A hot-rolled wire rod for 2330MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a hot-rolled wire rod for 2330MPa grade stranded wire and its manufacturing method. The method involves rolling high-carbon, high-silicon Cr-V wire rods 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 predominantly sorbite microstructure. A subsequent molten salt stage further increases the molten salt temperature and reduces the molten salt circulation rate, promoting the continued transformation of untransformed residual austenite into sorbite and isothermal tempering, while also promoting 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 consisting of tempered sorbite, ferrite, and melted sorbite. This method controls material costs, achieving a tensile strength of 1558~1608MPa and a reduction of area of ​​34%~39%, while balancing production efficiency and energy consumption. It eliminates the need for offline heat treatment, enabling 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 2330MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] Steel strand, as a stress-bearing and transmission mechanism, is widely used in energy, transportation, bridges, and construction. With the increasing market demand for lightweight and high-strength strands, there is a need to develop strands with higher strength grades. The microstructure and properties of the raw material, hot-rolled wire rod, are particularly important for improving the strength grade of the strand. Based on the production line capacity of steel mills, further improvements to the strand strength grade beyond 2200MPa become significantly more difficult, sometimes requiring offline heat treatment to achieve higher-grade strand production. However, this would substantially increase the cost of the strand and production energy consumption. Therefore, there is a need to develop a 2330MPa grade hot-rolled wire rod for strand that does not require offline heat treatment to meet the development needs of the steel industry and market demands.

[0003] Existing high-strength hot-rolled wire rods for stranded wire generally employ a high-carbon composition system. The technological bottlenecks restricting the development of 2330MPa grade hot-rolled wire rods for stranded wire include:

[0004] I. To improve the strength and toughness of hot-rolled wire rod, existing hot-rolled wire rods generally adopt high carbon, high silicon, and microalloyed compositions, combined with Steyrmo air-cooling production lines. For example, patent CN118814082B discloses a 2300MPa grade ultra-high strength steel strand steel and its production method, as well as a 2300MPa grade ultra-high strength steel strand, which adopts a C-Mn-Cr-V-Ti-Si composition design, combined with diffusion annealing, low-temperature final rolling and wire drawing, and fast-then-slow air cooling to produce sorbitic wire rod. However, on the one hand, the carbon content of hypereutectoid steel... Due to the high carbon content and the limitation of the maximum cooling capacity of the Stellmore air-cooling line, carbon easily aggregates along the austenite grain boundaries to form network carbides. These network carbides not only disrupt the continuity of the matrix but also affect the carbon source for phase transformation and the ductility and toughness of the matrix. To minimize the level of network carbides and promote the refinement of pearlite lamellars to form sorbite, strong air cooling is used after wire drawing. This further increases temperature control instability and the temperature difference between the air-receiving and air-receiving surfaces of the wire. Influenced by the segregation of high carbon and alloying elements, localized overcooling easily occurs, forming abnormally hard and brittle phases such as martensite. The phase separation significantly degrades the plasticity and toughness of the wire rod, increases the fluctuation of mechanical properties, and the limited cooling capacity also leads to uneven precipitation of microalloyed carbides, making it difficult to fully exert the strengthening effect of microalloying. High content of V, Ti, etc., results in high material cost. In order to minimize the influence of alloying element segregation, long-term diffusion annealing is also required, which affects production efficiency. On the other hand, high silicon can provide solid solution strengthening and promote the refinement of wire rod lamellars when combined with hardenable elements, but it will also slow down the diffusion rate of carbon and prolong the phase transformation incubation time. Due to the limitations of the length, continuous cooling and minimum cooling capacity of the Steyrmo air-cooling line, the wire rod has a limited time to pass through the phase transformation temperature range, and the microstructure is difficult to fully transform. The high proportion of soft ferrite or pearlite in the microstructure will result in a loss of strength. Low-temperature rolling and wire drawing treatment are also required to reduce the processing difficulty, but this will also affect rolling efficiency, increase rolling line load and wear. At the same time, due to the effect of rapid cooling in the early stage, the temperature difference between the wire rod surface and the core is further increased, and the thermal stress and phase transformation stress accumulate significantly. The stress concentration residue after phase transformation incubation leads to insufficient plasticity and toughness, making it difficult to overcome the production bottleneck of no-offline heat treatment.

[0005] II. To improve the strength and ductility of wire rods, although existing technologies indicate the use of salt bath treatment, for example, patent CN120400687A discloses wire rods, steel strands, and preparation methods for 2300~2360MPa grade stress corrosion resistant steel strands, using a C-Si-Mn-Cr-V composition system, combined with low-temperature rolling of small billets, online isothermal salt bath treatment, and low-rate online aging to obtain sorbite wire rods, on the one hand, the limitations of wire rod composition and the temperature control capability of one-stage salt bath treatment mean that a large molten salt circulation volume is required throughout the process to control the microstructure refinement, but the energy consumption is high, and low temperature control is also necessary. Rolling can reduce control difficulty, but it also increases wear on the rolling line and affects rolling efficiency. On the other hand, due to the limitations of wire rod composition and molten salt temperature in one-stage salt bath treatment, the treatment time should not be too long. The thermal stress and structural stress generated inside the wire rod also need to be combined with long-term low-speed online aging treatment, which affects production efficiency and increases the risk of precipitation of brittle martensite in the microstructure, affecting stable wire rod production. At the same time, as the initial base material, insufficient strength of the wire rod will also lead to an increase in subsequent drawing passes and the risk of wire breakage. In order to further improve the strength, adding alloying elements such as Nb will increase the material cost. 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 2330MPa grade stranded wire and its manufacturing method, which can control material costs, improve the matching of strength and plasticity and the uniformity of structure of hot-rolled wire rod, and take into account production efficiency and energy consumption, so as to meet the requirements of efficient and stable production of ultra-high strength stranded wire without offline heat treatment.

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

[0008] A method for manufacturing hot-rolled wire rod for 2330MPa grade stranded wire, the method comprising:

[0009] The wire rod is rolled into production line 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: 0.85%~1.05%, Mn: 0.60%~0.80%, Cr: 0.46%~0.58%, V: 0.036%~0.054%, 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 ≥905℃ and then processed online. Molten salt isothermal toughening treatment involves first subjecting the wire rod to a molten salt treatment at a cooling rate of ≥34℃ / s, causing it to transition from the austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. Then, a subsequent molten salt treatment is performed, increasing the molten salt temperature and reducing the molten salt circulation rate to promote the continued transformation of untransformed residual austenite into sorbite and isothermal tempering, as well as promoting the melting of some sorbite lamellars. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted sorbite.

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

[0011] (1) Carbon: C is an effective strengthening element and its price is relatively low. Through solid solution strengthening, it increases the resistance to dislocation slip, which can improve the stability of austenite, reduce the nose temperature of phase transformation, shift the continuous cooling transformation curve of steel to the right, increase the nucleation rate of sorbite, and refine the interlamellar spacing. 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 prolongs the phase transformation incubation period, increases the difficulty of controlling the martensite deterioration structure and the uniformity of the structure, and leads to increased fluctuations in the mechanical properties of wire rod. Therefore, in order to take into account the high strength requirements and the uniformity of the structure of 2330MPa grade stranded wire and facilitate 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 suppress the formation of coarse carbides during cooling, strongly suppress the precipitation and coarsening process of cementite, and prevent excessive thickening of sorbite lamellar cementite, so that the lamellar distribution is more uniform. It plays a role in refining the precipitated phase and improving the toughness of the material. However, excessive silicon will aggravate the compositional segregation, reduce the diffusion coefficient of carbon in austenite, prolong the incubation period of sorbite transformation, increase the difficulty of isothermal tempering and softening, and reduce production efficiency. Therefore, in order to adapt to the control of phase transformation structure by online molten salt isothermal toughening, the resistance to tempering softening should be appropriately improved, and the mass percentage of Si should be controlled at 0.85%~1.05%.

[0013] (3) Manganese: Mn can improve the hardenability of steel, reduce the critical cooling rate of steel, and expand the stable range of austenite so as to ensure that austenite is fully transformed into sorbite, and promote finer and more uniform cementite lamellae 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 the structure, the mass percentage of Mn is controlled at 0.60%~0.80%.

[0014] (4) Chromium: Cr can improve the hardenability of steel, reduce the sorbite transformation temperature of steel, slow down the diffusion rate of carbon atoms in austenite, and promote the decomposition of austenite into sorbite at a lower temperature so that the sorbite lamellars are finer. At the same time, it can form nanoscale carbides with C in synergy, which can be 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 rolling deformation resistance, the risk of precipitation of martensite and other deteriorated structures, and the risk of precipitation coarsening. At the same time, it will increase the difficulty of tempering softening and affect the efficiency of the production line. Therefore, in order to control the sorbite phase transformation, the uniformity of the structure and the resistance to tempering softening, the mass percentage of Cr is controlled at 0.46%~0.58%.

[0015] (5) Vanadium: As a microalloying element, V can be precipitated by strain-induced pinning of grain boundaries and refinement of grains during high-temperature hot rolling. The refined austenite grains provide more nucleation sites for subsequent sorbite transformation. Excess V can be dispersed and precipitated during medium-temperature isothermal processes to rapidly strengthen the matrix. While improving strength, it avoids plasticity deterioration. However, V is relatively expensive. Excessive addition is not conducive to controlling material costs and has the risk of coarsening. Therefore, the mass percentage of V is controlled at 0.036%~0.054%.

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

[0017] The aforementioned hot-rolled wire rod adopts a high-carbon, high-silicon composition system of C-Si-Mn-Cr-V, with trace amounts of V added to appropriately control material costs. The solid solution strengthening of C provides a strength foundation, while V provides nano-precipitation strengthening. Optimizing the elemental contents of Si, Cr, and Mn controls the wire rod's hardenability and provides appropriate resistance to tempering softening. This creates favorable conditions for the entire cross-section of the wire rod to enter the sorbite phase region, suppressing cementite coarsening and the formation of abnormal structures such as martensite, and facilitating rapid toughening. Furthermore, selecting a higher wire drawing temperature ensures the wire rod is in a high-temperature austenitic state, improving austenite composition uniformity, balancing austenite stability, and preventing premature precipitation of network carbides or proeutectoid ferrite. This provides favorable conditions for the subsequent uniform transformation of the entire cross-section of the wire rod to sorbite and promotes the precipitation of nano-carbides. The wire rod undergoes online molten salt isothermal toughening treatment directly without air cooling.

[0018] I. Compared to the limitations imposed by the maximum cooling capacity of the Stellmore air-cooled line, which makes it difficult to control abnormal structures such as network carbides and martensite, the core of the wire rod has large lamellar layers and is difficult to control the diffuse precipitation of nano-carbides. After the wire rod undergoes a preliminary molten salt treatment, it can be rapidly cooled. On the one hand, this allows the wire rod to quickly pass through the temperature range for network carbide precipitation from a high-temperature austenitic state, inhibiting carbon aggregation and network carbide formation, thus avoiding adverse effects on the uniformity of the microstructure and ductility. Combined with Si inhibiting cementite coarsening and Mn and Cr reducing the carbon diffusion rate, rapid cooling can promote the transformation of the wire rod into a finer sorbite microstructure with pearlite lamellar layers. As the treatment time increases, the temperature difference between the wire rod surface and the core can be further reduced, inhibiting the coarsening of the core lamellar layers and preventing the large core microstructure from affecting the matrix strength. On the other hand, when the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for further processing. Rapid and uniform heat exchange eliminates the temperature difference between the air-receiving and air-exposed surfaces compared to air cooling, preventing localized overcooling of the wire rod and the formation of uncontrollable martensitic abnormal structures. Uniform rapid cooling also enhances the precipitation potential of carbides such as Cr and V, avoiding uneven precipitation due to slow cooling rates and unstable temperature control. The precipitation of nano-carbides can anchor the sorbite structure, rapidly increasing matrix strength and providing a certain degree of resistance to tempering softening. Compared to single-stage salt bath treatment, which struggles to balance phase transformation structure control and production energy consumption, and has a higher wire drawing temperature, the first-stage molten salt treatment has a larger molten salt circulation volume than the second-stage molten salt treatment. This, combined with the wire rod's hardenability characteristics, allows the wire rod to stably and uniformly transform into a sorbite structure under rapid cooling, reducing limitations on rolling control. Simultaneously, it prepares the microstructure for rapid online toughening in the second-stage molten salt treatment by increasing the molten salt temperature and reducing the molten salt circulation volume.

[0019] II. Compared to high-carbon, high-silicon systems, where the continuous cooling and minimum cooling capacity of the Stellmore air-cooling line limit the control of sufficient phase transformation and insufficient structural strength and plasticity, the subsequent molten salt treatment of wire rods can increase the molten salt temperature. On the one hand, by appropriately increasing the temperature, carbon diffusion and precipitation in austenite can be enhanced, promoting the decomposition of untransformed residual austenite into sorbite, thus increasing the phase transformation rate. With extended treatment time, the temperature difference between the wire rod surface and core can be further reduced, mitigating the adverse effects of high silicon and alloy element segregation, promoting sufficient austenite phase transformation inoculation, reducing phase transformation stress and ferrite proportion, and preventing residual austenite from continuing to form martensitic low-temperature brittle structures during subsequent cooling. This improves matrix strength and structural uniformity, eliminating the need for diffusion annealing and shortening the production process. On the other hand, the wire rod remains at a high temperature after phase transformation, requiring prolonged isothermal tempering. Under heat treatment, partial melting of sorbite lamellars can be promoted, lattice distortion can be alleviated, stress concentration can be reduced, and the dispersion and distribution of nano-carbide can be promoted, achieving online toughening and improving the plasticity and strength matching of wire rod. Compared with the difficulty in balancing microstructure control, production energy consumption and efficiency under one-stage salt bath treatment, on the one hand, the molten salt temperature of the later stage molten salt treatment is higher and the temperature difference with the wire rod is smaller, which can appropriately reduce the molten salt circulation volume to control production energy consumption. On the other hand, the high temperature can accelerate the release of internal stress. Since Si, Cr and other elements provide a certain resistance to tempering softening, the treatment time can be appropriately extended to promote the full precipitation of nano-carbide, alleviate stress concentration, and avoid the adverse effects of austenite residue. The rapid online toughening of wire rod under the later stage molten salt treatment can reduce the limitation of slow cooling on the roller table. By utilizing the high temperature state of the wire rod after exiting the molten salt, the slow cooling on the roller table can promote further toughening of the wire rod microstructure and promote efficient and stable wire rod production.

[0020] Before rolling, a higher heating furnace temperature and an appropriate furnace time are selected to quickly alleviate the enrichment of carbon and silicon in the center of the billet and the depletion of its composition on the surface, improve segregation and enhance the rollability of the billet, and avoid grain coarsening caused by excessively high temperature or excessively long furnace time. In the preferred technical solution, before rolling, the heating furnace temperature is controlled at 1186~1236℃ and the furnace time is 165~255min.

[0021] The higher wire drawing temperature reduces the limitations on rolling. During rolling, a higher initial rolling temperature can be used to reduce deformation resistance, improve rolling efficiency, and reduce the load requirements and wear on the rolling line. By selecting a suitable final rolling temperature and final rolling reduction, V carbide precipitation and grain boundary pinning can be induced, promoting dynamic recrystallization and grain refinement during the final rolling process, accumulating appropriate deformation energy, and providing more active sites for sorbite nucleation in subsequent online molten salt isothermal toughening treatment. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1036~1086℃, the final rolling temperature at 905~955℃, and the final rolling reduction at 23%~28%.

[0022] The wire spinning temperature can be further controlled during the spinning process to suppress austenite grain coarsening. In a preferred embodiment, the wire spinning temperature is controlled to be 905~935℃.

[0023] The lower the molten salt temperature in the initial molten salt treatment, the faster the wire rod cooling rate is increased, the formation of network carbides is suppressed, and the precipitation and coarsening of cementite are inhibited. The low-temperature environment reduces the phase transformation rate and enhances the precipitation kinetics of nano-carbides, resulting in finer sorbite lamellars and more dispersed nano-carbides. With prolonged treatment time, the uniform transformation of the core structure can be further promoted, reducing the amount of residual austenite. The fine lamellar structure and the pinning effect of nano-carbides improve the matrix strength. However, excessively low molten salt temperatures reduce carbon diffusion, increase the temperature gradient and phase transformation stress from the wire rod surface to the core, and may even lead to the precipitation of abnormally brittle low-temperature structures. Prolonged treatment time will affect production efficiency and unnecessarily increase energy consumption. Conversely, higher molten salt temperatures accelerate carbon diffusion, shorten the phase transformation incubation period and phase transformation stress, and allow carbides to be uniformly distributed in the sorbite matrix. In this process, shortening the processing time can reduce production energy consumption and improve production efficiency. However, excessively high molten salt temperatures will reduce the cooling rate of the wire rod and the driving force of phase transformation, accelerating the growth rate of sorbite lamellars. With excessively short processing times, the amount of untransformed residual austenite increases, the number of nano-carbide nuclei is insufficient, and the fineness of the microstructure decreases, which will increase the difficulty of lamellar melting and affect the strength and plasticity. Therefore, the molten salt temperature and processing time of the first stage of molten salt treatment can be controlled to allow the wire rod to quickly bypass the network carbide precipitation temperature range from the high-temperature austenitic state and enter the sorbite phase region, inhibiting the formation of network carbides and forming a microstructure dominated by fine lamellar spacing sorbite, promoting the uniform dispersion and distribution of nano-carbides, and preparing 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 535~565℃, and the processing time is 92~135s.

[0024] Since there is a large temperature difference between the spinning temperature and the molten salt temperature of the previous molten salt treatment, a higher molten salt circulation rate can be selected to control the molten salt temperature rise and improve the consistency of the wire rod structure in continuous processing. In the preferred technical solution, the molten salt circulation rate of the previous molten salt treatment is 580~780t / h, and the molten salt temperature rise is ≤8℃.

[0025] The higher the molten salt temperature in the subsequent molten salt treatment, the more thermal power it provides, improving carbon diffusion and promoting the rapid transformation of untransformed residual austenite into sorbite, thus accelerating phase transformation incubation. With prolonged treatment time, while promoting full phase transformation, isothermal tempering can also facilitate the melting of some sorbite cementite plates, promoting dislocation slip and rearrangement, and allowing the nano-carbides formed by vanadium, chromium, and other elements with carbon to further disperse evenly, avoiding local aggregation, reducing stress concentration, and improving the wire rod's ductility and toughness, as well as the matrix's ability to coordinate deformation. However, excessively high molten salt temperatures are detrimental to inhibiting cementite coarsening and carbide growth and aggregation. With prolonged treatment time, excessive melting of cementite plates and loss of anchoring effect in nano-carbides will lead to excessive softening of the microstructure, affecting strength and ductility. Conversely, lower molten salt temperatures are beneficial for inhibiting cementite coarsening and nano-carbide aggregation and growth. Shorter treatment times allow for the slow release of internal stress. While moderate isothermal softening provides stability to the microstructure and reduces production energy consumption, excessively low molten salt temperatures inhibit carbon diffusion, affecting cementite lamellar melting. Short processing times result in significant residual stress and uneven carbide distribution, leading to substantial plasticity loss. Furthermore, the formation of abnormal microstructures from retained austenite during subsequent cooling can further increase mechanical property fluctuations, thereby increasing the risk of wire breakage during drawing. Therefore, subsequent molten salt treatment allows for better control of molten salt temperature and processing time. This promotes the transformation of untransformed retained austenite into fine lamellar sorbite, while simultaneously promoting prolonged isothermal tempering of the formed fine lamellar sorbite and causing partial cementite lamellar melting, inhibiting carbide growth and aggregation, thus regulating the strength-plasticity balance of the wire rod. In a preferred embodiment, the molten salt temperature for the subsequent molten salt treatment is 584–598°C, and the processing time is 120–220 seconds.

[0026] The temperature difference between the molten salt in the first stage and the second stage of molten salt treatment of the wire rod is small. Using a smaller molten salt circulation volume can control the molten salt temperature rise and reduce production energy consumption. In the preferred technical solution, the molten salt circulation volume of the second stage of molten salt treatment is 430~500t / h, and the molten salt temperature rise is ≤3℃.

[0027] Since the wire rod has undergone sufficient phase transformation and isothermal tempering for rapid online toughening after the subsequent molten salt treatment, the wire rod temperature is relatively high after exiting the molten salt. This reduces the restriction on slow cooling on the roller table and avoids the impact of excessively low slow cooling speed on the line speed and production efficiency. The slow cooling speed can be further controlled to promote further toughening of the wire rod structure and improve the softening effect of the wire rod during tempering. In the preferred technical solution, the slow cooling on the roller table controls the wire rod to cool to below 280°C at a slow cooling rate of 0.3~0.6°C / s before winding.

[0028] In the preferred technical solution, the slow cooling of the roller conveyor is achieved by closing the insulation cover or controlling the opening of the insulation cover, and blowing the hot air at ≥250°C during the online molten salt isothermal toughening treatment to the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the 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 2330MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method of hot-rolled wire rod for stranded wire of any one of the above-mentioned methods.

[0030] The aforementioned hot-rolled wire rod adopts a high-carbon, high-silicon composition design with Cr-V, featuring a low V content and no added Nb, Ti, or other alloying elements, thus controlling material costs. Its microstructure comprises a mixed structure mainly composed of tempered sorbite and melted sorbite, with a small amount of ferrite. Compared to pearlitic wire rods used for stranded wire, this design simplifies the composition, suppressing abnormal structures such as network carbides and martensite, preventing core lamellar coarsening, promoting full phase transformation, reducing ferrite content, and facilitating the full dispersion and precipitation of nano-carbides. The lamellar spacing of the sorbite is finer than that of pearlite; this finer lamellar structure increases dislocation barrier interfaces, improving strength and enhancing interfacial coordination. The deformation is adjusted to improve plasticity. Compared with the sorbitic wire rods obtained by the existing one-stage salt bath treatment, the above-mentioned hot-rolled wire rods have some sorbitic cementite plates melted and transformed into intermediate tempered sorbite and melted sorbite in the spheroidized structure. This can further release structural stress, alleviate stress concentration, and improve the structural plasticity and toughness. In addition, the dispersed precipitation of nano carbides can effectively hinder dislocation movement, which can give full play to the strengthening effect of carbon, improve the matrix strength and structural uniformity, increase the wire rod's draw hardening ability, and make up for the strength loss caused by the omitted components. This eliminates the need for offline heat treatment and reduces the number of subsequent stranding draw passes and the risk of wire breakage during drawing and twisting processes.

[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 fused sorbite, the better the ductility and toughness. In the preferred technical solution, the volume percentage of tempered sorbite is ≥58%, the lamellar spacing is 70~125nm, the volume percentage of ferrite is ≤5%, and the volume percentage of fused sorbite is ≥27%.

[0032] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference within the same coil of ≤49MPa. By suppressing the formation of network carbides, the continuity of the matrix can be improved, the risk of stress concentration cracks in subsequent drawing can be reduced, and the service reliability and durability can be improved. By suppressing abnormal structures and improving the uniformity of structures, the wire rod has a smaller mechanical property difference within the same coil of ≤49MPa, which can further improve the uniformity and stability of the subsequent drawing process, so that the finished steel strand is subjected to uniform stress and has stable performance in mass production.

[0033] In the preferred technical solution, the hot-rolled wire rod has a diameter of 7~15mm, a tensile strength of 1558~1608MPa, and a section reduction rate of 34%~39%. The hot-rolled wire rod has a wider diameter range, which can be adapted to different specifications of stranded wire. Combined with the high tensile strength, it can reduce the number of drawing passes, quickly achieve the target diameter and strength of the stranded wire, and improve processing efficiency. The high section reduction rate means that the wire rod does not need offline heat treatment and can undergo uniform plastic deformation during the drawing process, avoiding tearing and wire breakage caused by insufficient plasticity, so as to improve the yield and service life of the stranded wire.

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

[0035] (1) In view of the problem that the abnormal structure and phase transformation are difficult to control due to the limitations of production line or process capacity, and it is difficult to balance production energy consumption and efficiency, the present invention adopts the high carbon and high silicon composition design of Cr-V combined with online molten salt isothermal toughening technology. It can control the wire rod to cool down rapidly after the first stage of molten salt treatment, from the high temperature austenitic state to the sorbite phase region, suppressing network carbides and forming a structure dominated by sorbite. After the second stage of molten salt treatment, the molten salt temperature is increased and the molten salt circulation is reduced, which promotes the untransformed residual austenite to continue to transform into sorbite and isothermal tempering, promotes the melting of some sorbite lamellars, and makes the nano carbides dispersed and inhibits the formation of martensite. Finally, it is further toughened by the slow cooling of the roller table, which improves the strength performance matching and the uniformity of the structure of the wire rod. It can reduce the limitations on rolling and slow cooling of the roller table, balance production energy consumption and efficiency, promote the efficient and stable production of hot rolled wire rod, and has good industrial adaptability.

[0036] (2) In view of the problems of high cost, prominent abnormal structure, insufficient strength and plasticity, and poor uniformity of structure of hot-rolled wire rod for stranded wire, which require offline heat treatment, resulting in high cost and poor production efficiency of stranded wire, the present invention has a low V content and does not add alloy components such as Nb and Ti, which can control material cost. The microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can suppress abnormal structure of network carbides and martensite. The finer lamellar structure increases the dislocation barrier interface, and the partial melting of cementite lamellars further releases the structural stress and alleviates stress concentration. Combined with the dispersed precipitation of nano carbides to pin dislocations, it can effectively hinder dislocation movement. The hot-rolled wire rod can achieve a tensile strength of 1558~1608MPa and a section reduction rate of 34%~39%. It can be used to manufacture 2330MPa grade stranded wire and other application fields. No offline heat treatment is required, which can reduce the number of drawing passes and meet the requirements of efficient and stable production of ultra-high strength stranded wire. It has good market application prospects. Attached Figure Description

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

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

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

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

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

[0042] Example 1:

[0043] A preferred embodiment of the manufacturing method of the 2330MPa 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: 1%, Mn: 0.6%, Cr: 0.48%, V: 0.036%, P: 0.014%, S: 0.012%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → 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 7mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1186℃, and the furnace dwell time is 255 minutes. The initial rolling temperature is 1036℃, the final rolling temperature is 905℃, and the final rolling reduction is 28%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of network carbides or proeutectoid ferrite. This provides favorable conditions for the uniform transformation of the entire cross-section of the coils into sorbite and promotes the precipitation of nano carbides. Specifically, the wire drawing temperature is controlled at 905℃.

[0045] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After spinning, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 36°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the coarsening of network carbides, martensite, and core lamellars, forming a microstructure dominated by fine-laminated sorbite. This promotes the uniform dispersion and distribution of nano-carbides. The wire rod is then conveyed via rollers through the second salt bath for final molten salt treatment, where the molten salt temperature is increased and the molten salt circulation rate is reduced, promoting… The 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 and inhibits carbide growth and aggregation, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 535℃, the treatment time is 135s, the molten salt circulation rate is 580t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 598℃, the treatment time is 120s, the molten salt circulation rate is 430t / 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 (≥250℃) 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 270℃ at a slow cooling rate of 0.6℃ / 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 → Stellmor air cooling. Specifically, the heating furnace homogenization temperature is controlled at 1120℃, the furnace time is 270 min, the initial rolling temperature is 1010℃, the final rolling temperature is 820℃, and the wire drawing temperature is 820℃; the Stellmor forced air cooling uses an air volume of 260,000 m³ per fan. 3 At 80% capacity, fans 1 to 4 are turned on to cool the wire rod to 705℃ at a cooling rate of 7.8℃ / s. Then, fans 5 to 14 are turned on to 25% capacity to cool the wire rod to 261℃ at a cooling rate of 3.1℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0048] Comparative Example 2: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 1 is that: the heating furnace is heated to a uniform temperature of 1120°C, the furnace time is 270 min, the initial rolling temperature is 990°C, the final rolling temperature is 830°C, and the wire drawing temperature is 815°C; during the molten salt treatment before the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 29°C / s, and the finished hot-rolled wire rod is obtained after leaving the production line.

[0049] Example 2:

[0050] A preferred embodiment of the manufacturing method of the 2330MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.93%, Si: 1.05%, Mn: 0.69%, Cr: 0.46%, V: 0.051%, 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:

[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 9mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1210℃, and the furnace dwell time is 195 minutes. The initial rolling temperature is 1076℃, the final rolling temperature is 945℃, and the final rolling reduction is 26%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of network carbides or proeutectoid ferrite. This provides favorable conditions for the uniform transformation of the entire cross-section of the coil to sorbite and promotes the precipitation of nano carbides. Specifically, the wire drawing temperature is controlled at 925℃.

[0052] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After spinning, the wire rod is conveyed via rollers through the first stage of the salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 39°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This process inhibits the coarsening of network carbides, martensite, and core lamellars, forming a microstructure dominated by fine-laminated sorbite. This promotes the uniform dispersion and distribution of nano-carbides. 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 is increased and the molten salt circulation rate is reduced, promoting… The 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 and inhibits carbide growth and aggregation, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 545℃, the treatment time is 120s, the molten salt circulation rate is 620t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 594℃, the treatment time is 160s, the molten salt circulation rate is 470t / 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 (≥250℃) 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 272℃ 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 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 40℃ / s, the molten salt temperature of the first stage of the molten salt treatment is 500℃, 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 605℃, the treatment time is 80s, 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 2330MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.93%, Si: 0.85%, Mn: 0.77%, Cr: 0.58%, V: 0.054%, P: 0.012%, S: 0.012%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → 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 that allows for rolling plasticity. Controlling the furnace parameters promotes uniform diffusion of alloying elements and reduces segregation. After exiting the furnace, the billet is rolled into a 13.5mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1236℃, and the furnace dwell time is 165 minutes. The initial rolling temperature is 1086℃, the final rolling temperature is 955℃, and the final rolling reduction is 25%. The wire drawing process is used to convert the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is distributed on the roller table and conveyed along the roller table. A suitable wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding premature precipitation of network carbides or proeutectoid ferrite. This provides favorable conditions for the uniform transformation of the entire cross section of the wire rod to sorbite and promotes the precipitation of nano carbides. Specifically, the wire drawing temperature is controlled at 935℃.

[0059] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After spinning, the wire rod is conveyed via rollers through the first stage of the salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 39°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region to the sorbite phase region. This process inhibits the coarsening of network carbides, martensite, and core lamellars, forming a microstructure dominated by fine-laminated sorbite. This promotes the uniform dispersion and distribution of nano-carbides. 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 is increased and the molten salt circulation rate is reduced, promoting… The 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 and inhibits carbide growth and aggregation, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 555℃, the treatment time is 110s, the molten salt circulation rate is 705t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 588℃, the treatment time is 190s, the molten salt circulation rate is 485t / h, and the molten salt temperature rise is ≤3℃.

[0060] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥250℃) 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 275℃ at a slow cooling rate of 0.4℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product, 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 605°C, the treatment time is 230s, 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 525°C, the treatment time is 100s, 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 2330MPa 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: 0.9%, Si: 0.93%, Mn: 0.7%, Cr: 0.53%, V: 0.04%, P: 0.013%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt 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 VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1200℃, and the furnace dwell time is 225 minutes. The initial rolling temperature is 1056℃, the final rolling temperature is 925℃, and the final rolling reduction is 23%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread 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 provides favorable conditions for the uniform transformation of the entire cross-section of the coils into sorbite and promotes the precipitation of nano carbides. Specifically, the wire drawing temperature is controlled at 915℃.

[0066] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After spinning, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 34°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the coarsening of network carbides, martensite, and core lamellars, forming a microstructure dominated by fine-laminated sorbite. This promotes the uniform dispersion and distribution of nano-carbides. The wire rod is then conveyed via rollers through the second salt bath for final molten salt treatment, where the molten salt temperature is increased and the molten salt circulation rate is reduced, promoting… The 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 and inhibits carbide growth and aggregation, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 565℃, the treatment time is 92s, the molten salt circulation rate is 780t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 584℃, the treatment time is 220s, the molten salt circulation rate is 500t / 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 (≥250℃) from 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 cooled to 278℃ at a slow cooling rate of 0.3℃ / 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 265°C at a cooling rate of 1.4°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] As can be seen from the comparison results of Example 1 and Comparative Example 1, compared with the Stellmore air-cooled line, which produces a higher level of network carbides and martensitic abnormal structure, core lamellar coarsening, a larger proportion of ferrite in the structure, and a larger accumulation of thermal stress and phase transformation stress, resulting in insufficient final strength and plasticity and large fluctuations in mechanical properties, this invention adopts a high carbon and high silicon composition design of Cr-V combined with online molten salt isothermal toughening technology. It can control the rapid cooling of the wire rod after the first stage of molten salt treatment, suppressing network carbides and forming a structure dominated by sorbite. After the second stage of molten salt treatment, it promotes the continued transformation of untransformed residual austenite into sorbite and isothermal tempering, taking into account both production energy consumption and efficiency. As can be seen from the results of Examples 1 to 4, the hot-rolled wire rod can achieve a tensile strength of 1558~1608MPa and a section reduction rate of 34%~39%, which can be used to manufacture 2330MPa grade stranded wire and other application fields. No offline heat treatment is required, which can reduce the number of drawing passes and meet the requirements for efficient and stable production of ultra-high strength stranded wire.

[0073] As can be seen from the comparison between Example 1 and Comparative Example 2, selecting a higher wire drawing temperature can keep the wire rod in a high-temperature austenitic state, improve the uniformity of austenitic composition, balance the stability of austenitic material, and avoid premature precipitation of network carbides or proeutectoid ferrite. This provides favorable conditions for the uniform transformation of the wire rod's cross-section to sorbite and promotes the precipitation of nano carbides.

[0074] The comparison results between Example 2 and Comparative Example 3 show that the lower the molten salt temperature in the initial molten salt treatment, the better it is to increase the cooling rate of the wire rod, suppress the formation of network carbides, suppress the coarsening of cementite precipitation, reduce the phase transformation rate and increase the precipitation kinetics of nano-carbides through the low temperature environment, so that the sorbite lamellars are more compact and the nano-carbides are more dispersed. With the extension of the treatment time, it can further promote the uniform transformation of the core structure and reduce the amount of residual austenite. The matrix strength is improved through the fine lamellar structure and the pinning effect of nano-carbides. However, if the molten salt temperature is too low, it will reduce the diffusion of carbon, increase the temperature gradient from the wire rod surface to the core and the phase transformation stress. If the treatment time is too long, it will affect the production efficiency and unnecessarily increase the production energy consumption.

[0075] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the faster the carbon diffusion rate, the shorter the phase transformation incubation period and phase transformation stress, and the more uniformly the carbides are distributed in the sorbite matrix. With the reduction of treatment time, production energy consumption can be reduced and production efficiency can be improved. However, if the molten salt temperature is too high, the wire rod cooling rate and phase transformation driving force will be reduced, and the growth rate of sorbite lamellars will be accelerated. With the treatment time being too short, the amount of untransformed residual austenite increases, the number of nano carbide nuclei is insufficient, the density of the microstructure decreases, which will increase the difficulty of lamellar melting and affect the strength and plasticity.

[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 untransformed residual austenite into sorbite. With the extension of treatment time, while promoting the full phase transformation of the microstructure, isothermal tempering can promote the melting of some sorbite cementite lamellars, allowing the nano-carbides formed by vanadium, chromium and other elements with carbon to be further uniformly dispersed, avoiding local aggregation and reducing stress concentration. However, if the molten salt temperature is too high, it is not conducive to inhibiting cementite coarsening and carbide growth and aggregation. With the excessive treatment time, excessive melting of cementite lamellars and loss of the anchoring effect of nano-carbides will lead to excessive softening of the microstructure and affect the strength and plasticity properties.

[0077] As can be seen from the comparison results of Example 3 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the better it is to suppress the coarsening of cementite and the agglomeration and growth of nano carbides. With the shortening of the treatment time, the internal stress can be slowly released to provide moderate isothermal softening, which stabilizes the microstructure and reduces production energy consumption. However, if the molten salt temperature is too low, it will inhibit the diffusion ability of carbon and affect the melting of cementite lamellars. If the treatment time is too short, there will be a large residual stress in the microstructure and uneven distribution of carbides, which will lead to a large loss of plasticity. In fact, the residual austenite may form abnormal microstructures during subsequent cooling, which will further increase the fluctuation of mechanical properties.

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

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

Claims

1. A method of manufacturing a hot-rolled wire rod for a 2330 MPa grade strand, characterized by, 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: 0.85%~1.05%, Mn: 0.60%~0.80%, Cr: 0.46%~0.58%, V: 0.036%~0.054%, 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 ≥905℃, it undergoes online molten salt isothermal toughening treatment. This process involves first undergoing a preliminary molten salt treatment and then cooling the wire rod at a rate of ≥34℃ / s, causing it to transition 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. This process increases the molten salt temperature and reduces the molten salt circulation rate, promoting the continued 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 535~565℃, and the treatment time is 92~135s. The molten salt temperature in the subsequent molten salt treatment is 584~598℃, and the treatment time is 120~220s. The slow cooling via the roller conveyor controls the wire rod to cool to below 280℃ at a slow cooling rate of 0.3~0.6℃ / s before coiling.

2. The method of producing a hot rolled wire rod for 2330 MPa grade strand wire according to claim 1, characterized by, Before rolling, the heating furnace temperature is controlled at 1186~1236℃ and the furnace time is 165~255min.

3. The method of producing a hot rolled wire rod for 2330 MPa grade strand wire according to claim 1, characterized by, During the rolling process, the initial rolling temperature is controlled at 1036~1086℃, the final rolling temperature is controlled at 905~955℃, and the final rolling reduction is controlled at 23%~28%.

4. The method of producing a hot rolled wire rod for 2330 MPa grade strand wire according to claim 1, characterized by, During the spinning process, the spinning temperature is controlled at 905~935℃.

5. The method of producing a hot rolled wire rod for 2330 MPa grade strand wire according to claim 1, characterized by, The molten salt circulation rate of the front-end molten salt treatment is 580~780t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation rate of the rear-end molten salt treatment is 430~500t / h, and the molten salt temperature rise is ≤3℃.

6. A hot rolled wire rod for 2330 MPa grade strand, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of hot-rolled wire rod for 2330MPa grade strand as described in any one of claims 1 to 5.

7. The hot rolled wire rod for 2330 MPa class stranded wire according to claim 6, characterized by, The volume percentage of the tempered sorbite is ≥58%, the lamellar spacing is 70~125nm, the volume percentage of the ferrite is ≤5%, the volume percentage of the melted sorbite is ≥27%, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤49MPa.

8. The hot rolled wire rod for 2330 MPa grade strand as claimed in claim 6, wherein, The hot-rolled wire rod has a diameter of 7~15mm, a tensile strength of 1558~1608MPa, and a reduction of area of ​​34%~39%.

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