Hot-rolled wire rod for 2400 MPa bridge cable and manufacturing method of hot-rolled wire rod

Abstract

The invention relates to a hot-rolled wire rod for a 2400 MPa bridge cable and a manufacturing method of the hot-rolled wire rod, after high-carbon components of Cr-Mo-V are rolled and spun into a wire rod, the wire rod is subjected to online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at the cooling speed of more than or equal to 32 DEG C / s and enters a sorbite phase region from an austenite state to form a structure taking sorbite as a main part, and precipitation and growth of carbides are controlled; and finally, roller way slow cooling is conducted, the hot-rolled wire rod with the microscopic structure comprising a mixed structure composed of tempered sorbite, ferrite and fused pearlite is prepared, the strength and plasticity matching performance of the wire rod can be improved, the mechanical property fluctuation can be reduced, the tensile strength can reach 1640-1680 MPa, the percentage reduction of area can reach 27%-32%, the mechanical property same-circle difference is smaller than or equal to 45 MPa, and the tensile strength of the wire rod can reach 1640-1680 MPa, the percentage reduction of area can reach 27%-32%, and the mechanical property same-circle difference is smaller than or equal to 45 MPa. And rapid and stable production is facilitated, the method is used for manufacturing the 2400 MPa bridge cable, and the wire breaking risk can be reduced easily.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled wire rods, and in particular relates to a 2400MPa-grade hot-rolled wire rod for bridge cables and a manufacturing method thereof. Background Art

[0002] After being put into use, long-span bridges must not only bear the static load caused by their own weight, but also the dynamic load caused by vehicles passing by and bad weather. Therefore, long-span bridges are constantly developing in the direction of lightweight and ultra-high strengthening of bridge cables, which places extremely high requirements on the strength and plasticity of bridge cables used in long-span bridges. Ultra-high strengthening of bridge cables means ultra-high strengthening of the parent material wire rods for bridge cables. Existing parent material wire rods for bridge cables are often developed by alloying combined with air-cooling lines. There are still the following technical difficulties in manufacturing 2400MPa grade hot-rolled wire rods for bridge cables with higher strength and plasticity and smaller fluctuations in mechanical properties: 1. In order to improve the strength of wire rods for bridge cables, the alloying degree of wire rod components is relatively high. For example, patent CN110144521B discloses a high-strength and high-toughness bridge cable steel and a preparation method thereof. The wire rod is designed with C-Si-Mn-Co-V composition and is used to produce bridge cable steel with a tensile strength ≥ 2400Mpa. However, on the one hand, although the low carbon component can reduce the difficulty of controlling the network carbides of the wire rod on the air-cooling line, the alloy component content is relatively high and the material cost is relatively high; on the other hand, the high silicon, high manganese and high aluminum components make the risk of deteriorated structures such as martensite in the hot-rolled wire rod on the air-cooling line extremely high, which will lead to insufficient plastic toughness of the wire rod, large fluctuations in internal stress and mechanical properties, and easy to induce wire breakage during the wire drawing process.

[0003] Second, in order to take into account the material cost and improve the drawing performance of the wire rod, the wire rod for high-strength bridge cables in the prior art mostly adopts high-carbon pearlite steel. For example, the patent CN118880169A discloses a wire rod for high-strength bridge cables and its production method, which adopts 87SiMn component combined with low-temperature rolling and spinning, Stelmor air-cooled line cooling and more than 120 minutes of insulation corridor insulation to make a high sorbitization rate wire rod with a strength of more than 1400MPa, which is used for 2200~2300MPa strength level bridge cables. However, on the one hand, the 2400MPa-level bridge cables have higher requirements on the strength of the hot-rolled wire rod parent material. If the wire rod strength is insufficient, it is necessary to increase the drawing passes to improve the material strength. The plastic loss is large in the process, and the risk of wire breakage is prone to occur. In addition, the galvanizing process after drawing will bring a certain strength loss, and it is difficult to reach the 2400MPa-level bridge cable grade. In order to further strengthen the material, the carbon, silicon, manganese and other elements in the organization are further increased, which will increase the carbon element and alloy in the steel billet. Gold element segregation. Due to the limited maximum cooling capacity of the Stelmor air-cooling line, network carbides that are unfavorable to plasticity and organizational uniformity are more likely to form in the wire rod, and the cooling control capacity of the air-cooling line is unstable. In order to minimize the impact of network carbides and increase the air-cooling intensity, the temperature difference from the surface of the wire rod to the core will be further increased. Affected by segregation, the parts of the wire rod surface that cool too quickly are more likely to form hard and brittle abnormal organizations such as martensite, which increases the fluctuation of the mechanical properties of the wire rod and induces the problem of wire breakage during the wire rod drawing process. On the other hand, during the continuous cooling process, the wire rod passes through the phase transition temperature range for a short time, and the insufficient phase transition of the austenite organization will cause the wire rod to continue to form abnormal organizations during the subsequent cooling process. As the carbides coarsen and lose strong plastic properties during the phase transition, the wire rod is already in a low temperature state after the phase transition, which will increase the difficulty of softening. The organizational stress of the resulting troostite phase is also large, which will lead to insufficient plasticity of the wire rod. Long-term heat preservation will also make the wire rod online for too long, affecting production efficiency and cost. Summary of the invention

[0004] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 2400MPa-grade hot-rolled wire rod for bridge cables and a manufacturing method thereof, which can improve the strength-plasticity matching of the wire rod, reduce the fluctuation of mechanical properties, facilitate rapid and stable production, and be used for the manufacture of 2400MPa-grade bridge cables, which is beneficial to reducing the risk of wire breakage.

[0005] The technical solution adopted by the present invention to solve its technical problem is: A method for manufacturing a 2400MPa grade hot-rolled wire rod for bridge cables, the manufacturing method comprising: The hot-rolled wire rod is rolled to produce a wire rod according to its chemical composition, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.94% to 0.97%, Si: 0.70% to 0.90%, Mn: 0.85% to 1.05%, Cr: 0.57% to 0.67%, V: 0.040% to 0.055%, Mo: 0.25% to 0.45%, P≤0.015%, S≤0.015%, and the rest is Fe and inevitable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥930°C, it is subjected to an online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥32°C / s, entering the sorbite phase region from the austenite state, forming a sorbite-based structure and controlling the precipitation and growth of carbides, while isothermal toughening to relieve stress, and finally slowly cooled through a roller to produce a hot-rolled wire rod whose microstructure includes a mixed structure consisting of tempered sorbite, ferrite and fused pearlite.

[0006] The design basis of the chemical composition and mass percentage of the above hot rolled wire rod includes: (1) Carbon: The carbon element is an effective carbide strengthening element and austenite forming element. It is more economical than other elements. As the carbon content increases, the phase transition temperature will decrease, the bainite transformation will be delayed, and the sorbite microstructure phase transition during the online molten salt rapid cooling isothermal treatment will be promoted, thereby improving the material strength. After the hot-rolled wire rod is drawn into a steel wire bridge cable, it has a higher load-bearing capacity. 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 abnormal microstructure will increase, which is not conducive to the control of carbide precipitation. Therefore, in order to meet the high strength requirements of the 2400MPa bridge cable for the hot-rolled wire rod base material, control material costs, and reduce the difficulty of controlling network carbides and carbide precipitation, the mass percentage of C is controlled to be 0.94%~0.97%.

[0007] (2) Silicon: Si is a good deoxidizer. It can also inhibit the coarsening of grains during online molten salt rapid cooling isothermal treatment, shift the phase transformation curve to the right, and inhibit the diffusion of carbon in austenite, so that a troostite structure dominated by fine lamellar spacing can be quickly obtained, thereby improving the matrix strength. However, excessive silicon content will make the steel more susceptible to decarburization when heated at high temperatures, increase the risk of abnormal microstructure precipitation, reduce the toughness and microstructure uniformity of the steel, and be unfavorable for drawing. At the same time, it will increase the phase transformation temperature of the steel, which is unfavorable for the control of the dispersion and precipitation of vanadium-containing carbides in the troostite phase region. Therefore, in order to regulate the phase transformation temperature, promote the troostite phase transformation, and facilitate the control of carbide precipitation and growth, the Si content is appropriately increased, and the mass percentage of Si is controlled to be 0.70%~0.90%.

[0008] (3) Manganese: Mn has a strong affinity with carbon and can expand the austenite phase region, increase the stability of austenite, increase the hardenability of the wire rod, and reduce the phase transformation temperature. This is beneficial to the rapid nucleation of the troostite structure with fine interlamellar spacing and the dispersion and precipitation of vanadium-containing carbides during the online molten salt rapid cooling isothermal treatment, thereby improving the tensile strength of the wire rod, so that the bridge cable can withstand impact and vibration loads while bearing tension. However, when the Mn content is too high, it will aggravate the segregation of alloy elements and increase the risk of precipitation of low-temperature structures such as bainite and martensite in the wire rod. At the same time, it will reduce the activity of carbon, increase the difficulty of isothermal toughening and stress relief, and then lose the plasticity of the wire rod. Therefore, in order to make the hot-rolled wire rod have higher strength and reduce the difficulty of organizational uniformity, tempering control and carbide precipitation control, the Mn content is appropriately increased, and the mass percentage of Mn is controlled to be 0.85%~1.05%.

[0009] (4) Chromium: Cr is a strong carbide-forming element that can refine grains, improve the hardenability of steel, increase the stability of austenite, shift the phase transformation curve to the right, and is beneficial to the formation of troostite structure during cooling, which is beneficial to improving the matrix strength and reducing the strength loss during the subsequent hot-dip galvanizing process of steel wire. However, too high Cr content will aggravate component segregation, increase the risk of abnormal martensite structure precipitation, affect the uniformity of wire rod structure, and significantly increase the difficulty of improving wire rod plasticity, affecting the isothermal toughening stress relief effect, and further affecting the wire drawing and torsion performance of steel wire. The precipitation and coarsening of Cr will also cause loss of strong and plastic properties, causing cracks and drawing fractures. Therefore, in order to improve material strength and facilitate carbide precipitation control, the mass percentage of Cr is controlled to be 0.57%~0.67%.

[0010] (5) Vanadium: As a microalloying element, the V element can inhibit the coarsening of high-temperature austenite, hinder the growth of austenite grains, and thus refine the grains. At the same time, it can be dispersed and precipitated during the isothermal process. The fine dispersed vanadium precipitate phase can pin dislocations and grain boundaries, hinder dislocation movement and grain growth, and effectively improve the strength of the hot-rolled wire rod without reducing plasticity. However, the cost of the V element is relatively high. Excessive addition is not conducive to controlling the cost of the wire rod and has the risk of coarsening. When the vanadium precipitate phase coarsens, the inhibitory effect on dislocations and grain boundaries is weakened, and dislocations are more likely to bypass the precipitate phase, and the strength of the wire rod is reduced. At the same time, the coarsened vanadium precipitate phase may become a crack source, reducing the toughness of the material. Therefore, based on the role of the V element and cost considerations, the present invention controls the V element content to 0.040%~0.055%.

[0011] (6) Molybdenum: Mo can improve the hardenability of steel, increase the stability of supercooled austenite, shift the C curve to the right, slow down the transformation of austenite to ferrite, and effectively inhibit the coarsening of carbide precipitation phase, effectively improve the strengthening effect of precipitation phase, and maintain high strength during tempering. However, Mo is expensive. Therefore, based on the role of Mo and cost considerations, the mass percentage of Mo is controlled to 0.25%~0.45%.

[0012] (7) 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%.

[0013] The above-mentioned hot-rolled wire rod adopts the high-carbon chemical composition design of Cr-Mo-V, and further regulates the C curve and phase transition temperature by optimizing the Si, Mn and Cr components, which provides favorable conditions for promoting the nucleation of troostite with finer interlamellar spacing, reducing the difficulty of carbide precipitation and growth control, and reducing the difficulty of abnormal organization and isothermal stress relief when the wire rod undergoes online molten salt rapid cooling isothermal treatment. On this basis, a higher spinning temperature is selected to avoid the formation of network carbides in the spinning stage due to low-temperature spinning of the wire rod, and at the same time facilitates the improvement of the strength and plasticity matching of the material at a higher quenching temperature. The wire rod after spinning is not air-cooled but is isothermally treated with online molten salt rapid cooling: 1. Compared with the Stelmore air-cooled line, which has a limited maximum cooling capacity and uncontrollable wire rod cooling rate, abnormal structures such as network carbon and martensite are generated, which affect the refinement of the sorbite lamellae. On the one hand, the online molten salt rapid cooling isothermal can utilize the high heat exchange capacity of the molten salt to promote the rapid cooling of the coil from the high-temperature austenite state, and quickly pass through the secondary cementite precipitation temperature range of 700~800℃, so as to avoid the formation of network carbides in the structure, destroy the uniformity of the structure, and increase the brittleness of the wire rod; on the other hand, with the higher spinning temperature, the hardenability of the wire rod and the use of Si to inhibit grain coarsening, the wire rod can quickly enter the sorbite phase region for quenching, forming a large degree of undercooling , thereby promoting the incubation of troostite phase transformation and forming a structure dominated by troostite with fine lamellar spacing, in order to compensate for the adverse effect of the longer time required for the transformation of supercooled austenite after the C curve shifts to the right, thereby improving material strength and reducing online time. 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 rapid and uniform heat exchange, and there is no temperature difference problem between the windward side and the leeward side. The temperature difference from the upper edge to the core of the wire rod section can be further reduced, thereby avoiding the formation of abnormal martensitic structure of the wire rod that is unfavorable to the uniformity of the structure and drawing, improving the control of abnormal structure by high alloy content and element segregation, and reducing the fluctuation of mechanical properties.

[0014] 2. Compared with the Stelmore air-cooled line, which has a limited minimum cooling capacity, uncontrollable wire rod cooling rate, and the continuous cooling of the wire rod weakens the carbide strengthening effect, the obtained troostite structure has a low content and insufficient plasticity. On the one hand, the wire rod can be kept consistent with the molten salt temperature after molten salt treatment. Compared with the continuous cooling of the wire rod, it can prolong the time when the wire rod is at the troostite peak precipitation temperature, promote the full transformation of austenite to troostite structure, reduce the ferrite content, and improve the matrix strength. It can also promote the precipitation of a large number of carbides in the medium temperature range, and cooperate with Mo to inhibit the coarsening of the carbide precipitation phase, avoiding the formation of coarser and finer precipitation phases at the same time due to the uncontrollable wire rod cooling rate and continuous cooling, thereby taking into account the uniformity of the structure and effectively improving the strengthening effect of the precipitation phase. , improve the matrix strength. On the other hand, the temperature of the troostite phase region is relatively high, which can extend the time of the wire rod in the higher temperature isothermal zone, and carry out high-temperature isothermal toughening treatment on the troostite structure obtained by quenching, so as to promote the melting of the troostite structure lamellae to form melting pearlite and tempering to form tempered troostite structure with better plasticity, reduce the tissue stress, and improve the strength-plasticity matching of the wire rod. Longer insulation time in the insulation corridor can also reduce the online time of the wire rod and promote the rapid offline of the wire rod. The temperature of the wire rod after the online molten salt rapid cooling isothermal treatment is relatively high. Reducing the cooling rate of the wire rod can prevent the stress of the wire rod from increasing due to excessive cooling during the cooling process, further promote the toughening of the wire rod structure, and realize the tissue regulation and high-strength-plasticity matching of the wire rod.

[0015] Selecting a higher heating furnace soaking temperature and furnace time before rolling can promote uniform diffusion of alloy elements in the steel billet, reduce the deformation resistance of the material, and prepare for rolling. In the preferred technical solution, before rolling, the heating furnace soaking temperature is controlled to be 1180~1240℃, and the furnace time is ≥150min.

[0016] Due to the high spinning temperature, the restriction on rolling temperature can be reduced. A higher initial rolling temperature is selected to reduce the rolling force, so that the steel billet can be deformed more evenly when subjected to force. The final rolling temperature and final rolling reduction are coordinated to promote dynamic recrystallization during the final rolling process, refine the grains, and improve the strength and toughness of the matrix. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1060~1100℃, the final rolling temperature is 900~950℃, and the final rolling reduction is 23%~29%.

[0017] 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 555-580°C, the treatment time is 25-35s, and the molten salt circulation volume of the front-stage treatment is greater than that of the back-stage treatment. The lower the molten salt temperature of the front-stage treatment and the longer the treatment time, the greater the degree of supercooling can be formed, which promotes the full transformation of the high-temperature austenite structure to the troostite structure with finer interlamellar spacing, provides more driving force for the dispersion and precipitation of vanadium-containing carbides, and improves the matrix strength. However, the molten salt temperature of the front-stage treatment is too low, and there is a risk of precipitating bainite structure, and at the same time, it is lower than the precipitation of vanadium-containing carbides. The medium temperature range is not conducive to the precipitation of carbides. If the treatment time is too long, the holding time of the larger molten salt circulation volume will increase, increasing the production energy consumption; conversely, the higher the molten salt temperature of the front-stage treatment and the shorter the treatment time, the lower the quenching rate and the lower the wire rod production energy consumption. However, if the molten salt temperature is too high and the treatment time is too short, it will be unfavorable for the rapid phase transformation of high-temperature austenite to troostite. The spacing between troostite lamellae will increase, which will increase the difficulty of melting and lose the matrix strength. Therefore, the molten salt temperature and treatment time of the front-stage treatment can be controlled to promote the phase transformation of the wire rod to form a structure dominated by troostite with fine lamellae spacing, taking into account the production energy consumption and making organizational preparations for the back-stage treatment.

[0018] Since the temperature difference of the wire rod from the spinning temperature to the troostite phase region is large, selecting a larger molten salt circulation rate can control the molten salt temperature rise, improve the uniformity of the structure and reduce the fluctuation of the mechanical properties of the wire rod. In the preferred technical scheme, the molten salt circulation rate of the front-stage treatment is 500~700t / h, and the molten salt temperature rise is ≤8℃.

[0019] In the preferred technical solution, the molten salt temperature of the later stage treatment is 560-570°C, and the treatment time is 150-250s. The lower the molten salt temperature of the later stage treatment, the lower the tendency of carbide precipitation and growth, and the dispersion and precipitation of vanadium-containing carbides can be promoted. However, if the molten salt temperature is too low, it is difficult to increase the thermal capacity of isothermal toughening stress relief, which affects the plasticity of the wire rod. On the contrary, if the molten salt temperature of the later stage treatment is higher, the more flakes are melted, the better the stress relief and softening effect, and the plasticity of the wire rod is improved. However, if the molten salt temperature is too high, the precipitation of vanadium-containing carbides is affected, the carbide precipitation is coarsened, and the strength is increased. The loss is accelerated, which is not conducive to the strength and plasticity properties; the longer the treatment time of the later stage, the greater the toughening and stress relief effect, but if the treatment time is too long, there is a risk of carbide coarsening and excessive strength loss; conversely, the shorter the treatment time of the later stage, the lower the isothermal toughening effect, but if the treatment time is too short, the carbides will not have time to fully precipitate, and there will be high stress in the organization, which will lose the strength and plasticity properties. Therefore, the molten salt temperature and treatment time of the later stage molten salt can be further controlled to control the sufficient dispersion and precipitation of carbides, to isothermally toughen the quenched organization, and to improve the strength and plasticity matching of the wire rod.

[0020] The post-processing uses an appropriate molten salt circulation volume to reduce production energy consumption, while avoiding too small a molten salt circulation volume, accurately controlling the temperature, reducing the temperature rise of the molten salt, and further controlling the carbide precipitation and tempering effect. In the preferred technical solution, the molten salt circulation volume of the post-processing is 150~250t / h, and the molten salt temperature rise is ≤3°C.

[0021] Since the wire rod undergoes sufficient phase transformation after the online molten salt rapid isothermal treatment, the residual austenite can be prevented from further transforming into martensite structure. The roller slow cooling selects an appropriate cooling rate to prevent the wire rod from increasing stress due to excessively fast cooling rate during the cooling process, promotes further toughening of the wire rod structure, improves the softening effect of the wire rod and allows it to be quickly offline. In the preferred technical solution, the roller slow cooling controls the wire rod to slowly cool to below 300°C at a cooling rate of 0.5~1°C / s.

[0022] A 2400MPa hot-rolled wire rod for bridge cables, wherein the hot-rolled wire rod is manufactured by any one of the above-mentioned methods for manufacturing the 2400MPa hot-rolled wire rod for bridge cables.

[0023] The above-mentioned hot-rolled wire rod adopts Cr-Mo-V chemical composition design combined with online molten salt rapid cooling isothermal technology to produce a microstructure including a mixed structure composed of tempered troostite, ferrite and fused pearlite. Compared with traditional pearlite high carbon steel, the interlamellar spacing of troostite is finer than that of pearlite, and the strength and drawing hardening ability are higher. After the troostite structure is transformed into tempered troostite and fused pearlite through isothermal toughening and tempering, the tissue stress is reduced, the toughness and plasticity are significantly improved, and the comprehensive mechanical properties are better. Through online molten salt rapid cooling isothermal treatment, the abnormal tissues of network carbides and martensite are effectively controlled, the troostite fully transforms to reduce the proportion of soft phase ferrite in the tissue, and the carbides can be dispersed and precipitated to avoid coarsening, which can improve the strengthening effect and tissue uniformity of carbon, V and Cr elements, improve the strength and plasticity matching of the wire rod, and reduce the fluctuation of the mechanical properties of the wire rod.

[0024] The finer the interlamellar spacing of the tempered troostite, the higher the matrix strength. In a preferred technical solution, the interlamellar spacing of the tempered troostite is 70-100 nm, and the volume percentage of the tempered troostite is ≥74%.

[0025] The higher the volume percentage of the tempered troostite and the fused pearlite, the better the strength and plasticity. In the preferred technical solution, the volume percentage of the tempered troostite and the fused pearlite is ≥94%, and the volume percentage of the fused pearlite is 16%~20%.

[0026] In a preferred technical solution, the network carbide level of the hot-rolled wire rod is level 0, which can prevent the network carbide from weakening the bonding force between metals and avoid the adverse effect of the network carbide on plasticity.

[0027] The uniformity of the hot-rolled wire rod's structure is improved, while abnormal structures such as network carbides and martensite are avoided, and the mechanical properties fluctuate less. In the preferred technical solution, the same-circle difference in the mechanical properties of the hot-rolled wire rod is ≤45MPa.

[0028] In the preferred technical solution, the diameter of the hot-rolled wire rod is 10.0~16.0mm, the tensile strength is 1640~1680MPa, and the cross-sectional shrinkage rate is 27%~32%. The hot-rolled wire rod has high tensile strength and good cross-sectional shrinkage rate, which can reduce the drawing reduction rate and plastic loss during the process, reduce the risk of fracture during drawing and torsion, and is conducive to the stable manufacture of 2400MPa-level bridge cables.

[0029] Compared with the prior art, the beneficial effects of the present invention are at least: (1) In view of the fact that the existing wire rods for bridge cables have a high degree of alloying, and the risk of deterioration of network carbides, martensite and other deteriorated structures on the air-cooled line is extremely high, which easily induces wire breakage during the wire drawing process, the present invention combines the Cr-Mo-V chemical composition design with the online molten salt rapid cooling isothermal technology, and uses a higher quenching temperature and molten salt to control the wire rod from the high-temperature austenite state to quickly pass through the network carbide formation zone and enter the sorbite phase zone, forming a structure dominated by sorbite with fine lamellar spacing, and controls the wire rod in the isothermal zone at a higher temperature. Controlling the precipitation and growth of carbides can avoid the occurrence of abnormal martensitic structure, enhance the strengthening effect of carbon elements, promote the dispersion and precipitation of vanadium-containing carbides in the isothermal process, inhibit the coarsening of V and Cr carbide precipitation phases, and enhance the strengthening effect of the precipitation phase. At the same time, the sorbite structure obtained by quenching is subjected to high-temperature isothermal toughening and stress relief treatment, and finally the roller is slowly cooled to prevent stress increase and promote further toughening of the wire rod structure, which can improve the strength and plasticity matching of the wire rod, reduce the fluctuation of mechanical properties, facilitate rapid and stable production, and has good industrial adaptability.

[0030] (2) In view of the current situation that the ultra-high strengthening of ultra-high bridge cables is limited by the base material wire rod, and it is difficult to meet the stable production of 2400MPa bridge cables, the hot-rolled wire rod microstructure of the present invention includes a mixed structure composed of tempered troostite, ferrite and fused pearlite, which can improve the strengthening effect and organizational uniformity of carbon, V and Cr elements, improve the matching of strength and plasticity of the wire rod, and reduce the fluctuation of mechanical properties of the wire rod, so as to achieve a product tensile strength of 1640~1680MPa and a cross-sectional shrinkage rate of 27%~32%. It is used in the manufacture of 2400MPa ultra-high strength bridge cables and other application fields, can reduce the risk of fracture during drawing and torsion, promote the stable production of bridge cables, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which: Figure 1 is a metallographic structure diagram of Example 1 of the present invention; Figure 2 is the metallographic structure diagram of Example 2 of the present invention; Figure 3 It is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0032] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention, so as to propose the best way to implement the present invention, and are intended to be used to explain the present invention and are sufficient to enable those skilled in the art to implement the present invention, but cannot be understood as any limitation on the scope of the present invention, which is limited only by the appended claims; the organization and performance testing of the hot-rolled wire rods obtained in the following embodiments and comparative examples include: the tensile test is carried out using "GB-T 228.1-2021 Metallic Material Tensile Test Part 1: Room Temperature Test Method" to obtain the tensile strength and cross-sectional reduction rate; the organization test is carried out according to the metal microstructure detection method of GB / T13298 standard; the mechanical property same circle difference test method: take 2 circles of wire rod 5m away from the end of the coil, take the overlap area position as the base point, divide each circle of wire rod into 8 sections on average, take 1 tensile specimen on each section, and the strength extreme difference of the tensile specimen after the tensile test is the mechanical property same circle difference. Embodiment 1:

[0033] A preferred embodiment of the manufacturing method of the 2400MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.95%, Si: 0.88%, Mn: 1.05%, Cr: 0.67%, V: 0.043%, Mo: 0.33%, P: 0.014%, S: 0.015%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 16mm through a rolling line to promote dynamic recrystallization and grain refinement during the final rolling process. Specifically: the heating furnace is heated to 1225°C, the furnace time is 160min, the initial rolling temperature is 1090°C, the final rolling temperature is 940°C, and the final rolling reduction is 23%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large degree of undercooling, promoting the refinement of the sorbite structure layer and rapid nucleation, specifically: the wire-spinning temperature is controlled to be 945°C.

[0034] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 37°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and reduced. Low production energy consumption, control the wire rod in the isothermal range and control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal toughening and stress relief treatment on the quenched troostite structure to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-end treatment is 566℃, the treatment time is 27s, the molten salt circulation volume is 645t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the back-end treatment is 563℃, the treatment time is 225s, the molten salt circulation volume is 180t / h, and the molten salt temperature rise is ≤3℃.

[0035] The roller slow cooling process uses an open insulation cover to transport the wire rod through the second salt bath tank by a conveyor roller to prevent the wire rod from increasing stress due to excessive cooling speed during the cooling process, and promote further toughening of the wire rod structure and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to slowly cool to 284°C at a cooling rate of 1°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0036] Comparative Example 1: A method for manufacturing a hot-rolled wire rod, the manufacturing method differs from that of Example 1 in that: the spinning temperature is controlled to be 900°C, the wire rod is subjected to a front-end treatment during the online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at a cooling rate of 31°C / s, and the hot-rolled wire rod is obtained after it comes off the line. Embodiment 2:

[0037] A preferred embodiment of the manufacturing method of the 2400MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.95%, Si: 0.7%, Mn: 0.95%, Cr: 0.64%, V: 0.04%, Mo: 0.25%, P: 0.014%, S: 0.01%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 10mm through a rolling line to promote dynamic recrystallization and grain refinement during the final rolling process. Specifically: the heating furnace is heated to 1180°C, the furnace time is 180min, the initial rolling temperature is 1060°C, the final rolling temperature is 900°C, and the final rolling reduction is 29%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large degree of undercooling, promoting the refinement of the sorbite structure layer and rapid nucleation, specifically: the wire-spinning temperature is controlled to be 930°C.

[0038] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 32°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and reduced. Low production energy consumption, control the wire rod in the isothermal range and control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal toughening and stress relief treatment on the quenched troostite structure to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-end treatment is 580℃, the treatment time is 35s, the molten salt circulation volume is 500t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the back-end treatment is 570℃, the treatment time is 150s, the molten salt circulation volume is 250t / h, and the molten salt temperature rise is ≤3℃.

[0039] The roller slow cooling process uses an open insulation cover to transport the wire rod through the second salt bath tank by a conveyor roller to prevent the wire rod from increasing stress due to excessive cooling speed during the cooling process, and promote further toughening of the wire rod structure and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to slowly cool to 295°C at a cooling rate of 0.5°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0040] Comparative Example 2: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 2 in that the molten salt temperature of the front-end treatment is 595° C., the treatment time is 10 s, and the hot-rolled wire rod is obtained after going offline.

[0041] Comparative Example 3: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 2 in that the molten salt temperature of the front-end treatment is 520° C., the treatment time is 50 s, and the hot-rolled wire rod is obtained after going offline. Embodiment 3:

[0042] A preferred embodiment of the manufacturing method of the 2400MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.94%, Si: 0.82%, Mn: 0.9%, Cr: 0.57%, V: 0.052%, Mo: 0.41%, P: 0.015%, S: 0.015%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 12mm through a rolling line to promote dynamic recrystallization and grain refinement during the final rolling process. Specifically: the heating furnace is heated to 1210°C, the furnace time is 170min, the initial rolling temperature is 1080°C, the final rolling temperature is 915°C, and the final rolling reduction is 27%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large degree of undercooling, promoting the refinement of the sorbite structure layer and rapid nucleation, specifically: the wire-spinning temperature is controlled to be 935°C.

[0043] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 34°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and reduced. Low production energy consumption, control the wire rod in the isothermal range and control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal toughening and stress relief treatment on the quenched troostite structure to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-end treatment is 573°C, the treatment time is 30s, the molten salt circulation volume is 570t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the back-end treatment is 566°C, the treatment time is 185s, the molten salt circulation volume is 215t / h, and the molten salt temperature rise is ≤3°C.

[0044] The roller slow cooling process uses an open insulation cover to transport the wire rod through the second salt bath tank by a conveyor roller to prevent the wire rod from increasing stress due to excessive cooling speed during the cooling process, and promote further toughening of the wire rod structure and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to slowly cool to 287°C at a cooling rate of 0.8°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 3 shown.

[0045] Comparative Example 4: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 3 in that the molten salt temperature of the post-treatment is 595° C., the treatment time is 300 s, and the hot-rolled wire rod is obtained after the production line is closed.

[0046] Comparative Example 5: A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 3 in that the molten salt temperature of the post-treatment is 515° C., the treatment time is 100 s, and the hot-rolled wire rod is obtained after going offline. Embodiment 4:

[0047] A preferred embodiment of the manufacturing method of the 2400MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.97%, Si: 0.9%, Mn: 0.85%, Cr: 0.61%, V: 0.055%, Mo: 0.45%, P: 0.015%, S: 0.015%, and the rest are Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote the homogenization of alloy components, and after the steel billet comes out of the heating furnace, roll the steel billet into a wire with a diameter of 14mm through a rolling line to promote dynamic recrystallization and grain refinement during the final rolling process. Specifically: the heating furnace is heated to 1240°C, the furnace time is 150min, the initial rolling temperature is 1100°C, the final rolling temperature is 950°C, and the final rolling reduction is 25%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller, so that the wire rod is in a high-temperature austenite state, and a higher quenching temperature is used to provide favorable conditions for the subsequent formation of a large degree of undercooling, promoting the refinement of the sorbite structure layer and rapid nucleation, specifically: the wire-spinning temperature is controlled to be 955°C.

[0048] The online molten salt fast cooling isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 39°C / s, and quickly passes through the network carbide precipitation interval from the high-temperature austenite state to enter the sorbite phase region, forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-end treatment. The molten salt circulation volume of the rear-end treatment is appropriately reduced, and the temperature is accurately controlled and reduced. Low production energy consumption, control the wire rod in the isothermal range and control the precipitation and growth of carbides, and at the same time perform high-temperature isothermal toughening and stress relief treatment on the quenched troostite structure to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-end treatment is 555℃, the treatment time is 25s, the molten salt circulation volume is 700t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the back-end treatment is 560℃, the treatment time is 250s, the molten salt circulation volume is 150t / h, and the molten salt temperature rise is ≤3℃.

[0049] The roller slow cooling process uses an open insulation cover to transport the wire rod through the second salt bath tank by a conveyor roller to prevent the wire rod from increasing stress due to excessive cooling speed during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. Specifically: the wire rod is controlled to slowly cool to 290°C at a cooling rate of 0.7°C / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage.

[0050] Comparative Example 6: A method for manufacturing a hot-rolled wire rod, which differs from Example 4 in that: the method is manufactured according to the process flow of rolling → spinning → online molten salt rapid cooling isothermal treatment → air cooling → coiling, and the air cooling is carried out by opening a heat preservation cover, controlling the wire rod to be cooled to 282°C at a cooling rate of 1.9°C / s, and obtaining the hot-rolled wire rod after it is off the line. The hot-rolled wire rod has a tensile strength of 1689MPa, a cross-sectional shrinkage rate of 24%, and a mechanical property same-circle difference of 56MPa.

[0051] The hot rolled wire rods obtained in the above examples 1 to 4 and comparative examples 1 to 5 were subjected to microstructure and performance tests, and the comparative results obtained are shown in Table 1 below: Table 1. Comparison of microstructure and performance of hot rolled wire rods with different compositions and manufacturing methods

[0052] It can be seen from the results of Examples 1 to 4 that the present invention combines the Cr-Mo-V chemical composition design with the online molten salt rapid cooling isothermal technology, and the microstructure of the hot-rolled wire rod includes a mixed structure composed of tempered troostite, ferrite and fused pearlite, which can improve the strength-plasticity matching of the wire rod and reduce the fluctuation of the mechanical properties of the wire rod, so that the product tensile strength is 1640~1680MPa and the cross-sectional shrinkage rate is 27%~32%.

[0053] From the comparison results of Example 1 and Comparative Example 1, it can be seen that the selection of a higher spinning temperature facilitates the improvement of the strength-plasticity matching of the material at a higher quenching temperature.

[0054] 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 quenching rate and the energy consumption of wire rod production. However, if the molten salt temperature is too high and the treatment time is too short, it is not conducive to the rapid phase transformation of high-temperature austenite to troostite. The spacing between troostite lamellae increases, which increases the difficulty of melting and affects the strength and plasticity of the matrix.

[0055] From the comparison results of Example 2 and Comparative Example 3, 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 lower the molten salt temperature of the front-stage treatment and the longer the treatment time, the greater the supercooling can be formed, which promotes the full transformation of the high-temperature austenite structure to the troostite structure with finer interlamellar spacing, provides more driving force for the dispersion and precipitation of vanadium-containing carbides, and improves the matrix strength. However, if the molten salt temperature of the front-stage treatment is too low and the treatment time is too long, the maintenance time of the larger molten salt circulation amount will increase, increasing production energy consumption.

[0056] From the comparison results of Example 3 and Comparative Example 4, it can be seen that the higher the molten salt temperature of the later stage treatment and the longer the treatment time, the more the flake melting will increase, the better the stress relief softening effect will be, and the plasticity of the wire rod will be improved. However, if the molten salt temperature is too high and the treatment time is too long, there is a risk of carbide coarsening, excessive strength loss, and affecting plasticity.

[0057] 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 lower the tendency of carbide precipitation and growth, and the dispersion and precipitation of vanadium-containing carbides can be promoted. However, if the molten salt temperature is too low and the treatment time is too short, it is difficult to increase the thermal capacity of isothermal toughening stress relief. The carbides will not be fully precipitated in time, and higher stress will exist in the organization, which will lead to the loss of strength and plasticity.

[0058] From the comparison results of Example 4 and Comparative Example 6, it can be seen that the temperature of the wire rod after online molten salt rapid cooling isothermal treatment is relatively high. The use of roller slow cooling can prevent the wire rod from increasing stress due to excessive cooling during the cooling process, further promoting the toughening of the wire rod structure.

[0059] 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 2400MPa grade hot-rolled wire rod for bridge cables, characterized in that: The manufacturing method thereof comprises: The hot-rolled wire rod is rolled to produce a wire rod according to its chemical composition, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.94% to 0.97%, Si: 0.70% to 0.90%, Mn: 0.85% to 1.05%, Cr: 0.57% to 0.67%, V: 0.040% to 0.055%, Mo: 0.25% to 0.45%, P≤0.015%, S≤0.015%, and the rest is Fe and inevitable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥930°C, it is subjected to an online molten salt rapid cooling isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥32°C / s, entering the sorbite phase region from the austenite state, forming a sorbite-based structure and controlling the precipitation and growth of carbides, while isothermal toughening to relieve stress, and finally slowly cooled through a roller to produce a hot-rolled wire rod whose microstructure includes a mixed structure consisting of tempered sorbite, ferrite and fused pearlite.

2. The method for manufacturing the 2400MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: Before the rolling, the heating furnace is controlled to have a soaking temperature of 1180-1240° C. and the time in the furnace is ≥150 min.

3. The method for manufacturing the 2400MPa 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 1060-1100° C., the final rolling temperature is controlled to be 900-950° C., and the final rolling reduction is controlled to be 23%-29%.

4. The method for manufacturing the 2400MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: The online molten salt rapid cooling isothermal treatment is divided into a front-end treatment and a back-end treatment. The molten salt temperature of the front-end treatment is 555-580° C. and the treatment time is 25-35 seconds; the molten salt temperature of the back-end treatment is 560-570° C. and the treatment time is 150-250 seconds.

5. The method for manufacturing the 2400MPa grade hot rolled wire rod for bridge cables according to claim 4, characterized in that: The molten salt circulation volume of the front-end treatment is 500-700 t / h, and the molten salt temperature rise is ≤8°C; the molten salt circulation volume of the back-end treatment is 150-250 t / h, and the molten salt temperature rise is ≤3°C.

6. The method for manufacturing the 2400MPa grade hot rolled wire rod for bridge cables according to claim 4, characterized in that: The roller slow cooling controls the wire rod to be slowly cooled to below 300° C. at a cooling rate of 0.5-1° C. / s.

7. A 2400MPa grade hot-rolled wire rod for bridge cables, characterized in that: The hot-rolled wire rod is manufactured by the method for manufacturing 2400MPa-grade hot-rolled wire rod for bridge cables according to any one of claims 1 to 6.

8. The 2400MPa grade hot rolled wire rod for bridge cables according to claim 7, characterized in that: The interlamellar spacing of the tempered troostite is 70-100 nm, and the volume percentage of the tempered troostite and the fused pearlite is ≥94%.

9. The 2400MPa grade hot rolled wire rod for bridge cables according to claim 7, characterized in that: The network carbide grade of the hot-rolled wire rod is grade 0, and the mechanical property difference is ≤45MPa.

10. The 2400MPa grade hot rolled wire rod for bridge cables according to claim 7, characterized in that: The hot-rolled wire rod has a diameter of 10.0-16.0 mm, a tensile strength of 1640-1680 MPa, and a cross-sectional shrinkage of 27%-32%.

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