Hot-rolled steel sheet for vacuum train tube

Abstract

Disclosed is a hot-rolled steel sheet for a vacuum train tube, which is excellent in terms of yield strength, weldability, low-temperature toughness of a welded part, and a vibration damping ratio, while having no hot cracks. The hot-rolled steel sheet according to the present invention comprises, in weight%, C: 0.06% or less (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), and the balance being Fe and inevitable impurities.

Description

Hot rolled steel plate for vacuum train tubes

[0001] The present invention relates to a hot-rolled steel sheet for a vacuum train tube.

[0002] A vacuum train, also known as a hyperloop, is a system in which a magnetic levitation train moves inside a vacuum tube.

[0003] Vacuum trains can operate at ultra-high speeds because they eliminate friction with air or tracks, which are major sources of energy loss during train operation. With minimal energy loss, they can save up to 93% of energy compared to aircraft, making them a promising eco-friendly next-generation mode of transportation, and active research is underway worldwide.

[0004] The structure and materials of vacuum tubes used in high-speed vacuum trains affect the system's performance and cost. Currently, three main materials are being studied as tube materials for vacuum trains. One is concrete. While concrete tubes offer cost advantages, it is difficult to connect individual tubes, each approximately 10 meters long. Furthermore, the pores within the concrete can easily cause external gases to infiltrate the tubes, breaking the vacuum. Another material being extensively studied is composite materials such as carbon fiber. While these materials are lightweight and offer high performance, their high cost is a major drawback.

[0005] Currently, steel is the most promising material for vacuum train tubes. Steel can be mass-produced at low cost. It boasts high rigidity and strength, and is easy to process. Furthermore, it facilitates the assembly and welding of components between or between tubes, and it also exhibits an appropriate outgassing rate when maintaining a vacuum. However, because ultra-high-speed vacuum trains operate at significantly faster speeds than current high-speed trains, the safety of passengers and surrounding facilities must be the top priority. Currently, safety standards for ultra-high-speed vacuum trains have not been established, and the development of tube materials necessary to ensure their safety is also insufficient.

[0006] Therefore, there is an urgent need to develop a material for vacuum train tubes that has processability and outgassing rate suitable for vacuum train tubes while ensuring safety.

[0007] An object of the present invention is to provide a hot-rolled steel sheet for vacuum train tubes having excellent yield strength, weldability, low-temperature toughness of weld zone, vibration damping ratio, and no hot cracking.

[0008] The objectives of the present invention are not limited to those mentioned above. Other objectives and advantages of the present invention not mentioned above can be understood through the following description and will be more clearly understood through the embodiments of the present invention. Furthermore, it will be readily apparent that the objectives and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0009] The hot-rolled steel sheet according to the present invention is characterized in that it includes, in weight % in the alloy composition, C: 0.06% or less (exceeding 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and inevitable impurities, and includes polygonal ferrite of more than 90 area% and pearlite of less than 10 area% for a total of 100 area%.

[0010] The average grain size of the above polygonal ferrite crystal grains may be 8 to 20 μm.

[0011] The above hot-rolled steel sheet contains Cu precipitates, and the ε-Cu precipitate fraction (%) according to ASTM E 562 may be 0.3 to 1.5%.

[0012] Additionally, it is desirable that there be no hot cracks, and hot cracks are defined as having a depth of 50 ㎛ or more and a width of 5 ㎛ or more.

[0013] Additionally, the yield strength can be greater than 350 MPa.

[0014] Also, the Charpy impact energy at -20℃ is 47 J / cm 2 It could be strange.

[0015] The above hot-rolled steel plate was processed into a specimen with a length*width*thickness of 80mm*20mm*2mm, and the vibration damping ratio measured for a frequency of 1650Hz in the bending vibration mode was 200*10 -6 It could be strange.

[0016] In the weld formed by welding the hot-rolled steel plate by submerged arc welding, the Charpy impact energy of the weld at -20℃ is 47 J / cm 2 In addition, the fraction of the M (martensite)-A (austenite) phase included in the above weld may be 5 area% or less (including 0%).

[0017] A hot-rolled steel sheet according to another embodiment of the present invention is characterized in that it includes, in weight % in an alloy composition, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder being Fe and unavoidable impurities.

[0018] A hot-rolled steel sheet according to another embodiment of the present invention is characterized in that it includes, in weight %, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and unavoidable impurities, and the average grain size of polygonal ferrite crystal grains is 8 to 20 ㎛.

[0019] A hot-rolled steel sheet according to another embodiment of the present invention includes, in weight %, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder being Fe and unavoidable impurities, and is characterized in that it includes Cu precipitates and the fraction (%) of ε-Cu precipitates according to ASTM E 562 is 0.3 to 1.5%.

[0020] A hot-rolled steel sheet according to another embodiment of the present invention includes, in weight %, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder being Fe and unavoidable impurities, and is characterized by the absence of hot cracking.

[0021] A hot-rolled steel sheet according to another embodiment of the present invention is characterized in that it includes, in weight % in an alloy composition, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and unavoidable impurities, and has a yield strength of 350 MPa or more.

[0022] A hot-rolled steel sheet according to another embodiment of the present invention includes, in weight %, C: 0.06% or less (over 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and unavoidable impurities, and the Charpy impact energy at -20℃ is 47 J / cm 2 It is characterized by the following:

[0023] According to another embodiment of the present invention, a hot-rolled steel sheet includes, in weight % in an alloy composition, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder of Fe and unavoidable impurities, and after processing the hot-rolled steel sheet into a specimen having a length*width*thickness of 80 mm*20 mm*2 mm, a vibration damping ratio measured for a frequency of 1650 Hz in a bending vibration mode is 200*10 -6 It is characterized by the following:

[0024] According to another embodiment of the present invention, a hot-rolled steel sheet includes, in weight %, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and unavoidable impurities, and in a weld formed by welding a hot-rolled steel sheet by submerged arc welding, the Charpy impact energy of the weld at -20°C is 47 J / cm 2 And, it is characterized in that the fraction of M (martensite)-A (austenite) phase included in the above weld is 5 area% or less (including 0%).

[0025] The hot-rolled steel sheet for vacuum train tubes according to the present invention has excellent yield strength, weldability, low-temperature toughness of welded joints, and vibration damping ratio, and has the effect of preventing hot cracking during the manufacturing process.

[0026] In particular, the hot-rolled steel sheet for vacuum train tubes according to the present invention facilitates securing welded joint properties, making it easy to manufacture into tubes. Furthermore, its excellent elongation allows it to maintain structural stability even under internal and external pressure differences and stresses generated during train operation.

[0027] Moreover, its excellent vibration damping ratio rapidly reduces vibrations generated during vacuum train operation. Furthermore, it is easy to manufacture because it does not generate cracks during hot working.

[0028] In addition to the effects described above, specific effects of the present invention are described below while explaining specific details for carrying out the invention.

[0029] Fig. 1 is an optical microscope photograph of the microstructure of sample 1 of the present invention.

[0030] Figure 2 is an optical microscope photograph of EN-S355, a conventional structural steel.

[0031] The above-described objects, features, and advantages will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can easily practice the technical idea of ​​the present invention. In describing the present invention, if it is determined that a detailed description of known technologies related to the present invention may unnecessarily obscure the gist of the present invention, a detailed description thereof will be omitted. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to indicate the same or similar components.

[0032] Hereinafter, the phrase "any configuration is placed on (or below)" a component or "on (or below)" a component may mean that any configuration is placed in contact with the upper surface (or lower surface) of said component, and that other configurations may be interposed between said component and any configuration placed on (or below) said component.

[0033] Additionally, when it is described that a component is "connected," "coupled," or "connected" to another component, it should be understood that the components may be directly connected or connected to one another, but that other components may also be "interposed" between the components, or that each component may be "connected," "coupled," or "connected" through another component.

[0034] Hereinafter, hot-rolled steel sheets for vacuum train tubes according to some embodiments of the present invention will be described.

[0035] Vacuum trains, which run inside vacuum or sub-vacuum tubes, are a next-generation transportation method currently in the early stages of development. By eliminating friction between wheels and tracks and minimizing air resistance, vacuum trains effectively achieve high speeds and high efficiency. However, due to the high-speed nature of vacuum trains, there is a risk of major accidents if their safety is not adequately ensured. In particular, structural damage or collapse of the vacuum tube, as well as deformation of any part of the tube, can lead to major disasters. Therefore, materials for vacuum train tubes require even stricter safety standards. Through in-depth research, the inventors have discovered that the following properties are crucial for ensuring the safety of vacuum trains.

[0036] The first property required of materials for vacuum tubes is high strength. Since vacuum trains travel through the interior of vacuum tubes, materials for vacuum tubes must possess sufficient structural strength. Furthermore, because the interior of vacuum tubes must be maintained in a vacuum or sub-vacuum state, they must possess sufficient strength to prevent deformation of the tube's shape due to pressure differences between the interior and exterior.

[0037] The second property required for vacuum tube materials is vibration damping. A vacuum train consists of pods, each carrying tens or even hundreds of passengers, passing through the tube at intervals of tens of seconds or minutes. When a pod passes after a preceding pod, vibrations within the tube can be amplified, causing resonance and, in severe cases, even leading to tube failure. Therefore, applying a material with a vibration damping ratio above a certain level to a vacuum tube can effectively reduce vibrations within the tube after the preceding pod passes, contributing significantly to the safety of the vacuum train.

[0038] The third property required for vacuum tube materials is low-temperature toughness. Vacuum trains can operate in polar regions or deep-sea environments. Steel tends to break more easily in low or cryogenic environments, so when using steel in vacuum tubes, a certain level of low-temperature toughness is required to ensure safety. In particular, vacuum train tubes are manufactured into tubes through welding, requiring excellent low-temperature toughness not only in the base material but also in the welded joint.

[0039] The fourth property required for materials for vacuum tubes is the absence of hot cracking during the manufacturing process.

[0040] Materials for mass production must not only have good final properties but also be easy to manufacture.

[0041] This material is produced through continuous casting and hot rolling.

[0042] As in the present invention, when a copper-containing steel is heated to high temperatures during the manufacturing process, iron, a metallic element with a greater tendency to ionize than copper, is selectively oxidized. If the oxidation of iron exceeds the solid solubility limit of copper, this leads to the formation of liquid copper. This liquid copper penetrates along the grain boundaries within the material, causing cracks. Since cracks on the material surface can develop into larger defects during subsequent processing, it is crucial to ensure the absence of cracks within the material, i.e., hot cracks.

[0043] The inventor of the present invention, through in-depth research, has recognized that by strictly controlling the alloy composition content and microstructure of the steel plate, it is possible to achieve both excellent yield strength, vibration damping ratio, weldability, low-temperature toughness of the weld zone, and absence of hot cracking, and has thus derived the present invention.

[0044] The hot-rolled steel sheet according to the present invention is characterized in that it includes, in weight % in the alloy composition, C: 0.06% or less (exceeding 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and inevitable impurities, and includes polygonal ferrite of more than 90 area% and pearlite of less than 10 area% for a total of 100 area%.

[0045] Hereinafter, the steel composition included in a hot-rolled steel sheet according to one aspect of the present invention will be described in more detail. Hereinafter, unless otherwise specified, the percentage indicating the content of each element is based on weight.

[0046] Carbon (C): 0.06% or less (over 0%)

[0047] Carbon (C) is a component that significantly affects the vibration damping ratio of steel plates. As carbon content increases, internal friction decreases, sharply reducing the vibration damping ratio. Therefore, to achieve the objectives of the present invention, the upper limit of carbon content can be limited to 0.06%.

[0048] Preferably, it may contain 0.01 to 0.05% carbon (C).

[0049] Silicon (Si): 0.5 to 1.5 wt%

[0050] Silicon (Si) forms slag by combining with oxygen during the steelmaking process, and thus tends to be removed together with oxygen. In addition, silicon (Si) is also an element that effectively contributes to improving the strength of the material. Therefore, the present invention may include silicon (Si) of 0.5% or more for this effect. On the other hand, if the content of silicon (Si) is excessive, it may hinder the removal of surface scale, thereby lowering the surface quality of the product. In addition, if the content of silicon (Si) is excessive, it may promote the formation of the MA phase (martensite-austenite composite) in the weld, which may lower the low-temperature toughness of the weld. Therefore, the present invention may limit the content of silicon (Si) to 1.5% or less.

[0051] Preferably, it may contain 0.6 to 1.0% silicon (Si).

[0052] Manganese (Mn): 1.2 to 2.2 wt%

[0053] Manganese (Mn) is a component that improves the strength and hardenability of steel. Therefore, the present invention may contain 1.2% or more of manganese (Mn) to secure such effects. On the other hand, if the manganese (Mn) content is excessive, material deviation may occur due to center segregation, and crack propagation resistance may be poor. Furthermore, if the manganese (Mn) content is excessive, the toughness of the steel may be reduced. Therefore, the present invention may limit the manganese (Mn) content to 2.2% or less.

[0054] Preferably, it may contain 1.4 to 2.0% manganese (Mn).

[0055] Copper (Cu): 0.5 to 1.5 wt%

[0056] Copper (Cu) is an element that precipitates in steel, enhancing its strength. Therefore, the present invention may contain at least 0.5% Cu to achieve this effect. Copper causes segregation during manufacturing, and the Cu-enriched region lowers the melting point, forming a liquid film during casting and deteriorating high-temperature ductility. This reduced ductility can lead to cracking in the material during manufacturing.

[0057] Accordingly, the copper (Cu) content can be limited to 1.5% or less.

[0058] Preferably, it may contain 0.6 to 1.2% copper (Cu).

[0059] Nickel (Ni): 0.25 wt% or more

[0060] Nickel (Ni) is an element that prevents the high-temperature ductility degradation caused by copper (Cu), as mentioned above. It also helps improve the toughness of steel. To prevent this degradation, nickel (Ni) must be added at least 0.5 times the copper content.

[0061] From this perspective, it is desirable to include nickel (Ni) of 0.25% or more, and up to 3.0% in consideration of increased cost.

[0062] Titanium (Ti), niobium (Nb), vanadium (V): total 0.02 wt% (0% included)

[0063] Titanium (Ti), niobium (Nb), and vanadium (V) are elements that delay recrystallization of steel during rolling, thereby reducing grain size. However, as grain size decreases, vibration damping ratio deteriorates, and therefore, the combined content of these elements is limited to 0.02% or less in the present invention.

[0064] Preferably, the total content of titanium (Ti), niobium (Nb), and vanadium (V) may be 0%.

[0065] The hot-rolled steel sheet of the present invention may contain iron and other unavoidable impurities in addition to the aforementioned components. However, since unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the typical manufacturing process, they cannot be completely excluded. Since these impurities are readily apparent to anyone skilled in the art, their full contents are not specifically discussed herein. Furthermore, the addition of additional effective components other than those described above is not completely excluded.

[0066] The hot-rolled steel sheet of the present invention may include polygonal ferrite and pearlite as its microstructure. Low-temperature structures such as bainite and martensite are controlled to not be included as they may impair toughness.

[0067] Since the hot-rolled steel plate for vacuum train tubes targeted by the present invention has a thickness of 10 mm or more, low-temperature structures are formed only on the surface of the steel plate, and it is difficult for low-temperature structures to be sufficiently formed to the center of the steel plate.

[0068] Accordingly, the present invention configures the microstructure of the steel plate as a structure in which pearlite is dispersed in a polygonal ferrite matrix structure in order to suppress the occurrence of a property deviation along the thickness direction of the steel plate, 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%).

[0069] Here, the matrix structure can be interpreted as meaning a structure that occupies a fraction of 50% or more when observing the microstructure of a hot-rolled steel plate.

[0070] The ferrite structure of the present invention is polygonal ferrite (PF), which is a structure formed when austenite transformed at high temperature undergoes phase transformation in a region of 700°C or higher, and refers to ferrite that is completely polygonal (polygonal) in shape.

[0071] In terms of securing physical properties, the microstructure may include more than 90 area% of polygonal ferrite and less than 10 area% of pearlite for a total of 100 area%, and preferably may include 92 to 100 area% of polygonal ferrite.

[0072] In order to simultaneously secure the desired excellent yield strength, vibration damping ratio, low-temperature toughness and no hot cracking effects, the present invention can limit the average grain size of polygonal ferrite crystal grains to a certain range. As the average grain size of polygonal ferrite crystal grains decreases, the strength and toughness improve, but the vibration damping ratio may deteriorate. Therefore, in the present invention, the vibration damping ratio is secured by increasing the average grain size of polygonal ferrite crystal grains, and the yield strength and low-temperature impact toughness are secured by utilizing Cu precipitates formed under conditions in which the average grain size of polygonal ferrite crystal grains is controlled.

[0073] The larger the average grain size of the polygonal ferrite crystal grains, the more advantageous it is for securing a vibration damping ratio. Therefore, the average grain size of the polygonal ferrite crystal grains may be 8 to 20 ㎛, and preferably 10 to 18 ㎛.

[0074] If the average grain size of polygonal ferrite crystal grains exceeds 20㎛, the strength and low-temperature toughness of the material deteriorate, so the average grain size of polygonal ferrite crystal grains can be limited to 20㎛ or less.

[0075] It is preferable that the Cu precipitate include a Cu single precipitate called epsilon (ε)-Cu. The ε-Cu precipitate fraction (%) may be 0.3 to 1.5%, preferably 0.3 to 1.3%. The yield strength and low-temperature impact toughness can be improved by forming Cu precipitates under conditions in which the average grain size of polygonal ferrite crystal grains is controlled.

[0076] If the ε-Cu precipitate fraction (%) is less than 0.3%, the yield strength may decrease. Conversely, if the ε-Cu precipitate fraction (%) exceeds 1.5%, there is a problem of hot cracking occurring during the manufacturing process as the ε-Cu precipitate fraction increases.

[0077] The hot-rolled steel sheet for a vacuum train tube of the present invention can exhibit a yield strength of 350 MPa or more, and preferably can exhibit a yield strength of 356 to 500 MPa.

[0078] In addition, hot rolled steel sheets for vacuum train tubes have a Charpy impact energy of 47 J / cm at -20℃. 2 It can exhibit ideals, preferably 50 to 300 J / cm 2 may be, more preferably 50 to 208 J / cm 2 It could be.

[0079] In this way, the hot-rolled steel sheet for vacuum train tubes of the present invention can effectively secure the structural safety of vacuum train tubes by securing excellent strength and low-temperature toughness as a structural material.

[0080] The vibration damping ratio measured for a frequency of 1650 Hz after processing a hot-rolled steel plate into a specimen with a length*width*thickness of 80 mm*20 mm*2 mm and striking it in the flexural vibration mode was 200*10 -6 It can be more than 200*10, preferably -6 ~ 400*10 -6 It can be, more preferably 200*10 -6 ~ 350*10 -6 It could be.

[0081] The hot rolled steel sheet for vacuum train tube of the present invention is 200*10 -6 By indicating the vibration damping ratio above, vibration amplification within a vacuum tube can be effectively suppressed, and damage to a vacuum train tube due to vibration can be effectively prevented.

[0082] In the weld formed by welding the hot-rolled steel plate by submerged arc welding, the Charpy impact energy of the weld at -20℃ is 47 J / cm 2 It may be ideal, and the fraction of M (martensite)-A (austenite) phase included in the weld may be 5 area% or less (including 0%) with respect to the total 100 area%. The preferred fraction of the weld MA phase may be 3 area% or less, and the more preferred fraction of the weld MA phase may be 2 area% or less.

[0083] Typically, welds are composed of a hard phase, such as martensite, and the grain size is also large. The hard phase's microstructure can act as impact notch sites during Charpy impact energy evaluation, reducing the ductile fracture rate. Furthermore, as the average grain size increases, the impact toughness of the weld deteriorates compared to the base material.

[0084] However, in the present invention, the fraction of the M (martensite) - A (austenite) phase in the microstructure of the weld was minimized to 5 area% or less, thereby securing low-temperature toughness even in the weld. The microstructure of 95 area% or more in the weld varies depending on the location, and as it gets closer to the molten metal (weld metal), acicular ferrite and granular bainitic ferrite are formed, and as it gets closer to the base metal, polygonal ferrite and pearlite similar to the base metal begin to appear.

[0085] Acicular ferrite is a structure formed during the cooling or coiling process, with an internal structure resembling a wooden stick, called a lath. Consequently, this structure exhibits grain refinement and offers an excellent combination of strength and ductility. Acicular ferrite is also known as acicular ferrite or acicular acicular ferrite.

[0086] Granular bainitic ferrite is a structure formed during continuous cooling, and is composed of irregularly shaped bainitic ferrite and a carbon-enriched secondary phase such as the MA (martensite-austenite) phase, and has a high internal dislocation density.

[0087] Here, the weld zone is a location 1 mm away from the fusion line, and can be interpreted to mean both the weld metal zone and the heat-affected zone (HAZ).

[0088] Submerged arc welding is a method of welding by inserting a bare electrode wire into a fine granular flux deposited on the surface of the joint and using the arc heat generated between the wire and the base metal. In submerged arc welding, 20 kJ / cm is used on the inside. 2 The heat input was applied to the outside, and 22kJ / cm 2 The heat input can be applied.

[0089] Although the welding material used for welding in the present invention is not particularly limited, it is preferable to perform welding using a welding material that does not contain silicon (Si) if possible. This is because when performing welding using a welding material containing silicon (Si), there is a possibility that a large amount of hard MA may form between the welded area due to excessive hardening ability.

[0090] The hot-rolled steel sheet of the present invention can exhibit excellent yield strength, vibration damping ratio, and low-temperature toughness, and at the same time, the hot-rolled steel sheet can exhibit the effect of no hot cracking during the manufacturing process, and thus has the advantage of being applicable to materials for vacuum train tubes.

[0091] There is no hot cracking during the manufacturing process, and the hot-rolled steel sheet produced can be maintained without hot cracking.

[0092] Here, hot cracking refers to cracks that occur on the surface of a slab after casting or reheating, or cracks on the surface of a steel plate after hot rolling. Cracks can be defined as those that are 50 μm or more in depth and 5 μm or more in width.

[0093] A method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to one aspect of the present invention may include the steps of heating a slab containing, in wt%, C: 0.06% or less (more than 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder Fe and unavoidable impurities, at a heating temperature of 1100 to 1300°C; hot-rolling the heated slab at a finishing rolling temperature of 860 to 960°C to provide a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a coiling temperature of 600 to 700°C.

[0094] A steel slab having the above alloy composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to that of the hot-rolled steel sheet described above, the description of the alloy composition of the steel slab is replaced with the description of the alloy composition of the hot-rolled steel sheet described above.

[0095] The prepared steel slab can be heated at a heating temperature of 1100 to 1300°C. Considering the rolling load during hot rolling, the steel slab can be heated at a temperature range of 1100°C or higher. In particular, since the present invention seeks to introduce a microstructure of a certain size or larger, the preferable heating temperature of the steel slab can be 1200°C or higher, and the more preferable heating temperature of the steel slab can be 1250°C or higher.

[0096] On the other hand, if the steel slab heating temperature is excessively high, there is concern about deterioration of surface quality due to scale formation, so the present invention can limit the steel slab heating temperature to 1300°C or less.

[0097] A hot-rolled steel sheet can be provided by hot-rolling a heated steel slab at a finishing rolling temperature of 860 to 960°C. The steel sheet provided by the hot rolling of the present invention can have a thickness of 10 mm or more. During hot rolling, the grains are deformed as the material is rolled, but soon recrystallized. Through this process, the coarse and non-uniform structure becomes refined and homogenized. An important process variable during hot rolling is the finishing delivery temperature (FDT). This is because the fraction of the final microstructure and the grain size can be controlled according to the finishing delivery temperature. Here, the finishing delivery temperature (FDT) refers to the temperature at the end of rolling, and refers to the measured value of the steel sheet surface at the exit of the finishing rolling mill.

[0098] Since the present invention aims to control the final microstructure to a level above a certain size, hot rolling can be performed at a finishing rolling temperature of 860°C or higher. A preferred finishing rolling temperature may be 900°C or higher. On the other hand, if the finishing rolling temperature is excessively high, the final microstructure may be realized to be excessively coarse. Therefore, the present invention may limit the upper limit of the finishing rolling temperature to 960°C.

[0099] The hot-rolled steel sheet produced by hot rolling can be coiled at a coiling temperature (CT) of 600 to 700°C after water cooling. Since the present invention seeks to realize polygonal ferrite and pearlite structures as the final structure, coiling can be performed at a temperature range of 600°C or higher. Since the present invention seeks to realize a final microstructure of a certain size or larger, coiling is preferably performed at a temperature range of 630°C or higher. However, if the coiling temperature is excessively high, a coarse microstructure may be formed or the surface quality may be deteriorated, and therefore the present invention may limit the upper limit of the coiling temperature to 700°C.

[0100] The cooling rate of the water cooling and coiling steps can be performed at a conventional cooling rate.

[0101] Accordingly, the method for manufacturing a hot-rolled steel sheet for a vacuum train tube according to the present invention comprises heating a slab at a heating temperature of 1100 to 1300°C, performing hot rolling at a finishing rolling temperature of 860 to 960°C, and coiling the hot-rolled steel sheet at a coiling temperature of 600 to 700°C.

[0102] Here, specific examples of hot-rolled steel sheets for vacuum train tubes are as follows.

[0103] 1. Manufacturing of hot-rolled steel plates

[0104] After preparing a steel slab with a thickness of 250 mm having the alloy composition shown in Table 1 below, a hot-rolled steel plate with a thickness of 15 mm was manufactured by applying the process conditions shown in Table 2.

[0105] Alloy components not listed in Table 1 below refer to impurities and residual Fe.

[0106] In Table 2, FDT is the finishing rolling temperature and CT is the coiling temperature.

[0107] [Table 1]

[0108]

[0109] 2. Physical property evaluation method and results

[0110] 1) The microstructure and mechanical properties of each specimen were analyzed and listed in Tables 2 and 3.

[0111] The average grain size of polygonal ferrite was measured according to ASTM E112, and the polygonal ferrite fraction was measured according to ASTM E 562 using an image analyzer.

[0112] The microstructure of the parent material was measured using an optical microscope at 500x magnification after etching each specimen using the Nital etching method.

[0113] The ε-Cu precipitate fraction (%) was measured according to ASTM E 562 using SEM images at 10,000x magnification.

[0114] 2) Submerged arc welding was performed on each specimen using a welding material containing C: 0.052 wt%, Mn: 1.53 wt%, Ni: 1.3 wt%, Mo: 0.135 wt%, the remainder being Fe and other unavoidable impurities. During submerged arc welding, 20 kJ / cm was applied to the inside. 2 The heat input was applied to the outside, and 22kJ / cm 2 The heat input was applied. Charpy impact energy of the weld was measured at -20℃ according to KS B 0810, and the results are shown in Table 2.

[0115] For an area 1 mm away from the fusion line, a first etching was performed using a solution containing 5 g of EDTA and 0.5 g of NaF dissolved in 100 ml of distilled water, and a second etching was performed using a solution containing 25 g of NaOH and 5 g of picric acid dissolved in 100 ml of distilled water. The MA phase fraction of the weld was measured according to ASTM E 562.

[0116] 3) Mechanical properties were measured according to KS B 0802 and KS B 0810, and the measured yield strength is listed in Table 3.

[0117] 4) The vibration damping ratio was measured at room temperature using IMCE's RFDA LTV800 after preparing a specimen with a length*width*thickness of 80*20*2mm.

[0118] After striking in the bending vibration mode, the vibration damping ratio in the 1650 Hz region corresponding to the 1st mode among the vibration modes of the specimen was measured and analyzed, and the results are shown in Table 3.

[0119] 5) Hot crack measurement method and definition of crack: After hot rolling during the manufacturing process according to Table 2, the presence of cracks on the surface of the steel plate was visually checked, and hot cracks were defined as those with a depth of 50 ㎛ or more and a width of 5 ㎛ or more.

[0120] [Table 2]

[0121]

[0122] [Table 3]

[0123]

[0124] As described in Tables 1 to 3, Sample 1 satisfied all of the alloy composition and process conditions of the present invention, and exhibited excellent yield strength, vibration damping ratio, low-temperature toughness, and no hot cracking.

[0125] Figure 1 is an optical microscope photograph used to observe the microstructure of Sample 1.

[0126] Looking at the microstructure of Fig. 1, it shows that polygonal ferrite, which appears white, accounts for approximately 97%, and pearlite, which appears black, accounts for approximately 3%.

[0127] Specimens 1-1 to 1-9 have Ti+Nb+V: 0.02% or less, and satisfy all process conditions. Like Specimen 1, they exhibited excellent yield strength, vibration damping ratio, low-temperature toughness, and no hot cracking.

[0128] Sample 2 had a carbon (C) content exceeding 0.06%, a polygonal ferrite fraction of less than 90%, and a vibration damping ratio of 200*10 -6 It was shown to be less than.

[0129] Psalm 3 had a silicon (Si) content of less than 0.5% and a yield strength of less than 350 MPa.

[0130] Specimen 4 has a silicon (Si) content exceeding 1.5%, the fraction of M (martensite)-A (austenite) phase included in the weld exceeded 5 area%, and the impact toughness at low temperature was 47 J / cm 2 It was shown to be less than.

[0131] Psalm 5 had a manganese (Mn) content of less than 1.2% and a yield strength of less than 350 MPa.

[0132] Psalm 6 has a manganese (Mn) content exceeding 2.2%, and a vibration damping ratio of 200*10 -6 It was shown to be less than.

[0133] Psalm 7 had a copper (Cu) content of less than 0.5% and a nickel (Ni) content of less than 0.25%, and an ε-Cu precipitate fraction (%) of more than 0.3%, and thus a yield strength of less than 350 MPa.

[0134] Psalm 8 showed a copper (Cu) content exceeding 1.5%, and an ε-Cu precipitate fraction (%) exceeding 1.5%, which resulted in hot cracking.

[0135] When the Cu content exceeds 1.5%, during the manufacturing process, when heated to high temperatures, Fe, a metallic element with a greater tendency to ionize than Cu, is selectively oxidized. When Fe oxidation exceeds the solid solubility limit of Cu, this leads to the formation of liquid Cu, which then penetrates along the grain boundaries within the material, causing cracks and resulting in hot cracking.

[0136] Psalm 9 contains more than 0.02% of titanium (Ti), niobium (Nb), and vanadium (V), and has a vibration damping ratio of 200*10. -6 It was shown to be less than.

[0137] Specimen 10 exceeds the finishing rolling temperature and coiling temperature, and the average grain size of polygonal ferrite crystal grains exceeds 20㎛. As the average grain size of polygonal ferrite crystal grains becomes very large, the vibration damping ratio is secured, but the yield strength is less than 350MPa, and the impact toughness at low temperature is 47 J / cm. 2 It was shown to be less than.

[0138] Psalm 11 is below the finishing rolling temperature and coiling temperature, and the average grain size of polygonal ferrite crystal grains is less than 8㎛. As the average grain size of polygonal ferrite crystal grains becomes very small, the vibration damping ratio is 200*10. -6 It was shown to be less than.

[0139] Therefore, the specimen satisfying the alloy composition and process conditions of the present invention has a yield strength of 350 MPa or more and a strength of 200*10 -6 In addition to satisfying the vibration damping ratio above, the Charpy impact energy at -20℃ of the weld is 47 J / cm 2 It can be seen that specimens that satisfy the above and are free of hot cracks, but do not satisfy any one or more of the conditions limited by the present invention, do not simultaneously secure the desired properties.

[0140] Figure 2 is a photograph of the microstructure of EN-S355 taken using an optical microscope.

[0141] For comparison with conventional materials, tests were conducted under the same conditions for the conventional structural steel EN-S355 (0.15 wt% C + 1.2 wt% Mn + 0.025 wt% Nb), and the vibration damping ratio measured under the same conditions for EN-S355 was 60*10 -6 It was confirmed that it was only at the level of .

[0142] Therefore, according to the present invention, a hot-rolled steel sheet for a vacuum train tube having excellent yield strength, vibration damping ratio, and low-temperature toughness of a weld zone and no hot cracking can be provided.

[0143] Although the present invention has been described with reference to the drawings exemplified above, it is to be understood that the present invention is not limited to the embodiments and drawings disclosed herein, and that various modifications may be made by those skilled in the art within the scope of the technical idea of ​​the present invention. Furthermore, even if the operational effects according to the configuration of the present invention have not been explicitly described while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be acknowledged.

Claims

1. In the alloy composition, in weight%, C: 0.06% or less (over 0%), Si: 0.5 to 1.5%, Mn: 1.2 to 2.2%, Cu: 0.5 to 1.5%, Ni: 0.25% or more, Ti+Nb+V: 0.02% or less (including 0%), the remainder including Fe and inevitable impurities, A hot-rolled steel sheet containing more than 90 area% of polygonal ferrite and less than 10 area% of pearlite for a total of 100 area%.

2. In paragraph 1, A hot-rolled steel sheet having an average grain size of the polygonal ferrite crystal grains of 8 to 20 ㎛.

3. In paragraph 1, Hot-rolled steel sheet containing Cu precipitates and having an ε-Cu precipitate fraction (%) of 0.3 to 1.5% according to ASTM E 562.

4. In paragraph 1, Hot rolled steel plate without hot cracking. (Hot cracks are defined as those with a depth of 50 ㎛ or more and a width of 5 ㎛ or more.) 5. In paragraph 1, Hot rolled steel plate with a yield strength of 350 MPa or more.

6. In paragraph 1, Charpy impact energy at -20℃ is 47J / cm 2 Ideal hot rolled steel plate.

7. In paragraph 1, The above hot-rolled steel plate was processed into a specimen with a length*width*thickness of 80mm*20mm*2mm, and the vibration damping ratio measured for a frequency of 1650Hz in the bending vibration mode was 200*10 -6 Ideal hot rolled steel plate.

8. In paragraph 1, In the weld formed by welding the hot rolled steel plate by submerged arc welding, The Charpy impact energy at -20℃ of the above weld is 47 J / cm 2A hot-rolled steel plate having a fraction of M (martensite)-A (austenite) phase included in the welded portion of 5 area% or less (including 0%).

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