High-strength complex-phase hot-rolled wire rod for 2460mpa grade strand and manufacturing method thereof

Abstract

This invention relates to a high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire and its manufacturing method. The method involves rolling high-carbon Cr-V wire rods into wire rods, followed by online molten salt rapid quenching and isothermal toughening treatment. The wire rod undergoes a preliminary molten salt treatment followed by rapid cooling, transitioning from the austenitic state to the bainitic phase region, promoting partial austenite transformation to bainite. A subsequent molten salt treatment increases the molten salt temperature and reduces the molten salt circulation rate, promoting the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, comprising a multiphase microstructure. This method simplifies the composition system, enables multiphase microstructure control and online toughening, achieving a tensile strength of 1606~1666MPa and a reduction of area of ​​26%~32%, while also considering production efficiency. This eliminates the need for offline heat treatment, promoting efficient production of ultra-high-strength stranded wire.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire and its manufacturing method. Background Technology

[0002] Stranded wire is made from hot-rolled wire rod as the base material, processed through drawing, twisting, and other processes. Developing ultra-high-strength stranded wire can help promote the upgrading of transportation, engineering, and other fields towards lightweight and long-life applications. However, given the current rolling and controlled-cooling production line capabilities of steel mills, the hot-rolled wire rod used for high-strength stranded wire is generally pearlitic, with insufficient initial strength and limited draw hardening capacity. With the addition of carbon and alloy components in the steel grade, network carbides and brittle structures become more difficult to control, significantly increasing the risk of wire breakage during subsequent drawing. Therefore, the development of 2200MPa-grade ultra-high-strength stranded wire is rare, and the development of 2400MPa-grade and above ultra-high-strength stranded wire is even less common. Furthermore, offline heat treatment is required to reheat the hot-rolled wire rod for austenitization, which also leads to high energy consumption, long processes, and high costs in stranded wire production. As the special steel industry moves towards green, low-carbon, and high-quality development, there is a need to develop a 2460MPa-grade hot-rolled wire rod for stranded wire and its manufacturing method that can simplify the composition system, control the microstructure and toughness online, and eliminate the need for additional heat treatment, thus meeting the development needs of the steel industry and market demands.

[0003] Existing high-strength stranded wire rods generally adopt a high-carbon, high-silicon microalloying composition system and are produced using a Steyrmo air-cooling line after rolling. However, the following technical bottlenecks still exist in manufacturing hot-rolled wire rods for 2460MPa grade stranded wire:

[0004] I. To improve the strength of steel, the content of elements such as C, Si, and Mn is increased in wire rod, and alloying elements such as V, B, and Nb are added. However, on the one hand, the segregation of carbon and alloying elements during the solidification of the billet is intensified. Due to the limitation of the maximum cooling capacity of the Stellmore air-cooling line, a network of hard and brittle carbide phases will precipitate in the central carbon-rich area, affecting the plasticity and toughness of the wire rod and becoming a crack source during cold drawing. In order to minimize the influence of network carbides and promote the reduction of pearlite lamellar spacing to increase the matrix strength, the air-cooling intensity is increased. However, this will further increase the temperature difference between the air-receiving and air-receiving surfaces and between the surface and the core. The alloy carbide precipitation is uneven and has limited effect on improving strength and toughness. Although water bath cooling can increase the cooling rate of the wire rod, it will generate a large number of bubbles that adhere to the surface of the wire rod, affecting heat transfer. Due to the influence of permeability, localized overcooling can easily form uncontrollable hard and brittle structures such as bainite and martensite. These hard and brittle structures cannot be toughened during subsequent continuous cooling, leading to a sharp decrease in wire plasticity and increased fluctuations in mechanical properties, further exacerbating the risk of wire breakage during drawing. On the other hand, in the high-silicon chromium-containing system, the phase transformation incubation is prolonged. Due to the limited length and minimum cooling capacity of the Stellmore air-cooled wire, the phase transformation incubation time of the wire is relatively short, making it difficult to achieve a complete phase transformation. This easily leads to the formation of coarse lamellar pearlite in the core, resulting in a decrease in strength. Under continuous cooling, the wire is already at a low temperature after phase transformation incubation, and the accumulated thermal stress and structural stress from the phase transformation remain in the matrix, ultimately resulting in insufficient strength and ductility. This makes it difficult to overcome the production bottleneck of offline heat treatment and achieve the target strength of stranded wire.

[0005] II. To improve the strength and ductility of wire rods, existing technologies have proposed online salt bath treatment processes. For example, patent CN120400685A discloses wire rods, wire rods, and production methods for 2400~2460MPa grade stress corrosion resistant prestressed steel strands. These methods use a C-Si-Mn-Cr-V-Nb composition system and combine online salt bath isothermal treatment with online aging to produce sorbitic wire rods. However, these methods are limited by the wire rod composition, target microstructure, and the temperature of the one-stage salt bath treatment. This increases the difficulty of smelting and rolling, and the salt bath treatment time should not be too long. It also requires low-speed online aging treatment, which affects production efficiency and energy consumption. At the same time, there is still a risk of grain boundary cementite and martensite precipitation, which affects the efficient and stable production of hot-rolled wire rods. Summary of the Invention

[0006] This invention aims to at least partially solve one of the above-mentioned technical problems. This invention provides a high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire and its manufacturing method, which can simplify the composition system, realize the control of multiphase structure and online toughening, improve the strength and plasticity and structure uniformity of hot-rolled wire rod, and take into account production efficiency, so as to promote the efficient production of ultra-high strength stranded wire without offline heat treatment.

[0007] The technical solution adopted by this invention to solve its technical problem is:

[0008] A method for manufacturing high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire, the method comprising:

[0009] The wire rod is rolled into a production line based on the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.93%~0.98%, Si: 0.77%~0.97%, Mn: 0.77%~0.97%, Cr: 0.59%~0.72%, V: 0.049%~0.065%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is then spun into wire rod at a spinning temperature of ≥900℃. After online molten salt rapid quenching and isothermal toughening treatment, the wire rod first undergoes a front-end molten salt treatment and is cooled at a rate of ≥35℃ / s, transitioning from the austenitic state to the bainitic phase region, promoting the transformation of some austenite into bainite. Then, it undergoes a rear-end molten salt treatment, increasing the molten salt temperature and reducing the molten salt circulation rate, promoting the decomposition of untransformed residual austenite into sorbite and isothermal tempering. Finally, it undergoes slow cooling on a roller table, resulting in a hot-rolled wire rod with a microstructure consisting of a multiphase structure composed of tempered bainite and tempered sorbite.

[0010] The chemical composition and mass percentage of the above-mentioned hot-rolled wire rods are designed based on the following:

[0011] (1) Carbon: As an effective strengthening element, C is relatively cheaper and can improve the strength of steel through solid solution strengthening and carbide precipitation strengthening, reduce the bainite transformation initiation temperature, so as to promote the preferential phase transformation of some bainite in conjunction with rapid quenching, reduce the critical cooling rate of pearlite transformation, increase the nucleation rate of cementite, generate lamellar strengthening effect through pearlite lamellar spacing, and regulate the work hardening rate during cold drawing. However, excessive C content will aggravate carbon segregation in steel billet, easily generate brittle phases such as continuous network cementite and martensite, affect the section shrinkage rate of wire rod, and cannot meet the requirements of high compression ratio drawing. Therefore, in order to take into account the high strength requirements of 2460MPa grade strand, reduce performance fluctuations, and adapt to online molten salt rapid quenching isothermal toughening treatment, the mass percentage of C is controlled at 0.93%~0.98%.

[0012] (2) Silicon: Si can provide solid solution strengthening, reduce the diffusion coefficient of carbon atoms in austenite, delay the nucleation and growth of cementite, postpone the transformation of diffusion-type pearlite, and make bainite preferentially transform. In the later stage of molten salt treatment, it can inhibit the formation of coarse lamellar pearlite, refine the precipitate phase and improve the toughness of the material, inhibit the coarsening of cementite during tempering, and avoid excessive attenuation of sorbite lamellar strengthening and bainite precipitation strengthening. However, excessive silicon will increase the surface decarburization sensitivity, increase the deformation resistance and rolling load of steel, prolong the phase transformation incubation time, and lead to a decrease in the plasticity, toughness and production efficiency of hot-rolled wire rod. Therefore, in order to control the multiphase structure and ensure the plasticity of cold working, the mass percentage of Si is controlled at 0.77%~0.97%.

[0013] (3) Manganese: Mn can improve the hardenability of steel, so as to reduce the difference in cooling phase transformation between the surface and core of the wire rod, while lowering the critical point of austenite phase transformation, delaying the pearlite transformation, and working synergistically with Si and Cr to promote the smooth entry of supercooled austenite into the bainite phase region, reduce the diffusion coefficient of carbon atoms, and refine the bainite and sorbite lamellars after subsequent phase transformation. However, if the Mn content is too high, it will increase the element segregation during the solidification process of the billet, increase the overheating sensitivity of the steel, and make the austenite grains prone to abnormal growth. At the same time, it is easy to generate quenched martensite hard and brittle phase, resulting in uneven wire rod structure and unbalanced hardness gradient. Therefore, in order to facilitate the control of the multiphase structure of hot-rolled wire rod and improve the uniformity of the entire cross section of the wire rod, the mass percentage of Mn is controlled at 0.77%~0.97%.

[0014] (4) Chromium: Cr can improve the hardenability of steel so as to promote the synchronous phase transformation of the core and surface of the wire rod, shift the nose of the pearlite transformation to the right, promote the shearing nucleation of bainite, and can work synergistically with C and Mn to increase the nucleation rate of cementite, inhibit the growth rate of cementite lamellae and dissolve in cementite. It works synergistically with Si to ensure that the strengthening effect of sorbite lamellars does not decay during isothermal toughening. However, when the Cr content is too high, segregation will occur during solidification, increasing the deformation resistance of steel and the difficulty of controlling martensite. At the same time, it increases the difficulty of tempering, affecting production efficiency and energy consumption. Therefore, in order to balance the control of the multiphase structure of hot-rolled wire rod and rapid production, the resistance to tempering softening of wire rod should be appropriately controlled. The mass percentage of Cr is controlled at 0.59%~0.72%.

[0015] (5) Vanadium: As a microalloying element, V can suppress the abnormal growth of austenite grains during high-temperature rolling, thereby refining the bainite and sorbite lamellae after subsequent phase transformation. Excess V can be dispersed and precipitated during isothermal toughening, compensating for the strength reduction caused by matrix recovery during tempering. While improving strength, it also suppresses uneven deformation and internal stress concentration during cold drawing, reduces the wire breakage rate during drawing, and avoids brittle fracture caused by excessive work hardening rate. However, V has a high cost, and excessive addition is not conducive to controlling material cost. Therefore, based on the role of V and material cost considerations, V is added appropriately, and the mass percentage of V is controlled at 0.049%~0.065%.

[0016] (6) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0017] The aforementioned hot-rolled wire rod adopts a high-carbon Cr-V composition system, eliminating the need for Nb and other elements, thus simplifying the composition system. The critical point of austenite phase transformation is controlled by C, Si, Mn, and Cr. Si inhibits matrix recovery and cementite coarsening, Cr enhances the thermal stability of cementite, and V improves matrix strength through nanoprecipitation. This provides favorable conditions for delaying pearlite transformation, promoting preferential bainite transformation, adapting to multiphase microstructure control and microstructure refinement, and stabilizing isothermal tempering toughening. A relatively high wire drawing temperature is selected to keep the wire rod in a high-temperature austenitic state, avoiding excessively low temperatures that could lead to network carbide precipitation or insufficient cooling rate. This provides favorable conditions for subsequent preferential bainite transformation and microstructure refinement. After wire drawing, the wire rod undergoes online molten salt rapid quenching and isothermal toughening treatment without air cooling.

[0018] Firstly, compared to the limitations of the Stellmore air-cooled line's maximum cooling capacity, both air-cooled and water-cooled lines suffer from unstable temperature control, making it difficult to effectively suppress abnormal microstructures and control bainitic phase transformation. When wire rods undergo pre-treatment with molten salt, on the one hand, molten salt has a higher heat transfer coefficient than air and water. After pre-treatment with molten salt at a lower temperature, the wire rods can quickly pass through the secondary cementite precipitation temperature range, avoiding the precipitation of network carbides along austenite grain boundaries that could cleave the matrix, affecting ductility and toughness, and the carbon source for phase transformation. Rapid cooling allows the wire rods to bypass the pearlite transformation nose region, transitioning from a high-temperature austenitic state to the bainitic phase region. Combined with a compositional delay in pearlite transformation, this promotes the preferential transformation of a small portion of austenite into fine-structure, highly distorted bainite under rapid quenching, preventing prolonged residence in the high-temperature region that could lead to coarsening of alloy carbide precipitation. This prepares the ground for multiphase microstructure control, thereby improving matrix strength. On the other hand… Molten salt can cover the surface of the wire rod for uniform and rapid heat exchange. Compared with air-cooled lines, there is no temperature difference between the air-receiving and air-receiving surfaces. Compared with water-cooled lines, it does not produce a large number of bubbles adhering to the surface of the wire rod, affecting heat transfer. It can avoid the formation of brittle martensite abnormal structures in segregation zones or localized overcooling. It can reduce the temperature difference between the wire rod surface and the core, making the bainitic phase transformation controllable and reducing mechanical property fluctuations. Compared with the existing one-stage salt bath treatment, which is difficult to effectively suppress network carbides and control the rapid toughening of the structure, on the one hand, the molten salt temperature of the first stage of molten salt treatment is lower and the molten salt circulation volume is larger, which can effectively suppress the risk of network carbide precipitation caused by carbon segregation and regulate the precipitation behavior of nano-sized VC. On the other hand, through a small amount of bainitic phase transformation, the matrix strength can be improved, the wire rod's resistance to tempering softening and subsequent draw hardening can be enhanced, and the foundation for rapid phase transformation and toughening at higher temperatures in the second stage of molten salt treatment can be provided.

[0019] Second, compared to the limitations of the minimum cooling capacity and continuous cooling of the Stellmore air-cooling line, which makes it difficult to control the multiphase structure and perform online toughening, the wire rod undergoes a higher molten salt treatment in the later stage. On the one hand, as the treatment time increases, the wire rod gradually transforms into an isothermal state consistent with the molten salt temperature. This promotes the transformation of the untransformed residual austenite after the initial molten salt treatment into a sorbite structure with finer lamellar spacing at the sorbite transformation temperature. Combined with the wire rod's hardenability to suppress coarse lamellar cores, the excess V can be continuously and fully dispersed and precipitated during the isothermal process, thereby improving the matrix strength. On the other hand, being in a high-temperature state after the phase transformation, rather than a low-temperature state after continuous cooling, allows for online tempering of the mixed bainite and sorbite structure formed by the phase transformation. This releases phase transformation stress, optimizes dislocation configuration, improves bainite brittleness, and enhances strength and toughness. Compared to the existing single-stage salt bath treatment, which is difficult to rapidly toughen online, the subsequent molten salt treatment can be carried out at a higher temperature and with an appropriate extension of the treatment time because the rapid quenching process produces a small amount of bainite, which, combined with Si to suppress cementite coarsening and Cr to improve the resistance to tempering softening, can achieve this. On the one hand, the high temperature can promote carbon diffusion and accelerate the decomposition and phase transformation of the retained austenite. Under sufficient phase transformation, the retained austenite can avoid continuing to form a low-temperature brittle structure during subsequent cooling. On the other hand, the high temperature can appropriately reduce the bainite dislocation density and rapidly toughen the structure through isothermal treatment. This can reduce the restriction on slow cooling on the roller table. The wire rod is in a high-temperature state after exiting the molten salt treatment. Slow cooling on the roller table can continue the softening effect of the subsequent molten salt treatment and promote further toughening of the wire rod structure. However, it is not necessary to use an excessively low cooling rate, thereby achieving the control of the multiphase structure of hot-rolled wire rod and efficient and stable production.

[0020] Before rolling, selecting a higher heating furnace soaking temperature and an appropriate furnace time can promote the solid solution of alloying elements, reduce component segregation, and ensure that the entire cross-section of the billet is thoroughly heated. At the same time, it avoids excessively high temperature or excessively long furnace time, which can lead to coarsening of austenite grains, surface decarburization, or overheating. In the preferred technical solution, before rolling, the heating furnace soaking temperature is controlled at 1192~1242℃ and the furnace time is controlled at 160~260min.

[0021] During the rolling process, selecting a higher initial rolling temperature can reduce rolling deformation resistance and wear on the rolling line, thereby improving rolling efficiency, reserving a temperature drop allowance for subsequent rolling, and coordinating with an appropriate final rolling temperature and final rolling reduction to promote VC precipitation, pinning grain boundaries, and final rolling dynamic recrystallization to refine grains, providing sufficient nucleation sites for subsequent phase transformation. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1040~1080℃, the final rolling temperature at 915~955℃, and the final rolling reduction at 23%~28%.

[0022] During the wire spinning process, the wire spinning temperature can be further controlled to avoid abnormal grain growth or decarburization risk caused by excessively high wire spinning temperature. In a preferred technical solution, the wire spinning temperature is controlled at 900~940℃.

[0023] The lower the molten salt temperature in the initial molten salt treatment, the greater the heat exchange temperature difference between the wire rod and the molten salt, and the stronger the cooling driving force. This can shorten the residence time of the wire rod in the secondary cementite precipitation zone, avoid the precipitation of network carbides, inhibit premature pearlite phase transformation, and promote bainite shear transformation. As the treatment time increases, the bainite dislocation density increases and retains the nano-precipitation strengthening potential, which can improve the matrix strength. However, if the molten salt temperature is too low, although the bainite nucleation rate increases, the growth rate decreases, the transformation incubation period is prolonged, and there is even a risk of martensitic phase transformation. As the treatment time is too long, the bainite transformation amount exceeds the standard, the brittleness of the structure increases, and the production energy consumption increases, affecting subsequent tempering. Conversely, the higher the molten salt temperature, the better it is to increase the bainite transformation rate, avoid the risk of martensitic phase transformation, and improve the carbon content uniformity and thermodynamic stability of the retained austenite. Shorter processing times are beneficial for controlling bainitic phase transformation, reducing internal stress accumulation, and minimizing production energy consumption. However, excessively high molten salt temperatures result in a small heat exchange temperature difference between the wire rod and the molten salt, which is detrimental to suppressing the transformation of network carbides and pearlite. Insufficient driving force for bainitic phase transformation leads to coarsening and embrittlement of the microstructure. Furthermore, insufficient bainitic transformation volume during the initial molten salt treatment increases the difficulty of controlling the temperature rise in the subsequent molten salt treatment, negatively impacting the control of matrix strength and microstructure uniformity. Therefore, the initial molten salt treatment allows for control of the molten salt temperature and processing time, enabling the wire rod to rapidly transition from a high-temperature austenitic state to the bainitic phase region, promoting the rapid quenching and transformation of a small portion of austenite into bainite, and preparing the microstructure for the subsequent molten salt treatment. In a preferred embodiment, the molten salt temperature for the initial molten salt treatment is 440~470℃, and the processing time is 12~32s.

[0024] The pre-treatment molten salt process can use a higher molten salt circulation rate to control the molten salt temperature rise and improve the consistency of wire rod performance. In the preferred technical solution, the molten salt circulation rate of the pre-treatment molten salt process is 645~900t / h, and the molten salt temperature rise is ≤8℃.

[0025] The higher the molten salt temperature in the subsequent molten salt treatment, the higher the carbon diffusion rate, accelerating the nucleation and growth rate of the sorbite transformation, releasing the phase transformation internal stress and thermal stress generated by the rapid quenching in the preceding stage and the sorbite transformation in this stage. With longer treatment time, dislocation density can be reduced, optimizing the uniformity of sorbite lamellars, promoting the full dispersion and precipitation of microalloyed carbides, and resulting in more thorough online tempering and toughening, thus improving ductility and toughness. However, excessively high molten salt temperatures are detrimental to suppressing cementite lamellar coarsening, increasing the growth rate of the VC precipitate. With excessively long treatment time, over-tempering of the microstructure and continuous growth and coarsening of the VC precipitate will lead to a loss of strength and ductility. Conversely, lower molten salt temperatures can refine the sorbite lamellar spacing, promoting the dispersed precipitation of microalloyed carbides and providing a pinning dislocation effect. With shorter treatment time... While retaining high strength and reducing production energy consumption, excessively low molten salt temperatures result in insufficient alloy atom diffusion, prolonged transformation incubation period, and reduced tempering and toughening effect on bainitic structures. Furthermore, excessively short treatment times lead to significant residual stress, insufficient diffusion of microalloyed carbides, and even incomplete decomposition of retained austenite, causing hard and brittle phases and affecting ductility and toughness. Therefore, controlling the molten salt temperature and treatment time in the subsequent molten salt treatment promotes the transformation of untransformed retained austenite into sorbite, and isothermal tempering of the quenched bainitic and sorbitic structures promotes the dispersed distribution of microalloyed carbides, thereby regulating the strength-ductility matching and structural uniformity of the wire rod. In the preferred technical solution, the molten salt temperature in the subsequent molten salt treatment is 580~610℃, and the treatment time is 295~403s.

[0026] The downstream molten salt treatment can use an appropriate molten salt circulation rate to control the molten salt temperature rise and production energy consumption. In the preferred technical solution, the molten salt circulation rate of the downstream molten salt treatment is 515~615t / h, and the molten salt temperature rise is ≤3℃.

[0027] The roller conveyor slow cooling can further control the slow cooling speed. Slow cooling promotes further toughening of the wire rod structure and improves the tempering and softening effect of the wire rod, while also taking into account production efficiency. In the preferred technical solution, the roller conveyor slow cooling controls the wire rod to cool to below 290°C at a slow cooling speed of 0.45~0.75°C / s before winding.

[0028] In the preferred technical solution, the slow cooling of the roller conveyor adopts the control of the opening of the heat insulation cover, and blows the hot air at ≥265℃ in the online molten salt rapid quenching isothermal toughening treatment to the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the heat insulation cover. The heat energy of the online molten salt rapid quenching isothermal toughening treatment can be recovered and utilized through the hot air, thereby reducing production energy consumption and promoting the rapid production of wire rod.

[0029] A high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire, wherein the hot-rolled wire rod is manufactured by the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire described in any one of the above-mentioned methods.

[0030] The aforementioned hot-rolled wire rod adopts a Cr-V chemical composition design, which simplifies the composition system, reduces smelting difficulty and material costs, and forms a multiphase structure mainly composed of tempered sorbite with a small amount of tempered bainite. Compared with existing hot-rolled wire rods for strand with high sorbitivity, it can effectively suppress network carbides and brittle martensite structure, preventing them from disrupting the matrix continuity and causing tensile fracture or fatigue cracks. At the same time, the high dislocation density of bainite can strongly hinder dislocation slip and has higher strength than sorbite structure. After isothermal toughening and transformation into tempered bainite, it can retain The subsequent cold drawing process results in work hardening and a moderate decrease in dislocation density, effectively improving brittleness. Combined with the distribution of nano-sized VC particles in the matrix, the sorbite undergoes a full phase transformation, suppressing the coarse lamellar soft phase structure in the core. This reduces fluctuations in mechanical properties, leverages the strengthening effect of carbon and alloying elements, and uniformly enhances the matrix strength. The sorbite is isothermally toughened into an intermediate tempered sorbite structure transitioning to a spheroidized structure, which further improves stress concentration and enhances the coordinated deformation capability during high-compression cold drawing. Consequently, offline heat treatment can be eliminated, making it suitable for the development of ultra-high strength stranded wires.

[0031] The higher the proportion of tempered bainite and the finer the lamellar spacing of tempered sorbite, the higher the matrix strength; the higher the proportion of tempered sorbite, the higher the matrix ductility and toughness; in the preferred technical solution, the volume percentage of tempered bainite is 15%~23%, the volume percentage of tempered sorbite is 77%~85%, and the lamellar spacing of tempered sorbite is 85~130nm.

[0032] In the preferred technical solution, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same coil is ≤54MPa; this can avoid the network carbide from cutting the matrix and its adverse effects on plasticity and toughness, reduce the risk of wire breakage during drawing, improve the service fatigue life of the strand, and by suppressing abnormal structure and improving the uniformity of structure, the hot-rolled wire rod has smaller fluctuations in mechanical properties, which can further promote uniform cold drawing deformation, avoid internal stress concentration cracking, and maintain the uniformity of the overall mechanical properties of the strand.

[0033] In the preferred technical solution, the diameter of the hot-rolled wire rod is 8~15mm, the tensile strength is 1606~1666MPa, and the reduction of area is 26%~32%. The diameter of the hot-rolled wire rod can match the production requirements of different strand specifications. The high initial tensile strength can be combined with the draw hardening ability of the hot-rolled wire rod structure, reducing the number of drawing passes, quickly reaching the target strength, and reducing plastic loss during the process. At the same time, the high reduction of area can maintain sufficient torsional toughness and bending plasticity without offline heat treatment, reduce the wire breakage rate, meet the requirements of strand drawing and twisting, and thus promote the efficient and stable production of ultra-high strength strands.

[0034] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0035] (1) In view of the current situation that it is difficult to control abnormal structure and quickly regulate the toughness of the structure in the production of hot-rolled wire rod for stranded wire, the present invention adopts Cr-V chemical composition design combined with online molten salt rapid quenching isothermal toughening technology. The wire rod first undergoes a front-end molten salt treatment and rapid cooling, and enters the bainite phase region from the austenite state. This can effectively suppress abnormal structure of network carbides and martensite, promote the preferential transformation of some austenite to bainite, and suppress cementite coarsening. Then, after the rear-end molten salt treatment, the molten salt temperature is increased and the molten salt circulation is reduced. This promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, promotes the full dispersion and precipitation of microalloyed carbides, and rapidly toughens the structure. This can improve the strength and plasticity matching of the wire rod, reduce the restriction on the slow cooling of the roller table, and take into account both production efficiency and energy consumption. Finally, it controls the multiphase structure and structure uniformity of the hot-rolled wire rod, realizes the efficient and stable production of hot-rolled wire rod, and has good industrial adaptability.

[0036] (2) In view of the high cost, insufficient strength, plasticity and uniformity of existing hot-rolled wire rod materials for stranded wire, which require additional heat treatment and result in high development cost and energy consumption of ultra-high strength stranded wire, this invention can simplify the composition system and control the material cost. The microstructure includes a multiphase structure composed of tempered bainite and tempered sorbite, which can effectively suppress the network carbides and brittle martensite structure, avoid them from destroying the continuity of the matrix and causing drawing fracture or fatigue cracks. At the same time, the bainite phase change is controllable. After isothermal toughening, it is transformed into tempered bainite that is both strong and tough and tempered sorbite with further improved plasticity. Nanoscale VC particles are distributed in the matrix, which can effectively improve the strength, plasticity and uniformity of hot-rolled wire rod. It can achieve a tensile strength of 1606~1666MPa and a section reduction rate of 26%~32%. It can be used to manufacture 2460MPa grade stranded wire and other application fields without additional heat treatment, which can reduce the number of drawing passes and reduce the risk of wire breakage during drawing and twisting. It has good market application prospects. Attached Figure Description

[0037] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0038] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention;

[0039] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention;

[0040] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0041] The embodiments described below with reference to the accompanying drawings are exemplary and are merely for illustrative purposes and do not limit the description of the features and characteristics of the invention. They are intended to provide the best mode for carrying out the invention, to explain the invention, and to enable those skilled in the art to practice the invention. However, they should not be construed as limiting the scope of the invention in any way, which is defined only by the appended claims. The microstructure and performance testing of the hot-rolled wire rods obtained in the following embodiments and comparative examples includes: tensile testing using GB-T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, to obtain tensile strength and reduction of area; microstructure testing using the metal microstructure testing method of GB / T13298 standard; and mechanical property same-coil difference test method: two coils of wire rod are taken 5m from the end of the coil. Using the overlap area as the base point, each coil of wire rod is divided into 8 equal segments. One tensile specimen is taken from each segment. The difference in strength of the tensile specimens after tensile testing is the mechanical property same-coil difference.

[0042] Example 1:

[0043] A preferred embodiment of the manufacturing method of the 2460MPa grade high-strength multiphase hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.93%, Si: 0.77%, Mn: 0.94%, Cr: 0.70%, V: 0.059%, P: 0.012%, S: 0.012%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt rapid quenching and isothermal toughening → roller table slow cooling → coiling, specifically:

[0044] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity, promoting composition homogenization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote VC precipitation, grain refinement, and grain shaping. Specifically, the furnace soaking temperature is controlled at 1192℃, the furnace time at 160min, the initial rolling temperature at 1040℃, the final rolling temperature at 915℃, and the final rolling reduction at 28%. The wire rod drawing process is used to draw the wire rod from the rolling line into coil through a wire rod drawing machine. The coil is distributed on a roller conveyor and transported along the roller conveyor. A higher wire rod drawing temperature is selected to keep the coil in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for subsequent preferential transformation of bainite and microstructure refinement. Specifically, the wire rod drawing temperature is controlled at 900℃.

[0045] The online molten salt rapid quenching isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment. This allows the wire rod to cool at a rate of 35°C / s, rapidly bypassing the network carbide precipitation temperature range from the high-temperature austenitic state and entering the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and rapid quenching forms a small amount of bainite structure. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, increasing the molten salt temperature, reducing the molten salt circulation rate, and promoting... Untransformed residual austenite decomposes into sorbite. Isothermal tempering is performed on the quenched bainite and sorbite structures to promote the dispersed distribution of microalloyed carbides, thereby controlling the strength-plasticity matching and microstructure uniformity of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 470℃, the treatment time is 12s, the molten salt circulation rate is 645t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 580℃, the treatment time is 403s, the molten salt circulation rate is 515t / h, and the molten salt temperature rise is ≤3℃.

[0046] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥265℃) from the two salt bath tanks undergoing online molten salt rapid quenching and isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 280℃ at a slow cooling rate of 0.75℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure is shown in the figure below. Figure 1 As shown.

[0047] Comparative Example 1: A method for manufacturing hot-rolled wire rod, the difference between this method and Example 1 is that the manufacturing method follows a process flow of rolling → wire drawing → Steyrmo air cooling. Specifically, the heating furnace homogenization temperature is controlled at 1145℃, the furnace time is 220 min, the initial rolling temperature is 990℃, the final rolling temperature is 890℃, and the wire drawing temperature is 860℃; the Steyrmo forced air cooling uses an air volume of 260,000 m³ per fan. 3 At 75% capacity, fans 1 to 6 are turned on to cool the wire rod to 670℃ at a cooling rate of 6.9℃ / s. Then, fans 7 to 14 are turned on to 20% capacity to cool the wire rod to 269℃ at a cooling rate of 2.5℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0048] Comparative Example 2: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 1 is that: the heating furnace is controlled to have a uniform heating temperature of 1160°C, a furnace time of 230 min, an initial rolling temperature of 1000°C, a final rolling temperature of 850°C, and a wire drawing temperature of 830°C. During the initial molten salt treatment, the wire rod is cooled at a cooling rate of 29°C / s, and the finished hot-rolled wire rod is obtained after the wire rod is removed from the production line.

[0049] Example 2:

[0050] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%, Si: 0.96%, Mn: 0.97%, Cr: 0.59%, V: 0.053%, P: 0.014%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt rapid quenching and isothermal toughening → roller table slow cooling → coiling, specifically:

[0051] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity, promoting composition homogenization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 11mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote VC precipitation, grain refinement, and grain shaping. Specifically, the furnace soaking temperature is controlled at 1215℃, the furnace time at 260min, the initial rolling temperature at 1055℃, the final rolling temperature at 925℃, and the final rolling reduction at 26%. The wire rod drawing process is used to draw the wire rod from the rolling line into coil through a wire rod drawing machine. The coil is distributed on a roller conveyor and transported along the roller conveyor. A higher wire rod drawing temperature is selected to keep the coil in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for subsequent preferential transformation of bainite and microstructure refinement. Specifically, the wire rod drawing temperature is controlled at 915℃.

[0052] The online molten salt rapid quenching isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment. This allows the wire rod to cool at a rate of 37°C / s, rapidly bypassing the network carbide precipitation temperature range from the high-temperature austenitic state and entering the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and rapid quenching forms a small amount of bainite structure. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, increasing the molten salt temperature, reducing the molten salt circulation rate, and promoting... Untransformed residual austenite decomposes into sorbite. Isothermal tempering is performed on the quenched bainite and sorbite to promote the dispersed distribution of microalloyed carbides, thereby controlling the strength-plasticity matching and microstructure uniformity of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 461℃, the treatment time is 20s, the molten salt circulation rate is 755t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 589℃, the treatment time is 373s, the molten salt circulation rate is 545t / h, and the molten salt temperature rise is ≤3℃.

[0053] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥265℃) from the two salt bath tanks undergoing online molten salt rapid quenching and isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 285℃ at a slow cooling rate of 0.7℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure is shown in the figure below. Figure 2 As shown.

[0054] Comparative Example 3: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the pre-treatment of molten salt, the wire rod is cooled at a cooling rate of 39°C / s, the molten salt temperature of the pre-treatment is 410°C, the treatment time is 36s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0055] Comparative Example 4: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the pre-treatment of molten salt, the wire rod is cooled at a cooling rate of 30℃ / s, the molten salt temperature of the pre-treatment is 485℃, the treatment time is 8s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0056] Example 3:

[0057] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.96%, Si: 0.86%, Mn: 0.77%, Cr: 0.72%, V: 0.065%, P: 0.013%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt rapid quenching and isothermal toughening → roller table slow cooling → coiling, specifically:

[0058] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity, promoting compositional homogenization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 13.5mm through a rolling line. Appropriate rolling temperatures and reductions are selected to improve rolling efficiency and promote VC precipitation, grain refinement, and grain shaping. Specifically, the furnace soaking temperature is controlled at 1234℃, the furnace time at 230min, the initial rolling temperature at 1065℃, the final rolling temperature at 940℃, and the final rolling reduction at 24%. The wire rod drawing process is used to draw the wire rod from the rolling line into coil through a wire rod drawing machine. The coil is distributed on a roller conveyor and transported along the roller conveyor. A higher wire rod drawing temperature is selected to keep the coil in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for subsequent preferential transformation of bainite and microstructure refinement. Specifically, the wire rod drawing temperature is controlled at 930℃.

[0059] The online molten salt rapid quenching isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment. This allows the wire rod to cool at a rate of 39°C / s, rapidly bypassing the network carbide precipitation temperature range from the high-temperature austenitic state and entering the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and the rapid quenching forms a small amount of bainite structure. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, increasing the molten salt temperature, reducing the molten salt circulation rate, and promoting... Untransformed residual austenite decomposes into sorbite. Isothermal tempering is performed on the quenched bainite and sorbite structures to promote the dispersed distribution of microalloyed carbides, thereby controlling the strength-plasticity matching and microstructure uniformity of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 453℃, the treatment time is 26s, the molten salt circulation rate is 825t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 602℃, the treatment time is 330s, the molten salt circulation rate is 585t / h, and the molten salt temperature rise is ≤3℃.

[0060] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥265℃) from the two salt bath tanks undergoing online molten salt rapid quenching and isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 287℃ at a slow cooling rate of 0.6℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure is shown in the figure below. Figure 3 As shown.

[0061] Comparative Example 5: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that the molten salt temperature of the subsequent molten salt treatment is 615°C, the treatment time is 410s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0062] Comparative Example 6: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that the molten salt temperature of the subsequent molten salt treatment is 565°C, the treatment time is 180s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0063] Example 4:

[0064] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.98%, Si: 0.97%, Mn: 0.82%, Cr: 0.65%, V: 0.049%, P: 0.013%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt rapid quenching and isothermal toughening → roller table slow cooling → coiling, specifically:

[0065] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity, promoting composition homogenization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 15mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote VC precipitation, grain refinement, and grain shaping. Specifically, the furnace soaking temperature is controlled at 1242℃, the furnace time at 200min, the initial rolling temperature at 1080℃, the final rolling temperature at 955℃, and the final rolling reduction at 23%. The wire drawing process is used to transfer the wire rod from the rolling line to a wire rod through a wire drawing mechanism. The wire rod is distributed on a roller conveyor and transported along the roller conveyor. A higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for subsequent preferential transformation of bainite and microstructure refinement. Specifically, the wire drawing temperature is controlled at 940℃.

[0066] The online molten salt rapid quenching isothermal toughening process employs a two-stage salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 40℃ / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide precipitation temperature range, into the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and the rapid quenching forms a small amount of bainite structure. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, increasing the molten salt temperature, reducing the molten salt circulation rate, and promoting... Untransformed residual austenite decomposes into sorbite. Isothermal tempering is performed on the quenched bainite and sorbite to promote the dispersed distribution of microalloyed carbides, thereby controlling the strength-plasticity matching and microstructure uniformity of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 440℃, the treatment time is 32s, the molten salt circulation rate is 900t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 610℃, the treatment time is 295s, the molten salt circulation rate is 615t / h, and the molten salt temperature rise is ≤3℃.

[0067] The roller conveyor slow cooling process uses an adjustable insulation cover to blow hot air (≥265℃) from the two salt bath tanks undergoing online molten salt rapid quenching and isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is controlled to cool to 288℃ at a slow cooling rate of 0.45℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0068] Comparative Example 7: A method for manufacturing hot-rolled wire rod, the difference between which is that the manufacturing method follows the process of rolling → wire drawing → online molten salt rapid quenching and isothermal toughening → air cooling. The air cooling is achieved by opening the heat insulation cover, conveying the wire rod by the conveyor roller, and controlling the wire rod to cool to 265°C at a slow cooling rate of 1.6°C / s. After the wire rod is removed from the line, the finished hot-rolled wire rod is obtained.

[0069] The microstructure and properties of the hot-rolled wire rods obtained in Examples 1-4 and Comparative Examples 1-7 were tested, and the comparative results are shown in Table 1 below:

[0070] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0071]

[0072] As can be seen from the comparison results of Example 1 and Comparative Example 1, compared with the Stellmore air-cooled line, the strengthening effect is weakened due to the weak cooling rate during the cooling phase transformation, resulting in a higher level of abnormal structures such as network carbides and martensite, and poor ductility, toughness and structural uniformity. The present invention adopts Cr-V chemical composition design combined with online molten salt rapid quenching isothermal toughening technology, which can control the wire rod to first undergo molten salt treatment, effectively suppressing abnormal structures such as network carbides and martensite, and promoting the preferential transformation of some austenite to bainite. After the subsequent molten salt treatment, the untransformed residual austenite is decomposed into sorbite and isothermally tempered, promoting the full dispersion and precipitation of microalloyed carbides, rapidly toughening the structure, and controlling the multiphase structure and structural uniformity of hot-rolled wire rod. As can be seen from the results of Examples 1 to 4, a tensile strength of 1606~1666MPa and a reduction of area of ​​26%~32% can be achieved, which can be used to manufacture 2460MPa grade stranded wire and other application fields, so as to produce ultra-high strength stranded wire without offline heat treatment.

[0073] As can be seen from the comparison results between Example 1 and Comparative Example 2, selecting a higher spinning temperature keeps the wire rod in a high-temperature austenitic state, avoiding the precipitation of network carbides or insufficient cooling rate caused by excessively low temperature, thus providing favorable conditions for subsequent promotion of preferential transformation of bainite and microstructure refinement.

[0074] The comparison results between Example 2 and Comparative Example 3 show that the lower the molten salt temperature in the initial molten salt treatment, the shorter the residence time of the wire rod in the secondary cementite precipitation zone, avoiding the precipitation of network carbides, inhibiting the premature phase transformation of pearlite, and promoting the shear transformation of bainite. As the treatment time increases, the bainite dislocation density increases and the nano-precipitation strengthening potential is retained, which can improve the matrix strength. However, if the molten salt temperature is too low, although the bainite nucleation rate increases, the growth rate decreases. As the treatment time is too long, the bainite transformation amount exceeds the standard, the brittleness of the structure increases and the production energy consumption increases, affecting the subsequent tempering.

[0075] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature in the first stage of molten salt treatment, the better it is to increase the bainite transformation rate and avoid the risk of martensitic phase transformation. The shorter the treatment time, the better it is to control the bainite phase transformation, reduce the accumulation of internal stress and production energy consumption. However, if the molten salt temperature is too high, the heat exchange temperature difference between the wire rod and the molten salt is too small, which is not conducive to suppressing the transformation of network carbides and pearlite. The driving force of bainite phase transformation is insufficient, and the microstructure becomes coarse and brittle. As the treatment time is too short, the amount of bainite transformation is insufficient, and the difficulty of controlling the temperature rise in the second stage of molten salt treatment increases, which is not conducive to controlling the matrix strength and microstructure uniformity.

[0076] As can be seen from the comparison results of Example 3 and Comparative Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the higher the carbon diffusion rate, the faster the nucleation and growth rate of sorbite transformation, and the release of phase transformation internal stress and thermal stress generated by rapid quenching in the previous stage and sorbite transformation in this stage. With longer treatment time, the dislocation density can be reduced, the uniformity of sorbite lamellars can be optimized, the microalloyed carbides can be fully dispersed and precipitated, the online tempering and toughening can be more complete, and the ductility and toughness can be improved. However, if the molten salt temperature is too high, it is not conducive to suppressing the coarsening of cementite lamellars. The growth rate of VC precipitate phase increases. With excessive treatment time, the microstructure is over-tempered and the VC precipitate phase continues to grow and coarsen, which will lead to a loss of strength and plasticity.

[0077] As can be seen from the comparison results of Example 3 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the finer the sorbite lamellar spacing can be achieved, promoting the dispersed precipitation of microalloyed carbides and providing a pinning dislocation effect. With shorter treatment time, high strength characteristics are retained and production energy consumption is reduced. However, if the molten salt temperature is too low, the diffusion of alloy atoms is insufficient, the transformation incubation period is prolonged, and the tempering and toughening effect on the bainite structure decreases. With too short a treatment time, the structure has large residual stress, insufficient diffusion of microalloyed carbides, and even incomplete decomposition of residual austenite, resulting in a hard and brittle phase, which affects the ductile and tough properties.

[0078] As can be seen from the comparison results between Example 4 and Comparative Example 7, the slow cooling of the roller table can promote further toughening of the wire rod structure and improve the tempering and softening effect of the wire rod.

[0079] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire, characterized in that, Its manufacturing methods include: The wire rod is rolled into wire rod according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.93%~0.98%, Si: 0.77%~0.97%, Mn: 0.77%~0.97%, Cr: 0.59%~0.72%, V: 0.049%~0.065%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a wire drawing temperature of ≥900℃, it undergoes online molten salt rapid quenching and isothermal toughening treatment. This process involves first undergoing a preliminary molten salt treatment and then cooling at a rate of ≥35℃ / s to transition the wire rod from the austenitic state to the bainitic phase, promoting the transformation of some austenite into bainite. The wire rod then undergoes a subsequent molten salt treatment. The process involves increasing the molten salt temperature and reducing the molten salt circulation rate to promote the decomposition and transformation of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling via a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite. The molten salt temperature in the first stage of the molten salt treatment is 440–470°C, the treatment time is 12–32 s, the molten salt circulation rate is 645–900 t / h, and the molten salt temperature rise is ≤8°C. The molten salt temperature in the second stage of the molten salt treatment is 580–610°C, the treatment time is 295–403 s, the molten salt circulation rate is 515–615 t / h, and the molten salt temperature rise is ≤3°C. The slow cooling via the roller conveyor controls the wire rod to cool to below 290°C at a slow cooling rate of 0.45–0.75°C / s before coiling.

2. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1192~1242℃ and the furnace time is 160~260min.

3. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1040~1080℃, the final rolling temperature is controlled at 915~955℃, and the final rolling reduction is controlled at 23%~28%.

4. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to claim 1, characterized in that, During the silk-spinning process, the silk-spinning temperature is controlled at 900~940℃.

5. A high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of high-strength multiphase hot-rolled wire rod for 2460MPa grade strand as described in any one of claims 1 to 4.

6. The high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to claim 5, characterized in that, The volume percentage of tempered bainite is 15%~23%, the volume percentage of tempered sorbite is 77%~85%, the lamellar spacing of tempered sorbite is 85~130nm, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤54MPa.

7. The high-strength multiphase hot-rolled wire rod for 2460MPa grade stranded wire according to claim 5, characterized in that, The hot-rolled wire rod has a diameter of 8-15 mm, a tensile strength of 1606-1666 MPa, and a reduction of area of ​​26%-32%.

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