A hot-rolled wire rod for 2500 MPa class bridge cables and its manufacturing method

Abstract

The present invention relates to a hot-rolled wire rod for 2500 MPa-class bridge cables and a manufacturing method thereof. After hot-rolling and coiling the wire rod with a high-carbon composition of Nb-V-Mo into a wire rod, it undergoes on-line molten salt rapid cooling and isothermal treatment, causing the wire rod to cool at a cooling rate of ≥33 °C / s, entering the sorbite phase region from the austenite state, forming a structure mainly composed of sorbite and controlling the precipitation and growth of carbides. At the same time, isothermal toughening is carried out to remove stress. Finally, it is slowly cooled through a roller table to produce a hot-rolled wire rod with a mixed microstructure composed of tempered sorbite, ferrite, and fused sorbite. It can improve the content of strengthening elements without weakening the toughness of the material, taking into account the material cost, achieving a tensile strength of 1692-1735 MPa and a reduction of area of 25%-30%, and is used in application fields such as manufacturing 2500 MPa-class ultra-high-strength bridge cables, which is beneficial to reducing the risk of wire breakage during drawing and torsion.

Description

Technical Field

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

[0002] Under the development trend of lightweight, the strength grades of bridge cables and their base metal strength grades in the bridge field are constantly increasing. As the base metal of bridge cable steel wires, the quality of hot-rolled wire rods directly determines the final performance of bridge cables, and thus affects the safety, stability and service life of bridges. Therefore, in order to meet the application requirements of long-span bridges and ensure the reliable operation of bridges under complex environments and long-term loads, it is necessary to develop hot-rolled wire rods for bridge cables with a strength grade of 2500 MPa.

[0003] Due to the limitation of the controlled cooling capacity of the existing Stelmor air-cooling line, it is difficult to stably develop bridge cables with ultra-high strength grades of 2200 MPa and above at present. The main reasons why the existing hot-rolled wire rods cannot meet the production and application requirements of 2500 MPa bridge cables include:

[0004] First, in order to improve the strength of the base metal of hot-rolled wire rods, the content of alloying elements in the steel is relatively high. For example, a high-toughness bridge cable steel with a tensile strength ≥2500 Mpa disclosed in Patent CN110055392B adopts a medium-carbon high-manganese high-aluminum composition design of C-Mn-Al-Co-V. On the one hand, the high content of alloying elements brings higher material cost pressure. On the other hand, the existing hot-rolled wire rods for bridge cables generally adopt controlled cooling by the Stelmor air-cooling line after wire laying. Under the influence of alloy element segregation and the uncontrollability of the cooling rate of air-cooled wire rods, the wire rods are prone to generate brittle structures, resulting in a decrease in tissue uniformity and large fluctuations in mechanical properties. The wire rods have high hardness, high brittleness and poor plastic and toughness properties, and thus the risk of wire breakage is aggravated during the subsequent drawing process, making it difficult to stably produce bridge cable steel wires.

[0005] 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.

[0006] 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.

[0007] 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

[0008] The present invention aims to solve at least one of the above technical problems to some extent. The present invention provides a hot-rolled wire rod for 2500 MPa-class bridge cables and a manufacturing method thereof, which can increase the content of strengthening elements without weakening the toughness of the material, take into account the material cost, and are suitable for the stable production of 2500 MPa-class bridge cables.

[0009] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0010] A manufacturing method of a hot-rolled wire rod for 2500 MPa-class bridge cables, the manufacturing method includes:

[0011] Roll wire rods 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: 0.95% - 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 inevitable impurities; after the wire rods are spun into wire rods at a laying head temperature of ≥900 °C, they are subjected to online molten salt rapid cooling and isothermal treatment, so that the wire rods cool down at a cooling rate of ≥33 °C / s, enter the sorbite phase region from the austenite state, form a structure mainly composed of sorbite and control the precipitation and growth of carbides. At the same time, isothermal toughening is carried out to remove stress. Finally, slow cooling is carried out through a roller table to obtain a hot-rolled wire rod with a mixed structure composed of tempered sorbite, ferrite and fused sorbite in the microstructure.

[0012] The design basis for the chemical composition and mass percentage of the above hot-rolled wire rod includes:

[0013] (1) Carbon: The C element is an effective carbide strengthening element and austenite forming element, and its price is lower than that of other elements. As the carbon content increases, the transformation temperature of sorbite will decrease, the phase transformation driving force will increase, and the sorbite structure in the online molten salt rapid cooling and isothermal treatment process will become finer and more dispersed, improving the material strength. However, as the carbon content increases, the carbon segregation tendency during the solidification of the steel billet will increase, the tendency of decarburization and the precipitation of network carbides will increase, and the difficulty of controlling brittle phases and isothermal toughening to remove stress will increase. Therefore, in order to meet the high-strength requirements of the 2500 MPa-class bridge cable for the hot-rolled wire rod base material, control the material cost, and at the same time reduce the brittle phase and toughening control difficulty, the mass percentage of C is controlled at 0.95% - 0.98%.

[0014] (2) Silicon: The Si element is a good deoxidizer with significant solid-solution strengthening effect and can inhibit grain coarsening during online molten salt rapid cooling isothermal treatment, so as to quickly obtain a sorbite structure mainly with fine lamellar spacing and improve the tissue uniformity of the matrix strength. However, too high silicon content will make the steel more prone to decarburization during high-temperature heating, prolong the phase transformation incubation time, and is not conducive to controlling the precipitation and growth of carbides through a shorter treatment time during the online molten salt rapid cooling isothermal process. Therefore, in order to inhibit austenite grain growth, promote sorbite phase transformation, and facilitate the control of carbide precipitation and growth, the mass percentage of Si is controlled at 1.00% - 1.20%.

[0015] (3) Manganese: Mn has a strong affinity with carbon, can expand the austenite phase region, increase the stability of austenite, and at the same time increase the hardenability of the wire rod, lower the transformation temperature 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, enabling the bridge cable to resist impact and vibration loads while bearing tension. However, when the content of Mn is too high, it will increase element segregation, increase the precipitation risk of low-temperature structures such as bainite and martensite in the wire rod, and at the same time reduce the activity of carbon, increase the difficulty of isothermal toughening and stress relief, and affect the plastic and tough properties of the wire rod. Therefore, in order to promote sorbite phase transformation refinement, reduce tissue uniformity and tempering control difficulty, the mass percentage of Mn is controlled at 0.70% - 0.90%.

[0016] (4) Chromium: The Cr element is a strong carbide-forming element, can improve the hardenability of steel, lower the sorbite transformation temperature, make the sorbite structure finer, is beneficial to improving the matrix strength, and reduce the strength loss during the subsequent hot-dip galvanizing process of wire drawing for cable making. However, too high Cr content will exacerbate composition segregation, increase the risk of abnormal martensite structure precipitation, significantly increase the difficulty of improving the plasticity of the wire rod, affect the isothermal toughening and stress relief effect, and thus affect the wire drawing and torsion properties of the wire. The precipitation and coarsening of Cr will also result in the loss of strength and plasticity. Therefore, in order to improve material strength, facilitate the improvement of wire rod plasticity and carbide precipitation control, and appropriately reduce the Cr content, the mass percentage of Cr is controlled at 0.33% - 0.53%.

[0017] (5) Niobium: As a microalloying element, the Nb element can inhibit high-temperature austenite coarsening, refine austenite grains, improve the strength and toughness of steel, increase the yield strength of steel through precipitation strengthening, keep austenite stable at a lower temperature, provide more nucleation sites for sorbite phase transformation, and disperse and precipitate during the isothermal process, effectively improving the strength of hot-rolled wire rod without reducing plasticity. However, the cost of the Nb element is relatively high. Therefore, based on the role and cost of the Nb element, the content of the Nb element is controlled at 0.03% - 0.038%.

[0018] (6) Vanadium: As a microalloying element, V element can inhibit the coarsening of high-temperature austenite and hinder the growth of austenite grains, thereby refining the grains. At the same time, during the isothermal process, it can be strengthened by forming carbide and nitride precipitates, improving the strength and toughness of the steel, effectively increasing the strength of the hot-rolled wire rod without reducing the plasticity. However, the cost of V element is relatively high. Excessive addition is not conducive to controlling the cost of the wire rod and there is a risk of coarsening. When the vanadium precipitation phase coarsens, the strength effect will be lost. At the same time, the coarsened vanadium precipitation phase may become a crack source, reducing the toughness of the material. Therefore, considering the role and cost of V element, the content of V element is controlled at 0.03% - 0.04%.

[0019] (7) Molybdenum: Mo element can improve the hardenability of the steel, increase the stability of supercooled austenite, shift the C curve to the right, slow down the transformation of austenite to ferrite, refine the sorbite structure, and effectively inhibit the coarsening of carbide precipitation phases during the online molten salt rapid cooling isothermal treatment process, thereby effectively enhancing the strengthening effect of the precipitation phase and maintaining a relatively high strength during tempering. However, due to the high price of Mo element, based on the role and cost of Mo element, the mass percentage of Mo is controlled at 0.25% - 0.35%.

[0020] (8) Phosphorus and sulfur: P element and S element belong to impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% are controlled.

[0021] The above hot-rolled wire rod adopts a high-carbon composition design of Nb-V-Mo, optimizes the composition ratio of Si, Mn, and Cr, reduces the transformation temperature of sorbite and the difficulty of plasticity improvement, provides favorable conditions for refining the sorbite lamellar spacing, facilitating the isothermal tempering toughening of the wire rod and controlling the precipitation and growth of carbides, and can reduce the consumption of alloying elements and take into account the control of material costs. On this basis, by selecting an appropriate spinning temperature for the wire rod, it is possible to avoid the formation of network carbides during the spinning stage due to too low spinning temperature, and increase the supercooling degree with a relatively high quenching temperature, providing favorable conditions for the rapid nucleation of sorbite and the dispersion precipitation of Nb and V carbides. The wire rod after spinning enters the molten salt for online molten salt rapid cooling isothermal treatment without passing through air cooling:

[0022] I. Compared with the high-carbon composition system, it is difficult to control brittle phases such as network carbides and martensite on the Stelmor air-cooling line. 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 transfer. On the one hand, it can control the rapid cooling of the wire rod from the high-temperature austenite state, pass through the secondary cementite precipitation temperature range of 700 - 800 °C, and avoid the formation of network carbides along the grain boundaries, which would damage the tissue continuity and reduce the plastic and toughness properties of the material. On the other hand, there is no problem of temperature difference between the windward side and the leeward side. Under high-speed heat transfer, the temperature difference from the surface to the core of the wire rod is smaller, which can avoid entering the martensite phase region due to too fast cooling and forming brittle abnormal tissues, effectively controlling the brittle phases and achieving higher tissue uniformity, providing favorable conditions for the full play of the strengthening effect of carbon elements.

[0023] II. Compared with the high-carbon and high-silicon composition system, which has a long phase transformation incubation time, a lower content of sorbite, and greater tissue stress on the Stelmor air-cooling line. On the one hand, when the wire rod undergoes online molten salt rapid cooling and isothermal treatment, it can promote the rapid cooling of the wire rod to the temperature of the sorbite phase region, form a large degree of supercooling, inhibit grain coarsening, and promote the rapid transformation of the wire rod into sorbite with finer lamellar spacing, so as to improve the matrix strength and reduce the isothermal tempering difficulty of the sorbite quenched structure. After the wire rod is treated with molten salt, it 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 full precipitation of the wire rod at the peak precipitation temperature of sorbite, increase the content of sorbite in the tissue, reduce the content of ferrite, improve the matrix strength, and improve the adverse effect of increasing silicon content on the extension of phase transformation time. By fully transforming, austenite residue can be avoided, and abnormal hard brittleness will not continue to form during the subsequent slow cooling process on the roller table. On the other hand, the temperature in the sorbite phase region is relatively high, which can extend the treatment time of the wire rod at high temperature, promote the rapid isothermal toughening and stress relief of the sorbite structure formed by phase transformation, so as to improve the matching of plasticity and high strength of the wire rod. After the wire rod undergoes online molten salt rapid cooling and isothermal treatment, the temperature is relatively high. Using the roller table slow cooling can reduce the cooling rate of the wire rod, avoid the increase in stress caused by the too fast cooling of the wire rod, promote the further toughening of the wire rod tissue, and improve the softening effect of the wire rod.

[0024] III. Compared with the Stelmor air-cooling line, it is difficult to control the precipitation and growth of carbides due to unstable continuous cooling and controlled cooling. On the one hand, when the wire rod undergoes online molten salt rapid cooling and isothermal treatment, it can rapidly cool down, increase the precipitation driving force of Nb and V carbides, adapt the temperature in the sorbite phase region to the precipitation temperature range of carbide strengthening phases, and by isothermal control rather than continuous cooling, the time of the wire rod in the precipitation temperature range can be extended, promoting the full and uniform precipitation of Nb and V during the isothermal process, effectively improving 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. Cooperating with Mo to inhibit carbide coarsening and controlling a certain degree of isothermal tempering and stress relief of the sorbite structure of the wire rod can avoid the loss of strength and plasticity due to carbide coarsening, and thus give full play to the microalloy strengthening effect.

[0025] Selecting an appropriate soaking temperature and soaking time in the heating furnace before rolling can promote composition homogenization, reduce the influence of segregation, and at the same time avoid excessive heating temperature leading to a large amount of Nb solid solution, which affects subsequent precipitation strengthening, and avoid too long soaking time causing decarburization and burning loss. In the preferred technical solution, before rolling, the soaking temperature of the heating furnace is controlled at 1200 - 1250 °C, and the soaking time is 150 - 200 min.

[0026] 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%.

[0027] 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%.

[0028] 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.

[0029] 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℃.

[0030] In the preferred technical solution, the molten salt temperature for the post-treatment is 565 - 585 °C, and the treatment time is 100 - 150 s. The molten salt temperature for the post-treatment is in the sorbite phase region, but the molten salt circulation volume is smaller. The lower the molten salt temperature for the post-treatment, the more beneficial it is for the dispersion precipitation of carbide strengthening phases, improving 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, the toughening and stress relief effect decreases, and insufficient matrix plasticity will result. On the contrary, the higher the molten salt temperature, the more beneficial it is for 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 prolongs, the coarsening of carbides is adverse to the strength and plasticity of the wire rod. The longer the treatment time for the post-treatment, the better the toughening and stress relief effect, and the plasticity of the wire rod is improved. However, if the treatment time is too long, there is a risk of excessive loss of the wire rod strength and loss of strength and plasticity due to the coarsening of carbide precipitation. On the contrary, the shorter the treatment time, the lower the production energy consumption and the higher the matrix strength. However, if the treatment time is too short, there are large residual stresses in the wire rod, and the carbide strengthening phases cannot precipitate sufficiently, resulting in loss of strength and plasticity. Therefore, the molten salt temperature and treatment time for the post-treatment can be further controlled to regulate the tempering state of the structure, control the precipitation and growth of carbides, and improve the strength-plasticity matching of the wire rod.

[0031] Since the temperature difference between the molten salt temperatures of the pre-treatment and the post-treatment is small, with the release of heat energy during the phase transformation, an appropriate molten salt circulation volume can be selected to control the molten salt temperature rise and production energy consumption. In the preferred technical solution, the molten salt circulation volume for the post-treatment is 350 - 450 t / h, and the molten salt temperature rise ≤ 3 °C.

[0032] Since the austenite structure has undergone sufficient phase transformation and there is no problem of residual austenite transforming into brittle phases, but to avoid the physical stress contraction effect, an appropriate roller table slow cooling control is selected, which can prevent the stress increase caused by too fast cooling rate during the cooling process of the wire rod and promote the further toughening of the wire rod structure, improving the softening effect of the wire rod. In the preferred technical solution, the roller table slow cooling controls the wire rod to cool slowly at a cooling rate of 0.2 - 0.45 °C / s to below 280 °C.

[0033] In the preferred technical solution, the roller table slow cooling is achieved by closing the heat preservation cover, inputting the hot air during the online molten salt rapid cooling isothermal treatment process into the heat preservation cover, and controlling the slow cooling by the roller table to convey the wire rod into the heat preservation cover, which can recycle the heat energy in the molten salt treatment and reduce the production energy consumption.

[0034] A hot-rolled wire rod for 2500 MPa-class bridge cables, which is manufactured by the manufacturing method of the hot-rolled wire rod for 2500 MPa-class bridge cables described in any one of the above.

[0035] The above hot-rolled wire rods adopt a C-Si-Mn-Cr-Nb-V-Mo composition design combined with an online molten salt rapid cooling and isothermal technology. The microstructure is mainly tempered sorbite and fused sorbite, which can effectively control brittle tissues, reduce the ferrite content in the microstructure. The lamellar spacing of sorbite is small, the dislocation movement is hindered greatly, and the strength and hardness are higher than those of pearlite. After the isothermal tempering toughening treatment, some lamellae of sorbite fuse, the internal stress decreases, and it transforms into tempered sorbite and fused sorbite. The strength and hardness decrease somewhat, but the toughness and plasticity are significantly improved. It can undergo plastic deformation better when stressed and has good comprehensive mechanical properties. Strengthening precipitation phases such as Nb and V carbides are dispersed, which can improve the content of strengthening elements without weakening the toughness of the material, achieving a better match between the higher strength and good plasticity of the wire rods.

[0036] The larger the lamellar spacing of the tempered sorbite, the higher the matrix strength. In the preferred technical solution, the lamellar spacing of the tempered sorbite is 65 - 95 nm.

[0037] The higher the proportion of the tempered sorbite and fused sorbite in the microstructure, the better the strength-plasticity performance and drawing hardening ability of the matrix. In the preferred technical solution, the volume percentage of the tempered sorbite and fused sorbite ≥ 95%.

[0038] In the preferred technical solution, the reticulated carbide grade of the hot-rolled wire rods is grade 0, which can avoid the adverse effects of reticulated carbides on the microstructure uniformity and plasticity of the wire rods and reduce the risk of drawing wire breakage of the hot-rolled wire rods.

[0039] The brittle tissues of the hot-rolled wire rods are effectively controlled, the phase transformation and carbide precipitation are controlled more uniformly, and the mechanical property fluctuation is reduced, which is beneficial to reducing the risks of drawing and torsion cracking. In the preferred technical solution, the mechanical property difference within the same coil of the hot-rolled wire rods ≤ 47 MPa.

[0040] In the preferred technical solution, the diameter of the hot-rolled wire rods is 9.0 - 15.0 mm, the tensile strength is 1692 - 1735 MPa, and the reduction of area is 25% - 30%. The hot-rolled wire rods have a high tensile strength, can be rapidly drawn to reach the corresponding strength grade, reduce the plastic loss during the process, and at the same time, the good reduction of area can reduce the deformation resistance and the risk of drawing wire breakage, thus promoting the stable production of high-strength bridge cables.

[0041] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0042] (1)In view of the current situation that due to the limitation of the existing Stelmor air-cooled wire control cooling capacity, it is difficult to effectively control the microstructure of the wire rod, the cost of the wire rod is relatively high, the strength and plasticity are insufficient, and the tissue uniformity is poor. The present invention combines Nb-V-Mo chemical composition design with an online molten salt rapid cooling and isothermal technology to control the wire rod to quickly enter the sorbite phase region from the high-temperature austenite state, forming a structure mainly composed of fine lamellar spacing sorbite, effectively avoiding the generation of brittle tissues, improving the strengthening effect of carbon elements, controlling the precipitation and growth of carbides in the high-temperature isothermal range of the wire rod, controlling the dispersion precipitation of Nb and V during the isothermal process, effectively enhancing the strengthening effect of the precipitated phase without reducing plasticity, and performing high-temperature isothermal tempering and toughening stress relief treatment on the structure. Subsequently, slow cooling on the roller table promotes further toughening of the wire rod structure, improves the softening effect of the wire rod, realizes the regulation of the microstructure, improves the strength-plasticity matching of the wire rod, and has good industrial adaptability.

[0043] (2)In view of the current situation in the prior art that although increasing carbon elements and other alloying elements can rapidly increase the strength, the brittleness of the wire rod also increases rapidly. The hot-rolled wire rod of the present invention adopts a high-carbon composition system, which can take into account the material cost. The types of microstructures include a mixed structure composed of tempered sorbite, ferrite and fused sorbite. Using isothermal tempering to toughen and relieve stress can improve the content of strengthening elements without weakening the toughness of the material. The tensile strength of the product reaches 1692-1735 MPa, and the reduction of area is 25-30%. It is used in application fields such as manufacturing 2500 MPa grade ultra-high strength bridge cables, which is beneficial to reducing the risk of wire breakage during drawing and torsion, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The above and / or additional aspects and advantages of the present invention will become apparent and easy to understand from the description of the embodiments in conjunction with the following drawings, where:

[0045] Figure 1 is the metallographic structure diagram of Embodiment 1 of the present invention;

[0046] Figure 2 is the metallographic structure diagram of Embodiment 2 of the present invention;

[0047] Figure 3 is the metallographic structure diagram of Embodiment 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0048] The embodiments described below with reference to the accompanying drawings are exemplary, solely for the purpose of illustration and not limiting 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 be sufficient for 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, which 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 in mechanical properties within the same coil: take 2 coils of wire rods at a distance of 5 m from the end of the coil. Taking the lap area position as the base point, each coil of wire rod is evenly divided into 8 segments on average, and 1 tensile specimen is taken on each segment. The strength range of the tensile specimens taken after the tensile test is the difference in mechanical properties within the same coil. Example 1:

[0049] A preferred embodiment of the manufacturing method of the 2500MPa grade 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: 0.96%, 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 are Fe and inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt rapid cooling and isothermal treatment → slow cooling on the roller table → coiling. Specifically:

[0050] The rolling process is used to heat a steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the plasticity for rolling through a heating furnace to promote the homogenization of alloy components. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 14mm through a rolling line to promote the strain-induced precipitation of Nb and the dynamic recrystallization during the finishing rolling process and refine the grains. Specifically: control the soaking temperature of the heating furnace to be 1235°C, the residence time in the furnace to be 175min, the initial rolling temperature to be 1110°C, the initial rolling reduction to be 17%, the finishing rolling temperature to be 1015°C, and the finishing rolling reduction to be 27%. The wire laying process is used to make the wire rod exiting the rolling line into a coil through a wire laying machine. The coil is scattered on the roller table and transported along the roller table, making the coil in a high-temperature austenite state, providing favorable conditions for forming a large supercooling degree and promoting the rapid precipitation of sorbite structure and carbide strengthening phases at a relatively high quenching temperature. Specifically: control the wire laying temperature to be 940°C.

[0051] The online molten salt rapid cooling and isothermal 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 pre-treatment, so that the wire rod 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, forming a structure mainly composed of sorbite with a fine lamellar spacing. Then the wire rod is transported through the second-stage salt bath tank by a roller table for post-treatment. The circulation volume of the molten salt in the post-treatment is appropriately reduced, the temperature is accurately controlled and the production energy consumption is reduced. The wire rod is controlled to enter the high-temperature isothermal range, the precipitation and growth of carbides are controlled, and at the same time, the sorbite structure formed by quenching is subjected to high-temperature isothermal tempering and toughening stress relief treatment to improve the strength-plasticity matching of the wire rod. Specifically: the temperature of the molten salt in the pre-treatment is 559 °C, the treatment time is 66 s, the circulation volume of the molten salt is 720 t / h, and the temperature rise of the molten salt ≤ 9 °C; the temperature of the molten salt in the post-treatment is 579 °C, the treatment time is 114 s, the circulation volume of the molten salt is 420 t / h, and the temperature rise of the molten salt ≤ 3 °C.

[0052] In the roller table slow cooling process, the heat preservation cover is closed, and the hot air above the two-stage salt bath tank is input into the heat preservation cover. The wire rod transported by the conveying roller table through the second-stage salt bath tank enters the heat preservation cover to prevent the stress of the wire rod from increasing due to too fast cooling rate during the cooling process, promote the further toughening of the wire rod structure, and improve the softening effect of the wire rod. Specifically: the wire rod is 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 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.

[0053] Comparative Example 1:

[0054] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 1 is that: the soaking temperature of the heating furnace is controlled at 1185 °C, the time in the furnace is 220 min, the initial rolling temperature is 1040 °C, the final rolling temperature is 930 °C, the wire laying temperature is controlled at 850 °C, and during the online molten salt rapid cooling and isothermal treatment, the wire rod undergoes pre-treatment, so that the wire rod cools down at a cooling rate of 30 °C / s, and the hot-rolled wire rod is obtained after being taken off the production line. Example 2:

[0055] A preferred implementation manner of the manufacturing method of the 2500 MPa grade bridge cable hot-rolled wire rod 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 inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt rapid cooling and isothermal treatment → roller table slow cooling → coiling. Specifically:

[0056] The rolling process is used to heat a steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, promoting the homogenization of alloy components. After the steel billet exits the heating furnace, it is rolled into wire rods with a diameter specification of 15mm through a rolling line, promoting the strain-induced precipitation of Nb and the dynamic recrystallization and grain refinement during the finishing rolling process. Specifically: control the soaking temperature of the heating furnace at 1250°C, the residence time in the furnace at 150min, the initial rolling temperature at 1120°C, the initial rolling reduction at 15%, the finishing rolling temperature at 1030°C, and the finishing rolling reduction at 25%; 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, providing favorable conditions for forming a large supercooling degree and promoting the rapid precipitation of sorbite tissue and carbide strengthening phases at a relatively high quenching temperature. Specifically: control the wire laying temperature at 970°C.

[0057] The online molten salt rapid cooling and isothermal 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 treatment, cooling the coiled bars at a cooling rate of 37°C / s, quickly skipping the reticulated carbide precipitation range from the high-temperature austenite state into the sorbite phase region, forming a structure mainly composed of fine lamellar spacing sorbite. Then the coiled bars are transported through the second-stage salt bath tank by the roller table for the back-stage treatment. The molten salt circulation volume in the back-stage treatment is appropriately reduced to accurately control the temperature and reduce production energy consumption, control the coiled bars to enter the high-temperature isothermal range, control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal tempering and toughening stress relief treatment on the sorbite tissue formed by quenching to improve the strength-ductility matching of the coiled bars. Specifically: the molten salt temperature in the front-stage treatment is 571°C, the treatment time is 83s, the molten salt circulation volume is 750t / h, and the molten salt temperature rise ≤ 9°C; the molten salt temperature in the back-stage treatment is 570°C, the treatment time is 132s, the molten salt circulation volume is 450t / h, and the molten salt temperature rise ≤ 3°C.

[0058] 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 and passing through the second-stage salt bath tank enter the heat preservation cover to prevent the stress of the coiled bars from increasing due to too fast cooling rate during the cooling process, promoting the further toughening of the coiled bar structure and improving the softening effect of the coiled bars. Specifically: control the coiled bars to slowly cool to 287°C at a cooling rate of 0.25°C / s; The coiling process is used to coil the coiled bars into coils through a coiling drum, and after packaging and warehousing, the hot-rolled coil bar finished products are obtained. Its metallographic structure diagram is as Figure 2 shown.

[0059] Comparative Example 2:

[0060] A manufacturing method of hot-rolled wire rod, the difference between the manufacturing method and that of Example 2 is that: during the on-line molten salt rapid cooling and isothermal treatment, the wire rod undergoes pre-treatment, so that the wire rod cools down at a cooling rate of 35 °C / s. The molten salt temperature of the pre-treatment is 595 °C, and the treatment time is 35 s. After being taken off the production line, the hot-rolled wire rod is obtained.

[0061] Comparative Example 3:

[0062] A manufacturing method of hot-rolled wire rod, the difference between the manufacturing method and that of Example 2 is that: during the on-line molten salt rapid cooling and isothermal treatment, the wire rod undergoes pre-treatment, so that the wire rod cools down at a cooling rate of 42 °C / s. The molten salt temperature of the pre-treatment is 537 °C, and the treatment time is 150 s. After being taken off the production line, the hot-rolled wire rod is obtained. Example 3:

[0063] A preferred implementation manner of the manufacturing method of the 2500 MPa grade 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: 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; its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt rapid cooling and isothermal treatment → slow cooling on the roller table → coiling. Specifically:

[0064] The rolling process is used to heat the steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet that reaches the plastic state for rolling, promoting the homogenization of alloy components. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 9 mm through the rolling line, promoting the strain-induced precipitation of Nb and the dynamic recrystallization and grain refinement during the finishing rolling process. Specifically: control the soaking temperature of the heating furnace to be 1200 °C, the residence time in the furnace to be 200 min, the initial rolling temperature to be 1095 °C, the initial rolling reduction to be 20%, the finishing rolling temperature to be 980 °C, and the finishing rolling reduction to be 30%; the wire laying process is used to make the wire rod exiting the rolling line into a wire rod through the wire laying machine. The wire rod is scattered on the roller table and transported along the roller table, so that the wire rod is in a high-temperature austenite state, providing favorable conditions for forming a large supercooling degree and promoting the rapid precipitation of sorbite structure and carbide strengthening phase in the future. Specifically: control the wire laying temperature to be 900 °C.

[0065] The online molten salt rapid cooling isothermal 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 pre-treatment, so that the wire rod cools down at a cooling rate of 33 °C / s, quickly skips the network carbide precipitation interval from the high-temperature austenite state and enters the sorbite phase region, forming a structure mainly composed of sorbite with a fine lamellar spacing. Then the wire rod is transported through the second-stage salt bath tank by a roller table for post-treatment. The circulation volume of the molten salt in the post-treatment is appropriately reduced, the temperature is accurately controlled, and the production energy consumption is reduced. The wire rod is controlled to enter the high-temperature isothermal interval, the precipitation and growth of carbides are controlled, and at the same time, the sorbite structure formed by quenching is subjected to high-temperature isothermal tempering and toughening stress relief treatment to improve the strength-plasticity matching of the wire rod. Specifically: the temperature of the molten salt in the pre-treatment is 585 °C, the treatment time is 100 s, the circulation volume of the molten salt is 550 t / h, and the temperature rise of the molten salt ≤ 9 °C; the temperature of the molten salt in the post-treatment is 565 °C, the treatment time is 149 s, the circulation volume of the molten salt is 350 t / h, and the temperature rise of the molten salt ≤ 3 °C.

[0066] In the roller table slow cooling process, the heat preservation cover is closed, and the hot air above the two-stage salt bath tank is input into the heat preservation cover. The wire rod transported by the conveying roller table and passing through the second-stage salt bath tank enters the heat preservation cover, preventing the stress of the wire rod from increasing 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: 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 by a coiling drum, and after packaging and warehousing, the finished product of the hot-rolled wire rod is obtained, and its metallographic structure diagram is as Figure 3 shown.

[0067] Comparative Example 4:

[0068] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 3 is that: the temperature of the molten salt in the post-treatment is 595 °C, the treatment time is 225 s, and the hot-rolled wire rod is obtained after being taken off the production line.

[0069] Comparative Example 5:

[0070] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 3 is that: the temperature of the molten salt in the post-treatment is 525 °C, the treatment time is 40 s, and the hot-rolled wire rod is obtained after being taken off the production line. Example 4:

[0071] A preferred embodiment of the manufacturing method of the hot-rolled wire rod for 2500MPa-class bridge cables according to 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 inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt rapid cooling isothermal treatment → roller table slow cooling → coiling. Specifically:

[0072] The rolling process is used to heat the steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the plasticity for rolling through a heating furnace to promote the homogenization of alloy components. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 11mm through the rolling line to promote the strain-induced precipitation of Nb and the dynamic recrystallization and grain refinement during the finish rolling process. Specifically: control the soaking temperature of the heating furnace to be 1220°C, the residence time in the furnace to be 190min, the rough rolling temperature to be 1105°C, the rough rolling reduction to be 19%, the finish rolling temperature to be 995°C, and the finish rolling reduction to be 28%. The wire laying process is used to make the wire rod exiting the rolling line into a wire coil through a 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, providing favorable conditions for forming a large supercooling degree and promoting the rapid precipitation of sorbite tissue and carbide strengthening phases in the future with a relatively high quenching temperature. Specifically: control the wire laying temperature to be 930°C.

[0073] The on-line molten salt rapid cooling isothermal 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 pre-treatment, so that the wire coil cools down at a cooling rate of 36°C / s, quickly skips the reticular carbide precipitation interval from the high-temperature austenite state and enters the sorbite phase region, forming a tissue mainly composed of fine lamellar spacing sorbite. Then the wire coil is transported through the second-stage salt bath tank by the roller table for post-treatment. The molten salt circulation volume in the post-treatment is appropriately reduced to accurately control the temperature and reduce production energy consumption. Control the wire coil to enter the high-temperature isothermal interval, control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal tempering and toughening stress relief treatment on the sorbite tissue formed by quenching to improve the strength-plasticity matching of the wire coil. Specifically: the molten salt temperature in the pre-treatment is 550°C, the treatment time is 50s, the molten salt circulation volume is 630t / h, and the molten salt temperature rise ≤ 9°C; the molten salt temperature in the post-treatment is 585°C, the treatment time is 100s, the molten salt circulation volume is 385t / h, and the molten salt temperature rise ≤ 3°C.

[0074] For 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 conveyed by the conveying roller table enter the heat preservation cover, preventing the stress of the wire rods from increasing 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 rods. Specifically: the wire rods are slowly cooled at a cooling rate of 0.2 °C / s to 279 °C; the coiling process is used to coil the wire rods into coils by a coiling drum, and the finished hot-rolled wire rod products are obtained after packaging and warehousing.

[0075] Comparative Example 6:

[0076] A manufacturing method of hot-rolled wire rods, the difference between its manufacturing method and that of Example 4 lies in: for the roller table slow cooling process, the heat preservation cover is not tightly closed, and the wire rods passing through the second salt bath tank conveyed by the conveying roller table enter the heat preservation cover. The wire rods are cooled at a cooling rate of 1.3 °C / s to 299 °C, and the hot-rolled wire rods are obtained after being taken off the production line. The tensile strength of the hot-rolled wire rods is 1741 MPa, the reduction of area is 23%, and the mechanical property difference within the same coil is 55 MPa.

[0077] The hot-rolled wire rods obtained from the above Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to microstructure and property tests, and the comparison results obtained are shown in Table 1 below:

[0078] Table 1. Comparison results of microstructure and properties of different hot-rolled wire rod compositions and manufacturing methods

[0079]

[0080] From the results of Examples 1 to 4, it can be seen that the present invention adopts the Nb-V-Mo chemical composition design combined with the online molten salt rapid cooling and isothermal technology, which can achieve a volume percentage of tempered sorbite and fusing sorbite ≥ 95%. The tensile strength of the products is 1692 - 1735 MPa, the reduction of area is 25% - 30%, and the brittle structure is effectively controlled. It is used in application fields such as manufacturing 2500 MPa grade ultra-high strength bridge cables, which is beneficial to reducing the risks of drawing and torsional wire breakage.

[0081] From the comparison results of Example 1 and Comparative Example 1, it can be seen that selecting an appropriate spinning temperature for the wire rods can avoid the formation of network carbides during the spinning stage due to too low spinning temperature, avoid the limitation of the rolling temperature control caused by too low spinning temperature, and increase the supercooling degree with a higher quenching temperature, providing favorable conditions for the rapid nucleation of sorbite and the dispersion precipitation of Nb and V carbides.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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 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: 0.95%-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 to make the wire rod ≥33 The temperature is cooled at a cooling rate of 0.1°C / s from the austenite state into the sorbite phase region, forming a structure dominated by sorbite and controlling the precipitation and growth of carbides, and isothermally toughening to relieve stress. Finally, it is slowly cooled through a roller to form a hot-rolled wire rod with a microstructure including a mixed structure composed of tempered sorbite, ferrite and fused sorbite; 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, and the molten salt circulation amount of the front-stage treatment is greater than the molten salt circulation amount of the back-stage treatment.

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 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.

5. The method for manufacturing the 2500MPa grade 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 0.2-0.45° C. / s.

6. 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 5.

7. The 2500MPa grade hot rolled wire rod for bridge cables according to claim 6, 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%.

8. The 2500MPa grade 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 2500MPa grade hot rolled wire rod for bridge cables according to claim 6, 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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