A hot-rolled wire rod for 2300MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a hot-rolled wire rod for 2300MPa grade stranded wire and its manufacturing method. The method involves rolling high-Si, high-carbon components containing trace amounts of Nb into wire rods, followed by online molten salt isothermal toughening treatment. This process involves a pre-treatment molten salt stage followed by rapid cooling, transitioning the wire rod from an austenitic state to a sorbite phase, forming a structure dominated by sorbite. A subsequent molten salt stage further reduces the molten salt temperature and circulation rate, promoting the continued transformation of untransformed residual austenite into sorbite, followed by isothermal tempering and partial melting of sorbite lamellars. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure comprising tempered sorbite, ferrite, and melted sorbite. This achieves a tensile strength of 1530~1580MPa, a reduction of area of ​​35%~40%, and a mechanical property difference of ≤48MPa between different coils. This method balances material cost and production efficiency, eliminating the need for offline heat treatment and promoting efficient and stable production of ultra-high strength stranded wire.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a 2300MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] Steel strand possesses extremely high stress-bearing capacity and is primarily used in construction, railways, bridges, new energy, and power industries through anchoring, hoisting, lifting, and towing. The strength grade of steel strand is a crucial factor affecting steel consumption, cost, and energy consumption. Therefore, ultra-high strength strand has become a key research and development focus for manufacturers. Based on the current production capacity of steel mills and wire drawing plants, it is difficult to further increase the strength grade of steel strand after reaching 1960MPa. Offline heat treatment can help improve the strength grade, but it drastically increases production costs and energy consumption. Therefore, there is a need to develop a 2300MPa grade hot-rolled wire rod for strand and its manufacturing method that eliminates the need for offline heat treatment to meet the development needs of the steel industry and market demands.

[0003] Existing high-strength steel strand wire rods generally use a high-carbon composition system. The following technical bottlenecks still exist in manufacturing hot-rolled wire rods for 2300MPa grade strands:

[0004] I. Hot-rolled wire rod is generally produced in conjunction with a Stellmore air-cooling line. For example, patent CN119287269A discloses a green and environmentally friendly 2300MPa grade steel strand wire rod and its production method, which uses a C-Si-Mn-Cr-Al composition and combines controlled cooling, salt bath treatment, and aging treatment after rolling to produce sorbitic wire rod. However, on the one hand, due to the segregation of high carbon components in continuous casting and the limitation of the maximum cooling capacity of the post-rolling air-cooling line, the wire rod stays in the network carbide sensitive area for a long time. Carbon at the grain boundaries will continue to precipitate and grow into continuous network carbides, which not only ruptures the matrix but also deteriorates the ductility and toughness of the wire rod. On the other hand, increasing the air-cooling strength increases the temperature control instability, and the air-receiving and air-receiving surfaces of the wire rod, as well as the surface... The temperature difference between the core and the outer core will further increase. Due to the high alloy content and hardenability, local overcooling will form abnormal structures such as brittle martensite, affecting the uniformity and ductility of the matrix, and thus exacerbating the risk of wire breakage during subsequent drawing and twisting. On the other hand, due to the limitations of the length and cooling capacity of the air-cooling line, in order to obtain a high-strength matrix and increase the sorbite content in the microstructure, low-temperature rolling and strong air-cooling treatment are required during controlled rolling. This not only increases the number of abnormal structures, but also increases the instability of temperature control during the phase transformation process. Large residual stress will remain in the microstructure, and the sorbite content and uniformity are difficult to guarantee. Although salt bath treatment is combined, after air-cooling and coiling, it is necessary to unwind, heat to the austenitizing temperature, and then heat-treat. This involves many processes and high energy consumption, which will affect production costs and efficiency.

[0005] II. To improve the strength and ductility of wire rods, existing technologies employ online salt bath treatment. For example, patent CN120400687A discloses wire rods, strands, and preparation methods for 2300~2360MPa grade stress corrosion resistant steel strands, using a C-Si-Mn-Cr-V composition, combined with low-temperature rolling and online isothermal salt bath treatment, and online aging to produce sorbite wire rods. However, to promote the refinement of sorbite lamellars and improve matrix strength, the single-stage salt bath treatment limits the need for low-temperature rolling, which also introduces the risk of network carbide precipitation during low-temperature rolling. As wire rod specifications decrease, rolling becomes increasingly difficult. Speed ​​and efficiency are limited, which can easily exacerbate wear on the rolling line. At the same time, a large amount of molten salt circulation is required for overall treatment, resulting in high energy consumption. On the other hand, due to the influence of steel composition, the treatment time of one-stage salt bath treatment should not be too long in order to reduce the risk of cementite coarsening. However, large internal stress will be generated at low molten salt temperature, which will require low cooling rate aging treatment, affecting the speed and efficiency of the line exit. At the same time, the final strength and plasticity of hot-rolled wire rod are insufficient. In the subsequent drawing process, more drawing passes are required to harden and achieve the target strength. This process will increase plasticity loss, bring the risk of wire breakage, and affect the yield of stranded wire and 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 2300MPa grade stranded wire and its manufacturing method, which can improve the strength-plasticity matching and microstructure uniformity of hot-rolled wire rod, while taking into account material cost and production efficiency, so as to eliminate the need for offline heat treatment and promote the efficient and stable production of ultra-high strength 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 2300MPa grade stranded wire, the method comprising:

[0009] The wire rod is rolled into production wire according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.90%~0.95%, Si: 0.82%~1.02%, Mn: 0.65%~0.85%, Cr: 0.23%~0.43%, Nb: 0.026%~0.046%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is spun into wire rod at a spinning temperature of ≥910℃, and then subjected to… Online molten salt isothermal toughening treatment involves first subjecting the wire rod to a pre-treatment molten salt process with a cooling rate of ≥33℃ / s, transforming it from an austenitic state into a sorbite phase region, forming a microstructure dominated by sorbite. Then, a post-treatment molten salt process is performed, reducing the molten salt temperature and circulation rate to promote the further transformation of untransformed residual austenite into sorbite, followed by isothermal tempering and partial melting of sorbite lamellars. Finally, the wire rod undergoes slow cooling via 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 effective strengthening element and its price is relatively low. Through solid solution strengthening, it can induce lattice distortion to hinder dislocation movement, reduce the critical temperature of austenitization, improve the stability of austenite, and shift the continuous cooling transformation curve of steel to the right in order to reduce the diffusion rate of carbon, form a fine lamellar structure, and improve the strength of the matrix. However, excessive C content can easily cause compositional segregation, which can easily accumulate at the austenite grain boundaries and precipitate continuous network carbides. At the same time, it slows down the sorbite transformation rate, prolongs the phase transformation incubation period, and increases the difficulty of controlling the martensite deterioration structure, resulting in increased brittleness and mechanical property fluctuations of wire rod. Therefore, in order to take into account the high strength requirements of 2300MPa grade strand, improve the uniformity of the structure, and promote rapid production, the mass percentage of C is controlled at 0.90%~0.95%.

[0012] (2) Silicon: Si is a solid solution strengthening element of ferrite. It can slow down the diffusion rate of carbon to the grain boundary, inhibit the formation of coarse carbides during cooling, strongly inhibit the precipitation and coarsening process of cementite, avoid the formation of coarse lamellar pearlite with insufficient strength during cooling, make the lamellar distribution more uniform, and play a role in refining the precipitated phase and improving the toughness of the material. However, excessive silicon will lead to excessive solid solution strengthening, increased degree of lattice distortion of steel, significant increase in dislocation movement resistance, and performance fluctuation due to excessive residual austenite, which will increase the difficulty of isothermal tempering and reduce production efficiency. Therefore, in order to adapt to the regulation of phase transformation structure by online molten salt isothermal toughening, the resistance to tempering softening should be appropriately increased, and the Si content should be appropriately increased. The mass percentage of Si should be controlled at 0.82%~1.02%.

[0013] (3) Manganese: Mn can reduce the critical cooling rate of steel, expand the stable range of austenite, and improve hardenability to ensure that austenite is fully transformed into fine lamellar sorbite, reduce the formation of coarse grain structure, and promote finer and more uniform cementite lamellars in sorbite. As a solid solution strengthening element, it can help improve strength through lattice distortion, while hindering the diffusion of C atoms into cementite particles, improving the strength stability and work hardening rate after tempering. However, if the Mn content is too high, it will increase the segregation of alloying elements during the solidification process of steel billet, and increase the risk of martensite formation during cooling, leading to increased brittleness and mechanical property fluctuations. Therefore, in order to make hot-rolled wire rod have a high strength-plasticity combination and take into account the uniformity of structure, the mass percentage of Mn is controlled at 0.65%~0.85%.

[0014] (4) Chromium: Cr can provide solid solution strengthening effect, while significantly improving hardenability, reducing the sorbite transformation temperature of steel, delaying the diffusion rate of carbon atoms in austenite, and promoting the decomposition of austenite into sorbite at a lower temperature, so that the sorbite lamellars are finer and more uniformly distributed. At the same time, it can form nanoscale carbides with C, which are uniformly precipitated during molten salt treatment and tempering, which can increase the resistance to tempering softening and reduce strength loss. However, if the Cr content is too high, it will aggravate the segregation of alloying elements, increase the risk of precipitation of martensite and other deteriorated structures and dislocation movement resistance, and increase the difficulty of tempering softening and affect the production line efficiency. Therefore, in order to control the sorbite phase transformation, structure uniformity and resistance to tempering softening, and take into account production efficiency, the mass percentage of Cr is controlled at 0.23%~0.43%.

[0015] (5) Niobium: As a microalloying element, Nb can refine grains by precipitating strain-induced pinning grain boundaries during high-temperature hot rolling, inhibiting the growth of austenite grains at high temperatures, so as to reduce the hot deformation resistance of steel, reduce the risk of rolling cracks, and enable rapid rolling, laying the foundation for the formation of strong and tough sorbite by subsequent phase transformation. By refining austenite grains, it indirectly inhibits the formation of pearlite and reduces the fluctuation of the structure. During isothermal tempering, excess Nb will further precipitate nano-sized NbC. NbC has a high melting point and better stability than VC, which can provide a strong precipitation strengthening effect, allowing the wire rod to maintain good plasticity in multiple drawing passes, reducing the risk of wire breakage, and adapting to the processing requirements of subsequent steel strands. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling material costs. Therefore, the mass percentage of Nb is controlled at 0.026%~0.046%.

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

[0017] The aforementioned wire rod adopts a high-carbon, high-silicon composition system of C-Si-Mn-Cr-Nb, with trace amounts of Nb added to appropriately control material costs. Simultaneously, Si slows down carbon diffusion, Mn and Cr synergistically regulate wire rod hardenability and reduce the critical cooling rate of the steel, and Cr and Nb synergistically strengthen C with C carbides. This provides favorable conditions for reducing rolling limitations, promoting the full transformation of fine-laminated interlaminated sorbite, reducing the risk of cementite coarsening and abnormal structures, and improving strength and microstructure uniformity. Furthermore, a relatively high wire drawing temperature is selected to keep the wire rod in a high-temperature austenitic state, avoiding premature precipitation of network carbides or proeutectoid ferrite at excessively low wire drawing temperatures. This prepares the microstructure for rapid cooling to promote the subsequent transformation to a uniform sorbite structure. After wire drawing, the wire rod undergoes online molten salt isothermal toughening treatment without air cooling.

[0018] I. Compared to the limitations of high-carbon and Stellmore air-cooled wire lines, which suffer from high cooling capacity and temperature control instability, making it difficult to control abnormal structures such as network carbides and martensite, wire rods treated with molten salt in the initial stage can be rapidly cooled, bypassing the sensitive precipitation temperature range of network carbides. This suppresses the formation of network carbides and avoids their adverse effects on matrix uniformity and ductility. Furthermore, as the wire rod passes through the molten salt, the molten salt covers the surface for uniform heat exchange, eliminating the temperature difference between the air-receiving and air-receiving surfaces. This avoids the risk of brittle martensite structures caused by alloy element segregation and localized overcooling. Compared to existing single-stage salt bath treatments, which struggle to balance phase transformation control and microstructure uniformity, this method offers several advantages. On the one hand, the higher wire drawing temperature and the higher molten salt circulation rate in the first stage of molten salt treatment compared to the second stage allow the wire rod to cool down rapidly from its high-temperature austenitic state using the strong cooling capacity of the molten salt. This prevents the formation of harmful phases such as network carbides and martensite, thus increasing the phase transformation driving force. Combined with Si to suppress cementite coarsening, this results in fine and uniformly distributed sorbite lamellae, providing a high-strength microstructure. On the other hand, the higher molten salt temperature in the first stage of molten salt treatment compared to the second stage helps to mitigate the temperature difference between the wire rod surface and core. Combined with the wire rod's hardenability, this allows the sorbite microstructure to form simultaneously on the wire rod surface and in the core, providing a uniform microstructure for the second stage of molten salt treatment and reducing the difficulty and energy consumption of isothermal tempering.

[0019] II. Compared to the limitations imposed by the length and minimum cooling capacity of the Stellmore air-cooling line, which hinders the strengthening effect of microalloying, sufficient phase transformation, and online toughening, the wire rod gradually transforms to the same temperature as the molten salt during the subsequent molten salt treatment. This promotes the dispersed precipitation of Cr and Nb alloy carbides, preventing coarsening caused by uncontrollable temperature or excessively high temperatures. With prolonged treatment time, it further promotes the decomposition of retained austenite into fine lamellar sorbite, facilitating the full precipitation of alloy carbides, increasing the sorbite content and reducing the ferrite content in the microstructure. This fully leverages the strengthening effect of carbon and microalloying elements, preventing the continued formation of abnormal microstructures from retained austenite during subsequent cooling. Furthermore, as the wire rod undergoes isothermal tempering in the high-temperature range, it can… This process promotes the melting of cementite lamellars in some sorbite layers, further alleviating thermal and structural stresses and preventing stress concentration. Online toughening controls the balance between plasticity and strength. Compared to existing single-stage salt bath treatments, which struggle to balance production energy consumption and efficiency, this method addresses these issues. Firstly, the lower molten salt circulation volume in the later stages of molten salt treatment reduces production energy consumption. Secondly, by using Si to suppress cementite coarsening and the studding effect of dispersed alloy carbides, strength loss is compensated. Extending the isothermal tempering time allows for rapid toughening, enhancing the wire rod's strength and plasticity while reducing the restriction on slow cooling on the roller table. Since the wire rod is at a higher temperature after exiting the molten salt bath, slow cooling on the roller table promotes further toughening, thus balancing production energy consumption and efficiency and promoting efficient and stable hot-rolled wire rod production.

[0020] Before rolling, a higher heating furnace temperature and an appropriate furnace time can be used to promote uniform diffusion of components, reduce the influence of segregation, and ensure that Nb elements are fully dissolved to prepare for rolling. At the same time, excessively high temperature and excessively long furnace time can be avoided to prevent grain coarsening or overheating. In a preferred embodiment, before rolling, the heating furnace temperature is controlled at 1170~1230℃ and the furnace time is controlled at 160~250min.

[0021] Because the high wire drawing temperature and the stud effect of Nb carbides can suppress austenite grain coarsening, combined with molten salt isothermal toughening treatment, the limitations on rolling can be reduced. During rolling, a higher initial rolling temperature can reduce deformation resistance and wear on the rolling line. Combined with a larger initial rolling reduction, coarse austenite grains are broken up, internal defects are eliminated, and rolling speed and efficiency are improved. By selecting an appropriate final rolling temperature and final rolling reduction, Nb carbide precipitation is induced, and dynamic recrystallization during the final rolling process is promoted, resulting in finer and more stable austenite grains. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1070~1095℃, the initial rolling reduction is 38%~42.5%, the final rolling temperature is 915~950℃, and the final rolling reduction is 22%~27%.

[0022] The wire spinning temperature can be further controlled to avoid excessively high temperatures that could cause austenite grain growth. In a preferred embodiment, the wire spinning temperature is controlled to be 910~940℃.

[0023] Higher molten salt temperatures in the initial molten salt treatment are beneficial for reducing the temperature difference between the wire rod and the core, lowering phase transformation stress and internal stress accumulation. Shorter treatment times can suppress the risk of cementite coarsening and reduce production energy consumption. However, excessively high molten salt temperatures result in insufficient phase transformation driving force, increasing the risk of austenite transforming into coarse lamellar pearlite. Insufficient cooling rate may even increase the risk of network carbide precipitation, affecting the dispersed precipitation of alloy carbides. Conversely, excessively short treatment times will increase the phase transformation rate difference across the wire rod cross-section due to insufficient cooling, leading to fluctuations in mechanical properties. Simultaneously, the increase in residual austenite in the microstructure, which is retained in the subsequent molten salt treatment, will prolong phase transformation inoculation, resulting in a loss of strength and plasticity, or requiring a longer total online molten salt isothermal toughening treatment time, impacting production energy consumption and efficiency. Conversely, lower molten salt temperatures are beneficial for suppressing network carbides. Increased free energy difference between austenite and sorbite can increase the phase transformation driving force, resulting in finer sorbite lamellars. Extending the processing time can promote the full transformation of austenite into sorbite across the entire cross-section of the wire rod, and promote the dispersed precipitation of alloy carbides. However, if the molten salt temperature is too low, the phase transformation rate slows down and the phase transformation stress increases. As the processing time becomes too long, the production pace slows down and energy consumption increases. The accumulation of internal stress will increase the difficulty of the subsequent molten salt treatment, which is detrimental to rapid production. Therefore, the molten salt temperature and processing time of the first stage of molten salt treatment can be controlled to promote the wire rod to quickly bypass the network carbide precipitation temperature range from the high-temperature austenitic state and enter the sorbite phase region, inhibit the formation of network carbides and form a microstructure dominated by fine lamellar interlayer sorbite, promote the dispersed precipitation of alloy carbides, and reduce the accumulation of internal stress, so as to regulate the microstructure uniformity of the wire rod and prepare the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the first stage of molten salt treatment is 568~598℃, and the processing time is 35~135s.

[0024] Since the temperature difference between the spinning temperature and the molten salt temperature of the preceding molten salt treatment is large, a large molten salt circulation rate is selected for the preceding molten salt treatment in order to quickly remove heat and improve the uniformity of the structure. In the preferred technical solution, the molten salt circulation rate of the preceding molten salt treatment is 600~850t / h, and the molten salt temperature rise is ≤8℃.

[0025] Higher molten salt temperatures in the subsequent molten salt treatment accelerate the transformation of retained austenite, reduce limitations on the amount of molten salt circulating, and promote the full precipitation and uniform distribution of alloy carbides with extended treatment time. This facilitates the melting of some sorbite cementite lamellars, releases internal stress, and prevents surface cracks caused by stress concentration during subsequent drawing, thus reducing fluctuations in the performance of the billet. However, excessively high molten salt temperatures enhance atomic diffusion, causing fine cementite and alloy carbides to aggregate and grow, losing their dispersion strengthening effect and resulting in a loss of strength and ductility. Conversely, lower molten salt temperatures suppress cementite lamellar coarsening, resulting in finer sorbite lamellar spacing and a denser structure. Shorter treatment times prevent strength loss due to excessive cementite melting and alloy carbide growth. However, excessively low molten salt temperatures slow down the transformation rate of retained austenite. The reduced melting efficiency of cementite lamellars, coupled with insufficient processing time, leads to incomplete stress release in the sorbite structure, increasing the pressure of slow cooling on the roller conveyor. Incomplete precipitation of alloy carbides results in a loss of strength and plasticity, and even inadequate transformation of retained austenite increases the risk of abnormal microstructure during subsequent cooling. Therefore, the molten salt temperature and processing time of the subsequent molten salt treatment can be further controlled to promote the transformation of untransformed retained austenite into fine lamellar sorbite, while simultaneously promoting long-term isothermal tempering of the formed fine lamellar sorbite and inducing partial melting of cementite lamellars. This promotes the complete precipitation and uniform distribution of alloy carbides, thereby controlling the strength and plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 545~560℃, and the processing time is 150~300s.

[0026] Since the molten salt temperature in the later stage of molten salt treatment is lower than that in the earlier stage, the molten salt circulation rate in the later stage is also lower, which can control the molten salt temperature rise, further reduce the temperature difference between the wire rod surface and the core, and effectively reduce production energy consumption. In the preferred technical solution, the molten salt circulation rate in the later stage of molten salt treatment is 200~280t / h, and the molten salt temperature rise is ≤3℃.

[0027] Since the wire rod has been rapidly toughened by the subsequent molten salt treatment, the limitation on the slow cooling of the roller table can be reduced. The slow cooling of the roller table can further control the slow cooling rate, avoid excessive cooling that would lead to increased stress, promote further toughening of the wire rod structure, and improve the tempering and softening effect of the wire rod. At the same time, it takes into account the production time and production efficiency. In the preferred technical solution, the slow cooling of the roller table controls the wire rod to cool to below 280°C at a slow cooling rate of 0.25~0.55°C / s before winding.

[0028] In the preferred technical solution, the roller conveyor slow cooling adopts the method of closing the heat insulation cover or controlling the opening of the heat insulation cover, blowing the hot air of ≥260°C in the online molten salt isothermal toughening treatment to the conveyor roller conveyor, and the conveyor roller conveyor transports the wire rod through the heat insulation cover. The heat energy of the online molten salt isothermal toughening treatment can be recovered and utilized through the hot air, thereby reducing production energy consumption and promoting the rapid production of wire rod.

[0029] A hot-rolled wire rod for stranded wire of 2300MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method for hot-rolled wire rod of 2300MPa grade as described in any one of the above-mentioned methods.

[0030] The aforementioned hot-rolled wire rod is designed with a high-Si, high-carbon composition containing trace amounts of Nb, which simplifies the composition system, reduces smelting difficulty, and controls material costs. The microstructure consists of a mixture of tempered sorbite and melted sorbite, with a small amount of ferrite. Compared to air-cooled wire rods with a linear martensite structure, this design effectively suppresses abnormal network carbides and martensite, preventing their deterioration of microstructure uniformity and ductility. Simultaneously, it promotes complete phase transformation, reduces the ferrite content, and the sorbite has a finer lamellar spacing than pearlite. The uniform lamellar interface from the wire rod surface to the core effectively hinders dislocation slip. After isothermal tempering, the sorbite... Some cementite lamellars melt and transform into intermediate tempered sorbite and melted sorbite, which transition to spheroidized structure. The nanoscale alloy carbides dispersed in the lamellars further pin dislocations, which can give full play to the strengthening effect of carbon and alloying elements and further improve the matrix strength. At the same time, compared with the sorbite wire rod treated by one-stage salt bath, isothermal tempering can fully release stress, reduce interfacial stress concentration, and promote the uniform dispersion of alloy carbides, which can further improve the matching of plasticity and high strength, improve the uniformity of structure, and thus eliminate the need for offline heat treatment in the subsequent strand manufacturing process, and reduce the initiation of microcracks during drawing and twisting.

[0031] The higher the volume percentage of tempered sorbite and the finer the lamellar spacing, the higher the matrix strength. The higher the proportion of melted sorbite, the higher the ductility and toughness of the matrix. In the preferred technical solution, the volume percentage of tempered sorbite is ≥68%, the lamellar spacing is 80~130nm, the volume percentage of ferrite is ≤4%, and the volume percentage of melted sorbite is ≥18%.

[0032] In the preferred technical solution, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same coil is ≤48MPa. By effectively suppressing the network carbide, stress concentration during drawing can be reduced, stable strand production can be promoted, and service safety and life can be improved. The higher uniformity of the structure makes the mechanical property fluctuation of the hot-rolled wire rod smaller, which can make the force uniform during the drawing of the hot-rolled wire rod, further improving the stability of the cold working process, reducing stress concentration in strand applications, and improving the load-bearing safety of strands.

[0033] In the preferred technical solution, the hot-rolled wire rod has a diameter of 5.5~15mm, a tensile strength of 1530~1580MPa, and a reduction of area of ​​35%~40%. The hot-rolled wire rod has a wider range of specifications to meet the production needs of various steel strands. With the higher initial tensile strength, the number of drawing passes can be reduced, the target diameter and strength can be achieved quickly, and the plasticity loss during the process is smaller. With the higher reduction of area, the risk of wire breakage during drawing and twisting can be further reduced without offline heat treatment, promoting efficient and stable production of strands, reducing the initiation and propagation of fatigue cracks, and extending service life.

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

[0035] (1) In view of the shortcomings of existing hot-rolled wire rods for stranded wire, which are limited by the production process, have abnormal structure, are easy to weaken the strengthening effect, and are difficult to balance the toughening of the structure with production energy consumption and efficiency, this invention adopts a high-Si and high-carbon composition design with trace Nb and online molten salt isothermal toughening technology. This allows the wire rod to quickly bypass the network carbide region from the high-temperature austenitic state and enter the sorbite phase region after the first stage of molten salt treatment. This inhibits the formation of network carbides and forms a structure dominated by fine lamellar interlayer sorbite, controlling the uniformity of the structure. Then, after the second stage of molten salt treatment, the molten salt temperature and molten salt circulation are reduced, which promotes the untransformed residual austenite to continue to transform into fine lamellar interlayer sorbite. At the same time, it promotes the formation of fine lamellar interlayer sorbite for long-term isothermal tempering, causing some cementite lamellars to melt and the alloy carbides to be dispersed and evenly distributed. This controls the strength and plasticity matching of the wire rod, reduces the restrictions on rolling and slow cooling of the roller table, and promotes the efficient and stable production of hot-rolled wire rods.

[0036] (2) In view of the shortcomings of existing hot-rolled wire rods for stranded wire, such as insufficient strength and plasticity, large fluctuations in mechanical properties, and the need for offline heat treatment, the present invention can control material costs. At the same time, the microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can effectively suppress network carbides, martensite and coarse pearlite. The uniform fine lamellar interface effectively hinders dislocation slip. At the same time, the tempered state of the structure is controlled by the melting of cementite lamellars in the sorbite, which fully releases stress, reduces interfacial stress concentration and promotes uniform dispersion of alloy carbides. This can improve the strength and plasticity matching and microstructure uniformity of hot-rolled wire rods, achieving a tensile strength of 1530~1580MPa, a section reduction rate of 35%~40%, and a mechanical property difference of ≤48MPa between the same coils, so that offline heat treatment is not required and the production of ultra-high strength stranded wire is promoted efficiently and stably. Attached Figure Description

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

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

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

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

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

[0042] Example 1:

[0043] A preferred embodiment of the manufacturing method of the 2300MPa 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: 1.02%, Mn: 0.77%, Cr: 0.23%, Nb: 0.03%, P: 0.014%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → roller table slow cooling → coiling, specifically:

[0044] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 5.5mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1230℃, and the furnace time is 160 minutes. The rolling temperature is 1095℃, the initial rolling reduction is 42.5%, the final rolling temperature is 950℃, and the final rolling reduction is 27%. 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 network carbides or proeutectoid ferrite. This prepares the microstructure for rapid cooling to promote the transformation of the subsequent microstructure into a uniform sorbite microstructure. Specifically, the wire drawing temperature is controlled at 940℃.

[0045] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first stage of the salt bath for initial molten salt treatment. This process cools the wire rod at a rate of 33°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of alloy carbides and reduces internal stress accumulation. The wire rod is then conveyed via rollers through the second stage of the salt bath for final molten salt treatment, where the molten salt temperature and circulation rate are reduced, further promoting… Untransformed residual austenite continues to transform into fine lamellar interlamellar sorbite, while simultaneously promoting the formation of this fine lamellar interlamellar sorbite structure to undergo prolonged isothermal tempering. This also induces partial melting of cementite lamellars, promoting the complete precipitation and uniform distribution of alloy carbides, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 598℃, the treatment time is 35s, the molten salt circulation rate is 600t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 560℃, the treatment time is 300s, the molten salt circulation rate is 200t / h, and the molten salt temperature rise is ≤3℃.

[0046] 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 toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 278℃ at a slow cooling rate of 0.5℃ / 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.

[0047] Comparative Example 1: A method for manufacturing hot-rolled wire rod, the difference between this method and Example 1 is that the manufacturing method follows a process flow of rolling → wire drawing → Steyrmo air cooling. Specifically, the heating furnace homogenization temperature is controlled at 1135℃, the furnace time is 260 min, the initial rolling temperature is 1000℃, the initial rolling reduction is 30%, the final rolling temperature is 850℃, the final rolling reduction is 38%, and the wire drawing temperature is 820℃; the Steyrmo forced air cooling uses an air volume of 260,000 m³ per fan. 3 At 65% speed, fans 1 to 4 are turned on to cool the wire rod to 710℃ at a cooling rate of 6.6℃ / s. Then, fans 5 to 14 are turned on to 20% speed to cool the wire rod to 260℃ at a cooling rate of 3.7℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0048] Comparative Example 2: A method for manufacturing hot-rolled wire rod, the difference between this method and Example 1 is that: the heating furnace homogenization temperature is controlled at 1130℃, the furnace time is 280min, the initial rolling temperature is 990℃, the initial rolling reduction is 29%, the final rolling temperature is 830℃, the final rolling reduction is 40%, and the wire drawing temperature is 820℃; during the molten salt treatment before the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 29℃ / s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0049] Example 2:

[0050] A preferred embodiment of the manufacturing method of the 2300MPa 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: 0.99%, Mn: 0.65%, Cr: 0.35%, Nb: 0.026%, 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 isothermal toughening treatment → roller table slow cooling → coiling, specifically:

[0051] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 7.5mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1200℃, and the furnace dwell time is 180 minutes. The initial rolling temperature is 1080℃, the initial rolling reduction is 41%, the final rolling temperature is 940℃, and the final rolling reduction is 26%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of network carbides or proeutectoid ferrite. This prepares the microstructure for rapid cooling to promote the transformation of the subsequent microstructure into a uniform sorbite microstructure. Specifically, the wire drawing temperature is controlled at 930℃.

[0052] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first stage salt bath for initial molten salt treatment. This process cools the wire rod at a rate of 34°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of alloy carbides and reduces internal stress accumulation. The wire rod is then conveyed via rollers through the second stage salt bath for final molten salt treatment, where the molten salt temperature and circulation rate are reduced, further promoting… Untransformed residual austenite continues to transform into fine lamellar interlamellar sorbite, while simultaneously promoting the formation of this fine lamellar interlamellar sorbite through prolonged isothermal tempering. This also induces partial melting of cementite lamellars, promoting the complete precipitation and uniform distribution of alloy carbides, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of the molten salt treatment is 583℃, the treatment time is 58s, the molten salt circulation rate is 710t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of the molten salt treatment is 556℃, the treatment time is 270s, the molten salt circulation rate is 235t / h, and the molten salt temperature rise is ≤3℃.

[0053] 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 toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 276℃ at a slow cooling rate of 0.45℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product, the metallographic structure of which is shown in the figure below. Figure 2 As shown.

[0054] Comparative Example 3: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the first stage of the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 37°C / s, the molten salt temperature of the first stage of the molten salt treatment is 520°C, the treatment time is 150s, and the finished hot-rolled wire rod is obtained after going offline.

[0055] Comparative Example 4: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the first stage of the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 31℃ / s, the molten salt temperature of the first stage of the molten salt treatment is 600℃, the treatment time is 25s, and the finished hot-rolled wire rod is obtained after going offline.

[0056] Example 3:

[0057] A preferred embodiment of the manufacturing method of the 2300MPa 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: 0.94%, Si: 0.82%, Mn: 0.85%, Cr: 0.40%, Nb: 0.039%, 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 isothermal toughening treatment → roller table slow cooling → coiling, specifically:

[0058] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 13mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1185℃, and the furnace dwell time is 225 minutes. The rolling temperature is 1075℃, the initial rolling reduction is 39%, the final rolling temperature is 935℃, and the final rolling reduction is 23%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller table and conveyed along the roller table. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of network carbides or proeutectoid ferrite. This prepares the microstructure for rapid cooling to promote the transformation of the subsequent microstructure into a uniform sorbite microstructure. Specifically, the wire drawing temperature is controlled at 920℃.

[0059] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first stage salt bath for initial molten salt treatment. This process cools the wire rod at a rate of 35°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of alloy carbides and reduces internal stress accumulation. The wire rod is then conveyed via rollers through the second stage salt bath for final molten salt treatment, where the molten salt temperature and circulation rate are reduced, further promoting… Untransformed residual austenite continues to transform into fine lamellar interlamellar sorbite, while simultaneously promoting the formation of this fine lamellar interlamellar sorbite through prolonged isothermal tempering. This also induces partial melting of cementite lamellars, promoting the complete precipitation and uniform distribution of alloy carbides, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 573℃, the treatment time is 104s, the molten salt circulation rate is 800t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 550℃, the treatment time is 185s, the molten salt circulation rate is 265t / h, and the molten salt temperature rise is ≤3℃.

[0060] The slow cooling process on the roller conveyor employs a closed insulation cover, blowing hot air (≥260℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 274℃ at a slow cooling rate of 0.35℃ / 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.

[0061] Comparative Example 5: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that the molten salt temperature of the subsequent molten salt treatment is 600°C, the treatment time is 305s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0062] Comparative Example 6: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that the molten salt temperature of the subsequent molten salt treatment is 500°C, the treatment time is 110s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0063] Example 4:

[0064] A preferred embodiment of the manufacturing method of the 2300MPa 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: 0.95%, Si: 0.93%, Mn: 0.81%, Cr: 0.43%, Nb: 0.046%, P: 0.014%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → roller table slow cooling → coiling, specifically:

[0065] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 15mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1170℃, and the furnace dwell time is 250 minutes. The rolling temperature is 1070℃, the initial rolling reduction is 38%, the final rolling temperature is 915℃, and the final rolling reduction is 22%. 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 table and conveyed along the roller table. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of network carbides or proeutectoid ferrite. This prepares the microstructure for rapid cooling to promote the transformation of the subsequent microstructure into a uniform sorbite microstructure. Specifically, the wire drawing temperature is controlled at 910℃.

[0066] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first stage salt bath for initial molten salt treatment. This process cools the wire rod at a rate of 36°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This inhibits the formation of network carbides, martensite, and coarse pearlite, resulting in a microstructure dominated by fine-laminated sorbite. This promotes the dispersed precipitation of alloy carbides and reduces internal stress accumulation. The wire rod is then conveyed via rollers through the second stage salt bath for final molten salt treatment, where the molten salt temperature and circulation rate are reduced, further promoting… Untransformed residual austenite continues to transform into fine lamellar interlamellar sorbite, while simultaneously promoting the formation of this fine lamellar interlamellar sorbite structure to undergo prolonged isothermal tempering. This also induces partial melting of cementite lamellars, promoting the complete precipitation and uniform distribution of alloy carbides, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 568℃, the treatment time is 135s, the molten salt circulation rate is 850t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 545℃, the treatment time is 150s, the molten salt circulation rate is 280t / h, and the molten salt temperature rise is ≤3℃.

[0067] The slow cooling process of the roller conveyor uses a closed insulation cover to blow hot air at ≥260°C above the two salt bath tanks of the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering and softening effect of the wire rod. Specifically, the wire rod is controlled to be cooled to 272°C at a slow cooling rate of 0.25°C / s. The coiling process is used to coil the wire rod into coils through a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0068] Comparative Example 7: A method for manufacturing hot-rolled wire rod, the difference between which is that the manufacturing method is according to the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → air cooling. The air cooling process is carried out by opening the heat insulation cover and controlling the wire rod to cool to 260°C at a cooling rate of 1.45°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

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

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

[0071]

[0072] The comparison between Example 1 and Comparative Example 1 shows that, compared to the design using a high-Si, high-carbon composition with trace amounts of Nb, the Stellmore air-cooled line exhibits a higher level of network carbides and martensitic anomalous structures. This results in significant residual stress, and the content and uniformity of sorbite are difficult to guarantee. This invention, combined with online molten salt isothermal toughening technology, allows the wire rod to rapidly transition from a high-temperature austenitic state to the sorbite phase region after the initial molten salt treatment, quickly bypassing the network carbide zone. This suppresses the formation of network carbides and forms a structure dominated by fine-laminated interlamellar sorbite, controlling the uniformity of the structure. This is further addressed by the subsequent molten salt treatment. By reducing the molten salt temperature and molten salt circulation rate, the untransformed residual austenite is further transformed into fine lamellar interlamellar sorbite. At the same time, the formed fine lamellar interlamellar sorbite is subjected to long-term isothermal tempering, which causes some cementite lamellars to melt and the alloy carbides to be dispersed and evenly distributed, thereby controlling the strength and plasticity matching of the wire rod. As can be seen from the results of Examples 1 to 4, the tensile strength of hot-rolled wire rod can reach 1530~1580MPa, the reduction of area is 35%~40%, and the mechanical property difference between the same ring is ≤48MPa, so that offline heat treatment is not required and the production of ultra-high strength stranded wire is promoted efficiently and stably.

[0073] As can be seen from the comparison results of Example 1 and Comparative Example 2, selecting a higher wire drawing temperature keeps the wire rod in a high-temperature austenitic state, avoiding the premature precipitation of network carbides or proeutectoid ferrite due to excessively low wire drawing temperature. This prepares the microstructure for rapid cooling to promote the subsequent transformation of the microstructure into a uniform sorbite structure. At the same time, it can reduce the restrictions on rolling, reduce deformation resistance and wear on the rolling line, so as to broaden the specification range of hot-rolled wire rod.

[0074] As can be seen from the comparison results of Example 2 and Comparative Example 3, the lower the molten salt temperature in the first stage of molten salt treatment, the better it is to suppress network carbides. The increased free energy difference between austenite and sorbite can increase the phase transformation driving force, and the formed sorbite lamellars are finer. With the extension of the treatment time, it can promote the full transformation of austenite in the entire cross section of the wire rod into sorbite and promote the dispersion precipitation of alloy carbides. However, if the molten salt temperature is too low, the phase transformation rate will slow down and the phase transformation stress will increase. With the treatment time being too long, the production pace will slow down and the production energy consumption will increase. The accumulation of internal stress will increase the processing difficulty of the subsequent molten salt treatment, which is not conducive to rapid production.

[0075] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature in the later stage of molten salt treatment, the better it is to reduce the temperature difference between the wire rod and the core, reduce the phase transformation stress and internal stress accumulation. With the shortening of the treatment time, the risk of cementite coarsening can be suppressed and the production energy consumption can be reduced. However, if the molten salt temperature is too high, the phase transformation driving force is insufficient, and the risk of austenite transforming into coarse lamellar pearlite increases, affecting the dispersion precipitation of alloy carbides. If the treatment time is too short, the phase transformation rate difference on the wire rod cross-section will increase due to insufficient cooling, resulting in fluctuations in mechanical properties. At the same time, the amount of residual austenite in the microstructure increases and is retained in the later stage of molten salt treatment, which will prolong the phase transformation inoculation, resulting in the loss of strength and plasticity or requiring an extension of the total time of online molten salt isothermal toughening treatment, affecting production energy consumption and efficiency.

[0076] As can be seen from the comparison results between Example 3 and Comparative Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the faster the transformation of residual austenite, the lower the restriction on the amount of molten salt circulation, and the longer the treatment time, the more fully alloy carbides can be precipitated and evenly distributed, and the more some sorbite cementite lamellars can be melted, releasing internal stress and avoiding surface cracks caused by stress concentration during subsequent drawing, thus reducing the fluctuation of the through-strength performance. However, if the molten salt temperature is too high, the atomic diffusion ability is enhanced, and the fine cementite and alloy carbides will aggregate and grow, losing the dispersion strengthening effect and resulting in a loss of strength and plasticity.

[0077] As can be seen from the comparison results between Example 4 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the more effectively the cementite lamellar coarsening can be suppressed, resulting in finer sorbite lamellar spacing and a denser structure. With shorter treatment time, the strength loss caused by excessive cementite melting and alloy carbide growth can be avoided. However, if the molten salt temperature is too low, the transformation rate of retained austenite will slow down, and the cementite lamellar melting efficiency will decrease. With too short a treatment time, the internal stress of the sorbite structure will not be fully released, which will increase the pressure of the slow cooling process on the roller table. Incomplete precipitation of alloy carbides will lead to a loss of strength and plasticity. Even insufficient transformation of retained austenite will increase the risk of abnormal structure in subsequent cooling.

[0078] As can be seen from the comparison results of Example 4 and Comparative Example 7, the roller conveyor slow cooling can further control the slow cooling speed, avoid excessive cooling leading to increased stress, promote further toughening of the wire rod structure, improve the tempering and softening effect of the wire rod, and at the same time take into account the off-line time and production efficiency.

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

Claims

1. A method for manufacturing hot-rolled wire rod for 2300MPa grade stranded wire, characterized in that, Its manufacturing methods include: The wire rod is rolled into production wire according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.90%~0.95%, Si: 0.82%~1.02%, Mn: 0.65%~0.85%, Cr: 0.23%~0.43%, Nb: 0.026%~0.046%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a spinning temperature of ≥910℃, it undergoes online molten salt isothermal toughening treatment. This process involves the wire rod undergoing a preliminary molten salt treatment and then cooling at a rate of ≥33℃ / s, transitioning it from the austenitic state to the sorbite phase region, forming a sorbite-based phase. The wire rod, with a predominantly austenitic microstructure, undergoes a subsequent molten salt treatment to reduce the molten salt temperature and circulation rate, promoting the transformation of untransformed residual austenite into sorbite followed by isothermal tempering and promoting partial melting of sorbite lamellars. Finally, it undergoes slow cooling via a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted sorbite. The molten salt temperature in the initial molten salt treatment is 568–598°C, and the treatment time is 35–135 s. The molten salt temperature in the subsequent molten salt treatment is 545–560°C, and the treatment time is 150–300 s. The slow cooling via the roller conveyor controls the wire rod to cool to below 280°C at a slow cooling rate of 0.25–0.55°C / s before coiling.

2. The method for manufacturing hot-rolled wire rod for 2300MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1170~1230℃ and the furnace time is 160~250min.

3. The method for manufacturing hot-rolled wire rod for 2300MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1070~1095℃, the initial rolling reduction is 38%~42.5%, the final rolling temperature is 915~950℃, and the final rolling reduction is 22%~27%.

4. The method for manufacturing hot-rolled wire rod for 2300MPa grade stranded wire according to claim 1, characterized in that, During the spinning process, the spinning temperature is controlled at 910~940℃.

5. The method for manufacturing hot-rolled wire rod for 2300MPa grade stranded wire according to claim 1, characterized in that, The molten salt circulation rate of the front-end molten salt treatment is 600~850t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation rate of the rear-end molten salt treatment is 200~280t / h, and the molten salt temperature rise is ≤3℃.

6. A hot-rolled wire rod for 2300MPa grade stranded wire, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of hot-rolled wire rod for 2300MPa grade strand as described in any one of claims 1 to 5.

7. The hot-rolled wire rod for 2300MPa grade stranded wire according to claim 6, characterized in that, The volume percentage of tempered sorbite is ≥68%, the lamellar spacing is 80~130nm, the volume percentage of ferrite is ≤4%, the volume percentage of melted sorbite is ≥18%, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤48MPa.

8. The hot-rolled wire rod for 2300MPa grade stranded wire according to claim 6, characterized in that, The hot-rolled wire rod has a diameter of 5.5~15mm, a tensile strength of 1530~1580MPa, and a reduction of area of ​​35%~40%.

Images