A hot rolled wire rod for 2200mpa class strand and a manufacturing method thereof

Abstract

The present application relates to a kind of 2200MPa grade hot rolled wire rod for stranding wire and its manufacturing method, after Nb-Al high carbon component rolling spinning is wire rod, through on-line molten salt isothermal treatment, wire rod is first through the front section molten salt treatment and from austenite state into sorbite phase region, form the structure mainly with sorbite structure, again through the rear section molten salt treatment, improve molten salt temperature, reduce molten salt circulation amount, promote the residual austenite that is not transformed continue to be transformed into sorbite and isothermal tempering, promote part of sorbite lamella to be fused off, finally through slow cooling of roller bed, be made into the hot rolled wire rod of microstructure including tempered sorbite, ferrite and fused-off sorbite the mixed structure of composition, can inhibit abnormal structure, improve hot rolled wire rod strength, plasticity and production efficiency are considered, reach tensile strength is 1490~1540MPa, area reduction is 34%~39%, so that downstream user is exempted from offline heat treatment, promote stranding wire stable production.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled wire rod production technology, specifically relating to a 2200MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] With the continuous and rapid growth in demand from railway concrete bridges, coal mining engineering, building construction, wind power, and other fields, the strength requirements for steel used in strand are constantly increasing. Improving the strength grade of steel wire can directly reduce the amount of steel wire used, achieving energy conservation and emission reduction goals. After the strength grade of steel strand is increased to 2200MPa, the hot-rolled wire rod base material usually needs to be adjusted in conjunction with offline heat treatment to achieve the performance target. This not only involves more processes but also significantly increases production costs and energy consumption. Conversely, reducing the strength of hot-rolled wire rod can easily lead to insufficient work hardening increments, forcing the increase of drawing passes or diameter reduction, resulting in wire breakage risks and affecting production efficiency. Therefore, it is necessary to develop a high-strength hot-rolled wire rod with good plasticity for strands with a 2200MPa grade to meet the development needs of the steel industry and market demand.

[0003] The following technical bottlenecks still exist in manufacturing high-strength, high-ductility hot-rolled wire rods for 2200MPa grade stranded wire:

[0004] I. Existing stranded wire rods are generally produced using the Steyrmore air-cooled wire line. For example, patent CN113652610A discloses an ultra-high strength stranded wire rod for railway bridges and its production process, which uses an 87MnSi composition design combined with small-section rolling and Steyrmore cooling line to produce sorbitic wire rods. However, on the one hand, the steel has a high carbon content, and due to the limitation of the maximum cooling capacity of the Steyrmore air-cooled wire line, carbon elements are prone to precipitate at the grain boundaries to form network carbides. The presence of network carbides will rupture the matrix, reduce the plasticity and toughness of the material, and also lose the carbon source for phase transformation, increasing the fluctuation of mechanical properties. In order to suppress network carbides and improve air-cooling strength, as the wire rod diameter increases, the temperature difference between the air-receiving and air-receiving surfaces, and between the surface and the core, will be further increased due to alloy element segregation and quenching. The rolling process has several drawbacks. Localized overcooling of the wire rod can lead to the formation of brittle phases such as martensite, increasing the risk of coil breakage and wire breakage during drawing. Meanwhile, slow cooling in the core results in the formation of coarse pearlite, affecting the uniformity of the microstructure and the final mechanical properties. On the other hand, while decreasing the cross-sectional dimensions and increasing the rolling temperature helps reduce the risk of cracking, it also raises the wire drawing temperature. Limited by the length and minimum cooling capacity of the Steyrmore air-cooling line, the sorbite incubation time is shortened, leading to strength loss. Furthermore, the different phase transformation volumes of proeutectoid ferrite and pearlite result in insufficient austenite transformation, abnormal precipitation at low temperatures, and uneven phase transformation temperatures. This leads to a significant stress difference between the surface and the core, reducing the wire rod's ductility and toughness, and affecting the stability of its mechanical properties. Long-term heat treatment offers limited improvement and prolongs the production cycle.

[0005] II. Although existing technologies have proposed salt bath isothermal treatment processes for steel strand wire rods, such as the 2200~2260MPa grade corrosion-resistant prestressed steel strand wire rod, steel strand, and its production method disclosed in patent CN120400691A, which uses a C-Si-Mn-Cr-V composition design and combines online salt bath isothermal treatment after low-temperature rolling to produce sorbite wire rods, on the one hand, the controlled cooling capacity of the one-stage salt bath treatment is limited, and high temperature is not conducive to sorbite nucleation and lamellar refinement, resulting in strength loss; on the other hand, low-temperature isothermal treatment affects the atomic diffusion rate, resulting in a longer time for stress release in the microstructure, and is also affected by the alloy carbon... Due to limitations in chemical precipitation and stability, the isothermal process in the salt bath cannot be used for extended periods. This necessitates long-term online aging at low cooling rates to compensate for plasticity, which also impacts production efficiency. To control precise phase transformation and suppress brittle structures such as martensite, a single-stage molten salt treatment requires a large molten salt circulation volume, resulting in high overall energy consumption. On the other hand, to facilitate the precipitation of V, adapt to the wire drawing temperature, and accommodate the phase deformation nuclei of the molten salt treatment, low-temperature rolling is required. However, the high alloying elements result in greater resistance to deformation during the rolling of large-size billets, which also increases wear on the rolling line. Further increasing the alloy content to improve wire rod strength will further affect rolling and overall 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 stranded wire of 2200MPa grade and its manufacturing method, which can suppress abnormal structure, improve the strength of hot-rolled wire rod, and balance plasticity and production efficiency, so that downstream users can avoid offline heat treatment and promote stable production of stranded wire.

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

[0008] A method for manufacturing hot-rolled wire rod for 2200MPa 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: 0.88%~0.92%, Si: 0.56%~0.76%, Mn: 0.72%~0.92%, Cr: 0.38%~0.54%, Nb: 0.005%~0.025%, Al: 0.08%~0.15%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities. The wire rod is spun at a spinning temperature of ≥925℃. After being processed into wire rod, the wire rod undergoes an online molten salt isothermal treatment. This process involves first passing through a pre-treatment molten salt stage and cooling at a rate of ≥37℃ / s, causing it to transition from the austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. Then, a post-treatment molten salt stage is performed, increasing the molten salt temperature and reducing the molten salt circulation rate. This promotes the continued transformation of untransformed residual austenite into sorbite, followed by isothermal tempering and isothermal melting of some sorbite lamellars. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in 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: C is an interstitial solid solution strengthening element with a relatively lower price. It can be dissolved in austenite to improve lattice distortion. At the same time, it can work synergistically with Cr and Nb to enhance carbide stability, inhibit high-temperature coarsening, regulate austenite stability, and reduce the sorbite phase transformation temperature. This is beneficial to improve the phase transformation driving force, promote the precipitation of sufficient fine lamellar sorbite during phase transformation, reduce austenite residue in the previous molten salt treatment, and improve the strength stability after tempering by forming carbides. However, if the C content is too high, it will aggravate the center segregation, easily accumulate at the austenite grain boundary and precipitate continuous network carbides. At the same time, the phase transformation incubation period is prolonged, the uniformity of the structure and the difficulty of controlling the martensitic deterioration structure increase, resulting in increased wire brittleness and mechanical property fluctuations. Therefore, in order to take into account the high strength requirements of 2200MPa grade stranded wire, improve the uniformity of the structure and promote rapid production, the mass percentage of C is controlled at 0.88%~0.92%.

[0012] (2) Silicon: Si can work with Al to improve purity and is also a solid solution strengthening element. By dissolving in the matrix phase of sorbite, it can hinder dislocation movement, improve tensile strength, and reduce work hardening load during cold drawing. During the austenite-to-sorbite phase transformation, it can suppress proeutectoid ferrite and promote the nucleation of fine lamellar sorbite. During isothermal tempering, it can increase the activity of C in the retained austenite, promote the decomposition of the retained austenite, improve the high temperature stability of cementite, prevent cementite from coarsening due to the aggregation of C atoms, resist tempering softening, and avoid insufficient drawing strength due to softening. However, excessive silicon will promote decarburization, increase intragranular segregation, and inhibit the cross-slip of dislocations, making isothermal tempering softening more difficult and reducing production efficiency. Therefore, in order to adapt to the control of phase transformation structure by online molten salt strong isothermal, the resistance to tempering softening ability should be appropriately improved while taking into account production efficiency. The mass percentage of Si should be controlled at 0.56%~0.76%.

[0013] (3) Manganese: Mn is an austenite stabilizing element that can reduce the resistance to high-temperature deformation, expand the austenite region, and improve the hardenability of wire rod. In turn, it can improve strength and toughness by refining the sorbite lamellars, accelerate phase transformation kinetics, promote the complete decomposition of residual austenite, ensure sufficient phase transformation under rapid cooling, and at the same time hinder the diffusion of C atoms into cementite particles, thereby improving the strength stability 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, and increase the risk of forming bainite and martensite during the cooling process, leading to increased brittleness and mechanical property fluctuations, and affecting the uniformity of wire rod structure. Therefore, in order to take into account the high strength and plasticity of hot-rolled wire rod and provide a certain tempering stability, the mass percentage of Mn is controlled at 0.72%~0.92%.

[0014] (4) Chromium: Cr can provide solid solution strengthening effect, while enhancing hardenability and prolonging the stability of austenite. It shifts the continuous cooling transformation curve to the right, prolongs the incubation period of austenite to sorbite transformation, and makes the sorbite lamellar spacing finer during low-temperature transformation. At the same time, Cr has a strong affinity for C and can form fine and dispersed Cr-based carbides distributed in the sorbite lamellars. It can work with Nb to optimize the microstructure and continuously pin dislocations, slow down the diffusion rate of cementite, 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 microstructures, and reduce the activity of carbon during isothermal tempering. Excessive carbide nucleation density will increase the risk of coarsening and affect the drawability of steel. Therefore, in order to promote sorbite transformation and provide appropriate resistance to tempering softening, the mass percentage of Cr is controlled at 0.38%~0.54%.

[0015] (5) Niobium: During the hot rolling process, the carbonitrides of Nb can pin the austenite grain boundaries, inhibit the growth of austenite grains at high temperatures, reduce the deformation resistance during hot rolling, and reduce the risk of rolling cracks, so as to facilitate rapid rolling. At the same time, the refined grains can optimize the sorbite structure, which not only improves the strength through fine grain strengthening, but also indirectly improves the plasticity and reduces the structure fluctuation. During the molten salt treatment, Nb will further precipitate nano-sized NbC. NbC has a high melting point and better stability than VC. It can fill the strength gap by pinning dislocations, inhibit the coarsening of Cr carbides at grain boundaries, improve strength and toughness, make the dislocation distribution more uniform during the drawing process of the wire rod, reduce the cold work hardening rate, and improve the service toughness. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling the material cost. Therefore, the mass percentage of Nb is controlled at 0.005%~0.025%.

[0016] (6) Aluminum: Al can be used as a deoxidizer to improve the purity of molten steel, form aluminum nitride to pin grain boundaries, inhibit the growth of austenite grains, reduce the diffusion rate of C atoms through solid solution dragging, and improve the sufficiency of phase transformation. At the same time, after the grain boundary migration is blocked, the diffusion path of C atoms to cementite particles is extended, which can inhibit the formation of coarse carbides during cooling and the coarsening of cementite during isothermal tempering, thus playing a role in refining precipitates and improving the toughness of materials. However, if the Al content is too high, it will increase the risk of coarse inclusions and defects in the billet. Therefore, in order to control the degree of melting of sorbite lamellars to balance strength and plasticity and to inhibit high-temperature tempering brittleness to improve service toughness, the mass percentage of Al is controlled at 0.08%~0.15%.

[0017] (7) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% should be controlled.

[0018] The aforementioned hot-rolled wire rod adopts a high-carbon composition design of C-Si-Mn-Cr-Nb-Al. Carbon provides the strength foundation, Mn and Cr regulate the hardenability of the wire rod, lower the sorbite transformation temperature, and adapt to the molten salt temperature range. Si and Al suppress cementite coarsening, and the combined strengthening of Cr and Nb improves tempering stability. This provides favorable conditions for high-temperature wire drawing to suppress austenite grain growth, ensure the driving force of sorbite phase transformation, promote sorbite lamellar refinement, improve the sufficiency of phase transformation, and maintain high strength characteristics after tempering. Furthermore, using a higher wire drawing temperature promotes austenite homogenization and prevents premature carbon precipitation into network carbides and proeutectoid ferrite during wire drawing. This lays the foundation for subsequent rapid cooling to suppress grain boundary carbon aggregation and refine sorbite lamellars. After wire drawing, the wire rod undergoes direct online molten salt isothermal treatment.

[0019] I. Compared to the limitations of the Stellmore air-cooled line, which suffers from high cooling capacity and unstable temperature control, making it difficult to control network carbides and martensitic abnormal structures, the molten salt treatment of the wire rod offers several advantages. First, the high heat transfer capacity of the molten salt promotes rapid cooling of the wire rod, bypassing the precipitation temperature range of secondary cementite. This suppresses the risk of network carbides caused by carbon segregation, ensures the carbon source for subsequent phase transformation, and avoids the brittleness problems caused by network carbides. Second, the molten salt can cover the surface of the wire rod for uniform heat exchange, eliminating the temperature difference between the air-cooled and air-received surfaces compared to the air-cooled line. As the treatment time increases, it can reduce the temperature difference between the surface and the core when the wire rod size increases. Temperature gradients prevent localized overcooling from forming brittle martensite or slow cooling in the core from forming coarse pearlite, thus reducing the accumulation of thermal and phase transformation stresses. Compared to the limitations of single-stage molten salt treatment in controlling cooling, which makes it difficult to balance rolling efficiency and sorbite phase transformation, the first-stage molten salt treatment has a lower molten salt temperature and a larger molten salt circulation volume than the second-stage molten salt treatment. Combined with high-temperature rolling, it can quickly remove heat from the wire rod, increase undercooling, approach the nose temperature of sorbite phase transformation, inhibit lamellar coarsening, promote sorbite nucleation and lamellar refinement, and increase the dispersion precipitation of Cr and Nb carbides, providing higher matrix strength.

[0020] II. Compared to the limitations of continuous cooling and minimum cooling capacity of the Stellmore air-cooling line, which makes it difficult to control the full phase transformation of austenite and release structural stress online, the wire rod, after the first stage of molten salt treatment, can form a structure dominated by fine lamellar sorbite. The subsequent molten salt treatment, with a slightly higher temperature than the first stage, can increase the atomic diffusion rate, extend the treatment time in the sorbite phase region, promote the continued decomposition of untransformed residual austenite during phase transformation inoculation, transform it into sorbite, improve the matrix strength, and prevent residual austenite from continuing to form a low-temperature brittle structure during subsequent cooling. At the same time, the uniform and sufficient structural phase transformation reduces the accumulation of stress in the wire rod. The isothermal treatment in the high-temperature range further causes the cementite lamellars of the sorbite to gradually melt and release structural stress, avoiding stress concentration. Compared to the single-stage molten salt treatment... Limited cooling capacity makes it difficult to simultaneously address sorbite phase transformation, energy consumption, and softening control. The subsequent molten salt treatment, by increasing the molten salt temperature, enhances tempering thermal dynamics, accelerates atomic diffusion, and promotes the rapid precipitation and decomposition of C from the retained austenite into sorbite. This, combined with Si and Al to suppress cementite coarsening and the composite strengthening of pinned dislocations by Cr and Nb, improves tempering stability. This allows for rapid online control of wire rod toughness without excessive loss of matrix strength. Furthermore, the lower molten salt circulation rate in the subsequent molten salt treatment reduces overall production energy consumption. Since the wire rod has already undergone prolonged isothermal tempering and softening, the subsequent slow cooling on the roller conveyor can maintain the high-temperature state after exiting the molten salt, further promoting toughening of the wire rod structure. However, it eliminates the need for excessively low online aging rates, increasing the production speed and efficiency to achieve a balance between wire rod strength and plasticity, as well as efficient and stable production.

[0021] Before rolling, selecting a higher heating furnace temperature and an appropriate furnace time can promote uniform diffusion of alloy components, eliminate cross-sectional temperature gradients, release casting internal stress, improve rolling uniformity and rollability, and at the same time avoid austenite grain coarsening caused by excessively high temperature and excessively long furnace time. In the preferred technical solution, before rolling, the heating furnace homogenization temperature is controlled at 1180~1220℃ and the furnace time is controlled at 150~220min.

[0022] Because the high wire-drawing temperature reduces the limitation on rolling temperature, and in conjunction with the precipitation strengthening effect of Nb, a higher initial rolling temperature can be selected during rolling to improve rollability, reduce rolling deformation resistance and cracking risk, and increase rolling speed and efficiency. A larger initial rolling reduction can break up the heated austenite grains and eliminate casting defects. An appropriate final rolling temperature can be selected to connect with the wire-drawing temperature. The final rolling reduction can control dimensional accuracy, induce Nb carbonitride precipitation to pin grain boundaries, refine austenite grains, and strengthen the matrix, providing a good microstructure for phase transformation. In a preferred embodiment, the initial rolling temperature is controlled at 1030~1080℃, the initial rolling reduction is 34%~37.5%, the final rolling temperature is 925~955℃, and the final rolling reduction is 23%~28%.

[0023] During the wire spinning process, the wire spinning temperature can be further controlled to reduce the risk of austenite grain coarsening. In a preferred embodiment, the wire spinning temperature is controlled to be 925~950℃.

[0024] The lower the molten salt temperature in the initial molten salt treatment, the better it is for suppressing network carbides, reducing atomic diffusion rate, making cementite less coarsening, promoting the refinement of sorbite lamellars, and resulting in finer Cr and Nb carbide precipitation. With prolonged treatment time, the temperature difference between the wire rod surface and core can be reduced, promoting the uniform transformation of most austenite to sorbite, facilitating the dispersed precipitation of alloy carbides, and improving matrix strength. However, if the molten salt temperature is too low, exceeding the nose temperature of the sorbite phase transformation and the alloy carbide precipitation range, it will affect the nucleation and carbide precipitation rate, and may even produce low-temperature abnormal structures. Prolonged treatment time will increase production energy consumption. Conversely, higher molten salt temperatures can reduce the temperature difference between the wire rod surface and core, reduce structural stress and molten salt circulation, promote uniform diffusion of alloy carbides, and shorten the phase transformation incubation period. Shorter treatment time can reduce production energy consumption, but the molten salt temperature... Excessive temperature is detrimental to suppressing the coarsening of network carbides and cementite lamellars, as well as the rapid precipitation of alloy carbides. With insufficient treatment time, the amount of untransformed residual austenite in the microstructure increases, leaving it for the subsequent molten salt treatment stage. This reduces the amount of dispersed alloy carbides precipitated, diminishing the pinning effect on dislocations and accelerating softening, thus reducing matrix strength. Therefore, the molten salt temperature and treatment time of the initial molten salt treatment can be controlled to ensure the wire rod rapidly transitions from the high-temperature austenitic state to the sorbite phase region, suppressing network carbide formation and forming a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of Cr and Nb carbides, improves matrix strength, and prepares the microstructure for the subsequent molten salt treatment. In a preferred embodiment, the molten salt temperature of the initial molten salt treatment is 525~560℃, and the treatment time is 80~280s.

[0025] Since the spinning temperature differs significantly from the molten salt temperature of the preceding molten salt treatment, using a higher molten salt circulation rate can improve heat exchange efficiency, quickly remove heat from the wire rod, and control the consistency of fine lamellar structure formation and continuous processing. In the preferred technical solution, the molten salt circulation rate of the preceding molten salt treatment is 555~650t / h, and the molten salt temperature rise is ≤8℃.

[0026] The molten salt treatment in the latter stage has a higher molten salt temperature than the former. Higher molten salt temperatures accelerate the decomposition of residual austenite and increase atomic diffusion rates. With prolonged treatment time, the temperature gradient across the wire rod cross-section can be further reduced, promoting stress release and uniform diffusion of alloy carbides within the microstructure, thus improving the matrix's ductility and toughness. However, excessively high molten salt temperatures are detrimental to controlling the refinement of the sorbite lamellars formed during decomposition and inhibiting cementite coarsening. With prolonged treatment time, the amount of fractured sorbite in the microstructure increases, and cementite and alloy carbides aggregate and grow, reducing the anchoring effect and leading to a decrease in strength and ductility, while also increasing production energy consumption. Conversely, lower molten salt temperatures promote the refinement of the formed sorbite, supplement the precipitation of alloy carbides to improve the tempering stability of the steel, offset the softening effect caused by high temperatures, and retain the matrix strength. With shorter treatment times, further... While reducing production energy consumption, excessively low molten salt temperatures slow down the decomposition of retained austenite, prolonging the inoculation time and hindering the release of structural stress and uniform diffusion of alloy carbides. Conversely, excessively short treatment times result in insufficient sorbite melting, loss of ductility and toughness, and even the formation of low-temperature structures due to retained austenite during subsequent cooling, leading to fluctuations in mechanical properties. Therefore, subsequent molten salt treatment allows for control of molten salt temperature and treatment time, promoting the transformation of untransformed retained austenite into fine lamellar sorbite, facilitating the precipitation of alloy carbides, and promoting prolonged isothermal tempering of the formed fine lamellar sorbite structure. This process causes the melting of some sorbite cementite lamellars, thus regulating the strength-ductility balance of the wire rod. In a preferred embodiment, the molten salt temperature for the subsequent molten salt treatment is 572~592℃, and the treatment time is 160~250s.

[0027] Since the processing time of the first stage molten salt treatment is relatively long, the latent heat released by the phase change of the wire rod is small, and the temperature difference between the first stage molten salt treatment and the second stage molten salt treatment is small, the second stage molten salt treatment can appropriately reduce the molten salt circulation volume, accurately control the temperature and reduce production energy consumption. In the preferred technical solution, the molten salt circulation volume of the second stage molten salt treatment is 100~240t / h, and the molten salt temperature rise is ≤3℃.

[0028] Since the wire rod has undergone sufficient phase transformation and released structural stress through online molten salt isothermal treatment, the slow cooling on the roller table can further control the cooling rate, promote further toughening of the wire rod structure, improve the tempering and softening effect of the wire rod, and take into account production efficiency. In the preferred technical solution, the slow cooling on the roller table controls the wire rod to cool to below 280°C at a slow cooling rate of 0.4~0.75°C / s before winding.

[0029] 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 ≥260℃ in the online molten salt strong isothermal 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 strong isothermal treatment can be recovered and utilized through the hot air, thereby reducing production energy consumption and promoting the rapid production of wire rod.

[0030] A hot-rolled wire rod for stranded wire of 2200MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method for hot-rolled wire rod for stranded wire of 2200MPa grade as described in any one of the above-mentioned methods.

[0031] The aforementioned hot-rolled wire rod uses a high-carbon Nb-Al composition with trace amounts of Nb, which allows for appropriate control of material costs. Its microstructure comprises a mixture of tempered sorbite and melt-bonded sorbite, with a small amount of ferrite. Compared to sorbite wire rod used in air-cooled stranded wire, this structure effectively suppresses the brittleness and mechanical property fluctuations caused by network carbides and martensite. The sorbite has a finer lamellar spacing than pearlite; the smaller the lamellar spacing, the greater the resistance to dislocation movement. Furthermore, the strength is supplemented by nano-alloy carbides, which can suppress the strength loss and mechanical property fluctuations caused by pearlite. This process enhances the balance between strength and plasticity. Compared to existing salt-bath treated sorbitic wire rods, the high-temperature isothermal treatment of sorbite results in partial cementite lamellar melting. The elimination of internal stress gradients and the formation of melted sorbite significantly improve plasticity. Combined with the dispersed precipitation of alloy carbides and the formation of pinned dislocations, the superposition of fine grain strengthening and lamellar strengthening further preserves strength characteristics. This improves the balance between strength and plasticity and the uniformity of microstructure in hot-rolled wire rods under rapid production conditions, thereby reducing the risk of fracture during drawing and stranding processes without offline heat treatment and enhancing service safety.

[0032] The higher the proportion of tempered sorbite and the finer the lamellar spacing, the higher the matrix strength. The higher the proportion of melt-fractured sorbite, the better the ductility and toughness of the hot-rolled wire rod. In the preferred technical solution, the volume percentage of tempered sorbite is ≥52%, the lamellar spacing is 80~120nm, the volume percentage of ferrite is ≤4%, and the volume percentage of melt-fractured sorbite is ≥34%.

[0033] 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 ≤44MPa. This can avoid the adverse effects of network carbides on the uniformity and ductility of the matrix, improve the service stability of the strand, and reduce mechanical property fluctuations by suppressing abnormal structures and making the wire rod structure more uniform. The smaller mechanical property difference within the same coil is beneficial to improving the consistency and service reliability in drawing and twisting processes and reducing the risk of breakage.

[0034] In the preferred technical solution, the diameter of the hot-rolled wire rod is 5.5~15mm, the tensile strength is 1490~1540MPa, and the reduction of area is 34%~39%. When the hot-rolled wire rod is used as the base material to produce strands, the higher initial tensile strength is beneficial to reduce the number of drawing passes, reduce plastic loss during the drawing process, quickly reach the target strength, improve production efficiency, and support the high load requirements of strand service. The higher reduction of area can provide sufficient cold drawing plasticity reserve, further reduce the risk of deformation and wire breakage, improve the service resistance to brittle fracture, and thus achieve efficient and stable production without annealing.

[0035] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0036] (1) In view of the problems of abnormal structure risk, insufficient strength and plasticity, or high energy consumption and limited production efficiency in the production of existing stranded steel wire rod using Stellmore air-cooled line or salt bath, this invention adopts Nb-Al chemical composition design combined with online molten salt strong isothermal technology. It can use high temperature wire rolling, control the wire rod to pass through the first stage of molten salt treatment to quickly bypass the network carbide zone from the high temperature austenite state and enter the sorbite phase region, inhibit the formation of network carbide and form a structure mainly composed of fine lamellar interlayer sorbite, promote the precipitation of alloy carbides, and then pass through the second stage of molten salt treatment with higher temperature and lower molten salt circulation to promote the transformation of untransformed residual austenite into fine lamellar interlayer sorbite. At the same time, it promotes the formation of fine lamellar interlayer sorbite for long-term isothermal tempering and causes some cementite plates to melt, regulate the strength and plasticity matching of wire rod, reduce production energy consumption and the restrictions on rolling and slow cooling of roller table, take into account plasticity and production efficiency, and have good industrial adaptability.

[0037] (2) In view of the current situation that the existing stranded steel wire rod has insufficient strength and plasticity and requires offline heat treatment to achieve the performance target, the present invention can control the material cost. The microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can suppress abnormal structures such as network carbides, martensite and coarse pearlite. By the finer lamellar spacing of sorbite than pearlite, combined with the dispersed precipitation of alloy carbides to pin dislocations, the resistance to dislocation movement is improved and the strength characteristics are preserved. By partially melting the cementite lamellars and eliminating the internal stress gradient, the plasticity and microstructure uniformity are improved, thereby improving the strength of hot-rolled wire rod while taking into account plasticity, achieving a tensile strength of 1490~1540MPa and a section reduction rate of 34%~39%, so that downstream users can avoid offline heat treatment and promote the stable production of stranded wire, which has good market application prospects. Attached Figure Description

[0038] 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:

[0039] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention;

[0040] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention;

[0041] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0042] 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.

[0043] Example 1:

[0044] A preferred embodiment of the manufacturing method of the 2200MPa 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: 0.65%, Mn: 0.72%, Cr: 0.47%, Nb: 0.019%, Al: 0.13%, P: 0.015%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt strong isothermal treatment → slow cooling on roller table → coiling, specifically:

[0045] 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 avoids grain coarsening. After exiting the furnace, the billet is rolled into a 5.5mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet, improve rolling efficiency, and control the precipitation of Nb carbonitrides and grain refinement. Specifically, the furnace soaking temperature is controlled at 1195℃. The furnace time is 200 min, the initial rolling temperature is 1055℃, the initial rolling reduction is 37.5%, the final rolling temperature is 940℃, and the final rolling reduction is 28%. 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 precipitation of proeutectoid ferrite or network carbides. This provides a basis for suppressing carbon aggregation at grain boundaries and refining sorbite lamellars. Specifically, the wire drawing temperature is controlled at 930℃.

[0046] The online molten salt isothermal treatment 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 salt bath for the initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, and most of the high-temperature austenitic structure transforms into a microstructure dominated by fine-laminated sorbite, promoting the dispersed precipitation of Cr and Nb alloy carbides. The wire rod is then conveyed via rollers through the second salt bath for the final molten salt treatment, increasing the molten salt temperature and reducing the molten salt temperature. The circulation rate promotes the transformation of untransformed residual austenite into fine lamellar interlamellar sorbite, promotes the precipitation of alloy carbides, and simultaneously promotes isothermal tempering of the formed fine lamellar interlamellar sorbite, causing some of the cementite lamellars to melt and fracture, 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 560℃, the treatment time is 80s, the molten salt circulation rate is 555t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 592℃, the treatment time is 250s, the molten salt circulation rate is 100t / h, and the molten salt temperature rise is ≤3℃.

[0047] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥260℃) from above the two salt bath tanks in the online molten salt isothermal 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. Specifically, the wire rod is cooled to 276℃ at a slow cooling rate of 0.75℃ / 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.

[0048] 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 1150℃, the furnace time is 250 min, the initial rolling temperature is 1020℃, the initial rolling reduction is 25%, the final rolling temperature is 845℃, the final rolling reduction is 35%, and the wire drawing temperature is 840℃; the Steyrmo forced air cooling uses an air volume of 200,000 m³ / s per fan. 3 At 85% capacity, fans 1 to 4 are turned on to cool the wire rod to 695℃ at a cooling rate of 8.5℃ / s. Then, fans 5 to 14 are turned on to 30% capacity to cool the wire rod to 260℃ at a cooling rate of 3.3℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0049] Comparative Example 2: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 1 is that: the heating furnace homogenization temperature is controlled at 1145℃, the furnace time is 255min, the initial rolling temperature is 1000℃, the initial rolling reduction is 26%, the final rolling temperature is 845℃, the final rolling reduction is 34.5%, and the wire drawing temperature is 835℃; during the molten salt treatment before the online molten salt strong isothermal treatment 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 line.

[0050] Example 2:

[0051] A preferred embodiment of the manufacturing method of the 2200MPa 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: 0.92%, Si: 0.71%, Mn: 0.89%, Cr: 0.39%, Nb: 0.005%, Al: 0.11%, 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 strong isothermal treatment → slow cooling on roller table → coiling, specifically:

[0052] 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 avoids grain coarsening. After exiting the furnace, the billet is rolled into wire rod with a diameter of 8.5mm via a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet, improve rolling efficiency, and control the precipitation of Nb carbonitrides and grain refinement. Specifically, the furnace soaking temperature is controlled at 1180℃. The furnace time is 220 minutes, the initial rolling temperature is 1030℃, the initial rolling reduction is 36%, the final rolling temperature is 925℃, 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 spread 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 precipitation of proeutectoid ferrite or network carbides. This provides a basis for suppressing grain boundary carbon aggregation and refining sorbite lamellars. Specifically, the wire drawing temperature is controlled at 925℃.

[0053] The online molten salt isothermal treatment 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 salt bath for the initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, and most of the high-temperature austenitic structure transforms into a microstructure dominated by fine-laminated sorbite, promoting the dispersed precipitation of Cr and Nb alloy carbides. The wire rod is then conveyed via rollers through the second salt bath for the final molten salt treatment, increasing the molten salt temperature and reducing the molten salt temperature. The circulation rate promotes the transformation of untransformed residual austenite into fine lamellar sorbite, promotes the precipitation of alloy carbides, and simultaneously promotes isothermal tempering of the formed fine lamellar sorbite, causing some of the cementite lamellars to melt and fracture, 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 549℃, the treatment time is 135s, the molten salt circulation rate is 585t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 585℃, the treatment time is 220s, the molten salt circulation rate is 160t / h, and the molten salt temperature rise is ≤3℃.

[0054] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥260℃) from above the two salt bath tanks in the online molten salt isothermal 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. Specifically, the wire rod is cooled to 273℃ at a slow cooling rate of 0.62℃ / 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.

[0055] Comparative Example 3: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the initial molten salt treatment of the online molten salt strong isothermal treatment process, the wire rod is cooled at a cooling rate of 40℃ / s, the molten salt temperature of the initial molten salt treatment is 510℃, the treatment time is 285s, and the finished hot-rolled wire rod is obtained after going offline.

[0056] 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 treatment 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 575℃, the treatment time is 60s, and the finished hot-rolled wire rod is obtained after going offline.

[0057] Example 3:

[0058] A preferred embodiment of the manufacturing method of the 2200MPa 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: 0.88%, Si: 0.72%, Mn: 0.92%, Cr: 0.38%, Nb: 0.025%, Al: 0.08%, P: 0.012%, 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 strong isothermal treatment → slow cooling on roller table → coiling, specifically:

[0059] 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 avoids grain coarsening. After exiting the furnace, the billet is rolled into a 12mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet, improve rolling efficiency, and control the precipitation of Nb carbonitrides and grain refinement. Specifically, the furnace soaking temperature is controlled at 1220℃. The furnace time is 150 minutes, the initial rolling temperature is 1080℃, the initial rolling reduction is 35%, the final rolling temperature is 955℃, 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 precipitation of proeutectoid ferrite or network carbides. This provides a basis for suppressing grain boundary carbon aggregation and refining sorbite lamellars. Specifically, the wire drawing temperature is controlled at 950℃.

[0060] The online molten salt isothermal treatment 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 of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 40°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, and most of the high-temperature austenitic structure transforms into a microstructure dominated by fine-laminated sorbite, promoting the dispersed precipitation of Cr and Nb alloy carbides. The wire rod is then conveyed via rollers through the second stage of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing the molten salt temperature. The circulation rate promotes the transformation of untransformed residual austenite into fine lamellar sorbite, promotes the precipitation of alloy carbides, and simultaneously promotes isothermal tempering of the formed fine lamellar sorbite, causing some of the cementite lamellars to melt and fracture, 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 533℃, the treatment time is 195s, the molten salt circulation rate is 610t / 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 185s, the molten salt circulation rate is 210t / h, and the molten salt temperature rise is ≤3℃.

[0061] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥260℃) from above the two salt bath tanks in the online molten salt isothermal 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. Specifically, the wire rod is cooled to 272℃ at a slow cooling rate of 0.55℃ / 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.

[0062] 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 600℃, the treatment time is 300s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0063] 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 150s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0064] Example 4:

[0065] A preferred embodiment of the manufacturing method of the 2200MPa 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: 0.56%, Mn: 0.83%, Cr: 0.54%, Nb: 0.022%, Al: 0.15%, P: 0.013%, 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 strong isothermal treatment → slow cooling on roller table → coiling, specifically:

[0066] The rolling process is used to heat a 220mm × 220mm steel billet in a furnace to a high-temperature, rollable form. Furnace parameters are controlled to promote uniform diffusion of alloying elements and prevent grain coarsening. After exiting the furnace, the billet is rolled into 15mm diameter wire rods via a rolling line. Appropriate rolling temperatures and reductions are selected to promote rapid deformation of the billet, improve rolling efficiency, and control Nb carbonitride precipitation and grain refinement. Specifically, the furnace soaking temperature is controlled at 1210℃. The furnace time is 175 minutes, the initial rolling temperature is 1065℃, the initial rolling reduction is 34%, the final rolling temperature is 950℃, 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 spread 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 precipitation of proeutectoid ferrite or network carbides. This provides a basis for suppressing grain boundary carbon aggregation and refining sorbite lamellars. Specifically, the wire drawing temperature is controlled at 940℃.

[0067] The online molten salt isothermal treatment 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 of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 40°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, and most of the high-temperature austenitic structure transforms into a microstructure dominated by fine-laminated sorbite, promoting the dispersed precipitation of Cr and Nb alloy carbides. The wire rod is then conveyed via rollers through the second stage of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing the molten salt temperature. The circulation rate promotes the transformation of untransformed residual austenite into fine lamellar sorbite, promotes the precipitation of alloy carbides, and simultaneously promotes isothermal tempering of the formed fine lamellar sorbite, causing some of the cementite lamellars to melt and fracture, 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 525℃, the treatment time is 280s, the molten salt circulation rate is 650t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 572℃, the treatment time is 160s, the molten salt circulation rate is 240t / h, and the molten salt temperature rise is ≤3℃.

[0068] The slow cooling process of the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥260℃) from the two salt bath tanks of the online molten salt isothermal 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. Specifically, the wire rod is cooled to 270℃ at a slow cooling rate of 0.4℃ / 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.

[0069] 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 strong isothermal treatment → air cooling. The air cooling process is carried out by opening the heat preservation cover and controlling the wire rod to cool to 265°C at a cooling rate of 1.6°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0070] 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:

[0071] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0072]

[0073] As can be seen from the comparison results of Example 1 and Example 1, compared with the production using the Stellmore air-cooled line, carbon elements are more likely to precipitate at the grain boundaries to form network carbides. After increasing the air-cooling intensity, local overcooling is prone to forming brittle phases such as martensite, which brings the risk of coil breakage and wire breakage during drawing. Meanwhile, the core forms a coarse pearlite structure under slow cooling, and there is a large stress difference between the surface and the core, which affects the uniformity of the structure and the final mechanical properties. The present invention uses Nb-Al chemical composition design combined with online molten salt strong isothermal technology to control the wire rod to first undergo the pre-melting process. Salt treatment inhibits the formation of network carbides and forms a microstructure dominated by fine lamellar interlamellar sorbite. Subsequent molten salt treatment at higher temperatures and with lower molten salt circulation promotes the transformation of untransformed residual austenite into fine lamellar interlamellar sorbite. Simultaneous isothermal tempering regulates the strength-plasticity balance of the wire rod. As shown in the results of Examples 1-4, the hot-rolled wire rod achieves a tensile strength of 1490-1540 MPa and a reduction of area of ​​34%-39%, enabling downstream users to avoid offline heat treatment and promoting stable strand production.

[0074] As can be seen from the comparison results of Example 1 and Example 2, using a higher wire drawing temperature can promote austenite homogenization and prevent carbon from precipitating prematurely as network carbides and proeutectoid ferrite during wire drawing. This lays the foundation for subsequent rapid cooling to suppress grain boundary carbon aggregation and refine sorbite lamellars. At the same time, it can reduce the limitation on rolling temperature. Combined with the precipitation strengthening effect of Nb, it can improve rolling speed and efficiency and refine austenite grains.

[0075] As can be seen from the comparison results of Example 2 and Example 3, the lower the molten salt temperature in the initial molten salt treatment, the better it is to suppress network carbides, reduce the atomic diffusion rate, make it difficult for cementite to coarsen, promote the refinement of sorbite lamellars, and make the precipitates of Cr and Nb carbides finer. With the extension of the treatment time, the temperature difference between the wire rod surface and the core can be reduced, promoting the uniform transformation of most austenite to sorbite, promoting the dispersed precipitation of alloy carbides, and improving the matrix strength. However, if the molten salt temperature is too low, exceeding the nose temperature of sorbite phase transformation and the precipitation range of alloy carbides, it will affect the nucleation and carbide precipitation rate, and may even produce low-temperature abnormal structures. With the treatment time being too long, the production energy consumption will increase.

[0076] As can be seen from the comparison results of Example 2 and Example 4, the higher the molten salt temperature in the first stage of molten salt treatment, the lower the temperature difference between the wire rod surface and the core, the lower the structural stress and the amount of molten salt circulation, the more uniform the diffusion of alloy carbides, and the shorter the phase transformation incubation period. The shorter the treatment time, the lower the production energy consumption. However, if the molten salt temperature is too high, it is not conducive to suppressing the coarsening of network carbides, cementite lamellars, and the rapid precipitation of alloy carbides. As the treatment time is too short, the amount of untransformed residual austenite in the microstructure increases and is left to the later stage of molten salt treatment. The amount of dispersed precipitation of alloy carbides decreases, the pinning effect on dislocations is lost, and softening will be accelerated and the matrix strength will be lost.

[0077] As can be seen from the comparison results of Example 3 and Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the faster the decomposition of residual austenite and the higher the atomic diffusion rate. With the extension of treatment time, the temperature gradient on the cross-section of the wire rod can be further reduced, promoting stress release and uniform diffusion of alloy carbides in the microstructure and improving the plasticity and toughness of the matrix. However, if the molten salt temperature is too high, it is not conducive to controlling the refinement of the sorbite lamellars formed by decomposition and inhibiting the coarsening of cementite. With the longer treatment time, the amount of melted sorbite in the microstructure increases, and cementite and alloy carbides agglomerate and grow, which will reduce the anchoring effect, resulting in a decrease in strength and plasticity, and at the same time increase production energy consumption.

[0078] As can be seen from the comparison results of Example 3 and Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the more refined the sorbite formed can be, the more alloy carbides precipitated can be added to improve the tempering stability of the steel, offset the softening effect caused by high temperature, and retain the matrix strength. With the shortening of the treatment time, the production energy consumption can be further reduced. However, if the molten salt temperature is too low, the decomposition of the residual austenite will be slower, which will prolong the inoculation time. At the same time, it is not conducive to the release of structural stress and the uniform diffusion of alloy carbides. If the treatment time is too short, it will lead to insufficient sorbite melting, loss of ductile and tough properties, and even the formation of low-temperature structure in subsequent cooling due to residual austenite, resulting in fluctuations in mechanical properties.

[0079] As can be seen from the comparison results of Example 4 and Example 7, slow cooling on the roller table can promote further toughening of the wire rod structure, improve the tempering and softening effect of the wire rod, and take into account production efficiency.

[0080] 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 2200 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The chemical composition of the hot-rolled wire rod comprises the following components in percentage by mass: C: 0.88%-0.92%, Si: 0.56%-0.76%, Mn: 0.72%-0.92%, Cr: 0.38%-0.54%, Nb: 0.005%-0.025%, Al: 0.08%-0.15%, P≤0.015%, S≤0.015%, and the rest is Fe and inevitable impurities; after the wire rod is spun at a spinning temperature of ≥925 ℃, the wire rod is subjected to on-line molten salt isothermal treatment, the wire rod is first subjected to front-stage molten salt treatment and cooled at a cooling rate of ≥37 ℃ / s, and then is subjected to rear-stage molten salt treatment, so that the untransformed residual austenite is continuously transformed into sorbite and isothermally tempered, and part of the sorbite lamellas are melted and broken, and finally the wire rod is slowly cooled on a roller bed to obtain a hot-rolled wire rod with a microstructure comprising a mixture of tempered sorbite, ferrite and melted sorbite; the molten salt temperature of the front-stage molten salt treatment is 525-560 ℃, and the treatment time is 80-280 s; the molten salt temperature of the rear-stage molten salt treatment is 572-592 ℃, and the treatment time is 160-250 s.

2. The method of producing a hot-rolled wire rod for a 2200 MPa grade strand according to claim 1, characterized by, Before the rolling, the soaking temperature of the heating furnace is controlled to be 1180-1220 ℃, and the furnace time is 150-220 min.

3. The method of producing a hot-rolled wire rod for a 2200 MPa grade strand according to claim 1, characterized by, During the rolling, the initial rolling temperature is controlled to be 1030-1080 ℃, the initial rolling reduction is 34%-37.5%, the final rolling temperature is 925-955 ℃, and the final rolling reduction is 23%-28%.

4. The method of producing a hot-rolled wire rod for a 2200 MPa grade strand according to claim 1, characterized by, During the spinning, the spinning temperature is controlled to be 925-950 ℃.

5. The method of producing a hot-rolled wire rod for a 2200 MPa grade strand according to claim 1, characterized by, The molten salt circulation amount of the front-stage molten salt treatment is 555-650 t / h, and the molten salt temperature rise is ≤8 ℃; the molten salt circulation amount of the rear-stage molten salt treatment is 100-240 t / h, and the molten salt temperature rise is ≤3 ℃.

6. The method of producing a hot-rolled wire rod for a 2200 MPa grade strand according to claim 1, characterized by, The wire rod is cooled to below 280 ℃ at a slow cooling speed of 0.4-0.75 ℃ / s on the roller bed to perform coiling.

7. A hot rolled wire rod for 2200 MPa grade strand, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of the hot-rolled wire rod for a 2200 MPa grade twisted wire rod according to any one of claims 1-6.

8. The hot rolled wire rod for 2200 MPa class strand according to claim 7, characterized by, The volume percentage of the tempered sorbite is ≥52%, the lamellar spacing is 80-120 nm, the volume percentage of the ferrite is ≤4%, the volume percentage of the melted sorbite is ≥34%, the reticular carbide level of the hot-rolled wire rod is 0 level, and the mechanical property difference between turns is ≤44 MPa.

9. The hot rolled wire rod for 2200 MPa class strand according to claim 7, characterized by, The diameter of the hot-rolled wire rod is 5.5-15 mm, the tensile strength is 1490-1540 MPa, and the reduction of area is 34%-39%.

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