Hot-rolled steel sheet for vacuum train tube and method for producing the same

Abstract

According to one aspect of the present invention, there is provided a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness, 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 method for manufacturing the same, and more particularly, to a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness and suitable physical properties for a vacuum train tube, and a method for manufacturing the same.

Background Art

[0002] A vacuum train, so-called hyper tube train, is a system in which a maglev train moves inside a vacuum tube. Since there is no air or friction with the track, which is the main cause of energy loss during the running of the train, the vacuum train can run at ultra-high speed. Because of less energy loss and 93% energy saving compared to airplanes, it is attracting attention as an environmentally friendly next-generation transportation means, and active research is being conducted worldwide.

[0003] The structure and material of the vacuum tube used for an ultra-high-speed vacuum train affect the performance and cost of the system. Currently, there are roughly three types of materials being studied as tube materials for vacuum trains. One is concrete. Concrete tubes are advantageous in terms of cost, but it is difficult to join individual tubes of about 10 m to each other. Also, when a vacuum is realized due to pores inside the concrete, there is a drawback that external gas enters the inside of the tube and the vacuum degree easily becomes fragile. One of the other materials that has been studied a lot is a composite material such as carbon fiber. Composite materials such as carbon fiber are light and have high performance, but the highest drawback is the high cost.

[0004] Currently, the most promising material for the vacuum train tube 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 or weld accessories between tubes or to the tubes, and it also has an appropriate outgassing rate when maintaining a vacuum. However, since the ultra-high-speed vacuum train runs at a significantly higher speed compared to current high-speed trains, the stability of passengers and surrounding facilities should be given top priority. Currently, even the safety standards for ultra-high-speed vacuum trains have not been established, and the development of tube materials for ensuring the safety of ultra-high-speed vacuum trains is also in an inadequate state.

[0005] Therefore, it is an urgent situation to develop a material for the vacuum train tube that has workability and an outgassing rate suitable for the vacuum train tube while ensuring safety.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems 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, and low-temperature toughness and having physical properties suitable for a vacuum train tube, and a method for manufacturing the same.

[0008] The problems of the present invention are not limited to the above-described content. An ordinary technician will have no difficulty in understanding further problems of the present invention from the entire content of this specification.

Means for Solving the Problems

[0009] The hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention contains, by weight%, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 1.3%, manganese (Mn): 1.0 to 2.0%, and the balance Fe and other inevitable impurities, has a ferrite and pearlite composite structure as a microstructure, and can satisfy the following relational expressions 1 to 3. [Relational expression 1] 350 ≦ 11 + 394*D (-0.5) + 448*[C] + 94*[Si] + 69*[Mn] [Relational expression 2] 100 ≦ 186 - 210*D (-0.5) - 121*[C] - 13.2*[Si] + 13.7*[Mn] [Relational expression 3] 303.78 - 85.22*ln(D) > 27 In the above relational expressions 1 to 3, D means the average crystal grain size (μm) of ferrite in the hot-rolled steel sheet, and [C], [Si], and [Mn] respectively mean the contents (weight%) of carbon (C), silicon (Si), and manganese (Mn) in the hot-rolled steel sheet.

[0010] The microstructure of the hot-rolled steel sheet can consist of 60 to 90 area% of ferrite, 10 to 40 area% of pearlite, and other inevitable structures.

[0011] The total content of titanium (Ti), niobium (Nb), and vanadium (V) inevitably contained in the hot-rolled steel sheet can be less than 0.01% (including 0%).

[0012] The average crystal grain size (D) of the ferrite can be 10 to 30 μm.

[0013] The yield strength of the hot-rolled steel sheet is 350 MPa or more, the Charpy impact energy of the hot-rolled steel sheet based on -20 °C is 27 J or more, and after processing the hot-rolled steel sheet into a test piece with a length * width * thickness of 80 * 20 * 2 mm, the vibration damping ratio measured at a frequency of 1650 Hz in the flexural vibration mode is 100*10 -6It can be as described above.

[0014] The thickness of the hot-rolled steel sheet can be 10 mm or more.

[0015] According to one aspect of the present invention, a method for manufacturing a hot-rolled steel sheet for a vacuum train tube includes: heating a slab containing, by weight%, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 1.3%, manganese (Mn): 1.0 to 2.0%, and the balance 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 expression 4. [Relational Expression 4] 1 ≦ 0.0284 * [T1] + 0.071 * [T2] + 0.045 * [T3] - 131 ≦ 3 In the above relational expression 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).

[0016] The total content of titanium (Ti), niobium (Nb), and vanadium (V) inevitably contained in the slab can be less than 0.01% (including 0%).

[0017] The solutions to the above problems do not enumerate all the features of the present invention, and various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific implementation examples and examples.

Effects of the Invention

[0018] According to one aspect of the present invention, there can be provided a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness and suitable physical properties for a vacuum train tube, and a method for manufacturing the same.

[0019] The effects of the present invention are not limited to the above-described matters, and can be interpreted to include matters that can be reasonably inferred by an ordinary technician from the matters described in this specification.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0021] The present invention relates to a hot-rolled steel sheet for a vacuum train tube and a method for manufacturing the same. Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be deformed in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment is provided to explain the present invention in more detail to those having ordinary knowledge in the technical field to which the invention pertains.

[0022] A vacuum train is a next-generation transportation means that is currently in the initial stage of development as a train that runs inside a tube in a vacuum or sub-vacuum state. A vacuum train can effectively achieve high speed and high efficiency by removing the frictional resistance between the wheels and the track and minimizing the air resistance. However, due to the characteristics of a vacuum train that runs at ultra-high speed, if the stability of the vacuum train cannot be sufficiently ensured, there is a risk of a major accident occurring. In particular, not only when the vacuum tube is structurally damaged or collapses, but also when a change occurs in a partial shape of the tube, it may cause a huge tragedy. Therefore, stricter safety is required for the material for the vacuum train tube. As a result of in-depth research, the inventor of the present invention found that the following physical properties are major as the material for the vacuum tube to ensure the safety of the vacuum train.

[0023] The first physical property required for the material used in the vacuum tube is high strength characteristics. Since the vacuum train moves through the inside of the vacuum tube, the material for the vacuum tube is required to have sufficient strength as a structure. In addition, since the inside of the vacuum tube needs to be maintained in a vacuum or sub-vacuum state, it is required to have sufficient high strength characteristics so that the shape of the tube does not deform due to the pressure difference between the inside and the outside.

[0024] The second physical property required for the material used in the vacuum tube is vibration damping ability. In the vacuum train, pods carrying several to dozens of people pass through the inside of the vacuum tube at intervals of dozens of seconds to several minutes. After the passage of the leading pod, vibration may be amplified and resonance may occur inside the vacuum tube when the trailing pod passes through. In severe cases, it may even cause damage to the tube. Therefore, when applying a material with a vibration damping ratio above a certain level to the vacuum tube, the vibration inside the tube can be effectively reduced after the passage of the leading pod, which can effectively contribute to the stability of the vacuum train.

[0025] The third physical property required for the material used in the vacuum tube is low temperature toughness. The vacuum train may operate even in polar regions or deep seas. Since steel materials tend to be more easily damaged in low temperature or cryogenic environments, when applying steel materials to the vacuum tube, it is required to have low temperature toughness above a certain level to ensure stability.

[0026] Through in-depth research, the inventor of the present invention recognized that by strictly controlling the content and microstructure of the alloy composition of the steel plate, excellent yield strength, vibration damping ratio and low temperature toughness can be achieved simultaneously, and thus the present invention was derived.

[0027] Hereinafter, the hot-rolled steel plate for a vacuum train tube according to one aspect of the present invention will be described in more detail.

[0028] The hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention contains, by weight%, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 1.3%, manganese (Mn): 1.0 to 2.0%, and the balance Fe and other inevitable impurities, and has a ferrite and pearlite composite structure as a microstructure, and can satisfy the following relational expressions 1 to 3. [Relational expression 1] 350 ≦ 11 + 394*D (-0.5) + 448*[C] + 94*[Si] + 69*[Mn] [Relational expression 2] 100 ≦ 186 - 210*D (-0.5) - 121*[C] - 13.2*[Si] + 13.7*[Mn] [Relational expression 3] 303.78 - 85.22*ln(D) > 27 In the above relational expressions 1 to 3, D means the average crystal grain size (μm) of ferrite in the hot-rolled steel sheet, and [C], [Si], and [Mn] respectively mean the contents (weight%) of carbon (C), silicon (Si), and manganese (Mn) in the hot-rolled steel sheet.

[0029] Hereinafter, the steel composition contained in the hot-rolled steel sheet of the present invention will be described in more detail. Hereinafter, unless otherwise indicated, the % indicating the content of each element is based on weight.

[0030] Carbon (C): 0.15 to 0.25% Carbon (C) is a component that has a very significant impact on the strength of steel plates. The present invention can contain 0.15% or more of carbon (C) in order to ensure the strength required by the structure. On the other hand, when the carbon (C) content is excessive, the toughness of the material may decrease, the weldability may be inferior, and the yield ratio may increase. The lower limit of the preferred carbon (C) content can be 0.17%, and the lower limit of the more preferred carbon (C) content can be 0.20%. Also, when the carbon (C) content is excessive, it is difficult to coarsen the crystal grains. Therefore, the present invention can limit the upper limit of the carbon (C) content to 0.25%. The upper limit of the preferred carbon (C) content can be 0.23%, and the upper limit of the more preferred carbon (C) content can be 0.22%.

[0031] Silicon (Si): 0.3 - 1.3% Silicon (Si) tends to be removed together with oxygen because it forms slag by oxygenation bonding during the steelmaking stage. Also, silicon (Si) is a component that effectively contributes to improving the strength of the material. Therefore, the present invention can contain 0.3% or more of silicon (Si) for such an effect. The lower limit of the preferred silicon (Si) content can be 0.5%. On the other hand, when the silicon (Si) content is excessive, it may prevent the surface scale from falling off and reduce the surface quality of the product. Also, when the silicon (Si) content is excessive, the low-temperature toughness of the base material and the welded part decreases, increasing the risk of fracture during the use of the material. Therefore, the present invention can limit the silicon (Si) content to 1.3% or less. The upper limit of the preferred silicon (Si) content can be 1.0%.

[0032] Manganese (Mn): 1.0 - 2.0% Manganese (Mn) is a component that improves the strength and hardenability of steel. Therefore, the present invention can contain 1.0% or more of manganese (Mn) in order to ensure such an effect. On the other hand, when the content of manganese (Mn) is excessive, material deviation may occur due to segregation in the central part, and the crack propagation resistance may deteriorate. The lower limit of the preferable content of manganese (Mn) can be 1.2%, and the lower limit of the more preferable content of manganese (Mn) can be 1.5%. Also, when the content of manganese (Mn) is excessive, the toughness of the steel may decrease. Therefore, the present invention can limit the content of manganese (Mn) to 2.0% or less. The upper limit of the preferable content of manganese (Mn) can be 1.8%, and the upper limit of the more preferable content of manganese (Mn) can be 1.6%.

[0033] The hot-rolled steel sheet of the present invention can contain the remaining Fe and other inevitable impurities in addition to the components described above. However, in the normal manufacturing process, unintended impurities may inevitably be mixed in from the raw materials or the surrounding environment, so it is impossible to completely exclude them. Since these impurities are known to anyone with ordinary knowledge in the technical field, all of their contents are not particularly mentioned in this specification. Furthermore, the additional addition of effective components other than the components described above is not completely excluded.

[0034] 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 contained, 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 effectively contribute to improving the strength of steel by forming fine carbonitrides. However, since titanium (Ti), niobium (Nb), and vanadium (V) overly refine the microstructure of steel and act disadvantageously on ensuring the vibration damping ability, the present invention actively suppresses these components. Also, titanium (Ti), niobium (Nb), and vanadium (V) are expensive components and are not preferable from the economic aspect. The present invention can actively suppress the total content of these components to less than 0.01% even when these components are not artificially added and are inevitably added. The preferable total content of these components can be 0.005% or less, and the more preferable total content of these components can be 0%.

[0035] The hot-rolled steel sheet according to one aspect of the present invention can have a composite structure composed of ferrite and pearlite as the 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, a low yield ratio, and can exhibit excellent physical properties as structural materials. However, in the case of the hot-rolled steel sheet for a vacuum train tube according to an example of the present invention, since the thickness is thick at a level of 10 mm or more, physical property deviations may occur in the thickness direction of the steel sheet even when a low-temperature structure is introduced. 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 up to the center of the steel sheet.

[0036] Therefore, in order to reduce physical property deviation, the present invention configures the microstructure of the steel plate as a composite structure composed of ferrite and pearlite. Even if low-temperature structures such as bainite and martensite are inevitably formed, their fraction can be actively suppressed to 1 area% or less (including 0%). The fraction of the preferred low-temperature structure can be less than 0.5 area%. From the aspect of ensuring physical properties, the fraction of ferrite can be 60 - 90 area%, and the fraction of pearlite can be 10 - 40 area%.

[0037] In order to simultaneously ensure the target yield strength, vibration damping ratio, and low-temperature toughness, the present invention can limit the average crystal grain size of ferrite within a certain range. Since the larger the crystal grain size, the more advantageous it is to ensure the vibration damping ratio, the present invention can limit the average crystal grain size of ferrite to 10 μm or more. The preferred average crystal grain size can exceed 10 μm, and the more preferred average crystal grain size can be 15 μm or more. On the other hand, when the crystal grain size becomes excessively large, the strength and low-temperature toughness of the material deteriorate. Therefore, the present invention can limit the average crystal grain size of ferrite to 30 μm or less. The preferred average crystal grain size can be 25 μm or less.

[0038] As a result of in-depth research on the stability ensuring scheme of the material for the vacuum train tube by the inventor of the present invention, when controlling the contents of carbon (C), silicon (Si), and manganese (Mn) and the average crystal grain size of ferrite within a certain range in a low-alloy steel plate as in the present invention, it was recognized that it is possible to simultaneously ensure the yield strength, vibration damping ratio, and low-temperature toughness, and the following relational expressions 1 to 3 were derived. [Relational Expression 1] 350 ≦ 11 + 394*D (-0.5) + 448*[C] + 94*[Si] + 69*[Mn] [Relational Expression 2] 100 ≦ 186 - 210*D (-0.5) - 121*[C] - 13.2*[Si] + 13.7*[Mn] [Relational Expression 3] 303.78 - 85.22*ln(D) > 27 In the above relational expressions 1 to 3, D means the average crystal grain size (μm) of ferrite in the hot-rolled steel sheet, and [C], [Si], and [Mn] respectively mean the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) in the hot-rolled steel sheet.

[0039] Since the hot-rolled steel sheet for a vacuum train tube of the present invention satisfies all of the relational expressions 1 to 3, the desired yield strength, vibration damping ratio, and low-temperature toughness can be ensured simultaneously.

[0040] The hot-rolled steel sheet for a vacuum train tube 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 a vacuum train tube of the present invention can ensure appropriate strength and low-temperature toughness as a structural material, and effectively ensure the structural stability of the tube for a vacuum train.

[0041] The hot-rolled steel sheet for a vacuum train tube of the present invention can have a vibration damping ratio of 100*10 -6 or more. Here, the vibration damping ratio means the vibration damping ratio measured at a frequency of 1650 Hz after hitting a test piece having a length * width * thickness of 80*20*2 mm in a flexural vibration mode. Since the hot-rolled steel sheet for a vacuum train tube of the present invention has a vibration damping ratio of 100*10 -6 or more, the vibration amplification in the vacuum tube can be effectively suppressed, and the breakage of the tube for a vacuum train due to vibration can be effectively prevented.

[0042] 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, and low-temperature toughness and having physical properties suitable for a vacuum train tube.

[0043] Hereinafter, the manufacturing method of the hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention will be described in more detail.

[0044] The manufacturing method of the hot-rolled steel sheet for a maglev train tube according to one aspect of the present invention includes, by weight%, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 1.3%, manganese (Mn): 1.0 to 2.0%, heating a slab containing the remaining 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 winding the hot-rolled steel sheet at a winding temperature (T3) of 600°C to 700°C, and the heating temperature (T1), finish rolling temperature (T2), and winding temperature (T3) can satisfy the following relational expression 4. [Relational Expression 4] 1 ≤ 0.0284 * [T1] + 0.071 * [T2] + 0.045 * [T3] - 131 ≤ 3 In the above relational expression 4, [T1], [T2], and [T3] respectively mean the slab heating temperature (T1, °C), finish rolling temperature (T2, °C), and winding temperature (T3, °C).

[0045] Preparation and Heating of Steel Slab Prepare a steel slab having a predetermined alloy composition. Since the steel slab of the present invention has an alloy composition corresponding to the above-described hot-rolled steel sheet, the description of the alloy composition of the steel slab is replaced with the description of the alloy composition of the above-described hot-rolled steel sheet.

[0046] 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 in a temperature range of 1100°C or higher. In particular, in order to introduce a microstructure of a certain size or more in the present invention, the preferable heating temperature of the steel slab can be 1200°C or higher. A more preferable steel slab heating temperature can be 1250°C or higher. On the other hand, when the steel slab heating temperature is excessively high, there is a concern about the deterioration of surface quality due to scale formation, so the present invention can limit the steel slab heating temperature to 1300°C or lower.

[0047] Hot Rolling It is possible to provide a hot-rolled steel sheet by hot-rolling a heated steel slab at a finishing rolling temperature (T2) of 900°C to 1000°C. The steel sheet provided by the hot-rolling of the present invention can have a thickness of 10 μm or more.

[0048] During hot-rolling, while the material is being rolled, the crystal grains are deformed but are immediately recrystallized. Through such a process, the structure that was coarse and non-uniform is refined and homogenized. An important process parameter during hot-rolling is the finishing rolling temperature (FDT), which is the temperature at the end of rolling. This is because the crystal grain size of the final fine structure can be controlled by the finishing rolling temperature. In order to control the final fine structure to a level of a certain size or more, the present invention can perform hot-rolling at a finishing rolling temperature of 900°C or higher. A preferable finishing rolling temperature can be 950°C or higher. On the other hand, when the finishing rolling temperature is excessively high, the final fine structure can be realized to be excessively coarse. Therefore, the present invention can limit the upper limit of the finishing rolling temperature to 1000°C.

[0049] Coiling The hot-rolled steel sheet provided by hot-rolling can be coiled at a coiling temperature (T3) of 600°C to 700°C after passing through water cooling. In order to realize a composite structure of ferrite and pearlite as the final structure, coiling can be performed in a temperature range of 600°C or higher. In order to realize a final fine structure of a certain size or more, it is more preferable to coil in a temperature range of 650°C or higher. However, when the coiling temperature is excessively high, a coarse fine structure may be formed or the surface quality may deteriorate. Therefore, the present invention can limit the upper limit of the coiling temperature to 700°C.

[0050] The inventor of the present invention has conducted in-depth research on technical means for controlling the crystal grain size of the final microstructure. In the component system of the present invention, in order to control the crystal grain size of the final microstructure, not only 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 need to be controlled independently to satisfy a certain range, but also these slab heating temperature (T1), finish rolling temperature (T2), and coiling temperature (T3) need to be controlled within a certain range in relation to each other. It was confirmed that the following relational expression 4 was derived. [Relational Expression 4] 1261≦0.0284*[T1]+0.071*[T2]+0.045*[T3]-131≦3 In the above relational expression 4, [T1], [T2], and [T3] respectively represent the heating temperature (T1, °C) of the slab, the finish rolling temperature (T2, °C), and the coiling temperature (T3, °C).

[0051] Therefore, the method for manufacturing a hot-rolled steel sheet for a maglev train tube according to one aspect of the present invention not only heats the slab at a heating temperature (T1) of 1100°C to 1300°C, performs hot rolling at a finish rolling temperature (T2) of 900°C to 1000°C, and coils the hot-rolled steel sheet at a coiling temperature (T3) of 600°C to 700°C, but also controls the process conditions so that the slab heating temperature (T1), finish rolling temperature (T2), and coiling temperature (T3) satisfy relational expression 4, thus effectively realizing the target microstructure of the hot-rolled steel sheet.

[0052] The hot-rolled steel sheet manufactured by the above-described manufacturing method can satisfy the following relational expressions 1 to 3. [Relational Expression 1] 350≦11+394*D (-0.5) +448*[C]+94*[Si]+69*[Mn] [Relational Expression 2] 100≦186-210*D (-0.5) -121*[C]-13.2*[Si]+13.7*[Mn] [Relational Expression 3] 303.78-85.22*ln(D)>27 In the above relational expressions 1 to 3, D represents the average ferrite crystal grain size (μm) of the hot-rolled steel sheet, and [C], [Si], and [Mn] respectively represent the contents (wt%) of carbon (C), silicon (Si), and manganese (Mn) in the hot-rolled steel sheet.

[0053] Moreover, the hot-rolled steel sheet manufactured by the above-described 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 a vibration damping ratio measured at a frequency of 1650 Hz in a flexural vibration mode for a test piece with a length * width * thickness of 80 * 20 * 2 mm of 100 * 10 -6 The above level can be satisfied.

[0054] Therefore, according to one aspect of the present invention, it is possible to provide a method for manufacturing a hot-rolled steel sheet having excellent yield strength, vibration damping ratio, and low-temperature toughness and suitable physical properties for a vacuum train tube.

Example

[0055] Hereinafter, the hot-rolled steel sheet for a vacuum train tube and its manufacturing method of the present invention will be described in more detail through specific examples. It should be noted that the following examples are only for understanding the present invention and not for specifying the scope of rights of the present invention. The scope of rights of the present invention can be determined by the matters described in the claims and matters reasonably inferred therefrom.

[0056] (Example) After preparing a steel slab with a thickness of 250 mm having the alloy composition shown in Table 1 below, a hot-rolled steel sheet with a thickness of 15 mm was manufactured by applying the process conditions shown in Table 2. Alloy components not described in Table 1 below mean impurities and the balance Fe, and the "-" indication means a case close to 0 wt% within the error range.

[0057]

Table 1

[0058]

Table 2

[0059] The microstructure and mechanical properties of each test piece were analyzed and described in Table 3, and whether or not the relational expressions 1 to 3 of each test piece were satisfied was also described in Table 3. For the microstructure, after etching each test piece by the Nital etching method, it was measured using an optical microscope at a magnification of 500. The crystal grain size of ferrite was measured according to ASTM E112. Fig. 1 is an optical micrograph used for observing the microstructure of test piece 1, and Fig. 2 is an optical micrograph used for observing the microstructure of test piece 5.

[0060] 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 were also described in Table 3.

[0061] After preparing a test piece with a length * width * thickness of 80 * 20 * 2 mm, the vibration damping ratio was measured at room temperature using the RFDA LTV800 of IMCE. After hitting in the flexural vibration mode, among the vibration modes of the corresponding test piece, the vibration damping ratio in the 1650 Hz region corresponding to mode 1 was measured and analyzed, and the results were also described in Table 3. st was measured and analyzed, and the results were also described in Table 3.

[0062]

Table 3

[0063] As described in Tables 1 to 3, the test pieces that satisfy 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 Charpy impact energy at -20°C of 27 J or more, and a vibration damping ratio of 100 * 10 -6 or more, while the test pieces that do not satisfy any one or more of the conditions restricted by the present invention have a yield strength of 350 MPa or more, a Charpy impact energy at -20°C of 27 J or more, and a vibration damping ratio of 100 * 10 -6It can be seen that the above vibration damping ratios are not simultaneously satisfied.

[0064] Also, for comparison with the conventional material, tests were conducted on EN-S355, a conventional structural steel, under the same conditions. In the case of EN-S355, it was confirmed that the vibration damping ratio measured under the same conditions was only at the level of 60×10 -6 . FIG. 3 is a microstructural observation photograph of EN-S355 taken using an optical microscope.

[0065] 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, and low-temperature toughness and having physical properties suitable for a vacuum train tube, and a method for manufacturing the same.

[0066] The present invention has been described in detail through the examples above, but examples in different forms are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. by weight, carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 1.3%, and manganese (Mn): 1.0 to 2.0%, consisting of the balance Fe and other inevitable impurities, having a microstructure consisting of 60 to 90 area% ferrite, 10 to 40 area% pearlite, and other inevitable structures, the average crystal grain size (D) of the ferrite is 10 to 30 μm, a hot-rolled steel sheet for a vacuum train tube that satisfies the following relational expressions 1 to 3. [Relational Expression 1] 350 ≤ 11 + 394 * D （－０．５） + 448 * [C] + 94 * [Si] + 69 * [Mn] [Relational Expression 2] 100 ≤ 186 - 210 * D （－０．５） -121 * [C] - 13.2 * [Si] + 13.7 * [Mn] [Relational Expression 3] 303.78 - 85.22 * ln(D) > 27 In the above relational expressions 1 to 3, D means the average crystal grain size (μm) of the ferrite in the hot-rolled steel sheet, and [C], [Si], and [Mn] respectively mean the contents (by weight) of carbon (C), silicon (Si), and manganese (Mn) in the hot-rolled steel sheet.

2. 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%), the hot-rolled steel sheet for a vacuum train tube according to Claim 1.

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

4. The thickness of the hot-rolled steel sheet is 10 mm or more, the hot-rolled steel sheet for a vacuum train tube according to Claim 1.

5. By weight, it contains carbon (C): 0.15 to 0.25%, silicon (Si): 0.3 to 1.3%, and manganese (Mn): 1.0 to 2.0%, and a slab consisting of the balance Fe and other inevitable impurities is heated at a heating temperature (T １ ) in the range of 1100 °C to 1300 °C; Hot rolling the heated slab at a finish rolling temperature (T ２ ) of 900°C to 1000°C to provide a hot-rolled steel sheet; and The step of coiling the hot-rolled steel sheet at a coiling temperature (T ３ ) of 600°C to 700°C is included, The heating temperature (T １ ), the finish rolling temperature (T ２ ), and the coiling temperature (T ３ ) satisfy the following relational expression 4. The method for manufacturing a hot-rolled steel sheet for a maglev train tube according to claim 1. [Relational Expression 4] 1 ≤ 0.0284 * [T １ + 0.071 * [T ２ + 0.045 * [T ３ - 131 ≤ 3 In the relational expression 4, [T １ , [T ２ , and [T ３ respectively represent the heating temperature (T １ , °C), the finish rolling temperature (T ２ , °C), and the coiling temperature (T ３ , °C) of the slab.

6. The total content of titanium (Ti), niobium (Nb), and vanadium (V) inevitably contained in the slab is less than 0.01% (including 0%), the method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to Claim 5.

Images