Hot-rolled steel plate for vacuum train tubes

Abstract

This invention discloses a hot-rolled steel sheet for vacuum train tubes that exhibits excellent yield strength, weldability, low-temperature toughness of the welded portion, vibration damping ratio, and is free from hot cracking. The hot-rolled steel sheet according to this invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities.

Description

Technical Field

[0001] This invention relates to a hot-rolled steel plate for vacuum train tubes. Background Technology

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

[0003] Vacuum trains eliminate air resistance and friction with the track, which are major causes of energy loss, allowing them to operate at ultra-high speeds. With minimal energy loss—saving up to 93% compared to aircraft—they are attracting significant attention as an environmentally friendly next-generation mode of transportation, and active research is underway globally.

[0004] The structure and materials of vacuum tubes used in ultra-high-speed vacuum trains significantly impact system performance and cost. Currently, materials studied for vacuum train tubes generally fall into three categories. The first is concrete. Concrete tubes offer cost advantages, but connecting individual tubes of approximately 10 meters in length is difficult. Furthermore, due to the pores within concrete, external gases can easily penetrate the tubes during vacuum operation, potentially disrupting the vacuum level. Another category of extensively studied materials is composite materials such as carbon fiber. These materials are lightweight and possess excellent performance, but their biggest drawback is their high cost.

[0005] Currently, steel is considered the most promising material for vacuum train piping. Steel is a low-cost, mass-producible material with high rigidity and strength, and is easy to process. Furthermore, steel is easy to assemble or weld between pipes and between pipes and auxiliary components, and its degassing rate while maintaining a vacuum is at a suitable level. However, ultra-high-speed vacuum trains operate at significantly higher speeds than current high-speed trains, therefore, the safety of passengers and surrounding facilities must be a top priority. Currently, safety standards for ultra-high-speed vacuum trains have not even been established, and the development of piping materials to ensure the safety of ultra-high-speed vacuum trains is also insufficient.

[0006] Therefore, there is an urgent need to develop a material for vacuum train pipelines that has both processability and degassing rate suitable for vacuum train pipelines, and can ensure safety. Summary of the Invention

[0007] (a) Technical problems to be solved The purpose of this invention is to provide a hot-rolled steel plate for vacuum train tubes that has excellent yield strength, weldability, low-temperature toughness of welded parts, vibration damping ratio, and is free from hot cracking.

[0008] The objectives of this invention are not limited to those mentioned above. Other objectives and advantages of this invention not mentioned can be understood through the following description and can be more clearly understood through embodiments of this invention. Furthermore, those skilled in the art will readily recognize that the objectives and advantages of this invention can be achieved through the means and combinations thereof described in the claims.

[0009] (II) Technical Solution The hot-rolled steel sheet according to the present invention is characterized in that the alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities; and by area percent, it comprises more than 90 area percent of polygonal ferrite and less than 10 area percent of pearlite.

[0010] The average grain size of the polygonal ferrite can be 8-20 μm.

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

[0012] In addition, it is preferred to have no hot cracks, where hot cracks are defined as cracks with a depth of more than 50 μm and a width of more than 5 μm.

[0013] In addition, the yield strength can be above 350 MPa.

[0014] Furthermore, the Charpy impact energy based on -20℃ can be 47 J / cm². 2 above.

[0015] The hot-rolled steel plate is processed into length width Thickness 80mm 20mm After inserting a 2mm specimen, the vibration attenuation ratio measured at a frequency of 1650Hz in bending vibration mode can reach 200. 10 -6 above.

[0016] In the welded section formed by submerged arc welding of the hot-rolled steel plate, the Charpy impact energy of the welded section, based on -20°C, is 47 J / cm². 2 The above-mentioned weld portion contains a fraction of M (martensite)-A (austenite) phases of 5% or less (including 0%).

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

[0018] The hot-rolled steel sheet according to another embodiment of the present invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities, and the average grain size of the polygonal ferrite grains is 8-20 μm.

[0019] The hot-rolled steel sheet according to another embodiment of the invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities, said hot-rolled steel sheet containing Cu precipitates, and the ε-Cu precipitate fraction (%) according to ASTM E 562 is 0.3-1.5%.

[0020] The hot-rolled steel sheet according to another embodiment of the present invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities, and is free from hot cracking.

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

[0022] The hot-rolled steel sheet according to another embodiment of the present invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities, and has a Charpy impact energy of 47 J / cm² based on -20°C. 2 above.

[0023] A hot-rolled steel sheet according to another embodiment of the present invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities, wherein the hot-rolled steel sheet is processed into a length of width Thickness 80mm 20mm After inserting a 2mm specimen, the vibration attenuation ratio measured at a frequency of 1650Hz in bending vibration mode was 200. 10 -6 above.

[0024] According to another embodiment of the invention, the hot-rolled steel sheet is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities. In the welded portion formed by submerged arc welding of the hot-rolled steel sheet, the Charpy impact energy of the welded portion, based on -20°C, is 47 J / cm². 2 The above-mentioned weld portion contains a fraction of M (martensite)-A (austenite) phase of 5% or less (including 0%).

[0025] (III) Beneficial Effects The hot-rolled steel sheet for vacuum train tubes according to the present invention has excellent yield strength, weldability, low-temperature toughness of welded parts, vibration damping ratio, and no hot cracking during manufacturing.

[0026] In particular, the hot-rolled steel sheet for vacuum train tubes according to the present invention easily ensures the performance of the welded parts, thus facilitating tube forming into a pipe shape. Furthermore, its excellent elongation allows it to maintain structural stability under pressure differences between the inside and outside of the pipe, as well as stresses generated during train operation.

[0027] Furthermore, its vibration damping ratio is extremely excellent, which can quickly dampen the vibrations generated during the operation of the vacuum train. In addition, no cracks are generated during the hot working process, making it easy to manufacture.

[0028] In addition to the effects described above, the specific effects of the present invention will be described together with the following description of the specific aspects of implementing the invention. Attached Figure Description

[0029] Figure 1 This is an optical microscope photograph of the fine tissue of the specimen 1 of the present invention.

[0030] Figure 2 These are optical microscope images of existing structural steel EN-S355. Detailed Implementation

[0031] The aforementioned objectives, features, and advantages will be described in detail below with reference to the accompanying drawings, which will enable those skilled in the art to readily implement the technical concept of this invention. In describing this invention, if it is determined that a detailed description of the known technologies related to this invention might unnecessarily obscure the main points of the invention, such detailed description will be omitted. Hereinafter, preferred embodiments according to the invention will be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings are used to refer to the same or similar constituent elements.

[0032] The following description of setting any structure on the "upper (or lower) part" or "above (or below)" of a constituent element not only refers to any structure being in contact with the upper (or lower) surface of the constituent element, but also to the possibility of intervening other structures between the constituent element and any structure set on (or below) the constituent element.

[0033] Furthermore, when a constituent element is described as being "connected", "combined", or "joined" with other constituent elements, it should be understood that the constituent elements can be directly connected or joined to each other, or that other constituent elements can be "intervened" between each constituent element, or that each constituent element can be "connected", "combined", or "joined" through other constituent elements.

[0034] The following will describe the hot-rolled steel sheet for vacuum train tubes according to several embodiments of the present invention.

[0035] Vacuum trains, operating within tubes in a vacuum or near-vacuum state, represent a next-generation transportation technology currently in its early stages of development. By eliminating frictional resistance between wheels and rails and minimizing air resistance, vacuum trains can effectively achieve high speeds and high efficiency. However, due to the characteristics of ultra-high-speed operation, large-scale accidents could occur if safety is not adequately guaranteed. In particular, not only structural damage or collapse of the vacuum tube, but even local deformation of the tube's shape can trigger catastrophic accidents. Therefore, the materials used in vacuum train tubes need to meet even more stringent safety requirements. Through in-depth research, the inventors have discovered that the following physical properties of materials used in vacuum tubes are crucial for ensuring the safety of vacuum trains.

[0036] The first physical property required for materials used in vacuum pipelines is high strength. Vacuum trains travel inside vacuum pipelines, therefore the materials used in these pipelines, as structural elements, must possess sufficient strength. Furthermore, the interior of the vacuum pipeline must maintain a vacuum or near-vacuum state, thus requiring sufficiently high strength to prevent deformation of the pipeline due to pressure differences.

[0037] The second required physical property for materials used in vacuum tubes is vibration damping capability. During vacuum train operation, pods carrying several to dozens of passengers pass through the vacuum tube at intervals of tens of seconds to several minutes. Vibrations within the vacuum tube can be amplified and resonate after the preceding pod has passed, potentially leading to tube damage in severe cases. Therefore, applying materials with a vibration damping ratio of a certain level to the vacuum tube can effectively reduce vibrations within the tube after the preceding pod has passed, significantly improving the safety of the vacuum train.

[0038] The third required physical property for materials used in vacuum pipelines is low-temperature toughness. Vacuum trains can operate in polar or deep-sea regions. Steel materials are more prone to damage in low or ultra-low temperature environments; therefore, when steel materials are used in vacuum pipelines, they are required to have a certain level of low-temperature toughness to ensure safety. In particular, since vacuum train pipelines are manufactured into a pipeline shape through welding, not only the base material but also the welded parts must possess excellent low-temperature toughness.

[0039] The fourth physical property required for materials used in vacuum pipes is that they do not crack during the manufacturing process.

[0040] Materials used in mass production not only need to have good final physical properties, but also need to be easy to manufacture.

[0041] This material will be produced through continuous casting and hot rolling processes.

[0042] As described in this invention, when the Cu-containing steel is heated to a high temperature during manufacturing, Fe, a metallic element with a greater tendency to ionize than Cu, undergoes selective oxidation. This Fe oxidation causes the Cu content to exceed the solid solution limit, leading to the formation of liquid Cu. This liquid Cu penetrates along the grain boundaries within the material, initiating cracks. Cracks on the material surface may develop into larger defects during subsequent processing; therefore, the absence of internal cracks, i.e., the absence of thermal cracks, is crucial.

[0043] Through in-depth research, the inventors discovered that by strictly controlling the alloy composition and microstructure of the steel plate, excellent yield strength, vibration damping ratio, weldability, low-temperature toughness of the welded part, and absence of hot cracks can be achieved simultaneously. Based on this, the present invention was completed.

[0044] The hot-rolled steel sheet according to the present invention is characterized in that its alloy composition, by weight percent, comprises: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities, and by area percent, the hot-rolled steel sheet comprises more than 90 area percent of polygonal ferrite and less than 10 area percent of pearlite.

[0045] The steel composition of the hot-rolled steel sheet according to one aspect of the present invention will now be described in more detail. Unless otherwise specified, all percentages of element content are by weight.

[0046] Carbon (C): less than 0.06% (more than 0%) Carbon (C) is a component that has a significant impact on the vibration damping ratio of steel plates. As the carbon content increases, internal friction decreases, and the vibration damping ratio drops sharply. Therefore, to achieve the objectives of this invention, the upper limit of the carbon content can be limited to 0.06%.

[0047] Preferably, it may contain 0.01-0.05% carbon (C).

[0048] Silicon (Si): 0.5-1.5% by weight Silicon (Si) combines with oxygen during the steelmaking process to form slag, thus tending to be removed along with the oxygen. Furthermore, silicon (Si) is also a component that effectively contributes to improving material strength. Therefore, to achieve the above effects, the present invention may contain 0.5% or more silicon (Si). On the other hand, if the silicon (Si) content is too high, it will hinder the removal of surface iron oxide scale, potentially reducing the surface quality of the product. Additionally, if the silicon (Si) content is too high, it will promote the formation of the MA phase (martensite-austenite complex) in the welded area, potentially reducing the low-temperature toughness of the welded area. Therefore, the present invention limits the silicon (Si) content to 1.5% or less.

[0049] Preferably, it may contain 0.6-1.0% silicon (Si).

[0050] Manganese (Mn): 1.2-2.2% by weight Manganese (Mn) is a component that improves the strength and hardenability of steel. Therefore, to ensure the above effects, the present invention may contain more than 1.2% manganese (Mn). On the other hand, if the manganese (Mn) content is too high, it will cause material deviation due to center segregation and may lead to a deterioration in crack propagation resistance. In addition, an excessively high manganese (Mn) content may also reduce the toughness of steel. Therefore, the present invention limits the manganese (Mn) content to 2.2% or less.

[0051] Preferably, it may contain 1.4-2.0% manganese (Mn).

[0052] Copper (Cu): 0.5-1.5% by weight Copper (Cu) is an element that precipitates in steel and enhances its strength. Therefore, to ensure the above-mentioned effects, this invention may contain more than 0.5% Cu. During manufacturing, Cu segregates, lowering the melting point of Cu-rich regions and forming a liquid film during casting, which deteriorates high-temperature ductility. This reduction in ductility can lead to material cracking during manufacturing.

[0053] Therefore, the copper (Cu) content can be limited to below 1.5%.

[0054] Preferably, it may contain 0.6-1.2% copper (Cu).

[0055] Nickel (Ni): 0.25% by weight or more Nickel (Ni) is an element that can suppress the decrease in high-temperature ductility caused by copper (Cu), and also helps to improve the toughness of steel. To prevent the decrease in high-temperature ductility, the amount of nickel added needs to be at least 0.5 times the copper content.

[0056] Based on this, it can contain more than 0.25% nickel (Ni), and considering the increase in cost, it is preferable to contain up to 3.0% nickel (Ni).

[0057] Titanium (Ti), Niobium (Nb), Vanadium (V): Total 0.02% by weight (inclusive) Titanium (Ti), niobium (Nb), and vanadium (V) are elements that delay recrystallization and refine grain size in steel during rolling. However, grain refinement leads to a deterioration in vibration damping ratio; therefore, the total amount of these elements added is limited to less than 0.02% in this invention.

[0058] Preferably, the total content of titanium (Ti), niobium (Nb), and vanadium (V) can be 0%.

[0059] In addition to the above-mentioned components, the balance of the hot-rolled steel sheet of the present invention may be Fe and other unavoidable impurities. However, during conventional manufacturing processes, unintentionally added impurities inevitably mix in from raw materials or the surrounding environment, and therefore cannot be completely eliminated. These impurities are known to those skilled in the art, and therefore their contents are not specifically mentioned in this specification. Furthermore, the possibility of adding additional active ingredients beyond the above-mentioned components is not completely excluded.

[0060] The microstructure of the hot-rolled steel sheet of this invention may include polygonal ferrite and pearlite. Low-temperature microstructures such as bainite and martensite pose a risk of impairing toughness, therefore their formation must be controlled to prevent their formation.

[0061] The thickness of the hot-rolled steel plate for vacuum train tubes desired by this invention can reach more than 10 mm. Therefore, low-temperature structures may only be formed on the surface of the steel plate, and it is difficult to generate low-temperature structures sufficiently in the center of the steel plate.

[0062] Therefore, in order to suppress the deviation of physical properties in the thickness direction of the steel plate, the present invention sets the microstructure of the steel plate to a structure in which pearlite is dispersed in a polygonal ferrite matrix. Even if low-temperature structures such as bainite and martensite are inevitably generated, their fraction can be actively suppressed to below 1% (including 0%).

[0063] The matrix structure here can be interpreted as the fraction of the structure that accounts for more than 50% of the area when observing the fine structure of hot-rolled steel sheets.

[0064] The ferrite microstructure of the present invention is polygonal ferrite (PF). Polygonal ferrite refers to ferrite that is completely polygonal in shape and is formed by a phase transformation of austenite that has been deformed at high temperature in a region above 700°C.

[0065] From the perspective of ensuring physical properties, the microstructure can contain more than 90% polygonal ferrite and less than 10% pearlite, with a total area percentage of 100%, preferably containing 92-100% polygonal ferrite.

[0066] To simultaneously achieve the desired excellent yield strength, vibration damping ratio, low-temperature toughness, and crack-free properties, this invention restricts the average grain size of the polygonal ferrite within a certain range. Smaller average grain size of the polygonal ferrite results in superior strength and toughness, but may degrade the vibration damping ratio. Therefore, this invention ensures the vibration damping ratio by increasing the average grain size of the polygonal ferrite, while simultaneously ensuring yield strength and low-temperature impact toughness through Cu precipitates formed under controlled average grain size conditions.

[0067] The larger the average grain size of polygonal ferrite, the better it is to ensure the vibration damping ratio. Therefore, the average grain size of polygonal ferrite can be 8-20 μm, preferably 10-18 μm.

[0068] If the average grain size of polygonal ferrite exceeds 20 μm, the strength and low-temperature toughness of the material will deteriorate. Therefore, the average grain size of polygonal ferrite can be limited to below 20 μm.

[0069] The Cu precipitates preferably include elemental Cu precipitates known as ε-Cu. The fraction (%) of ε-Cu precipitates can be 0.3-1.5%, preferably 0.3-1.3%. By generating Cu precipitates under controlled average grain size conditions of polygonal ferrite, yield strength and low-temperature impact toughness can be improved.

[0070] If the ε-Cu precipitate fraction (%) is less than 0.3%, the yield strength may decrease. On the other hand, if the ε-Cu precipitate fraction (%) exceeds 1.5%, hot cracking may occur during the manufacturing process as the ε-Cu precipitate fraction increases.

[0071] The hot-rolled steel sheet for vacuum train tubes of the present invention can exhibit a yield strength of more than 350 MPa, and preferably a yield strength of 356-500 MPa.

[0072] Furthermore, the hot-rolled steel plate for vacuum train tubes of the present invention can achieve a Charpy impact energy of 47 J / cm² based on -20°C. 2 The preferred values ​​are 50-300 J / cm³. 2 More preferably, it can be 50-208 J / cm. 2 .

[0073] As described above, the hot-rolled steel plate used for vacuum train pipes in this invention, as a structural material, has excellent strength and low-temperature toughness, which can effectively ensure the structural safety of the pipes used in vacuum trains.

[0074] Process hot-rolled steel plates into lengths width Thickness 80mm 20mm After inserting a 2mm specimen, impact was performed in flexural vibration mode. The vibration attenuation ratio measured at a frequency of 1650Hz was 200. 10 -6 The above can be preferably 200. 10 -6 Up to 400 10 -6 More preferably, it can be 200 10 -6 Up to 350 10 -6 .

[0075] The hot-rolled steel plate for vacuum train tubes of the present invention has 200 10 -6 The above vibration attenuation ratio can effectively suppress vibration amplification in vacuum pipes and effectively prevent damage to vacuum train pipes caused by vibration.

[0076] In the welded section formed by submerged arc welding of the hot-rolled steel plate, the Charpy impact energy of the welded section, based on -20°C, can be 47 J / cm². 2 Based on the overall area percentage (100%), the fraction of the M (martensite)-A (austenite) phase contained in the welded portion can be 5% or less (including 0%). Preferably, the fraction of the MA phase in the welded portion can be 3% or less, and more preferably 2% or less.

[0077] Typically, welded sections are composed of hard phases such as martensite, and the grain size is relatively large. In Charpy impact energy testing, the fine structure of the hard phase may act as impact notch sites, reducing the ductile fracture rate. Furthermore, the larger the average grain size of the welded section, the worse its impact toughness is compared to the base material.

[0078] However, in this invention, by minimizing the M (martensite)-A (austenite) phase fraction in the microstructure of the welded portion to below 5% by area, low-temperature toughness is also ensured in the welded portion. The microstructure in the welded portion, exceeding 95% by area, varies with location; closer to the weld metal, acicular ferrite and granular bainitic ferrite are more likely to form; closer to the base metal, polygonal ferrite and pearlite similar to those in the base metal begin to appear.

[0079] Acicular ferrite is a microstructure formed during cooling or winding, composed of lath-like structures. Therefore, this microstructure exhibits grain refinement and offers an excellent balance between strength and plasticity. Acicular ferrite is also known as acicular ferrite or acicular ferrite.

[0080] Granular bainitic ferrite is a microstructure formed during continuous cooling. It consists of irregularly shaped bainitic ferrite and carbon-rich second phases such as martensite-austenite (MA), and has a high internal dislocation density.

[0081] Here, the welded part refers to the position 1mm away from the fusion line, which can be interpreted as including both the deposited metal part and the heat-affected zone (HAZ).

[0082] Submerged arc welding (SAW) is a welding method in which a photoelectric electrode wire is embedded in a fine-particle flux deposited on the joint surface, and welding is performed using the heat of an electric arc generated between the base metal and the welding wire. During SAW, the inner side can be welded at 20 kJ / cm². 2 The heat input on the outside can be 22 kJ / cm². 2 Heat input.

[0083] There are no particular restrictions on the welding materials used in this invention, but it is preferable to use welding materials that do not contain silicon (Si) as much as possible. This is because if silicon (Si) welding materials are used, excessive hardenability may lead to the formation of a large amount of hard MA phase in the weld.

[0084] The hot-rolled steel sheet of the present invention exhibits excellent yield strength, vibration damping ratio, and low-temperature toughness, while also being free from hot cracking during manufacturing. Therefore, it has the advantage of being applicable to materials used in vacuum train pipelines.

[0085] It produces no hot cracks during manufacturing, and the hot-rolled steel sheets produced can always remain free of hot cracks.

[0086] Hot cracks here refer to cracks that occur on the surface of slabs after casting and reheating, or cracks on the surface of hot-rolled steel plates. Cracks can be defined as defects with a depth of more than 50 μm and a width of more than 5 μm.

[0087] A method for manufacturing a hot-rolled steel sheet for vacuum train tubes according to one aspect of the present invention may include the following steps: heating a slab at a heating temperature of 1100-1300°C, wherein the slab comprises, by weight %,: C: less than 0.06% (more than 0%), Si: 0.5-1.5%, Mn: 1.2-2.2%, Cu: 0.5-1.5%, Ni: more than 0.25%, Ti+Nb+V: less than 0.02% (inclusive of 0%), the balance being Fe and unavoidable impurities; hot-rolling the heated slab at a finishing rolling temperature of 860-960°C to provide a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a coiling temperature of 600-700°C.

[0088] Prepare a steel billet having the above-described alloy composition. The steel billet of the present invention has an alloy composition corresponding to that of the above-described hot-rolled steel sheet; therefore, the description of the alloy composition of the steel billet is replaced by the description of the alloy composition of the above-described hot-rolled steel sheet.

[0089] The prepared steel billet can be heated at a temperature of 1100-1300°C. Considering the rolling load during hot rolling, the steel billet can be heated in a temperature range above 1100°C. In particular, the present invention aims to introduce a fine microstructure of a certain size or higher, therefore the heating temperature of the steel billet is preferably above 1200°C, and more preferably above 1250°C.

[0090] On the other hand, if the heating temperature of the steel billet is too high, the surface quality may be reduced due to the formation of iron oxide scale. Therefore, the present invention can limit the heating temperature of the steel billet to below 1300°C.

[0091] Heated steel billets can be hot-rolled at a finishing temperature of 860-960°C to provide hot-rolled steel sheets. The thickness of the steel sheets provided by hot rolling according to this invention can be 10 mm or more. During hot rolling, the grains of the material deform during rolling, but recrystallization occurs immediately afterward. Through this process, the coarse and inhomogeneous microstructure is refined and homogenized. An important process parameter in hot rolling is the finishing delivery temperature (FDT), because the final microstructure fraction and grain size can be controlled by the finishing delivery temperature. Here, the finishing delivery temperature (FDT) refers to the temperature at which rolling is completed, i.e., the measured value on the surface of the steel sheet at the exit of the finishing mill.

[0092] The present invention aims to control the final microstructure to a level above a certain size, thus allowing hot rolling at a finishing rolling temperature above 860°C. The finishing rolling temperature is preferably above 900°C. On the other hand, if the finishing rolling temperature is too high, the final microstructure may be too coarse; therefore, the present invention limits the upper limit of the finishing rolling temperature to 960°C.

[0093] Hot-rolled steel sheets, after water cooling, can be coiled at a coiling temperature (CT) of 600-700°C. This invention aims to obtain polygonal ferrite and pearlite as the final microstructure, therefore coiling can be performed within a temperature range above 600°C. This invention aims to obtain a final fine microstructure of a certain size, therefore coiling is preferably performed within a temperature range above 630°C. However, excessively high coiling temperatures may result in coarse fine microstructures or deteriorate surface quality; therefore, this invention limits the upper limit of the coiling temperature to 700°C.

[0094] The cooling rate for the water cooling and winding steps can be the conventional cooling rate.

[0095] Therefore, the manufacturing method of the hot-rolled steel sheet for vacuum train tubes according to the present invention is as follows: heating the slab at a heating temperature of 1100-1300°C, hot rolling at a finishing rolling temperature of 860-960°C, and coiling the hot-rolled steel sheet at a coiling temperature of 600-700°C.

[0096] As described above, the implementation scheme for hot-rolled steel plates used in vacuum train tubes is as follows.

[0097] 1. Manufacturing of hot-rolled steel plates Prepare a steel billet with the alloy composition shown in Table 1 and a thickness of 250 mm, and then manufacture a hot-rolled steel sheet with a thickness of 15 mm using the process conditions in Table 2.

[0098] The alloy composition not listed in Table 1 below consists of impurities and the balance Fe.

[0099] In Table 2, FDT represents the finishing rolling temperature and CT represents the winding temperature.

[0100] [Table 1] 2. Physical performance evaluation methods and results 1) Analyze the microstructure and mechanical and physical properties of each specimen, and record them in Tables 2 and 3.

[0101] The average grain size of polygonal ferrite was measured according to ASTM E 112, and the polygonal ferrite fraction was measured using an image analyzer according to ASTM E 562.

[0102] The microstructure of the base material was etched using the Nital etching method on each specimen, and then measured using a 500x optical microscope.

[0103] The ε-Cu precipitate fraction (%) was measured using scanning electron microscopy (SEM) images at 10,000x magnification, according to ASTM E 562.

[0104] 2) Submerged arc welding was performed on each specimen using welding material containing C: 0.052 wt%, Mn: 1.53 wt%, Ni: 1.3 wt%, Mo: 0.135 wt%, with the balance being Fe and other unavoidable impurities. During submerged arc welding, a welding flux of 20 kJ / cm² was applied to the inside. 2 The heat input on the outside is 22 kJ / cm². 2 The heat input was measured according to KS B 0810. The Charpy impact energy of the welded part at -20°C was measured, and the results are recorded in Table 3.

[0105] For the area 1 mm from the fusion line, a first etching was performed using a solution of 5 g EDTA and 0.5 g NaF dissolved in 100 ml distilled water. Then, a second etching was performed using a solution of 25 g NaOH and 5 g picric acid dissolved in 100 ml distilled water. The MA phase fraction of the weld was measured according to ASTM E 562.

[0106] 3) The mechanical and physical properties were measured according to KS B 0802 and KS B 0810, and the measured yield strength is recorded in Table 3.

[0107] 4) For the damping ratio, the preparation length width Thickness 80 20 A 2mm sample was used, and then the measurement was performed at room temperature using an IMCE RFDA LTV800.

[0108] After impact in bending vibration mode, the vibration attenuation ratio in the 1650Hz range corresponding to the first-order mode of the specimen was measured and analyzed, and the results are recorded in Table 3.

[0109] 5) Hot crack measurement method and definition of crack: After hot rolling in the manufacturing process according to Table 2, the presence of cracks on the surface of the steel plate is visually confirmed. Hot crack is defined as a crack with a depth of more than 50 μm and a width of more than 5 μm.

[0110] [Table 2] [Table 3] As shown in Tables 1 to 3, Specimen 1 simultaneously meets the alloy composition and process conditions of the present invention, exhibiting excellent yield strength, vibration damping ratio, low-temperature toughness, and no hot cracking.

[0111] Figure 1This is an optical microscope photograph used to observe the fine tissue of specimen 1.

[0112] from Figure 1 The fine structure shows that approximately 97% is white polygonal ferrite and approximately 3% is black pearlite.

[0113] The Ti+Nb+V content of specimens 1-1 to 1-9 is below 0.02%, and all of them meet the process conditions, the same as specimen 1. They exhibit excellent yield strength, vibration damping ratio, low temperature toughness and no hot cracking.

[0114] Specimen 2 has a carbon (C) content exceeding 0.06%, a polygonal ferrite fraction below 90%, and a vibration damping ratio less than 200. 10 -6 .

[0115] The silicon (Si) content of sample 3 is less than 0.5%, and the yield strength is less than 350 MPa.

[0116] The silicon (Si) content of specimen 4 exceeds 1.5%, the M (martensite)-A (austenite) phase fraction in the welded part exceeds 5% by area, and the impact toughness at low temperature is less than 47 J / cm. 2 .

[0117] The manganese (Mn) content of specimen 5 is less than 1.2%, and the yield strength is less than 350 MPa.

[0118] The manganese (Mn) content of sample 6 exceeds 2.2%, and the vibration damping ratio is less than 200. 10 -6 .

[0119] The copper (Cu) content of specimen 7 is less than 0.5%, the nickel (Ni) content is less than 0.25%, and the ε-Cu precipitate fraction (%) exceeds 0.3%, therefore the yield strength is less than 350 MPa.

[0120] The copper (Cu) content of sample 8 exceeded 1.5%, and the ε-Cu precipitate fraction (%) exceeded 1.5%, thus resulting in hot cracks.

[0121] When the Cu content exceeds 1.5%, during the manufacturing process and heating to high temperatures, Fe, a metal element with a greater tendency to ionize than Cu, will undergo selective oxidation. The oxidation of Fe causes the Cu content to exceed the solid solution limit, thus forming liquid Cu. This liquid Cu will penetrate along the grain boundaries inside the material, initiating cracks, thus producing hot cracks.

[0122] The combined content of titanium (Ti), niobium (Nb), and vanadium (V) in sample 9 exceeds 0.02%, and the vibration damping ratio is less than 200. 10 -6.

[0123] The finishing and coiling temperatures of sample 10 exceeded the specified ranges, and the average grain size of the polygonal ferrite exceeded 20 μm. While the vibration damping ratio was maintained due to the significant increase in the average grain size of the polygonal ferrite, the yield strength was less than 350 MPa, and the impact toughness at low temperatures was less than 47 J / cm². 2 .

[0124] The finishing rolling temperature and coiling temperature of test piece 11 were below the specified range, and the average grain size of the polygonal ferrite was less than 8 μm. As the average grain size of the polygonal ferrite decreased significantly, the vibration damping ratio was less than 200. 10 -6 .

[0125] Therefore, it can be seen that the test pieces that meet the alloy composition and process conditions of this invention not only meet the requirements of a yield strength of over 350 MPa and a strength of 200 MPa, but also meet the requirements of a high yield strength of over 350 MPa and a high yield strength of over 200 MPa. 10 -6 The above vibration damping ratio also meets the Charpy impact energy requirement of 47 J / cm² for the welded section at -20°C. 2 The above-mentioned specimens, which do not meet any one or more of the conditions specified in this invention, and have no thermal cracks, cannot simultaneously achieve the target physical properties.

[0126] Figure 2 These are microscopic images of EN-S355 taken using an optical microscope.

[0127] To compare with existing materials, tests were conducted on existing structural steel EN-S355 (0.15 wt% C + 1.2 wt% Mn + 0.025 wt% Nb) under the same conditions. The results confirmed that EN-S355 exhibited a vibration damping ratio of only 60 under the same conditions. 10 -6 The level.

[0128] Therefore, according to the present invention, a hot-rolled steel sheet for vacuum train tubes can be provided, which has excellent yield strength, vibration damping ratio, low-temperature toughness of welded parts, and no hot cracking.

[0129] As described above, the present invention has been illustrated with reference to the exemplary drawings. However, the present invention is not limited to the embodiments and drawings disclosed in this specification. It is obvious that those skilled in the art can make various modifications within the scope of the technical concept of the present invention. Furthermore, although the effects of the configuration of the present invention have not been explicitly described or explained in the foregoing description of the embodiments of the present invention, the predictable effects that can be achieved by such configuration should also be recognized.

Claims

1. A hot-rolled steel sheet comprising, in mass%, C: 0.06% or less and 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 and including 0%, the balance of Fe and unavoidable impurities, comprising, in the entirety of 100 area%, more than 90 area% of polygonal ferrite and less than 10 area% of pearlite.

2. The hot-rolled steel sheet according to claim 1, wherein, The average grain size of the grains of the polygonal ferrite is 8-20 μm.

3. The hot-rolled steel sheet according to claim 1, wherein, The hot-rolled steel sheet comprises Cu precipitates, and the ε-Cu precipitate fraction in % according to ASTM E 562 is 0.3-1.5%.

4. The hot-rolled steel sheet according to claim 1, wherein, The hot-rolled steel sheet is free of hot cracks, defined as cracks having a depth of 50 μm or more and a width of 5 μm or more.

5. The hot-rolled steel sheet according to claim 1, wherein, The yield strength is 350 MPa or more.

2. The hot-rolled steel sheet according to claim 1, wherein the average grain size of the grains of the polygonal ferrite is 10-20 μm.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the ε-Cu precipitate fraction in % according to ASTM E 562 is 0.5-1.5%.

4. The hot-rolled steel sheet according to any one of claims 1-3, wherein the hot-rolled steel sheet is free of hot cracks, defined as cracks having a depth of 50 μm or more and a width of 5 μm or more.

5. The hot-rolled steel sheet according to any one of claims 1-4, wherein the yield strength is 400 MPa or more.

6. The hot-rolled steel sheet according to any one of claims 1-5, wherein the yield strength is 450 MPa or more.

7. The hot-rolled steel sheet according to any one of claims 1-6, wherein the yield strength is 500 MPa or more.

8. The hot-rolled steel sheet according to any one of claims 1-7, wherein the yield strength is 550 MPa or more.

9. The hot-rolled steel sheet according to any one 6. The hot-rolled steel sheet according to claim 1, wherein, Charpy impact energy at -20°C of 47 J / cm 2 The above.

7. The hot-rolled steel sheet according to claim 1, wherein, The hot-rolled steel plate is processed into a test piece having a length width thickness of 80 mm 20 mm 2 mm, the vibration attenuation ratio measured in the bending vibration mode for a frequency of 1650 Hz is 200 10 -6 or more.

8. The hot-rolled steel sheet according to claim 1, wherein, The welded portion formed by submerged-arc welding the hot-rolled steel sheet has a Charpy impact energy of 47 J / cm at -20°C 2 The fraction of M (martensite)-A (austenite) phase included in the welded portion is 5 area% or less, including 0%.

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