A hot rolled wire rod for 2600mpa class strand and a manufacturing method thereof

Abstract

This invention relates to a hot-rolled wire rod for 2600MPa grade stranded wire and its manufacturing method. The method employs a high Si-Al chemical composition design. After the rolled wire is coiled, it undergoes online molten salt rapid cooling isothermal control treatment in a salt bath, cooling the wire rod at a rate of ≥32℃ / s, controlling the molten salt temperature at 553~583℃, and the treatment time at 240~400s. It then undergoes slow cooling on a roller conveyor, with the wire rod cooled slowly to below 280℃ at a rate of 0.4~0.9℃ / s, resulting in a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melt-bonded sorbite. This method controls material costs, suppresses abnormal microstructures, and regulates the strength-plasticity balance of the wire rod, achieving a tensile strength of 1644~1694MPa and a reduction of area of ​​32%~37%, while also considering production efficiency. It is suitable for manufacturing 2600MPa grade stranded wire and other applications, eliminating the need for offline heat treatment and reducing the risk of wire breakage.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a 2600MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] With the development of prestressing and concrete technology, the prestressing strength applied to concrete bridges has exceeded that of long-span cable-stayed bridges of the same period. Therefore, improving the strength of steel strands and anchoring systems for railway bridges is an important direction for future development. Simultaneously, as bridge spans continue to increase, higher requirements are being placed on the strength of steel strands. In recent years, 1960MPa-grade high-strength strands have been widely used, while 2200MPa~2400MPa-grade ultra-high-strength strands have emerged, but higher strength grades are rarely developed. Furthermore, the hot-rolled wire rods currently used for ultra-high-strength strands all require offline heat treatment, which is not only costly and energy-intensive but also hinders the large-scale application of ultra-high-strength strands. Therefore, it is necessary to develop a 2600MPa-grade hot-rolled wire rod for strands that meets the comprehensive requirements of strength, plasticity, and processing performance, without requiring offline heat treatment, to achieve energy and material savings, thereby meeting the development and market demands of the steel industry.

[0003] Existing hot-rolled wire rods for stranded wire are generally produced using the Steyrmo air-cooled line. The following technical bottlenecks still exist in manufacturing 2600MPa grade hot-rolled wire rods for stranded wire: I. Existing hot-rolled wire rods for ultra-high strength strands mostly use high-carbon and high-silicon compositions. Limited by the maximum cooling capacity of the Steyrmore air-cooled wire line, during the slow cooling of hypereutectoid steel from the austenite region, carbon atoms diffuse and segregate to the grain boundaries, forming a higher-level continuous network of cementite. This network of carbides disrupts the continuity of the matrix, making it extremely easy for cracks to initiate along the network of carbides during downstream drawing, leading to brittle fracture and insufficient torsional performance. To refine the sorbite lamellars and improve… Strength: Existing technologies typically enhance the hardenability of steel by increasing the Mn and Cr content. However, if the air-cooling strength is increased to suppress network carbides and refine lamellar layers, it will further exacerbate the temperature difference between the surface and core of the wire rod, the temperature difference between the overlapping and non-overlapping areas, and the instability of temperature control. Under the condition of C, Mn, and Cr element segregation, local overcooling can easily induce hard and brittle abnormal phases such as martensite. Martensite has high hardness, and severe stress concentration will occur at the interface between the two phases during deformation, thereby causing brittle fracture of the coil or wire breakage during drawing.

[0004] II. Existing ultra-high strength wire rods employ microalloying technologies such as V and Ti. However, due to insufficient cooling, unstable temperature control, and continuous cooling conditions in the Steyrmo air-cooling line, the precipitation of microalloyed carbides is uneven, resulting in limited strengthening effects. Excessive addition of microalloying elements significantly increases the material cost of the wire rod. To improve microstructure uniformity, the decarburization sensitivity of high-silicon billets increases under high-temperature heating. Simultaneously, high silicon content reduces the diffusion rate of carbon atoms in austenite, leading to a prolonged phase transformation incubation period and a decrease in the overall transformation rate. Under continuous cooling conditions, although air-cooling strength is reduced during phase transformation to improve sorbitization, the phase transformation is asynchronous due to the minimum cooling capacity of the Steyrmo air-cooling line and the influence of continuous cooling. The sorbite lamellar spacing fluctuates significantly, and residual microstructure stress in the wire rod results in insufficient plasticity and toughness. Furthermore, insufficient transformation leads to excessive residual austenite content. During subsequent slow cooling or drawing processes, the residual austenite may transform into martensite or cause brittle fracture due to excessive microstructure stress, making offline heat treatment unavoidable. Summary of the Invention

[0005] 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 2600MPa grade stranded wire and its manufacturing method. It adopts a high Si-Al chemical composition design, which can control material costs, suppress abnormal structure, regulate the strength and plasticity matching of wire rod, and take into account production efficiency. It can be used to manufacture 2600MPa grade stranded wire and other application fields, without the need for offline heat treatment and reducing the risk of wire breakage.

[0006] The technical solution adopted by this invention to solve its technical problem is: The first aspect of the present invention is to provide a method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand, the method comprising: The hot-rolled wire rod is rolled into finished products according to its chemical composition. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.96%~1.01%, Si: 0.79%~0.97%, Mn: 0.88%~1.08%, Cr: 0.50%~0.70%, Nb: 0.015%~0.035%, V: 0.028%~0.038%, Al: 0.22%~0.42%, Mo: 0.22%~0.42%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is spun into coils at a spinning temperature of ≥904℃ and then enters a salt bath for online molten salt rapid cooling isothermal control treatment, so that the coils are cooled at a cooling rate of ≥32℃ / s, the molten salt temperature is controlled at 553~583℃, and the treatment time is 240~400s; then it is slowly cooled by roller conveyor, and the coils are slowly cooled to below 280℃ at a cooling rate of 0.4~0.9℃ / s, so as to make hot-rolled coils with a microstructure consisting of a mixture of tempered sorbite, ferrite and melted sorbite.

[0007] In the preferred technical solution, before rolling, a rectangular steel billet with a side length of 180mm~220mm is manufactured by continuous casting process, the superheat of the tundish is controlled to be ≤25℃, the current of the electromagnetic stirring in the crystallizer is 305~355A and the frequency is 1~3Hz; the current of the electromagnetic stirring at the end is controlled to be 500~550A, the frequency is 7~9Hz, and the pulling speed is 0.13~0.33m / min.

[0008] In a preferred embodiment, a rectangular steel billet with a side length of 180mm to 220mm is heated before rolling. During heating, the uniform heating temperature of the heating furnace is controlled at 1200 to 1260℃, and the furnace time is 185 to 245 minutes.

[0009] In the preferred technical solution, during the rolling process, the initial rolling temperature is 1037~1087℃, the final rolling temperature is 914~954℃, and the final rolling reduction is 21%~26%.

[0010] In a preferred embodiment, the spinning temperature during spinning is 904~944℃.

[0011] In the preferred technical solution, the online molten salt rapid cooling isothermal control process is divided into a pre-stage molten salt treatment and a post-stage molten salt treatment. The molten salt temperature in the pre-stage molten salt treatment is 553~583℃, the treatment time is 142~200s, and the molten salt circulation volume is larger than that in the post-stage molten salt treatment. The molten salt temperature in the post-stage molten salt treatment is 568~583℃, and the treatment time is 100~200s.

[0012] In the preferred technical solution, the molten salt circulation rate of the front-end molten salt treatment is 500~600t / h, and the molten salt temperature rise is ≤8℃.

[0013] In the preferred technical solution, the molten salt circulation rate of the downstream molten salt treatment is 320~420t / h, and the molten salt temperature rise is ≤3℃.

[0014] In a preferred embodiment, during the slow cooling process of the roller conveyor, the wire rod enters the conveyor roller conveyor after exiting the salt tank, and the hot air at ≥230°C from the salt tank is blown onto the conveyor roller conveyor.

[0015] A second aspect of the present invention is to provide a hot-rolled wire rod for 2600MPa grade steel strand, said hot-rolled wire rod being manufactured by any one of the above-described methods for manufacturing hot-rolled wire rod for 2600MPa grade steel strand.

[0016] In the preferred embodiment, the diameter of the hot-rolled wire rod is 5.5~15mm, the volume ratio of tempered sorbite and melted sorbite is ≥97%, and the lamellar spacing of the tempered sorbite is 55~75nm.

[0017] In the preferred embodiment, the hot-rolled wire rod has a network carbide level of 0, is free of martensite, and has a mechanical property difference of ≤50MPa between different rings.

[0018] In the preferred embodiment, the hot-rolled wire rod has a tensile strength of 1644~1694MPa and a reduction of area of ​​32%~37%.

[0019] Compared with the prior art, the beneficial effects of the present invention are at least as follows: (1) In view of the problems that existing hot-rolled wire rods for high-strength strands are difficult to control in terms of network carbide and martensite abnormal structure under the Stellmore air-cooling line process, as well as insufficient pearlite phase transformation plasticity and microstructure uniformity, this invention uses a high Si-Al chemical composition design combined with online molten salt rapid cooling isothermal control technology to enable the wire rod to quickly cross the network carbide formation range from the high-temperature austenitic state to the sorbite phase region, thereby inhibiting the formation of network carbide, avoiding the formation of abnormal structures such as martensite, and rapidly forming a microstructure dominated by fine lamellar sorbite structure. This promotes long-term isothermal tempering of the fine lamellar sorbite structure, causing some cementite lamellars to melt and transform into spheroids, and microalloyed carbides to be fully precipitated and dispersed to control the strength and plasticity matching of the wire rod. Finally, the slow cooling of the roller table promotes further toughening of the wire rod structure and improves the tempering softening effect. No offline heat treatment is required, which can promote the efficient and stable production of hot-rolled wire rods and has good industrial application prospects.

[0020] (2) In view of the current situation that the existing hot-rolled wire rod materials for high-strength stranded wire are expensive, have large fluctuations in strength, plasticity or mechanical properties, and require offline heat treatment, the hot-rolled wire rod of this invention adopts a high Si-Al chemical composition design, which can control material cost and suppress abnormal structure. The microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite. Combined with the nano carbides precipitated by Nb, V and Mo microalloying elements, it can compensate for the strength loss caused by cementite fracture, improve the high strength and plasticity matching of hot-rolled wire rod, and continuously provide dispersion strengthening during cold drawing process. The network carbide level of hot-rolled wire rod is 0 grade, there is no martensite, the mechanical property difference between the same coil is ≤50MPa, the tensile strength is 1644~1694MPa, and the reduction of area is 32%~37%. It is suitable for manufacturing 2600MPa grade stranded wire and other application fields. No offline heat treatment is required, which can reduce the risk of wire breakage and has good market application prospects. Attached Figure Description

[0021] 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: Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention.

[0022] Figure 2This is a metallographic diagram of Embodiment 2 of the present invention.

[0023] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0024] 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.

[0025] To address the problems of high-carbon, high-silicon content combined with the Stellmore air-cooled production line, such as high network carbide levels in hot-rolled wire rods, leading to abnormal martensitic structures, asynchronous phase transformations, high structural stress, and insufficient strength and plasticity, this invention utilizes the residual heat of the wire rod itself. Through high Si-Al chemical composition design combined with online molten salt rapid cooling isothermal control technology, a 2600MPa grade hot-rolled wire rod for stranded wire and its manufacturing method are developed online. A preferred embodiment of the 2600MPa grade hot-rolled wire rod for stranded wire described in this invention is as follows: the chemical composition and mass percentage of the hot-rolled wire rod... The composition includes: C: 0.96%~1.01%, Si: 0.79%~0.97%, Mn: 0.88%~1.08%, Cr: 0.50%~0.70%, Nb: 0.015%~0.035%, V: 0.028%~0.038%, Al: 0.22%~0.42%, Mo: 0.22%~0.42%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities; the microstructure consists of a mixed structure composed of tempered sorbite, ferrite, and melted sorbite.

[0026] The chemical composition and mass percentage of the above-mentioned hot-rolled wire rods are designed based on the following: (1) Carbon: C is a strengthening element with a relatively low price. It can provide solid solution strengthening and increase the volume fraction of cementite in the sorbite structure. Through the hindrance of dislocation slip by cementite, combined with the rapid cooling process of molten salt to refine the lamellar spacing of sorbite, it can effectively improve the work hardening rate in the subsequent large plastic deformation process of cold drawing by means of phase boundary strengthening mechanism. However, if the C content is too high, dendritic segregation and central macro segregation are prone to occur during solidification, which increases the tendency of network carbide precipitation. The segregation enrichment area will also generate hard and brittle abnormal structures such as martensite due to increased hardenability, which deteriorates the plasticity and toughness of the matrix and the uniformity of the structure. At the same time, it prolongs the incubation period of sorbite transformation and causes a decrease in production efficiency. Therefore, in order to take into account the high strength and production efficiency of 2600MPa grade stranded wire, the mass percentage of C is controlled at C: 0.96%~1.01%.

[0027] (2) Silicon: Si can be dissolved in the ferrite matrix to produce a solid solution strengthening effect, thereby improving the matrix strength; at the same time, it can enhance the diffusion activation energy of carbon, and work with Al to inhibit the coarsening and spheroidization process of cementite, significantly enhancing the tempering stability of steel. Under high temperature and short time tempering conditions, sorbite only undergoes lamellar melting to improve plasticity, and will not cause a decrease in strength due to excessive coarsening of cementite, thereby playing a role in refining the precipitated phase and improving the toughness of the material; however, excessive silicon content will exacerbate the surface decarburization tendency of steel, and delay the pearlite transformation, causing the phase transformation curve to shift to the right, prolonging the isothermal treatment and toughening time, which is not conducive to online high-efficiency production; therefore, in order to take into account the wire rod strength, online molten salt rapid cooling isothermal control treatment, and reduce the pressure of slow cooling treatment on the roller table, the mass percentage of Si is controlled at 0.79%~0.97%.

[0028] (3) Manganese: Mn can regulate the hardenability of wire rod, reduce the critical cooling rate of pearlite transformation, prolong the incubation period of supercooled austenite, avoid the formation of coarse pearlite due to insufficient core cooling rate, resulting in low strength, thereby improving the uniformity of cross-sectional structure. Mn is partially dissolved in the ferrite matrix, producing a substitution solid solution strengthening effect, which can improve the hardness and stability of cementite and reduce the risk of early fracture caused by uneven stress on strands. However, excessive Mn content will aggravate the tendency of component segregation during the solidification process of the billet, increase the risk of martensite formation during cooling, and lead to increased fluctuations in mechanical properties and wire breakage during drawing. Therefore, in order to refine the sorbite lamellars and reduce the difference in microstructure lamellars from the surface of the wire rod to the core, the Mn content should be appropriately increased, and the mass percentage of Mn should be controlled at 0.88%~1.08%.

[0029] (4) Chromium: Cr is a strong hardenable and medium-strong carbide forming element. It can reduce the nose temperature of pearlite transformation, increase the phase transformation undercooling, and improve the nucleation rate of sorbite. The phase boundary strengthening brought about by lamellar refinement can simultaneously improve the plastic deformation coordination. Chromium can be dissolved in cementite to form alloy carbides, which can improve the structural stability and strength of cementite, inhibit the aggregation and spheroidization of cementite during high-temperature tempering, and improve tempering stability. After drawing, higher dislocation density and work hardening effect can be obtained. However, excessive Cr content will aggravate the risk of element segregation, increase the risk of precipitation of martensite and other deteriorated structures, increase the difficulty of tempering softening, and affect the efficiency of the line. Therefore, in order to refine the lamellar spacing of sorbite and improve the resistance to tempering softening, the mass percentage of Cr is controlled at 0.50%~0.70%.

[0030] (5) Niobium: As a microalloying element, Nb can induce precipitates to pin austenite grain boundaries during high-temperature hot rolling, inhibit abnormal growth of austenite grains at high temperatures, obtain uniform and fine original austenite grains, increase sorbite nucleation sites, and improve the nucleation rate. During isothermal processes, the niobium dissolved in solid solution will disperse in the form of nano-precipitates, producing a dispersion strengthening effect and compensating for the strength loss during tempering. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling material costs. It will also increase the high-temperature deformation resistance of steel, increase rolling load and production difficulty. Therefore, the mass percentage of Nb is controlled at 0.015%~0.035%.

[0031] (6) Vanadium: As a microalloying element, V has high solubility in austenite. During rolling, it can be combined with niobium to refine austenite grains, increase sorbite nucleation density, and refine microstructure. Excess V can be dispersed and precipitated during the intermediate temperature isothermal process, and dispersed in the ferrite matrix, significantly improving strength and stress relaxation resistance of strand. However, V is relatively expensive, and excessive addition is not conducive to controlling material costs. At the same time, too much or too coarse precipitate will hinder dislocation coordinated deformation. Therefore, the mass percentage of V is controlled at 0.028%~0.038%.

[0032] (7) Aluminum: Al is a solid solution strengthening element of ferrite. It can inhibit the formation of coarse carbides during cooling, increase the nose temperature of pearlite transformation, increase the driving force of phase transformation, and accelerate the diffusion rate of carbon atoms. In synergy with Si, it can refine the precipitated phase and improve the toughness of the material. While ensuring the tempering stability and solid solution strengthening effect, it avoids the increase in decarburization sensitivity caused by higher silicon content. At the same time, it can keep the wire rod with good torsional and bending toughness, thereby improving the toughness and stability of the strand during long-term service. However, if the Al content is too high, it will generate large-sized non-metallic inclusions, reduce the fatigue life of the strand, reduce the fluidity of the molten steel, aggravate the risk of nodule formation in continuous casting, and cause problems such as casting interruption and increased surface defects of the billet. Therefore, the mass percentage of Al is controlled at 0.22%~0.42%.

[0033] (8) Molybdenum: The addition of Mo strongly inhibits the pearlite transformation, shifting the pearlite transformation curve to the right. At the same time, it reduces the diffusion ability of carbide-forming elements, thereby delaying the formation of carbide precipitation, inhibiting the precipitation of related second-phase particles, and preventing the precipitated phase from ripening and growing at high temperatures. This element forms a multi-scale precipitation strengthening synergy with Nb and V, ensuring the strengthening effect over a wide temperature range, making the stranded wire stable in structure and performance and with better low relaxation performance during long-term service. However, Mo is relatively expensive, and excessive addition is not conducive to controlling material costs. At the same time, due to its effect of delaying the pearlite transformation, it will also have an adverse effect on rapid production. Therefore, the mass percentage of Mo is controlled at 0.22%~0.42%.

[0034] (9) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0035] The aforementioned hot-rolled wire rod is designed with a high Si-Al chemical composition and trace amounts of Nb and V to effectively control material costs. Basic strength is provided through solid solution treatment and cementite strengthening with high carbon elements; the addition of high Mn and Cr improves the hardenability of the wire rod, reduces the critical cooling rate for pearlite transformation, and enhances the stability of supercooled austenite; high Si and Al suppress cementite coarsening and improve tempering stability. Combined with molten salt treatment, this increases the phase transformation driving force, refines the sorbite lamellar spacing, and improves the stability of the production process. Simultaneously, nano-precipitates increase the nucleation rate and synergistically offset the phase transformation delay effects of elements such as silicon, chromium, and molybdenum, ensuring that the total sorbite transformation time is controlled within the suitable range of the molten salt process. This results in a fully transformed and uniformly structured sorbite microstructure, providing favorable conditions for balancing production efficiency.

[0036] The microstructure of the aforementioned hot-rolled wire rod is mainly composed of tempered sorbite and melted sorbite, containing a small amount of ferrite, forming a mixed structure. The network carbide level is 0, and there is no martensite structure. The lamellar spacing of the sorbite is finer than that of pearlite. The high sorbitization rate can improve the matrix strength through phase boundary strengthening. After isothermal tempering, the microstructure transforms into intermediate tempered sorbite and melted sorbite that transition to spheroidized structure. Dislocations can slip along the cementite interstices, significantly alleviating local stress concentration and improving the uniform plastic deformation capacity of the matrix. Combined with the nano carbides precipitated by Nb, V, and Mo microalloying elements, it can compensate for the strength loss caused by cementite fracture, improve the high strength and plasticity matching of the hot-rolled wire rod, and continuously provide dispersion strengthening during cold drawing. During the strand twisting process, the high torsional and high bending properties imparted by the mixed structure can effectively prevent the initiation of microcracks or brittle fracture on the surface of the wire, thereby eliminating the need for offline heat treatment and improving the finished product qualification rate and fatigue resistance.

[0037] The manufacturing method of the above-mentioned 2600MPa grade hot-rolled wire rod for steel strand includes: rolling the wire rod into a production line according to the chemical composition of the hot-rolled wire rod; after the wire rod is spun into a wire rod at a spun temperature of ≥904℃, it enters a salt bath for online molten salt rapid cooling isothermal control treatment, so that the wire rod is cooled at a cooling rate of ≥32℃ / s, the molten salt temperature is controlled at 553~583℃, and the treatment time is 240~400s; then it is slowly cooled by roller table, and the wire rod is slowly cooled to below 280℃ at a cooling rate of 0.4~0.9℃ / s, so as to produce a hot-rolled wire rod with a microstructure including a mixed microstructure of tempered sorbite, ferrite and melted sorbite.

[0038] The aforementioned hot-rolled wire rod, based on a high Si-Al composition, employs a higher wire-drawing temperature to keep the wire rod in a high-temperature austenitizing state, ensuring sufficient solid solution of the components. This avoids premature precipitation of secondary cementite due to excessively low temperatures and allows for greater undercooling during molten salt treatment, promoting phase transformation. Simultaneously, it reduces the deformation resistance of high-carbon, high-alloy rolled products, simplifying rolling. After the wire rod is drawn into wire, it is directly fed into the salt bath for online molten salt rapid cooling and isothermal control without air cooling. I. Compared to the Stellmore air-cooled line, which is limited by factors such as maximum cooling capacity, air-cooling method, and unstable temperature control, making it difficult to effectively control abnormal structures such as network carbides and martensite and affect the transformation of sorbite, this invention has the following advantages: On the one hand, the heat transfer coefficient of molten salt is much higher than that of air, which can compress the time for the wire rod to pass through the sensitive area of ​​network carbide precipitation to within a few seconds. Carbon atoms cannot fully diffuse and agglomerate, and cementite is also difficult to grow continuously along the grain boundaries, thereby effectively suppressing the problem of abnormal precipitation of network carbides caused by high carbon content; on the other hand, the wire rod During molten salt treatment, the molten salt can uniformly cover the surface of the wire rod for heat exchange, which helps to reduce the cross-sectional temperature difference between the wire rod surface and the core, as well as the circumferential temperature difference between the overlapping and non-overlapping areas of the wire rod. This avoids the formation of hard and brittle abnormal phases such as martensite due to local supercooling caused by the solidification segregation of C, Mn, and Cr elements, which would lead to severe stress concentration. At the same time, it improves the synchronicity of phase transformation across the entire cross-section. In addition, the nano-carbides of Nb and V can be dispersed and precipitated between phases, avoiding the strength loss caused by uneven precipitation under slow cooling and unstable temperature control conditions.

[0039] II. Compared to the continuous temperature drop of wire rods during continuous cooling in the Stellmore air-cooled line with high carbon and high silicon content, which affects the microalloying strengthening efficiency and leads to asynchronous phase transformation, large fluctuations in microstructure and properties, and insufficient strength and plasticity, this invention has the following advantages: Firstly, rapid cooling of the wire rod can increase the phase transformation driving force, and together with Si and Al, it can suppress cementite coarsening and promote the transformation of high-temperature austenite into fine lamellar sorbite in a short time. As the processing time is extended, the wire rod gradually transforms to the molten salt temperature instead of continuous cooling. Combined with the wire rod hardenability and synchronous phase transformation, it can improve the sorbite nucleation rate, reduce microstructure stress, and avoid the formation of coarse lamellar pearlite due to slow core cooling, thus avoiding strength loss. It also offsets the phase transformation delay effect brought by Si, Cr, and Mo, and under sufficient phase transformation, it can avoid the residual austenite in the later stages. During continued cooling, martensite and other low-temperature brittle structures continue to form. On the other hand, the high temperature of the molten salt allows for the full precipitation of Nb and V nanocarbides during the isothermal process. Combined with the wire rod's resistance to tempering softening, this promotes the melting of some cementite flakes, and the structural stress generated by the phase transformation is released simultaneously with the thermal stress from rapid cooling. The use of Mo to inhibit the precipitation and coarsening of nanocarbides during the isothermal process compensates for the loss of tempering strength. Under long-term isothermal tempering, the wire rod's ductility and toughness can be controlled to match its high strength. After exiting the molten salt, the wire rod is slowly cooled by the roller conveyor using its residual heat, further improving the toughening effect. This balances production efficiency and energy consumption, promoting efficient and stable production of hot-rolled wire rod. The resulting hot-rolled wire rod has good strength, plasticity, and uniformity, and does not require further offline heat treatment. It can be directly used for cold drawing and twisting to produce stranded wire.

[0040] Furthermore, before rolling, a rectangular steel billet with a side length of 180mm~220mm is manufactured using a continuous casting process. Protective casting is employed to prevent secondary oxidation of the molten steel. The superheat of the tundish is controlled to ≤25℃. At low superheat, the proportion of equiaxed crystals is increased, suppressing the penetrating growth of columnar crystals and reducing dendrite segregation from the initial stage of solidification. The current of the electromagnetic stirring in the crystallizer is controlled at 305~355A, and the frequency at 1~3Hz, acting on the initial stage of solidification. A low-frequency design is used to break up the primary dendrites at the solidification front, preventing the billet shell from being eroded by excessively high current or frequency. This can lead to surface cracks or the formation of subcutaneous inclusions due to the entrainment of protective slag. The current of the electromagnetic stirring at the end is controlled at 500~550A and the frequency is 7~9Hz. It acts on the mushy zone at the end of solidification and adopts a medium-high frequency design to break up coarse secondary dendrites, mix the solute enriched in the center, and reduce macroscopic segregation in the center. This avoids the inability to stir the molten steel in the mushy zone due to insufficient current or frequency, and also prevents internal microcracks caused by excessive current. The casting speed is controlled at 0.13~0.33m / min. The low casting speed is suitable for the solidification characteristics of high carbon and high alloy steel and improves the uniformity of the composition of the core of the billet.

[0041] Furthermore, before rolling, rectangular steel billets with side lengths of 180mm to 220mm are heated. During heating, the homogenization temperature of the heating furnace is controlled at 1200 to 1260℃, and the furnace time is 185 to 245 minutes. Through high-temperature diffusion, the steel billet is completely burned through from the surface to the core, further reducing residual dendrite segregation in the billet, continuing the homogenization effect of the continuous casting process, and effectively controlling the austenite grain size and surface decarburization. This avoids excessive coarsening of austenite grains, surface decarburization, and accelerated oxidation and burning due to excessively high homogenization temperature or excessive furnace time, thereby providing austenitic high-temperature steel billets with uniform temperature and stable state across the entire cross-section for subsequent rolling.

[0042] Furthermore, during the rolling process, the initial rolling temperature is 1037~1087℃, the final rolling temperature is 914~954℃, and the final rolling reduction is 21%~26%. By selecting appropriate rolling temperatures and deformation amounts, a higher initial rolling temperature can reduce the rolling deformation resistance, facilitate the precipitation of Nb and V carbides, promote dynamic recrystallization during the final rolling process, and refine the grains.

[0043] Furthermore, the wire drawing temperature is 904~944℃; a higher wire drawing temperature is selected to ensure that the wire rod is in a high-temperature austenitizing state and to prevent the precipitation of network carbides; at the same time, the wire drawing temperature should not be too high, so as to prevent the austenite grains from continuing to grow during the transportation process and losing strength, or to increase the energy consumption for subsequent salt bath temperature fluctuation control due to excessive heat brought in by the high temperature.

[0044] Furthermore, the online molten salt rapid cooling isothermal control process is divided into a pre-stage molten salt treatment and a post-stage molten salt treatment. The molten salt temperature in the pre-stage molten salt treatment is 553~583℃, the treatment time is 142~200s, and the molten salt circulation volume is larger than that in the post-stage molten salt treatment. The molten salt temperature in the post-stage molten salt treatment is 568~583℃, and the treatment time is 100~200s.

[0045] Considering that using a large molten salt circulation volume throughout the single-stage molten salt treatment process would increase production energy consumption, while using a small molten salt circulation volume would increase the molten salt temperature rise and affect the uniformity of the microstructure, there are shortcomings such as difficulty in balancing rapid cooling and energy consumption, and difficulty in balancing efficiency and plasticity. Therefore, this invention can further adopt a segmented treatment process with a front-stage molten salt treatment and a rear-stage molten salt treatment: The initial molten salt treatment controls the wire rod to cool at a rate of ≥32℃ / s, rapidly transitioning it from the high-temperature austenitic state through the network carbide region to the sorbite phase region. This suppresses network carbide formation and, with the aid of significant undercooling, fosters a microstructure dominated by fine-lamellar sorbite, enhancing the dispersion and precipitation of microalloyed carbides. It avoids excessively low molten salt temperatures to prevent excessive undercooling from leading to elemental segregation and the formation of a hard, brittle microstructure. It also avoids excessively high molten salt temperatures to prevent increased difficulty in controlling network carbides, insufficient undercooling, decreased sorbite nucleation rate, increased sorbite lamellar spacing, and negative impacts on alloy carbide precipitation and fine-grain strengthening effects. Furthermore, it avoids excessively short treatment times, which result in insufficient cooling rate in the wire rod core, reduced network carbide suppression, asynchronous phase transformation initiation, insufficient sorbite nucleation, lamellar coarsening, and decreased microstructure uniformity. Finally, it avoids excessively long treatment times, which increase production energy consumption.

[0046] The subsequent molten salt treatment reduces the molten salt circulation rate to promote the continued transformation of untransformed residual austenite into fine lamellar interlamellar sorbite. Simultaneously, it allows the already formed fine lamellar interlamellar sorbite to undergo prolonged isothermal tempering, and promotes the transformation of some cementite lamellars into spheroidized structures. Microalloyed carbides are fully precipitated and dispersed, effectively releasing phase transformation stress and thus regulating the strength-ductility balance of the wire rod. This avoids excessively low molten salt temperatures or treatment times, which can lead to insufficient carbon atom diffusion rates, resulting in incomplete phase transformation, insufficient microalloyed carbide precipitation, or poor cementite melting, limiting ductility improvement. Conversely, excessively high molten salt temperatures or excessively long treatment times can prevent excessive spheroidization and coarsening of cementite, or agglomeration and coarsening of microalloyed carbides, ultimately leading to a decrease in strength and ductility.

[0047] Furthermore, the molten salt circulation rate of the front-end molten salt treatment is 500~600t / h, and the molten salt temperature rise is ≤8℃. Using a higher molten salt circulation rate can effectively control the cooling rate of the wire rod and the molten salt temperature rise, avoiding the decrease in the cooling rate of the wire rod and the impact on the uniformity of the structure during continuous processing due to too low circulation rate, while preventing the equipment load and energy consumption from increasing due to too high circulation rate.

[0048] Furthermore, the molten salt circulation rate of the subsequent molten salt treatment is 320~420t / h, and the molten salt temperature rise is ≤3℃. The heat exchange load of the subsequent molten salt treatment is less than that of the preceding molten salt treatment. The smaller circulation rate makes it easier to achieve precise temperature control, ensure uniformity of the structure, and avoid energy waste caused by a large molten salt circulation rate.

[0049] Furthermore, during the slow cooling process on the roller conveyor, after the wire rod exits the salt tank, it enters the conveyor roller conveyor, where hot air at ≥230°C from the salt tank is blown onto the conveyor roller conveyor. For example, after the wire rod exits the molten salt tank, it enters the conveyor roller conveyor, where the insulation cover on the conveyor roller conveyor is opened, and hot air from the salt tank is blown onto the conveyor roller conveyor, controlling the wire rod to be slowly cooled to below 280°C at a cooling rate of 0.4~0.9°C / s before winding. During this slow cooling process, the residual heat of the wire rod itself can be used to further promote the toughening of the structure and improve the tempering and softening effect. The hot air used comes from the heat dissipation of the salt tank and the residual heat from the volatilization of the medium. By recovering and utilizing the residual heat, energy consumption and operating costs are effectively reduced.

[0050] To further illustrate the present invention, a preferred embodiment of the hot-rolled wire rod for 2600MPa grade steel strand is further described using Example 1 as an example. The hot-rolled wire rod manufacturing method of Example 1 follows the process flow of continuous casting → heating → rolling → wire drawing → online molten salt rapid cooling isothermal control treatment → roller table slow cooling → coiling. Specifically: The continuous casting process involves injecting molten steel with a chemical composition matching that of hot-rolled wire rod into a tundish. The tundish then distributes the molten steel to various crystallizers of the continuous casting machine via a nozzle. The continuous casting produces steel billets with a length × width of 220mm × 220mm. During the continuous casting process, protective casting is used to prevent secondary oxidation of the molten steel. The tundish superheat is controlled at 20°C, the current of the electromagnetic stirring in the crystallizer is 350A, and the frequency is 3Hz. The current of the electromagnetic stirring at the end is controlled at 530A, the frequency is 8Hz, and the casting speed is 0.25m / min to improve the uniformity of the core composition of the billet.

[0051] The heating process is used to feed the continuously cast steel billet into the heating furnace and heat it to a high temperature that can be rolled into plasticity. The heating furnace is heated in sequence according to the preheating section, the first heating section, the second heating section and the soaking section. The soaking temperature of the heating furnace is controlled at 1230°C and the time in the furnace is 220 minutes.

[0052] The rolling process uses a rolling line to roll high-temperature steel billets into wire rods with a diameter of 13.5 mm. During rolling, the initial rolling temperature is controlled at 1050℃, the final rolling temperature is 954℃, and the final rolling reduction is 21%.

[0053] The spinning process is used to turn the wire from the rolling line into coils through the spinning mechanism. The spinning temperature is controlled at 930°C. The coils are spread on the roller table and conveyed to the salt tank along the closely arranged roller table.

[0054] The online molten salt rapid cooling isothermal control process employs a two-section salt tank with internal molten salt. After spinning, the wire rod is conveyed by rollers through the first section of the salt tank for the initial molten salt treatment, which cools the wire rod at a rate of 35℃ / s. The molten salt temperature in the initial molten salt treatment is controlled at 583℃, the treatment time is 180s, the molten salt circulation rate is 580t / h, and the molten salt temperature rise is ≤8℃. Then, the wire rod is conveyed by rollers through the second section of the salt tank for the final molten salt treatment. The molten salt temperature in the final molten salt treatment is 575℃, the treatment time is 190s, the molten salt circulation rate is 370t / h, and the molten salt temperature rise is ≤3℃.

[0055] The slow cooling process on the roller conveyor involves opening the insulation cover, conveying the wire rod via the conveyor rollers, and blowing hot air (≥230℃) from the salt bath onto the conveyor rollers, controlling the wire rod to cool slowly to 262℃ at a cooling rate of 0.65℃ / s. The coiling process involves coiling the wire rod into coils using a coiling drum, packaging and storing them to obtain the finished hot-rolled wire rod, the metallographic structure of which is shown in the figure below. Figure 1 As shown.

[0056] Other embodiments differ from Example 1 in chemical composition and process parameters. The chemical composition of each embodiment is shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0057] Table 1. Chemical composition of each embodiment

[0058] Comparative Examples 1 and 2 have the same chemical composition as Example 1; Comparative Example 3 has the same chemical composition as Example 2; Comparative Example 4 has the same chemical composition as Example 3; Comparative Example 5 has the same chemical composition as Example 4; The process parameters of each example and comparative example are shown in Table 2 below.

[0059] Table 2. Process parameters for each embodiment and comparative example

[0060] The microstructure and performance of the hot-rolled wire rods obtained in the above embodiments and comparative examples were tested, and the comparison results are shown in Table 3 below.

[0061] Table 3. Microstructure and property test results of different hot-rolled wire rods

[0062] As shown in the table above, compared to high-carbon, high-silicon wire rods, which are prone to developing network carbides and martensite abnormal structures in the Steyr air-cooling line process, and whose microalloying element precipitation is affected by continuous cooling temperature fluctuations, resulting in low strengthening efficiency, delayed pearlite phase transformation, high cold deformation resistance, and insufficient plasticity and microstructure uniformity, this invention, through high Si-Al chemical composition design combined with online molten salt rapid cooling isothermal control technology, can effectively control material costs. The online molten salt rapid cooling isothermal control preferably controls the wire rod to cool at a rate ≥32℃ / s, with a molten salt temperature of 553~583℃ and a processing time of 240~400s. This allows the wire rod to rapidly transition from the high-temperature austenitic state to the sorbite phase region, thereby inhibiting network carbide formation and avoiding the formation of martensite, etc. The process produces an abnormal microstructure, primarily composed of fine-laminated interlaminated sorbite. Based on this, a prolonged isothermal tempering process is initiated to induce partial melting and spheroidization of cementite lamellars. Microalloyed carbides are then fully dispersed to regulate the strength-ductility balance of the wire rod. Finally, slow cooling via roller conveyors further toughens the wire rod, enhancing the tempering and softening effect. This process eliminates the need for offline heat treatment, achieving a network carbide level of 0 and no martensite in the hot-rolled wire rod. Mechanical properties with a difference of ≤50 MPa between coils, tensile strength of 1644~1694 MPa, and reduction of area of ​​32%~37% are achieved. This process balances production efficiency and is suitable for applications such as manufacturing 2600 MPa grade stranded wire. The elimination of offline heat treatment reduces the risk of wire breakage, achieving energy and material savings.

[0063] The comparison results between Comparative Examples 1 and 2 and Example 1 show that excessively high spinning temperature, excessively low molten salt temperature, and excessively short processing time will affect the interlamellar spacing and thus the mechanical properties of the wire rod. It is necessary to avoid excessively low molten salt temperature and excessive supercooling, which will lead to the formation of hard and brittle structures in the element segregation enrichment zone. At the same time, it is necessary to avoid excessively short processing time, which will result in insufficient cooling rate of the wire rod core, reduced suppression effect of network carbides, asynchronous phase transformation initiation, insufficient number of sorbite nuclei, lamellar coarsening, and reduced structure uniformity. Therefore, the preferred molten salt temperature for the initial molten salt treatment is 553~583℃, and the processing time is 142~200s.

[0064] Comparisons between Comparative Example 3 and Example 2, and between Comparative Example 4 and Example 3, show that appropriately reducing the molten salt circulation rate in the later stage of molten salt treatment, and increasing the molten salt temperature and treatment time, results in larger lamellar structures, lower mechanical strength, and poorer reduction of area. It is necessary to avoid excessively low molten salt temperatures or excessively short treatment times, which would lead to insufficient carbon atom diffusion rates, resulting in incomplete phase transformation, insufficient precipitation of microalloyed carbides, or poor cementite melting effect. Simultaneously, it is necessary to avoid excessively high molten salt temperatures or excessively long treatment times, which would lead to a decrease in strength and plasticity. Therefore, the preferred molten salt temperature for the later stage of molten salt treatment is 568~583℃, and the preferred treatment time is 100~200s.

[0065] As can be seen from the comparison results between Comparative Example 5 and Example 4, slow cooling on the roller table can further promote the toughening of the structure. The faster the slow cooling speed, the slightly higher the strength and the lower the shrinkage rate. Therefore, it is preferable to control the wire rod to be slowly cooled to below 280°C at a cooling rate of 0.4~0.9°C / s before winding.

[0066] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand, characterized in that, Its manufacturing methods include: The hot-rolled wire rod is rolled into finished products according to its chemical composition. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.96%~1.01%, Si: 0.79%~0.97%, Mn: 0.88%~1.08%, Cr: 0.50%~0.70%, Nb: 0.015%~0.035%, V: 0.028%~0.038%, Al: 0.22%~0.42%, Mo: 0.22%~0.42%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is spun into coils at a spinning temperature of ≥904℃ and then enters a salt bath for online molten salt rapid cooling isothermal control treatment, so that the coils are cooled at a cooling rate of ≥32℃ / s, the molten salt temperature is controlled at 553~583℃, and the treatment time is 240~400s; then it is slowly cooled by roller conveyor, and the coils are slowly cooled to below 280℃ at a cooling rate of 0.4~0.9℃ / s, so as to make hot-rolled coils with a microstructure consisting of a mixture of tempered sorbite, ferrite and melted sorbite.

2. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 1, characterized in that, Before rolling, a rectangular steel billet with a side length of 180mm~220mm is manufactured by continuous casting. The superheat of the tundish is controlled to be ≤25℃. The current of the electromagnetic stirring in the crystallizer is 305~355A and the frequency is 1~3Hz. The current of the electromagnetic stirring at the end is controlled to be 500~550A and the frequency is 7~9Hz. The casting speed is 0.13~0.33m / min.

3. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 1, characterized in that, Before rolling, a rectangular steel billet with a side length of 180mm to 220mm is heated. During heating, the uniform heating temperature of the heating furnace is controlled at 1200 to 1260℃, and the furnace time is 185 to 245 minutes.

4. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is 1037~1087℃, the final rolling temperature is 914~954℃, and the final rolling reduction is 21%~26%.

5. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 1, characterized in that, The spinning temperature is 904~944℃.

6. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 1, characterized in that, The online molten salt rapid cooling isothermal control process is divided into a pre-stage molten salt treatment and a post-stage molten salt treatment. The molten salt temperature in the pre-stage molten salt treatment is 553~583℃, the treatment time is 142~200s, and the molten salt circulation volume is larger than that in the post-stage molten salt treatment. The molten salt temperature in the post-stage molten salt treatment is 568~583℃, and the treatment time is 100~200s.

7. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 6, characterized in that, The molten salt circulation rate of the front-end molten salt treatment is 500~600t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation rate of the rear-end molten salt treatment is 320~420t / h, and the molten salt temperature rise is ≤3℃.

8. The method for manufacturing hot-rolled wire rod for 2600MPa grade steel strand according to claim 1, characterized in that, During the slow cooling process of the roller conveyor, the wire rod enters the conveyor roller conveyor after exiting the salt tank, and the hot air at ≥230℃ from the salt tank is blown onto the conveyor roller conveyor.

9. A hot-rolled wire rod for 2600MPa grade steel strand, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of hot-rolled wire rod for 2600MPa grade steel strand as described in any one of claims 1 to 8.

10. The hot-rolled wire rod for 2600MPa grade steel strand according to claim 9, characterized in that, The hot-rolled wire rod has a diameter of 5.5~15mm, a volume ratio of tempered sorbite and melted sorbite of ≥97%, a lamellar spacing of 55~75nm in tempered sorbite, a network carbide grade of 0, no martensite, a mechanical property difference of ≤50MPa between rings, a tensile strength of 1644~1694MPa, and a reduction of area of ​​32%~37%.

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