A hot-rolled wire rod for 2260MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a hot-rolled wire rod for 2260MPa grade stranded wire and its manufacturing method. The wire rod is designed and rolled from high-carbon C-Si-Mn-Cr-Mo-Nb-Al, and then undergoes online molten salt isothermal toughening treatment. The wire rod first undergoes a preliminary molten salt treatment and rapid cooling, transitioning from the austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. A subsequent molten salt treatment reduces the molten salt circulation rate, promoting the further transformation of untransformed residual austenite into sorbite, followed by isothermal tempering and promoting partial melting of sorbite lamellars. Finally, it undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered sorbite, ferrite, and melted sorbite, achieving a tensile strength of 1534~1584MPa and a reduction of area of ​​33%~38%. This wire rod is suitable for manufacturing 2260MPa grade stranded wire and other applications, requiring no additional heat treatment process.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a 2260MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] Ultra-high strength stranded wire, as an important development direction for stranded wire, has received increasing attention and research from steel mills and stranded wire manufacturers in recent years. Hot-rolled wire rod, the raw material for ultra-high strength stranded wire, is a key factor in its grade improvement. However, due to the constraints of steel mill production lines, hot-rolled wire rod usually requires additional heat treatment after production to adjust its microstructure and properties in order to achieve the production of ultra-high strength stranded wire. This process is not only complex and costly but also inefficient. Therefore, it is necessary to develop a 2260MPa grade hot-rolled wire rod for stranded wire and its manufacturing method that does not require additional heat treatment to meet the development needs of the steel industry and market demand.

[0003] Existing stranded steel wire rods generally use a high-carbon C-Si-Mn composition system. Combined with the Steyrmo air-cooling line for producing high-strength wire rods, the technical bottlenecks restricting the development of hot-rolled wire rods for stranded wires that require no additional heat treatment and achieve a strength of 2260 MPa include:

[0004] I. To improve the strength of hot-rolled wire rod, existing technologies generally increase the C, Si, and Mn content in the rod, combined with a strong air-cooling process after low-temperature rolling and wire drawing, to obtain a sorbite structure with fine lamellar spacing. However, on the one hand, the high carbon content increases the carbon supersaturation in austenite. During the Steyrmore air-cooling process, carbon preferentially segregates at the austenite grain boundaries to form continuous network carbides. The network carbides have high hardness, which leads to a decrease in the plasticity and toughness of the wire rod. During bending and twisting, they become stress concentration sources, causing cracks to propagate rapidly along the grain boundaries. The wire rod cannot withstand the deformation during stranding and twisting. Limited by the maximum cooling capacity of the Steyrmore air-cooling line, the improvement after increasing the air-cooling strength is limited. On the other hand, reducing the carbon content and further increasing the content of other alloys will lead to higher material costs. Furthermore, the addition of alloying elements such as Mn and Cr results in higher segregation and hardenability of the wire rod, which is a significant factor in the problem. Promoting microstructure refinement and minimizing the level of network carbides, while increasing air-cooling strength, will further increase temperature control instability. The large temperature difference between the air-receiving and air-exposed surfaces of the wire rod, and between the surface and the core, makes it prone to local overcooling and transformation into hard and brittle martensite, leading to deterioration of strength uniformity. During cold drawing, it cannot undergo plastic deformation, causing wire breakage or overall failure. Although alloying elements such as Cr, V, and Ti can help improve strength through fine grain strengthening and precipitation strengthening, they are difficult to form uniform and fine carbides due to the unstable temperature control and continuous cooling of the air-cooling line, resulting in limited strengthening performance. At the same time, after phase transformation inoculation, the wire rod is in a low-temperature state with large residual microstructure stress, resulting in limited improvement of ductility and toughness by expensive microalloys. The residual stress is superimposed on the external load during cold drawing, which can easily lead to local stress exceeding the limit, requiring additional heat treatment to adjust the microstructure and properties of hot-rolled wire rod.

[0005] II. To obtain higher plasticity, existing technologies employ online salt bath treatment. For example, patent CN120400686A discloses 2200-2260MPa grade ultra-high stress corrosion resistant prestressed steel strand wire rod, steel strand, and their production method, which uses a C-Si-Mn-Cr-V-Nb composition design and combines online salt bath isothermal treatment after low-temperature rolling and online aging to prepare sorbitic wire rod. However, to improve the sorbitization rate and reduce the lamellar gap across the entire cross-section, a single-stage salt bath cooling requires a large molten salt circulation volume, resulting in high overall energy consumption. Furthermore, this is affected by alloy composition, ... Due to limitations in low-temperature rolling and phase transformation control, reducing the molten salt circulation rate can lead to further decreases in the initial strength of the wire rod if the sorbite lamellars become coarser and unevenly distributed. This increases the fluctuation in mechanical properties, resulting in an increase in subsequent drawing passes and a greater risk of plasticity loss and wire breakage. On the other hand, due to limitations in alloy composition and one-stage salt bath treatment, the treatment time for one-stage salt bath treatment should not be too long in order to accommodate the strengthening effect of microalloying and suppress cementite coarsening to avoid excessive strength loss. This necessitates combining it with low-temperature rolling to improve the phase deformation nucleation base, affecting rolling efficiency. It also requires slow online aging at low cooling rates for extended periods, impacting production efficiency. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the above-mentioned technical problems. The present invention provides a hot-rolled wire rod for 2260MPa grade stranded wire and its manufacturing method, which can suppress abnormal structure, improve the strength-plasticity matching of wire rod, and take into account production energy consumption and efficiency, so as to improve the yield and production efficiency of high-strength stranded wire without additional heat treatment process.

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

[0008] A method for manufacturing hot-rolled wire rod for 2260MPa grade stranded wire, the method comprising:

[0009] The wire rod is rolled into production wire according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.89%~0.93%, Si: 0.82%~0.95%, Mn: 0.55%~0.75%, Cr: 0.37%~0.57%, Mo: 0.1%~0.3%, Nb: 0.010%~0.025%, Al: 0.28%~0.4%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities; the wire rod is manufactured with a chemical composition of ≥90%. After being spun into wire rod at a wire rod temperature of 8℃, the wire rod undergoes online molten salt isothermal toughening treatment. The wire rod first undergoes a preliminary molten salt treatment and is cooled at a rate of ≥35℃ / s, transitioning from the austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. Then, it undergoes a subsequent molten salt treatment, reducing the molten salt circulation rate to promote the continued transformation of untransformed residual austenite into sorbite and isothermal tempering, promoting the melting of some sorbite lamellars. Finally, it undergoes slow cooling on a roller table to produce a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted 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: The price of C element is relatively lower. It can be incorporated into the matrix through interstitial solid solution, causing lattice distortion to hinder dislocation movement. At the same time, it can regulate the stability of austenite and promote the rapid transformation of austenite into fine lamellar sorbite. It can form dispersed carbides in synergy with Cr, Mo and Nb, further improving the strength by pinning dislocations. It can also maintain the long-term strength of the material in service by relying on the stability of carbides. However, if the C content is too high, it will aggravate the center segregation, easily enrich and precipitate continuous network carbides at the austenite grain boundaries. At the same time, the phase transformation incubation period is prolonged and the control of martensite deterioration structure is more difficult, resulting in increased wire brittleness and mechanical property fluctuations. Therefore, in order to take into account the high strength requirements of 2260MPa grade stranded wire, improve the uniformity of structure and promote rapid production, the mass percentage of C is controlled at 0.89%~0.93%.

[0012] (2) Silicon: Si is a solid solution strengthening element of ferrite. It can suppress the formation of coarse carbides during cooling, hinder the diffusion of carbon atoms into cementite, strongly suppress the precipitation and coarsening process of cementite, and play a role in refining the precipitated phase and improving the toughness of the material. However, excessive silicon will promote decarburization, increase intragranular segregation, increase the bainite transformation temperature, and cause performance fluctuations due to excessive residual austenite, making isothermal tempering more difficult and reducing production efficiency. Therefore, in order to adapt to the control of phase transformation structure by online molten salt isothermal toughening, appropriately improve the resistance to tempering softening and take into account production efficiency, the mass percentage of Si is controlled at 0.82%~0.95%.

[0013] (3) Manganese: Mn is an austenite stabilizing element that can lower the critical temperature of austenite to pearlite transformation and the free energy of austenite, prolong the residence time of austenite in the medium temperature range, and provide sufficient time for the transformation of sorbite in the core. As a substitutional solid solution element, it can enhance strength through lattice distortion, while hindering the diffusion of C atoms into cementite particles, improving the strength stability and work hardening rate after tempering. However, if the Mn content is too high, it will increase the segregation of alloying elements during the solidification process of the billet, slow down the sorbite transformation kinetics during the cooling process, increase the risk of forming bainite and martensite, and lead to increased brittleness and mechanical property fluctuations, affecting the uniformity of wire rod structure. Therefore, in order to make the hot-rolled wire rod have a high strength-plasticity combination and take into account the uniformity of structure, the mass percentage of Mn is controlled at 0.55%~0.75%.

[0014] (4) Chromium: Cr can provide solid solution strengthening effect, while significantly improving hardenability, increasing the recrystallization temperature of austenite, and reducing the diffusion coefficient of carbon in austenite, so as to suppress proeutectoid ferrite and refine the interlamellar spacing of sorbite. At the same time, it can form nanoscale carbides in synergy with C, which are uniformly precipitated during molten salt treatment and tempering, which can increase the resistance to tempering softening and enable the wire rod to maintain high strength after tempering. However, excessive Cr content will aggravate the segregation of alloying elements, increase the risk of precipitation of martensite and other deteriorated structures, and increase the risk of carbide coarsening. Grain boundary coarse carbides are the preferred initiation source of fatigue cracks, affecting the pull-out plasticity and service life of steel. Therefore, in order to control the sorbite phase transformation and microstructure uniformity and provide appropriate resistance to tempering softening, the mass percentage of Cr is controlled at 0.37%~0.57%.

[0015] (5) Molybdenum: The addition of Mo strongly inhibits the pearlite transformation, causing the pearlite transformation curve to shift to the right, so as to refine the sorbite and melt sorbite structure, reduce stress concentration, improve the deformation coordination of the material, and reduce the diffusion ability of carbide forming elements, thereby delaying the formation of carbide precipitation, inhibiting the precipitation of related second phase particles, and avoiding the aging and growth of precipitated phases at high temperatures. Through the synergistic effect with Cr and Nb, the strength is stable for a long time. However, if the content of Mo is too high, it will prolong the phase transformation incubation period, affect the uniformity of carbide distribution, and increase the material cost. Therefore, based on the role of Mo and material cost considerations, the mass percentage of Mo is controlled at 0.1%~0.3%.

[0016] (6) Niobium: During the hot rolling process, Nb can pin the austenite grain boundaries by forming high-temperature stable carbonitrides, inhibiting the growth of austenite grains at high temperatures, reducing the risk of rolling cracks, and enabling rapid rolling. At the same time, the high dislocation density of the deformed austenite can increase the nucleation rate of sorbite, indirectly inhibiting pearlite formation and reducing microstructure fluctuations by refining austenite grains. During the isothermal tempering process, Nb will further precipitate nanoscale NbC. NbC has a high melting point and better stability than VC, which can provide a strong precipitation strengthening effect and optimize the tempering process. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling material costs. Therefore, the mass percentage of Nb is controlled at 0.010%~0.025%.

[0017] (7) Aluminum: Al can reduce harmful inclusions, improve the purity of steel, form aluminum nitride to pin grain boundaries and inhibit austenite grain coarsening. The solid solution strengthening of Al has a smaller effect on plasticity than that of Si. At the same time, Al can hinder carbon diffusion through solid solution, reduce the growth rate of cementite lamellars, inhibit the formation of proeutectoid ferrite during cooling, reduce the diffusion coefficient of C, inhibit cementite coarsening during isothermal tempering, and play a role in refining precipitates and improving the toughness of materials. However, excessive Al content will increase the risk of coarse inclusions and billet defects, and increase the rolling mill load. Therefore, in order to adapt to rolling and appropriately inhibit the precipitation and coarsening process of cementite, the mass percentage of Al is controlled at 0.28%~0.4%.

[0018] (8) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0019] The aforementioned hot-rolled wire rod adopts a high-carbon composition design of C-Si-Mn-Cr-Mo-Nb-Al, which avoids excessive alloying elements, appropriately controls material costs, provides a strength basis through interstitial solid solution of carbon, and provides a carbon source for microalloyed carbides. Si and Al supplement solid solution strengthening and inhibit cementite coarsening, while Mn and Cr regulate the hardenability of the wire rod. Nb, Mo, and Cr precipitation strengthening improves tempering stability. This provides favorable conditions for inhibiting grain coarsening and coarse pearlite, refining sorbite lamellar structure, promoting uniform microstructure transformation and melting of sorbite-cementite lamellars, improving strength-plasticity matching, and increasing production efficiency. On this basis, a higher wire drawing temperature is selected, which keeps the wire rod in a high-temperature austenitic state, where carbon and microalloying elements are in solid solution, avoiding excessively low temperatures, local carbon enrichment forming network carbides, or premature phase transformation. At the same time, it provides a high nucleation driving force for subsequent sorbite phase transformation and microalloyed carbide precipitation. After wire drawing, the wire rod does not undergo air cooling but is directly subjected to online molten salt isothermal toughening treatment.

[0020] I. Compared to the limitations imposed by the maximum cooling capacity and temperature control instability of the Stellmore air-cooled line, which makes it difficult to control network carbides and martensitic abnormal structures and exert the strengthening effect of microalloying, the high heat transfer capacity of molten salt can promote rapid cooling of the wire rod. When the wire rod passes through the initial molten salt treatment, it can quickly bypass the precipitation temperature range of network carbides from the high-temperature austenitic state, suppressing the risk of network carbides caused by high carbon content. 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 uniform heat exchange, eliminating the temperature difference problem between the air-receiving and air-receiving surfaces. This avoids the segregation of alloying elements and the abnormal martensite structure caused by local overcooling. On the one hand, rapid cooling can increase the degree of undercooling, which, together with the effects of Si and Al, can suppress cementite coarsening. This allows the wire rod to transform into a sorbite structure with finer lamellar spacing after entering the pearlite phase region. It also promotes the dispersed precipitation of Cr, Nb, and Mo nanocarbides instead of uneven coarsening on the air-cooling line. The nanocarbides can pin grain boundaries and suppress grain coarsening. As the processing time increases, the temperature gradient from the wire rod surface to the core can be further reduced, suppressing lamellar coarsening in the core and forming a structure dominated by fine lamellar spacing. This promotes the full precipitation of nanocarbides, fully leveraging the microalloying strengthening effect to improve the matrix strength.

[0021] Second, compared to the limitations of the Stellmore air-cooling line's minimum cooling capacity and continuous cooling, which makes it difficult to improve the sorbitization rate and regulate the microstructure toughness, the wire rod temperature gradually changes to match the molten salt temperature during the later stage of molten salt treatment. This promotes the continued transformation of untransformed residual austenite into fine lamellar interlamellar sorbite, ensuring sufficient phase transformation inoculation, improving the sorbitization rate, and preventing the residual austenite from continuing to form low-temperature brittle abnormal structures during subsequent cooling. Simultaneously, the relatively high temperature after phase transformation inoculation allows for prolonged isothermal tempering of the formed fine lamellar interlamellar sorbite, causing some of the cementite in the sorbite to melt and release microstructure stress, thereby regulating the microstructure toughness online and improving the balance between the wire rod's plasticity and high strength. Compared to existing one-stage online salt bath treatment, which struggles to balance microstructure state, production energy consumption, and efficiency, the later stage of molten salt treatment allows for a smaller temperature difference between the wire rod and the molten salt after the initial molten salt treatment, thus appropriately reducing the molten salt circulation. On the one hand, the higher circulation rate of the molten salt treatment in the front stage reduces production energy consumption. On the other hand, the molten salt treatment of the wire rod with a higher circulation rate suppresses abnormal structures such as network carbides, martensite, and coarse pearlite, while promoting the refinement of lamellar structure and the precipitation of microalloyed carbides. This provides a high-strength foundation and reduces the limitations on rolling. By suppressing cementite coarsening and reducing the diffusion ability of carbide-forming elements with Mo, the precipitation and uniform distribution of alloy carbides can improve tempering stability, compensate for the strength loss caused by online softening, and thus extend the subsequent molten salt treatment time, quickly release structural stress, improve the strength-plasticity matching of the wire rod, promote the uniform dispersion of alloy carbides, and improve the uniformity of structure and performance stability. The wire rod is at a higher temperature after exiting the molten salt treatment, and the slow cooling on the roller table can promote further toughening of the wire rod structure. This avoids the slow treatment of online aging at a low cooling rate, thereby controlling the structural state, taking into account both production energy consumption and efficiency, and promoting efficient and stable production of wire rod.

[0022] During the rolling process, a higher heating furnace homogenization temperature and an appropriate furnace dwell time can be used to promote the solid solution and uniform diffusion of alloying elements, reduce compositional segregation, and improve rollability. At the same time, excessively high temperatures and excessively long furnace dwell times can be avoided to prevent austenite grain coarsening. In a preferred embodiment, before rolling, the heating furnace homogenization temperature is controlled at 1193~1233℃ and the furnace dwell time is controlled at 158~228min.

[0023] Higher wire drawing temperatures and online molten salt isothermal treatment can reduce the limitations on rolling. Higher initial rolling temperatures can reduce the deformation resistance of the billet. Combined with a larger initial rolling reduction, the coarse-grained structure of the casting is broken up and the rolling efficiency is improved. With appropriate final rolling temperatures and final rolling reductions, deformation energy is accumulated and stored. Through strain-induced precipitation of nailed grain boundaries in NbC, dynamic recrystallization is promoted during the final rolling process, refining the grains and strengthening the matrix. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1028~1068℃, the initial rolling reduction is 36%~40.5%, the final rolling temperature is 925~955℃, and the final rolling reduction is 23%~28%.

[0024] During the wire spinning process, the wire spinning temperature can be further controlled to suppress grain coarsening in conjunction with the pinning effect of NbC. In a preferred embodiment, the wire spinning temperature is controlled at 908~924℃.

[0025] The lower the molten salt temperature in the initial molten salt treatment, the faster the wire rod cools down, the higher the supercooling, the suppression of network carbides and coarse pearlite, the faster the austenite transforms into sorbite with finer lamellar spacing, and the lower the difficulty of lamellar melting. As the treatment time increases, the lower carbon diffusion rate at low temperatures makes it difficult for cementite to coarsen, promoting uniform lamellar separation and providing sufficient nucleation time for microalloyed carbides, thus promoting their nano-dispersed precipitation. However, excessively low molten salt temperatures will increase the temperature difference between the wire rod surface and core, as well as the phase transformation thermal stress, which is detrimental to the rapid precipitation of microalloyed carbides and the suppression of brittle structures such as martensite. Furthermore, excessively long treatment times unnecessarily increase production energy consumption. Conversely, higher molten salt temperatures accelerate carbon diffusion in austenite and reduce carbon-induced melting. Segregation leads to localized lamellar anomalies and improves microstructure uniformity. Shorter processing times can reduce production energy consumption. However, excessively high molten salt temperatures and short processing times are detrimental to suppressing the coarsening of network carbides, cementite lamellars, and microalloyed carbides, affecting matrix strength and increasing the difference in cross-sectional microstructure. Therefore, the molten salt temperature and processing time of the initial molten salt treatment can be controlled to control the wire rod to quickly transition from the high-temperature austenitic state to the pearlite phase region, suppressing the formation of network carbides and forming a microstructure dominated by fine lamellar spacing sorbite, promoting the dispersed precipitation of microalloyed carbides, and preparing the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the initial molten salt treatment is 547~587℃, and the processing time is 83~223s.

[0026] Since the temperature difference between the spinning temperature and the molten salt temperature in the preceding molten salt treatment is large, a larger molten salt circulation rate can be used in the preceding molten salt treatment to control the molten salt temperature rise, maintain a high cooling rate to suppress abnormal structures, and improve the uniformity of structures in continuous processing. In the preferred technical solution, the molten salt circulation rate in the preceding molten salt treatment is 550~730t / h, and the molten salt temperature rise is ≤8℃.

[0027] The higher the molten salt temperature in the subsequent molten salt treatment, the faster the carbon diffusion and the shorter the transformation time of the retained austenite. With extended treatment time, the temperature difference between the wire rod surface and core can be further reduced. At high temperatures, the cementite lamellar fracture of the sorbite is more easily distributed along the grain, promoting the full precipitation and uniform dispersion of microalloyed carbides, and thus toughening the microstructure online. However, excessively high molten salt temperatures are detrimental to inhibiting cementite coarsening and microalloyed carbide growth. With extended treatment time, high temperatures accelerate cementite growth, weakening the dislocation hindering effect, leading to a loss of strength and plasticity and increased production energy consumption. Conversely, lower molten salt temperatures prolong austenite stability and promote the transformation of the microstructure into fine lamellar sorbite. Shorter treatment times inhibit the aggregation and growth of microalloyed carbides, maintaining strength properties. However, excessively low molten salt temperatures... If the processing time is too short, it will hinder the full transformation of residual austenite into sorbite, the full release of structural stress, and the complete precipitation of microalloyed carbides. This will lead to strength loss, affect the plasticity of the matrix, increase the fluctuation of mechanical properties, and easily cause local stress to exceed the limit when combined with external loads during cold drawing. Therefore, the molten salt temperature and processing time of the subsequent molten salt treatment can be controlled to promote the continued transformation of untransformed residual austenite into fine lamellar interlamellar sorbite. At the same time, the formed fine lamellar interlamellar sorbite is subjected to long-term isothermal tempering, which causes some cementite lamellars to melt and microalloyed carbides to be fully precipitated and uniformly dispersed, so as to regulate the strength and plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 551~571℃, and the processing time is 155~200s.

[0028] The temperature difference between the molten salt in the first stage and the second stage of molten salt treatment of the wire rod is small. The second stage of molten salt treatment can appropriately reduce the molten salt circulation volume, control the molten salt temperature rise, reduce production energy consumption, and promote uniform toughening of the structure. In the preferred technical solution, the molten salt circulation volume of the second stage of molten salt treatment is 375~530t / h, and the molten salt temperature rise is ≤3℃.

[0029] The roller conveyor slow cooling can further control the slow cooling speed, promote the slow release of stress after the wire rod exits the molten salt, promote further toughening of the structure, avoid stress increase caused by rapid cooling, and avoid affecting the off-line efficiency due to excessively slow cooling. In the preferred technical solution, the roller conveyor slow cooling controls the wire rod to cool to below 280°C at a slow cooling speed of 0.8~1.25°C / s before winding.

[0030] 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 of ≥220℃ in the online molten salt 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 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.

[0031] A hot-rolled wire rod for stranded wire of 2260MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method of the hot-rolled wire rod for stranded wire of 2260MPa grade as described in any one of the above-mentioned methods.

[0032] The aforementioned hot-rolled wire rods are designed with a high-carbon composition containing trace amounts of Mo, Nb, and Al, which can appropriately control material costs. The microstructure of the wire rods consists mainly of tempered sorbite and melted sorbite, containing a small amount of ferrite. Compared with the pearlitic wire rods used for air-cooled wire strands, it can effectively suppress network carbides, martensite, and coarse pearlite in the core, making the cementite distributed as fine lamellar sorbite, improving the uniformity of the microstructure and the sorbitization rate. Combined with the dispersed and uniform precipitation of microalloyed carbides, it can enhance the strengthening effect of carbon and microalloying elements. Through the microstructure in Wire toughening can further improve ductility and toughness. Compared with existing salt bath treated wire rods for stranding, the wire rods avoid cementite coarsening and precipitate growth at high temperatures. Through partial cementite melting of sorbite, the wire rods form transitional tempered sorbite that transforms into spheroidized structure. This further reduces residual stress, improves the matching of wire rod ductility and high strength, reduces mechanical property fluctuations, local stress and crack propagation risks, and eliminates the need for offline heat treatment. This reduces the risk of wire breakage during stranding and drawing and twisting, and improves yield and processing efficiency.

[0033] The higher the volume percentage of tempered sorbite and the finer the lamellar spacing, the higher the matrix strength. The higher the volume percentage of fused sorbite, the better the matrix ductility and toughness. In the preferred technical solution, the volume percentage of tempered sorbite is ≥56%, the lamellar spacing is 90~140nm, the volume percentage of ferrite is ≤4%, and the volume percentage of fused sorbite is ≥30%.

[0034] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference within the same coil of ≤47MPa. This can effectively avoid the adverse effects of network carbides on the ductility and toughness of the wire rod, and avoid the risk of intergranular fracture during cold working and service of the stranded wire. By suppressing abnormal structure, achieving uniform phase transformation of the structure, and uniform distribution of microalloyed carbides, the uniformity of the structure can be improved and the fluctuation of mechanical properties can be reduced. The lower mechanical property difference within the same coil can further improve the consistency of stranded wire processing and fatigue resistance, and reduce the risk of fracture.

[0035] In the preferred technical solution, the diameter of the hot-rolled wire rod is 8~15mm, the tensile strength is 1534~1584MPa, and the reduction of area is 33%~38%. The hot-rolled wire rod has high tensile strength, which can reduce the number of drawing passes and quickly reach the target strength, thereby improving production efficiency and reducing plasticity loss during the process. The hot-rolled wire rod has a high reduction of area, which does not require additional heat treatment, thereby reducing the risk of wire breakage and internal defects during drawing and twisting, and improving the yield and service performance of stranded wire.

[0036] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0037] (1) In view of the current situation where hot-rolled wire rods for ultra-high strength stranded wire are constrained by the process and have abnormal structures, making it difficult to balance the strength and plasticity of the structure and production efficiency, this invention adopts a high-carbon composition design containing trace amounts of Mo, Nb and Al, combined with online molten salt isothermal toughening technology. This can control the wire rod to quickly move from the high-temperature austenitic state to the pearlite phase region after the first stage of molten salt treatment, thereby inhibiting the abnormal formation of network carbides and martensite, and forming a structure dominated by fine lamellar interlayer sorbite structure. This promotes the dispersion and precipitation of microalloyed carbides. After the second stage of molten salt treatment, the amount of molten salt circulation is reduced, which promotes the transformation of untransformed residual austenite into fine lamellar interlayer sorbite structure. At the same time, the fine lamellar interlayer sorbite structure formed isothermally tempered for a long time, causing some cementite lamellars to melt and microalloyed carbides to be fully precipitated and uniformly dispersed. This can control the strength and plasticity matching of the wire rod, reduce the restrictions on rolling and slow cooling of the roller table, promote the efficient and stable production of hot-rolled wire rods, and has good industrial adaptability.

[0038] (2) In view of the shortcomings of existing hot-rolled wire rods, such as insufficient strength and plasticity, large fluctuations in mechanical properties, and the need for additional heat treatment to adjust the microstructure and properties of hot-rolled wire rods, resulting in many processes, high costs and low production efficiency for downstream users, this invention can appropriately control material costs. The microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can effectively suppress network carbides, martensite and coarse pearlite in the core, improve the uniformity of the microstructure and the sorbitization rate. With the uniform precipitation of microalloyed carbides, the strengthening effect of carbon and microalloying elements can be improved. The residual stress is further reduced by tempering state control, and the plasticity and high strength of wire rod are matched. The tensile strength is 1534~1584MPa, the reduction of area is 33%~38%, and the mechanical property difference between the same coil is ≤47MPa. It can be used to manufacture 2260MPa grade stranded wire and other application fields, so that no additional heat treatment process is required, improving the yield and production efficiency of high-strength stranded wire, and has good market application prospects. Attached Figure Description

[0039] 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:

[0040] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention;

[0041] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention;

[0042] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0043] 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.

[0044] Example 1:

[0045] A preferred embodiment of the manufacturing method of the 2260MPa grade 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: 0.92%, Mn: 0.55%, Cr: 0.37%, Mo: 0.3%, Nb: 0.017%, Al: 0.4%, 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 isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0046] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 8mm via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1205℃, and the furnace dwell time is 205 min. n, the initial rolling temperature is 1038℃, the initial rolling reduction is 40.5%, the final rolling temperature is 936℃, and the final rolling reduction is 28%; the wire drawing process is used to turn the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is spread on the roller table and conveyed along the roller table. A suitable wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding premature phase transformation or network carbides, and providing a high nucleation driving force for subsequent sorbitic phase transformation and microalloyed carbide precipitation. Specifically, the wire drawing temperature is controlled at 914℃.

[0047] The online molten salt 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 causes the wire rod to cool at a rate of 35°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite with fine interlamellar spacing. This promotes the dispersed precipitation of Cr, Nb, and Mo microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for final molten salt treatment, reducing the molten salt circulation rate and promoting further molten salt precipitation. The untransformed residual austenite continues to transform into fine lamellar sorbite, while the formed sorbite undergoes isothermal tempering, causing some of the cementite lamellars to melt and the microalloyed carbides to fully precipitate and uniformly disperse, thereby controlling the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 587℃, the treatment time is 83s, the molten salt circulation rate is 550t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 571℃, the treatment time is 200s, the molten salt circulation rate is 375t / h, and the molten salt temperature rise is ≤3℃.

[0048] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥220℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 279℃ at a slow cooling rate of 1.25℃ / 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, the metallographic structure of which is shown in the figure below. Figure 1 As shown.

[0049] 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 → Steyrmore air cooling. Specifically, the heating furnace homogenization temperature is controlled at 1165℃, the furnace time is 252 min, the initial rolling temperature is 1010℃, the initial rolling reduction is 27.5%, the final rolling temperature is 835℃, the final rolling reduction is 36.5%, and the wire drawing temperature is 805℃; the Steyrmore forced air cooling uses an air volume of 200,000 m³ / s per fan. 3 At 70% capacity, fans 1 to 5 are turned on to cool the wire rod to 699℃ at a cooling rate of 7.2℃ / s. Then, fans 6 to 14 are turned on to 20% capacity to cool the wire rod to 263℃ at a cooling rate of 2.6℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0050] Comparative Example 2: A method for manufacturing hot-rolled wire rod, the difference between this method and Example 1 is that: the heating furnace homogenization temperature is controlled at 1145℃, the furnace time is 260min, the initial rolling temperature is 1000℃, the initial rolling reduction is 25%, the final rolling temperature is 830℃, the final rolling reduction is 35%, and the wire drawing temperature is 805℃; during the molten salt treatment before the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 30℃ / s, and the finished hot-rolled wire rod is obtained after leaving the production line.

[0051] Example 2:

[0052] A preferred embodiment of the manufacturing method of the 2260MPa grade 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: 0.90%, Si: 0.82%, Mn: 0.68%, Cr: 0.55%, Mo: 0.15%, Nb: 0.025%, Al: 0.31%, P: 0.014%, 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 isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0053] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 11mm via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1233℃, and the furnace dwell time is 158 minutes. The initial rolling temperature is 1068℃, the initial rolling reduction is 39%, the final rolling temperature is 955℃, and the final rolling reduction is 26%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature phase transformation or network carbides, and providing a high nucleation driving force for subsequent sorbitic phase transformation and microalloyed carbide precipitation. Specifically, the wire drawing temperature is controlled at 924℃.

[0054] The online molten salt 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 causes the wire rod to cool at a rate of 36°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of Cr, Nb, and Mo microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for final molten salt treatment, reducing the molten salt circulation rate and promoting… Untransformed residual austenite continues to transform into fine lamellar sorbite, while the formed sorbite undergoes isothermal tempering, causing some of the cementite lamellars to melt and the microalloyed carbides to fully precipitate and uniformly disperse, thereby controlling the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 569℃, the treatment time is 115s, the molten salt circulation rate is 590t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 563℃, the treatment time is 180s, the molten salt circulation rate is 390t / h, and the molten salt temperature rise is ≤3℃.

[0055] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥220℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 276℃ at a slow cooling rate of 1.1℃ / 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, the metallographic structure of which is shown in the figure below. Figure 2 As shown.

[0056] Comparative Example 3: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the first stage of the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 39°C / s, the molten salt temperature of the first stage of the molten salt treatment is 515°C, the treatment time is 230s, and the finished hot-rolled wire rod is obtained after going offline.

[0057] Comparative Example 4: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the first stage of the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 31℃ / s, the molten salt temperature of the first stage of the molten salt treatment is 600℃, the treatment time is 75s, and the finished hot-rolled wire rod is obtained after going offline.

[0058] Example 3:

[0059] A preferred embodiment of the manufacturing method of the 2260MPa grade 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: 0.91%, Si: 0.83%, Mn: 0.75%, Cr: 0.49%, Mo: 0.1%, Nb: 0.022%, Al: 0.36%, P: 0.014%, 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 isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0060] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 14mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1228℃, and the furnace dwell time is 172 minutes. The initial rolling temperature is 1057℃, the initial rolling reduction is 37%, the final rolling temperature is 944℃, and the final rolling reduction is 24%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature phase transformation or network carbides, and providing a high nucleation driving force for subsequent sorbitic phase transformation and microalloyed carbide precipitation. Specifically, the wire drawing temperature is controlled at 920℃.

[0061] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first stage salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 38°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite with fine interlamellar spacing. This promotes the dispersed precipitation of Cr, Nb, and Mo microalloyed carbides. The wire rod is then conveyed via rollers through the second stage salt bath for final molten salt treatment, reducing the molten salt circulation rate and promoting… Untransformed residual austenite continues to transform into fine lamellar sorbite, while the formed sorbite undergoes isothermal tempering, causing some of the cementite lamellars to melt and the microalloyed carbides to fully precipitate and uniformly disperse, thereby controlling the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 554℃, the treatment time is 165s, the molten salt circulation rate is 640t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 557℃, the treatment time is 170s, the molten salt circulation rate is 445t / h, and the molten salt temperature rise is ≤3℃.

[0062] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥220℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 273℃ at a slow cooling rate of 0.95℃ / 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, the metallographic structure of which is shown in the figure below. Figure 3 As shown.

[0063] 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 596°C, the treatment time is 205s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0064] 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 520°C, the treatment time is 85s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0065] Example 4:

[0066] A preferred embodiment of the manufacturing method of the 2260MPa grade 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: 0.89%, Si: 0.95%, Mn: 0.73%, Cr: 0.57%, Mo: 0.28%, Nb: 0.01%, Al: 0.28%, 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 isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0067] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 15mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1193℃, and the furnace dwell time is 228 minutes. The initial rolling temperature is 1028℃, the initial rolling reduction is 36%, the final rolling temperature is 925℃, and the final rolling reduction is 23%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature phase transformation or network carbides, and providing a high nucleation driving force for subsequent sorbitic phase transformation and microalloyed carbide precipitation. Specifically, the wire drawing temperature is controlled at 908℃.

[0068] The online molten salt 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 causes the wire rod to cool at a rate of 37°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of Cr, Nb, and Mo microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for final molten salt treatment, reducing the molten salt circulation rate and promoting… Untransformed residual austenite continues to transform into fine lamellar sorbite, while the formed sorbite undergoes isothermal tempering, causing some of the cementite lamellars to melt and the microalloyed carbides to fully precipitate and uniformly disperse, thereby controlling the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 547℃, the treatment time is 223s, the molten salt circulation rate is 730t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 551℃, the treatment time is 155s, the molten salt circulation rate is 530t / h, and the molten salt temperature rise is ≤3℃.

[0069] The slow cooling process of the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥220℃) from the two salt bath tanks of the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering and softening effect of the wire rod. Specifically, the wire rod is cooled to 270℃ at a slow cooling rate of 0.8℃ / 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.

[0070] Comparative Example 7: A method for manufacturing hot-rolled wire rod, the difference between which is that the manufacturing method is according to the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → air cooling. The air cooling process is carried out by opening the heat insulation cover and controlling the wire rod to cool to 264°C at a cooling rate of 1.7°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0071] 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:

[0072] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0073]

[0074] The comparison between Example 1 and Comparative Example 1 shows that, compared to the design using a high-carbon composition containing trace amounts of Mo, Nb, and Al, the Stellmore air-cooled line produces a higher level of network carbides and martensitic anomalous structures, a higher proportion of ferrite, and coarse pearlite in the core, making it difficult to exert the strengthening effect of carbon and microalloying. This results in insufficient strength and plasticity, and significant fluctuations in mechanical properties. This invention, combined with online molten salt isothermal toughening technology, can control the wire rod to quickly transition from a high-temperature austenitic state to the pearlite phase region after the initial molten salt treatment, rapidly bypassing the network carbide zone. This suppresses the formation of network carbides and martensitic anomalous structures, resulting in a microstructure dominated by fine lamellar interlayer sorbite, promoting the dispersed precipitation of microalloyed carbides. After the subsequent molten salt treatment... The process involves reducing the molten salt circulation rate to promote the transformation of untransformed residual austenite into fine lamellar sorbite. Simultaneously, it promotes prolonged isothermal tempering of the formed fine lamellar sorbite, causing some cementite lamellars to melt and microalloyed carbides to fully precipitate and uniformly disperse. Tempering further reduces residual stress, improving the balance between wire rod plasticity and high strength. As seen in Examples 1-4, hot-rolled wire rods can achieve tensile strengths of 1534-1584 MPa, a reduction of area of ​​33%-38%, and a mechanical property difference of ≤47 MPa between coils. This allows for applications such as manufacturing 2260 MPa grade stranded wire, eliminating the need for additional heat treatment processes and improving the yield and production efficiency of high-strength stranded wire.

[0075] As can be seen from the comparison results of Example 1 and Comparative Example 2, selecting a higher wire drawing temperature can keep the wire rod in a high-temperature austenitic state, where carbon and microalloying elements are dissolved in solid solution, avoiding excessively low temperatures, local carbon enrichment to form network carbides, or premature phase transformation. At the same time, it provides a high nucleation driving force for subsequent sorbitic phase transformation and alloy carbide precipitation.

[0076] 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 faster the wire rod cools down, the higher the supercooling, the more it suppresses network carbides and coarse pearlite, the more it promotes the transformation of austenite to sorbite with finer lamellar spacing, and the lower the difficulty of lamellar melting. As the treatment time increases, the diffusion rate of carbon elements is lower at low temperatures, and cementite is difficult to coarsen, which can promote the uniformity of lamellars and provide sufficient nucleation time for microalloyed carbides, promoting the nano-dispersion precipitation of microalloyed carbides. However, if the molten salt temperature is too low, it will increase the temperature difference and phase transformation thermal stress from the wire rod surface to the core, which is not conducive to the rapid precipitation of microalloyed carbides. At the same time, as the treatment time is too long, it will unnecessarily increase the production energy consumption.

[0077] As can be seen from the comparison results of Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the faster the diffusion of carbon in austenite, the less local lamellar abnormalities caused by carbon segregation, and the better the uniformity of the microstructure. The shorter the treatment time, the lower the production energy consumption. However, if the molten salt temperature is too high and the treatment time is too short, it will be detrimental to suppressing the coarsening of network carbides, cementite lamellars and microalloyed carbides, which will affect the matrix strength and increase the difference in cross-sectional microstructure.

[0078] As can be seen from the comparison results between Example 3 and Comparative Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the faster the diffusion of carbon and the shorter the transformation time of the residual austenite. With the extension of the treatment time, the temperature difference between the wire rod surface and the core can be further reduced. At high temperature, the cementite lamellar fracture of sorbite is more likely to be distributed along the grain, which can promote the full precipitation and uniform dispersion of microalloyed carbides and toughen the structure in the line. However, if the molten salt temperature is too high, it is not conducive to suppressing cementite coarsening and microalloyed carbide growth. With the extension of the treatment time, the high temperature accelerates cementite growth, the dislocation inhibition effect weakens, which will lead to loss of strength and plasticity and increase production energy consumption.

[0079] 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 longer the austenite stability can be extended, and the microstructure can be promoted to transform into fine lamellar sorbite. The shorter the treatment time, the more the microalloyed carbide agglomeration and growth can be suppressed, and the strength performance can be maintained. However, if the molten salt temperature is too low and the treatment time is too short, it will be detrimental to the full transformation of residual austenite into sorbite, the full release of microstructure stress, and the full precipitation of microalloyed carbides. This will lead to strength loss, affect the plasticity of the matrix, and increase the fluctuation of mechanical properties.

[0080] As can be seen from the comparison results of Example 4 and Comparative Example 7, online molten salt isothermal toughening treatment can reduce the limitation of slow cooling on the roller table. The slow cooling on the roller table can promote the further slow release of stress after the wire rod exits the molten salt, and promote further toughening of the structure.

[0081] 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 of manufacturing a hot-rolled wire rod for a 2260 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The manufacturing method of the hot rolled wire rod comprises the following steps: rolling a wire rod according to the chemical composition of a hot rolled coil, wherein the chemical composition of the hot rolled coil and the mass percentage of the hot rolled coil are as follows: C: 0.89% to 0.93%, Si: 0.82% to 0.95%, Mn: 0.55% to 0.75%, Cr: 0.37% to 0.57%, Mo: 0.1% to 0.3%, Nb: 0.010% to 0.025%, Al: 0.28% to 0.4%, P: ≤0.014%, S: ≤0.014%, and the rest is Fe and inevitable impurities; after the wire rod is spun into a coil according to a spinning temperature of ≥908 ℃, the wire rod is subjected to on-line molten salt isothermal toughening treatment, the wire rod is first subjected to front-stage molten salt treatment and is cooled at a cooling speed of ≥35 ℃ / s, and the wire rod is changed from an austenite state into a sorbite phase region to form a structure mainly composed of sorbite structure, then the wire rod is subjected to rear-stage molten salt treatment, the molten salt circulation amount is reduced, the untransformed residual austenite is continuously transformed into sorbite and is subjected to isothermal tempering, and part of the sorbite lamellas is melted and broken, and finally the wire rod is subjected to roller slow cooling to be manufactured into a hot rolled coil with a microstructure composed of a mixed structure of tempered sorbite, ferrite and melted and broken sorbite; the molten salt temperature of the front-stage molten salt treatment is 547 to 587 ℃, and the treatment time is 83 to 223 s; the molten salt temperature of the rear-stage molten salt treatment is 551 to 571 ℃, and the treatment time is 155 to 200 s.

2. The method of producing a hot rolled wire rod for a 2260 MPa grade strand according to claim 1, characterized by, Before the rolling, the soaking temperature of a heating furnace is controlled to be 1193 to 1233 ℃, and the furnace time is 158 to 228 min.

3. The method of producing a hot rolled wire rod for 2260 MPa grade strand wire according to claim 1, characterized by, During the rolling, the initial rolling temperature is controlled to be 1028 to 1068 ℃, the initial rolling reduction is 36% to 40.5%, the final rolling temperature is 925 to 955 ℃, and the final rolling reduction is 23% to 28%.

4. The method of producing a hot rolled wire rod for a 2260 MPa grade strand wire according to claim 1, characterized by, During the spinning, the spinning temperature is controlled to be 908 to 924 ℃.

5. The method of producing a hot rolled wire rod for a 2260 MPa grade strand wire according to claim 1, characterized by, The molten salt circulation amount of the front-stage molten salt treatment is 550 to 730 t / h, and the molten salt temperature rise is ≤8 ℃; the molten salt circulation amount of the rear-stage molten salt treatment is 375 to 530 t / h, and the molten salt temperature rise is ≤3 ℃.

6. The method of producing a hot rolled wire rod for a 2260 MPa grade strand wire according to claim 1, characterized by, The roller slow cooling controls the coil to be cooled to below 280 ℃ at a slow cooling speed of 0.8 to 1.25 ℃ / s and to be coiled.

7. A hot rolled wire rod for 2260 MPa grade strand, characterized in that, The hot rolled coil is manufactured by the manufacturing method of the hot rolled coil for a 2260 MPa grade twisted wire rod according to any one of claims 1 to 6.

8. The hot rolled wire rod for 2260 MPa grade strand according to claim 7, characterized by, The volume percentage of the tempered sorbite is ≥56%, the lamellar spacing is 90 to 140 nm, the volume percentage of the ferrite is ≤4%, the volume percentage of the melted and broken sorbite is ≥30%, the reticular carbide level of the hot rolled coil is 0 level, and the mechanical property same circle difference is ≤47 MPa.

9. The hot rolled wire rod for 2260 MPa grade strand as claimed in claim 7, wherein, The diameter of the hot rolled coil is 8 to 15 mm, the tensile strength is 1534 to 1584 MPa, and the reduction of area is 33% to 38%.

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