (Ceq) can be 0.4 - 0.53. [Relational Expression 1] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the above relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively represent the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and 0 is substituted when these components are not intentionally added.

[0066] On the other hand, when manufacturing a slab from molten steel using a continuous casting process or a semi - continuous casting process, water cooling is performed until the temperature of the slab surface layer reaches 800 - 850°C at a cooling rate of 0.01 - 3°C / s from immediately after it exits the mold at a casting speed of 0.06 - 0.1 m / min, and then air cooling is carried out during secondary cooling. When the surface cooling rate is excessively slow, the growth of austenite crystal grains continues to progress, so the AGS of the surface layer cannot be finely controlled. Also, when it exceeds 3°C / s, there is a possibility of surface cracks occurring during the cooling process due to the generation of a surface hard phase and an increase in the temperature gradient between the surface and the interior. If the target temperature is also less than 800°C, a local low - temperature transformation structure may be formed. If it exceeds 850°C, it is difficult to control the AGS to 1000 μm or less required by the present invention, so appropriate surface quality cannot be ensured.

[0067] "Heating and Primary Positioning" Next, in the present invention, after heating the manufactured slab in the temperature range of 1100 to 1300°C, the first upsetting is performed with a forging ratio of 1.3 to 2.4.

[0068] The manufactured slab can be heated in the temperature range of 1100 to 1300°C. As described above, the thickness of the slab can be 500 mm or more, and the preferred thickness can be 700 mm or more.

[0069] In order to redissolve the complex carbonitrides of titanium (Ti) and niobium (Nb) or the coarse crystals of TiNb(C,N) formed during casting, it is necessary to heat the slab at a certain temperature range or higher. Also, in order to homogenize the structure by heating and maintaining the slab above the recrystallization temperature before the first upsetting forging and to ensure that the forging finish temperature is sufficiently high to minimize the surface cracks that may occur during the forging process, it is preferable to heat the slab at a certain temperature range or higher. Therefore, the slab heating of the present invention is preferably performed in the temperature range of 1100°C or higher.

[0070] On the other hand, when the slab heating temperature is excessively high, excessive high-temperature oxidation scale may occur, and the manufacturing cost may increase excessively due to high-temperature heating and maintenance. Therefore, the slab heating of the present invention is preferably performed in the range of 1300°C or lower.

[0071] On the other hand, upsetting is a method of performing severe plastic deformation perpendicular to the length direction as the axis. The forging ratio during the first upsetting is appropriately 1.3 to 2.4, and preferably can be 1.5 to 2.0. Here, the forging ratio refers to the ratio of the cross-sectional area that changes by forging. In such a first upsetting, when the size of the forging surface to be punched is initially 700 mm × 1800 mm, it can be 1000 to 1200 mm × 1800 to 2000 mm.

[0072] When the forging ratio during the first upsetting is less than 1.3, it is difficult to sufficiently crimp the Porosity remaining in the center of the slab. Therefore, the porosity required for the final product of the present invention is at an appropriate level of 0.1 mm3 Since it is difficult to control below / g, it is not easy to ensure the low-temperature impact toughness at the center. On the other hand, when the forging ratio exceeds 2.4 during the first setting, buckling occurs during the forging process, making it difficult to control the appropriate shape required for the surface quality and flange products. Therefore, a forging ratio of 1.3 to 2.4 is appropriate during the first setting.

[0073] "Bloom forging (forging on both the upper and lower surfaces)" And in the present invention, bloom forging is performed on the material after the above first setting with a forging ratio of 1.5 to 2.0.

[0074] Bloom forging is a method of further compressing the material after the first setting and processing it in the bloom form, and it is a method of expanding the area while processing in a certain direction of width or length on both the upper and lower surfaces. In the case of the above bloom forging, when the size of the forging surface at the end of forging is 1000 - 1200 mm × 1800 - 2000 mm initially, it can be 1450 - 1850 mm × 2100 - 2500 mm. In the case of bloom forging, a forging ratio of 1.5 to 2.0 is appropriate. This is because when the forging ratio is less than 1.5, it is difficult to ensure the appropriate void quality required in the present invention similar to the upset forging, and when it exceeds 2.0, there is a possibility of surface cracks occurring.

[0075] The forging progress direction is possible in both the length direction and the width direction. However, in the case of the length direction, since the casting structure is more densely configured, the elongation rate of the surface layer structure is high and the workability can be excellent. Therefore, bloom forging in the length direction can be more appropriate from the perspective of surface cracks than in the width direction.

[0076] "Reheating and round forging - second setting" And in the present invention, the material after the above bloom forging is reheated in the temperature range of 1100 - 1300 °C, and then round forging is performed with a forging ratio of 1.65 - 2.25, and then second setting is performed with a forging ratio of 1.3 - 2.3.

[0077] When the bloom forging is completed, the surface temperature of the bloom is 950 °C or lower. If the processing continues, surface cracks or material fracture may occur. Therefore, after the bloom forging, the material can be heated again to the temperature range of 1100 - 1300 °C. As described above, it is preferable to heat to 1100 °C or higher for reasons such as re-solution of crystallized substances, tissue homogenization, and prevention of surface cracks, and it is preferable to control the temperature below 1300 °C due to problems such as excessive scale and coarsening of crystal grains.

[0078] In the case of a bloom whose heating-up has ended, round forging is performed to process it into a circular shape of the flange frame, and then secondary positioning is applied again. When the above-mentioned round forging and secondary positioning are completed, the size of the product can be 1450 - 1850Φ × 1300 - 1700 mm. The forging ratios during round forging and secondary positioning can be 1.65 - 2.25 and 1.3 - 2.3 respectively. When the forging ratios during round forging and secondary positioning are below the levels required in the present invention, it is difficult to control the central porosity in the final product to 0.1 mm 3 / g or less, so it is not easy to ensure the low-temperature impact toughness of the central part. When exceeding the forging ratio standard, problems such as buckling, generation of surface cracks, and shape defects make it impossible to process into the desired flange product form.

[0079] After the above-mentioned secondary positioning is completed, round forging can be applied again for shape control, and then it can be heated under the same conditions as the above-mentioned reheating temperature.

[0080] "Tertiary positioning and hole machining" Then, in the present invention, after performing tertiary positioning on the material on which the above-mentioned secondary positioning has been performed at a forging ratio of 2.0 - 2.8, hole machining is carried out.

[0081] The material processed into the above cylindrical form can be processed to an appropriate flange thickness by the third setting before drilling (piercing). When the third setting is completed, the size of the product can be 2300 - 2800Φ × 400 - 800 mm. The forging ratio of the third setting can be 2.0 - 2.8. If the forging ratio is insufficient or excessive, problems such as the inability to control the above-mentioned residual voids and surface cracks / shape control may occur. After the third setting is completed, a hole can be made in the central part of the material using a 500 - 1000Φ punch.

[0082] "Reheating and ring forging" Subsequently, in the present invention, the drilled material is reheated to a temperature range of 1100 - 1300°C and then ring forged with a forging ratio of 1.0 - 1.6.

[0083] The drilled material is reheated again to the above-mentioned temperature range of 1100 - 1300°C and then can be processed into the final flange ring form. The maximum thickness of the flange made of the above steel can be 200 - 500 mm, the inner diameter can be 4000 - 7000 mm, and the outer diameter can be 5000 - 8000 mm. Since ring forging is a process where final shape and dimension control are more important than void crimping, severe plastic working is not applied. Therefore, the forging ratio can be 1.0 - 1.6, and more preferably 1.2 - 1.4.

[0084] On the other hand, in all the forging processes presented in the present invention, the deformation rate can be 1 / s - 4 / s. When the deformation rate is less than 1 / s, there is a possibility that the temperature of the finish forging decreases and surface cracks occur. On the other hand, when applying a high deformation rate exceeding 4 / s in the non-recrystallized region, surface cracks may be induced due to a decrease in elongation rate caused by excessive local work hardening.

[0085] "Normalizing heat treatment" Finally, in the present invention, the forged flange product can be subjected to a normalizing heat treatment in which the forged flange product is heated to a temperature range of 820 to 930 °C, which is the product center temperature measurement standard, maintained for 5 to 600 minutes, and then air-cooled to room temperature.

[0086] During the above normalizing heat treatment, if the heating temperature is less than 820 °C or the holding time is less than 5 minutes, the re-solution of carbides generated during cooling after forging and impurity elements segregated at grain boundaries may not occur smoothly, and the low-temperature toughness of the steel after heat treatment may be significantly reduced. On the other hand, during the above normalizing heat treatment, if the heating temperature exceeds 930 °C or the holding time exceeds 600 minutes, the grain size of the ferrite matrix phase of the ferrite-pearlite composite structure may exceed 30 μm required in the present invention, or the strength and low-temperature impact toughness may deteriorate due to the coarsening of precipitation phases such as Nb(C,N) and V(C,N).

[0087] On the other hand, in the present invention, it is preferable to perform normalizing heat treatment on the ring-forged flange material under the condition that the LMP defined by the following relational expression 2 satisfies 20 to 33.

[0088] The normalizing heat treatment and the holding time can be expressed by the Larson-Miller Parameter formula 2 as follows (Reference: F.R. Larson and J.Miller: Trans. ASME, 1952, vol. 74, pp. 765-75). In order to refine the size of the pearlite colony and satisfy the impact toughness required in the present invention, the LMP for the normalizing temperature and time conditions can be 20 to 23. [Relational Expression 2] LMP = T(Logt + 20) × (1 / 1000) In the above relational expression 2, T is the normalizing heat treatment temperature based on the Kelvin standard, t is the heat treatment time, and the exponent of log is 10.

[0089] When the above LMP is less than 20, there are drawbacks that the Austenite single-phase region may not be heated sufficiently, or the diffusion of the Solute may not occur uniformly, resulting in material variations. When the LMP exceeds 23, Ferrite and Pearlite Coloney are excessively coarsely generated, making it difficult to ensure the low-temperature impact toughness required by the present invention.

[0090] And in the present invention, when welding is performed after the above annealing heat treatment, post-weld heat treatment (Post-Weld Heat Treatment), stress relieving heat treatment (Stress Relieving Heat Treatment), or tempering heat treatment can be carried out. Such post-weld heat treatment can be performed in the range where the value defined by the above relational expression 2 is LMP 19.3 or less. When the LMP exceeds 19.3, the size of the grain boundary cementite increases and exceeds 5 μm required by the present invention, and thus the impact toughness may deteriorate. Therefore, when welding is performed, it is preferable that the LMP of the subsequent heat treatment is 19.3 or less.

Example

[0091] The present invention will be described in detail by the following examples. However, it should be noted that the following examples are for illustrating and explaining the present invention in more detail, and do not limit the scope of the rights of the present invention.

[0092] (Example)

Table 1

[0093] A slab with a thickness of 700 mm having the alloy components shown in Table 1 above was produced. Using such a slab, after preparing a slab through cooling according to the process conditions shown in Table 2 below, a final 320 mmt Flange was produced through a forging process (reheating and first setting, bloom forging, reheating - second setting, third setting, reheating and ring forging) and a normalizing heat treatment. For processes other than those described in Table 2, process conditions that satisfy the scope of the present invention were applied in all cases.

[0094] Thereafter, the physical property values of each of the above-produced test pieces were measured and shown in Table 3 below. Here, the prior austenite grain size and the polygonal ferrite (PF) fraction of the slab surface layer were measured using an image automatic analyzer by collecting test pieces from the surface layer structure after casting.

[0095] And the ferrite grain size of the steel material was also measured using an image automatic analyzer by collecting test pieces from the final steel material structure. In this example, in all of the inventive examples and comparative examples, the product microstructure was a mixed structure of ferrite and pearlite.

[0096] Also, the yield / tensile strength was evaluated by a normal temperature tensile test, and in the case of the yield strength, 0.2% Offset was applied. Also, the impact toughness for each test piece used the average of the absorbed energy values measured three times at the corresponding temperature through a Charpy V-Notch Test.

[0097] Also, in the cross-section of the steel material, the number of NbC precipitates, etc. was measured using TEM. NbC precipitates were confirmed by the diffraction pattern and EDX mapping of NbC, and the number of NbC precipitates located at 1 μm 2 was counted.

[0098] The porosity of the product center was measured by measuring the density (g / mm 3 ) and taking the reciprocal (mm 3 / g).

[0099] After visually observing the surface of each test piece, grinding was performed at the point where surface cracks were formed, and the grinding length until the cracks disappeared was measured as the surface crack length. In the case of through cracks, it corresponded to the case where the cracks were not confined to the surface layer but penetrated deeply to the inside, and the total length into which the cracks flowed was measured by cutting the cross-section.

[0100]

Table 2

[0101]

Table 3

[0102] On the other hand, Comparative Examples 1 to 15 and 21 to 22 satisfy the alloy composition proposed by the present invention but do not satisfy the manufacturing conditions, and the strength and low-temperature impact toughness values are at a low level because they do not satisfy the characteristics such as the prior austenite grain size of the slab, the polygonal ferrite fraction, or the porosity at the center, or the ferrite grain size in the flange product state proposed by the present invention. Also, even when the material is good or when the forging ratio conditions are not satisfied at each stage of forging, poor surface quality characteristics in the product state can be confirmed due to the occurrence of surface cracks or through cracks.

[0103]

[0104] ​On the other hand, Comparative Examples 16 to 20 satisfy the manufacturing conditions proposed by the present invention but do not satisfy the alloy composition, and it can be seen that the quality level is low, such as exceeding strength (not reaching impact toughness) or falling short of strength.

[0105] As described above, in the detailed description of the present invention, the preferred embodiments of the present invention have been described. However, for those having ordinary knowledge in the technical field to which the present invention pertains, it goes without saying that various modifications can be made without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the claims described below but also by those equivalent thereto.

Claims

1. by weight, 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%, with the balance being Fe and other unavoidable impurities, and satisfying the range of 0.35 to 0.55 for Ceq according to the following relational expression 1, 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 μm or less, the maximum size of cementite present at ferrite-ferrite and / or ferrite-pearlite grain boundaries being 5 μm or less, In the central part of the product, which is a region with a thickness ranging from 3 / 8t to 5 / 8t in the thickness direction from the steel surface (where t represents the thickness of the steel material in mm), the porosity is 0.1 mm ３ / g or less, and Among the precipitates observed in the cross-section of the steel material, there are 5 or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per 1 μm. ２ Steel material for extremely thick flanges. 【Relational Expression 1】 Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the above relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively represent the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and when these components are not intentionally added, 0 is substituted.

2. The steel material according to Claim 1, having a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorbed energy value in the Charpy impact test at -50°C of 50 J or more for the extremely thick steel material for flanges.

3. The extremely thick steel material for flanges according to Claim 1, wherein the depth of the maximum surface crack of the steel material is 0.1 mm or less (including 0).

4. The extremely thick steel material for flanges according to Claim 1, wherein the fraction of cementite present at ferrite-ferrite or ferrite-pearlite grain boundaries is 3 area % or less.

5. By weight, 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%, containing the balance Fe and other inevitable impurities, satisfying the range of Ceq of 0.35 to 0.55 according to the following relational expression 1, and producing a slab having a thickness of 500 mm or more; After heating the produced slab in the temperature range of 1100 to 1300 °C, performing a first setting with a forging ratio of 1.3 to 2.4; After the first setting, performing bloom forging with a forging ratio of 1.5 to 2.0; Reheating the bloom-forged material in the temperature range of 1100 to 1300 °C, subsequently performing round forging with a forging ratio of 1.65 to 2.25, and then performing a second setting with a forging ratio of 1.3 to 2.3; After performing a third setting on the material on which the second setting has been performed with a forging ratio of 2.0 to 2.8, performing hole machining; After reheating the hole-machined material in the temperature range of 1100 to 1300 °C, performing ring forging with a forging ratio of 1.0 to 1.6; and Heating the ring-forged material in the temperature range of 820 to 930 °C based on the center temperature measurement standard and maintaining it for 5 to 600 minutes, and then performing a normalizing heat treatment of air-cooling to room temperature; A method for manufacturing a thick steel material for a flange, including: [Relational Expression 1] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively mean the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and when these components are not intentionally added, 0 is substituted.

6. The method for manufacturing a thick steel material for a flange according to claim 5, wherein the slab is manufactured using a continuous casting process or a semi-continuous casting process.

7. After the slab is manufactured and before forging, the prior austenite grain size of the slab surface layer is 1000 μm or less, and the microstructure of the slab surface layer before forging is composed of a composite structure of 15% or more polygonal ferrite and the balance bainite. The method for manufacturing an extra-thick steel material for a flange according to claim 5.

8. When the size of the forging surface punched during the first setting is initially 700 mm × 1800 mm, it is 1000 to 1200 mm × 1800 to 2000 mm. The method for manufacturing an extra-thick steel material for a flange according to claim 5.

9. In the case of bloom forging, when the size of the forging surface at the end of forging is initially 1000 to 1200 mm × 1800 to 2000 mm, it is 1450 to 1850 mm × 2100 to 2500 mm. The method for manufacturing an extra-thick steel material for a flange according to claim 5.

10. When the round forging and the second setting are completed, the size of the product is 1450 to 1850Φ × 1300 to 1700 mm. The method for manufacturing an extra-thick steel material for a flange according to claim 5.

11. When the third setting is completed, the size of the product is 2300 to 2800Φ × 400 to 800 mm. The method for manufacturing an extra-thick steel material for a flange according to claim 5.

12. The maximum thickness of the flange made of the steel material can be 200 to 500 mm, the inner diameter is 4000 to 7000 mm, and the outer diameter is 5000 to 8000 mm. The method for manufacturing an extra-thick steel material for a flange according to claim 5.

13. During the normalizing heat treatment, heat treatment is performed so that the LMP defined by the following relational expression 2 satisfies 20 to 33. The method for manufacturing an extra-thick steel material for a flange according to claim 5. [Relational expression 2] LMP = T (Logt + 20) × (1 / 1000) In the relational expression 2, T is the reference temperature in Kelvin, t is the time, and the exponent of log is 10.

14. When welding is performed on the steel material after the normalizing heat treatment, the method for manufacturing an extra-thick steel material for a flange according to claim 5 further includes a step of performing post-weld heat treatment, stress relieving heat treatment, or tempering heat treatment.

15. The post-weld heat treatment is performed in a range where the value defined by the following relational expression 2 is LMP 19.3 or less. The method for manufacturing an extra-thick steel material for a flange according to claim 14. [[Relationship formula 2]]LMP = T(Logt + 20)×(1 / 1000) In the relationship formula 2, T is the Kelvin reference temperature, t is the time, and the exponent of log is 10.

16. In terms of weight percentage, C: 0.05 - 0.2%, Si: 0.05 - 0.5%, Mn: 1.0 - 2.0%, Al: 0.005 - 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 - 0.07%, V: 0.001 - 0.3%, Ti: 0.001 - 0.03%, Cr: 0.01 - 0.3%, Mo: 0.01 - 0.12%, Cu: 0.01 - 0.6%, Ni: 0.05 - 1.0%, Ca: 0.0005 - 0.004%, containing the remaining Fe and other inevitable impurities, when manufacturing a slab using a molten steel satisfying the range of 0.35 - 0.55 for Ceq according to the following relationship formula 1, the step of manufacturing a slab by performing secondary cooling on the slab discharged from the mold at a cooling rate of 0.01 - 3°C / s to a temperature range of 800 - 850°C; The step of heating the manufactured slab to a temperature range of 1100 - 1300°C and then performing primary upsetting at a forging ratio of 1.3 - 2.4; The step of bloom forging at a forging ratio of 1.5 - 2.0 after the primary upsetting; The step of reheating the bloom-forged material to a temperature range of 1100 - 1300°C, subsequently performing round forging at a forging ratio of 1.65 - 2.25, and then performing secondary upsetting at a forging ratio of 1.3 - 2.3; The step of performing tertiary upsetting on the material after the secondary upsetting at a forging ratio of 2.0 - 2.8 and then performing hole machining; The step of reheating the hole-machined material to a temperature range of 1100 - 1300°C and then performing ring forging at a forging ratio of 1.0 - 1.6; and The step of performing a normalizing heat treatment of heating the ring-forged material to a temperature range of 820 - 930°C based on the central part temperature measurement reference, maintaining it for 5 - 600 minutes, and then air-cooling to room temperature; A method for manufacturing a thick steel material for a flange, including. [[Relationship formula 1]] Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 In the relationship formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively mean the contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, and when these components are not intentionally added, 0 is substituted.

17. The normalizing heat treatment is a method for manufacturing an extra-thick steel material for a flange according to claim 16, which is heat-treated so that the LMP defined by the following relational expression 2 satisfies 20 to 33 and has excellent low-temperature impact toughness. [Relational Expression 2] LMP = T (Log t + 20) × (1 / 1000) In the relational expression 2, T is the reference temperature on the Kelvin scale, t is the time, and the exponent of log is 10.

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