Hot-rolled wire rod for 2500 MPa bridge cable and manufacturing method of hot-rolled wire rod

Abstract

The invention relates to a hot-rolled wire rod for a 2500 MPa bridge cable and a manufacturing method of the hot-rolled wire rod, which are characterized in that after Nb-V-Mo high-carbon component is rolled and spun into a wire rod, the wire rod is cooled at a cooling speed of more than or equal to 33 DEG C / s through online molten salt rapid cooling isothermal treatment, and enters a sorbite phase region from an austenite state to form a structure mainly comprising sorbite and control the precipitation and growth of carbides; the hot-rolled wire rod with a microscopic structure comprising a mixed structure composed of tempered sorbite, ferrite and fused sorbite is prepared through roller way slow cooling, the content of strengthening elements can be increased without weakening the toughness of the material, the material cost is considered, the tensile strength is 1692-1735 MPa, the percentage reduction of area is 25%-30%, and the tensile strength of the steel wire rod is 1692-1735 MPa. The method is used for the application field of manufacturing of 2500 MPa-grade ultrahigh-strength bridge cables and the like, and the wire breaking risk in the drawing and twisting process can be reduced easily.

Description

Technical Field

[0001] The invention belongs to the field of hot-rolled wire rods, and in particular relates to a 2500MPa-grade hot-rolled wire rod for bridge cables and a manufacturing method thereof. Background Art

[0002] Under the development direction of lightweight, the strength grade of bridge cables and their parent materials in the bridge field are constantly rising. As the parent material of bridge cable steel wire, the quality of hot-rolled wire rod directly determines the final performance of the bridge cable, which in turn affects the safety, stability and service life of the bridge. Therefore, in order to meet the application requirements of large-span bridges and ensure the reliable operation of bridges under complex environments and long-term loads, it is necessary to develop 2500MPa hot-rolled wire rod for bridge cables.

[0003] Due to the limitation of the cooling capacity of the existing Stelmor air-cooled line, it is difficult to stably develop bridge cables with ultra-high strength grades of 2200MPa and above. The main reasons why the existing hot-rolled wire rods cannot meet the production and application of 2500MPa bridge cables include: 1. In order to improve the strength of the hot-rolled wire rod base material, the alloy component content in the steel is relatively high. For example, patent CN110055392B discloses a high-toughness bridge cable steel with a tensile strength ≥2500Mpa. The wire rod is designed with a medium-carbon, high-manganese, and high-aluminum component of C-Mn-Al-Co-V. However, on the one hand, the high content of alloy components brings higher material cost pressure. On the other hand, the existing hot-rolled wire rods for bridge cables generally adopt Stelmor air-cooled wire cooling after wire drawing. Under the influence of the segregation of alloy elements and the uncontrollable cooling rate of the air-cooled wire rod, the wire rod is prone to produce brittle structure, resulting in a decrease in structural uniformity and large fluctuations in mechanical properties. The wire rod has high hardness, high brittleness, and poor plasticity and toughness, which in turn aggravates the risk of wire breakage in the subsequent drawing process, making it difficult to stably produce bridge cable steel wire.

[0004] 2. Carbon is an effective strengthening element for hot-rolled wire rod base material, and its cost is lower than other alloying elements. Existing high-strength wire rods for bridge cables generally use high-carbon pearlite steel, but when the carbon content exceeds 0.77%, the probability of brittle structure generation increases significantly. On this basis, in order to further improve the strength of the wire rod, the carbon and other alloying elements are continuously increased. Although the strength can be quickly improved, on the one hand, due to the limited maximum cooling capacity of the Stelmor air-cooled line, excessively high carbon concentration is easy to precipitate along the austenite grain boundaries in the form of carbides, forming a thick cementite network. The presence of network carbides It weakens the bonding force between metals, reduces the strength and toughness of steel, especially the impact toughness decreases significantly and the brittleness increases, which in turn aggravates the risk of wire breakage during drawing. On the other hand, the hardenability of the wire rod is improved. In order to minimize the level of network carbides, strong air cooling treatment will be selected after wire drawing, but it also brings about the uncontrollability of air volume and air temperature. The temperature difference between the surface and the core of the wire rod is further increased. The position where the wire rod is cooled too quickly is more likely to form hard and brittle phases such as bainite and martensite. Therefore, the brittleness of the wire rod also increases rapidly, weakening the toughness and organizational uniformity of the material, making the risk of wire breakage during subsequent drawing and torsion of the wire rod extremely high.

[0005] 3. As a fine lamellar pearlite structure, troostite has good drawing deformation and work hardening characteristics. However, due to the limited minimum cooling control capacity of the Stelmor air cooling line, as the content of alloy elements in the structure increases, the phase transformation time of the wire rod through the troostite phase region during continuous cooling is short, which limits the full phase transformation of the troostite. On the one hand, the residual austenite may continue to transform into a brittle martensite structure during the subsequent cooling process, increasing the fluctuation of the mechanical properties of the wire rod. On the other hand, the troostite content in the structure is low, the interlamellar spacing is large, and the ferrite content is high. After the phase transformation, the troostite has a higher stress, which makes the wire rod insufficient in strength and plasticity, and it is difficult to meet the ultra-high strength grade requirements of the bridge cable.

[0006] 4. In order to further improve the strength of the wire rod, micro-alloy elements such as Nb, V, and Ti are added to the wire rod to refine the grains and strengthen the wire rod through precipitation. However, due to the cooling capacity of the air-cooling line, the wire rod passes through the temperature range of the strengthening phase precipitation for a short time, and it is difficult to fully exert its strengthening effect, which will lead to a large amount of alloy and high cost. In order to exert the effects of Nb and V, the existing technology will reduce the spinning temperature, but it also brings limitations to the rolling and network carbide control; on the other hand, due to the uncontrollable cooling of the air-cooling line, it is easier to form coarser phases and finer phases at the same time, which not only affects the uniformity of the organization, but also the coarser phase is detrimental to the strength and plasticity of the wire rod, and it is difficult to achieve the ultra-high strength grade and stable production requirements of the bridge cable. Summary of the invention

[0007] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 2500MPa grade hot-rolled wire rod for bridge cables and a manufacturing method thereof, which can increase the content of strengthening elements without weakening the toughness of the material and take into account the material cost, and is suitable for the stable production of 2500MPa grade bridge cables.

[0008] The technical solution adopted by the present invention to solve its technical problem is: A method for manufacturing a 2500MPa grade hot-rolled wire rod for bridge cables, the manufacturing method comprising: The hot-rolled wire rod is rolled to produce a wire rod according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%~0.98%, Si: 1.00%~1.20%, Mn: 0.70%~0.90%, Cr: 0.33%~0.53%, Nb: 0.03%~0.038%, V: 0.03%~0.04%, Mo: 0.25%~0.35%, P≤0.015%, S≤0.015 %, and the rest are Fe and unavoidable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥900°C, it is subjected to an online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥33°C / s, enters the sorbite phase region from the austenite state, forms a sorbite-based structure, and controls the precipitation and growth of carbides, and isothermally toughened to relieve stress, and finally is slowly cooled through a roller to produce a hot-rolled wire rod whose microstructure includes a mixed structure consisting of tempered sorbite, ferrite and fused sorbite.

[0009] The design basis of the chemical composition and mass percentage of the above hot rolled wire rod includes: (1) Carbon: The carbon element is an effective carbide strengthening element and austenite forming element. It is cheaper than other elements. As the carbon content increases, the troostite transformation temperature decreases, the driving force of phase transformation increases, and the troostite structure becomes finer and more dispersed during the online molten salt rapid cooling isothermal treatment, thereby improving the material strength. However, as the carbon content increases, the carbon segregation tendency during the solidification of the steel billet increases, the tendency of decarburization and precipitation of network carbides increases, and the difficulty of brittle phase control and isothermal toughening stress relief increases. Therefore, in order to meet the high strength requirements of the 2500MPa bridge cable for the hot-rolled wire rod base material, control the material cost, and reduce the difficulty of brittle phase and toughening control, the mass percentage of C is controlled to be 0.95%~0.98%.

[0010] (2) Silicon: Si is a good deoxidizer with significant solid solution strengthening effect. It can also inhibit grain coarsening during online molten salt rapid cooling isothermal treatment, so that a troostite structure with fine interlamellar spacing can be quickly obtained, thereby improving the structural uniformity of matrix strength. However, excessive silicon content will make steel more susceptible to decarburization when heated at high temperatures, prolong the phase transformation incubation time, and be detrimental to the control of carbide precipitation and growth through a shorter treatment time during the online molten salt rapid cooling isothermal process. Therefore, in order to inhibit austenite grain growth, promote troostite phase transformation, and facilitate the control of carbide precipitation and growth, the mass percentage of Si is controlled to be 1.00%~1.20%.

[0011] (3) Manganese: Mn has a strong affinity with carbon and can expand the austenite phase region, increase the stability of austenite, increase the hardenability of the wire rod, reduce the temperature of the transformation from austenite to sorbite, refine the sorbite structure, improve the strength and toughness of sorbite, and thus improve the tensile strength of the wire rod, so that the bridge cable can withstand impact and vibration loads while bearing tension. However, when the Mn content is too high, it will increase element segregation and increase the risk of precipitation of low-temperature structures such as bainite and martensite in the wire rod. At the same time, it will reduce the activity of carbon, increase the difficulty of isothermal toughening stress relief, and affect the plastic and toughness properties of the wire rod. Therefore, in order to promote the refinement of the sorbite phase transformation, reduce the uniformity of the structure and the difficulty of tempering control, the mass percentage of Mn is controlled to be 0.70%~0.90%.

[0012] (4) Chromium: Cr is a strong carbide-forming element that can improve the hardenability of steel, reduce the troostite transformation temperature, make the troostite structure finer, and improve the matrix strength. It can also reduce the strength loss during the subsequent hot-dip galvanizing process of steel wire. However, too high Cr content will aggravate the component segregation, increase the risk of abnormal martensite structure precipitation, significantly increase the difficulty of improving the plasticity of wire rod, affect the isothermal toughening stress relief effect, and further affect the wire drawing and torsion performance of steel wire. The precipitation and coarsening of Cr will also cause loss of strong and plastic properties. Therefore, in order to improve the material strength, facilitate the improvement of wire rod plasticity and control the precipitation of carbides, the Cr content is appropriately reduced, and the mass percentage of Cr is controlled to be 0.33%~0.53%.

[0013] (5) Niobium: As a microalloying element, Nb can inhibit the coarsening of high-temperature austenite, refine austenite grains, improve the strength and toughness of steel, and increase the yield strength of steel through precipitation strengthening. It can keep austenite stable at a lower temperature and provide more nucleation sites for troostite transformation. At the same time, it can be dispersed and precipitated during the isothermal process, effectively improving the strength of hot-rolled wire rod without reducing plasticity. However, the cost of Nb is relatively high. Therefore, based on the role of Nb and cost considerations, the Nb content is controlled at 0.03%~0.038%.

[0014] (6) Vanadium: As a microalloying element, V can inhibit the coarsening of high-temperature austenite and hinder the growth of austenite grains, thereby refining the grains. At the same time, it can improve the strength and toughness of steel by forming carbides and nitrides during the isothermal process, effectively improving the strength of hot-rolled wire rods without reducing plasticity. However, the cost of V is relatively high. Excessive addition is not conducive to controlling the cost of wire rods and has the risk of coarsening. When the vanadium precipitate phase coarsens, it will lose its strength effect. At the same time, the coarsened vanadium precipitate phase may become a crack source, reducing the toughness of the material. Therefore, based on the role of V and cost considerations, the V content is controlled at 0.03%~0.04%.

[0015] (7) Molybdenum: Mo can improve the hardenability of steel, increase the stability of supercooled austenite, shift the C curve to the right, slow down the transformation of austenite to ferrite, refine the troostite structure, and effectively inhibit the coarsening of carbide precipitation phase during online molten salt rapid cooling isothermal treatment, thereby effectively improving the strengthening effect of the precipitation phase and maintaining a high strength during tempering. However, Mo is expensive. Therefore, based on the role of Mo and cost considerations, the mass percentage of Mo is controlled to be 0.25%~0.35%.

[0016] (8) Phosphorus and sulfur: P and S are impurity elements. The lower the better. Therefore, P is controlled to be ≤ 0.015% and S is ≤ 0.015%.

[0017] The above-mentioned hot-rolled wire rod adopts the high carbon component design of Nb-V-Mo, optimizes the composition ratio of Si, Mn and Cr, reduces the sorbite phase transformation temperature and the difficulty of plasticity improvement, provides favorable conditions for refining the spacing between sorbite lamellae, facilitating the isothermal tempering and toughening of the wire rod and controlling the precipitation and growth of carbides, and can reduce the amount of alloy components and take into account the material cost control. On this basis, the wire rod selects an appropriate wire-spinning temperature to avoid the formation of network carbides in the wire-spinning stage due to the wire-spinning temperature being too low, and increases the supercooling degree with a higher quenching temperature, which provides favorable conditions for the rapid nucleation of sorbite and the dispersion and precipitation of Nb and V carbides. After the wire-spinning, the wire rod enters the molten salt for online molten salt rapid cooling isotherm without air cooling: 1. Compared with the high-carbon component system, the brittle phases such as network carbides and martensite on the Stelmor air cooling line are difficult to control. When the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for efficient and uniform heat exchange. On the one hand, the wire rod can be controlled to quickly cool down from the high-temperature austenite state and pass through the secondary cementite precipitation temperature range of 700~800℃, avoiding the formation of network carbides along the grain boundaries to destroy the continuity of the organization and reduce the plasticity and toughness of the material; on the other hand, there is no temperature difference problem between the windward side and the leeward side. Under high-speed heat exchange, the temperature difference from the surface of the wire rod to the core is smaller, which can avoid entering the martensite phase region due to excessive cooling to form brittle abnormal organization, so that the brittle phase is effectively controlled and the organization is more uniform, which provides favorable conditions for giving full play to the strengthening effect of carbon element.

[0018] 2. Compared with the high-carbon and high-silicon composition system, the phase transformation incubation time is long on the Stelmor air-cooling line, the sorbite content is low, and the organizational stress is large. On the one hand, when the wire rod undergoes online molten salt rapid cooling isothermal treatment, it can promote the wire rod to quickly cool down to the sorbite phase temperature, form a large degree of supercooling, inhibit grain coarsening, and promote the wire rod to quickly transform to sorbite with finer interlamellar spacing, so as to improve the matrix strength and reduce the difficulty of isothermal tempering of the sorbite quenched structure. After the molten salt treatment, the wire rod can be transformed to the same temperature as the molten salt, which can extend the isothermal time of the wire rod in the sorbite phase region, promote the wire rod to fully precipitate at the sorbite peak precipitation temperature, increase the sorbite content in the organization, and reduce Low ferrite content improves matrix strength and improves the adverse effect of increasing silicon content on the extension of phase transformation time. Through sufficient phase transformation, austenite residue is avoided and abnormal hardness and brittleness continue to form during the subsequent roller slow cooling process. On the other hand, the temperature of the troostite phase region is relatively high, which can extend the treatment time of the wire rod at high temperature and promote the rapid isothermal toughening and stress relief of the troostite structure formed by phase transformation to improve the plasticity and high strength matching of the wire rod. The temperature of the wire rod is relatively high after online molten salt rapid cooling isothermal treatment. Roller slow cooling can reduce the cooling rate of the wire rod, avoid excessive cooling of the wire rod and cause stress increase, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod.

[0019] 3. Compared with the Stelmore air-cooled line, which is difficult to control the precipitation and growth of carbides due to continuous cooling and unstable controlled cooling, on the one hand, the wire rod can be cooled quickly during the online molten salt rapid cooling isothermal process, which increases the precipitation driving force of Nb and V carbides. By adjusting the temperature of the troostite phase region to adapt to the precipitation temperature range of the carbide strengthening phase, the time the wire rod is in the precipitation temperature range can be extended through isothermal control rather than continuous cooling, which promotes the full dispersion and precipitation of Nb and V in the isothermal process, and effectively improves the strength of the hot-rolled wire rod. On the other hand, the temperature difference from the surface to the core of the wire rod is smaller, and Mo is used to inhibit the coarsening of carbides. The troostite structure of the wire rod is controlled to undergo a certain degree of isothermal tempering to relieve stress, which can avoid the coarsening of carbides and the loss of strength and plasticity, thereby giving full play to the strengthening effect of microalloying.

[0020] Selecting appropriate heating furnace soaking temperature and furnace time before rolling can promote composition homogenization and reduce segregation effects, while avoiding excessively high heating temperature that leads to large amounts of Nb solid solution affecting subsequent precipitation strengthening, and avoiding decarburization and burning caused by excessively long furnace time. In the preferred technical solution, before rolling, the heating furnace soaking temperature is controlled to be 1200-1250°C, and the furnace time is 150-200min.

[0021] Selecting a suitable initial rolling temperature during the rolling can prevent excessive growth of austenite grains, ensure good plasticity of the steel billet and facilitate rolling, and provide more nucleation sites for strain-induced precipitation of Nb with a suitable initial rolling reduction. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1095~1120°C, and the initial rolling reduction is 15%~20%.

[0022] During the rolling, a suitable final rolling temperature and final rolling reduction are selected to retain more deformation energy storage, promote dynamic recrystallization during the final rolling process, refine the grains, and provide favorable conditions for the precipitation of Nb and microstructure refinement during the subsequent cooling process. In the preferred technical solution, during the rolling, the final rolling temperature is controlled to be 980~1030℃, and the final rolling reduction is 25%~30%.

[0023] In the preferred technical solution, the online molten salt rapid cooling isothermal treatment is divided into a front-stage treatment and a back-stage treatment. The molten salt temperature of the front-stage treatment is 550-585°C, the treatment time is 50-100s, the molten salt circulation volume of the front-stage treatment is greater than the molten salt circulation volume of the back-stage treatment, and the molten salt temperature of the front-stage treatment is in the troostite phase region. The lower the molten salt temperature, the more favorable it is for the refinement of the troostite lamellae and the rapid dispersion and precipitation of Nb and V carbides, which can improve the matrix strength. However, if the molten salt temperature is too low, there is a risk of precipitating bainite and increasing the difficulty of subsequent softening and temperature control. On the contrary, the higher the molten salt temperature, the lower the demand for the molten salt circulation volume and the lower the production energy consumption, but the molten salt temperature is too low. If the salt temperature is too high, it will be disadvantageous for the rapid nucleation of troostite and the dispersion and precipitation of carbide strengthening phase. The longer the treatment time of the front stage treatment is, the more conducive it is to the full transformation of austenite structure to troostite with fine lamellar spacing. However, if the treatment time is too long, the production energy consumption will increase, which is disadvantageous for cost control. On the contrary, the shorter the treatment time of the front stage treatment is, the lower the production energy consumption will be. However, if the treatment time is too short, it will not be conducive to the formation of a large degree of supercooling to promote the rapid nucleation of troostite. Therefore, the molten salt temperature and treatment time of the front stage treatment can be controlled to promote the formation of a structure dominated by troostite with fine lamellar spacing in the wire rod, promote the dispersion and precipitation of carbide strengthening phase, control the production energy consumption, and make organizational preparations for the subsequent molten salt treatment.

[0024] Since the temperature difference of the wire rod from the spinning temperature to the troostite phase temperature is large, selecting a higher molten salt circulation rate can reduce the molten salt temperature rise, which is convenient for the continuous production phase change control of the wire rod. In the preferred technical scheme, the molten salt circulation rate of the front-stage treatment is 550~750t / h, and the molten salt temperature rise is ≤9℃.

[0025] In the preferred technical solution, the molten salt temperature of the latter treatment is 565-585°C, and the treatment time is 100-150s. The molten salt temperature of the latter treatment is in the troostite phase region, but the molten salt circulation volume is smaller. The lower the molten salt temperature of the latter treatment, the more conducive it is to the dispersion and precipitation of the carbide strengthening phase and the improvement of the matrix strength. However, if the molten salt temperature is too low, it is difficult to provide more thermal power for the isothermal tempering of the structure, and the toughening and stress relief effect is reduced, which will lead to insufficient matrix plasticity. On the contrary, the higher the molten salt temperature, the more conducive it is to the isothermal tempering of the wire rod and the plasticity of the wire rod can be improved. However, if the molten salt temperature is too high, it is not conducive to controlling the precipitation of carbides. As the treatment time increases, The longer the treatment time is, the coarsening of carbides is not good for the strength and plasticity of wire rod; the longer the treatment time of the later stage treatment, the better the toughening and stress relief effect, and the plasticity of the wire rod is improved, but if the treatment time is too long, there is a risk of excessive loss of wire rod strength, carbide precipitation and coarsening, and loss of strength and plasticity. On the contrary, the shorter the treatment time is, the lower the production energy consumption and the higher the matrix strength is, but if the treatment time is too short, there will be large residual stress in the wire rod, and the carbide strengthening phase cannot be fully precipitated, which will cause loss of strength and plasticity. Therefore, the molten salt temperature and treatment time of the later stage treatment can be further controlled, the tempering state of the organization can be adjusted, the precipitation and growth of carbides can be controlled, and the strength and plasticity matching of the wire rod can be improved.

[0026] Since the temperature difference of the molten salt in the front-end treatment and the back-end treatment is small, as the phase change releases heat energy, an appropriate molten salt circulation rate can be selected to control the molten salt temperature rise and production energy consumption. In the preferred technical scheme, the molten salt circulation rate of the back-end treatment is 350~450t / h, and the molten salt temperature rise is ≤3℃.

[0027] Due to the sufficient phase transformation of the austenite structure, there is no problem of residual austenite transforming into a hard and brittle phase. However, in order to avoid the shrinkage effect of physical stress, appropriate roller slow cooling control is selected to prevent the increase of stress in the wire rod due to excessive cooling rate during the cooling process, and promote further toughening of the wire rod structure and improve the softening effect of the wire rod. In the preferred technical solution, the roller slow cooling controls the wire rod to slowly cool to below 280°C at a cooling rate of 0.2~0.45°C / s.

[0028] In the preferred technical solution, the roller slow cooling adopts a closed insulation cover, and the hot air in the online molten salt rapid cooling isothermal treatment process is input into the insulation cover. The wire rod is transported by the roller into the insulation cover to control the slow cooling, which can recycle the heat energy in the molten salt treatment and reduce production energy consumption.

[0029] A 2500MPa hot-rolled wire rod for bridge cables, wherein the hot-rolled wire rod is manufactured by any one of the above-mentioned methods for manufacturing the 2500MPa hot-rolled wire rod for bridge cables.

[0030] The above-mentioned hot-rolled wire rod adopts the C-Si-Mn-Cr-Nb-V-Mo composition design combined with the online molten salt rapid cooling isothermal technology. The microstructure is mainly tempered troostite and fused troostite, which can effectively control the brittle structure and reduce the ferrite content in the microstructure. The troostite has a fine interlamellar spacing, large dislocation movement hindrance, and higher strength and hardness than pearlite. After isothermal tempering and toughening treatment, some troostite lamellae are melted, the internal stress is reduced, and it is transformed into tempered troostite and fused troostite. The strength and hardness are reduced, but the toughness and plasticity are significantly improved. It can better undergo plastic deformation when subjected to stress and has good comprehensive mechanical properties. Strengthening precipitation phases such as Nb and V carbides are dispersed, which can increase the content of strengthening elements without weakening the toughness of the material, thereby achieving a match between higher strength and good plasticity of the wire rod.

[0031] The larger the interlamellar spacing of the tempered troostite is, the higher the matrix strength is. In a preferred technical solution, the interlamellar spacing of the tempered troostite is 65-95 nm.

[0032] The higher the proportion of the tempered troostite and the fused troostite in the microstructure, the better the matrix strength-plasticity and drawing hardening ability. In the preferred technical solution, the volume percentage of the tempered troostite and the fused troostite is ≥95%.

[0033] In the preferred technical solution, the network carbide level of the hot-rolled wire rod is level 0, which can avoid the adverse effects of the network carbide on the uniformity and plasticity of the wire rod structure and reduce the risk of wire breakage during drawing of the hot-rolled wire rod.

[0034] The brittle structure of the hot-rolled wire rod is effectively controlled, the phase transformation and carbide precipitation are more evenly controlled, and the fluctuation of mechanical properties is reduced, which is beneficial to reducing the risk of drawing and torsional cracking. In the preferred technical solution, the same-circle difference of the mechanical properties of the hot-rolled wire rod is ≤47MPa.

[0035] In the preferred technical solution, the diameter of the hot-rolled wire rod is 9.0~15.0mm, the tensile strength is 1692~1735MPa, and the cross-sectional shrinkage rate is 25%~30%. The hot-rolled wire rod has a high tensile strength and can be quickly drawn to the corresponding strength grade, reducing plastic loss in the process. At the same time, the good cross-sectional shrinkage rate can reduce deformation resistance and the risk of wire breakage during drawing, thereby promoting the stable production of high-strength bridge cables.

[0036] Compared with the prior art, the beneficial effects of the present invention are at least: (1) In view of the current situation that the microstructure of wire rod is difficult to effectively control due to the limitation of the cooling capacity of the existing Stelmor air-cooling line, the wire rod has high cost, insufficient strength and plasticity, and poor uniformity of structure, the present invention controls the wire rod to quickly enter the troostite phase region from the high-temperature austenite state by combining the online molten salt rapid cooling isothermal technology through the design of Nb-V-Mo chemical composition, so as to form a structure dominated by troostite with fine lamellar spacing, effectively avoid the generation of brittle structure, and improve the strengthening effect of carbon element, so as to control the precipitation and growth of carbides in the high-temperature isothermal range of the wire rod, control the dispersion precipitation of Nb and V in the isothermal process, effectively improve the strengthening effect of the precipitated phase without reducing the plasticity, and perform high-temperature isothermal tempering on the structure, toughening and stress relief treatment, and then roller slow cooling to promote further toughening of the wire rod structure, improve the softening effect of the wire rod, realize microstructure regulation, and improve the strength and plasticity matching of the wire rod, which has good industrial adaptability.

[0037] (2) In view of the current situation that the strength can be rapidly improved by increasing carbon and other alloying elements in the prior art, but the brittleness of the wire rod also increases rapidly, the hot-rolled wire rod of the present invention adopts a high-carbon component system, which can take into account the material cost. The microstructure type includes a mixed structure composed of tempered troostite, ferrite and fused troostite. Isothermal tempering is used to relieve stress, which can increase the content of strengthening elements without weakening the toughness of the material, achieving a product tensile strength of 1692~1735MPa and a cross-sectional shrinkage rate of 25~30%. It is used in the manufacture of 2500MPa ultra-high strength bridge cables and other application fields, which is beneficial to reduce the risk of wire breakage during drawing and torsion, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which: Figure 1 is a metallographic structure diagram of Example 1 of the present invention; Figure 2 is the metallographic structure diagram of Example 2 of the present invention; Figure 3 It is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0039] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention, so as to propose the best way to implement the present invention, and are intended to be used to explain the present invention and are sufficient to enable those skilled in the art to implement the present invention, but cannot be understood as any limitation on the scope of the present invention, which is limited only by the appended claims; the organization and performance testing of the hot-rolled wire rods obtained in the following embodiments and comparative examples include: the tensile test is carried out using "GB-T 228.1-2021 Metallic Material Tensile Test Part 1: Room Temperature Test Method" to obtain the tensile strength and cross-sectional reduction rate; the organization test is carried out according to the metal microstructure detection method of GB / T13298 standard; the mechanical property same circle difference test method: take 2 circles of wire rod 5m away from the end of the coil, take the overlap area position as the base point, divide each circle of wire rod into 8 sections on average, take 1 tensile specimen on each section, and the strength extreme difference of the tensile specimen after the tensile test is the mechanical property same circle difference. Embodiment 1:

[0040] A preferred embodiment of the manufacturing method of the 2500MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.96%, Si: 1.15%, Mn: 0.9%, Cr: 0.41%, Nb: 0.03%, V: 0.032%, Mo: 0.35%, P: 0.015%, S: 0.015%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 14mm through a rolling line to promote the strain-induced precipitation of Nb and dynamic recrystallization and grain refinement during the final rolling process. Specifically, the heating furnace is heated to 1235°C, the furnace time is 175min, the initial rolling temperature is 1110°C, the initial rolling reduction is 17%, the final rolling temperature is 1015°C, and the final rolling reduction is 27%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large undercooling degree and the promotion of rapid precipitation of sorbite structure and carbide strengthening phase. Specifically, the wire-spinning temperature is controlled to be 940°C.

[0041] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 36°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and the production is reduced. Energy consumption, control the wire rod to enter the high-temperature isothermal zone, control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal tempering, toughening and stress relief treatment on the sorbite structure formed by quenching to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-end treatment is 559°C, the processing time is 66s, the molten salt circulation volume is 720t / h, and the molten salt temperature rise is ≤9°C; the molten salt temperature of the back-end treatment is 579°C, the processing time is 114s, the molten salt circulation volume is 420t / h, and the molten salt temperature rise is ≤3°C.

[0042] The roller slow cooling process adopts closing the insulation cover, inputting the hot air above the two salt bath tanks into the insulation cover, and conveying the wire rod passing through the second salt bath tank into the insulation cover by the conveying roller, so as to prevent the wire rod from increasing stress due to too fast cooling speed during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to be slowly cooled to 275°C at a cooling rate of 0.3°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and obtain the hot-rolled wire rod product after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0043] Comparative Example 1: A method for manufacturing a hot-rolled wire rod, the manufacturing method differs from Example 1 in that: the heating furnace soaking temperature is controlled to be 1185°C, the furnace time is 220min, the initial rolling temperature is 1040°C, the final rolling temperature is 930°C, the spinning temperature is controlled to be 850°C, the wire rod is subjected to a front-end treatment during the online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at a cooling rate of 30°C / s, and the hot-rolled wire rod is obtained after it is off the line. Embodiment 2:

[0044] A preferred embodiment of the manufacturing method of the 2500MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.96%, Si: 1.2%, Mn: 0.85%, Cr: 0.53%, Nb: 0.036%, V: 0.03%, Mo: 0.29%, P: 0.01%, S: 0.013%, and the rest are Fe and unavoidable impurities; the manufacturing method thereof is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 15mm through a rolling line to promote the strain-induced precipitation of Nb and dynamic recrystallization and grain refinement during the final rolling process. Specifically, the heating furnace is heated to 1250°C, the furnace time is 150min, the initial rolling temperature is 1120°C, the initial rolling reduction is 15%, the final rolling temperature is 1030°C, and the final rolling reduction is 25%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large undercooling degree and the promotion of rapid precipitation of sorbite structure and carbide strengthening phase. Specifically, the wire-spinning temperature is controlled to be 970°C.

[0045] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 37°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation amount of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and the production is reduced. Energy consumption, control the wire rod to enter the high temperature isothermal zone, control the precipitation and growth of carbides, and at the same time perform high temperature isothermal tempering, toughening and stress relief treatment on the sorbite structure formed by quenching to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front stage treatment is 571°C, the processing time is 83s, the molten salt circulation volume is 750t / h, and the molten salt temperature rise is ≤9°C; the molten salt temperature of the back stage treatment is 570°C, the processing time is 132s, the molten salt circulation volume is 450t / h, and the molten salt temperature rise is ≤3°C.

[0046] The roller slow cooling process adopts closing the insulation cover, inputting the hot air above the two salt bath tanks into the insulation cover, and conveying the wire rod passing through the second salt bath tank into the insulation cover by the conveying roller, so as to prevent the wire rod from increasing stress due to too fast cooling speed during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to be slowly cooled to 287°C at a cooling rate of 0.25°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and obtain the hot-rolled wire rod product after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0047] Comparative Example 2: A method for manufacturing a hot-rolled wire rod, the manufacturing method differs from Example 2 in that: during the online molten salt rapid cooling isothermal treatment, the wire rod undergoes a front-end treatment to cool the wire rod at a cooling rate of 35°C / s, the molten salt temperature of the front-end treatment is 595°C, the treatment time is 35s, and the hot-rolled wire rod is obtained after going offline.

[0048] Comparative Example 3: A method for manufacturing a hot-rolled wire rod, the manufacturing method differs from Example 2 in that: during the online molten salt rapid cooling isothermal treatment, the wire rod undergoes a front-end treatment to cool the wire rod at a cooling rate of 42°C / s, the molten salt temperature of the front-end treatment is 537°C, the treatment time is 150s, and the hot-rolled wire rod is obtained after going offline. Embodiment 3:

[0049] A preferred embodiment of the manufacturing method of the 2500MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.95%, Si: 1.09%, Mn: 0.7%, Cr: 0.33%, Nb: 0.033%, V: 0.04%, Mo: 0.25%, P: 0.012%, S: 0.014%, and the rest are Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 9mm through a rolling line to promote the strain-induced precipitation of Nb and dynamic recrystallization and grain refinement during the final rolling process. Specifically, the heating furnace soaking temperature is controlled to be 1200°C, the furnace time is 200min, the initial rolling temperature is 1095°C, the initial rolling reduction is 20%, the final rolling temperature is 980°C, and the final rolling reduction is 30%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large undercooling degree and the promotion of rapid precipitation of sorbite structure and carbide strengthening phase. Specifically, the wire-spinning temperature is controlled to be 900°C.

[0050] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 33°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and the production energy is reduced. consumption, control the wire rod to enter the high temperature isothermal zone, control the precipitation and growth of carbides, and at the same time carry out high temperature isothermal tempering, toughening and stress relief treatment on the sorbite structure formed by quenching to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front treatment is 585℃, the processing time is 100s, the molten salt circulation volume is 550t / h, and the molten salt temperature rise is ≤9℃; the molten salt temperature of the back treatment is 565℃, the processing time is 149s, the molten salt circulation volume is 350t / h, and the molten salt temperature rise is ≤3℃.

[0051] The roller slow cooling process adopts closing the insulation cover, inputting the hot air above the two salt bath tanks into the insulation cover, and conveying the wire rod passing through the second salt bath tank into the insulation cover by the conveying roller, so as to prevent the wire rod from increasing stress due to too fast cooling speed during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to slowly cool to 267°C at a cooling rate of 0.45°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and obtain the hot-rolled wire rod product after packaging and storage. Its metallographic structure diagram is as follows Figure 3 shown.

[0052] Comparative Example 4: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 3 in that the molten salt temperature of the post-treatment is 595° C., the treatment time is 225 s, and the hot-rolled wire rod is obtained after going offline.

[0053] Comparative Example 5: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 3 in that the molten salt temperature of the post-treatment is 525° C., the treatment time is 40 s, and the hot-rolled wire rod is obtained after the production line is closed. Embodiment 4:

[0054] A preferred embodiment of the manufacturing method of the 2500MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.98%, Si: 1%, Mn: 0.77%, Cr: 0.49%, Nb: 0.038%, V: 0.03%, Mo: 0.32%, P: 0.012%, S: 0.013%, and the rest are Fe and unavoidable impurities; the manufacturing method thereof is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 11mm through a rolling line to promote the strain-induced precipitation of Nb and dynamic recrystallization and grain refinement during the final rolling process. Specifically, the heating furnace is heated to 1220°C, the furnace time is 190min, the initial rolling temperature is 1105°C, the initial rolling reduction is 19%, the final rolling temperature is 995°C, and the final rolling reduction is 28%; the wire-spinning process is used to make the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large undercooling degree and the promotion of rapid precipitation of sorbite structure and carbide strengthening phase. Specifically, the wire-spinning temperature is controlled to be 930°C.

[0055] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 36°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and the production is reduced. Energy consumption, control the wire rod to enter the high temperature isothermal zone, control the precipitation and growth of carbides, and at the same time perform high temperature isothermal tempering, toughening and stress relief treatment on the sorbite structure formed by quenching to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-end treatment is 550℃, the treatment time is 50s, the molten salt circulation volume is 630t / h, and the molten salt temperature rise is ≤9℃; the molten salt temperature of the back-end treatment is 585℃, the treatment time is 100s, the molten salt circulation volume is 385t / h, and the molten salt temperature rise is ≤3℃.

[0056] The roller slow cooling process adopts closing the insulation cover, inputting the hot air above the two salt bath tanks into the insulation cover, and conveying the wire rod passing through the second salt bath tank by the conveying roller into the insulation cover, so as to prevent the wire rod from increasing stress due to too fast cooling speed during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to be slowly cooled to 279°C at a cooling rate of 0.2°C / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and obtain the hot-rolled wire rod finished product after packaging and storage.

[0057] Comparative Example 6: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which differs from that of Example 4 in that: the roller slow cooling process adopts an open insulation cover, and the wire rod passing through the second salt bath tank is transported by a conveyor roller into the insulation cover, and the wire rod is controlled to be cooled to 299°C at a cooling rate of 1.3°C / s. After the hot-rolled wire rod is off the line, the tensile strength of the hot-rolled wire rod is 1741MPa, the cross-sectional shrinkage rate is 23%, and the mechanical property same-circle difference is 55MPa.

[0058] The hot rolled wire rods obtained in the above examples 1 to 4 and comparative examples 1 to 5 were subjected to microstructure and performance testing, and the comparative results obtained are shown in Table 1 below: Table 1. Comparison of microstructure and performance of hot rolled wire rods with different compositions and manufacturing methods

[0059] It can be seen from the results of Examples 1 to 4 that the present invention adopts Nb-V-Mo chemical composition design combined with online molten salt rapid cooling isothermal technology to achieve a volume percentage of tempered troostite and fused troostite of ≥95%, a product tensile strength of 1692~1735MPa, a cross-sectional shrinkage rate of 25%~30%, and an effective control of the brittle structure. It is used in the manufacture of 2500MPa-level ultra-high strength bridge cables and other application fields, which is beneficial to reduce the risk of wire breakage due to drawing and torsion.

[0060] From the comparison results of Example 1 and Comparative Example 1, it can be seen that the appropriate spinning temperature of the wire rod can avoid the formation of network carbides during the spinning stage due to too low spinning temperature, avoid the control of rolling temperature being restricted by too low spinning temperature, increase the supercooling degree with a higher quenching temperature, and provide favorable conditions for the rapid nucleation of troostite and the dispersion precipitation of Nb and V carbides.

[0061] From the comparison results of Example 2 and Comparative Example 2, it can be seen that the molten salt circulation amount of the front-stage treatment is greater than the molten salt circulation amount of the rear-stage treatment. The higher the molten salt temperature of the front-stage treatment and the shorter the treatment time, the lower the production energy consumption. However, the molten salt temperature is too high and the treatment time is too short, which is not conducive to forming a large degree of supercooling to promote the rapid nucleation of troostite, increases the difficulty of softening the wire rod, and increases the fluctuation of the mechanical properties of the wire rod.

[0062] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the lower the molten salt temperature of the front-stage treatment is, the more beneficial it is to the refinement of the troostite lamellae and the rapid dispersion and precipitation of Nb and V carbides, which can improve the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, the difficulty of subsequent softening will be increased, the production energy consumption will increase, and it will be unfavorable to control the production cost.

[0063] From the comparison results of Example 3 and Comparative Example 4, it can be seen that the higher the molten salt temperature and the longer the treatment time of the later stage treatment are, the more conducive to the isothermal tempering of the wire rod and the better the plasticity of the wire rod. However, if the molten salt temperature is too high, it is not conducive to controlling the precipitation of carbides. As the treatment time increases, the coarsening of carbides is detrimental to the strength and plasticity of the wire rod.

[0064] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the lower the molten salt temperature of the later stage treatment, the more conducive it is to the dispersion and precipitation of the carbide strengthening phase and the improvement of the matrix strength. However, if the molten salt temperature is too low and the treatment time is too short, it is difficult to provide more thermal power for the isothermal tempering of the structure, the toughening and stress relief effect is reduced, and large residual stress is left in the wire rod. The carbide strengthening phase cannot be fully precipitated, which will lead to insufficient strength and plasticity of the matrix.

[0065] From the comparison results of Example 4 and Comparative Example 6, it can be seen that the selection of appropriate roller slow cooling control can prevent the increase of stress in the wire rod due to excessively fast cooling rate during the cooling process, improve the softening effect of the wire rod, and recycle the heat energy in the molten salt treatment to reduce production energy consumption.

[0066] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. All equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing a 2500MPa grade hot-rolled wire rod for bridge cables, characterized in that: The manufacturing method thereof comprises: The hot-rolled wire rod is rolled to produce a wire rod according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%~0.98%, Si: 1.00%~1.20%, Mn: 0.70%~0.90%, Cr: 0.33%~0.53%, Nb: 0.03%~0.038%, V: 0.03%~0.04%, Mo: 0.25%~0.35%, P≤0.015%, S≤0.015 %, and the rest are Fe and unavoidable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥900°C, it is subjected to an online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥33°C / s, enters the sorbite phase region from the austenite state, forms a sorbite-based structure, and controls the precipitation and growth of carbides, and isothermally toughened to relieve stress, and finally is slowly cooled through a roller to produce a hot-rolled wire rod whose microstructure includes a mixed structure consisting of tempered sorbite, ferrite and fused sorbite.

2. The method for manufacturing the 2500MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: Before the rolling, the soaking temperature of the heating furnace is controlled to be 1200-1250° C., and the time in the furnace is 150-200 minutes.

3. The method for manufacturing the 2500MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: During the rolling, the initial rolling temperature is controlled to be 1095-1120° C., the initial rolling reduction is 15%-20%, the final rolling temperature is controlled to be 980-1030° C., and the final rolling reduction is controlled to be 25%-30%.

4. The method for manufacturing the 2500MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: The online molten salt rapid cooling isothermal treatment is divided into a front-stage treatment and a back-stage treatment. The molten salt temperature of the front-stage treatment is 550-585°C, and the treatment time is 50-100s; the molten salt temperature of the back-stage treatment is 565-585°C, and the treatment time is 100-150s; the molten salt circulation amount of the front-stage treatment is greater than the molten salt circulation amount of the back-stage treatment.

5. The method for manufacturing the 2500MPa grade hot rolled wire rod for bridge cables according to claim 4, characterized in that: The molten salt circulation volume of the front-end treatment is 550-750t / h, and the molten salt temperature rise is ≤9°C; the molten salt circulation volume of the back-end treatment is 350-450t / h, and the molten salt temperature rise is ≤3°C.

6. The method for manufacturing the 2500MPa grade hot rolled wire rod for bridge cables according to claim 4, characterized in that: The roller slow cooling controls the wire rod to be slowly cooled to below 280° C. at a cooling rate of 0.2-0.45° C. / s.

7. A 2500MPa grade hot-rolled wire rod for bridge cables, characterized in that: The hot-rolled wire rod is manufactured by the method for manufacturing 2500MPa-grade hot-rolled wire rod for bridge cables according to any one of claims 1 to 6.

8. The 2500MPa grade hot rolled wire rod for bridge cables according to claim 7, characterized in that: The interlamellar spacing of the tempered troostite is 65-95 nm, and the volume percentage of the tempered troostite and the fused troostite is ≥95%.

9. The 2500MPa grade hot rolled wire rod for bridge cables according to claim 7, characterized in that: The network carbide grade of the hot-rolled wire rod is grade 0, and the mechanical property difference is ≤47MPa.

10. The 2500MPa grade hot rolled wire rod for bridge cables according to claim 7, characterized in that: The hot-rolled wire rod has a diameter of 9.0-15.0 mm, a tensile strength of 1692-1735 MPa, and a cross-sectional shrinkage of 25%-30%.

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