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

Abstract

The present invention relates to a hot-rolled steel sheet for vacuum train tubes that has excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the welded joint, and a manufacturing method thereof. The hot-rolled steel sheet for vacuum train tubes according to the present invention contains, by weight, carbon (C): 0.03 to 0.11%, silicon (Si): 0.3 to 1.5%, cobalt (Co): 1 to 3%, manganese (Mn): 1.2 to 2.2%, the balance being Fe, and other unavoidable impurities, and the final microstructure has a composite structure containing ferrite and pearlite.

Description

[Technical Field]

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

[0002] A vacuum train system, also known as a hypertube train system, is a system in which a magnetically levitated train moves 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. These vacuum trains have little energy loss and can save more than 93% of the energy used by aircraft, so they are attracting attention as an environmentally friendly means of next-generation transportation, and as a result, active research is being conducted around the world.

[0003] The structure and materials of the vacuum tubes used in ultra-high-speed vacuum trains affect the system's performance and cost. Currently, there are roughly three types of materials being researched for vacuum train tube materials.

[0004] One of the materials used to make vacuum train tubes is concrete. Concrete vacuum tubes are advantageous in terms of cost, but it is not easy to join individual tubes of around 10m each. Another drawback is that when a vacuum is created, the pores inside the concrete allow external gases to seep into the tubes, easily destroying the vacuum.

[0005] Another tube material that has been the subject of much research is composite materials such as carbon fiber, which are lightweight and offer high performance, but are expensive.

[0006] Therefore, currently, the most promising material for vacuum train tubes is steel. Steel is a material that can be mass-produced at low cost. Steel has high rigidity and strength and is easy to process. Steel is also easy to assemble or weld accessories between or to the vacuum tubes, and it has an appropriate degassing rate when maintaining a vacuum.

[0007] 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. Therefore, there is an urgent need to develop materials for vacuum train tubes that have the processability and degassing rate suitable for use in vacuum train tubes and ensure safety.

[0008] A related prior document is Korean Patent Publication No. 10-2009-0086232 (published on August 11, 2009), which describes a member with excellent damping capacity, a manufacturing method thereof, and a steel plate used as a member with excellent damping capacity. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a hot-rolled steel sheet for vacuum train tubes, which has excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of welds, and has physical properties suitable for use in vacuum train tubes, and a manufacturing method thereof.

[0010] 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 general content of this specification. [Means for solving the problem]

[0011] In order to achieve the above object, the hot-rolled steel sheet for vacuum train tubes according to an embodiment of the present invention contains, by weight, carbon (C): 0.03 to 0.11%, silicon (Si): 0.3 to 1.5%, cobalt (Co): 1 to 3%, manganese (Mn): 1.2 to 2.2%, the balance being Fe, and other inevitable impurities, and is characterized in that the final microstructure has a composite structure containing ferrite and pearlite, and satisfies the following relational formulas 1 and 2.

[0012] [Equation 1] 355≦(11+394×D -0.5 )+(448×[C])+(94×[Si])+(36.5×[Co])+(69×[Mn])+(3,429×[Nb])

[0013] [Equation 2] 150≦(186-210×D -0.5 )-(121×[C])+(13.2×[Si])+(31.1×[Co])-(13.7×[Mn])-(4,723×[Nb]) (In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.)

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

[0015] In this case, the other unavoidable structures preferably include at least one low-temperature structure of bainite and martensite, and the low-temperature structure preferably accounts for 5% or less in area ratio.

[0016] The average crystal grain size of the ferrite may be 7 to 15 μm.

[0017] The hot-rolled steel sheet has a yield strength (YS) of 350 MPa or more and a Charpy impact energy at -20°C of 50 J or more.

[0018] The hot-rolled steel sheet has a vibration damping ratio of 150×10 measured at a frequency of 1,650 Hz in bending vibration mode. -6 More preferably, it is equal to or greater than this.

[0019] In a weld formed by welding the hot-rolled steel sheets by submerged arc welding, it is preferable that the Charpy impact energy of the weld at -20°C is 50 J or more, and the fraction of MA phase contained in the weld is 5% or less in terms of area ratio.

[0020] The thickness of the hot-rolled steel plate is preferably 10 mm or more.

[0021] In order to achieve the above object, a method for manufacturing a hot-rolled steel sheet for a vacuum tube according to an embodiment of the present invention includes the steps of reheating a steel slab containing, by weight, 0.03-0.11% carbon (C), 0.3-1.5% silicon (Si), 1-3% cobalt (Co), 1.2-2.2% manganese (Mn), the balance being Fe, and other inevitable impurities, at 1,100°C to 1,300°C; hot-rolling the reheated steel slab; and cooling the hot-rolled hot-rolled steel sheet to 600-700°C and coiling it, wherein the hot-rolled steel sheet has a final microstructure having a composite structure containing ferrite and pearlite and satisfies the following Relational Formula 1 and Relational Formula 2:

[0022] [Equation 1] 355≦(11+394×D -0.5 )+(448×[C])+(94×[Si])+(36.5×[Co])+(69×[Mn])+(3,429×[Nb])

[0023] [Equation 2] 150≦(186-210×D -0.5 )-(121×[C])+(13.2×[Si])+(31.1×[Co])-(13.7×[Mn])-(4,723×[Nb]) (In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.)

[0024] The hot rolling can be carried out under the condition of a finish rolling temperature of 860°C to 960°C.

[0025] The final microstructure of the hot-rolled steel sheet may be composed of 60 to 95% by area of ​​ferrite, 5 to 40% by area of ​​pearlite, and other unavoidable structures.

[0026] In this case, the other unavoidable structures preferably include at least one low-temperature structure of bainite and martensite, and the low-temperature structure preferably accounts for 5% or less in area ratio.

[0027] The average crystal grain size of the ferrite may be 7 to 15 μm.

[0028] The hot-rolled steel sheet has a yield strength (YS) of 350 MPa or more and a Charpy impact energy at -20°C of 50 J or more.

[0029] The hot-rolled steel sheet has a vibration damping ratio of 150×10 measured at a frequency of 1,650 Hz in bending vibration mode. -6 More preferably, it is equal to or greater than this.

[0030] In a weld formed by welding the hot-rolled steel sheets by submerged arc welding, it is preferable that the Charpy impact energy of the weld at -20°C is 50 J or more, and the fraction of MA phase contained in the weld is 5% or less in terms of area ratio.

[0031] The thickness of the hot-rolled steel plate is preferably 10 mm or more. [Effects of the Invention]

[0032] The hot-rolled steel sheet for vacuum train tubes and its manufacturing method according to the present invention not only has a yield strength of 350 MPa or more and a Charpy impact energy of 50 J or more at -20°C, but also has a vibration damping ratio of 150 x 10 measured at a frequency of 1,650 Hz in a flexural vibration mode using a test specimen having a length, width, and thickness of 80 x 20 x 2 mm. -6 The above is shown.

[0033] In addition, the hot-rolled steel sheet for vacuum train tubes and the manufacturing method thereof according to the present invention, when welded by submerged arc welding, has a Charpy impact energy of 50 J or more at -20°C at the weld, and the proportion of MA phases contained in the weld is 5% or less in terms of area ratio.

[0034] As a result, the hot-rolled steel sheet for vacuum train tubes and the manufacturing method thereof according to the present invention have excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the welded joint, and have physical properties suitable for use in vacuum train tubes.

[0035] The effects of the present invention are not limited to the above-mentioned matters, but can be construed to include matters that a person of ordinary skill in the art can reasonably infer from the matters described in this specification. DETAILED DESCRIPTION OF THE INVENTION

[0036] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the following examples. However, the present invention is not limited to the examples disclosed below, and may be embodied in various different forms. However, these examples are provided to complete the disclosure of the present invention and to fully convey the scope of the invention to those skilled in the art, and the present invention is defined only by the scope of the claims. The same reference symbols throughout the specification refer to the same elements.

[0037] Hereinafter, a hot-rolled steel sheet for a vacuum train tube and a manufacturing method thereof according to a preferred embodiment of the present invention will be described in detail.

[0038] "Hot-rolled steel sheets for vacuum train tubes" A vacuum train is a next-generation transportation method currently in the early stages of development that runs inside a tube under vacuum or near-vacuum conditions. Vacuum trains eliminate friction between the wheels and tracks, minimizing air resistance, making them a transportation method that can effectively achieve high speeds and high efficiency.

[0039] However, due to the nature of vacuum trains, which operate at ultra-high speeds, serious accidents could occur if the safety of the vacuum train is not adequately ensured. In particular, not only could a vacuum tube be structurally damaged or collapsed, but a partial deformation of the tube could also cause a catastrophe. Therefore, stricter safety standards are required for materials used in vacuum train tubes.

[0040] As a result of extensive research, the inventors of the present invention have found that the following physical properties are essential for the material of the vacuum tube to ensure the safety of the vacuum train.

[0041] The primary physical property required for materials for vacuum train tubes is high strength. Because vacuum trains travel through the interior of vacuum tubes, materials for vacuum tubes must have sufficient strength as a structure. Furthermore, because the interior of a vacuum tube must be maintained in a vacuum or near-vacuum state, the material must have sufficient strength to prevent deformation of the vacuum tube due to the pressure difference between the inside and outside.

[0042] The second physical property required for vacuum tube materials is vibration damping. 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 can even cause damage to the vacuum tube in extreme cases.

[0043] Therefore, if a material with a vibration damping ratio above a certain level is applied to the vacuum tube, it can effectively reduce vibrations within the vacuum tube after the preceding pod passes, thereby effectively contributing to the safety of the vacuum train.

[0044] The third physical property required for materials for vacuum tubes is low-temperature toughness. Vacuum trains may operate in polar regions or deep underwater. Steel materials tend to be more susceptible to damage in low-temperature or extremely low-temperature environments. Therefore, when steel materials are used for vacuum tubes, they must have a certain level of low-temperature toughness to ensure safety. In particular, because vacuum train tubes are manufactured into tube shape by welding, excellent low-temperature toughness is required not only for the base material but also for the welded joints.

[0045] The inventors of the present invention have conducted extensive research and have recognized that by strictly controlling the alloy composition and content ratio of the hot-rolled steel sheet and the final microstructure, it is possible to achieve excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the weld, all at the same time, and have arrived at the present invention.

[0046] Therefore, the hot-rolled steel sheet for vacuum train tubes according to an embodiment of the present invention contains, by weight, carbon (C): 0.03 to 0.11%, silicon (Si): 0.3 to 1.5%, cobalt (Co): 1 to 3%, manganese (Mn): 1.2 to 2.2%, the balance being Fe, and other unavoidable impurities, and the final microstructure has a composite structure containing ferrite and pearlite, and satisfies the following relational formulas 1 and 2.

[0047] [Equation 1] 355≦(11+394×D -0.5 )+(448×[C])+(94×[Si])+(36.5×[Co])+(69×[Mn])+(3,429×[Nb])

[0048] [Equation 2] 150≦(186-210×D -0.5)-(121×[C])+(13.2×[Si])+(31.1×[Co])-(13.7×[Mn])-(4,723×[Nb])

[0049] In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.

[0050] Furthermore, the final microstructure of the hot-rolled steel sheet of the present invention is composed of 60 to 95% by area of ​​ferrite, 5 to 40% by area of ​​pearlite, and other unavoidable structures, wherein the other unavoidable structures include at least one low-temperature structure of bainite and martensite, and the low-temperature structure preferably accounts for 5% or less by area.

[0051] Furthermore, the average crystal grain size of the ferrite is preferably 7 to 15 μm.

[0052] The hot-rolled steel sheet of the present invention has a yield strength (YS) of 350 MPa or more and a Charpy impact energy at -20°C of 50 J or more.

[0053] The hot-rolled steel sheet of the present invention has a vibration damping ratio of 150×10 measured at a frequency of 1,650 Hz in bending vibration mode. -6 More preferably, it is equal to or greater than this.

[0054] In a weld formed by welding hot-rolled steel sheets by submerged arc welding, it is preferable that the Charpy impact energy of the weld at -20°C is 50 J or more, and the fraction of MA phase contained in the weld is 5% or less in terms of area ratio.

[0055] The thickness of the hot-rolled steel sheet of the present invention may be 10 mm or more.

[0056] The steel composition and composition ratio of the hot-rolled steel sheet for vacuum train tubes according to the embodiment of the present invention will be described in more detail below. Unless otherwise specified, the percentage indicating the content of each element is based on weight.

[0057] "Carbon (C): 0.03~0.11%" Carbon (C) is a component that has a significant effect on the strength of steel plate.

[0058] The carbon (C) is preferably added in a content ratio of 0.03 to 0.11 wt % of the total weight of the hot-rolled steel sheet according to the embodiment of the present invention, and more preferably in a range of 0.05 to 0.09 wt %.

[0059] If the amount of carbon (C) added is less than 0.03 wt%, it may be difficult to ensure the strength required for the structure. Conversely, if the amount of carbon (C) added is too much, exceeding 0.11 wt%, the toughness of the material may decrease, weldability may deteriorate, and the yield ratio may increase. Furthermore, if the amount of carbon (C) added is too much, exceeding 0.11 wt%, it may be difficult to coarsen the crystal grains.

[0060] "Silicon (Si): 0.3 to 1.5%" Silicon (Si) is an element that can increase magnetic hysteresis and vibration damping ratio. It also tends to be removed along with oxygen because it combines with oxygen to form slag during the steelmaking process. Silicon (Si) is also an element that effectively contributes to improving the strength of materials.

[0061] Silicon (Si) is preferably added in an amount of 0.3 to 1.5 wt % of the total weight of the hot-rolled steel sheet according to an embodiment of the present invention, and more preferably, 0.5 to 1.0 wt %.

[0062] If the amount of silicon (Si) added is less than 0.3 wt%, it is difficult to achieve the above effects. On the other hand, if the amount of silicon (Si) added is more than 1.5 wt%, it may hinder the sloughing off of surface scale, which may degrade the quality of the product surface. Furthermore, if the amount of silicon (Si) added is excessive, more than 1.5 wt%, it may promote the formation of MA phase (martensite-austenite complex) in the weld, which may reduce the low-temperature toughness of the weld.

[0063] "Cobalt (Co): 1.0~3.0%" The present invention maximizes the phenomenon of external vibration damping due to friction generated when domain walls move. To this end, adding alloy elements that increase the magnetic hysteresis or magnetostriction constant generated when domain walls move is useful. Cobalt (Co) is an effective element that can increase magnetic hysteresis without decreasing magnetic permeability and improve vibration damping capacity.

[0064] Therefore, it is preferable that cobalt (Co) is added in a content ratio of 1.0 to 3.0 wt% of the total weight of the hot-rolled steel sheet for vacuum train tube according to the present invention, and a more preferable range is 1.5 to 2.5 wt%.

[0065] When the amount of cobalt (Co) added is less than 1.0 wt%, the amount may be small and the effect of improving vacuum damping capacity may be effectively exhibited. Conversely, when the amount of cobalt (Co) added exceeds 3.0 wt%, there is a high risk of cost increase due to the increase in the amount of expensive cobalt added. Furthermore, when the amount of cobalt (Co) added exceeds 3.0 wt%, it is not preferable because it may cause side effects due to the formation of precipitates.

[0066] "Manganese (Mn): 1.2-2.2%" Manganese (Mn) is an element that improves the strength and hardenability of steel.

[0067] Manganese (Mn) is preferably added in an amount of 1.2 to 2.2 wt % of the total weight of the hot-rolled steel sheet for vacuum train tubes according to the present invention, and more preferably, 1.5 to 2.0 wt %.

[0068] If the amount of manganese (Mn) added is less than 1.2 wt%, it is difficult to effectively improve strength and hardenability. Conversely, if the amount of manganese (Mn) added is more than 2.2 wt%, material variations occur due to segregation in the center, and crack propagation resistance may deteriorate. Furthermore, if the amount of manganese (Mn) added is too much, more than 2.2 wt%, the toughness of the steel may decrease.

[0069] On the other hand, niobium (Nb), titanium (Ti), and vanadium (V) are elements that improve the strength of steel sheets by forming fine carbides. Therefore, niobium (Nb), titanium (Ti), and vanadium (V) are used in various products to ensure strength. However, these precipitates hinder the growth of crystal grains and the movement of domain walls, thereby deteriorating the vibration damping ratio. Therefore, in the present invention, niobium (Nb), titanium (Ti), and vanadium (V) are not added.

[0070] In addition to the above-mentioned components, the hot-rolled steel sheet for vacuum train tubes according to the embodiments of the present invention may contain the remainder Fe and other inevitable impurities. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to completely eliminate them. These impurities are known to anyone with ordinary skill in the art, and therefore, the entire contents of these impurities are not separately mentioned in this specification. Furthermore, the addition of other effective components in addition to the above-mentioned components is not completely excluded.

[0071] The hot-rolled steel sheet for vacuum train tubes according to the embodiment of the present invention may have a composite structure including ferrite and pearlite as the final microstructure, and the hot-rolled steel sheet for vacuum train tubes according to the embodiment of the present invention minimizes the formation of low-temperature structures such as bainite and martensite.

[0072] These 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, the hot-rolled steel sheet for vacuum train tubes targeted in the present invention is quite thick, at 10 mm or more, and even if a low-temperature structure is introduced, variations in physical properties occur in the thickness direction of the steel sheet. Therefore, the low-temperature structure is formed only on the surface of the steel sheet, and it is difficult to sufficiently form a low-temperature structure throughout the thickness center of the steel sheet. For this reason, in the present invention, the formation of low-temperature structures, such as bainite and martensite, is intentionally suppressed as much as possible.

[0073] Therefore, in order to minimize the variation in physical properties between the surface and the center of the thickness of the hot-rolled steel sheet for vacuum train tubes according to the embodiment of the present invention, the microstructure of the steel sheet is composed of a composite structure including ferrite and pearlite, but even if low-temperature structures such as bainite and martensite are inevitably formed, their fraction is suppressed as much as possible to 5% or less in area ratio, more preferably 3% or less, and even more preferably 1% or less. From the viewpoint of ensuring physical properties, the fraction of ferrite may be 60 to 95% in area ratio, and the fraction of pearlite may be 5 to 40% in area ratio.

[0074] In order to simultaneously ensure the desired yield strength, vibration damping ratio, and low-temperature toughness, the present invention limits the average ferrite grain size to a certain range of 7 to 15 μm. Since a larger average ferrite grain size is more advantageous for ensuring the vibration damping ratio, the ferrite grain size is limited to a minimum of 7 μm. On the other hand, if the average ferrite grain size is too large, the strength and low-temperature toughness of the material deteriorate, so the average ferrite grain size is limited to a maximum of 15 μm.

[0075] 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, when the carbon (C), silicon (Si), cobalt (Co), and manganese (Mn) contents and the average ferrite grain size are controlled within a certain range of 7 to 15 μm, it is possible to simultaneously ensure the yield strength, vibration damping ratio, and low-temperature toughness of the weld, and have arrived at the following Relational Formulas 1 and 2. Relational Formulas 1 and 2 indicate that the yield strength is 350 MPa or more depending on the alloying elements and the average ferrite grain size, and that the yield strength is 150×10 -6 The present inventors have designed the formulas to predict whether a hot-rolled steel sheet will exhibit a vibration damping ratio of 350 MPa or more. Relational formula 1 relates mainly to the yield strength, and relational formula 2 relates mainly to the vibration damping ratio measured at a frequency of 1,650 Hz in the bending vibration mode. When both relations 1 and 2 are satisfied, the hot-rolled steel sheet will exhibit a yield strength of 350 MPa or more, and a vibration damping ratio of 150 × 10 -6 If relational expression 1 is not satisfied, the yield strength of the hot-rolled steel sheet is predicted to be less than 350 MPa, and if relational expression 2 is not satisfied, the vibration damping ratio of the hot-rolled steel sheet is predicted to be less than 150 × 10 -6 It is predicted that the number of cases will fall short of 100.

[0076] [Equation 1] 355≦(11+394×D -0.5 )+(448×[C])+(94×[Si])+(36.5×[Co])+(69×[Mn])+(3,429×[Nb])

[0077] [Equation 2] 150≦(186-210×D -0.5 )-(121×[C])+(13.2×[Si])+(31.1×[Co])-(13.7×[Mn])-(4,723×[Nb])

[0078] In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.

[0079] As described above, the hot-rolled steel sheet for vacuum train tubes according to the embodiment of the present invention satisfies the relational expressions 1 and 2, and therefore can simultaneously ensure the desired yield strength, vibration damping ratio, and low-temperature toughness of the weld.

[0080] As mentioned above, vacuum train tubes require a design thickness and strength for structural stability. For this reason, in this invention, we determined that tube design would be easy only if the yield strength was 355 MPa or more, and developed a material with a value of 355 or more in relational expression 1. We also developed a steel material with a vibration damping ratio predicted from relational expression 2 of 150 or more. Small trains called pods move inside the vacuum train tube every two minutes, and if the vibrations do not dampen quickly, there is a risk of destruction due to resonance. For this reason, as shown in relational expression 2, a vibration damping ratio of 150 x 10 -6 If the above is ensured, the risk of these disruptions will be significantly reduced.

[0081] Furthermore, the hot-rolled steel sheet for vacuum train tubes according to the embodiments of the present invention may have a yield strength of 350 MPa or more and a Charpy impact energy of 50 J or more at -20°C. More specifically, the hot-rolled steel sheet for vacuum train tubes according to the embodiments of the present invention may have a yield strength of 350 to 500 MPa and a Charpy impact energy of 50 to 200 J at -20°C.

[0082] Therefore, the hot-rolled steel sheet for vacuum train tubes according to the embodiment of the present invention can ensure strength and low-temperature toughness suitable for use as a structural material, thereby effectively ensuring the structural safety of vacuum train tubes.

[0083] The hot rolled steel sheet for vacuum train tubes according to the embodiment of the present invention is 150×10 -6 The vibration damping ratio here means the vibration damping ratio measured at a frequency of 1,650 Hz after hitting a test piece having a length, width and thickness of 80 mm x 20 mm x 2 mm in a flexural vibration mode.

[0084] The hot rolled steel sheet for vacuum train tubes according to the embodiment of the present invention is 150×10 -6 Because of the above vibration damping ratio, the amplification of vibrations within the vacuum tube can be effectively suppressed, and damage to the vacuum train tube due to vibration can be effectively prevented.

[0085] When the hot-rolled steel sheet according to the present invention is welded using submerged arc welding, the Charpy impact energy of the weld at -20°C may be 50 J or more, and the fraction of the MA phase (martensite-austenite complex) contained in the weld may be 5% or less in area ratio. Preferably, the fraction of the MA phase in the weld may be 3% or less in area ratio, and even more preferably, the fraction of the MA phase in the weld may be 1% or less in area ratio.

[0086] Here, the welded portion is a position 1 mm away from the fusion line, and can be interpreted to include both the weld metal portion and the heat-affected zone (HAZ).

[0087] 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) as much as possible, because when welding is performed using a welding material that contains silicon (Si), there is a possibility that a large amount of light MA phase will be formed in the weld due to excessive hardening ability.

[0088] Therefore, the present invention can 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.

[0089] "Method of manufacturing hot-rolled steel sheets for vacuum train tubes" Hereinafter, a method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to an embodiment of the present invention will be described.

[0090] A method for manufacturing a hot rolled steel sheet for a vacuum train tube according to an embodiment of the present invention includes a reheating step, a hot rolling step, and a cooling and coiling step.

[0091] "Reheat" In the reheating stage, a steel slab containing, by weight, carbon (C): 0.03-0.11%, silicon (Si): 0.3-1.5%, cobalt (Co): 1-3%, manganese (Mn): 1.2-2.2%, the balance being Fe, and other unavoidable impurities is reheated at 1,100°C to 1,300°C.

[0092] In this stage, during hot rolling, the steel slab can be heated in a temperature range of 1,100°C or higher, taking into consideration the rolling load. In particular, in the present invention, in order to introduce a microstructure of a certain size or larger, a preferred steel slab heating temperature may be 1,200°C or higher, and an even more preferred steel slab heating temperature may be 1,250°C or higher. On the other hand, if the steel slab heating temperature is too high, exceeding 1,300°C, there is a risk of deterioration in surface quality due to the formation of scale.

[0093] "Hot rolling" In the hot rolling stage, the reheated steel slab is hot rolled at a finish rolling temperature of 860°C to 960°C.

[0094] In the present invention, the steel plate provided by hot rolling may have a thickness of 10 mm or more.

[0095] During hot rolling, the crystal grains deform as the steel slab is rolled, but immediately recrystallize. Through this process, the coarse and uneven structure becomes finer and more homogenous. 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 grain size of the final microstructure can be controlled depending on the finishing delivery temperature. Since the present invention aims to control the final microstructure to a certain size or higher, it is preferable to perform hot rolling at a finishing delivery temperature of 860°C or higher, and a more preferable finishing delivery temperature may be 900°C or higher. On the other hand, if the finishing delivery temperature is too high, exceeding 960°C, the final microstructure may become too coarse and is therefore not preferred.

[0096] "Cooling and winding" In the cooling and coiling stage, the hot-rolled steel sheet is cooled to 600 to 700°C and then coiled.

[0097] The hot-rolled steel sheet may be water-cooled and then coiled at a coiling temperature of 600°C to 700°C. Since the present invention aims to realize a composite structure containing ferrite and pearlite as the final microstructure, it is preferable to carry out coiling at a temperature range of 600°C or higher. Since the present invention aims to realize a final microstructure of a certain size or larger, it is more preferable to carry out coiling at a temperature range of 630°C or higher. However, if the coiling temperature is too high, exceeding 700°C, a coarse microstructure may be formed or the surface quality may deteriorate.

[0098] The hot-rolled steel sheet manufactured by the manufacturing method according to the embodiment of the present invention described above can satisfy the following Relational Expressions 1 and 2.

[0099] [Equation 1] 355≦(11+394×D -0.5 )+(448×[C])+(94×[Si])+(36.5×[Co])+(69×[Mn])+(3,429×[Nb])

[0100] [Equation 2] 150≦(186-210×D -0.5 )-(121×[C])+(13.2×[Si])+(31.1×[Co])-(13.7×[Mn])-(4,723×[Nb])

[0101] In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.

[0102] In this way, the hot-rolled steel sheet manufactured by the manufacturing method according to the embodiment of the present invention satisfies the relational expressions 1 and 2, and therefore can simultaneously ensure the desired yield strength, vibration damping ratio, and low-temperature toughness of the weld.

[0103] Therefore, the hot-rolled steel sheet manufactured by the manufacturing method according to the embodiment of the present invention not only has a yield strength of 350 MPa or more and a Charpy impact energy of 50 J or more at -20°C, but also has a vibration damping ratio of 150 x 10 measured at a frequency of 1,650 Hz in a flexural vibration mode using a test specimen having a length, width, and thickness of 80 x 20 x 2 mm. -6 The above is shown.

[0104] Furthermore, when the hot-rolled steel sheet manufactured by the manufacturing method according to the embodiment of the present invention is welded by submerged arc welding, the weld has a Charpy impact energy of 50 J or more at -20°C, and the fraction of MA phase contained in the weld has an area ratio of 5% or less. Here, the weld may refer to a position 1 mm away from the fusion line.

[0105] As a result, the hot-rolled steel sheet manufactured by the manufacturing method according to the embodiment of the present invention has excellent yield strength, vibration damping ratio, weldability, and low-temperature toughness of the weld, and has physical properties suitable for use in vacuum train tubes. [Example]

[0106] The structure and operation of the present invention will be described in more detail below through preferred examples of the present invention, which are presented as preferred examples of the present invention and are not to be construed as limiting the present invention in any sense.

[0107] The contents not described here will not be explained here because they can be easily inferred by those skilled in the art.

[0108] 1. Preparation of specimens Steel slabs having a thickness of 250 mm and having alloy compositions A to F listed in Table 1 below were prepared, and then hot-rolled steel sheets were manufactured using the process conditions listed in Table 2 below. The balance in Table 1 is iron (Fe) and other unavoidable impurities, and hot-rolled steel sheets (specimens No. 1 to 15) having a thickness of 2.0 mm were manufactured using the process conditions listed in Table 2. Alloy components not listed in Table 1 below represent impurities and the balance Fe.

[0109] (unit: weight %) [Table 1]

[0110] [Table 2]

[0111] 2. Evaluation of physical properties Table 3 shows the results of evaluating the physical properties of specimens 1 to 15.

[0112] 1) Analysis of microstructure and mechanical properties The microstructure and mechanical properties of each specimen were analyzed and are listed in Tables 2 and 3. The microstructure was measured using an optical microscope at 500x magnification after etching each specimen using Nital etching. The ASTM# of ferrite was measured according to ASTM E112.

[0113] 2) Measurement of vibration damping ratio The vibration damping ratio was measured at room temperature using IMCE's RFDA LTV800 after preparing a specimen with length, width, and thickness of 80mm x 20mm x 2mm. After striking in flexural vibration mode, the vibration damping ratio in the 1,650Hz range, which corresponds to the first mode of the vibration mode of the specimen, was measured and analyzed, and the results are shown in Table 3.

[0114] 3) Evaluation of weldability and low-temperature toughness of welds Submerged arc welding was performed on each specimen using a welding material containing 0.052 wt% C, 1.53 wt% Mn, 1.3 wt% Ni, 0.135 wt% Mo, the balance being Fe, and other unavoidable impurities.

[0115] During submerged arc welding, the inside is 20kJ / cm 2 The heat input is applied to the outside, and 22 kJ / cm 2 The Charpy impact toughness of the welds was measured at −20° C. by KS B 0810, and the results are shown in Table 3.

[0116] [Table 3]

[0117] As shown in Tables 1 to 3, specimens 1 and 10 to 15 satisfying the alloy composition, process conditions, and relations 1 and 2 of the present invention have a yield strength of 350 MPa or more and a yield strength of 150 × 10 -6 Not only does it satisfy the above vibration damping ratio, but it can also be confirmed that the Charpy impact energy of the weld at -20°C is 50J or more.

[0118] On the other hand, it can be seen that specimens 2 to 9, which do not satisfy the alloy composition, process conditions, and at least one of the relational expressions 1 and 2 of the present invention, are unable to simultaneously secure the desired physical properties. In particular, in relation to the relational expressions 1 and 2, specimen 6, which does not satisfy relational expression 1, had a yield strength of less than 350 MPa. Furthermore, specimens 3, 5, 7, and 9, which do not satisfy relational expression 2, had a vibration damping ratio of 150×10 -6 Furthermore, specimen 7, to which niobium (Nb) was added, showed a significantly lower vibration damping ratio than the other specimens.

[0119] On the other hand, for comparison with conventional materials, tests were conducted under the same conditions on the existing structural steel material EN-S355. In the case of EN-S355, the vibration damping ratio measured under the same conditions was 60 × 10 -6 It was confirmed that the level was only

[0120] As can be seen from the above experimental results, it was proven that Specimen 1, which satisfies the alloy composition, process conditions, and relations 1 and 2 of the present invention, has excellent yield strength, vibration damping ratio, and low-temperature toughness of the weld, and has physical properties suitable for use in vacuum train tubes.

[0121] Although the present invention has been described above with reference to its preferred embodiments, various modifications and variations may be made by those skilled in the art. These modifications and variations are within the scope of the present invention as long as they do not deviate from the scope of the technical concept provided by the present invention. Therefore, the scope of the present invention should be determined by the following claims.

Claims

1. A hot-rolled steel sheet for vacuum train tubes, In weight percent, carbon (C): 0.03 to 0.11%, silicon (Si): 0.3 to 1.5%, cobalt (Co): 1 to 3%, manganese (Mn): 1.2 to 2.2%, the balance being Fe, and other unavoidable impurities; The final microstructure has a composite structure including ferrite and pearlite, The following relational expressions 1 and 2 are satisfied: Hot rolled steel plate. [Relationship 1] 355≦(11+394×D －０．５ )+(4448×[C])+(94×[Si])+(36.5×["])+(69×[M])+(3,429×[Nb]) [Relationship 2] 150≦(186-210×D －０．５ )-(121×[C])+(13.2×[Si])+(31.1×["])-(13.7×[M])-(4,723×[Nb]) (In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.)

2. The hot-rolled steel sheet has a final microstructure consisting of 60 to 95% ferrite and 5 to 40% pearlite in terms of area ratio, and other unavoidable structures. The hot-rolled steel sheet according to claim 1.

3. The other unavoidable structures include at least one low-temperature structure of bainite and martensite, and the low-temperature structure accounts for 5% or less in area ratio. The hot-rolled steel sheet according to claim 2.

4. The average grain size of the ferrite is 7 to 15 μm. The hot-rolled steel sheet according to claim 1.

5. The hot-rolled steel sheet has a yield strength (YS) of 350 MPa or more and a Charpy impact energy of 50 J or more at −20° C. The hot-rolled steel sheet according to claim 1.

6. The hot-rolled steel sheet has a vibration damping ratio of 150×10 measured at a frequency of 1,650 Hz in a bending vibration mode. －６ That's all. The hot-rolled steel sheet according to claim 1.

7. In a weld formed by welding the hot-rolled steel plates by submerged arc welding, the Charpy impact energy of the weld at −20° C. is 50 J or more, and the fraction of the M-A phase contained in the weld is 5% or less in area ratio. The hot-rolled steel sheet according to claim 1.

8. The thickness of the hot-rolled steel plate is 10 mm or more. The hot-rolled steel sheet according to claim 1.

9. A method for manufacturing a hot rolled steel sheet for a vacuum train tube, comprising: A step of reheating a steel slab containing, by weight, 0.03 to 0.11% carbon (C), 0.3 to 1.5% silicon (Si), 1 to 3% cobalt (Co), 1.2 to 2.2% manganese (Mn), the balance being Fe, and other unavoidable impurities at 1,100°C to 1,300°C; hot rolling the reheated steel slab; and cooling the hot-rolled steel sheet to 600 to 700°C and coiling it; The hot-rolled steel sheet has a final microstructure having a composite structure containing ferrite and pearlite, The following relational expressions 1 and 2 are satisfied: method. [Relationship 1] 355≦(11+394×D －０．５ )+(4448×[C])+(94×[Si])+(36.5×["])+(69×[M]])+(3,429×[Nb]) [Relationship 2] 150≦(186-210×D －０．５ )-(121×[C])+(13.2×[Si])+(31.1×["])-(13.7×[M])-(4,723×[Nb]) (In the above Relational Formulas 1 and 2, D represents the average grain size of ferrite in the hot-rolled steel sheet, and [ ] represents the weight percentage of each element.)

10. The hot rolling is performed under the condition of a finish rolling temperature of 860 ° C to 960 ° C.

10. The method of claim 9.

11. The hot-rolled steel sheet has a final microstructure consisting of 60 to 95% ferrite and 5 to 40% pearlite in terms of area ratio, and other unavoidable structures.

10. The method of claim 9.

12. The other unavoidable structures include at least one low-temperature structure of bainite and martensite, and the low-temperature structure accounts for 5% or less in area ratio. The method of claim 11.

13. The average grain size of the ferrite is 7 to 15 μm.

10. The method of claim 9.

14. The hot-rolled steel sheet has a yield strength (YS) of 350 MPa or more and a Charpy impact energy of 50 J or more at −20° C.

10. The method of claim 9.

15. The hot-rolled steel sheet has a vibration damping ratio of 150×10 measured at a frequency of 1,650 Hz in a bending vibration mode. －６ That's all.

10. The method of claim 9.

16. In a weld formed by welding the hot-rolled steel plates by submerged arc welding, the Charpy impact energy of the weld at −20° C. is 50 J or more, and the fraction of the M-A phase contained in the weld is 5% or less in area ratio.

10. The method of claim 9.

17. The thickness of the hot-rolled steel plate is 10 mm or more.

10. The method of claim 9.

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