Hot-rolled steel sheet for tube of hyper-tube train and method for manufacturing same

Abstract

The present invention relates to a steel material suitable as a material for a vacuum tube and a manufacturing method therefor. A steel material for a vacuum tube according to an aspect of the present invention comprises: 0.03-0.11% of C, 0.5-1.5% of Si, 1.2-2.2% of Mn, 0-0.01% of Ti, 0-0.05% of Nb, 0-0.2% of V, and the balance being Fe and inevitable impurities, and has a microstructure composed of ferrite and pearlite, wherein the ASTM grain size number of ferrite is 9-12.

Description

Hot-rolled steel sheet for vacuum train tubes and method of manufacturing the same

[0001] The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same, and more specifically, to a hot-rolled steel sheet having properties suitable for vacuum train tubes, such as excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the weldment, and a method for manufacturing the same.

[0002] A vacuum train, also known as a hypertube train, is a system in which a magnetic levitation train moves inside a vacuum tube. Because vacuum trains eliminate friction with air or tracks—which are major causes of energy loss during operation—they are capable of ultra-high-speed operation. Due to minimal energy loss, they can achieve 93% energy savings compared to aircraft, making them a highly regarded eco-friendly next-generation mode of transportation, and active research is being conducted worldwide.

[0003] The structure and materials of vacuum tubes used in high-speed vacuum trains affect the system's performance and cost. Currently, there are roughly three main materials being researched for vacuum train tubes. One is concrete. While concrete tubes are advantageous in terms of cost, joining individual tubes of approximately 10 meters in length is not easy. Additionally, due to the pores within the concrete, external gases can intrude into the tube when a vacuum is created, causing the vacuum level to easily break. Another material undergoing extensive research is composite materials, such as carbon fiber. While composite materials like carbon fiber are lightweight and high-performance, their high cost is considered their biggest drawback.

[0004] Currently, steel is the most promising material for vacuum train tubes. Steel is a material that can be mass-produced at a low cost. It possesses high rigidity and strength and is easy to process. Furthermore, it is a material that allows for easy assembly or welding of fittings between or within tubes, and it also offers an appropriate degassing rate when maintaining a vacuum. However, since ultra-high-speed vacuum trains operate at significantly higher speeds than current high-speed trains, the safety of passengers and surrounding facilities must be the top priority. Currently, safety standards for ultra-high-speed vacuum trains have not even been established, and the development of tube materials to ensure their safety is also insufficient.

[0005] Therefore, there is an urgent need to develop a material for vacuum train tubes that possesses processability and degassing rates suitable for vacuum train tubes while ensuring safety.

[0006] According to one aspect of the present invention, a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the weldment, and a method for manufacturing the same can be provided.

[0007] The problems of the present invention are not limited to those described above. A person skilled in the art will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0008] A hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.03~0.11%, silicon (Si): 0.5~1.5%, manganese (Mn): 1.2~2.2%, titanium (Ti): 0~0.1%, niobium (Nb): 0~0.05%, vanadium (V): 0~0.2%, and the remainder being Fe and unavoidable impurities, wherein the sum of the contents of titanium (Ti), niobium (Nb), and vanadium (V) is greater than 0 and is 0.35%, and the microstructure comprises a ferrite and pearlite composite structure, satisfying the following equations 1 and 2.

[0009] Equation 1: 355 ≤ -169.8 + 27.3*[ASTM#] + 264.3*[C] + 113.7*[Si] + 44.7*[Mn] + 3429.4*[Nb] + 1237*[Ti] + 611*[V]

[0010] Equation 2: 0.05 ≤ 3.3*[Ti] + 10*[Nb] + 1.6*[V] ≤ 0.5

[0011] (Here, ASTM# refers to the ASTM grain size number, and [C], [Si], [Mn], [Ti], [Nb], and [V] respectively represent the content (weight%) of carbon (C), silicon (Si), manganese (Mn), titanium (Ti), niobium (Nb), and vanadium (V) contained in the hot-rolled steel sheet.)

[0012] In addition, a hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention may have a microstructure comprising 60 to 95 area percent ferrite, 5 to 40 area percent pearlite, and the remainder of the microstructure.

[0013] In addition, in one embodiment of the present invention, the hot-rolled steel sheet for vacuum train tubes may have an ASTM grain size number of 9 to 12 for the ferrite.

[0014] In addition, in one embodiment of the present invention, the hot-rolled steel sheet for vacuum train tubes may have a room temperature yield strength of 350 MPa or more.

[0015] In addition, in one embodiment of the present invention, the hot-rolled steel sheet for a vacuum train tube may have a Charpy impact energy of 50J or more at -20℃.

[0016] In addition, a hot-rolled steel plate for a vacuum train tube according to one embodiment of the present invention has a vibration damping ratio of 100*10 measured at a frequency of 1650 Hz in the flexural vibration mode of the hot-rolled steel plate. -6 It could be more than that.

[0017] In addition, in one embodiment of the present invention, a hot-rolled steel plate for a vacuum train tube may have a Charpy impact energy of 50 J or more at -20°C in a weld formed by welding the hot-rolled steel plate by submerged arc welding, and the fraction of a martensite-austenite composite included in the weld may be 5% or less.

[0018] In addition, in one embodiment of the present invention, the hot-rolled steel plate for a vacuum train tube may have a thickness of 10 mm or more.

[0019] A method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to another embodiment of the present invention comprises the steps of: heating a slab containing, in weight percent, carbon (C): 0.03~0.11%, silicon (Si): 0.5~1.5%, manganese (Mn): 1.2~2.2%, titanium (Ti): 0~0.1%, niobium (Nb): 0~0.05, vanadium (V): 0~0.2%, and the remainder being Fe and unavoidable impurities, at a heating temperature of 1100°C to 1300°C; hot-rolling the heated slab at a finishing rolling temperature of 860°C to 960°C to provide a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a coiling temperature of 600°C to 700°C.

[0020] In addition, in one embodiment of the present invention, a method for manufacturing a hot-rolled steel sheet for a vacuum train tube comprises, after the coiling step, having a room temperature yield strength of the hot-rolled steel sheet of 350 MPa or more, a Charpy impact energy of the hot-rolled steel sheet at -20°C of 50 J or more, and a vibration damping ratio of 100 * 10 measured at a frequency of 1650 Hz in a flexural vibration mode. -6 It can be controlled beyond this.

[0021] According to one aspect of the present invention, a hot-rolled steel sheet having properties suitable for vacuum train tubes, such as excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the weldment, and a method for manufacturing the same may be provided.

[0022] The effects of the present invention are not limited to the matters described above and may be interpreted to include matters that a person skilled in the art can reasonably infer from the matters described in this specification.

[0023] Figure 1 is an optical microscope image used to observe the microstructure of specimen No. 1.

[0024] Figure 2 is an optical microscope image of EN-S355, a conventional structural steel.

[0025] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0026] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0027] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0028] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0029] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0030] Vacuum trains are next-generation modes of transportation currently in the early stages of development, running inside tubes in a vacuum or sub-vacuum state. By eliminating frictional resistance between wheels and tracks and minimizing air resistance, vacuum trains are a means of transportation capable of effectively achieving high speeds and high efficiency. However, due to the nature of vacuum trains operating at ultra-high speeds, there is a risk of major accidents if their safety is not sufficiently ensured. In particular, since massive disasters can occur not only when the vacuum tube is structurally damaged or collapses, but also when deformation occurs in parts of the tube's shape, materials for vacuum train tubes require even stricter safety standards. To ensure the safety of vacuum trains, the following physical properties are critical for vacuum tube materials.

[0031] The first physical property required for materials used in vacuum tubes is high strength. Since vacuum trains travel through the interior of vacuum tubes, the materials used in vacuum tubes are required to have sufficient strength as a structural component. Additionally, since the interior of the vacuum tube must be maintained in a vacuum or near-vacuum state, it is required to have sufficient high strength to prevent deformation of the tube's shape due to pressure differences between the interior and exterior.

[0032] The second physical property required for materials used in vacuum tubes is vibration damping capability. In vacuum trains, pods carrying several to tens of passengers pass through the interior of the vacuum tube at intervals ranging from tens of seconds to minutes. When a subsequent pod passes after a preceding pod has passed, vibrations within the vacuum tube can be amplified, potentially causing resonance; in severe cases, this can even lead to tube failure. Therefore, applying a material with a vibration damping ratio exceeding a certain level to the vacuum tube can effectively reduce vibrations within the tube after the passage of the preceding pod, thereby effectively contributing to the safety of the vacuum train.

[0033] The third physical property required for vacuum tube materials is low-temperature toughness. Vacuum trains can operate in polar regions or deep seas. Since steel materials tend to fracture more easily in low or cryogenic environments, when steel is applied to vacuum tubes, it is required to possess a certain level of low-temperature toughness to ensure safety. In particular, because vacuum train tubes are fabricated into tube shapes through welding, excellent low-temperature toughness is required not only in the base material but also in the weldment.

[0034] Accordingly, according to the present invention, by strictly controlling the alloy composition content and microstructure of the steel plate, it is possible to provide a hot-rolled steel plate that achieves excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the weldment.

[0035] First, a hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention will be described.

[0036] A hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.03~0.11%, silicon (Si): 0.5~1.5%, manganese (Mn): 1.2~2.2%, titanium (Ti): 0~0.1%, niobium (Nb): 0~0.05%, vanadium (V): 0~0.2%, and the remainder being Fe and unavoidable impurities, wherein the sum of the contents of titanium (Ti), niobium (Nb), and vanadium (V) is greater than 0 and is 0.35%, and the microstructure comprises a ferrite and pearlite composite structure, satisfying the following equations 1 and 2.

[0037] Equation 1: 355 ≤ -169.8 + 27.3*[ASTM#] + 264.3*[C] + 113.7*[Si] + 44.7*[Mn] + 3429.4*[Nb] + 1237*[Ti] + 611*[V]

[0038] Equation 2: 0.05 ≤ 3.3*[Ti] + 10*[Nb] + 1.6*[V] ≤ 0.5

[0039] (Here, ASTM# refers to the ASTM grain size number, and [C], [Si], [Mn], [Ti], [Nb], and [V] respectively represent the content (weight%) of carbon (C), silicon (Si), manganese (Mn), titanium (Ti), niobium (Nb), and vanadium (V) contained in the hot-rolled steel sheet.)

[0040] Hereinafter, the reason for the numerical limitation of the alloy component content in the embodiments of the present invention will be explained.

[0041] Carbon (C): 0.03~0.11%

[0042] Carbon (C) is a component that has a very significant effect on the strength of steel plates. The present invention may include 0.03% or more of carbon (C) to secure the strength required by the structure. On the other hand, if the carbon (C) content is excessive, the toughness of the material decreases, weldability deteriorates, and the yield ratio may increase. In addition, if the carbon (C) content is excessive, it is difficult to coarsen the crystal grains; therefore, the present invention may limit the upper limit of the carbon (C) content to 0.11%. Preferably, it may be 0.05 to 0.09%, and more preferably, it may be included in an amount of 0.06 to 0.08%.

[0043] Silicon (Si): 0.5~1.5%

[0044] Silicon (Si) tends to be removed along with oxygen because it combines with oxygen during the steelmaking process to form slag. Additionally, silicon (Si) is a component that effectively contributes to improving the strength of the material. Therefore, the present invention may include 0.5% or more of silicon (Si) to achieve these effects. On the other hand, if the silicon (Si) content is excessive, it may hinder the detachment of surface scale, thereby degrading the surface quality of the product. Furthermore, if the silicon (Si) content is excessive, it may promote the formation of the MA phase (martensite-austenite complex) in the weld area, which may reduce the low-temperature toughness of the weld area; therefore, the present invention may limit the silicon (Si) content to 1.5% or less. Preferably, it may be 0.8% to 1.3%, and more preferably, it may be included in an amount of 0.9% to 1.2%.

[0045] Manganese (Mn): 1.2~2.2%

[0046] Manganese (Mn) is a component that improves the strength and hardenability of steel. Therefore, the present invention may include 1.2% or more of manganese (Mn) to secure such effects. On the other hand, if the manganese (Mn) content is excessive, material variation may occur due to central segregation, and crack propagation resistance may be inferior. In addition, since the toughness of steel may decrease if the manganese (Mn) content is excessive, the present invention may limit the manganese (Mn) content to 2.2% or less. Preferably, it may be 1.5 to 2.0%, and more preferably, it may be included in an amount of 1.6 to 1.9%.

[0047] Titanium (Ti): 0~0.1%, Niobium (Nb): 0~0.05%, Vanadium (V): 0~0.2%, Sum of contents greater than 0 0.35%

[0048] Titanium (Ti), niobium (Nb), and vanadium (V) are elements that contribute to grain refinement by forming nitrides and carbides. The present invention includes these alloying elements in an amount greater than 0 and greater than 0.35% to simultaneously secure strength and toughness through these effects. However, if the content of titanium (Ti) exceeds 0.1%, if niobium (Nb) exceeds 0.05%, or if vanadium exceeds 0.2%, coarse precipitates are formed, which may actually lower toughness; therefore, the present invention limits the content of titanium (Ti) to 0.1% or less.

[0049] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0050] Equation 1: 355 ≤ -169.8 + 27.3*[ASTM#] + 264.3*[C] + 113.7*[Si] + 44.7*[Mn] + 3429.4*[Nb] + 1237*[Ti] + 611*[V]

[0051] In order to secure strength suitable for use in vacuum train tubes according to the present invention, the range of Equation 1 above must be satisfied. Specifically, Equation 1 above is intended to predict and control yield strength, and is an equation that considers the solid solution strengthening effect and grain refinement effect of each alloy element, and is an equation intended to control the yield to 355 MPa or higher, which is required by considering the structural stability and deformation control aspects of the vacuum tube. That is, if Equation 1 above is not satisfied, the deformation of the tube caused by various loads that may occur during operation cannot be limited to within the elastic region, and thus a problem with stability may occur.

[0052] Equation 2: 0.05 ≤ 3.3*[Ti] + 10*[Nb] + 1.6*[V] ≤ 0.5

[0053] As described above, titanium (Ti), niobium (Nb), and vanadium (V) are elements that contribute to grain refinement by forming nitrides and carbides, and Equation 2 above is an equation that controls the total content of these refinement elements. If the value of Equation 2 exceeds 0.5, there is a problem where hardenability in the weld heat-affected zone increases excessively, leading to increased susceptibility to low-temperature cracking, and in terms of microstructure, a large amount of coarse precipitates are formed, which may cause a decrease in the toughness of the base material. Furthermore, if the value of Equation 2 is less than 0.05, the growth of austenite grains is not effectively suppressed, which may result in the formation of a coarse final microstructure, causing a decrease in both strength and low-temperature toughness; therefore, it must be controlled within the range of Equation 2.

[0054] A hot-rolled steel sheet according to one aspect of the present invention may include a composite structure comprising ferrite, pearlite, and residual structure as its microstructure. Although low-temperature structures such as bainite and martensite may inevitably be included as residual structures, the present invention can actively suppress the formation of low-temperature structures such as bainite and martensite. Low-temperature structures such as bainite and martensite possess high strength and a low yield ratio, thereby exhibiting excellent physical properties as structural materials. However, since the hot-rolled steel sheet for vacuum train tubes targeted by the present invention is thick at a level of 10 mm or more, even if a low-temperature structure is introduced, variations in physical properties occur along the thickness direction of the steel sheet. This is because the low-temperature structure is formed only on the surface of the steel sheet, and it is difficult to sufficiently generate the low-temperature structure up to the center of the steel sheet.

[0055] Accordingly, the present invention comprises a microstructure of a steel plate containing a composite structure of ferrite and pearlite to reduce variations in physical properties, and suppresses the formation of low-temperature structures such as bainite and martensite, but even if they are inevitably formed, their fraction can be controlled to 1 area% or less (including 0%). In terms of securing physical properties, the fraction of ferrite may be 60 to 95 area%, and the fraction of pearlite may be 5 to 40 area%.

[0056] In order to simultaneously secure the desired yield strength, vibration damping ratio, and low-temperature toughness, the present invention may limit the average grain size of the ferrite, expressed by the ASTM grain size number, to a certain range. Since a lower ASTM# is advantageous for securing the vibration damping ratio, the present invention may limit the ASTM# to 12 or less. On the other hand, since the strength and low-temperature toughness of the material deteriorate if the ASTM# is excessively small, the present invention may limit the ASTM# to 9 or more.

[0057] The present invention can provide a hot-rolled steel sheet that simultaneously secures yield strength, vibration damping ratio, and low-temperature toughness of the weldment by controlling the content of carbon (C), silicon (Si), manganese (Mn), titanium (Ti), and the above-mentioned relationships 1 and 2, as well as the ASTM# of niobium (Nb) and vanadium (V) and ferrite within a certain range.

[0058] Specifically, the hot-rolled steel sheet for vacuum train tubes of the present invention satisfies equations 1 and 2, so that the desired yield strength, vibration damping ratio, and low-temperature toughness of the weldment can be secured simultaneously.

[0059] The hot-rolled steel sheet for vacuum train tubes of the present invention can have a yield strength of 350 MPa or more and a Charpy impact energy of -20°C of 50 J or more. Accordingly, the hot-rolled steel sheet for vacuum train tubes of the present invention secures strength and low-temperature toughness suitable as a structural material, thereby effectively ensuring the structural safety of vacuum train tubes.

[0060] The hot-rolled steel plate for the vacuum train tube of the present invention is 100*10 -6 It can have a vibration damping ratio greater than or equal to the above. Here, the vibration damping ratio refers to the vibration damping ratio measured at a frequency of 1650 Hz after striking a specimen with length*width*thickness of 80*20*2 mm in the flexural vibration mode. The hot-rolled steel sheet for vacuum train tubes according to the present invention is 100*10 -6 Since it has a vibration damping ratio of the above, vibration amplification within the vacuum tube can be effectively suppressed, and damage to the vacuum train tube caused by vibration can be effectively prevented.

[0061] When a hot-rolled steel sheet according to one aspect of the present invention is welded using submerged arc welding, the Charpy impact energy of the weldment at -20°C may be 50 J or more, and the fraction of the MA phase included in the weldment may be 5 area% or less (including 0%). A preferred fraction of the MA phase in the weldment may be 3 area% or less, and a more preferred fraction of the MA phase in the weldment may be 1 area% or less. Here, the weldment is a location 1 mm away from the fusion line and can be interpreted to mean including both the weld metal part and the heat-affected zone (HAZ).

[0062] Although the welding material used in the present invention is not particularly limited, it is preferable to perform welding using a welding material that does not contain silicon (Si) as much as possible. This is because if welding is performed using a welding material containing silicon (Si), there is a possibility that a large amount of hard MA phase may be formed in the weld area due to excessive hardenability.

[0063] Hereinafter, a method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention will be described in more detail.

[0064] A method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention comprises the steps of: heating a slab containing, in weight percent, carbon (C): 0.03~0.11%, silicon (Si): 0.5~1.5%, manganese (Mn): 1.2~2.2%, titanium (Ti): 0~0.1%, niobium (Nb): 0~0.05, vanadium (V): 0~0.2%, and the remainder being Fe and unavoidable impurities, at a heating temperature of 1100°C to 1300°C; hot-rolling the heated slab at a finishing rolling temperature of 860°C to 960°C to provide a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a coiling temperature of 600°C to 700°C.

[0065] Steps for preparing and heating steel slabs

[0066] A steel slab having the above-mentioned predetermined alloy composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the aforementioned hot-rolled steel sheet, the description of the alloy composition of the steel slab is replaced by the description of the alloy composition of the aforementioned hot-rolled steel sheet.

[0067] The prepared steel slab can be heated at a heating temperature of 1100°C to 1300°C. Considering the rolling load during hot rolling, the steel slab can be heated in a temperature range of 1100°C or higher. In particular, since the present invention aims to introduce a microstructure of a certain size or larger, the preferred heating temperature of the steel slab may be 1200°C or higher. A more preferred heating temperature of the steel slab may be 1250°C or higher. On the other hand, if the heating temperature of the steel slab is excessively high, the deterioration of surface quality due to scale formation may become a problem; therefore, the present invention may limit the heating temperature of the steel slab to 1300°C or lower.

[0068] hot rolling step

[0069] The heated slab is hot-rolled at a finishing rolling temperature of 860°C to 960°C to produce a hot-rolled steel sheet, preferably at 900°C to 960°C. The lower limit of the finishing rolling temperature is set to 860°C to ensure a uniform microstructure by completing rolling above the ferrite transformation start temperature. If the finishing rolling temperature is below 860°C, a non-uniform microstructure may be formed due to rolling in the ferrite region, which may cause variations in the mechanical properties of the final product. Meanwhile, the upper limit of the finishing rolling temperature is restricted to 960°C to appropriately control grain growth. If the finishing delivery temperature (FDT) exceeds 960°C, grains may grow excessively during the cooling process after rolling is completed, which may be disadvantageous in achieving the target mechanical properties.

[0070] The steel plate provided by hot rolling of the present invention can have a thickness of 10 mm or more.

[0071] Winding stage

[0072] The hot-rolled steel sheet is coiled at a temperature range of 600°C to 700°C, and preferably at a temperature range of 630°C or higher. The lower limit of the coiling temperature is set to 600°C to complete the ferrite-pearlite transformation and secure a uniform microstructure. If the coiling temperature is below 600°C, hard phases such as bainite may be formed due to incomplete transformation, which may be disadvantageous in achieving the target vibration damping characteristics. On the other hand, the upper limit of the coiling temperature is restricted to 700°C to suppress grain growth and prevent coarsening of precipitates. If the coiling temperature exceeds 700°C, grains may grow excessively during the slow cooling process of the coil, and fine precipitates may coarsen, leading to a decrease in strength and toughness; therefore, the temperature is controlled within the above range.

[0073] Hereinafter, the hot-rolled steel sheet for vacuum train tubes and the method for manufacturing the same according to the present invention will be explained in more detail through specific embodiments. It should be noted that the following embodiments are for the purpose of understanding the present invention only and are not intended to specify the scope of the rights of the present invention. The scope of the rights of the present invention may be determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0074] (Example)

[0075] After preparing a steel slab with a thickness of 250 mm with the alloy composition of Table 1 below, a hot-rolled steel sheet with a thickness of 15 mm was manufactured by applying the process conditions of Table 2. Alloy components not listed in Table 1 below refer to the remainder Fe and unavoidable impurities.

[0076] Alloy (wt%) CsiMnTiNbVA 0.071 1.800.0150B 0.021 1.800.0150C 0.2011.80.0500D 0.071.81.8000.1E 0.070.21.800.0150F 0.071100.0150G 0.0711.8000

[0077] The microstructure and mechanical properties of each specimen were analyzed and are listed in Table 2. The microstructure was measured using an optical microscope at 500x magnification after etching each specimen using the Nital etching method. The ASTM # of the ferrite was measured according to ASTM E112. Figure 1 is an optical microscope image used to observe the microstructure of Specimen 1. Mechanical properties were measured according to KS B 0802 and KS B 0810, and the measured yield strength is listed in Table 2.

[0078] The vibration damping ratio was measured at room temperature using IMCE’s RFDA LTV800 after preparing a specimen with dimensions of 80*20*2 mm (length*width*thickness). After striking in the flexural vibration mode, the vibration damping ratio in the 1650 Hz range, corresponding to the 1st mode of the specimen's vibration modes, was measured and analyzed, and the results are listed in Table 2 below.

[0079] Submerged arc welding was performed on each specimen using a welding material containing C: 0.052 wt%, Mn: 1.53 wt%, Ni: 1.3 wt%, Mo: 0.135 wt%, and the remainder being Fe and unavoidable impurities. During submerged arc welding, the inside pressure was 20 kJ / cm² 2 A heat input of was applied, and 22 kJ / cm² was applied to the outside. 2The heat input was applied. The Charpy impact toughness of the weldment at -20°C was measured according to KS B 0810, and the results are listed in Table 2 below. For the area 1 mm away from the fusion line, a first etching was performed using a solution of 5 g of EDTA and 0.5 g of NaF dissolved in 100 ml of distilled water, followed by a second etching using a solution of 25 g of NaOH and 5 g of picric acid dissolved in 100 ml of distilled water, and the MA phase fraction was measured according to ASTM E 562.

[0080] Specimen No. Steel Grade Relationship Formula 1 Relationship Formula 2 FDT (°C) CT (°C) ASTM #Room Temperature Yield Strength (MPa) Impact Toughness (Jcm -2 )Vibration damping ratio(*10 -6 )1A394.600.15910670113991871102B375.920.1591067010.83221911183C480. 320.16591067012.5511208714D487.030.1691067010.7490351005E298.180.15 91067010.82991011236F350.650.1591067010.7345891027G329.51091067010. 5333911158A337.270.159707108.934091319A454.660.1584060013.248012066

[0081] As described in Tables 1 and 2 above, Specimen No. 1 satisfying the alloy composition, process conditions, and Equations 1 and 2 of the present invention has a yield strength of 350 MPa or higher, 100*10 -6It can be seen that specimens No. 2 to 7, which satisfy the above vibration damping ratio and a Charpy impact energy of 50J or more at -20°C of the weldment, but do not satisfy one or more of the conditions limited by the present invention, fail to satisfy one or more of the composition, Equation 1, and / or Equation 2, and thus fail to simultaneously secure the desired yield strength, vibration damping ratio, and impact toughness. In addition, it can be seen that specimens No. 8 and 9 have a composition that falls within the scope of the present invention, but fail to simultaneously secure yield strength, vibration damping ratio, and impact toughness because the Equation or ASTM grain size number does not satisfy the scope of the present invention.

[0082] In addition, for comparison with conventional materials, tests were conducted on the existing structural steel EN-S355 under the same conditions, and in the case of EN-S355, the vibration damping ratio measured under the same conditions was 60*10 -6 It was confirmed that it was merely at the level of... Figure 2 is a micrograph of the microstructure of EN-S355 taken using an optical microscope.

[0083] Accordingly, according to one aspect of the present invention, a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness of the weldment, and a method for manufacturing the same can be provided.

[0084] Although the present invention has been described in detail through embodiments above, other forms of embodiments are also possible. Therefore, the technical concept and scope of the claims described below are not limited to the embodiments.

[0085] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

1. In weight%, it comprises carbon (C): 0.03–0.11%, silicon (Si): 0.5–1.5%, and manganese (Mn): 1.2–2.2%, titanium (Ti): 0–0.1%, niobium (Nb): 0–0.05%, vanadium (V): 0–0.2%, and the remainder being Fe and unavoidable impurities, wherein the sum of the contents of titanium (Ti), niobium (Nb), and vanadium (V) is greater than 0 and 0.35%, and The microstructure includes a ferrite and pearlite composite structure, and Hot-rolled steel sheet for vacuum train tubes satisfying the following relationships 1 and 2. Equation 1: 355 ≤ -169.8 + 27.3*[ASTM#] + 264.3*[C] + 113.7*[Si] + 44.7*[Mn] + 3429.4*[Nb] + 1237*[Ti] + 611*[V] Equation 2: 0.05 ≤ 3.3*[Ti] + 10*[Nb] + 1.6*[V] ≤ 0.5 (Here, ASTM# refers to the ASTM grain size number, and [C], [Si], [Mn], [Ti], [Nb], and [V] respectively represent the content (weight%) of carbon (C), silicon (Si), manganese (Mn), titanium (Ti), niobium (Nb), and vanadium (V) contained in the hot-rolled steel sheet.) 2. In Paragraph 1, The microstructure of the above hot-rolled steel sheet comprises 60 to 95 area percent ferrite, 5 to 40 area percent pearlite, and the remainder being a microstructure for a vacuum train tube.

3. In Paragraph 1, Hot-rolled steel sheet for vacuum train tubes, the ASTM grain size number of the ferrite above is 9 to 12.

4. In Paragraph 1, A hot-rolled steel sheet for vacuum train tubes, wherein the room temperature yield strength of the above hot-rolled steel sheet is 350 MPa or higher.

5. In Paragraph 1, Hot-rolled steel sheet for vacuum train tubes, wherein the Charpy impact energy of the above hot-rolled steel sheet at -20℃ is 50J or more.

6. In Paragraph 1, The vibration damping ratio measured at a frequency of 1650 Hz in the flexural vibration mode of the above hot-rolled steel sheet is 100*10 -6 Lee Sang-in, hot-rolled steel sheet for vacuum train tubes.

7. In Paragraph 1, The Charpy impact energy at -20°C of the weld formed by welding the hot-rolled steel plate by submerged arc welding is 50J or more, and A hot-rolled steel sheet for vacuum train tubes, wherein the fraction of the martensite-austenite composite included in the welded portion is 5 area% or less.

8. In Paragraph 1, A hot-rolled steel plate for vacuum train tubes, the thickness of which is 10 mm or more.

9. In the method for manufacturing hot-rolled steel sheets for vacuum train tubes according to paragraph 1, The above manufacturing method comprises the step of heating a slab containing, in weight percent, carbon (C): 0.03~0.11%, silicon (Si): 0.5~1.5%, manganese (Mn): 1.2~2.2%, titanium (Ti): 0~0.1%, niobium (Nb): 0~0.05%, vanadium (V): 0~0.2%, and the remainder being Fe and unavoidable impurities, at a heating temperature of 1100℃ to 1300℃; A step of providing a hot-rolled steel sheet by hot-rolling the heated slab at a finishing rolling temperature of 860°C to 960°C; and A method for manufacturing a hot-rolled steel sheet for a vacuum train tube, comprising the step of winding the hot-rolled steel sheet at a winding temperature of 600°C to 700°C.

10. In Paragraph 9, After the above-mentioned coiling step, the room temperature yield strength of the hot-rolled steel sheet is 350 MPa or higher, the Charpy impact energy of the hot-rolled steel sheet at -20°C is 50 J or higher, and the vibration damping ratio measured at a frequency of 1650 Hz in the flexural vibration mode is 100*10 -6 A method for manufacturing hot-rolled steel sheets for vacuum train tubes controlled above.

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