Extra-thick steel material for steam drums with excellent surface quality and lamellar tear resistance, and its manufacturing method

Abstract

To provide an extra-thick steel material for steam drums having excellent surface quality and lamellar tear resistance, and a manufacturing method thereof. [Solution] By weight, C: 0.2-0.3%, 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: 0.001-0.02%, V: 0.001-0.03%, Ti: 0.001-0.03%, Cr: 0.01-0.3%, Mo: 0.01-0.12%, Cu: 0.01-0.4%, Ni: 0.05-0.4%, Ca: 0.0005-0.00 The steel has a matrix structure consisting of a composite structure of ferrite and pearlite with an average grain size of 20 μm or less, a surface layer region from the surface to 10 mm in the thickness direction, and a porosity of 0.1 mm or less in the center region from 3 / 8t to 5 / 8t (where t means the thickness (mm) of the steel material). 3 / g or less, and among the precipitates observed in the cross section of steel after post-weld heat treatment (PWHT), fine VC precipitates with a diameter of 5 to 15 nm are found to be 1 μm or less. 2 There can be five or more per prize.

Description

[Technical Field]

[0001] The present invention relates to an extra-heavy steel material for steam drums having excellent surface quality and lamellar tearing resistance, and a manufacturing method thereof. More particularly, the present invention relates to an extra-heavy steel material for steam drums having excellent surface quality and lamellar tearing resistance that can be used in petrochemical power generation facilities, boilers, etc., and a manufacturing method thereof. [Background technology]

[0002] A boiler steam drum used in power generation equipment is a container that stores steam evaporated from a boiler under constant pressure and separates it into water. Waste heat boilers are often used to utilize heat generated by chemical and combustion reactions, and a steam drum is always required when a waste heat boiler is installed. To meet the demand for increased steam boiler efficiency, the steel used for larger boilers and larger storage capacities is continually increasing. As the thickness of steel increases, the total rolling reduction decreases, resulting in a larger microstructure and a tendency for material defects such as inclusions and segregations to deteriorate the quality. Therefore, to improve the internal and external soundness of steel, there is a trend toward reducing the concentration of impurities such as non-metallic inclusions and segregations, and minimizing cracks and voids on the surface and within the material.

[0003] In particular, in the case of extremely thick materials with a thickness of over 100 mm, the rolling reduction ratio is not as high as that of thin materials, so unsolidified shrinkage pores that occur during continuous casting or casting are not sufficiently compressed during the rough rolling process and remain in the form of residual voids in the center of the product.

[0004] When a structure is subjected to stress in the thickness direction, these residual voids act as starting points for cracks, eventually causing damage to the entire equipment in the form of lamellar tears. Therefore, a process of sufficiently compressing the central void to ensure that no residual voids exist is required prior to rolling.

[0005] Related Patent Document 1 provides a method for applying heavy reduction in the rough rolling process of thick plates, utilizing a technique for determining a critical reduction for each thickness at which plate jamming occurs from a heavy reduction for each pass set to approximate the design allowable values ​​(load and torque) of the rolling mill, a technique for allocating the reduction by adjusting the thickness ratio index for each pass to ensure the target thickness of the rough rolling mill, and a technique for correcting the reduction based on the critical reduction for each thickness to prevent plate jamming. The method provides a manufacturing method that can apply an average reduction of approximately 27.5% in the final three passes of rough rolling based on a standard of 80 mmt. However, in this rolling method, the average reduction is measured across the entire product thickness, and in the case of extremely thick materials with a maximum thickness of 233 mmt or more, it is technically difficult to apply high deformation to the center where residual porosity exists.

[0006] Another method for manufacturing extremely thick products is to use a forging machine, which has a higher effective deformation per pass than a rolling mill. Patent Document 2 proposes a method in which a continuously cast slab extracted from a heating furnace is vertically set and subjected to a full-width forging reduction of 400 mm or more, and width forging is performed within two passes, which is within the buckling limit reduction, to remove porosity from the edges and center in the width direction and increase the deformation rate in the center. This method effectively closes the residual voids in the center, which was a problem in Patent Document 1, and improves the lamellar tear resistance of the product.

[0007] However, localized deformation concentration during width forging can cause surface defects. In particular, if surface or subsurface defects exist in the slab before forging, the defects can propagate during the forging process, further degrading the surface quality of the product after rolling.

[0008] Meanwhile, Patent Document 3 discloses that a thick, high-strength steel plate of 100 mmt or more and having a yield strength of 620 MPa or more can be manufactured through the steps of heating a material provided with a predetermined alloy composition to 1200 to 1350°C, hot forging with a cumulative reduction of 25% or more, heating to between the Ac3 point and 1200°C, hot rolling with a cumulative reduction of 40% or more, reheating to between the Ac3 point and 1050°C, quenching at a temperature of the Ac3 point or higher to the lower of 350°C or lower or the Ar3 point or lower, and tempering at a temperature of 450°C to 700°C.

[0009] However, the high carbon equivalent (Ceq) and hardenability index (DI) of the aforementioned ultra-high strength steel sheets make them vulnerable to surface cracks during casting, and the process conditions are difficult to apply to steel for steam drums, which are manufactured using normalizing heat treatment. Furthermore, when the carbon equivalent (Ceq) and hardenability index (DI) are high, cracks are likely to occur on the surface of the cast slab due to the formation of hard structures in the surface during the secondary cooling process of steelmaking. These cracks can propagate during the forging process, potentially degrading the surface quality of the final product.

[0010] Therefore, a method of forging has been proposed to close the void in the center and improve the internal integrity of the final product, but no practical method has been presented to ensure both the appropriate material quality and excellent surface quality of the steel material for steam drums. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 10-2012-0075246 [Patent Document 2] Korean Patent Publication No. 10-2012-0074039 [Patent Document 3] Korean Patent Publication No. 10-2017-0095307 Summary of the Invention [Problem to be solved by the invention]

[0012] According to the present invention, it is possible to provide an extra-heavy steel material for steam drums having excellent surface quality and lamellar tear resistance, and a method for manufacturing the same.

[0013] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the overall content of this specification. [Means for solving the problem]

[0014] The extra heavy-gauge steel material according to the present invention contains, by weight, C: 0.2 to 0.3%, 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 0.02%, V: 0.001 to 0.03%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.4%, Ni: 0.05 to 0.4%, Ca: 0.0005 to 0.12%, and The steel has a matrix structure consisting of a composite structure of ferrite and pearlite with an average grain size of 20 μm or less, a surface layer region from the surface to 10 mm in the thickness direction, and a porosity of 0.1 mm or less in the center region from 3 / 8t to 5 / 8t (where t means the thickness (mm) of the steel material). 3 / g or less, and among the precipitates observed in the cross section of steel after post-weld heat treatment (PWHT), fine VC precipitates with a diameter of 5 to 15 nm are found to be 1 μm or less. 2 There can be five or more per prize.

[0015] [Equation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0016] In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent the contents (wt%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel, respectively, and if these elements are not intentionally added, 0 is substituted.

[0017] The thickness of the steel material may be 133 to 250 mm.

[0018] The tensile strength of the steel material may be 550 to 690 MPa.

[0019] The steel material may have a through-thickness reduction of area (ZRA) of 35% or more.

[0020] The maximum depth of the surface cracks of the steel material may be 0.1 mm or less (including 0).

[0021] The method for producing an extra-heavy-gauge steel material according to the present invention comprises, by weight %, C: 0.2 to 0.3%, 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 0.02%, V: 0.001 to 0.03%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.02%, and 0.4%, Ni: 0.05-0.4%, Ca: 0.0005-0.004%, the remainder being Fe and other inevitable impurities, and Ceq according to the following relational expression 1 satisfies the range of 0.5-0.6, the average grain size of prior austenite is 500 μm or less, and the thickness is 650 mm or more; The heated slab is subjected to a first forging process at a cumulative reduction of 3 to 15% and a deformation rate of 1 / s to 4 / s to obtain a first intermediate material having a thickness of 450 to 550 mm; the first intermediate material is secondarily heated to a temperature range of 1000 to 1200°C; and the secondarily heated first intermediate material is secondarily forged at a cumulative reduction of 3 to 30% and a deformation rate of 1 / s to 4 / s to obtain a second intermediate material having a thickness of 300 to 340 mm. a step of tertiarily heating the secondary intermediate material to a temperature range of 1000 to 1200°C; a step of hot-rolling the tertiarily heated secondary intermediate material to a temperature range of 900 to 1100°C to provide a hot-rolled material having a thickness of 133 to 233 mm; and a normalizing heat treatment step of heating the hot-rolled material to a temperature range of 820 to 900°C, holding the hot-rolled material for 10 to 40 minutes, and then air-cooling it to room temperature.

[0022] [Equation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0023] In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the contents (wt%) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel slab, respectively, and if these elements are not intentionally added, 0 is substituted.

[0024] The porosity of the center of the secondary intermediate material is 0.1 mm 3 / g or less.

[0025] The maximum depth of the surface cracks of the hot-rolled material may be 2 μm or less (including 0 μm).

[0026] The method may further include welding the normalizing heat treated steel material and performing a further heat treatment (PWHT) to remove residual stress in the welded steel material.

[0027] The above solution to the problem does not list all of the features of the present invention, and the various features of the present invention and the advantages and effects thereof can be understood in detail by referring to the following specific implementation examples. [Effects of the Invention]

[0028] According to the present invention, it is possible to provide an extra-heavy steel material for steam drums having excellent surface quality and lamellar tear resistance, and a method for manufacturing the same.

[0029] The effects of the present invention are not limited to the above-mentioned matters, but can be construed as including technical effects that can be inferred by a person skilled in the art from the matters described below. DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention relates to an extra-heavy steel material for steam drums having excellent surface quality and lamellar tear resistance, and a manufacturing method thereof. Preferred embodiments of the present invention will be described below. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

[0031] The extra-heavy steel material for steam drums of the present invention, which has excellent surface quality and lamellar tear resistance, will be described in more detail below.

[0032] The extra heavy steel material for steam drums of the present invention contains, by weight %, C: 0.2 to 0.3%, 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 0.02%, V: 0.001 to 0.03%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.4%, Ni: 0.05 to 0.4%, Ca: 0.00 The steel contains 0.05 to 0.004% of ferrite, with the remainder consisting of Fe and other unavoidable impurities, and the Ceq value calculated by the following relational expression 1 satisfies the range of 0.5 to 0.6. The steel has a composite structure of ferrite and pearlite with an average grain size of 20 μm or less as the matrix structure. The fraction of hard structure in the surface layer, which is the region from the surface to 10 mm in the thickness direction, is 5 area % or less, and the porosity in the center, which is the region of 3 / 8t to 5 / 8t (where t means the thickness (mm) of the steel material), is 0.1 mm or less. 3 / g or less, and among the precipitates observed in the cross section of steel after post-weld heat treatment (PWHT), fine VC precipitates with a diameter of 5 to 15 nm are found to be 1 μm or less. 2 There can be five or more per prize.

[0033] [Equation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0034] In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent the contents (wt%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel, respectively, and if these elements are not intentionally added, 0 is substituted.

[0035] The alloy composition of the present invention will be described in more detail below. Unless otherwise specified, % and ppm given in the alloy composition are based on weight.

[0036] Carbon (C): 0.20~0.30% Carbon (C) is the most important element for ensuring basic strength, so it must be contained within an appropriate range in steel. To achieve this effect, 0.20% or more of carbon (C) can be added. Preferably, 0.22% or more of carbon (C) can be added. However, if the carbon (C) content exceeds a certain level, the pearlite fraction increases during normalizing heat treatment, which can excessively increase the strength and hardness of the base material. This can lead to surface cracks during forging and reduced lamellar tear resistance in the final product. Therefore, in the present invention, the carbon (C) content can be limited to 0.30%, and a more preferable upper limit of the carbon (C) content can be 0.26%.

[0037] Silicon (Si): 0.05 to 0.50% Silicon (Si) is a substitutional element that improves the strength of steel through solid solution strengthening and has a strong deoxidizing effect, making it an essential element for the production of clean steel. Therefore, silicon (Si) can be added in an amount of 0.05% or more, and more preferably 0.20% or more. However, if silicon (Si) is added in large amounts, it can form a martensite-austenite (MA) phase, excessively increasing the strength of the ferrite matrix and deteriorating the surface quality of extra-thick steel products. Therefore, the upper limit of the silicon (Si) content can be limited to 0.50%. A more preferable upper limit of the silicon (Si) content may be 0.40%.

[0038] Manganese (Mn): 1.0-2.0% Manganese (Mn) is a useful element that improves strength through solid solution strengthening and improves hardenability by forming 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 may be 1.1% or more. However, manganese (Mn) forms elongated non-metallic inclusions, MnS, together with sulfur (S), which reduces toughness and acts as a factor in reducing elongation during tension in the thickness direction, which may cause a rapid deterioration in lamellar tear resistance. Therefore, the manganese (Mn) content is preferably controlled to 2.0% or less, and more preferably 1.5% or less.

[0039] Aluminum (Al): 0.005 to 0.1% Aluminum (Al), along with silicon (Si), is one of the powerful deoxidizing agents in the steelmaking process, and to achieve this effect, it is preferable to add 0.005% or more. A more preferable lower limit of the aluminum (Al) content may be 0.01%. On the other hand, if the aluminum (Al) content is excessive, the Al2O3 fraction in the oxidized inclusions generated as a result of deoxidation increases excessively, causing the inclusions to become coarse in size, making them difficult to remove during refining, which may be a factor in reducing lamellar tear resistance. Therefore, it is preferable to control the aluminum (Al) content to 0.1% or less. A more preferable aluminum (Al) content may be 0.07% or less.

[0040] 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 embrittlement at grain boundaries or by forming coarse inclusions, so in order to improve 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.

[0041] Niobium (Nb): 0.001-0.02% Niobium (Nb) is an element that precipitates in the form of NbC or NbCN to improve the strength of the base metal. Furthermore, niobium (Nb) that dissolves during high-temperature reheating precipitates very finely in the form of NbC during rolling, suppressing austenite recrystallization and thereby refining the structure. Therefore, niobium (Nb) is preferably added in an amount of 0.001% or more, and a more preferred niobium (Nb) content may be 0.005% or more. On the other hand, if niobium (Nb) is added in excess, undissolved niobium (Nb) is generated in the form of TiNb(C,N), which may impair lamellar tear resistance. Therefore, the upper limit of the niobium (Nb) content is preferably limited to 0.02%. A more preferred niobium (Nb) content may be 0.017% or less.

[0042] Vanadium (V): 0.001 to 0.03% Since most of the vanadium (V) dissolves again during reheating, its strengthening effect due to precipitation or dissolution during subsequent rolling is minimal. However, it precipitates as very fine carbonitrides during subsequent heat treatments such as PWHT, thereby improving strength. To fully achieve this effect, 0.001% or more vanadium (V) must be added. A more preferable lower limit for the vanadium (V) content is 0.01%. However, excessive vanadium (V) content excessively increases the strength and hardness of the base metal and weld, which can cause surface cracks during steam drum processing. It also significantly increases manufacturing costs and is therefore not commercially viable. Therefore, the vanadium (V) content can be limited to 0.03% or less. A more preferable vanadium (V) content is 0.02% or less.

[0043] Titanium (Ti): 0.001 to 0.03% Titanium (Ti) precipitates as TiN during reheating, suppressing grain growth in the base metal and weld heat-affected zone and significantly improving low-temperature toughness. To achieve this effect, it is preferable to add 0.001% or more of titanium (Ti). However, excessive addition of titanium (Ti) can lead to clogging of the continuous casting nozzle and reduced low-temperature toughness due to crystallization in the center. Furthermore, titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates in the center of the thickness, reducing the elongation of the product and potentially reducing the lamellar tear resistance of the final material. Therefore, the titanium (Ti) content can be 0.03% or less. A preferred titanium (Ti) content may be 0.025% or less, and a more preferred titanium (Ti) content may be 0.018% or less.

[0044] Chromium (Cr): 0.01 to 0.30% Chromium (Cr) is an element that increases the yield strength and tensile strength by increasing hardenability and forming a low-temperature transformation structure. It also has the effect of preventing a decrease in strength by slowing down the decomposition rate of cementite during tempering after quenching and post-weld heat treatment. To achieve this effect, 0.01% or more of chromium (Cr) can be added. On the other hand, if the chromium (Cr) content is excessive, M 23 The size and fraction of Cr-rich coarse carbides such as C6 increase, reducing the impact toughness of the product, and the solid solubility of niobium (Nb) and the fraction of fine precipitates such as NbC in the product decrease, which may result in a decrease in strength of the product. Therefore, in the present invention, the upper limit of the chromium (Cr) content can be limited to 0.30%. A preferred upper limit of the chromium (Cr) content may be 0.25%.

[0045] Molybdenum (Mo): 0.01 to 0.12% Molybdenum (Mo) is an element that increases grain boundary strength and has a significant effect on solid solution strengthening in ferrite, effectively contributing to increased strength and ductility of products. Molybdenum (Mo) also prevents a decrease in toughness due to the grain boundary segregation of impurity elements such as phosphorus (P). To achieve this effect, 0.10% or more of Mo can be added. However, since Mo is an expensive element, excessive addition can significantly increase manufacturing costs. Therefore, the upper limit of the Mo content can be set to 0.12%.

[0046] Copper (Cu): 0.01 to 0.40% Copper (Cu) is an advantageous element in the present invention because it not only significantly improves the strength of the matrix phase through solid solution strengthening in ferrite, but also has the effect of suppressing corrosion in a wet hydrogen sulfide atmosphere. To achieve this effect, copper (Cu) may be contained in an amount of 0.01% or more. A more preferred copper (Cu) content may be 0.03% or more. However, excessive copper (Cu) content increases the likelihood of star cracks being induced on the surface of the steel sheet, and since copper (Cu) is an expensive element, this may result in significant increases in manufacturing costs. Therefore, in the present invention, the upper limit of the copper (Cu) content may be limited to 0.40%. A preferred upper limit of the copper (Cu) content may be 0.35%.

[0047] Nickel (Ni): 0.05 to 0.40% Nickel (Ni) is an element that effectively contributes to improving strength by increasing stacking faults at low temperatures and facilitating cross-slip of dislocations, thereby improving impact toughness and hardening ability. To achieve these effects, 0.05% or more of nickel (Ni) can be added. A preferred nickel (Ni) content is 0.10% or more. However, since excessive nickel (Ni) addition can increase production costs due to its high cost, the upper limit of the nickel (Ni) content can be limited to 0.40%. A preferred upper limit of the nickel (Ni) content can be 0.35%.

[0048] Calcium (Ca): 0.0005-0.0040%, Adding calcium (Ca) after deoxidation with aluminum (Al) inhibits the formation of MnS by combining with sulfur (S), which forms MnS inclusions, and also inhibits the occurrence of cracks due to hydrogen-induced cracking by forming spherical CaS. To ensure that sulfur (S) contained as an impurity is fully converted into CaS, it is preferable to add 0.0005% or more of calcium (Ca). However, if the amount added is excessive, CaS is formed, and the remaining calcium (Ca) combines with oxygen (O) to form coarse oxidized inclusions, which may elongate and break during rolling, resulting in reduced lamellar tear resistance. Therefore, the upper limit of the calcium (Ca) content can be limited to 0.0040%.

[0049] The extra-heavy steel material for steam drums of the present invention contains the aforementioned components, with the remainder consisting of Fe and other inevitable impurities. However, since unintended impurities may be unavoidably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to completely eliminate these impurities. Since these impurities are known to anyone with ordinary skill in the art, the present specification will not specifically mention all of them. Furthermore, the addition of additional effective components other than those described above is not completely excluded.

[0050] The extra heavy-duty steel material for steam drums of the present invention can satisfy the Ceq range of 0.5 to 0.6 according to the following relational expression 1.

[0051] [Equation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0052] In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent the contents (wt%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel, respectively, and if these elements are not intentionally added, 0 is substituted.

[0053] The extra-heavy steel material for steam drums of the present invention has a thickness of 133 to 250 mm, and can therefore effectively cope with the trend toward larger steam drums.

[0054] The surface layer of the extra-heavy steel material for steam drums of the present invention may be composed of a composite structure of ferrite and pearlite with an average grain size of 20 μm or less. The extra-heavy steel material for steam drums according to one aspect of the present invention restricts the introduction of hard structures into the surface layer of the steel material, thereby suppressing the maximum depth of surface cracks in the final product to 0.1 mm or less. That is, the extra-heavy steel material for steam drums of the present invention actively suppresses the formation of hard structures such as martensite and bainite in the surface layer of the steel material, and even if these hard structures are inevitably formed, their fraction can be actively suppressed to 5 area % or less (including 0%). The fraction of hard structures in the surface layer of the steel material may preferably be 3% or less (including 0%). Here, the surface layer of the steel material may refer to the region from the surface of the steel material to a depth of 10 mm in the thickness direction.

[0055] When the cross section of the extra-heavy steel material for steam drums of the present invention is observed after post-weld heat treatment (PWHT), fine VC precipitates with a diameter of 5 to 15 nm are observed. 2 The minimum number of VC particles per specimen is 5. VC is formed in the form of carbide or carbonitride in the temperature range of 600 to 700°C, causing precipitation strengthening. Therefore, the present invention allows specimens to maintain an appropriate strength of 550 MPa or more even after heat treatment at high temperatures.

[0056] The extra-thick steel material for steam drums of the present invention has a porosity of 0.1 mm at the center of the steel material. 3 / g or less. Therefore, the extra-heavy steel material for steam drums of the present invention can effectively ensure lamellar tear resistance. Here, the center of the steel material means 3 / 8t to 5 / 8t (t: thickness of the steel material, mm), and the porosity of the center can be confirmed by measuring the density and taking the reciprocal.

[0057] The extra-heavy steel material for steam drums of the present invention may have a tensile strength of 550 to 690 MPa and a through-thickness reduction of area (ZRA) of 35% or more. Furthermore, the extra-heavy steel material for steam drums according to one aspect of the present invention may have a maximum surface crack depth of 0.1 mm or less in the final product state. Here, the depth of the surface crack can be determined by visually determining whether or not a surface crack is present, grinding the surface until the crack disappears at that point, and measuring the depth from the surface to the point removed by grinding.

[0058] The method for producing an extra-heavy steel material for a steam drum according to the present invention will be described in more detail below.

[0059] The extra-heavy steel material for steam drums of the present invention contains, by weight, C: 0.2-0.3%, 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: 0.001-0.02%, V: 0.001-0.03%, Ti: 0.001-0.03%, Cr: 0.01-0.3%, Mo: 0.01-0.12%, and Cu: 0.01-0. The present invention relates to a method for manufacturing a slab having a thickness of 650 mm or more, a primary heating of the slab at a temperature range of 1100 to 1300°C, and a secondary heating of the slab at a temperature range of 1100 to 1300°C. the first forging of the slab at a cumulative reduction of 3 to 15% and a deformation rate of 1 / s to 4 / s to obtain a first intermediate material having a thickness of 450 to 550 mm; the second heating of the first intermediate material to a temperature range of 1000 to 1200°C; and the second forging of the second-heated first intermediate material at a cumulative reduction of 3 to 30% and a deformation rate of 1 / s to 4 / s to obtain a second intermediate material having a thickness of 300 to 340 mm. The hot-rolled material can be manufactured through the following steps: tertiary heating the secondary intermediate material to a temperature range of 1000 to 1200°C; hot rolling the tertiarily heated secondary intermediate material to a temperature range of 900 to 1100°C to provide a hot-rolled material having a thickness of 133 to 233 mm; and normalizing the hot-rolled material to a temperature range of 820 to 900°C, maintaining the temperature for 10 to 40 minutes, and then air-cooling it to room temperature.

[0060] [Equation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0061] In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the contents (wt%) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel slab, respectively, and if these elements are not intentionally added, 0 is substituted.

[0062] Slab Preparation The inventors of the present invention have conducted extensive research into a method for manufacturing extra-thick steel products with excellent surface quality and physical properties suitable for steam drums. In particular, in order to ensure the strength and surface quality of the final steel product in slabs manufactured with a thickness of 650 mm or more, it is necessary to control the carbon equivalent (Ceq) of the slab within a certain range, and the prior austenite grain size of the slab is also an effective condition, which led to the development of the present invention.

[0063] The slab of the present invention has an alloy composition corresponding to that of the aforementioned steel material, and therefore the description of the alloy composition of the slab is substituted for the description of the alloy composition of the aforementioned steel material. The alloy composition of the slab used in the present invention satisfies the requirements for ensuring a tensile strength of 550 to 690 MPa and a zero reduction of area (ZRA) of 35% or more.

[0064] The casting speed of large section casters that produce slabs with a thickness of 650mm or more is 0.06~0.1m / min, which is significantly slower than the general casters that produce slabs with a thickness of 250~400mm (casting speed: 0.4~1.5m / min). Therefore, when producing slabs with a thickness of 650mm or more, the time they are held in the mold is relatively longer, creating an environment where austenite can grow even coarser.

[0065] As the grain size of the primary austenite increases, the manganese (Mn) segregation index at the austenite grain boundaries increases, reducing the grain boundary strength and increasing hardenability. This results in a higher proportion of hard bainite and martensite in the surface layer of the slab, rather than soft ferrite and pearlite. Because hard structures have low uniform elongation, intergranular cracking can easily occur when thermal deformation, external deformation, or stress is applied. Therefore, if the grain size of the prior austenite on the surface of the slab is large, intergranular cracking on the slab surface can occur more actively, and the depth of the cracks can further increase during subsequent high-deformation processes such as forging and rolling. Therefore, to prevent surface cracks in the final product, it is very important to control the grain size of the prior austenite to an appropriate level.

[0066] The average prior austenite grain size of a slab can be derived from the following relational expression 2, and the present invention can effectively suppress grain boundary cracks by limiting the average prior austenite grain size of a slab to 500 μm or less. The average prior austenite grain size of a slab may be preferably 400 μm or less, and more preferably 350 μm or less.

[0067] [Equation 2] D (Prior austenite grain size of cast slab) = 3600 × exp {-(89098 + 3581 × [C] + 1211 × [Ni] + 1443 × [Cr] + 4043 × [Mo]) / (RT)} × t 0.18

[0068] In the above Relational Formula 2, [C], [Ni], [Cr], and [Mo] respectively represent the contents (wt%) of C, Ni, Cr, and Mo contained in the steel slab, R represents 8.314 J / mol / K, T represents the casting temperature (K), and t represents the casting time (s).

[0069] One way to reduce the grain size of prior austenite is to increase the carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) contents, which have solute dragging and pinning effects. However, when the carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) contents are increased, the carbon equivalent (Ceq) also increases, which can lead to the formation of low-temperature transformation structures during the slab cooling process. Therefore, in the present invention, the carbon equivalent (Ceq) of the steel slab can be limited to 0.6 or less according to the following Relation 1. The preferred carbon equivalent (Ceq) is 0.5 to 0.6.

[0070] [Equation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0071] In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] mean the contents (wt%) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel slab, respectively, and if these elements are not intentionally added, 0 is substituted.

[0072] Primary heating of the slab The prepared slab can be heated in the temperature range of 1100 to 1300° C. As mentioned above, the thickness of the slab may be 650 mm or more, and the preferred thickness may be 700 mm or more.

[0073] In order to redissolve titanium (Ti) and niobium (Nb) composite carbonitrides or TiNb(C,N) coarse crystals formed during casting, the slab must be heated above a certain temperature range. Furthermore, it is preferable to heat the slab before primary forging above the recrystallization temperature to homogenize the structure and to minimize surface cracks that may occur during the forging process by ensuring a sufficiently high forging completion temperature. Therefore, the primary heating of the slab in the present invention is preferably performed at a temperature range of 1100°C or higher.

[0074] On the other hand, if the heating temperature of the slab is too high, high-temperature oxide scale may be excessively generated, and the manufacturing cost may increase excessively due to high-temperature heating and maintenance. Therefore, it is preferable that the primary heating of the slab in the present invention is performed at a temperature of 1300°C or less.

[0075] Primary forging The primarily heated slab can be subjected to a primary forging process with a cumulative reduction of 3 to 15% and a deformation rate of 1 / s to 4 / s to obtain a primary intermediate material, where 1 / s means that the deformation section per second is 100% deformed.

[0076] The primary forging is the step in which the heated slab is forged to a thickness of 450 to 550 mm to produce the final width of the secondary intermediate material. Since high-deformation, low-speed forging is essential to fully compress the voids, the primary forging can be carried out under conditions of a cumulative reduction of 3 to 15% and a deformation speed of 1 / s to 4 / s.

[0077] If the cumulative reduction in the first forging is less than 3%, the voids remaining in the slab cannot be fully closed, resulting in residual voids, which can reduce the lamellar tear resistance of the final product. A preferred cumulative reduction in the first forging may be 5% or more, and a more preferred cumulative reduction in the first forging may be 7% or more. However, if the cumulative reduction exceeds 15% below the non-recrystallization temperature, at which dislocation density is not recovered or offset by recrystallization, the work hardening of overlapping dislocations can significantly reduce the uniform elongation of the surface, potentially resulting in surface cracks during the forging process. A preferred cumulative reduction in the first forging may be 13% or less, and a more preferred cumulative reduction in the first forging may be 11% or less.

[0078] Secondary heating and secondary forging The primary intermediate material is secondarily heated to a temperature range of 1000-1200°C and secondarily forged with a cumulative reduction of 3-30% and a deformation rate of 1 / s-4 / s to obtain a secondary intermediate material with a thickness of 300-340mm. The maximum depth of surface cracks in the secondary intermediate material may be 5μm or less.

[0079] Secondary forging is the step of heating the primary intermediate material to a temperature range of 1000-1200°C and forging it to process it into the desired thickness and length of the final secondary intermediate material. As with primary forging, high deformation, low speed forging is essential in secondary forging to sufficiently reduce the porosity in the center of the secondary intermediate material. Therefore, secondary forging can be performed by applying a cumulative reduction of 3-30% and a deformation speed of 1 / s-4 / s. To reduce the porosity in the center of the secondary intermediate material to 0.1mm, 3 / g or less.

[0080] If the cumulative reduction amount in the second forging is insufficient, the microvoids remaining after the first forging may not be completely closed. Furthermore, if deformation is applied to the end of an elliptically closed void, the notch effect may result in a decrease in physical properties compared to a circular void. Therefore, a cumulative reduction of 3% or more is required during the second forging to fully close the voids. However, if the cumulative reduction amount is excessive, surface cracks may occur due to work hardening of the surface layer, so the upper limit of the cumulative reduction amount can be limited to 30%.

[0081] The deformation rate of the secondary forging may be 1 / s to 4 / s, similar to the primary forging. A deformation rate of less than 1 / s reduces the temperature during finish forging, which may cause surface cracks. On the other hand, applying a high deformation rate of more than 4 / s in the unrecrystallized region may reduce elongation and induce surface cracks.

[0082] Third heating and hot rolling After the forging process is completed, the secondary intermediate material can be tertiary heated to a temperature range of 1000~1200℃.

[0083] Titanium (Ti) or niobium (Nb) composite carbonitrides or TiNb (C, N) coarse crystals formed during casting are redissolved, and the secondary intermediate material is heated to and maintained at the recrystallization temperature or higher before hot rolling to homogenize the structure. Tertiary heating can be performed at a temperature range of 1000°C or higher to ensure the rolling end temperature is sufficiently high and minimize the crushing of inclusions during the rolling process.

[0084] On the other hand, if the secondary intermediate material is heated at an excessively high temperature, oxidation scale at high temperatures may become a problem, and the increase in manufacturing costs due to high-temperature heating and holding may become a problem. Therefore, in the present invention, the upper limit of the tertiary heating temperature can be limited to 1200°C.

[0085] The tertiarily heated secondary intermediate material can be hot-rolled at a temperature range of 900 to 1100°C to provide a hot-rolled material having a thickness of 133 to 233 mm. The maximum depth of surface cracks in the hot-rolled material can be 2 μm or less.

[0086] If the finish hot rolling temperature is less than 900°C, the deformation resistance increases excessively as the temperature decreases, making it difficult to sufficiently refine the austenite grains in the center of the product in the thickness direction, which may result in a decrease in the lamellar tear resistance of the final product. On the other hand, if the hot rolling temperature exceeds 1100°C, the austenite grains become excessively coarse, which may result in a decrease in strength and impact toughness. Therefore, the hot rolling temperature is preferably 900 to 1100°C.

[0087] Normalizing heat treatment After the hot rolling is completed, the hot-rolled material can be subjected to a normalizing heat treatment in which the material is heated to a temperature range of 820 to 900°C, held at that temperature for 10 to 40 minutes, and then air-cooled to room temperature.

[0088] If the heating temperature during normalizing is less than 820°C or the holding time is less than 10 minutes, the carbides formed during cooling after rolling and the impurity elements segregated at the grain boundaries do not redissolve smoothly, which can significantly reduce the through-thickness elongation (ZRA) and low-temperature toughness of the steel after heat treatment.On the other hand, if the heating temperature during normalizing is more than 900°C or the holding time is more than 40 minutes, the austenite and precipitate phases such as Nb(C,N) and V(C,N) will coarsen, which can reduce the lamellar tear resistance.

[0089] Post-weld heat treatment (PWHT) After normalizing, the product can be welded and further heat treated (ASME section VIII-Division 1, Table UCS-56) to remove residual stresses. For example, a 180 mm thick steel plate can be subjected to post-weld heat treatment at 635°C for 370 minutes.

[0090] The present invention will be described in more detail below with reference to examples. However, it should be noted that the examples described below are intended to illustrate and further embody the present invention, and are not intended to limit the scope of the present invention.

[0091] (Example) A 700 mm thick cast slab was produced having the alloy composition shown in Table 1. Primary forging, secondary forging, hot rolling, and normalizing heat treatment were performed according to the process conditions shown in Table 2. The primary heating temperature was 1200°C, the secondary heating temperature was 1100°C, and the tertiary heating temperature was 1050°C, and the normalizing time was 30 minutes. The thickness of the primary intermediate material was 550 mm, and the thickness of the secondary intermediate material was 400 mm. Other than the process conditions listed in Table 2, process conditions within the scope of the present invention were applied.

[0092] [Table 1]

[0093] [Table 2]

[0094] The physical properties of each specimen were then measured and are listed in Table 3. The microstructure of each specimen was observed using an SEM, confirming that all specimens had a composite matrix of ferrite and pearlite with an average grain size of 20 μm or less. The fraction of the surface hard structure was determined by revealing the MA in the surface structure specimens using LePera etching and then measuring its size using an automatic image analyzer. The porosity in the core was determined by measuring the density at the center of the specimen. The tensile strength and through-thickness reduction of area (ZRA) of each specimen were also measured using a tensile tester. Additionally, after visually inspecting the surface of each specimen, the specimen was ground at the point where a surface crack had formed. The grinding depth until the crack disappeared was measured as the surface crack depth. VC precipitates were analyzed using a TEM replica, and the VC crystal structure was confirmed by first measuring the diffraction pattern. VC precipitates can be easily found on a TEM image because the (001) plane is parallel to the (001) plane of ferrite and the <0110> direction of the VC precipitates is parallel to the <0100> direction of the ferrite, forming a Baker-Nutting orientation relationship. 2 ×200nm 2 By using multiple images, 2 The number of VC precipitates per sample was counted.

[0095] [Table 3]

[0096] As can be seen from Tables 1 to 3, in the case of Examples 1 to 5, which satisfy the alloy compositions and manufacturing conditions proposed by the present invention, excellent tensile strength, lamellar tear resistance (ZRA quality) and surface quality can be ensured.

[0097] However, in the case of Comparative Examples 1 to 4, although the alloy composition proposed by the present invention is satisfied, the manufacturing conditions are not satisfied, and the type and fraction of the microstructure of the surface layer or the porosity characteristics of the central portion proposed by the present invention are not satisfied, so it can be seen that the strength, ZRA, and surface quality characteristics are at low levels.

[0098] In the case of Comparative Examples 5 to 7, although the manufacturing conditions proposed by the present invention were satisfied, the alloy composition was not satisfied, and the conditions proposed by the present invention, such as the type and fraction of microstructure, and the porosity in the central part, were not satisfied, so the strength, ZRA, and surface quality were at low levels.In the case of Comparative Example 8, the number of VC precipitates proposed by the present invention was not satisfied, so the tensile strength was at a relatively low level.

[0099] Although the present invention has been described in detail with reference to the above-described embodiments, other embodiments are possible, and the technical spirit and scope of the following claims are not limited to the following embodiments.

Claims

1. The alloy contains, by weight, C: 0.2 to 0.3%, 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 0.02%, V: 0.001 to 0.03%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.4%, Ni: 0.05 to 0.4%, Ca: 0.0005 to 0.004%, and the remainder being Fe and other unavoidable impurities, Ceq according to the following relational expression 1 satisfies the range of 0.5 to 0.6, The steel sheet has a composite structure of ferrite and pearlite having an average grain size of 20 μm or less as a matrix structure, and the fraction of hard structures in the surface layer portion, which is the region from the surface to 10 mm in the thickness direction, is 5 area % or less, The porosity of the center part, which is the area of ​​3 / 8t to 5 / 8t (where t means the thickness of the steel material (mm)), is 0.1 mm ３ / g or less, The thickness is 133 to 250 mm, Among the precipitates observed on the cross section of steel after post-weld heat treatment (PWHT) at a temperature of 600 to 700°C for 370 minutes, fine VC precipitates with a diameter of 5 to 15 nm were observed. ２ 1. An extra-thick steel material before welding, characterized in that there are five or more holes per hole. [Relationship 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15 In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] respectively mean the contents (wt%) 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. 2. The extra-thick steel material before welding according to claim 1, wherein the tensile strength of the steel material is 550 to 690 MPa.

3. 2. The extra-heavy steel material before welding according to claim 1, wherein the steel material has a reduction in area (ZRA) in the thickness direction of the steel material of 35% or more.

4. 2. The extra-thick steel material before welding according to claim 1, wherein the maximum depth of the surface cracks of the steel material is 0.1 mm or less (including 0).

5. In weight percent, C: 0.2 to 0.3%, 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 0.02%, V: 0.001 to 0.03%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.4% , Ni: 0.05 to 0.4%, Ca: 0.0005 to 0.004%, the remainder being Fe and other inevitable impurities, Ceq according to the following relational expression 1 satisfies the range of 0.5 to 0.6, the average grain size of prior austenite is 500 μm or less, and a thickness is 650 mm or more; a step of primarily heating the slab at a temperature range of 1100 to 1300°C; a step of performing a primary forging process on the primarily heated slab at a cumulative reduction of 3 to 15% and a deformation rate of 1 / s to 4 / s to provide a primary intermediate material having a thickness of 450 to 550 mm; Secondarily heating the primary intermediate material to a temperature range of 1000 to 1200°C; Secondly forging the second-heated primary intermediate material at a cumulative reduction of 3 to 30% and a deformation rate of 1 / s to 4 / s to provide a secondary intermediate material having a thickness of 300 to 340 mm; thirdly heating the secondary intermediate material to a temperature range of 1000 to 1200°C; hot-rolling the tertiarily heated secondary intermediate material at a temperature range of 900 to 1100°C to provide a hot-rolled material having a thickness of 133 to 250 mm; a normalizing heat treatment step in which the hot-rolled material is heated to a temperature range of 820 to 900 ° C. and held for 10 to 40 minutes, and then air-cooled to room temperature; Including, The porosity of the center of the secondary intermediate material is 0.1 mm ３ 2. The method for manufacturing an extra-heavy-gauge steel material before welding according to claim 1, wherein the thickness of the steel material is 1 / g or less. [Relationship 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15 In the above Relational Formula 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively represent the contents (wt%) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel slab, and when these components are not intentionally added, 0 is substituted.

6. 6. The method for manufacturing an extra-heavy-gauge steel product before welding according to claim 5, wherein the maximum depth of the surface cracks of the hot-rolled material is 2 μm or less (including 0 μm).

7. A step of manufacturing an extra-thick steel material before welding according to claim 5; Welding the extra-thick steel material before welding; and performing a post-weld heat treatment (PWHT) to remove residual stress in the welded steel material.

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