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

Abstract

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

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, electrical resistivity and low-temperature toughness, and physical properties suitable for use in vacuum train tubes, and a manufacturing method thereof. [Background technology]

[0002] A vacuum train, or hyper tube train, is a system in which a train travels on magnetic levitation inside a vacuum tube. Vacuum trains can operate at ultra-high speeds because they do not have friction with the air or tracks, which are the main causes of energy loss when a train runs. Because of the low energy loss, they can achieve 93% energy savings compared to aircraft, and they have been attracting attention as an environmentally friendly means of transportation of the next generation, with active research 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 tube materials for vacuum trains. One of these is concrete. Concrete tubes are advantageous in terms of cost, but it is difficult to join the individual tubes, which are about 10 meters long. In addition, due to the pores inside the concrete, when a vacuum is achieved, external gases can enter the inside of the tube, making it easy for the vacuum to be broken. Another material that is being researched extensively is composite materials such as carbon fiber. Composite materials such as carbon fiber are lightweight and have high performance, but their biggest drawback is their high cost.

[0004] 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. It is also a material that is easy to assemble and weld when connecting accessories between tubes or to tubes, and has an appropriate outgassing rate when maintaining a vacuum. However, since ultra-high-speed vacuum trains run at significantly faster speeds than current high-speed trains, the safety of passengers and surrounding facilities should be considered as the top priority. Currently, even safety standards for ultra-high-speed vacuum trains have not been established, and the development of tube materials to ensure the safety of ultra-high-speed vacuum trains is insufficient. In addition, while vacuum trains need to be guaranteed high efficiency to meet the needs of the times, the development of tube materials to maximize the energy efficiency of vacuum trains is also insufficient.

[0005] Therefore, there is an urgent need to develop a material for vacuum train tubes that has suitable processability and outgassing rate for use in vacuum train tubes, while also ensuring safety and achieving high efficiency. [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 aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, electrical resistivity and low-temperature toughness, and physical properties suitable for use in vacuum train tubes, and a method for manufacturing the same.

[0008] The object of the present invention is not limited to the above-mentioned contents, 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 vacuum train tubes according to one aspect of the present invention contains, by weight, 0.03 to 0.25% carbon (C), 1.5 to 2.5% silicon (Si), 0.8 to 1.8% manganese (Mn), with the remainder being Fe and other unavoidable impurities, has a composite structure of ferrite and pearlite as a microstructure, and can satisfy the following Relational Formulas 1 to 3.

[0010] [Equation 1] 350≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0011] [Equation 2] 100≦186-210×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0012] [Equation 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si] In the above Relational Formulas 1 to 3, D means the average crystal grain size (μm) of ferrite in the heat-rolled steel sheet, and [C], [Si] and [Mn] mean the contents (wt%) of carbon (C), silicon (Si) and manganese (Mn) in the heat-rolled steel sheet, respectively.

[0013] The above-mentioned hot-rolled steel sheet can satisfy the following Relational Expression 4.

[0014] [Equation 4] 303.78-85.22×ln(D)>27 In the above Relational Formula 4, D means the average crystal grain size (μm) of ferrite in the above heat-rolled steel sheet.

[0015] The microstructure of the above-mentioned hot-rolled steel sheet may be composed of 60 to 90 area % of ferrite, 10 to 40 area % of pearlite, and other unavoidable structures.

[0016] The total content of titanium (Ti), niobium (Nb) and vanadium (V) that are inevitably contained in the above-mentioned hot-rolled steel sheet may be less than 0.01% (including 0%).

[0017] The average crystal grain size (D) of the ferrite may be 10 to 30 μm.

[0018] The hot-rolled steel sheet has a yield strength of 350 MPa or more, a Charpy impact energy of 27 J or more at -20°C, and a vibration damping ratio of 100×10 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 x 20 x 2 mm. -6 The electrical resistivity is 30×10 -8 It can be Ωm or more.

[0019] The hot-rolled steel sheet may have a thickness of 10 mm or more.

[0020] A method for producing a hot rolled steel sheet for a vacuum train tube according to one aspect of the present invention includes the steps of heating a slab containing, by weight, 0.15-0.25% carbon (C), 0.3-1.3% silicon (Si), 1.0-2.0% 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 heating temperature (T1), the finish rolling temperature (T2) and the coiling temperature (T3) can satisfy the following Relational Formula 5.

[0021] [Equation 5] 1≦0.0284×[T1]+0.071×[T2]+0.045×[T3]-131≦3 In the above Relational Formula 5, [T1], [T2], and [T3] respectively mean the slab heating temperature (T1, °C), the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C).

[0022] The total content of titanium (Ti), niobium (Nb) and vanadium (V) inevitably contained in the slab may be less than 0.01% (including 0%).

[0023] The slab can satisfy the following relation 3.

[0024] [Equation 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si] In the above Relational Formula 3, [C], [Si] and [Mn] respectively mean the contents (wt%) of carbon (C), silicon (Si) and manganese (Mn) in the above heat-rolled steel sheet.

[0025] The above solutions to the problems described above are not intended to be exhaustive, and the various features of the present invention and their associated advantages and benefits can be more fully understood with reference to the following specific implementations and examples. Effect of the Invention

[0026] According to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, electrical resistivity and low-temperature toughness, and physical properties suitable for use in vacuum train tubes, and a method for manufacturing the same.

[0027] The effects of the present invention are not limited to the above-mentioned matters, but can be interpreted as including matters that a person of ordinary skill in the art can reasonably deduce from the matters described in this specification. [Brief description of the drawings]

[0028] [Figure 1] 1 is an optical microscope photograph used for observing the microstructure of test piece 1. [Diagram 2] This is an optical microscope image of EN-S355, an existing structural steel material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The present invention relates to a hot rolled steel sheet for vacuum train tubes and a manufacturing method thereof, and preferred embodiments of the present invention will be described below. The embodiments of 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 further explain the present invention to those skilled in the art to which the present invention pertains.

[0030] A vacuum train is a next-generation transportation means that runs in a tube in a vacuum or semi-vacuum state and is currently in the early stages of development. A vacuum train is a transportation means that can effectively achieve high speed and high efficiency by eliminating friction resistance between the wheels and the tracks and minimizing air resistance. However, due to the characteristics of a vacuum train that runs at ultra-high speeds, if the safety of the vacuum train is not sufficiently ensured, a major accident may occur. In particular, not only if the vacuum tube is structurally damaged or collapsed, but also if a part of the tube is deformed in shape, a major disaster may occur, so stricter safety is required for the tube material for the vacuum train. As a result of intensive research by the inventor of the present invention, it has been found that the following physical properties are important for the vacuum tube material in order to ensure the safety of a vacuum train.

[0031] To ensure safety, the first physical property required for vacuum tube materials is high strength. Since vacuum trains move through the inside of vacuum tubes, vacuum tube materials are required to have sufficient strength as a structure. In addition, since the inside of a vacuum tube needs to be maintained in a vacuum or near-vacuum state, the material must have sufficient strength to prevent the tube from deforming due to the pressure difference between the inside and outside.

[0032] The second physical property required for vacuum tube materials to ensure safety is vibration damping capacity. In a vacuum train, pods carrying several to several tens of people pass through the inside of the vacuum tube at intervals of several tens of seconds to several minutes. After the preceding pod has passed, vibrations inside the vacuum tube can be amplified when a following pod passes, causing resonance, which in severe cases can even lead to damage to the tube. Therefore, if a material with a vibration damping ratio above a certain level is applied to the vacuum tube, it can effectively reduce vibrations inside the tube after the preceding pod has passed, effectively contributing to the safety of the vacuum train.

[0033] The third physical property required for vacuum tube materials to ensure safety is low-temperature toughness. Vacuum trains may operate in polar regions or deep underwater. Steel materials tend to break more easily in low-temperature or extremely low-temperature environments, so when steel materials are used for vacuum tubes, they are required to have a certain level of low-temperature toughness to ensure safety.

[0034] In addition, in recent years, there has been a rapid increase in the demand for environmentally friendly means of transportation worldwide, so there is a need to maximize the energy efficiency of vacuum tube trains as well. The electromagnetic suspension (EMS) method uses the attractive force between electromagnets to levitate trains, while the electrodynamic suspension (EDS) method uses the repulsive force between a superconductor and a magnet to levitate trains. Among these, the EDS method can form a stronger magnetic field around the train than the EMS method. When a train passes through the tube, changes occur in the magnetic field, forming induced currents in the tube, which can result in energy loss. Therefore, it is necessary to increase the electrical resistance of the tube material to reduce such energy loss, and it is necessary for the tube material to have a certain level of electrical resistivity (ρ) to ensure energy efficiency.

[0035] 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, electrical resistivity, and low-temperature toughness at the same time by strictly controlling the alloy composition content and microstructure of a steel plate, and have thus devised the present invention.

[0036] Hereinafter, a hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention will be described in more detail.

[0037] A hot-rolled steel sheet for vacuum train tubes according to one aspect of the present invention contains, by weight, 0.03-0.25% carbon (C), 1.5-2.5% silicon (Si), 0.8-1.8% manganese (Mn), the remainder being Fe and other unavoidable impurities, has a ferrite and pearlite composite structure as a microstructure, and can satisfy the following Relational Formulas 1 to 3, and can additionally satisfy the following Relational Formula 4.

[0038] [Equation 1] 350≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0039] [Equation 2] 100≦186-210×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0040] [Equation 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si]

[0041] [Equation 4] 303.78-85.22×ln(D)>27 In the above Relational Formulas 1 to 4, D means the average crystal grain size (μm) of ferrite in the above heat-rolled steel sheet, and [C], [Si] and [Mn] mean the contents (wt%) of carbon (C), silicon (Si) and manganese (Mn) in the above heat-rolled steel sheet, respectively.

[0042] The steel composition contained in the hot-rolled steel sheet of the present invention will be described in more detail below. Unless otherwise specified, % indicating the content of each element is based on weight.

[0043] Carbon (C): 0.03~0.25% Carbon (C) is a component that has a great influence on the strength of a steel plate. The present invention can contain 0.03% or more of carbon (C) to ensure the strength required by the structure. The lower limit of the carbon (C) content may be 0.05%, and more preferably 0.07%. On the other hand, if the carbon (C) content is excessive, the toughness of the material may decrease, the weldability may decrease, and the yield ratio may increase. In addition, if the carbon (C) content is excessive, it is difficult to coarsen the crystal grains, so the present invention can limit the upper limit of the carbon (C) content to 0.25%. The upper limit of the carbon (C) content may be 0.2%, and more preferably 0.15%.

[0044] Silicon (Si): 1.5-2.5% Silicon (Si) tends to be removed together with oxygen since it combines with oxygen to form slag during steelmaking. Silicon (Si) is also a component that effectively contributes to improving the strength and electrical resistivity of the material. Therefore, the present invention may contain 1.5% or more silicon (Si) for such effects. A preferred lower limit of the silicon (Si) content may be 1.6%, and a more preferred lower limit of the silicon (Si) content may be 1.8%. Meanwhile, if the silicon (Si) content is excessive, it may prevent the surface scale from falling off, thereby degrading the surface quality of the product. Also, if the silicon (Si) content is excessive, the low-temperature toughness of the base material and the welded part may decrease, increasing the risk of fracture during use of the material, so the present invention may limit the silicon (Si) content to 2.5% or less. A preferred upper limit of the silicon (Si) content may be 2.3%, and a more preferred upper limit of the silicon (Si) content may be 2.0%.

[0045] Manganese (Mn): 0.8-1.8% Manganese (Mn) is a component that improves the strength and hardenability of steel. Therefore, in order to secure such effects, the present invention may contain 0.8% or more of manganese (Mn). A preferable lower limit of the manganese (Mn) content may be 1.0%, and a more preferable lower limit of the manganese (Mn) content may be 1.1%. Meanwhile, if the manganese (Mn) content is excessive, the material variation may occur due to segregation in the center, and the crack propagation resistance may decrease. In addition, if the manganese (Mn) content is excessive, the toughness of the steel may decrease, so the present invention may limit the manganese (Mn) content to 1.8% or less. A preferable upper limit of the manganese (Mn) content may be 1.6%, and a more preferable upper limit of the manganese (Mn) content may be 1.5%.

[0046] The hot-rolled steel sheet of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and therefore, it is not possible to completely exclude them. Since these impurities are known to anyone having ordinary knowledge in this technical field, the entire contents of the impurities will not be specifically mentioned in this specification. Furthermore, the addition of further effective components in addition to the above-mentioned components is not completely excluded.

[0047] The hot-rolled steel sheet of the present invention actively suppresses the addition of titanium (Ti), niobium (Nb) and vanadium (V), and even if these components are inevitably included, the total content can be limited to less than 0.01% (including 0%). Titanium (Ti), niobium (Nb) and vanadium (V) are typical precipitation strengthening elements, and are components that generate fine carbonitrides and effectively contribute to improving the strength of steel. However, titanium (Ti), niobium (Nb) and vanadium (V) excessively refine the microstructure of steel, which acts against ensuring vibration damping capacity, so the present invention actively suppresses these components. In addition, titanium (Ti), niobium (Nb) and vanadium (V) are expensive components, and are not preferable from the viewpoint of economy. The present invention does not artificially add these components, and even if they are inevitably added, the total content of these components can be actively suppressed to less than 0.01%. The total content of these components may preferably be 0.005% or less, and more preferably, the total content of these components may be 0%.

[0048] The hot-rolled steel sheet according to one aspect of the present invention may have a composite structure consisting of ferrite and pearlite as a microstructure. 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 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 according to one example of the present invention has a thickness of 10 mm or more, even if a low-temperature structure is introduced, there is a possibility that physical property variations will 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 to form a sufficient low-temperature structure up to the center of the steel sheet.

[0049] Therefore, in the present invention, in order to reduce the variation in physical properties, the microstructure of the steel sheet is constituted by a composite structure of ferrite and pearlite, and even if low-temperature structures such as bainite and martensite are inevitably formed, the fraction thereof can be actively suppressed to 1 area % or less (including 0%). From the viewpoint of ensuring the physical properties, the fraction of ferrite may be 60 to 90 area %, and the fraction of pearlite may be 10 to 40 area %.

[0050] In order to simultaneously secure the desired yield strength, vibration damping ratio, and low-temperature toughness, the present invention can limit the average crystal grain size of ferrite to a certain range. Since a larger crystal grain size is more advantageous for securing the vibration damping ratio, the present invention can limit the average crystal grain size of ferrite to 10 μm or more. A preferred average crystal grain size may be more than 10 μm, and a more preferred average crystal grain size may be 15 μm or more. On the other hand, if the crystal grain size is excessively large, the strength and low-temperature toughness of the material are reduced, so the present invention can limit the average crystal grain size of ferrite to 30 μm or less. The upper limit of the preferred average crystal grain size may be 25 μm.

[0051] The inventors of the present invention have conducted intensive research into methods for ensuring the stability and energy efficiency of materials for vacuum train tubes, and as a result, have recognized that in low alloy steel sheets such as those of the present invention, when the contents of carbon (C), silicon (Si) and manganese (Mn) and the average grain size of ferrite are controlled within a certain range, it is possible to simultaneously ensure the yield strength, vibration damping ratio and electrical resistivity, and have derived the following Relational Formulas 1 to 3.

[0052] [Equation 1] 350≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0053] [Equation 2] 100≦186-210×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0054] [Equation 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si] In the above Relational Formulas 1 to 3, D means the average crystal grain size (μm) of ferrite in the above heat-rolled steel sheet, and [C], [Si] and [Mn] mean the contents (wt%) of carbon (C), silicon (Si) and manganese (Mn) in the above heat-rolled steel sheet, respectively.

[0055] The hot-rolled steel sheet for a vacuum train tube of the present invention satisfies the relational expressions 1 to 3 simultaneously, and therefore can simultaneously ensure the desired yield strength, vibration damping ratio, and electrical resistivity.

[0056] In addition, the inventors of the present invention recognized that in a steel plate having the composition system of the present invention, when the average crystal grain size of ferrite is controlled within a certain range, it is possible to ensure low temperature toughness, and were able to additionally derive the following Relational Formula 4.

[0057] [Equation 4] 303.78-85.22×ln(D)>27 In the above Relational Formula 4, D means the average crystal grain size (μm) of ferrite in the above heat-rolled steel sheet.

[0058] The hot-rolled steel sheet for vacuum train tubes of the present invention additionally satisfies Relational Formula 4, and therefore can effectively ensure the desired low-temperature toughness.

[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 -20°C Charpy impact energy of 27 J or more. Therefore, the hot-rolled steel sheet for vacuum train tubes of the present invention can ensure strength and low-temperature toughness suitable for use as a structural material, and can effectively ensure the structural safety of vacuum train tubes.

[0060] The hot rolled steel sheet for vacuum train tubes of the present invention is 100×10 -6The 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. The hot-rolled steel sheet for vacuum train tubes of the present invention has a vibration damping ratio of 100 x 10 -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.

[0061] The hot rolled steel sheet for vacuum train tubes of the present invention is 30×10 -8 Since it can have an electrical resistivity of Ωm or more, it can effectively ensure energy efficiency when operating a vacuum train.

[0062] Therefore, according to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, electrical resistivity and low-temperature toughness, and having physical properties suitable for use in vacuum train tubes.

[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 producing a hot rolled steel sheet for a vacuum train tube according to one aspect of the present invention includes the steps of heating a slab containing, by weight, 0.03-0.25% carbon (C), 1.5-2.5% silicon (Si), 0.8-1.8% 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 heating temperature (T1), the finish rolling temperature (T2) and the coiling temperature (T3) can satisfy the following Relational Formula 5.

[0065] [Equation 5] 1≦0.0284×[T1]+0.071×[T2]+0.045×[T3]-131≦3 In the above Relational Formula 5, [T1], [T2], and [T3] respectively mean the slab heating temperature (T1, °C), the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C).

[0066] Preparation and heating of steel slabs A steel slab having a predetermined alloy composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the above-mentioned hot-rolled steel sheet, the description of the alloy composition of the steel slab is substituted for the description of the alloy composition of the above-mentioned hot-rolled steel sheet.

[0067] The prepared steel slab may be heated at a heating temperature (T1) of 1100°C to 1300°C. Considering the rolling load during hot rolling, the steel slab may 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 heating temperature of the steel slab may be 1200°C or higher. More preferably, the 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, there is a concern that the surface quality may be deteriorated due to the generation of scale, and therefore, the present invention may limit the heating temperature of the steel slab to 1300°C or lower.

[0068] Hot rolling The heated steel slab can be hot rolled to a finish rolling temperature (T2) of 900° C. to 1000° C. to provide a hot rolled steel sheet. The steel sheet provided by the hot rolling of the present invention can have a thickness of 10 μm or more.

[0069] During hot rolling, the grains are deformed by rolling the material, but are quickly recrystallized. Through this process, the coarse and non-uniform 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 grain size of the final microstructure can be controlled by the finishing delivery temperature. In the present invention, hot rolling can be performed at a finishing delivery temperature of 900°C or higher in order to control the final microstructure to a certain size or higher. A preferred finishing delivery temperature may be 950°C or higher. On the other hand, if the finishing delivery temperature is excessively high, the final microstructure may be realized as being excessively coarse, so the present invention can limit the upper limit of the finishing delivery temperature to 1000°C.

[0070] Winding The hot-rolled steel sheet provided 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 since a composite structure of ferrite and pearlite is to be realized as the final structure. In the present invention, coiling is more preferably performed at a temperature range of 650°C or higher since a final fine structure of a certain size or more is to be realized. However, if the coiling temperature is excessively high, coarse fine structures may be formed or surface quality may deteriorate, so the present invention can limit the upper limit of the coiling temperature to 700°C.

[0071] The inventors of the present invention have conducted extensive research into technical means for controlling the crystal grain size of the final microstructure, and have confirmed that in order to control the crystal grain size of the final microstructure in the component system of the present invention, not only must the heating temperature (T1) when heating a steel slab, the finish rolling temperature (T2) when hot rolling, and the coiling temperature (T3) when coiling a hot-rolled steel sheet be controlled independently to satisfy certain ranges, but also that these slab heating temperature (T1), finish rolling temperature (T2), and coiling temperature (T3) must be controlled within certain ranges in conjunction with one another, and have derived the following Relational Formula 5.

[0072] [Equation 5] 1481≦0.0284×[T1]+0.071×[T2]+0.045×[T3]-131≦3 In the above Relational Formula 5, [T1], [T2], and [T3] respectively mean the slab heating temperature (T1, °C), the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C).

[0073] Therefore, in the manufacturing method of hot-rolled steel sheet for vacuum train tubes according to one aspect 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 hot-rolled steel sheet at a coiling temperature (T3) of 600°C to 700°C, but also the process conditions are controlled so that the slab heating temperature (T1), the finish rolling temperature (T2), and the coiling temperature (T3) satisfy Relation 4, thereby effectively achieving the targeted microstructure of the hot-rolled steel sheet.

[0074] The hot-rolled steel sheet produced by the above-mentioned production method can satisfy the following Relational Expressions 1 to 3, and can further satisfy the following Relational Expression 4.

[0075] [Equation 1] 350≦11+394×D (-0.5) +448×[C]+94×[Si]+69×[Mn]

[0076] [Equation 2] 100≦186-210×D (-0.5) -121×[C]-13.2×[Si]+13.7×[Mn]

[0077] [Equation 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si]

[0078] [Equation 4] 303.78-85.22×ln(D)>27 In the above Relational Formulas 1 to 4, D means the average crystal grain size (μm) of ferrite in the above heat-rolled steel sheet, and [C], [Si] and [Mn] mean the contents (wt%) of carbon (C), silicon (Si) and manganese (Mn) in the above heat-rolled steel sheet, respectively.

[0079] In addition, the hot-rolled steel sheet manufactured by the above-mentioned manufacturing method has a yield strength of 350 MPa or more and a -20°C Charpy impact energy of 27 J or more. In addition, the vibration damping ratio measured at a frequency of 1650 Hz in a flexural vibration mode using a test piece with a length x width x thickness of 80 x 20 x 2 mm is 100 x 10 -6 or higher, and the electrical resistivity is 30×10 -8 It can meet the level of Ωm or more.

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

[0081] 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 below with reference to specific examples. It should be noted that the following examples are for the purpose of understanding the present invention and are not intended to specify 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 in Table 1 was prepared, and then a 15 mm thick hot rolled steel sheet was manufactured by applying the process conditions in Table 2. Alloy components not listed in Table 1 below represent impurities and the balance Fe, and "-" represents a value close to 0 wt% within the margin of error.

[0083] [Table 1]

[0084] [Table 2]

[0085] The microstructure and mechanical properties of each test piece were analyzed and are shown in Table 3. Whether or not each test piece satisfied Relational Formulas 1 to 4 is also shown in Table 3. The microstructure was measured using an optical microscope at 500x magnification after etching each test piece by the Nital etching method. The ferrite grain size was measured according to ASTM E112. Figure 1 shows the optical microscope photograph used to observe the microstructure of test piece 1.

[0086] The mechanical properties were measured according to KS B 0802 and KS B 0810, and the measured yield strength, yield ratio, and Charpy impact toughness at -21°C are also shown in Table 3. The electrical resistivity was measured according to KS C IEC 60404, and the values ​​are shown in Table 3.

[0087] 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 the test piece in flexural vibration mode, one of the vibration modes of the test piece was st The vibration damping ratio in the 1650 Hz region corresponding to the mode was measured and analyzed, and the results are shown in Table 3.

[0088] [Table 3]

[0089] As shown in Tables 1 to 3, the test pieces satisfying the alloy composition, process conditions, and relational expressions 1 to 4 of the present invention have a yield strength of 350 MPa or more, a -20°C Charpy impact energy of 27 J or more, and a tensile strength of 30 × 10 -8 Electrical resistivity of Ωm or more and 100×10 -6In contrast, a test piece that does not satisfy one or more of the conditions restricted by the present invention has a yield strength of 350 MPa or more, a Charpy impact energy of -20°C of 27 J or more, and a vibration damping ratio of 30 x 10 -8 Electrical resistivity of Ωm or more and 100×10 -6 It can be seen that the above vibration damping ratios are not satisfied simultaneously.

[0090] In addition, for comparison with conventional materials, tests were also conducted under the same conditions on EN-S355, an existing structural steel material. The vibration damping ratio measured under the same conditions for EN-S355 was 60×10 -6 It was confirmed that the level of the change was only at that level. Figure 2 shows a photograph taken with an optical microscope used to observe the microstructure of EN-S355.

[0091] Therefore, according to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, electrical resistivity and low-temperature toughness, 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 embodiments, other embodiments are possible and the technical spirit and scope of the claims set forth below should not be limited to the embodiments.

Claims

1. In weight percent, carbon (C): 0.03-0.25%, silicon (Si): 1.5-2.5%, and manganese (Mn): 0.8-1.8%, with the remainder being Fe and other unavoidable impurities; The microstructure is a composite structure of ferrite and pearlite, The following Relational Formulas 1 to 3 are satisfied, The hot-rolled steel sheet for vacuum train tubes, wherein the average crystal grain size (D) of the ferrite is 10 to 30 μm. [Relationship 1] 350≦11+394×D （－０．５） +448×[C]+94×[Si]+69×[Mn] [Relationship 2] 100≦186-210×D （－０．５） -121×[C]-13.2×[Si]+13.7×[Mn] [Relationship 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si] In the Relational Formulas 1 to 3, D represents an average crystal grain size (μm) of ferrite in the heat-rolled steel sheet, and [C], [Si], and [Mn] represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) in the heat-rolled steel sheet, respectively.

2. The hot-rolled steel sheet for vacuum train tubes according to claim 1, wherein the hot-rolled steel sheet satisfies the following Relational Formula 4: [Relationship 4] 303.78-85.22×ln(D)>27 In the above Relational Formula 4, D means the average crystal grain size (μm) of ferrite in the heat-rolled steel sheet.

3. The hot-rolled steel sheet for vacuum train tubes according to claim 1, wherein the microstructure of the hot-rolled steel sheet is composed of 60-90 area % ferrite, 10-40 area % pearlite and other unavoidable structures.

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

5. The yield strength of the hot-rolled steel plate is 350 MPa or more, The Charpy impact energy of the hot-rolled steel sheet at -20 ° C. is 27 J or more, The hot-rolled steel sheet was processed into a test piece having a length x width x thickness of 80 x 20 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 That's all. Electrical resistivity is 30 x 10 －８ The hot rolled steel sheet for vacuum train tubes according to claim 1, having a compressive strength of Ωm or more.

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. The slab, which contains, by weight, 0.03-0.25% carbon (C), 1.5-2.5% silicon (Si), 0.8-1.8% manganese (Mn), and the remainder 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 wire with a The heating temperature (T １ ), finishing rolling temperature (T ２ ) and winding temperature (T ３ 2. The method for producing a hot-rolled steel sheet for a vacuum train tube according to claim 1, wherein: [Relationship 5] 1≦0.0284×[T １ ]+0.071×[T ２ ]+0.045×[T ３ ]-131≦3 In the above-mentioned Relational Formula 5, [T １ ], [T ２ ] and [T ３ ] are the slab heating temperature (T1, °C), the finish rolling temperature (T ２ , °C) and winding temperature (T ３ , °C).

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

9. The method for producing a hot-rolled steel sheet for a vacuum train tube according to claim 7, wherein the slab satisfies the following Relational Expression 3: [Relationship 3] 30≦9.5+5.2×[C]+5.8×[Mn]+13.1×[Si] In the above Relational Formula 3, [C], [Si] and [Mn] respectively represent the contents (wt%) of carbon (C), silicon (Si) and manganese (Mn) in the hot-rolled steel sheet.

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