Hot-rolled steel sheet for vacuum train tubes and its manufacturing method

Abstract

According to one embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of a weld, and physical properties suitable for use in vacuum train tubes, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof, and more particularly to a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of a welded portion, and physical properties suitable for use in vacuum train tubes, and a manufacturing method thereof. [Background technology]

[0002] A vacuum train, also known as a hypertube train, is a system in which a magnetically levitated train travels inside a vacuum tube. Vacuum trains are capable of ultra-high speed operation because they are free from friction with the air and tracks, which are the main causes of energy loss when a train moves. Because they have little energy loss and can achieve 93% energy savings compared to aircraft, they have been attracting attention as an environmentally friendly means of next-generation transportation, and active research is being conducted around the world.

[0003] The structure and materials of the vacuum tubes used in high-speed vacuum trains affect the system's performance and cost. Currently, there are three main materials being researched as materials for vacuum train tubes. One is concrete. Concrete tubes are advantageous in terms of cost, but it is difficult to join individual tubes of around 10 meters each. Another drawback is that the pores inside the concrete allow external gases to enter the tubes when a vacuum is created, easily destroying the vacuum. Another material that is being researched extensively is composite materials such as carbon fiber. Composite materials such as carbon fiber are lightweight and high-performance, but their biggest drawback is their high cost.

[0004] Currently, steel is the most promising material for vacuum train tubes. Steel is a material that can be mass-produced at low cost. It has high rigidity and strength and is easy to process. Steel is also a material that makes it easy to assemble or weld accessories between or to the tubes, and it also has an appropriate degassing rate for maintaining a vacuum. However, because ultra-high-speed vacuum trains operate at significantly faster speeds than current high-speed trains, the safety of passengers and surrounding facilities must be given top priority. Currently, safety standards for ultra-high-speed vacuum trains have not even been established, and the development of tube materials to ensure the safety of ultra-high-speed vacuum trains is insufficient.

[0005] Therefore, there is an urgent need to develop a material for vacuum train tubes that has the processability and degassing rate suitable for vacuum train tubes, while also ensuring safety. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Registration No. 10-2106353 (Announced on May 4, 2020) Summary of the Invention [Problem to be solved by the invention]

[0007] According to one embodiment of the present invention, a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of welds, and physical properties suitable for vacuum train tubes, and a manufacturing method thereof can be provided.

[0008] 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]

[0009] A hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention may contain, by weight, carbon (C): 0.03 to 0.11%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.2 to 2.2%, the remainder being Fe and other unavoidable impurities, have a ferrite and pearlite composite structure as a microstructure, and satisfy the following relational expressions 1 to 3.

[0010] [Equation 1] 355≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0011] [Equation 2] 100≦186-240×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0012] [Equation 3] 27≦476-95.22×ln(D)-220×[C]-88×[Si]

[0013] In the above Relational Formulas 1 to 3, D represents the average particle size (μm) of ferrite contained in the hot-rolled steel sheet, and [C], [Si], and [Mn] represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) contained in the hot-rolled steel sheet, respectively.

[0014] The microstructure of the hot-rolled steel sheet may be composed of 60 to 95 area % of ferrite, 5 to 40 area % of pearlite, and other unavoidable structures.

[0015] The total amount of titanium (Ti), niobium (Nb), and vanadium (V) inevitably contained in the above-mentioned hot-rolled steel sheet may be less than 0.01% (including 0%).

[0016] The average particle size (D) of the ferrite may be 10 to 20 μm.

[0017] The hot-rolled steel sheet has a yield strength of 350 MPa or more, a -20°C standard Charpy impact energy of 27 J or more, and a vibration damping ratio of 100 x 10 measured at a frequency of 1650 Hz in a flexural vibration mode after processing the hot-rolled steel sheet into a test piece having a length x width x thickness of 80 mm x 20 mm x 2 mm. -6 It may be more than that.

[0018] In a weld formed by welding the above hot-rolled steel sheets by submerged arc welding, the -20°C standard Charpy impact energy of the weld may be 27 J or more, and the fraction of MA phase contained in the weld may be 5 area % or less (including 0%).

[0019] The thickness of the hot-rolled steel plate may be 10 mm or more.

[0020] A method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention includes the steps of heating a slab containing, by weight, 0.03 to 0.11% carbon (C), 1.0 to 2.0% silicon (Si), 1.2 to 2.2% manganese (Mn), with the remainder being Fe and other inevitable impurities, at a heating temperature (T1) of 1100°C to 1300°C; hot-rolling the heated slab at a finish rolling temperature (T2) of 900°C to 1000°C to provide a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a coiling temperature (T3) of 600°C to 700°C, wherein the finish rolling temperature (T2) and the coiling temperature (T3) may satisfy the following relational expression 4:

[0021] [Equation 4] 10≦-101.9+0.103×[T2]+0.0339×[T3]-61.9×[C]-190.2×[Nb]≦20

[0022] In the above Relational Formula 4, [T1], [T2], and [T3] respectively represent the slab heating temperature (T1, °C), the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C), and [C] and [Nb] respectively represent the contents (% by weight) of carbon (C) and niobium (Nb) contained in the above hot-rolled steel sheet.

[0023] The total content of titanium (Ti), niobium (Nb) and vanadium (V) inevitably contained in the slab may be less than 0.01% (including 0%).

[0024] 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 more detail with reference to the following specific embodiments and examples. [Effects of the Invention]

[0025] According to one embodiment of the present invention, a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of welds, and physical properties suitable for vacuum train tubes, and a manufacturing method thereof can be provided.

[0026] The effects of the present invention are not limited to the above-mentioned matters, but can be construed as including matters that a person skilled in the art can reasonably infer from the matters described in this specification. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a micrograph of a weld formed by welding a base material containing 1.5 wt % silicon (Si) with a welding material containing no silicon (Si). [Figure 2] 1 is a micrograph of a weld formed by welding a base material containing 2.0 wt % silicon (Si) with a welding material containing 0.3 wt % silicon (Si). [Figure 3] 1 is an optical microscope photograph used for observing the microstructure of Test Specimen 1. [Figure 4] This is an optical microscope photograph of EN-S355, an existing structural steel material. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention relates to a hot-rolled steel sheet for vacuum train tubes and a manufacturing method thereof. Preferred embodiments of the present invention will be described below. The present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to more fully explain the present invention to those skilled in the art.

[0029] A vacuum train is a next-generation transportation mode currently in the early stages of development. Vacuum trains are trains that run inside tubes under vacuum or near-vacuum conditions. Vacuum trains are a transportation mode that can effectively achieve high speeds and high efficiency by eliminating frictional resistance between the wheels and tracks and minimizing air resistance. However, due to the nature of vacuum trains, which operate at ultra-high speeds, serious accidents may occur if the safety of the vacuum train is not adequately ensured. In particular, strict safety standards are required for the materials used in vacuum train tubes, as a major disaster may occur not only if the vacuum tubes are structurally damaged or collapsed, but also if a portion of the tube is deformed. Through extensive research, the inventors of the present invention have found that the following physical properties are essential for vacuum tube materials to ensure the safety of vacuum trains.

[0030] The primary physical property required for vacuum tube materials is high strength. Because vacuum trains travel through the inside of vacuum tubes, vacuum tube materials must have sufficient strength as a structure. Furthermore, because the inside of vacuum tubes must be maintained in a vacuum or near-vacuum state, the material must have sufficient strength to prevent deformation of the tube due to the pressure difference between the inside and outside.

[0031] The second physical property required for vacuum tube materials is vibration damping capacity. Vacuum trains have pods carrying several to several dozen people passing through the inside of the vacuum tube at intervals of several tens of seconds to several minutes. When a trailing pod passes after a preceding pod has passed, vibrations within the vacuum tube can be amplified, causing resonance, which in severe cases could even lead to tube damage. Therefore, applying a material with a vibration damping ratio above a certain level to the vacuum tube can effectively reduce vibrations within the tube after the preceding pod has passed, thereby contributing to the safety of the vacuum train.

[0032] The third physical property required for materials for vacuum tubes is low-temperature toughness. Vacuum trains may operate in polar regions or deep underwater. Since steel materials tend to break more easily in low-temperature or extremely low-temperature environments, when steel materials are used for vacuum tubes, they must have a certain level of low-temperature toughness to ensure safety. In particular, since tubes for vacuum trains are manufactured in tube form through welding, excellent low-temperature toughness is required not only in the base material but also in the welds.

[0033] The inventors of the present invention have conducted extensive research and have recognized that it is possible to achieve excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of a weld by strictly controlling the alloy composition and microstructure of a steel plate, and have thus devised the present invention.

[0034] Hereinafter, a hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention will be described in more detail.

[0035] A hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention may contain, by weight, carbon (C): 0.03 to 0.11%, silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.2 to 2.2%, the remainder being Fe and other unavoidable impurities, have a ferrite and pearlite composite structure as a microstructure, and satisfy the following relational expressions 1 to 3.

[0036] [Equation 1] 355≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0037] [Equation 2] 100≦186-240×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0038] [Equation 3] 27≦476-95.22×ln(D)-220×[C]-88×[Si]

[0039] In the above Relational Formulas 1 to 3, D represents the average particle size (μm) of ferrite contained in the hot-rolled steel sheet, and [C], [Si], and [Mn] represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) contained in the hot-rolled steel sheet, respectively.

[0040] The steel composition contained in the hot-rolled steel sheet of the present invention will be described in more detail below. Unless otherwise specified, % representing the content of each element is based on weight.

[0041] Carbon (C): 0.03~0.11% Carbon (C) is a component that has a significant effect on the strength of steel sheets. The present invention can contain 0.03% or more of carbon (C) to ensure the strength required for structures. However, excessive carbon (C) content can reduce the toughness of the material, reduce weldability, and increase the yield ratio. In addition, excessive carbon (C) content can make it difficult to coarsen grains, so the present invention can limit the upper limit of the carbon (C) content to 0.11%.

[0042] Silicon (Si): 1.0-2.0% Silicon (Si) combines with oxygen to form slag during steelmaking, so it tends to be removed along with oxygen. Silicon (Si) also effectively contributes to improving the strength of materials. Therefore, the present invention may contain 1.0% or more silicon (Si) to achieve this effect. However, excessive silicon (Si) content may hinder the removal of surface scale, thereby reducing the surface quality of the product. Furthermore, excessive silicon (Si) content may promote the formation of a martensite-austenite complex (MA) phase in the weld, reducing the low-temperature toughness of the weld. Therefore, the present invention limits the silicon (Si) content to 2.0% or less.

[0043] Manganese (Mn): 1.2-2.2% Manganese (Mn) is an element that improves the strength and hardenability of steel. Therefore, to ensure this effect, the present invention may contain 1.2% or more manganese (Mn). However, excessive manganese (Mn) content may cause material variations due to segregation in the center, reducing crack propagation resistance. Furthermore, excessive manganese (Mn) content may reduce the toughness of the steel, so the present invention limits the manganese (Mn) content to 2.2% or less.

[0044] In addition to the above-mentioned components, the hot-rolled steel sheet of the present invention may contain the remaining 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 entire content of these impurities will not be specifically mentioned in this specification. Furthermore, the addition of further effective components other than the above-mentioned components is not completely excluded.

[0045] The hot-rolled steel sheet of the present invention minimizes the addition of titanium (Ti), niobium (Nb), and vanadium (V). Even if these elements are unavoidably included, their total content can be limited to less than 0.01% (including 0%). Titanium (Ti), niobium (Nb), and vanadium (V) are typical precipitation strengthening elements that effectively contribute to improving the strength of steel by forming fine carbonitrides. However, because titanium (Ti), niobium (Nb), and vanadium (V) excessively refine the microstructure of steel, which has a detrimental effect on ensuring vibration damping capacity, the present invention aims to minimize the addition of these elements. Furthermore, titanium (Ti), niobium (Nb), and vanadium (V) are expensive elements, and are therefore undesirable from an economic standpoint. The present invention does not artificially add these elements, and even if they are unavoidably added, their total content can be minimized to less than 0.01%. A preferred total content of these components may be 0.005% or less, and a more preferred total content of these components may be 0%.

[0046] A hot-rolled steel sheet according to one embodiment of the present invention may have a composite structure consisting of ferrite and pearlite as a microstructure. The present invention can minimize the formation of low-temperature structures such as bainite and martensite. Low-temperature structures such as bainite and martensite have high strength and a low yield ratio, and can exhibit excellent physical properties as structural materials. However, since the hot-rolled steel sheet for vacuum train tubes intended 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 in 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 for the low-temperature structure to be sufficiently formed in the center of the steel sheet.

[0047] Therefore, in the present invention, in order to reduce variations in physical properties, the microstructure of the steel sheet is constituted by a composite structure consisting of ferrite and pearlite, and even if low-temperature structures such as bainite and martensite are inevitably formed, their fraction can be suppressed as much as possible to 1 area % or less (including 0%). From the viewpoint of ensuring physical properties, the fraction of ferrite may be 60 to 95 area %, and the fraction of pearlite may be 5 to 40 area %.

[0048] In order to simultaneously ensure the desired yield strength, vibration damping ratio, and low-temperature toughness, the present invention can limit the average grain size of ferrite to a certain range. Since a larger grain size is more advantageous for ensuring the vibration damping ratio, the present invention can limit the average grain size of ferrite to 10 μm or more. On the other hand, if the grain size is too large, the strength and low-temperature toughness of the material will deteriorate, so the present invention can limit the average grain size of ferrite to 20 μm or less.

[0049] As a result of extensive research into methods for ensuring the stability of materials for vacuum train tubes, the inventors of the present invention have recognized that in low-alloy steel plates such as those of the present invention, by controlling the contents of carbon (C), silicon (Si), and manganese (Mn) and the average grain size of ferrite within certain ranges, it is possible to simultaneously ensure yield strength, vibration damping ratio, and low-temperature toughness of welds, and have derived the following Relational Formulas 1 to 3.

[0050] [Equation 1] 355≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0051] [Equation 2] 100≦186-240×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0052] [Equation 3] 27≦476-95.22×ln(D)-220×[C]-88×[Si]

[0053] In the above Relational Formulas 1 to 3, D represents the average particle size (μm) of ferrite contained in the hot-rolled steel sheet, and [C], [Si], and [Mn] represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) contained in the hot-rolled steel sheet, respectively.

[0054] The hot-rolled steel sheet for vacuum train tubes of the present invention satisfies all of the relational expressions 1 to 3, and therefore can simultaneously ensure the desired yield strength, vibration damping ratio, and low-temperature toughness of the weld.

[0055] The hot-rolled steel sheet for vacuum train tubes of the present invention may have a yield strength of 350 MPa or more and a -20°C Charpy impact energy of 27 J or more, thereby ensuring strength and low-temperature toughness appropriate for a structural material and effectively ensuring the structural safety of vacuum train tubes.

[0056] The hot rolled steel sheet for vacuum train tubes of the present invention is 100 x 10 -6 The hot rolled steel sheet for vacuum train tubes of the present invention may have a vibration damping ratio of 100×10 or more. Here, the vibration damping ratio means the vibration damping ratio measured at a frequency of 1650 Hz after striking a test piece having a length x width x thickness of 80 x 20 x 2 mm in a flexural vibration mode. -6 Because of the above vibration damping ratio, vibration amplification within the vacuum tube can be effectively suppressed, and damage to the vacuum train tube due to vibration can be effectively prevented.

[0057] When a hot-rolled steel sheet according to one embodiment of the present invention is welded using submerged arc welding, the -20°C standard Charpy impact energy of the weld may be 27 J or more, and the MA phase fraction in the weld may be 5 area % or less (including 0%). The MA phase fraction in the weld may be preferably 3 area % or less, and more preferably 1 area % or less. Here, the weld is a position 1 mm away from the fusion line, and can be interpreted to include both the weld metal and the heat-affected zone (HAZ).

[0058] In the present invention, the welding material used for welding is not particularly limited, but it is preferable to use a welding material that does not contain silicon (Si). This is because welding using a welding material containing silicon (Si) can result in the formation of a large amount of hard MA phase in the weld due to excessive hardening ability. Figure 1 is a micrograph of a weld formed by welding a base metal containing 1.5 wt% silicon (Si) with a welding material that does not contain silicon (Si), and Figure 2 is a micrograph of a weld formed by welding a base metal containing 2.0 wt% silicon (Si) with a welding material containing 0.3 wt% silicon (Si). In Figure 2, a large amount of white regions (MA phase) are observed at the grain boundaries, while in Figure 1, no MA phase is observed.

[0059] Therefore, according to one embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness of welded joints, and having physical properties suitable for use in vacuum train tubes.

[0060] Hereinafter, a method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention will be described in more detail.

[0061] A method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention includes the steps of heating a slab containing, by weight, 0.03 to 0.11% carbon (C), 1.0 to 2.0% silicon (Si), 1.2 to 2.2% manganese (Mn), the remainder being Fe and other inevitable impurities, at a heating temperature (T1) of 1100°C to 1300°C, hot-rolling the heated slab at a finish rolling temperature (T2) of 900°C to 1000°C to provide a hot-rolled steel sheet, and coiling the hot-rolled steel sheet at a coiling temperature (T3) of 600°C to 700°C, wherein the finish rolling temperature (T2) and the coiling temperature (T3) may satisfy the following relational expression 4:

[0062] [Equation 4] 10≦-101.9+0.103×[T2]+0.0339×[T3]-61.9×[C]-190.2×[Nb]≦20

[0063] In the above Relational Formula 4, [T1], [T2], and [T3] respectively represent the slab heating temperature (T1, °C), the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C), and [C] and [Nb] respectively represent the contents (% by weight) of carbon (C) and niobium (Nb) contained in the above hot-rolled steel sheet.

[0064] Steel slab preparation and heating A steel slab having a predetermined alloy composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to that of the above-mentioned hot-rolled steel sheet, the description of the alloy composition of the steel slab replaces the description of the alloy composition of the above-mentioned hot-rolled steel sheet.

[0065] The prepared steel slab can be heated at a heating temperature (T1) of 1100°C to 1300°C. Considering the rolling load during hot rolling, the steel slab can be heated at 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 steel slab heating temperature may be 1200°C or higher. A more preferred steel slab heating temperature may be 1250°C or higher. On the other hand, if the steel slab heating temperature is excessively high, there is a concern that the surface quality may be reduced due to the formation of scale. Therefore, in the present invention, the steel slab heating temperature can be limited to 1300°C or lower.

[0066] hot rolling The heated steel slab can be hot rolled at a finish rolling temperature (T2) of 900°C to 1000°C to provide a hot rolled steel sheet. The steel sheet provided by hot rolling of the present invention may have a thickness of 10 µm or more.

[0067] During hot rolling, the crystal grains deform as the material is rolled, but then quickly recrystallize. Through this process, the coarse and uneven structure is refined and homogenized. An important process variable during hot rolling is the finishing delivery temperature (FDT), which is the temperature at the end of rolling. This is because the finishing delivery temperature can control the grain size of the final microstructure. In the present invention, hot rolling can be performed at a finishing delivery temperature of 900°C or higher to control the final microstructure to a certain size or higher. A preferred finishing delivery temperature is 950°C or higher. However, if the finishing delivery temperature is too high, the final microstructure may become excessively coarse. Therefore, in the present invention, the upper limit of the finishing delivery temperature can be limited to 1000°C.

[0068] Winding The hot-rolled steel sheet obtained by hot rolling can be water-cooled and then coiled at a coiling temperature (T3) of 600°C to 700°C. In the present invention, coiling can be performed at a temperature range of 600°C or higher to achieve a composite structure of ferrite and pearlite as the final structure. In the present invention, coiling is more preferably performed at a temperature range of 650°C or higher to achieve a final microstructure of a certain size or larger. However, if the coiling temperature is excessively high, a coarse microstructure may be formed or the surface quality may deteriorate, and therefore the upper limit of the coiling temperature can be limited to 700°C in the present invention.

[0069] The inventors of the present invention have conducted extensive research into technical means for controlling the grain size of the final microstructure, and have confirmed that in order to control the grain size of the final microstructure in the chemical composition of the present invention, the heating temperature (T1) during heating of the steel slab, the finish rolling temperature (T2) during hot rolling, and the coiling temperature (T3) during coiling of the hot-rolled steel sheet must be controlled independently to satisfy certain ranges, and further that the finish rolling temperature (T2) and the coiling temperature (T3) must be controlled within certain ranges in cooperation with each other, and have thus derived the following Relational Formula 4.

[0070] [Equation 4] 10≦-101.9+0.103×[T2]+0.0339×[T3]-61.9×[C]-190.2×[Nb]≦20

[0071] In the above Relational Formula 4, [T1], [T2], and [T3] respectively represent the slab heating temperature (T1, °C), the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C), and [C] and [Nb] respectively represent the contents (% by weight) of carbon (C) and niobium (Nb) contained in the above hot-rolled steel sheet.

[0072] Therefore, in a manufacturing method of a hot-rolled steel sheet for a vacuum train tube according to one embodiment of the present invention, not only is a slab heated at a heating temperature (T1) of 1100°C to 1300°C, hot-rolled at a finish rolling temperature (T2) of 900°C to 1000°C, and the hot-rolled steel sheet coiled at a coiling temperature (T3) of 600°C to 700°C, but the process conditions are also controlled so that the finish rolling temperature (T2) and the coiling temperature (T3) satisfy Relational Formula 4, thereby effectively achieving the targeted microstructure of the hot-rolled steel sheet.

[0073] The hot-rolled steel sheet produced by the above-described production method may satisfy the following relational expressions 1 to 3.

[0074] [Equation 1] 355≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0075] [Equation 2] 100≦186-240×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0076] [Equation 3] 27≦476-95.22×ln(D)-220×[C]-88×[Si]

[0077] In the above Relational Formulas 1 to 3, D represents the average particle size (μm) of ferrite contained in the hot-rolled steel sheet, and [C], [Si], and [Mn] represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) contained in the hot-rolled steel sheet, respectively.

[0078] The hot-rolled steel sheet manufactured by the above manufacturing method not only has a yield strength of 350 MPa or more and a -20°C Charpy impact energy of 27 J or more, but also has a vibration damping ratio of 100 x 10 measured at a frequency of 1650 Hz in a flexural vibration mode using a test specimen with dimensions of 80 x 20 x 2 mm. -6 above levels can be met.

[0079] Furthermore, when the hot-rolled steel sheet manufactured by the above-mentioned manufacturing method is welded by submerged arc welding, the Charpy impact energy at -20°C of the weld may be 27 J or more, and the fraction of MA phase contained in the weld may be 5 area % or less (including 0%). Here, the weld may refer to a position 1 mm away from the fusion line.

[0080] Therefore, according to one embodiment of the present invention, a method for manufacturing a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness of a weld, and physical properties suitable for use in vacuum train tubes, can be provided. [Example]

[0081] Hereinafter, the hot-rolled steel sheet for vacuum train tubes and the manufacturing method thereof according to the present invention will be described in more detail through specific examples. It should be noted that the following examples are provided for the purpose of understanding the present invention and are not intended to define the scope of the present invention. The scope of the present invention can be determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0082] (Example) A 250 mm thick steel slab having the alloy composition shown in Table 1 below was prepared, and then a 15 mm thick hot-rolled steel sheet was manufactured using the process conditions shown in Table 2. Alloy elements not listed in Table 1 below represent impurities and the balance Fe, and "-" indicates that the alloy element is close to 0 wt% within the margin of error.

[0083] [Table 1]

[0084] [Table 2]

[0085] The microstructure and mechanical properties of each test specimen were analyzed and are listed in Table 3. Table 3 also lists whether each test specimen satisfied Relational Formulas 1 to 3. The microstructure was measured using an optical microscope at 500x magnification after etching each test specimen using the Nital etching method. The ferrite grain size was measured in accordance with ASTM E112. Figure 3 shows an optical microscope photograph used to observe the microstructure of test specimen 1. Mechanical properties were measured in accordance with KS B 0802 and KS B 0810, and the measured yield strength is also listed in Table 3.

[0086] The vibration damping ratio was measured at room temperature using IMCE's RFDA LTV800 after preparing a test piece with length x width x thickness of 80 x 20 x 2 mm. After striking in flexural vibration mode, the test piece was subjected to one of the vibration modes. st The vibration damping ratio in the 1650 Hz range corresponding to the mode was measured and analyzed, and the results are also shown in Table 3.

[0087] Submerged arc welding was performed on each test piece using a welding material containing 0.052 wt% C, 1.53 wt% Mn, 1.3 wt% Ni, 0.135 wt% Mo, and the remainder Fe and other unavoidable impurities. During submerged arc welding, 20 kJ / cm was applied to the inside. 2The heat input is applied to the outside, and 22 kJ / cm 2 A heat input of 10 ...

[0088] [Table 3]

[0089] As shown in Tables 1 to 3, the test pieces satisfying the alloy composition, process conditions and relations 1 to 4 of the present invention have a yield strength of 350 MPa or more and a yield strength of 100 × 10 -6 In addition to satisfying the above vibration damping ratio, the -20°C standard Charpy impact energy of the weld is 27 J or more. However, it can be seen that a test piece that does not satisfy one or more of the conditions restricted by the present invention cannot simultaneously secure the desired physical properties.

[0090] In addition, for comparison with conventional materials, tests were also conducted under the same conditions on EN-S355, an existing structural steel material. In the case of EN-S355, the vibration damping ratio measured under the same conditions was 60 x 10 -6 Figure 4 is a photograph of the microstructure of EN-S355 taken using an optical microscope.

[0091] Therefore, according to one embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness of the weld, and physical properties suitable for use in vacuum train tubes, and a manufacturing method thereof.

[0092] 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. In weight percent, it contains carbon (C): 0.03 to 0.11%, silicon (Si): 1.0 to 2.0%, and manganese (Mn): 1.2 to 2.2%, with the remainder being Fe and other unavoidable impurities; It has a microstructure consisting of 60 to 95 area % ferrite, 5 to 40 area % pearlite and other unavoidable structures, A hot-rolled steel sheet for vacuum train tubes that satisfies the following Relational Formulas 1 to 3. [Relationship 1] 355≦11+394×D （－０．５） +448×[C]+94×[Si]+69×[Mn] [Relationship 2] 100≦186-240×D （－０．５） -121×[C]-13.2×[Si]+13.7×[Mn] [Relationship 3] 27≦476-95.22×ln(D)-220×[C]-88×[Si] In Relational Formulas 1 to 3, D represents the average particle size (μm) of ferrite contained in the hot-rolled steel sheet, and [C], [Si], and [Mn] represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) contained in the hot-rolled steel sheet, respectively.

2. 2. The hot-rolled steel sheet for vacuum train tubes according to claim 1, wherein the total amount of titanium (Ti), niobium (Nb), and vanadium (V) inevitably contained in the hot-rolled steel sheet is less than 0.01% (including 0%).

3. The hot-rolled steel sheet for vacuum train tubes according to claim 1, wherein the average particle size (D) of the ferrite is 10 to 20 μm.

4. The yield strength of the hot-rolled steel plate is 350 MPa or more, The hot-rolled steel sheet has a -20°C standard Charpy impact energy of 27 J or more, The hot-rolled steel sheet was processed into a test piece having a length x width x thickness of 80 mm x 20 mm x 2 mm, and the vibration damping ratio measured at a frequency of 1650 Hz in a flexural vibration mode was 100 x 10 －６ The hot rolled steel sheet for a vacuum train tube according to claim 1 .

5. In a weld formed by welding the hot-rolled steel plates by submerged arc welding, The -20°C standard Charpy impact energy of the weld is 27 J or more, The hot-rolled steel sheet for vacuum train tubes according to claim 1, wherein the fraction of the MA phase contained in the weld is 5% by area or less (including 0%).

6. 2. The hot-rolled steel sheet for vacuum train tubes according to claim 1, wherein the thickness of the hot-rolled steel sheet is 10 mm or more.

7. A slab containing, by weight, carbon (C): 0.03 to 0.11%, silicon (Si): 1.0 to 2.0%, and manganese (Mn): 1.2 to 2.2%, with the remainder being Fe and other unavoidable impurities, is heated at a temperature (T １ ) heating the mixture; The heated slab is subjected to a finish rolling temperature (T ２ ) to provide a hot-rolled steel sheet; The hot-rolled steel sheet is subjected to a coiling temperature of 600°C to 700°C (T ３ and winding the film on the film. The finish rolling temperature (T ２ ) and winding temperature (T ３ 2. The method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to claim 1, wherein: [Relationship 4] 10≦-101.9+0.103×[T ２ ]+0.0339×[T ３ ]-61.9×[C]-190.2×[Nb]≦20 In the above-mentioned relational expression 4, [T １ ], [T ２ ] and [T ３ ] are the slab heating temperatures (T １ , ° C.), finish rolling temperature (T ２ , °C) and winding temperature (T ３ , ° C.), and [C] and [Nb] respectively mean the contents (wt%) of carbon (C) and niobium (Nb) contained in the hot-rolled steel sheet.

8. 8. The method for manufacturing a hot-rolled steel sheet for vacuum train tubes according to claim 7, wherein the total amount of titanium (Ti), niobium (Nb), and vanadium (V) inevitably contained in the slab is less than 0.01% (including 0%).

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