A high-strength duplex hot-rolled wire rod for 2400 MPa class bridge cables and its manufacturing method

Abstract

The present invention relates to a high-strength dual-phase hot-rolled wire rod for 2400 MPa class bridge cables and a manufacturing method thereof. After hot-rolling the wire rod by using a high-carbon chemical composition of Mo-Al-V and then coiling it into a wire rod, it undergoes an on-line molten salt end quenching and toughening treatment. The wire rod first passes through the front-section molten salt and is cooled at a cooling rate of ≥35 °C / s, causing partial austenite structure to transform into quenched martensite. Then it passes through the rear-section molten salt and is heated to the temperature in the sorbite phase region, controlling the untransformed austenite to transform into sorbite and isothermally tempered. Finally, it is slowly cooled through a roller table to produce a hot-rolled wire rod with a dual-phase structure composed of tempered martensite and tempered sorbite in its microstructure. It can achieve the control of network carbide and other brittle phase structures, improve the strength and microstructure uniformity of the wire rod, balance the material cost and good plasticity, reaching a tensile strength of 1665~1705 MPa and an area reduction of 26%~31%. The production efficiency is relatively high, which is beneficial to the stable production of 2400 MPa class bridge cables.

Description

Technical Field

[0001] The present invention belongs to the technical field of hot-rolled wire rods, and particularly relates to a high-strength duplex hot-rolled wire rod for 2400 MPa-class bridge cables and a manufacturing method thereof. Background Art

[0002] Cold-drawn pearlitic steel has good drawing performance and work hardening ability, and has always been the mainstream steel type in the wire product industry. The strength grade of cold-drawn pearlitic steel wire for bridge cables depends on the chemical composition, microstructure of the base wire rod and its matching drawing process. The higher the content of carbon element and alloy elements in the base wire rod, the higher the strength grade of cold-drawn pearlitic steel wire that can be developed. However, the existing cold-drawn pearlitic steel wire cannot meet the development and application of 2400 MPa-class ultra-high-strength bridge cables. The main reasons are as follows:

[0003] First, in order to improve the strength of the wire rod, the content of alloy elements in the wire rod is relatively high. For example, a high-strength and high-toughness bridge cable steel and its preparation method disclosed in Patent CN110144521B use a low-carbon steel wire rod with a high alloy content and containing Co to produce a bridge cable steel with a tensile strength ≥ 2400 Mpa. However, on the one hand, the alloy content is relatively high, and the material cost is relatively high. On the other hand, the existing wire rods generally use the Stelmor air-cooling line controlled cooling after wire laying. Due to the limitation of the microstructure control ability of the existing hot-rolled wire rod production line, after the content of alloy elements in the base wire rod is increased, affected by the segregation of alloy elements and the uncontrollability of air-cooling speed, it is easy to make the control of other hard and brittle phase microstructures poor, and the risk of drawing wire breakage and subsequent cable-making wire breakage is relatively high.

[0004] II. To balance material costs and drawing ability, high-carbon pearlitic steel is mostly used for the wire rods for high-strength bridge cables at present. For example, a wire rod for high-strength bridge cables and its production method disclosed in CN118880169A adopt 87SiMn combined with low-temperature rolling and wire laying, the Stelmor air-cooling line and the heat preservation corridor process to increase the sorbite content of the wire rod and are used for bridge cables with a strength level of 2200-2300 MPa. However, on the one hand, there is a disadvantage that the original strength of the cold-drawn pearlitic steel is insufficient, and the plastic loss and wire breakage risk will increase during the drawing and strength improvement process, restricting the development and application of 2400 MPa-class bridge cables. If the carbon element and alloy element contents in the wire rod are further increased, affected by carbon element segregation and the maximum cooling capacity of the Stelmor air-cooling line, carbides will easily form in a network morphology distribution at the grain boundaries of the wire rod. The network carbides will cut the matrix, resulting in a significant reduction in the strength, especially the toughness of the material, and cracks are likely to be initiated at the carbide network. In order to minimize the network carbide level, after increasing the air-cooling intensity, the uncontrollability of the air-cooling line and the temperature difference between the surface and the core of the wire rod will further increase, making it more difficult to control the brittle phase structure, affecting the tissue uniformity, and further triggering the wire breakage risk during drawing; on the other hand, affected by the cooling control ability of the air-cooling line and the high content of alloy elements, the phase transformation incubation time of the wire rod is short. After the wire rod undergoes sorbite phase transformation, it is already in a low-temperature state with high tissue stress and insufficient plasticity of the wire rod. Martensite brittle phases may also continue to form during subsequent cooling due to retained austenite, further increasing the wire breakage risk during drawing, while long-term heat preservation treatment will affect the on-line time, production efficiency and production energy consumption.

[0005] III. To increase the strength of the wire rod, micro-alloying elements such as V are selected in the prior art. However, affected by the uncontrollability and cooling ability of the air-cooling line, on the one hand, the time for the wire rod to pass through the precipitation phase is short and the precipitation driving force is weak, affecting the full precipitation of the strengthening phase. On the other hand, affected by the temperature difference between the surface and the core of the wire rod, both coarser and finer precipitation phases will exist in the wire rod, affecting the tissue uniformity. At the same time, the coarsening of the precipitation phase will reduce the plasticity of the wire rod, affecting the exertion of the strengthening effect and the material cost. Summary of the Invention

[0006] The present invention aims to solve at least one of the above technical problems to some extent. The present invention provides a high-strength duplex hot-rolled wire rod for 2400 MPa-class bridge cables and its manufacturing method, which can achieve the control of network carbides and other brittle phase structures, improve the strength and tissue uniformity of the wire rod, balance the material cost and good plasticity, have high production efficiency, and are conducive to the stable production of 2400 MPa-class bridge cables.

[0007] The technical solution adopted by the present invention to solve its technical problems is:

[0008] A manufacturing method of a high-strength duplex hot-rolled wire rod for 2400 MPa-class bridge cables, the manufacturing method includes:

[0009] The wire rod is rolled according to the chemical composition of the hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.94% - 0.97%, Si: 0.80% - 0.95%, Mn: 0.75% - 0.95%, Cr: 0.35% - 0.45%, V: 0.050% - 0.075%, Mo: 0.30% - 0.50%, Al: 0.2% - 0.4%, P ≤ 0.015%, S ≤ 0.015%, and the rest are Fe and unavoidable impurities. After the wire rod is spun at a laying head temperature of ≥ 880 °C to form a wire rod, it undergoes online molten salt end quenching and toughening treatment. The wire rod first passes through the front-section molten salt and cools at a cooling rate of ≥ 35 °C / s, causing partial austenite structure to transform into quenched martensite. Then it passes through the rear-section molten salt and is heated to the temperature of the sorbite phase region, controlling the untransformed austenite to transform into sorbite and isothermal tempering. Finally, it is slowly cooled through the roller table to produce a hot-rolled wire rod with a duplex structure composed of tempered martensite and tempered sorbite in the microstructure.

[0010] The design basis for the chemical composition and mass percentage of the above hot-rolled wire rod includes:

[0011] (1) Carbon: The C element is an effective carbide strengthening element and austenite forming element, and its price is relatively lower. With the increase of carbon content, the stability of austenite can be improved, the martensite transformation temperature can be reduced, which promotes short-time quenching by cooling below the temperature of the sorbite phase region during the front-section molten salt treatment, promotes the transformation of partial austenite structure into quenched martensite, and is beneficial to the formation of pearlite with finer lamellar spacing, namely sorbite structure, during the rear-section molten salt treatment, and increases the amount of cementite in sorbite and the material strength. However, too much carbon content will increase the carbon segregation tendency during the solidification of the steel billet, increase the decarburization sensitivity and the precipitation tendency of network carbide, affecting the plastic and toughness properties of the material. Therefore, in order to improve the strength of the hot-rolled wire rod base material, control the material cost, and at the same time reduce the difficulty of controlling the tissue uniformity and the difficulty of improving plasticity, the mass percentage of C is controlled at 0.94% - 0.97%.

[0012] (2) Silicon: The Si element is the main deoxidizing element in steel, which can inhibit the grain coarsening during the front-section molten salt treatment, promote the nucleation of quenched martensite, and improve the matrix strength. During the sorbite phase transformation process in the rear-section molten salt treatment, it can inhibit the growth and aggregation of cementite, making the cementite lamellae in sorbite finer and more uniform. However, too high a silicon content will increase the surface decarburization tendency of the steel during high-temperature heating, prolong the time required for phase transformation incubation, and reduce the toughness of the steel. Therefore, in order to refine grains, facilitate short-time martensite phase transformation and short-time tempering control of the duplex structure, the mass percentage of Si is controlled at 0.80% - 0.95%.

[0013] (3) Manganese: As an austenite-forming element, Mn can increase the hardenability of the wire rod, inhibit the formation of ferrite, shift the bainite transformation towards lower temperatures, promote the pearlite transformation, and increase the strength of pearlite. This is beneficial for improving the tensile strength of the wire rod and enhancing the comprehensive mechanical properties of the steel. It enables the bridge cable to withstand tensile forces while resisting impact and vibration loads. However, when the Mn content is too high, it will exacerbate alloy element segregation, reduce the activity of carbon, increase the difficulty of stress relief during isothermal tempering, increase the susceptibility to overheating of the steel, and reduce the toughness and plasticity of the steel. Therefore, to facilitate the control of the duplex structure of the hot-rolled wire rod, reduce the difficulty of controlling tissue uniformity and short-time tempering, the mass percentage of Mn is controlled at 0.75% - 0.95%.

[0014] (4) Chromium: The Cr element can improve the hardenability of the steel, lower the martensite transformation temperature, promote the formation of quenched martensite, increase the amount of quenched martensite transformation during the previous molten salt treatment, and enhance the stability of austenite. During the pearlite transformation, it can refine the pearlite colonies and cementite lamellae, increase the strength of pearlite, and reduce the strength loss during the subsequent hot-dip galvanizing process of wire rope production. However, when the Cr content is too high, it will exacerbate compositional segregation, increase the difficulty of controlling tissue uniformity, the increase in the number and uneven distribution of carbides will reduce the toughness of the steel, significantly increase the difficulty of isothermal stress relief of the wire rod, and affect the short-time tempering toughening effect, thereby affecting the drawing and torsion properties of the wire. Therefore, to facilitate the control of the duplex structure, plasticity, and rapid production, the mass percentage of Cr is controlled at 0.35% - 0.45%.

[0015] (5) Vanadium: As a microalloying element, the V element can effectively inhibit the coarsening of the wire rod grains during rolling. At the same time, it can precipitate dispersively during the online molten salt end quenching toughening process, providing a strong precipitation strengthening effect, thereby enhancing the strength level of the hot-rolled wire rod without reducing plasticity. However, the cost of the V element is relatively high, and excessive addition is not conducive to controlling the cost of the wire rod and there is a risk of coarsening. Considering the role and cost of the V element, the content of the V element is controlled at 0.050% - 0.075%.

[0016] (6) Molybdenum: The Mo element can improve the hardenability of the steel, promote the formation of martensite, and inhibit the bainite transformation. Mo can form carbides in combination with carbon, which can play a role in dispersion strengthening. At the same time, it can effectively inhibit the coarsening of cementite, Cr, and V precipitation phases and other strengthening phases, preventing the coarsening of precipitation phases during the online molten salt end quenching toughening process and reducing the plasticity of the wire rod. However, the cost of the Mo element is relatively high, and excessive addition is not conducive to controlling the cost of the wire rod. Considering the role and cost of the Mo element, the content of the Mo element is controlled at 0.30% - 0.50%.

[0017] (7) Aluminum: Al can prevent austenite grains from growing during heating and rolling, allowing the wire rod to obtain a fine grain structure. The fine austenite grains can form a uniform structure after cooling and transformation. At the same time, it can inhibit the coarsening of cementite and refine the sorbite structure, which is beneficial to improving the strength and toughness of steel. However, too high Al content will increase the risk of inclusions, deteriorate the cold working performance of the wire rod, and reduce the fatigue performance of the steel. Therefore, the Al content is increased and the mass percentage of Al is controlled to be 0.2%~0.4%.

[0018] (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%.

[0019] The above-mentioned hot-rolled wire rod adopts the high carbon chemical composition design of Mo-Al-V, combined with the optimized ratio of Si, Mn and Cr components, to regulate the hardenability of the wire rod, the temperature at which martensite begins to transform and the sorbite phase region, so as to provide favorable conditions for the wire rod to undergo a short-term martensite phase transformation below the sorbite phase region, to fully transform and temper in a short time in the sorbite phase region, and to facilitate the control of the precipitation and growth of carbides. On this basis, it avoids the formation of network carbides in the spinning stage due to too low a spinning temperature, improves the uniformity of austenite, and provides favorable conditions for the subsequent short-term uniform martensite phase transformation. After spinning, the wire rod is directly subjected to online molten salt end quenching and toughening treatment without air cooling:

[0020] 1. Compared with the Stelmor air-cooled line, which has the highest cooling capacity and uncontrollability, resulting in uncontrollable network carbides and hard-brittle phase structures, the wire rod can be quickly cooled by utilizing the high heat exchange capacity of the molten salt when it is treated with molten salt. On the one hand, it can promote the wire rod to quickly pass through the secondary cementite precipitation temperature range of 700~800℃ from the high-temperature austenite state, avoiding the formation of network carbides that are unfavorable to drawing and plasticity. On the other hand, the wire rod can be controlled by the front-stage molten salt treatment. At an ultra-high cooling rate, it quickly enters the troostite phase region from the high-temperature austenite state, forms a certain amount of highly undercooled residual austenite, undergoes a short-term martensite phase transformation, promotes the transformation of part of the austenite to quenched martensite, and improves the matrix strength. At the same time, when the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for heat exchange, and there is no temperature difference problem between the wind-receiving side and the winded side. Due to the high heat exchange, the temperature difference from the wire rod surface to the core is also smaller, making the transformation of the hard and brittle phase quenched martensite more uniform and controllable.

[0021] II. Compared with the limited minimum cooling capacity of the Stelmor air-cooled line and the continuous cooling of the wire rod, which result in insufficient phase transformation of the wire rod, high tissue stress, and poor control of carbide precipitation, after the wire rod undergoes the front-section molten salt treatment, it can enter the rear-section molten salt and be heated to the sorbite phase region for treatment. On the one hand, the wire rod can be transformed to the same temperature as the molten salt, extending the treatment time of the wire rod in the sorbite phase region, promoting the full transformation of the residual austenite that has not transformed after the front-section molten salt treatment into sorbite tissue, refining the sorbite tissue, increasing the matrix strength, so as to improve the adverse effect of the increase in the phase transformation incubation time caused by a relatively high Si content, and avoiding the continued transformation into abnormally hard and brittle tissue due to residual austenite during subsequent cooling. The temperature difference from the surface to the core of the wire rod is also smaller, which can make the sorbite phase transformation uniform and reduce the fluctuation of mechanical properties. On the other hand, the temperature in the sorbite phase region is relatively high, which can extend the isothermal treatment time of the wire rod at high temperature, provide more thermal power for tempering softening, control the short-time tempering of the formed quenched martensite and sorbite tissue to a certain extent, improve the plastic and tough properties of the quenched martensite, reduce the dislocation density of the quenched martensite, transform it into tempered martensite that is both strong and tough, reduce the stress of the sorbite tissue, and transform it into tempered sorbite tissue to achieve the regulation of the duplex tissue. At the same time, Mo is used to inhibit the coarsening of strengthening phases such as cementite, Cr and V precipitation phases, extend the time of the wire rod in the temperature range where strengthening phases such as Cr and V precipitation phases are dispersed and precipitated, promote the full and uniform precipitation of carbide strengthening phases, avoid the loss of plastic and tough properties due to the coarsening of carbides, and give full play to the strengthening effect. At the same time, the long-time heat preservation and corridor treatment can also reduce the on-line time and promote efficient production.

[0022] After the wire rod undergoes the rear-section molten salt treatment, it still maintains a relatively high temperature. Using roller table slow cooling can extend the high-temperature time of the wire rod, prevent the wire rod from suffering plastic loss due to stress contraction caused by too fast cooling, and can promote the further toughening of the wire rod tissue, improving the high-strength and plasticity matching of the wire rod.

[0023] Selecting an appropriate soaking temperature and soaking time in the heating furnace before rolling can improve the tissue uniformity and plasticity of the steel billet, facilitate rolling, reduce the influence of segregation, and at the same time avoid burn-out, decarburization or deterioration of the surface quality of the steel billet due to too long soaking time. In the preferred technical solution, before rolling, the soaking temperature of the heating furnace is controlled at 1180 - 1250 °C, and the soaking time is 150 - 220 min.

[0024] Selecting appropriate rolling temperature and reduction ratio during rolling, and cooperating with V element to increase the recrystallization temperature of the metal, inhibit grain growth, and promote dynamic recrystallization during the finishing rolling process, so as to refine and improve the metal tissue. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1060 - 1110 °C, the finishing rolling temperature is controlled at 980 - 1010 °C, and the finishing rolling reduction is 22% - 26%.

[0025] In a preferred technical solution, the molten salt temperature of the front-stage molten salt is 485-505°C, and the treatment time is 8-18 s. The molten salt temperature of the front-stage molten salt is below the sorbite phase region. The lower the molten salt temperature of the front-stage molten salt and the longer the treatment time, the more the amount of retained austenite with large supercooling degree increases, the short-time transformation of martensite accelerates, the amount of quenched martensite increases, and the matrix strength is higher. However, if the molten salt temperature is too low and the treatment time is too long, the amount of retained austenite is too little, which is not conducive to the subsequent sorbite phase transformation and short-time tempering, and will affect the plasticity of the matrix; on the contrary, the higher the molten salt temperature of the front-stage molten salt and the shorter the treatment time, the less favorable it is for the martensite phase transformation, leaving more retained austenite for the subsequent sorbite phase transformation, and the plasticity of the matrix increases. However, if the molten salt temperature is too high and the treatment time is too short, it will affect the martensite phase transformation and the matrix strength will be lost. Therefore, the molten salt temperature and treatment time of the front-stage molten salt can be controlled to promote the transformation of part of austenite into quenched martensite, making organizational preparations for the subsequent molten salt treatment.

[0026] Since the temperature difference between the wire rod dropping from the high-temperature austenite state to below the sorbite phase region temperature is relatively large, selecting a larger molten salt circulation rate can control the molten salt temperature rise and control the quenching during the continuous treatment process. At the same time, the larger the specification, the larger the molten salt circulation flow rate per unit time, which can further reduce the temperature difference from the surface to the core of the wire rod. In a preferred technical solution, the molten salt circulation rate of the front-stage molten salt is 460-650 t / h, and the molten salt temperature rise ≤ 5°C.

[0027] In the preferred technical solution, the molten salt temperature of the latter-stage molten salt is 565-585°C, and the treatment time is 78-128 s. The molten salt temperature of the latter-stage molten salt is in the austenite phase region of the coil bar. The lower the molten salt temperature, the greater the supercooling degree can be formed, which promotes the refinement of the lamellar spacing of sorbite, provides more driving force for the dispersion precipitation of vanadium-containing carbides, and improves the matrix strength. However, if the molten salt temperature is too low, it is difficult to provide more thermal power for isothermal tempering, which will lead to a decrease in the toughening effect and an extension of the treatment time, which is not conducive to controlling the plasticity of the coil bar and production efficiency. On the contrary, the higher the molten salt temperature of the latter-stage molten salt, the more thermal power can be provided for tempering, promoting short-time tempering of the duplex structure and improving the strength-ductility performance matching of the coil bar. However, if the molten salt temperature is too high, the lamellar spacing of sorbite becomes coarser, affecting carbide precipitation and resulting in a loss of the strength of the coil bar. With the extension of the treatment time, there is a risk of excessive loss of matrix strength and coarsening of carbide precipitation, resulting in a loss of strength-ductility performance; the longer the treatment time of the latter-stage molten salt, the more sufficient the carbide precipitation and the stronger the isothermal toughening effect, and the plasticity of the coil bar increases. However, if the treatment time is too long, the production energy consumption increases, and there is also a risk of coarsening of carbide precipitation. On the contrary, if the treatment time is too short, the production energy consumption can be reduced, promoting rapid production. However, if the treatment time is too short, the tempering effect of the duplex structure decreases, resulting in greater brittleness, and the insufficient precipitation of carbide will cause a loss of strength-ductility performance. Therefore, the molten salt temperature and treatment time of the latter-stage molten salt can be controlled to control the transformation of the untransformed high-temperature austenite of the coil bar into sorbite. At the same time, after isothermal tempering, the formed duplex structure is controlled to undergo a certain degree of short-time tempering, improving the strength-ductility matching of the coil bar and promoting rapid production.

[0028] The latter-stage molten salt can select an appropriate molten salt circulation rate to control the molten salt temperature rise, further reduce the temperature difference from the edge to the core of the coil bar, and improve the tissue uniformity. In the preferred technical solution, the molten salt circulation rate of the latter-stage molten salt is 400-600 t / h, and the molten salt temperature rise ≤ 3°C.

[0029] In the preferred technical solution, the roller table slow cooling controls the coil bar to slowly cool at a cooling rate of 0.1-0.4°C / s to below 280°C. The roller table slow cooling can select to close the heat preservation cover or input the hot air during the online molten salt end quenching and toughening treatment process into the heat preservation cover for recycling heat energy, controlling the coil bar to cool slowly step by step, preventing excessive internal stress in the coil bar caused by too fast cooling rate during the cooling process, and promoting further toughening of the coil bar tissue and improving the softening effect of the coil bar.

[0030] A high-strength duplex hot-rolled coil bar for 2400 MPa-class bridge cables, which is manufactured by the manufacturing method of the high-strength duplex hot-rolled coil bar for 2400 MPa-class bridge cables described in any one of the above.

[0031] The above hot-rolled wire rods are designed with Mo-Al-V chemical composition and combined with the on-line molten salt end quenching toughening technology. Different from general cold-drawn pearlitic steels, the microstructure of the hot-rolled wire rods includes a duplex structure composed of tempered martensite and tempered sorbite. Quenched martensite has higher strength, hardness, and lattice distortion compared to sorbite. After tempering treatment, the quenched martensite becomes tempered martensite, the degree of lattice distortion is reduced, high strength is still retained, internal stress is eliminated to a certain extent, and toughness and plasticity are effectively improved. The structure of sorbite is finer and has a smaller lamellar spacing than pearlite, and its strength is better than that of pearlite. It has good drawing performance and work hardening ability. After tempering, the carbide distribution in sorbite is more uniform, dislocation movement is easier, and internal stress decreases, making tempered sorbite have good comprehensive mechanical properties. The duplex structure formed in this way can make up for the disadvantage of insufficient original strength of cold-drawn pearlitic steel, further improve the material strength, and at the same time take into account good plasticity and tissue uniformity.

[0032] The larger the volume percentage of the tempered martensite, the higher the strength of the wire rod. In the preferred technical solution, the volume percentage of the tempered martensite is 57% - 63%.

[0033] The finer the lamellar spacing of the tempered sorbite, the higher the strength of the wire rod. In the preferred technical solution, the lamellar spacing of the tempered sorbite is 80 - 110 nm.

[0034] In the preferred technical solution, the reticulated carbide grade of the hot-rolled wire rod is grade 0, which can avoid the reticulated carbide from splitting the matrix structure, improve the toughness and plasticity of the material, and is beneficial to improving the drawing performance.

[0035] The hot-rolled wire rod can effectively avoid the formation of reticulated carbide by C element and transform the quenched martensite into tempered martensite with both high strength and plasticity. The carbide strengthening phases are evenly distributed, and the mechanical properties fluctuate less, which is beneficial to improving the drawing and torsion performance. In the preferred technical solution, the mechanical property difference within the same coil of the hot-rolled wire rod is ≤ 47 MPa.

[0036] In the preferred technical solution, the diameter of the hot-rolled wire rod is 10.0 - 16.0 mm, the tensile strength is 1665 - 1705 MPa, and the reduction of area is 26% - 31%. The relatively high initial tensile strength enables the hot-rolled wire rod to reach the strength grade faster after being drawn as the base material. At the same time, the good plasticity enables the wire rod to undergo large deformation without cracking when stressed, which can reduce the risk of wire breakage during the drawing process, and thus stably develop 2400 MPa grade bridge cables.

[0037] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0038] (1)In view of the current situation that after the content of carbon element and alloy elements in the base material wire rod increases, the control of network carbon and other hard and brittle phase structures is poor, and the ability to control the microstructure is limited. The present invention designs the chemical composition of Mo-Al-V and combines the online molten salt end quenching toughening technology. The front-section molten salt is used to control the wire rod to rapidly pass from the high-temperature austenite state at an ultra-high cooling rate through the temperature range for the formation of network carbide and enter the area below the sorbite phase region, undergoing short-time martensite phase transformation. The rear-section molten salt is used to control the wire rod to enter the sorbite phase region for isothermal phase transformation. At the same time, after isothermal tempering, the formed duplex structure is controlled to undergo a certain degree of short-time tempering, improving the strength-plasticity matching of the wire rod. Finally, slow cooling on the roller table improves the softening effect of the wire rod, enabling the control of network carbide and other hard and brittle phase structures, improving the strength and tissue uniformity of the wire rod, taking into account material costs and good plasticity, with high production efficiency and good industrial adaptability.

[0039] (2)In view of the disadvantage of insufficient original strength of the existing cold-drawn pearlitic steel and the current situation of gradually restricting the development and application of ultra-high-strength bridge cables, the microstructure of the present invention includes a duplex structure composed of tempered martensite and tempered sorbite, which can effectively prevent the formation of network carbide by C element, transform quenched martensite into tempered martensite with both strength and plasticity, utilize the strong precipitation strengthening effect provided by the dispersion precipitation of V element, and utilize Mo element to prevent the coarsening of precipitation phases and reduce the plasticity of the wire rod, maximizing the strengthening effect of carbon element, making the carbide strengthening phases evenly distributed, and combining the tempering state regulation of the duplex structure to improve the strength-plasticity matching of the wire rod, with a tensile strength of 1665 - 1705 MPa and an area reduction of 26% - 31%, and is used in application fields such as manufacturing 2400 MPa grade ultra-high-strength bridge cables, having good market application prospects. Brief Description of the Drawings

[0040] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the following description of the embodiments in conjunction with the accompanying drawings, where:

[0041] Figure 1 is the metallographic structure diagram of Embodiment 1 of the present invention;

[0042] Figure 2 is the metallographic structure diagram of Embodiment 2 of the present invention;

[0043] Figure 3 is the metallographic structure diagram of Embodiment 3 of the present invention. Detailed Embodiments

[0044] The embodiments described below with reference to the accompanying drawings are exemplary and are merely for illustrative purposes and do not limit the description of the features and characteristics of the present invention. To present the best mode of implementing the present invention, it is intended to explain the present invention and is sufficient to enable those skilled in the art to implement the present invention. It should not be construed as any limitation on the scope of the present invention, and the scope of the present invention is only defined by the appended claims; the organization and performance testing of the hot-rolled wire rods obtained from the following embodiments and comparative examples include: the tensile test is carried out in accordance with "GB-T 228.1-2021 Metallic materials-Tensile testing-Part 1: Method of test at room temperature" to obtain the tensile strength and reduction of area; the microstructure detection is carried out in accordance with the metallic microstructure detection method of the GB / T13298 standard; the method for testing the difference of mechanical properties within the same coil: take 2 coils of wire rods at 5 m from the end of the coil, take the lap area position as the base point, evenly divide each coil of wire rods into 8 segments on average, take 1 tensile specimen on each segment, and the strength difference after the tensile test of the taken tensile specimens is the difference of mechanical properties within the same coil. Example 1:

[0045] A preferred embodiment of the manufacturing method of the 2400 MPa grade high-strength duplex hot-rolled wire rod for bridge cables according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.95%, Si: 0.88%, Mn: 0.95%, Cr: 0.38%, V: 0.05%, Mo: 0.45%, Al: 0.39%, P: 0.012%, S: 0.013%, and the rest are Fe and unavoidable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt end quenching and toughening treatment → slow cooling on the roller table → coiling, specifically:

[0046] The rolling process is used to heat the steel billet with a specification of 220 mm × 220 mm into a high-temperature steel billet that reaches the plastic state for rolling, promote the homogenization of alloy components, reduce segregation. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 16 mm through the rolling line. Select appropriate rolling temperature and reduction ratio to promote dynamic recrystallization, refine grains, and strengthen and toughen the matrix during the finishing rolling process. Specifically: control the soaking temperature of the heating furnace to be 1250 °C, the residence time in the furnace to be 150 min, the initial rolling temperature to be 1100 °C, the finishing rolling temperature to be 1010 °C, and the finishing rolling reduction ratio to be 22%; the wire laying process is used to make the wire rod exiting the rolling line into a coil through the wire laying machine. The coil is scattered on the roller table and transported along the roller table, so that the coil is in the high-temperature austenite state, improve the austenite uniformity, and provide favorable conditions for the short-time uniform phase transformation of martensite. Specifically: control the wire laying temperature to be 920 °C.

[0047] The online molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 38 °C / s, quickly skips the reticular carbide precipitation range from the high-temperature austenite state and enters the sorbite phase region or below, forming a certain amount of retained austenite with a large degree of supercooling, promoting the transformation of part of the austenite structure into quenched martensite, and undergoing short-time martensite phase transformation. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the back-stage molten salt treatment. The temperature of the back-stage molten salt is raised to the sorbite phase region temperature, controlling the untransformed austenite to transform into sorbite and isothermal tempering, promoting the massive dispersion precipitation of vanadium carbides, controlling the growth of carbide strengthening phases, and after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering, improving the strength-ductility matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 492 °C, the treatment time is 16 s, the molten salt circulation rate is 650 t / h, and the molten salt temperature rise ≤ 5 °C; the molten salt temperature of the back-stage molten salt is 579 °C, the treatment time is 95 s, the molten salt circulation rate is 600 t / h, and the molten salt temperature rise ≤ 3 °C.

[0048] The roller table slow cooling process adopts closing the heat preservation cover, inputting the hot air above the two-stage salt bath tank into the heat preservation cover, and the wire rod transported by the conveying roller table through the second-stage salt bath tank enters the heat preservation cover, preventing the wire rod from having too fast a cooling rate during cooling, resulting in increased stress, promoting the further toughening of the wire rod structure, and improving the softening effect of the wire rod. Specifically: controlling the wire rod to slowly cool at a cooling rate of 0.4 °C / s to 274 °C; the coiling process is used to coil the wire rod into a coil by a coiling drum, and after packaging and warehousing, the finished hot-rolled wire rod is obtained, and its metallographic structure diagram is as Figure 1 shown.

[0049] Comparative Example 1:

[0050] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 1 lies in: controlling the soaking temperature of the heating furnace to be 1150 °C, the time in the furnace to be 200 min, the initial rolling temperature to be 1020 °C, the final rolling temperature to be 920 °C, the wire laying temperature to be 830 °C, and the wire rod to cool down at a cooling rate of 30 °C / s during the front-stage molten salt treatment, and obtaining the hot-rolled wire rod after being taken off the production line. Example 2:

[0051] A preferred implementation manner of the manufacturing method of the 2400 MPa grade high-strength duplex hot-rolled wire rod for bridge cables described in the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.95%, Si: 0.95%, Mn: 0.78%, Cr: 0.35%, V: 0.062%, Mo: 0.5%, Al: 0.28%, P: 0.012%, S: 0.012%, and the rest are Fe and inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt end quenching and toughening treatment → roller table slow cooling → coiling. Specifically:

[0052] The rolling process is used to heat a billet with a specification of 220mm×220mm into a high-temperature billet that reaches the plastic state for rolling through a heating furnace, promoting the homogenization of alloy components and reducing segregation. After the billet exits the heating furnace, it is rolled into wire rods with a diameter specification of 14mm through a rolling line. Appropriate rolling temperature and deformation amount are selected to promote dynamic recrystallization, refine grains, and strengthen and toughen the matrix during the finishing rolling process. Specifically: control the soaking temperature of the heating furnace at 1235°C, the residence time in the furnace at 180min, the initial rolling temperature at 1090°C, the finishing rolling temperature at 1000°C, and the finishing rolling reduction at 23%; The wire laying process is used to make the wire rods exiting the rolling line into coiled bars through a wire laying machine. The coiled bars are scattered on the roller table and transported along the roller table, keeping the coiled bars in the high-temperature austenite state, improving the austenite uniformity, and providing favorable conditions for the short-time uniform phase transformation of martensite later. Specifically: control the wire laying temperature at 910°C.

[0053] The on-line molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with molten salt inside. The coiled bars after wire laying are transported through the first-stage salt bath tank by the roller table for the front-stage molten salt treatment, cooling the coiled bars at a cooling rate of 37°C / s, quickly passing through the network carbide precipitation range from the high-temperature austenite state into the sorbite phase region, forming a certain amount of large supercooled austenite remnants, promoting the transformation of part of the austenite structure into quenched martensite, and undergoing short-time martensite phase transformation. Then the coiled bars are transported through the second-stage salt bath tank by the roller table for the rear-stage molten salt treatment. The temperature of the rear-stage molten salt is raised to the sorbite phase region temperature, controlling the untransformed austenite to transform into sorbite and isothermal tempering, promoting the massive dispersion precipitation of vanadium carbides, controlling the growth of carbide strengthening phases, and after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering to improve the strength-plasticity matching of the coiled bars. Specifically: the molten salt temperature of the front-stage molten salt is 498°C, the treatment time is 12s, the molten salt circulation volume is 615t / h, and the molten salt temperature rise ≤ 5°C; the molten salt temperature of the rear-stage molten salt is 569°C, the treatment time is 107s, the molten salt circulation volume is 530t / h, and the molten salt temperature rise ≤ 3°C.

[0054] The roller table slow cooling process uses a closed heat preservation cover to input the hot air above the two-stage salt bath tank into the heat preservation cover. The coiled bars transported by the conveying roller table through the second-stage salt bath tank enter the heat preservation cover to prevent the coiled bars from cooling too fast during the cooling process, resulting in increased stress, promoting further toughening of the coiled bar structure, and improving the softening effect of the coiled bars. Specifically: control the coiled bars to cool slowly at a cooling rate of 0.25°C / s to 271°C; The coiling process is used to coil the coiled bars into coils through a coiling drum, and after packaging and warehousing, the finished hot-rolled coiled bars are obtained, and their metallographic structure diagram is as Figure 2 shown.

[0055] Comparative Example 2:

[0056] A manufacturing method of hot-rolled wire rod, the difference between its manufacturing method and that of Example 2 lies in that: during the front-stage molten salt treatment, the wire rod is cooled at a cooling rate of 36 °C / s, the molten salt temperature of the front-stage molten salt is 510 °C, the treatment time is 5 s, and the hot-rolled wire rod is obtained after being taken off the production line.

[0057] Comparative Example 3:

[0058] A manufacturing method of hot-rolled wire rod, the difference between its manufacturing method and that of Example 2 lies in that: during the front-stage molten salt treatment, the wire rod is cooled at a cooling rate of 39 °C / s, the molten salt temperature of the front-stage molten salt is 480 °C, the treatment time is 25 s, and the hot-rolled wire rod is obtained after being taken off the production line. Example 3:

[0059] A preferred implementation manner of the manufacturing method of the 2400 MPa grade high-strength duplex hot-rolled wire rod for bridge cables according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.94%, Si: 0.8%, Mn: 0.82%, Cr: 0.42%, V: 0.075%, Mo: 0.39%, Al: 0.2%, P: 0.015%, S: 0.015%, and the rest are Fe and inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt end quenching and toughening treatment → slow cooling on the roller table → coiling, specifically:

[0060] The rolling process is used to heat a steel billet with a specification of 220 mm × 220 mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, promoting the homogenization of alloy components and reducing segregation. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 10 mm through a rolling line. Appropriate rolling temperature and deformation amount are selected to promote dynamic recrystallization and grain refinement during the finishing rolling process, and strengthen and toughen the matrix. Specifically: control the soaking temperature of the heating furnace to be 1180 °C, the time in the furnace to be 220 min, the initial rolling temperature to be 1060 °C, the finishing rolling temperature to be 980 °C, and the finishing rolling reduction to be 26%; the wire laying process is used to make the wire rod exiting the rolling line into a wire rod through a wire laying machine, and the wire rod is scattered on the roller table and transported along the roller table, making the wire rod in a high-temperature austenite state, improving the austenite uniformity, and providing favorable conditions for the short-time uniform phase transformation of martensite. Specifically: control the wire laying temperature to be 880 °C.

[0061] The online molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with internal molten salt. The wire rod after wire drawing is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 35 °C / s, quickly skips the reticulated carbide precipitation range from the high-temperature austenite state and enters the pearlite phase region or below, forming a certain amount of retained austenite with a large degree of supercooling, promoting the transformation of part of the austenite structure into quenched martensite, and undergoing short-time martensite phase transformation. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the back-stage molten salt treatment. The temperature of the back-stage molten salt is raised to the pearlite phase region temperature, controlling the untransformed austenite to transform into pearlite and isothermal tempering, promoting the massive dispersion precipitation of vanadium carbides, controlling the growth of carbide strengthening phases, and after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering, improving the strength-ductility matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 505 °C, the treatment time is 8 s, the molten salt circulation rate is 460 t / h, and the molten salt temperature rise ≤ 5 °C; the molten salt temperature of the back-stage molten salt is 565 °C, the treatment time is 128 s, the molten salt circulation rate is 400 t / h, and the molten salt temperature rise ≤ 3 °C.

[0062] The roller table slow cooling process adopts closing the heat preservation cover, inputting the hot air above the two-stage salt bath tank into the heat preservation cover, and the wire rod transported by the conveying roller table passing through the second-stage salt bath tank enters the heat preservation cover, preventing the wire rod from increasing stress due to too fast cooling rate during the cooling process, promoting the further toughening of the wire rod structure, and improving the softening effect of the wire rod. Specifically: controlling the wire rod to slowly cool to 279 °C at a cooling rate of 0.13 °C / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and after packaging and warehousing, the finished hot-rolled wire rod is obtained, and its metallographic structure diagram is as Figure 3 shown.

[0063] Comparative Example 4:

[0064] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 3 is that: the molten salt temperature of the back-stage molten salt is 590 °C, the treatment time is 200 s, and the hot-rolled wire rod is obtained after being taken off the production line.

[0065] Comparative Example 5:

[0066] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 3 is that: the molten salt temperature of the back-stage molten salt is 535 °C, the treatment time is 55 s, and the hot-rolled wire rod is obtained after being taken off the production line. Example 4:

[0067] A preferred embodiment of the manufacturing method of the high-strength duplex hot-rolled wire rod for 2400 MPa-class bridge cables of the present invention. The chemical composition and mass percentage of the hot-rolled wire rod include C: 0.97%, Si: 0.92%, Mn: 0.75%, Cr: 0.45%, V: 0.065%, Mo: 0.3%, Al: 0.4%, P: 0.015%, S: 0.012%, and the rest are Fe and inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt end quenching and toughening treatment → slow cooling on the roller table → coiling. Specifically:

[0068] The rolling process is used to heat a steel billet with a specification of 220 mm × 220 mm into a high-temperature steel billet that reaches the plastic state for rolling, promoting the homogenization of alloy components and reducing segregation. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 12 mm through the rolling line. Appropriate rolling temperature and reduction ratio are selected to promote dynamic recrystallization and grain refinement during the finishing rolling process, and strengthen and toughen the matrix. Specifically: control the soaking temperature of the heating furnace to be 1210 °C, the residence time in the furnace to be 200 min, the rough rolling temperature to be 1070 °C, the finishing rolling temperature to be 990 °C, and the finishing rolling reduction to be 25%; The wire laying process is used to make the wire rod exiting the rolling line into a wire coil through the wire laying machine. The wire coil is scattered on the roller table and transported along the roller table, making the wire coil in the high-temperature austenite state, improving the austenite uniformity, and providing favorable conditions for the short-time uniform phase transformation of martensite. Specifically: control the wire laying temperature to be 895 °C.

[0069] The on-line molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with molten salt inside. The wire coil after wire laying is transported through the first-stage salt bath tank by the roller table for the front-stage molten salt treatment, so that the wire coil cools down at a cooling rate of 36 °C / s, quickly skips the network carbide precipitation range from the high-temperature austenite state and enters the sorbite phase region or below, forming a certain amount of retained austenite with a large degree of supercooling, promoting the transformation of part of the austenite structure into quenched martensite, and performing short-time martensite phase transformation. Then the wire coil is transported through the second-stage salt bath tank by the roller table for the rear-stage molten salt treatment. The temperature of the rear-stage molten salt is raised to the sorbite phase region temperature, controlling the untransformed austenite to transform into sorbite and isothermal tempering, promoting the massive dispersion precipitation of vanadium-containing carbides, controlling the growth of carbide strengthening phases, and after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering to improve the strength-plasticity matching of the wire coil. Specifically: the molten salt temperature of the front-stage molten salt is 485 °C, the treatment time is 18 s, the molten salt circulation volume is 545 t / h, and the molten salt temperature rise ≤ 5 °C; the molten salt temperature of the rear-stage molten salt is 585 °C, the treatment time is 78 s, the molten salt circulation volume is 470 t / h, and the molten salt temperature rise ≤ 3 °C.

[0070] In the roller table slow cooling process, the heat preservation cover is closed, and the hot air above the two salt bath tanks is input into the heat preservation cover. The wire rods passing through the second salt bath tank by the conveying roller table enter the heat preservation cover, preventing the wire rods from having too fast cooling rate during the cooling process, which may cause stress increase, promoting further toughening of the wire rod structure, and improving the softening effect of the wire rod. Specifically: the wire rods are slowly cooled at a cooling rate of 0.3 °C / s to 268 °C; the coiling process is used to coil the wire rods into coils by a coiling drum, and after packaging and warehousing, the finished hot-rolled wire rods are obtained.

[0071] Comparative Example 6:

[0072] A manufacturing method of hot-rolled wire rods, the difference between its manufacturing method and that of Example 4 lies in: in the roller table slow cooling process, the heat preservation cover is not closed tightly, and the wire rods are slowly cooled at a cooling rate of 0.9 °C / s to 280 °C. After being taken off the production line, the hot-rolled wire rods are obtained. The tensile strength of the hot-rolled wire rods is 1713 MPa, the reduction of area is 24%, and the mechanical property difference within the same coil is 48 MPa.

[0073] The hot-rolled wire rods obtained from the above Examples 1 to 4 and Comparative Examples 1 to 5 are subjected to microstructure and property tests, and the comparison results obtained are shown in Table 1 below:

[0074] Table 1. Comparison results of microstructure and properties of different hot-rolled wire rod compositions and manufacturing methods

[0075]

[0076] It can be seen from the results of Examples 1 to 4 that through the Mo-Al-V chemical composition design in combination with the online molten salt end quenching toughening technology of the present invention, the formation of network carbide by C element can be effectively avoided, and the quenched martensite is transformed into tempered martensite with both strength and plasticity. The microstructure includes a duplex structure composed of tempered martensite and tempered sorbite, which can maximize the strengthening effect of carbon element, make the carbide strengthening phase evenly distributed, achieving a tensile strength of 1665 - 1705 MPa and a reduction of area of 26% - 31%, which is beneficial to the stable production of 2400 MPa grade bridge cables.

[0077] It can be seen from the comparison results between Example 1 and Comparative Example 1 that by selecting an appropriate spinning temperature, the formation of network carbide during the spinning stage can be avoided due to too low spinning temperature, improving the austenite uniformity, and providing favorable conditions for the short-time uniform phase transformation of martensite later.

[0078] It can be seen from the comparison results between Example 2 and Comparative Example 2 that the molten salt temperature of the front-section molten salt is below the sorbite phase region. The higher the molten salt temperature of the front-section molten salt and the shorter the treatment time, the smaller the proportion of tempered martensite and the larger the proportion of tempered sorbite in the structure, and the plasticity of the matrix increases. However, if the molten salt temperature of the front-section molten salt is too high and the treatment time is too short, it will affect the martensite phase transformation and the matrix strength will be lost.

[0079] From the comparison results between Example 2 and Comparative Example 3, it can be seen that the lower the molten salt temperature and the longer the treatment time in the front-stage molten salt, the more the amount of retained austenite with large supercooling degree increases, the short-time transformation of martensite accelerates, the amount of quenched martensite increases, and the matrix strength is higher. However, if the molten salt temperature is too low and the treatment time is too long, it is not conducive to the subsequent sorbite transformation and short-time tempering. The proportion of tempered sorbite in the structure is too small, which will affect the plasticity of the matrix.

[0080] From the comparison results between Example 3 and Comparative Example 4, it can be seen that the higher the molten salt temperature and the longer the treatment time in the rear-stage molten salt, the more heat power can be provided for tempering, the carbide precipitation is sufficient, and the isothermal toughening effect is enhanced, and the plasticity of the wire rod increases. However, if the molten salt temperature is too high and the treatment time is too long, the interlamellar spacing of sorbite becomes coarser, and there is a risk of excessive loss of matrix strength and coarsening of carbide precipitation, resulting in loss of strength and plasticity.

[0081] From the comparison results between Example 3 and Comparative Example 5, it can be seen that the lower the molten salt temperature in the rear-stage molten salt, the finer the interlamellar spacing of sorbite can be promoted, providing more driving force for the dispersion precipitation of vanadium-containing carbides. The shorter the treatment time, the lower the production energy consumption and the promotion of rapid production. However, if the molten salt temperature is too low and the treatment time is too short, the tempering effect of the duplex structure decreases, resulting in greater brittleness, and the insufficient precipitation of carbides will cause loss of strength and plasticity.

[0082] From the comparison results between Example 4 and Comparative Example 6, it can be seen that the slow cooling of the wire rod by roller table can control the wire rod to cool gradually and slowly, prevent excessive internal stress in the wire rod caused by too fast cooling rate during the cooling process, and promote further toughening of the wire rod structure, improving the softening effect of the wire rod.

[0083] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for manufacturing a 2400MPa grade high-strength multiphase hot-rolled wire rod for bridge cables, characterized in that: The manufacturing method thereof comprises: The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.94%-0.97%, Si: 0.80%-0.95%, Mn: 0.75%-0.95%, Cr: 0.35%-0.45%, V: 0.050%-0.075%, Mo: 0.30%-0.50%, Al: 0.2%-0.4%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; the wire rod is spun into a wire rod at a spinning temperature of ≥880°C, and then subjected to online The molten salt terminal quenching and toughening treatment is to make the wire rod first pass through the front section of molten salt and cool down at a cooling rate of ≥35°C / s, so that part of the austenite structure is transformed into quenched martensite, and then pass through the rear section of molten salt to heat up to the troostite phase temperature, control the untransformed austenite to be transformed into troostite and isothermally tempered, and finally pass through a roller to slowly cool to obtain a hot-rolled wire rod with a microstructure including a complex phase structure composed of tempered martensite and tempered troostite; the molten salt temperature of the front section of molten salt is 485~505°C, and the treatment time is 8~18s; the molten salt temperature of the rear section of molten salt is 565~585°C, and the treatment time is 78~128s.

2. The method for manufacturing 2400MPa grade high-strength multi-phase 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 1180-1250° C., and the time in the furnace is 150-220 min.

3. The method for manufacturing the 2400MPa grade high-strength multi-phase 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 1060-1110° C., the final rolling temperature is controlled to be 980-1010° C., and the final rolling reduction is controlled to be 22%-26%.

4. The method for manufacturing 2400MPa grade high-strength multi-phase hot-rolled wire rod for bridge cables according to claim 1, characterized in that: The molten salt circulation volume of the front-stage molten salt is 460-650 t / h, and the molten salt temperature rise is ≤5°C; the molten salt circulation volume of the rear-stage molten salt is 400-600 t / h, and the molten salt temperature rise is ≤3°C.

5. The method for manufacturing 2400MPa grade high-strength multi-phase hot-rolled wire rod for bridge cables according to claim 1, characterized in that: The roller slow cooling controls the wire rod to be slowly cooled to below 280° C. at a cooling rate of 0.1-0.4° C. / s.

6. A 2400MPa grade high-strength multiphase hot-rolled wire rod for bridge cables, characterized in that: The hot-rolled wire rod is manufactured by the method for manufacturing 2400MPa-grade high-strength multi-phase hot-rolled wire rod for bridge cables as described in any one of claims 1 to 5.

7. The 2400MPa high-strength multi-phase hot-rolled wire rod for bridge cables according to claim 6, characterized in that: The volume percentage of the tempered martensite is 57% to 63%, and the interlamellar spacing of the tempered troostite is 80 to 110 nm.

8. The 2400MPa high-strength multi-phase hot-rolled wire rod for bridge cables according to claim 6, characterized in that: The network carbide grade of the hot-rolled wire rod is grade 0, and the mechanical property difference is ≤47MPa.

9. The 2400MPa high-strength multi-phase hot-rolled wire rod for bridge cables according to claim 6, characterized in that: The hot-rolled wire rod has a diameter of 10.0-16.0 mm, a tensile strength of 1665-1705 MPa, and a cross-sectional shrinkage of 26%-31%.

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