Very thick steel plate for steam drum having excellent surface quality and resistance to lamellar tearing and method of manufacturing the same

Abstract

According to one aspect of the present application, an ultra-heavy steel plate for a steam drum having excellent surface quality and resistance to lamellar tearing and a method of manufacturing the same can be provided.

Description

Technical Field

[0001] This disclosure relates to steels that can be used in petrochemical power generation equipment, boilers, etc., and methods for manufacturing them, and more particularly, to extremely thick steels for steam boiler drums with excellent surface quality and resistance to lamellar tearing, and methods for manufacturing them. Background Technology

[0002] A boiler steam drum used in power generation equipment is a container that separates steam and water by storing steam evaporated from the boiler under certain pressure. Waste heat boilers are widely used to utilize heat generated by chemical or combustion reactions. A steam drum is essential when installing a waste heat boiler. In response to the need to improve the efficiency of steam boilers, the thickness of steel used for large-scale and high-capacity storage purposes is constantly increasing. With the increase in steel thickness, the overall rolling reduction decreases, and therefore the microstructure increases, and defects in the material, such as inclusions or segregations, tend to degrade the material. Therefore, in order to improve the integrity of the steel's internal and external structure, there is a trend to reduce the concentration of impurities such as non-metallic inclusions and segregations, or to control surface and internal cracks and voids to the limit.

[0003] In particular, in the case of ultra-thick materials with a thickness exceeding 100 mmt, compared with thin materials, due to the low rolling reduction rate, the uncured shrinkage cavities generated during continuous casting or casting are not sufficiently compressed during the rough rolling process and remain in the center of the product as residual voids.

[0004] When the microstructure is subjected to thickness and axial stress, these residual voids act as crack initiation points and can eventually damage the entire device in the form of lamellar tearing. Therefore, a process is required in the steps prior to rolling to fully compress the central voids and eliminate residual voids.

[0005] Related patent document 1 describes a technique for applying strong pressure during the roughing process of thick plates. It utilizes a technique to determine the ultimate reduction rate for each thickness at the point of plate seizure by using a reduction rate per pass set close to the design tolerances (load and torque) of the rolling mill. It also describes a technique for distributing the reduction rate by adjusting the index of the thickness ratio per pass to ensure the target thickness of the roughing mill, and a technique for changing the rolling reduction rate based on the ultimate rolling reduction rate for each thickness to prevent plate seizure. Therefore, it provides a manufacturing method capable of applying an average reduction rate of approximately 27.5% in the last three passes of a roughing process based on 80 mmt. However, in the case of this rolling method, the average reduction rate is measured across the entire product thickness, and in the case of ultra-thick materials with a maximum thickness of 233 mmt or greater, applying high strain to the center where residual voids exist is technically difficult.

[0006] Another method for manufacturing ultra-thick materials is to use a forging mill with a higher effective strain rate per pass than a rolling mill. In Patent Document 2, a billet taken from a heating furnace is placed vertically to produce a forging reduction of 400 mm or more across the entire width, and width forging passes are performed in two passes at a reduction rate that serves as the condition for the yielding limit reduction. This provides a method for eliminating porosity at the edges and center in the width direction and increasing core strain. Furthermore, since the residual void in the center (a problem in Patent Document 1) can be effectively compressed, the product's resistance to lamellar tearing can be improved.

[0007] However, surface defects may occur during the width forging process due to localized deformation concentration. In particular, if surface or subsurface defects exist in the cast steel state prior to forging, these defects will propagate during the forging process, and the surface quality may further deteriorate in the finished product state after rolling.

[0008] On the other hand, Patent Document 3 discloses that a thick high-strength steel plate with a yield strength of 620 MPa or greater and a thickness of 100 mmt or more can be manufactured by the following process: heating a material provided with a predetermined alloy composition at 1200°C to 1350°C, hot forging with a cumulative reduction rate of 25% or greater, heating to Ac3 point or higher and 1200°C or lower, hot rolling with a cumulative rolling reduction rate of 40% or greater, reheating to Ac3 point or higher and 1050°C or lower, rapid cooling from a temperature at Ac3 point or higher to a lower temperature of 350°C or lower or Ar3 point or lower, and tempering at a temperature of 450°C to 700°C.

[0009] However, the aforementioned process conditions cannot be readily applied to ultra-high-strength steel plates that are prone to surface cracking during casting due to their high carbon equivalent (Ceq) and hardenability index (DI), or to steels used for steam boiler drums that are also manufactured through normalizing heat treatment. Furthermore, when both carbon equivalent (Ceq) and hardenability index (DI) are high, surface cracks are easily generated in the cast steel during the secondary cooling process of steelmaking due to the formation of hard structures on the surface, and the surface quality of the final product may deteriorate due to crack propagation during the forging process.

[0010] Therefore, a forging method has been proposed to improve the internal integrity of the final product by compressing the central air gap, but no practical method has yet been proposed to ensure both suitable material quality and excellent surface quality of the steel used for steam boiler drums.

[0011] (Existing technical literature)

[0012] (Patent Document 1) Korean Patent Application Publication No. 10-2012-0075246 (published on July 6, 2012)

[0013] (Patent Document 2) Korean Patent Application Publication No. 10-2012-0074039 (published on July 5, 2012)

[0014] (Patent Document 3) Korean Patent Application Publication No. 10-2017-0095307 (published on August 22, 2017) Summary of the Invention

[0015] Technical issues

[0016] One aspect of this disclosure is to provide extremely thick steel for steam boiler drums with excellent surface quality and resistance to lamellar tearing, and a method for manufacturing the same.

[0017] One aspect of this disclosure is not limited to the foregoing. Other subjects of this disclosure will be readily apparent to those skilled in the art from the general content of this specification.

[0018] Technical solution

[0019] According to one aspect of this disclosure, the extra-thick steel comprises, by 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: 0.0 1% to 0.4%, Ni: 0.05% to 0.4%, Ca: 0.0005% to 0.004%, with the balance being Fe and other unavoidable impurities, wherein Ceq according to the following relationship 1 satisfies the range of 0.5 to 0.6, a ferrite and pearlite composite structure with an average grain size of 20 μm or less is included as the base structure, and the hard tissue fraction in the surface layer portion is 5% by area or less, said surface layer portion being a region extending from the surface to 10 mm in the thickness direction, with a porosity of 0.1 mm in the central portion. 3 / g or less, the central portion being a region of 3 / 8t to 5 / 8t (where t is the steel thickness (mm)), and fine VC precipitates with a diameter of 5nm to 15nm per 1μm in the precipitates observed in the steel cross-section after post-weld heat treatment (PWHT). 2 Five or more.

[0020] [Relation 1]

[0021] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0022] In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (by weight%) of C, Mn, Cr, Mo, V, Ni and Cu in the steel, respectively, and 0 is used instead when these components are not intentionally added.

[0023] The thickness of the steel can range from 133mm to 250mm.

[0024] The tensile strength of steel can range from 550 MPa to 690 MPa.

[0025] The sectional reduction of area (ZRA) of steel in the thickness direction can be 35% or greater.

[0026] The maximum surface crack depth of steel can be 0.1 mm or less (including 0).

[0027] According to one aspect of this disclosure, a method of manufacturing extremely thick steel comprises: providing a slab with a thickness of 650 mm or greater, said slab comprising, by 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: 0.00% to 0.02%. 1% 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%, with the balance being Fe and other unavoidable impurities, wherein Ceq according to the following relation 1 satisfies the range of 0.5 to 0.6, the average grain size of the original austenite is 500 μm or less, and the thickness is 650 mm or greater; at 1100 °C to A first heating of the slab at 1300°C is performed; a first forging process is conducted on the first heated slab at a cumulative reduction rate of 3% to 15% and a strain rate of 1 / second to 4 / second to provide a first intermediate material with a thickness of 450 mm to 550 mm; a second heating of the first intermediate material is performed at 1000°C to 1200°C; a second forging process is conducted on the second heated first intermediate material at a cumulative reduction rate of 3% to 30% and a strain rate of 1 / second to 4 / second to provide... A second intermediate material with a thickness of 300 mm to 340 mm; a third heating of the second intermediate material at a temperature of 1000°C to 1200°C; hot rolling of the third-heated second intermediate material at a temperature of 900°C to 1100°C to provide a hot-rolled material with a thickness of 133 mm to 233 mm; and a normalizing heat treatment operation in which the hot-rolled material is heated at a temperature of 820°C to 900°C after hot rolling, held for 10 minutes to 40 minutes, and then air-cooled to room temperature.

[0028] [Relation 1]

[0029] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0030] In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (by weight%) of C, Mn, Cr, Mo, V, Ni and Cu in the steel billet, respectively, and 0 is used instead when these components are not intentionally added.

[0031] The porosity of the central portion of the second intermediate material can be 0.1 mm. 3 / g or less.

[0032] The maximum surface crack depth of hot-rolled steel can be 2 μm or less (including 0).

[0033] It may also include the following operations: welding the normalized steel; and performing additional heat treatment (PWHT) to remove residual stress in the welded steel.

[0034] The means used to solve the above problems do not list all the features of this disclosure, and the various features, advantages and effects of this disclosure will be understood in more detail by referring to the following specific implementation schemes.

[0035] Beneficial effects

[0036] According to one aspect of this disclosure, there is a method for manufacturing extremely thick steel for steam boiler drums with excellent surface quality and resistance to lamellar tearing.

[0037] The effects of this disclosure are not limited to the foregoing, and can be interpreted as including the technical effects that a person skilled in the art could infer from the details described below. Detailed Implementation

[0038] This disclosure relates to extremely thick steel for steam boiler drums with excellent surface quality and resistance to lamellar tearing, and methods for manufacturing the same. Preferred embodiments of this disclosure are described below. Embodiments of this disclosure can be modified in various ways, and the scope of this disclosure should not be construed as limited to the embodiments described below. These embodiments are provided to those skilled in the art to further illustrate this disclosure.

[0039] In the following sections, an extremely thick steel for steam boiler drums with excellent surface quality and resistance to lamellar tearing will be described in more detail based on one aspect of this disclosure.

[0040] The ultra-thick steel according to one aspect of this disclosure comprises, by 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: 0.0 1% to 0.4%, Ni: 0.05% to 0.4%, Ca: 0.0005% to 0.004%, with the balance being Fe and other unavoidable impurities, wherein Ceq according to the following relationship 1 satisfies the range of 0.5 to 0.6, a ferrite and pearlite composite structure with an average grain size of 20 μm or less is included as the base structure, and the hard tissue fraction in the surface layer portion is 5% by area or less, said surface layer portion being a region extending from the surface to 10 mm in the thickness direction, with a porosity of 0.1 mm in the central portion. 3 / g or less, the central portion being a region of 3 / 8t to 5 / 8t (where t is the steel thickness (mm)), and fine VC precipitates with a diameter of 5nm to 15nm per 1μm in the precipitates observed in the steel cross-section after post-weld heat treatment (PWHT). 2 Five or more.

[0041] [Relation 1]

[0042] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0043] In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (by weight%) of C, Mn, Cr, Mo, V, Ni and Cu in the steel, respectively, and 0 is used instead when these components are not intentionally added.

[0044] The alloy composition of this disclosure will be described in more detail below. Unless otherwise stated, all percentages and ppm values ​​in the following descriptions of alloy composition are by weight.

[0045] Carbon (C): 0.20% to 0.30%

[0046] Since carbon (C) is the most important element for ensuring basic strength, it needs to be included in steel within an appropriate range, and 0.20% or more of carbon (C) can be added to obtain this additional effect. Preferably, 0.22% or more of carbon (C) can be added. On the other hand, if the carbon (C) content exceeds a certain level, the pearlite fraction increases during normalizing heat treatment, and the strength and hardness of the base material may be excessively exceeded, leading to surface cracks during the forging process and deteriorating the resistance of the final product to lamellar tearing. Therefore, in this disclosure, the carbon (C) content can be limited to 0.30%, and a more preferred upper limit of 0.26% of the carbon (C) content can be.

[0047] Silicon (Si): 0.05% to 0.50%

[0048] Silicon (Si) is a substitutional element that improves the strength of steel through solid solution strengthening and has a strong deoxidizing effect, thus making it an essential element for the manufacture of clean steel. Therefore, silicon (Si) can be added in amounts of 0.05% or more, more preferably 0.20% or more. On the other hand, with the addition of large amounts of silicon (Si), an MA (martensitic-austenitic) phase is formed and the strength of the ferrite matrix increases excessively, which may degrade the surface quality of ultra-thick products. Therefore, the upper limit of its content can be limited to 0.50%. A more preferred upper limit of silicon (Si) content is 0.40%.

[0049] Manganese (Mn): 1.0% to 2.0%

[0050] Manganese (Mn) is a useful element that improves strength and hardenability through solid solution strengthening, thereby enabling the formation of low-temperature transformation phases. Therefore, to ensure a tensile strength of 550 MPa or greater, it is preferable to add 1.0% or more manganese (Mn). More preferably, the manganese (Mn) content can be 1.1% or more. On the other hand, manganese (Mn) forms MnS (which is a non-metallic inclusion that is elongated together with sulfur (S), thereby reducing toughness and acting as a factor in reducing elongation during stretching in the thickness direction, and thus potentially contributing to a rapid deterioration of resistance to lamellar tearing. Therefore, it is preferable to control the manganese (Mn) content to 2.0% or less, and more preferably, the manganese (Mn) content can be 1.5% or less.

[0051] Aluminum (Al): 0.005% to 0.1%

[0052] Besides silicon (Si), aluminum (Al) is also a strong deoxidizer in the steelmaking process, and it is preferably added in an amount of 0.005% or more to achieve this effect. A more preferred lower limit for the aluminum (Al) content is 0.01%. On the other hand, if the aluminum (Al) content is too high, the Al2O3 fraction in the oxide inclusions generated due to deoxidation increases excessively, and their size becomes coarser. This can lead to difficulties in removing inclusions during refining, which may be a factor reducing resistance to lamellar tearing. Therefore, it is preferable to control the aluminum (Al) content to 0.1% or less. A more preferred aluminum (Al) content is 0.07% or less.

[0053] Phosphorus (P): 0.010% or less (inclusive), Sulfur (S): 0.0015% or less (inclusive)

[0054] Phosphorus (P) and sulfur (S) are elements that cause brittleness at grain boundaries or form coarse inclusions that cause brittleness. Therefore, in order to improve resistance to brittle crack propagation, phosphorus (P) is preferably limited to 0.010% or less, and sulfur (S) is not limited to 0.0015% or less.

[0055] Niobium (Nb): 0.001% to 0.02%

[0056] Niobium (Nb) is an element that precipitates in the form of NbC or NbCN to improve the strength of the base material. Furthermore, niobium (Nb) dissolved during high-temperature reheating precipitates very finely as NbC during rolling to suppress austenite recrystallization, thus refining the microstructure. Therefore, niobium (Nb) is preferably added in an amount of 0.001% or more, and more preferably, the niobium (Nb) content can be 0.005% or more. On the other hand, if niobium (Nb) is added in excess, undissolved niobium (Nb) is produced in the form of TiNb(C,N) and becomes a factor suppressing resistance to lamellar tearing. Therefore, it is preferable to limit the upper limit of the niobium (Nb) content to 0.02%. More preferably, the niobium (Nb) content can be 0.017% or less.

[0057] Vanadium (V): 0.001% to 0.03%

[0058] Since almost all of the vanadium (V) is redissolved during reheating, the strengthening effect through precipitation or solidification during subsequent rolling is not significant. However, in subsequent heat treatment processes such as PWHT, it precipitates as very fine carbonitrides and has the effect of improving strength. To fully obtain these effects, 0.001% or more of vanadium (V) needs to be added. A more preferred lower limit for the vanadium (V) content can be 0.01%. On the other hand, if the content is too high, the strength and hardness of the base material and weld zone will be excessively increased, which may act as a factor for surface cracking, for example, during the processing of the steam boiler drum, and is not commercially beneficial because manufacturing costs may rise rapidly. Therefore, the vanadium (V) content can be limited to 0.03% or less. A more preferred vanadium (V) content can be 0.02% or less.

[0059] Titanium (Ti): 0.001% to 0.03%

[0060] Titanium (Ti) precipitates as TiN during reheating and inhibits grain growth in the heat-affected zone of the base material and weld, and is a component that greatly improves low-temperature toughness. To achieve this effect, it is preferable to add 0.001% or more of titanium (Ti). On the other hand, if excessive titanium (Ti) is added, the low-temperature toughness may be reduced due to continuous casting nozzle blockage or central crystallization. Furthermore, since titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates in the center of the thickness, thereby reducing the elongation of the product, the final material's resistance to lamellar tearing may deteriorate. Therefore, the titanium (Ti) content can be 0.03% or less. A preferred titanium (Ti) content can be 0.025% or less, and a more preferred titanium (Ti) content can be 0.018% or less.

[0061] Chromium (Cr): 0.01% to 0.30%

[0062] Chromium (Cr) is a component that increases yield strength and tensile strength by forming a low-temperature transformation structure through improved hardenability. Furthermore, it is also effective in preventing strength reduction by slowing the decomposition rate of cementite during tempering after rapid cooling or during heat treatment after welding. For this effect, 0.01% or more of chromium (Cr) can be added. On the other hand, if the chromium (Cr) content is too high, it will result in the formation of Cr-rich coarse carbides such as M... 23 As the size and fraction of C6 and other elements increase, the impact toughness of the product may decrease, and the reduction in product strength may be problematic due to the decreased solid solubility of niobium (Nb) and the reduced fraction of fine precipitates such as NbC in the product. Therefore, in this disclosure, 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%.

[0063] Molybdenum (Mo): 0.01% to 0.12%

[0064] Molybdenum (Mo) is an element that improves grain boundary strength and has a high solid solution strengthening effect in ferrite, effectively contributing to the improvement of product strength and ductility. Furthermore, Molybdenum (Mo) also has the effect of preventing toughness degradation due to grain boundary segregation of impurity elements such as phosphorus (P). For this effect, 0.10% or more of Molybdenum (Mo) can be added. However, since Molybdenum (Mo) is an expensive element, excessive addition can significantly increase manufacturing costs; therefore, the upper limit of Molybdenum (Mo) content can be limited to 0.12%.

[0065] Copper (Cu): 0.01% to 0.40%

[0066] Copper (Cu) is an advantageous element in this disclosure because it can significantly improve the strength of the matrix phase through solid solution strengthening in ferrite and also has the effect of inhibiting corrosion in a moist hydrogen sulfide atmosphere. For this effect, 0.01% or more of copper (Cu) may be included. A more preferred copper (Cu) content may be 0.03% or more. However, if the copper (Cu) content is too high, the likelihood of star-shaped cracks forming on the surface of the steel sheet increases, and since copper (Cu) is an expensive element, there may be a problem of significantly increased manufacturing costs. Therefore, in this disclosure, the upper limit of the copper (Cu) content can be limited to 0.40%. A preferred upper limit of the copper (Cu) content may be 0.35%.

[0067] Nickel (Ni): 0.05% to 0.40%

[0068] Nickel (Ni) is an element that effectively improves impact toughness and hardenability to increase strength by increasing stacking faults at low temperatures to promote cross-slip of dislocations. For this effect, 0.05% or more of nickel (Ni) can be added. A preferred nickel (Ni) content is 0.10% or more. On the other hand, if excessive nickel (Ni) is added, manufacturing costs may increase due to high costs, and therefore the upper limit of the nickel (Ni) content can be limited to 0.40%. A preferred upper limit of the nickel (Ni) content is 0.35%.

[0069] Calcium (Ca): 0.0005% to 0.0040%,

[0070] When calcium (Ca) is added after deoxidation with aluminum (Al), the calcium (Ca) combines with sulfur (S) to form MnS inclusions, thereby inhibiting MnS formation. Simultaneously, it suppresses the formation of hydrogen-induced cracks by forming spherical CaS. To fully form sulfur (S)CaS contained as an impurity, it is preferable to add 0.0005% or more of calcium (Ca). However, if too much is added, the remaining calcium (Ca) after CaS formation combines with oxygen (O) to form coarse oxide inclusions, which are elongated and destroyed during rolling, potentially contributing to deterioration of resistance to lamellar tearing. Therefore, the upper limit of the calcium (Ca) content can be limited to 0.0040%.

[0071] In addition to the components described above, the extremely thick steel used for steam boiler drums in this disclosure may also contain iron and other unavoidable impurities in the balance. However, since unintended impurities from raw materials or the surrounding environment may inevitably mix in during normal manufacturing processes, they cannot be completely excluded. Since these impurities are known to those skilled in the art, not all of them are specifically mentioned in this specification. Furthermore, the addition of effective components other than those described above is not entirely excluded.

[0072] In the extremely thick steel for steam boiler drums according to one aspect of this disclosure, Ceq can satisfy the range of 0.5 to 0.6 according to the following relation 1.

[0073] [Relation 1]

[0074] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0075] In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] respectively refer to the content (by weight%) of C, Mn, Cr, Mo, V, Ni, and Cu in the steel, and 0 is used instead if these components are not intentionally added.

[0076] Since the thickness of the extremely thick steel used for the steam boiler drum according to one aspect of this disclosure is 133 mm to 250 mm, it can effectively respond to the trend of increasing the size of the steam boiler drum.

[0077] According to one aspect of this disclosure, the surface layer of the extremely thick steel for a steam boiler drum can be formed of a ferrite and pearlite composite structure with an average grain size of 20 μm or less. Since the extremely thick steel for a steam boiler drum according to one aspect of this disclosure restricts the introduction of hard structures into the surface layer of the steel, the maximum surface crack depth of the final product can be suppressed to 0.1 mm or less. That is, in the extremely thick steel for a steam boiler drum according to one aspect of this disclosure, the formation of hard structures such as martensite, bainite, etc., on the surface layer of the steel is actively suppressed, and even if the formation of these hard materials is unavoidable, their fraction can be actively suppressed to 5% area or less (including 0%). Preferably, the hard structure fraction of the surface layer of the steel can be 3% or less (including 0%). In this case, the surface layer of the steel can refer to the region from the surface of the steel to 10 mm in the thickness direction.

[0078] When observing the cross-section of steel that has undergone post-weld heat treatment (PWHT), according to one aspect of this disclosure, the extremely thick steel for steam boiler drums can contain 1 μm of material per 1 μm. 2 At least five or more fine VC precipitates with a diameter of 5 nm to 15 nm are present. VC forms as carbides or carbonitrides in the temperature range of 600°C to 700°C, thereby causing precipitation hardening. Therefore, in this disclosure, even after heat treatment of the sample at high temperatures, a suitable strength of 550 MPa or greater can be maintained.

[0079] In the extremely thick steel for a steam boiler drum according to one aspect of this disclosure, the porosity at the center of the steel can be 0.1 mm. 3 / g or less. Therefore, the extremely thick steel used for steam boiler drums according to one aspect of this disclosure can effectively ensure resistance to lamellar tearing. In this case, the steel center refers to 3 / 8t to 5 / 8t (t: steel thickness, mm), and the center porosity can be determined by measuring the density and taking its reciprocal.

[0080] The ultra-thick steel for steam boiler drums according to one aspect of this disclosure can have a tensile strength of 550 MPa to 690 MPa and a reduction of area (ZRA) of 35% or greater in the thickness direction. Furthermore, the ultra-thick steel for steam boiler drums according to one aspect of this disclosure can have a maximum surface crack depth of 0.1 mm or less in the final product state. In this case, the depth of the surface crack can be determined by visually identifying the presence of the surface crack, then grinding at the corresponding point where the crack exists until the crack disappears, and by measuring the depth from the surface layer to the location removed by grinding.

[0081] The method of manufacturing extremely thick steel for steam boiler drums according to one aspect of this disclosure will be described in more detail below.

[0082] The extra-thick steel for steam boiler drums according to one aspect of this disclosure can be manufactured by providing a slab with a thickness of 650 mm or greater, said slab comprising, by 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%, with the balance being Fe and other unavoidable impurities, wherein Ceq according to Equation 1 below satisfies a range of 0.5 to 0.6, the average grain size of the original austenite is 500 μm or less, and the thickness is 650 mm or greater; in 1 A first heating of the slab is performed at a temperature of 100°C to 1300°C; a first forging process is performed on the first heated slab at a cumulative reduction rate of 3% to 15% and a strain rate of 1 / second to 4 / second to provide a first intermediate material with a thickness of 450 mm to 550 mm; the first intermediate material is then subjected to a second heating at a temperature of 1000°C to 1200°C; and the second heating of the first intermediate material is then subjected to a second forging process at a cumulative reduction rate of 3% to 30% and a strain rate of 1 / second to 4 / second to provide a first intermediate material with a thickness of 450 mm to 550 mm. Provide a second intermediate material with a thickness of 300 mm to 340 mm; subject the second intermediate material to a third heating at a temperature of 1000°C to 1200°C; provide a hot-rolled material with a thickness of 133 mm to 233 mm by hot rolling the third-heated second intermediate material at a temperature of 900°C to 1100°C; and perform a normalizing heat treatment operation after hot rolling by heating the hot-rolled material at a temperature of 820°C to 900°C for 10 minutes to 40 minutes and then air cooling to room temperature.

[0083] [Relation 1]

[0084] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0085] In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (by weight%) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel billet, respectively, and are replaced by 0 when these components are not intentionally added.

[0086] Slab preparation

[0087] The inventors of this disclosure have conducted in-depth research on methods for manufacturing extremely thick steel with both physical properties suitable for steam boiler drums and excellent surface quality. In particular, in manufacturing slabs with a thickness of 650 mm or greater, in order to ensure the strength and surface quality of the final steel, the carbon equivalent (Ceq) of the slab should be controlled within a certain range, and the original austenite grain size of the slab should also be considered as an effective condition for obtaining this disclosure.

[0088] Since the slabs of this disclosure are provided with an alloy composition corresponding to the aforementioned steel, the description of the alloy composition of the aforementioned steel is used instead of the description of the alloy composition of the slab. The alloy composition of the slabs used in this disclosure corresponds to the necessary conditions to ensure a tensile strength of 550 MPa to 690 MPa and a reduction of area (ZRA) of 35% or greater.

[0089] Because the casting speed of a single-sided casting machine used to produce slabs with a thickness of 650 mm or greater is 0.06 m / min to 0.1 m / min, casting is carried out at a significantly lower speed than that of a typical casting machine (casting speed: 0.4 m / min to 1.5 m / min) used to produce slabs with a thickness of 250 mm to 400 mm. Therefore, when producing slabs with a thickness of 650 mm or greater, the austenite is placed in an environment where it can grow coarser due to the relatively long holding time in the mold.

[0090] As the initial austenite grain size increases, the manganese (Mn) segregation index at the austenite grain boundaries increases. Furthermore, due to decreased grain boundary strength and increased hardenability, the fraction of hardened bainite and martensite, rather than softer ferrite and pearlite, increases in the surface layer of the slab. Because hard structures have low uniform elongation, intergranular cracking may easily occur when hot deformation, external deformation, or stress is applied. Therefore, when the initial austenite grain size in the surface layer of the slab is large, intergranular cracking on the slab surface may occur more actively, and the crack ingress depth may further increase in subsequent high-deformation processes such as forging and rolling. Therefore, controlling the initial austenite grain size to an appropriate level or smaller is crucial to suppress surface cracking in the final product.

[0091] The average grain size of the original austenite in the slab can be derived from the following Equation 2, and in this disclosure, intergranular cracking can be effectively suppressed by limiting the average grain size of the original austenite in the slab to 500 μm or less. A preferred average grain size of the original austenite in the slab is 400 μm or less, and a more preferred average grain size is 350 μm or less.

[0092] [Relationship 2]

[0093] D (original austenite grain size of the slab after casting) = 3600*exp{-(89098+3581*[C]+1211*[Ni]+1443*[Cr]+4043*[Mo]) / (RT)}*t 0.18

[0094] In Equation 2 above, [C], [Ni], [Cr], and [Mo] refer to the contents (wt%) of C, Ni, Cr, and Mo in the steel billet, respectively, and R is 8.314 J / mol / K. T refers to the casting temperature (K) and t refers to the casting time (seconds).

[0095] As a method for reducing the grain size of the original austenite, there are methods that highly design the composition of carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) to have solute dragging or pinning effects. However, with the increase of these carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) compositions, the carbon equivalent (Ceq) also increases, and a low-temperature transformation structure may occur during the cooling process of the slab. Therefore, in this disclosure, the carbon equivalent (Ceq) of the billet according to the following relationship 1 can be limited to 0.6 or less. A preferred carbon equivalent (Ceq) can be 0.5 to 0.6.

[0096] [Relation 1]

[0097] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15

[0098] In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] respectively refer to the content (by weight%) of C, Mn, Cr, Mo, V, Ni and Cu in the steel billet, and 0 is used instead when these components are not intentionally added.

[0099] First heating of slab

[0100] The prepared slab can be heated in a temperature range of 1100°C to 1300°C. As mentioned above, the thickness of the slab can be 650 mm or more, and preferably 700 mm or more.

[0101] To redissolve the titanium (Ti) or niobium (Nb) composite carbonitrides or coarse crystals of TiNb(C,N) formed during casting, the slab needs to be heated to a temperature range above a certain threshold. Furthermore, the microstructure is homogenized by heating the slab to or above the recrystallization temperature before the first forging and maintaining this temperature. It is preferable to heat the slab to a temperature range above a certain threshold to ensure a sufficiently high forging end temperature, thereby minimizing surface layer cracks that may occur during the forging process. Therefore, the first heating of the slab in this disclosure is preferably performed at a temperature range of 1100°C or higher.

[0102] On the other hand, if the slab heating temperature is too high, excessive high-temperature oxide scale may be generated, and the increase in manufacturing costs due to high-temperature heating and holding may be excessive. Therefore, the first heating of the slab in this disclosure is preferably carried out in the range of 1300°C or lower.

[0103] First Forging

[0104] The first intermediate material can be provided by first forging a first heated slab at a cumulative reduction rate of 3% to 15% and a strain rate of 1 / second to 4 / second. In this case, 1 / second means that the cross-section is 100% deformed per second.

[0105] The first forging is the operation of forging a heated slab, which has undergone a first heating process, to a thickness of 450 mm to 550 mm and then machining it to the width of the final second intermediate material. Since low-speed forging with high strain is essential for sufficient compression of the voids, the first forging can be performed under conditions of a cumulative reduction rate of 3% to 15% and a strain rate of 1 / second to 4 / second.

[0106] If the cumulative reduction rate of the first forging is less than 3%, the remaining voids in the slab cannot be sufficiently compressed, resulting in residual voids and potentially deteriorating the final product's resistance to lamellar tearing. A preferred cumulative reduction rate for the first forging can be 5% or greater, and a more preferred cumulative reduction rate can be 7% or greater. However, when the cumulative reduction rate (not offset by recrystallization) exceeds 15% at the dislocation density recovery or non-recrystallization temperature, the uniform elongation of the surface is significantly reduced due to work hardening of overlapping dislocations, and surface cracks may occur during forging. A preferred cumulative reduction rate for the first forging can be 13% or less, and a more preferred cumulative reduction rate can be 11% or less.

[0107] Second heating and second forging

[0108] The first intermediate material is subjected to a second heating in the temperature range of 1000°C to 1200°C, and a second forging process is performed at a cumulative reduction rate of 3% to 30% and a strain rate of 1 / s to 4 / s to obtain a second intermediate material with a thickness of 300 mm to 340 mm. The maximum surface crack depth of the second intermediate material can be 5 μm or less.

[0109] The second forging involves heating the first intermediate material to a temperature range of 1000°C to 1200°C and forging it to obtain the desired thickness and length of the final second intermediate material. Similar to the first forging, high-strain, low-speed forging is essential in the second forging to sufficiently reduce the central porosity of the second intermediate material. Therefore, the second forging can be performed by applying a cumulative reduction rate of 3% to 30% and a strain rate of 1 / s to 4 / s. The central porosity of the second intermediate material can be 0.1 mm. 3 / g or less.

[0110] If the cumulative reduction rate in the second forging is insufficient, the remaining micro-voids after the first forging may not be fully compressed. Furthermore, when strain is applied to the endpoints of the elliptical-shaped voids, the physical properties may be inferior to those of the circular void form due to the notch effect. Therefore, sufficient compression of the voids is required during the second forging with a cumulative reduction rate of 3% or greater. However, if the cumulative reduction rate is too high, surface cracks may occur due to work hardening; therefore, the upper limit of the cumulative reduction rate can be limited to 30%.

[0111] Similar to the first forging, the strain rate for the second forging can range from 1 / s to 4 / s. At strain rates less than 1 / s, the finishing forging temperature decreases, and surface cracks may occur. On the other hand, applying a high strain rate exceeding 4 / s in the non-recrystallization region can lead to reduced elongation and surface cracking.

[0112] Third heating and hot rolling

[0113] The second intermediate material after forging can be heated a third time within a temperature range of 1000℃ to 1200℃.

[0114] The composite carbonitrides of titanium (Ti) or niobium (Nb) formed during casting, or the coarse crystals of TiNb(C,N), are re-dissolved, and the microstructure is homogenized by heating a second intermediate material to the recrystallization temperature or higher and holding it before hot rolling. A third heating can be performed at a temperature range of 1000°C or higher to minimize the breakage of inclusions during rolling by ensuring a sufficiently high rolling end temperature.

[0115] On the other hand, if the second intermediate material is heated to an excessively high temperature, the oxide scale at high temperatures may be problematic, and the increased manufacturing costs due to high-temperature heating and holding may also be a problem. Therefore, in this disclosure, the upper limit of the third heating temperature may be limited to 1200°C.

[0116] Hot-rolled materials with thicknesses from 133 mm to 233 mm can be provided by hot rolling a second intermediate material that has undergone a third heating process within a temperature range of 900°C to 1100°C. The maximum surface crack depth of hot-rolled steel can be 2 μm or less.

[0117] If the hot rolling temperature is below 900°C, the deformation resistance increases excessively as the temperature decreases, making it difficult to sufficiently refine the central austenite grains in the thickness direction of the product. This may degrade the final product's resistance to lamellar tearing. On the other hand, if the hot rolling temperature exceeds 1100°C, there is a concern that the strength and impact toughness may deteriorate due to the excessively coarse austenite grains. Therefore, the preferred hot rolling temperature is between 900°C and 1100°C.

[0118] Normalizing heat treatment

[0119] Normalizing heat treatment can be performed by heating the hot-rolled steel to a temperature range of 820°C to 900°C after hot rolling, holding it for 10 to 40 minutes, and then air cooling to room temperature.

[0120] During normalizing heat treatment, if the heating temperature is below 820°C or the holding time is less than 10 minutes, the thickness elongation (ZRA) and low-temperature toughness of the steel may be significantly reduced because carbides generated during cooling after rolling or impurity elements segregated at grain boundaries do not dissolve smoothly. On the other hand, in the case of normalizing heat treatment, if the heating temperature exceeds 900°C or the holding time exceeds 40 minutes, the resistance to lamellar tearing may deteriorate due to the coarsening of austenite and precipitates such as Nb(C,N) and V(C,N).

[0121] Post-weld heat treatment (PWHT)

[0122] In post-weld heat treatment, additional heat treatment (ASME Section VIII-Part 1, Table UCS-56) can be performed to normalize the weld and remove residual stress. As an example, for steel with a thickness of 180 mm, post-weld heat treatment can be performed at 635°C for 370 minutes.

[0123] Invention Embodiments

[0124] The present disclosure will be described in more detail below by way of examples. However, it should be noted that the examples described below are for illustrative purposes only and are not intended to limit the scope of the disclosure.

[0125] (Example)

[0126] Cast steel with a thickness of 700 mm is manufactured, having the alloy composition shown in Table 1. First forging, second forging, hot rolling, and normalizing heat treatment are performed according to the process conditions in Table 2. Typically, a first heating temperature of 1200°C, a second heating temperature of 1100°C, and a third heating temperature of 1050°C are applied, and a normalizing time of 30 minutes is typically used. For the thickness of the first intermediate material, a condition of 550 mm is applied, and for the thickness of the second intermediate material, a condition of 400 mm is applied. In addition to the process conditions described in Table 2, process conditions that satisfy the scope of this disclosure are also applied.

[0127] [Table 1]

[0128]

[0129] [Table 2]

[0130]

[0131] Subsequently, the mechanical properties of each sample were measured and listed in Table 3. The microstructure of each sample was observed using SEM, and it was determined that all samples possessed a ferrite and pearlite composite microstructure with an average grain size of 20 μm or less as the base microstructure. For the hard microstructure portion of the surface layer, the MA was exposed from the surface layer microstructure sample by LePera etching, and then the dimensions were measured using an automated image analyzer. The central porosity was determined by measuring the density at the center of the sample. Furthermore, the tensile strength and reduction of area (ZRA) in the thickness direction of each sample were measured using a tensile testing machine. Additionally, after visually observing the surface of each sample, the points where surface cracks formed were ground, and the grinding depth until the cracks disappeared was measured as the surface crack depth. TEM replicas were used to analyze the VC precipitates, and the crystal structure of VC was determined by first measuring the diffraction patterns. Since the (001) plane in the VC precipitate is parallel to the (001) plane of the ferrite, the

[110] direction of the VC precipitate forms a Baker-Nutting orientation parallel to the

[100] direction of the ferrite, which can be easily observed in TEM images. For statistical processing, several 200 nm images were used. 2 ×200nm 2 To calculate per 1μm image 2 The amount of VC precipitates.

[0132] [Table 3]

[0133]

[0134] As can be seen from Tables 1 to 3, in the cases of Examples 1 to 5 of the Invention, the alloy composition and manufacturing conditions proposed in this disclosure are satisfied, and it can be seen that excellent tensile strength, resistance to lamellar tearing (ZRA quality) and surface quality can be ensured.

[0135] However, in Comparative Examples 1 to 4, the alloy composition meets the requirements of this disclosure, but the manufacturing conditions are not met. It can be seen that the strength, ZRA, and surface quality characteristics are low because the type and fraction of the surface layer microstructure or the central porosity characteristics do not meet the requirements of this disclosure.

[0136] In Comparative Examples 5 to 7, since the manufacturing conditions proposed in this disclosure are met, but the alloy composition is not, it can be seen that the strength, ZRA, and surface quality are low because the conditions proposed in this disclosure, such as the type and fraction of microstructure and the porosity of the center, are not met. In Comparative Example 8, since the amount of VC precipitates proposed in this disclosure is not met, it can be seen that the tensile strength is relatively low.

[0137] While the present disclosure has been described in detail through the above embodiments, other types of implementations are also possible. Therefore, the spirit and scope of the appended claims are not limited to the described embodiments.

Claims

1. A type of steel, comprising, by 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%, balance Fe and other unavoidable impurities. Wherein, Ceq according to the following relation 1 satisfies the range of 0.5 to 0.6; The microstructure comprises a ferrite and pearlite composite microstructure with an average grain size of 20 μm or less, forming the base microstructure, and a surface layer portion comprising a hard microstructure fraction of 5% by area or less, wherein the surface layer portion is a region extending from the surface to 10 mm in the thickness direction. The porosity of the central part is 0.1 mm. 3 / g or less, the central portion being a region of 3 / 8t to 5 / 8t, where t is the steel thickness in mm. And among the precipitates observed in the steel cross-section after post-weld heat treatment (PWHT), fine VC precipitates with a diameter of 5 nm to 15 nm were found per 1 μm. 2 5 or more; [Relation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15, in, In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the content of C, Mn, Cr, Mo, V, Ni and Cu in the steel by weight % respectively, and 0 is used instead when these components are not intentionally added.

2. The steel according to claim 1, wherein the thickness of the steel is from 133 mm to 250 mm.

3. The steel according to claim 1, wherein the tensile strength of the steel is from 550 MPa to 690 MPa.

4. The steel according to claim 1, wherein the reduction of area (ZRA) of the steel in the thickness direction is 35% or greater.

5. The steel according to claim 1, wherein the surface crack depth of the steel is 0 mm to 0.1 mm.

6. A method for manufacturing steel, comprising: Slabs with a thickness of 650 mm or greater are provided for use, the slabs comprising, by 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%, with the balance being Fe and other unavoidable impurities, wherein Ceq according to the following relation 1 satisfies the range of 0.5 to 0.6, the average grain size of the original austenite is 500 μm or less, and the thickness is 650 mm or greater; The slab is first heated at a temperature of 1100°C to 1300°C; A first intermediate material with a thickness of 450 mm to 550 mm is provided by first forging a first heated slab with a cumulative reduction rate of 3% to 15% and a strain rate of 1 / s to 4 / s. The first intermediate material is subjected to a second heating at a temperature of 1000°C to 1200°C; A second intermediate material with a thickness of 300 mm to 340 mm is provided by second forging the first intermediate material after second heating at a cumulative reduction rate of 3% to 30% and a strain rate of 1 / s to 4 / s. The second intermediate material is subjected to a third heating at a temperature of 1000°C to 1200°C; Hot-rolled materials with a thickness of 133 mm to 233 mm are provided by hot rolling a second intermediate material that has undergone a third heating process at a temperature of 900°C to 1100°C; and After hot rolling is completed, the hot-rolled material is heated at a temperature of 820°C to 900°C and held for 10 to 40 minutes, and then air-cooled to room temperature in a normalizing heat treatment operation. [Relation 1] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15, In Equation 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the weight percentage of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel billet, respectively, and are replaced by 0 when these components are not intentionally added.

7. The method for manufacturing steel according to claim 6, wherein the porosity of the central portion of the second intermediate material is 0.1 mm. 3 / g or less.

8. The method for manufacturing steel according to claim 6, wherein the surface crack depth of the hot-rolled steel is 0 μm to 2 μm.

9. The method for manufacturing steel according to claim 6, further comprising: Welding steel that has undergone normalizing heat treatment; as well as Further post-weld heat treatment (PWHT) is performed to remove residual stress in the welded steel.

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