A hot-rolled wire rod for 1960MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a hot-rolled wire rod for stranded wire with a strength of 1960MPa and its manufacturing method. The method involves rolling a high-carbon C-Si-Mn-Cr-B wire rod into wire, followed by rapid online molten salt isothermal treatment. The wire rod undergoes a pre-treatment molten salt process, cooling at a rate of ≥32℃ / s to transition from austenitic to sorbitic phase, forming a predominantly sorbitic microstructure. A subsequent molten salt treatment lowers the molten salt temperature, promoting the transformation of untransformed residual austenite into sorbite, while simultaneously promoting 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 consisting of tempered sorbite, ferrite, and melted sorbite, achieving a tensile strength of 1305~1355MPa and a reduction of area of ​​37%~42%. This material is used as a base material for manufacturing high-strength stranded wire and other applications, reducing the risk of wire breakage.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a 1960MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] Steel strand, as a crucial load-bearing tool in high-stress environments, is widely used in bridges, railways, construction, and new energy fields. With the trend towards lightweighting, the strength grade of steel strand needs continuous improvement. Current methods for increasing the strength grade of steel strand often involve adding carbon (C) and hardenability-enhancing elements to the hot-rolled wire rod base material. Some even involve adjusting the microstructure and properties of the wire rod through multiple processes such as offline quenching. However, the combination of C and hardenability-enhancing elements can easily lead to abnormal microstructures such as martensite in the hot-rolled wire rod, insufficient material plasticity, and high alloy costs. Furthermore, multiple offline quenching processes increase production costs and reduce efficiency. Therefore, there is a need to develop a 1960MPa grade hot-rolled wire rod with uniform microstructure and a balanced strength and plasticity, along with its manufacturing method, to control production costs, reduce the risk of wire breakage during subsequent drawing processes, and meet the environmental requirements for higher load-bearing stress and longer service life.

[0003] Existing high-strength stranded steel hot-rolled wire rods are generally produced using carbon, hardenability elements, etc., combined with the Steyrmo air-cooling line. The factors limiting the improvement of their microstructure uniformity and strength-plasticity include:

[0004] I. To improve the strength of wire rod for stranded steel, existing technologies increase the content of carbon or hardenability elements, and combine this with air-cooling to produce wire rod with a predominantly sorbitic structure. For example, patent CN112301287B discloses a prestressed steel strand, wire rod for prestressed steel strand, and its production method, which uses a C-Si-Mn-Cr-V-Al-Ti-B-Cu-Ni composition design, combined with rapid-then-slow cooling and high-temperature coiling on a Steyrmo air-cooling line after wire drawing, resulting in a wire rod microstructure. The main components are sorbite and a small amount of pearlite. However, because the wire rod is made of high-carbon steel, it exacerbates center segregation during continuous casting solidification. When high-carbon steel austenite transforms into pearlite or sorbite, if the cooling rate is too low, carbon has sufficient time to diffuse to the grain boundaries and precipitate carbides. Moreover, carbides are more likely to accumulate on coarse grain boundaries, forming a continuous network. The formation of network carbides will disrupt the continuity of the matrix, leading to increased brittleness of the wire rod. During drawing, cracks are easily generated at the grain boundaries, reducing the wire rod's plasticity. The properties and fatigue performance of steel strands affect their final safety in use. Therefore, in order to minimize the level of network carbides and promote the refinement of pearlite lamellars and improve strength, strong air cooling is used in the early stage of air cooling after wire drawing. However, the control of network carbides is limited by the maximum cooling capacity of the air cooling line. On the other hand, the increase in air cooling intensity also increases the temperature difference between the air-receiving and air-receiving surfaces of the wire rod, and between the overlapping and non-overlapping areas. Due to the uncontrollability of air volume and air temperature, as the wire rod size decreases, the surface is more prone to forming brittle abnormal structures due to local overcooling, increasing the brittleness and mechanical property fluctuations of the wire rod. The minimum cooling capacity of the air cooling line and the slow cooling of the core will lead to coarsening of the pearlite lamellars. The incomplete transformation of residual austenite under high temperature coiling may lead to the continued formation of hard and brittle structures such as martensite or bainite during subsequent cooling. After mixing with pearlite, it reduces the overall plasticity. During drawing, the hard and brittle areas will experience plasticity depletion first, while the core will be unable to bear the load due to insufficient strength, resulting in a rapid decrease in overall plasticity and a high wire breakage rate.

[0005] II. To control the production cost and strength of wire rod for stranded steel, existing technologies employ methods such as adding alloys that refine grain size (e.g., V) and strong cooling treatments like water cooling. For example, patent CN119082607A discloses a 1960MPa grade steel strand wire rod and its preparation method, steel wire, and steel strand. This design utilizes a C-Si-Mn-Cr-V composition, combining strong water mist cooling with air cooling to create a pearlitic structure wire rod with a strength of 1220MPa~1260MPa. However, this requires solid solution strengthening with elements like Mn to improve the steel's basic properties, which is affected by the carbon and alloying element content. The decrease in quantity affects the initial strength of the wire rod, and the cold drawing strengthening of the steel strand requires a larger reduction rate, which leads to faster plasticity loss and increased wire breakage rate during the process. On the other hand, a large number of bubbles are generated during the water mist cooling process and adhere to the surface of the wire rod, which increases the cooling difference of the wire rod and makes it more prone to brittle abnormal structure. As the wire rod size increases, the temperature difference between the wire rod surface and the core will further increase, resulting in less VC precipitation or coarsening, while affecting the uniformity of the phase transformation structure, resulting in limited strengthening effect and large structural stress, which in turn leads to fluctuations in tensile strength and insufficient plasticity, making it difficult to provide stable base material properties. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the above-mentioned technical problems. The present invention provides a hot-rolled wire rod for stranded wire of grade 1960MPa and its manufacturing method, which can control material cost and microstructure uniformity, reduce the level of network carbides and the risk of abnormal microstructure, improve the high strength and plasticity matching of wire rod, take into account production efficiency, provide stable stranded steel base material performance, and help reduce the risk of wire breakage.

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

[0008] A method for manufacturing hot-rolled wire rod for 1960MPa 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: 0.79%~0.83%, Si: 0.20%~0.40%, Mn: 0.55%~0.65%, Cr: 0.20%~0.40%, B: 0.001%~0.008%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a spinning temperature of ≥885℃, it undergoes rapid rolling... The wire rod undergoes a molten salt isothermal treatment. First, the wire rod is cooled at a rate of ≥32℃ / s, transitioning from the austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. Then, the wire rod undergoes a second molten salt treatment to lower the molten salt temperature, promoting the transformation of untransformed residual austenite into sorbite. Simultaneously, it promotes isothermal tempering and partial melting of sorbite lamellars. Finally, the wire rod is slowly cooled on a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted sorbite.

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

[0011] (1) Carbon: C is the main strengthening element in high carbon steel. It is relatively cheaper. Carbon forms interstitial solid solution in the iron matrix, which can improve the strength of steel. As an austenite stabilizing element, it can reduce the phase transformation temperature of austenite to sorbite, so that the phase transformation can be carried out at a lower temperature. In addition, it can refine the lamellar spacing of sorbite in conjunction with rapid molten salt treatment, provide basic strength, slow down the coarsening of cementite during isothermal tempering, and avoid excessive strength loss. However, if the C content is too high, it will make it more difficult to control the center segregation, network carbides and coarse carbides in high carbon steel alloy billets, affecting toughness and cold working plasticity. Therefore, in order to take into account the high strength requirements of 1960MPa grade strand, reduce the difficulty of controlling the uniformity of the structure, and facilitate the low-temperature short-time phase transformation and rapid isothermal treatment of online molten salt isothermal treatment, the carbon content is appropriately reduced. The mass percentage of C is controlled at 0.79%~0.83%.

[0012] (2) Silicon: Si can be used as a deoxidizing element. Through solid solution strengthening, it can dissolve in the iron matrix to produce lattice distortion and hinder dislocation movement. In the austenite to sorbite phase transformation process, silicon can hinder the diffusion of carbon atoms into cementite, inhibit the growth of cementite, refine the interlamellar spacing of sorbite and improve the tempering stability of steel, thereby synergistically improving the strength of steel with carbon. However, excessive silicon will promote decarburization, over-stabilize ferrite and prolong the phase transformation time, increase the difficulty of softening, and be detrimental to rapid phase transformation and tempering in online molten salt isothermal process. Therefore, in order to adapt to the control of phase transformation structure in rapid online molten salt isothermal treatment, the mass percentage of Si is controlled at 0.20%~0.40%.

[0013] (3) Manganese: Mn is an element that expands the austenite region, can increase the recrystallization temperature of austenite, can inhibit the growth of austenite grains during rolling, and can significantly improve the hardenability of steel. It can stabilize austenite through solid solution strengthening, reduce the phase transformation temperature of austenite to sorbite, refine sorbite lamellae, and prevent ferrite or coarse pearlite from precipitating first in the core of the wire rod, thereby improving the strengthening effect of carbon. However, if the Mn content is too high, it will increase the segregation of central alloying elements during the solidification process of the billet, and it is easy to form low-temperature martensite structure during the cooling process, which increases the risk of abnormal structure precipitation and material cost, resulting in excessive hardness of the structure. At the same time, it is not conducive to the rapid melting of sorbite and tempering softening in the later stage of molten salt treatment, thereby reducing the plasticity of the wire rod. Therefore, in order to take into account the high strength and plasticity of hot-rolled wire rod and reduce the difficulty of controlling the uniformity of the structure, the Mn content should be appropriately reduced, and the mass percentage of Mn should be controlled at 0.55%~0.65%.

[0014] (4) Chromium: Cr is a strong carbide-forming element. It easily forms alloy carbides and is distributed in the sorbite lamellars to improve the hardness of the structure. At the same time, it can prolong the stability of austenite and enhance the hardenability of austenite. Combined with the rapid cooling of online molten salt, it inhibits the premature decomposition of austenite into ferrite or coarse pearlite during the cooling process, so that the continuous cooling transformation curve of steel shifts to the right. Under the same cooling rate, it can easily form fine lamellar sorbite, which improves the work hardening rate of wire rod in the subsequent wire drawing process. However, if the Cr content is too high, it will aggravate the segregation of alloying elements, increase the risk of abnormal structure precipitation, performance fluctuation and material cost. At the same time, it reduces the activity of carbon in steel, which is not conducive to sorbite melting and tempering softening, resulting in increased difficulty in improving plasticity and affecting the rapid processing of online molten salt. Therefore, the mass percentage of Cr is controlled at 0.20%~0.40%.

[0015] (5) Boron: Trace amounts of boron preferentially accumulate in steel at the austenite grain boundaries to form a low-energy interface layer, which can effectively improve the hardenability of steel, reduce the excessive dependence on carbon and chromium content, inhibit the phase transformation nuclei of austenite to pearlite / ferrite, and, together with Mn and Cr, avoid the phase transformation lag or incomplete phase transformation caused by insufficient hardenability of wire rod. It is beneficial for rapid online molten salt treatment so that a uniform structure dominated by sorbite can be formed throughout the cross section. At the same time, during isothermal tempering, boron will not interfere with the melting and tempering softening of sorbite lamellars. However, the price of boron is high, and excessive addition is not conducive to controlling material costs. At the same time, excessive boron will form brittle borides, and the hard and brittle phases are distributed along the grain boundaries, which will reduce the toughness and cold working plasticity of steel. Therefore, the mass percentage of boron is controlled at 0.001%~0.008%.

[0016] (6) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015%.

[0017] The aforementioned wire rod adopts a high-carbon composition design of C-Si-Mn-Cr-B, with a slightly lower C or alloy content compared to existing stranded steel wire rods. It contains no V or other elements, with trace amounts of B added to control material costs. The synergistic effect of C and Si enhances the basic strength, while manganese expands the austenite region, chromium lowers the phase transformation temperature, and boron inhibits grain boundary phase transformation. This regulates the wire rod's hardenability, sorbite phase transformation temperature range, and resistance to tempering softening. It provides favorable conditions for delaying the decomposition of austenite in the core, promoting rapid, short-term phase transformation in molten salt to form a uniform structure, reducing sorbite lamellar spacing, and decreasing softening difficulty. Furthermore, a higher wire drawing temperature ensures the wire rod is in a high-temperature austenitized state, preventing premature precipitation of network carbides. After wire drawing, the wire rod undergoes rapid online molten salt treatment without passing through a Stellmore air-cooling line.

[0018] I. Compared to the Stellmore air-cooling phase transformation process of high-carbon steel wire rod, where the network carbides or abnormal structures are difficult to suppress, thus weakening the strengthening effect and affecting the uniformity of the structure, the molten salt treatment at the beginning stage can rapidly remove heat from the wire rod by utilizing the high thermal conductivity of molten salt, which is much higher than that of air. Under high cooling rates, the austenite rapidly cools to the sorbite phase transformation region. By utilizing the characteristic of lowering the sorbite phase transformation temperature under compositional design, the atomic diffusion coefficient is drastically reduced, and austenite grain coarsening is suppressed. The grain boundary area increases, and carbon is less likely to accumulate in large quantities at the grain boundaries, thus preventing carbon from accumulating on the grain boundaries to form coarse and continuous network carbides. This avoids the adverse effects of network carbides on strength, plasticity, and structural uniformity, and promotes the formation of dispersed fine lamellar cementite. 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 uniform heat exchange. Compared to air cooling, there is no overlap between the air-receiving and air-receiving surfaces. Compared to water cooling, there is no significant air bubble interference in heat transfer due to the temperature difference at the non-overlapping joints. As the hardenability of the wire rod can delay the decomposition of austenite in the core, the uniform high heat transfer can promote the simultaneous entry of the wire rod surface and core into the sorbitic phase transformation range. The phase transformation occurs at a lower temperature, with greater supercooling, weakened atomic diffusion ability, increased carbide nucleation rate, and slower growth rate, thus forming finer sorbitic lamellae. Even if the wire rod specification increases, it can avoid the formation of coarse pearlite in the core due to excessively low cooling rate, which would result in strength loss. Moreover, the finer the lamellae, the higher the strength of the wire rod, thus providing basic strength and compensating for the strength loss caused by a proper decrease in C or alloy content. At the same time, the isothermal characteristics of molten salt can stabilize the wire rod temperature within the sorbitic phase transformation range. Even if the wire rod specification decreases, it can avoid the formation of low-temperature brittle structures such as martensite due to local component segregation and local supercooling, which would affect the uniformity and plasticity of the microstructure.

[0019] Second, compared to controlling material costs, the Stellmore air-cooled phase transformation process involves greater structural stress and makes it difficult to balance strong plasticity and rapid, stable production. On the one hand, the initial molten salt treatment of the wire rod can form a microstructure dominated by fine lamellar spacing, which improves strength stability through uniform strengthening, reduces internal stress caused by microstructural differences, and appropriately reduces softening difficulty. The subsequent molten salt treatment of the wire rod can promote temperature consistency between the wire rod and the molten salt, and through isothermal treatment rather than continuous cooling, promotes the rapid melting and spheroidization transformation of some sorbite and cementite lamellars after the phase transformation, further releasing structural stress and improving the matching of wire rod plasticity and high strength; On the other hand, In terms of the latter stage of molten salt treatment, the molten salt temperature is appropriately reduced compared to the former stage. Combined with rapid treatment, this can promote full phase transformation of the microstructure and avoid excessive softening of the microstructure, thus reducing strength loss. After the wire rod exits the molten salt, it undergoes slow cooling on the roller table. The high-temperature coiling will not continue to form an abnormally brittle microstructure due to austenite residue, promoting stable production. Furthermore, the high temperature state of the wire rod after exiting the molten salt can be used to further toughen the wire rod microstructure and improve the tempering softening effect. At the same time, there is no need to use excessively long continuous cooling treatment, which would affect production efficiency. Thus, while controlling material costs and production efficiency, the strength-plasticity matching of the wire rod is improved, providing stable stranded steel base material properties.

[0020] Before rolling, selecting an appropriate heating furnace homogenization temperature and furnace dwell time can improve austenite uniformity and rolling plasticity, promote composition homogenization, reduce compositional segregation in the billet, and avoid austenite grain coarsening and decarburization caused by excessively high temperature and excessively long furnace dwell time. In the preferred technical solution, before rolling, the heating furnace homogenization temperature is controlled at 1150~1200℃ and the furnace dwell time is controlled at 200~280min.

[0021] During the rolling process, using an appropriate initial rolling temperature can reduce cracks and rolling line wear defects caused by insufficient plasticity of the rolled workpiece due to excessively low initial rolling temperature. It also avoids grain coarsening during the rolling process due to excessively high initial rolling temperature. Using an appropriate final rolling temperature and final rolling reduction can accumulate sufficient deformation energy, promote dynamic recrystallization and grain refinement during the final rolling process, and avoid excessive residual stress caused by excessively low final rolling temperature and excessive final rolling reduction. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1040~1070℃, the final rolling temperature is controlled at 910~940℃, and the final rolling reduction is controlled at 25%~30%.

[0022] During the wire spinning process, the wire spinning temperature can be further controlled to reduce the risk of austenite grain coarsening. In a preferred embodiment, the wire spinning temperature is controlled to be 885~920℃.

[0023] The molten salt temperature in the initial molten salt treatment is within the sorbite phase region. Lower molten salt temperatures are beneficial for increasing undercooling, promoting rapid cooling of the wire rod, inhibiting the formation of network carbides, and reducing carbon atom diffusion rates, thus promoting the nucleation and refinement of sorbite lamellars. With prolonged treatment time, it facilitates the full transformation of austenite to sorbite and improves the uniformity of the microstructure across the entire cross-section. However, excessively low molten salt temperatures will increase internal stress accumulation, and may even cause austenite to bypass the sorbite phase region and form brittle structures such as bainite and martensite, increasing the wire rod's brittleness. Prolonged treatment time will also increase production energy consumption. Furthermore, the higher molten salt temperature in the initial molten salt treatment will accelerate wire rod softening, leading to a certain strength loss. Conversely, higher molten salt temperatures can reduce the temperature difference and stress accumulation from the wire rod surface to the core, reducing the risk of brittle abnormal microstructure precipitation and facilitating subsequent microstructure softening. Shortening the processing time can reduce production energy consumption and retain a small amount of residual austenite in the microstructure. However, excessively high molten salt temperatures are detrimental to suppressing network carbides and grain boundary embrittlement. At the same time, the accelerated atomic diffusion rate makes the lamellar structure more prone to coarsening, resulting in a loss of strength properties. If the processing time is too short, insufficient carbon atom diffusion will increase the risk of coarsening of the core structure, causing fluctuations in mechanical properties and a decrease in strength. Therefore, the molten salt temperature and processing time of the first stage of molten salt treatment can be controlled to control the wire rod to quickly bypass the network carbide formation zone from the high-temperature austenitic state and enter the sorbite phase region, suppressing the formation of network carbides and forming a microstructure dominated by fine lamellar spacing sorbite, thus 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 490~540℃, and the processing time is 35~75s.

[0024] The temperature difference between the coiling temperature and the molten salt temperature in the preceding molten salt treatment is relatively large. Using a larger molten salt circulation rate in the preceding molten salt treatment can control the molten salt temperature rise, promote rapid cooling of the wire rod, reduce the temperature difference between different parts of the wire rod, and further reduce the risk of structural differences and mechanical property fluctuations. In the preferred technical solution, the molten salt circulation rate in the preceding molten salt treatment is 500~650t / h, and the molten salt temperature rise is ≤8℃.

[0025] The molten salt temperature in the subsequent molten salt treatment is appropriately lower than that in the preceding molten salt treatment. Higher molten salt temperatures and longer treatment times increase atomic diffusion capacity, providing more thermal dynamics to promote the melting and spheroidization of cementite lamellars in the sorbite. This releases structural stress, improves the ductility and toughness of the wire rod, and strengthens the matrix and improves structural uniformity through the dispersed distribution of melted sorbite. However, excessively high molten salt temperatures are detrimental to controlling the continued transformation of untransformed retained austenite into refined sorbite. Furthermore, increased atomic diffusion capacity, with prolonged treatment time, can drive the melted structure to grow along grain boundaries, forming coarse carbides. This excessive softening reduces strength, and the coarse carbides can become stress concentration points, further reducing ductility. Conversely, lower molten salt temperatures promote the transformation of retained austenite into sorbite, reducing the proportion of ferrite and atomic diffusion. Speed, as the processing time is shortened, can reduce production energy consumption and avoid excessive softening and loss of strength; however, if the molten salt temperature is too low and the processing time is too short, it will affect the melting of cementite lamellars, leaving large residual stress in the structure, which will affect the plasticity of the wire rod, increase the risk of deformation and cracking during drawing, and even form abnormal structures during subsequent cooling due to austenite residue. Therefore, the molten salt temperature and processing time of the later stage of molten salt treatment can be controlled to promote the continued transformation of untransformed residual austenite into fine lamellar sorbite structure, and at the same time promote the formation of fine lamellar sorbite structure to undergo long-term isothermal tempering, causing some cementite lamellars to melt and transform into spheroidization. Rapid processing can regulate the strength and plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the later stage of molten salt treatment is 400~445℃, and the processing time is 35~95s.

[0026] The temperature difference between the molten salt in the pre-treatment and post-treatment is small. Appropriately reducing the molten salt circulation rate in the post-treatment can control the molten salt temperature rise, improve the uniformity of the structure, and reduce production energy consumption. In the preferred technical solution, the molten salt circulation rate in the post-treatment is 240~350t / h, and the molten salt temperature rise is ≤3℃.

[0027] The roller conveyor slow cooling can further control the slow cooling rate, promote further toughening of the wire rod structure, and improve the tempering and softening effect of the wire rod. In the preferred technical solution, the roller conveyor slow cooling controls the wire rod to cool to below 280°C at a slow cooling rate of ≤0.5°C / s before winding. In order to improve the production efficiency of the wire rod, the slow cooling rate can be further preferred to be 0.3~0.5°C / s.

[0028] In the preferred technical solution, the slow cooling of the roller conveyor is achieved by controlling the opening of the heat insulation cover and blowing hot air of ≥250°C from the rapid online molten salt isothermal treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the heat insulation cover, which can recover and utilize the heat energy during the production process, reduce production energy consumption, and increase the wire rod off-line speed.

[0029] A hot-rolled wire rod for stranded wire of 1960MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method for hot-rolled wire rod for stranded wire of 1960MPa grade as described in any one of the above-mentioned methods.

[0030] The aforementioned hot-rolled wire rod adopts a high-carbon chemical composition design with Cr-B, featuring relatively low carbon or alloy content, a relatively simple composition design, and trace amounts of B, thus appropriately controlling material costs. Compared to existing air-cooled hot-rolled wire rods with a martensitic microstructure used in stranded steel, the microstructure of the aforementioned hot-rolled wire rod includes a mixed structure dominated by tempered sorbite and melted sorbite, containing a small amount of ferrite. This eliminates hard and brittle structures such as network carbides and martensite, avoiding the cutting effect of network carbides and abnormally hard and brittle structures, as well as the resulting plasticity loss and strength fluctuations. Through the transformation of austenite to sorbite with finer lamellar spacing, coarse pearlite is suppressed and the proportion of ferrite is reduced, which can reduce strength loss and performance fluctuations caused by microstructure differences, enhance the strengthening effect of carbon, and improve the overall quality of the wire rod. The improved microstructure and uniform strengthening allow for higher initial strength of the wire rod at lower alloy content. Simultaneously, the partial melting of cementite lamellars within the sorbite transforms it into melted sorbite and a transitional tempered sorbite that transforms into a spheroidized microstructure. This further reduces stress concentration, improves matrix plasticity, and maintains strength characteristics, enhancing the balance between high strength and plasticity in hot-rolled wire rod. This avoids reliance on large reduction ratios during drawing due to insufficient initial strength of the hot-rolled wire rod base material, thus reducing plasticity loss during drawing. The improved plasticity and microstructure uniformity also result in better coordination of microstructure deformation during drawing, with synchronized stress deformation in all regions and a smoother plasticity loss. This reduces the generation of grain boundary crack initiation, comprehensively lowering the risk of wire breakage and improving the final safety and service performance of high-strength steel strand.

[0031] In the microstructure, the higher the volume percentage of tempered sorbite and the finer the lamellar spacing, the fine and uniform lamellar structure can hinder dislocation movement, improve matrix strength, and reduce fatigue crack initiation. In the preferred technical solution, the volume percentage of tempered sorbite is ≥82%, the lamellar spacing is 75~120nm, and the volume percentage of ferrite is ≤8%.

[0032] The decrease in the network carbide level and the improvement in the uniformity of the structure in the hot-rolled wire rod can reduce the fluctuation of the tensile strength of the wire rod. 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 ≤32MPa. This can improve the consistency of the wire rod's processing performance and the product qualification rate, avoid local over-hardening that leads to wire breakage during drawing, make the stranded wire more uniformly stressed during service, and avoid the risk of overall failure caused by insufficient local strength.

[0033] In the preferred technical solution, the diameter of the hot-rolled wire rod is 8.0~16.0mm, the tensile strength is 1305~1355MPa, and the reduction of area is 37%~42%. The higher the tensile strength of the wire rod, the fewer drawing passes and the less plasticity loss during the drawing process can be reduced, and the target strength can be reached quickly. The wire rod has high plasticity, which is manifested as a high reduction of area, which can reduce the risk of wire breakage during drawing and avoid brittle fracture of the stranded wire due to insufficient plasticity during the twisting process.

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

[0035] (1) In view of the current situation that the Stellmore air-cooling line is difficult to control network carbides, abnormal structure, structure uniformity and plasticity in the production of high carbon steel hot-rolled wire rod of steel strand, which weakens its strengthening effect, this invention combines the high carbon chemical composition design of Cr-B with rapid online molten salt isothermal technology. First, after the first stage of molten salt treatment, the wire rod after wire drawing is controlled to quickly bypass the network carbides zone from the high temperature austenitic state and enter the sorbite phase region, which inhibits the formation of network carbides and forms a structure dominated by fine lamellar interlayer sorbite structure. Then, after the second stage of molten salt treatment, the untransformed residual austenite is promoted. The martensite continues to transform into fine lamellar interlamellar sorbite, while the formed fine lamellar interlamellar sorbite undergoes prolonged isothermal tempering, and some of the cementite lamellars of the sorbite melt, thereby regulating the strength-plasticity balance of the wire rod. Finally, slow cooling on the roller conveyor promotes further toughening of the wire rod structure, improving the tempering softening effect of the wire rod. This process can control material costs and microstructure uniformity, reduce the level of network carbides and the risk of abnormal structures, improve the high strength-plasticity balance of the wire rod, take into account production efficiency, provide stable stranded steel base material properties, and has good industrial adaptability.

[0036] (2) In response to the current situation where the high alloy cost, high risk of abnormal structures such as network carbides and martensite, and high risk of wire breakage during the drawing process of hot-rolled wire rod are caused by the combination of C and hardenability elements to improve the strength grade of steel strand, the present invention has a C or alloy content that is appropriately reduced compared with existing stranded steel wire rods, does not contain elements such as V, and adds trace amounts of B, which can appropriately control the material cost. The microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can suppress the precipitation of abnormal structures such as network carbides and martensite. At the same time, through uniform fine lamellar layers Sorbite maximizes the strengthening effect of carbon and provides basic strength. Through tempering, stress is further released and the microstructure is regulated, improving the overall strength and plasticity of the wire rod. This results in a tensile strength of 1305~1355MPa and a reduction of area of ​​37%~42% for hot-rolled wire rod. It is used as a base material in applications such as manufacturing 1960MPa grade stranded wire. It helps reduce the risk of wire breakage during drawing and twisting, conforms to the development of lightweighting, meets the requirements of higher load-bearing stress and longer service life, and 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. Example 1:

[0042] A preferred embodiment of the manufacturing method of the 1960MPa 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: 0.79%, Si: 0.33%, Mn: 0.63%, Cr: 0.40%, B: 0.005%, 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 → rapid online molten salt isothermal → slow cooling on roller table → coiling, specifically:

[0043] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high temperature that allows for rolling plasticity. The furnace parameters are controlled to promote uniform diffusion of alloy components and prevent decarburization. After exiting the furnace, the billet is rolled into a 16mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet. The final rolling process is controlled to dynamically recrystallize and refine the grains. Specifically, the furnace soaking temperature is controlled at 1... The rolling temperature is 170℃, the furnace time is 250 min, the initial rolling temperature is 1055℃, the final rolling temperature is 920℃, and the final rolling reduction is 25%. 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 the precipitation of network carbides during the wire drawing stage, and providing favorable conditions for the rapid nucleation of sorbite in the later stage. Specifically, the wire drawing temperature is controlled at 895℃.

[0044] The rapid online molten salt isothermal process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 38°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region, inhibiting the formation of network carbides and resulting in a microstructure dominated by fine-laminated sorbite. The wire rod is then conveyed via rollers through the second salt bath for subsequent molten salt treatment, where the molten salt temperature and circulation rate are reduced, promoting the transformation of untransformed residual austenite. The body continues to transform into a fine lamellar sorbite structure, while promoting the formation of the fine lamellar sorbite structure to undergo long-term isothermal tempering, and causing some cementite lamellars to melt and fracture, so as to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 490℃, the treatment time is 75s, the molten salt circulation rate is 650t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 400℃, the treatment time is 35s, the molten salt circulation rate is 350t / h, and the molten salt temperature rise is ≤3℃.

[0045] 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 rapid online molten salt isothermal treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering and softening effect. Specifically, the wire rod is cooled to 275℃ at a slow cooling rate of 0.3℃ / 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 1As shown, the microstructure consists of 89% tempered sorbite, 5% ferrite, and 6% melted sorbite by volume. The interlamellar spacing of the tempered sorbite is 76 nm, the network carbide grade is 0, the mechanical property difference is 32 MPa, the tensile strength is 1350 MPa, and the reduction of area is 37%.

[0046] Comparative Example 1:

[0047] A method for manufacturing hot-rolled wire rod differs from Example 1 in that it follows a process flow of rolling → wire drawing → Steyrmo air cooling. Specifically, the heating furnace is controlled at a uniform heating temperature of 1140°C, with a furnace time of 200 min, an initial rolling temperature of 1030°C, a final rolling temperature of 900°C, and a wire drawing temperature of 880°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 75% capacity, fans 1-5 are turned on to cool the wire rod to 670℃ at a cooling rate of 7.2℃ / s. Then, fans 6-14 are turned on to 35% capacity to cool the wire rod to 270℃ at a cooling rate of 3.8℃ / s. After cooling, the hot-rolled wire rod is obtained. The microstructure consists of 36% sorbite, 48% pearlite, and 16% ferrite by volume. The interlamellar spacing of the sorbite is 142nm, the interlamellar spacing of the pearlite is 225nm, the network carbide grade is 2, the mechanical property difference between the same ring is 87MPa, the tensile strength is 1130MPa, and the reduction of area is 29%.

[0048] As can be seen from the comparison results between Example 1 and Comparative Example 1, compared with reducing the carbon and alloy element content, it is difficult to control the network carbides, abnormal structure, structural uniformity and plasticity. The slow cooling of the core will lead to coarsening of the pearlite lamellars, weakening its strengthening effect and causing large fluctuations in mechanical properties. However, the present invention, through the design of high carbon chemical composition of Cr-B combined with rapid online molten salt isothermal technology, can suppress the formation of network carbon and form a structure dominated by fine lamellar sorbite, suppress coarse pearlite structure, reduce the proportion of ferrite, effectively improve the strengthening effect of carbon, and avoid the formation of low-temperature brittle abnormal structures such as martensite. By performing long-term isothermal tempering on the fine lamellar sorbite structure, the cementite lamellars of some sorbite are induced to melt and transform into spheroidization, which can control the structural uniformity, reduce the fluctuation of mechanical properties, and improve the high strength and plasticity matching of wire rod.

[0049] Comparative Example 2:

[0050] A method for manufacturing hot-rolled wire rod differs from that in Example 1 in that: the heating furnace is heated to a uniform temperature of 1080°C, the furnace time is 300 min, the initial rolling temperature is 990°C, the final rolling temperature is 870°C, and the wire drawing temperature is 840°C. During the molten salt treatment before the rapid online molten salt isothermal process, the wire rod is cooled at a rate of 31°C / s, and the finished hot-rolled wire rod is obtained after the process is completed. Example 2:

[0051] A preferred embodiment of the manufacturing method of the 1960MPa 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: 0.79%, Si: 0.35%, Mn: 0.55%, Cr: 0.36%, B: 0.001%, P: 0.015%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → rapid online molten salt isothermal → slow cooling on roller table → coiling, specifically:

[0052] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity. The furnace parameters are controlled to promote uniform diffusion of alloy components and avoid decarburization. After exiting the furnace, the steel billet is rolled into wire rod with a diameter of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the steel billet. The dynamic recrystallization of the final rolling process is controlled to refine the grains. Specifically, the furnace soaking temperature is controlled at 1150℃, the furnace time is controlled at 280min, the initial rolling temperature is controlled at 1040℃, the final rolling temperature is controlled at 910℃, and the final rolling reduction is controlled at 30%. The wire drawing process is used to convert the wire rod exiting the rolling line into wire rod through a wire drawing machine. The wire rod is distributed on the roller conveyor and conveyed along the roller conveyor. An appropriate wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding the precipitation of network carbides during the wire drawing stage, and providing favorable conditions for the rapid nucleation of sorbite. Specifically, the wire drawing temperature is controlled at 885℃.

[0053] The rapid online molten salt isothermal process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 32°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region, inhibiting the formation of network carbides and resulting in a microstructure dominated by fine-laminated sorbite. The wire rod is then conveyed via rollers through the second salt bath for subsequent molten salt treatment, where the molten salt temperature and circulation rate are reduced, promoting the transformation of untransformed residual austenite. The body continues to transform into a fine lamellar interlamellar sorbite structure, while promoting the formation of the fine lamellar interlamellar sorbite structure to undergo long-term isothermal tempering, and causing some cementite lamellars to melt and fracture, so as to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage molten salt treatment is 540℃, the treatment time is 35s, the molten salt circulation rate is 500t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage molten salt treatment is 445℃, the treatment time is 95s, the molten salt circulation rate is 240t / h, and the molten salt temperature rise is ≤3℃.

[0054] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥250℃) from above the two salt bath tanks in the rapid online molten salt isothermal treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 271℃ 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.

[0055] Comparative Example 3:

[0056] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the rapid online molten salt isothermal process, the wire rod is cooled at a rate of 37°C / s, the molten salt temperature of the initial molten salt treatment is 485°C, the treatment time is 90s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0057] Comparative Example 4:

[0058] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the rapid online molten salt isothermal process, the wire rod is cooled at a rate of 30°C / s, the molten salt temperature of the initial molten salt treatment is 565°C, the treatment time is 30s, and the finished hot-rolled wire rod is obtained after the process is completed. Example 3:

[0059] A preferred embodiment of the manufacturing method of the 1960MPa 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: 0.81%, Si: 0.40%, Mn: 0.65%, Cr: 0.25%, B: 0.005%, P: 0.012%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → rapid online molten salt isothermal → slow cooling on roller table → coiling, specifically:

[0060] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. The furnace parameters are controlled to promote uniform diffusion of alloy components and prevent decarburization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 11mm via a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet. The final rolling process is controlled to dynamically recrystallize and refine the grains. Specifically, the furnace soaking temperature is controlled at 11... The rolling temperature is 90℃, the furnace time is 235 minutes, the initial rolling temperature is 1065℃, the final rolling temperature is 930℃, and the final rolling reduction is 28.5%. 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 the precipitation of network carbides during the wire drawing stage, and providing favorable conditions for the rapid nucleation of sorbite in the later stage. Specifically, the wire drawing temperature is controlled at 910℃.

[0061] The rapid online molten salt isothermal process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region, inhibiting the formation of network carbides and resulting in a microstructure dominated by fine-laminated sorbite. The wire rod is then conveyed via rollers through the second salt bath for subsequent molten salt treatment, where the molten salt temperature and circulation rate are reduced, promoting the transformation of untransformed residual austenite. The body continues to transform into a fine lamellar sorbite structure, while promoting the formation of the fine lamellar sorbite structure to undergo long-term isothermal tempering, and causing some cementite lamellars to melt and fracture, so as to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 509℃, the treatment time is 42s, the molten salt circulation rate is 575t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 425℃, the treatment time is 80s, the molten salt circulation rate is 285t / h, and the molten salt temperature rise is ≤3℃.

[0062] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥250℃) from above the two salt bath tanks in the rapid online molten salt isothermal treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering and softening effect. Specifically, the wire rod is cooled to 273℃ at a slow cooling rate of 0.4℃ / 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.

[0063] Comparative Example 5:

[0064] 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 465°C, the treatment time is 130s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0065] Comparative Example 6:

[0066] 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 395°C, the treatment time is 30s, and the finished hot-rolled wire rod is obtained after the process is completed. Example 4:

[0067] A preferred embodiment of the manufacturing method of the 1960MPa 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: 0.83%, Si: 0.20%, Mn: 0.59%, Cr: 0.20%, B: 0.008%, P: 0.014%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → rapid online molten salt isothermal → slow cooling on roller table → coiling, specifically:

[0068] 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. The furnace parameters are controlled to promote uniform diffusion of alloy components and prevent decarburization. After exiting the furnace, the billet is rolled into a 14mm diameter wire rod on a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet. The final rolling process is controlled to dynamically recrystallize and refine the grains. Specifically, the furnace soaking temperature is controlled at 1... The rolling temperature is 200℃, the furnace time is 200min, the initial rolling temperature is 1070℃, 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 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 the precipitation of network carbides during the wire drawing stage, and providing favorable conditions for the rapid nucleation of sorbite in the later stage. Specifically, the wire drawing temperature is controlled at 920℃.

[0069] The rapid online molten salt isothermal process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region, inhibiting the formation of network carbides and resulting in a microstructure dominated by fine-laminated sorbite. The wire rod is then conveyed via rollers through the second salt bath for subsequent molten salt treatment, where the molten salt temperature and circulation rate are reduced, promoting the transformation of untransformed residual austenite. The body continues to transform into a fine lamellar sorbite structure, while promoting the formation of the fine lamellar sorbite structure to undergo long-term isothermal tempering, and causing some cementite lamellars to melt and fracture, so as to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 522℃, the treatment time is 60s, 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 410℃, the treatment time is 65s, the molten salt circulation rate is 310t / h, and the molten salt temperature rise is ≤3℃.

[0070] The slow cooling process of the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥250℃) from the two salt bath tanks of the rapid online molten salt isothermal treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering and softening effect. Specifically, the wire rod is cooled to 276℃ at a slow cooling rate of 0.45℃ / 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.

[0071] Comparative Example 7:

[0072] A method for manufacturing hot-rolled wire rod differs from that in Example 4 in that the method follows a process flow of rolling → wire drawing → rapid online molten salt isothermal cooling → air cooling. The air cooling process involves opening the insulation cover and controlling the wire rod to cool to 265°C at a slow cooling rate of 1.1°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0073] The microstructure and properties of the hot-rolled wire rods obtained in Examples 1-4 and Comparative Examples 2-7 were tested, and the comparative results are shown in Table 1 below:

[0074] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0075]

[0076] As can be seen from the comparison results between Example 1 and Comparative Example 2, using a higher wire drawing temperature to ensure that the wire rod is in a high-temperature austenitizing state can prevent the premature precipitation of network carbides, provide favorable conditions for increasing the undercooling degree, promoting the rapid nucleation of sorbite structure and lamellar refinement, and at the same time reduce the limitation on rolling temperature in order to improve rolling efficiency.

[0077] As can be seen from the comparison results between Example 2 and Comparative Example 3, the molten salt temperature in the first stage of molten salt treatment is in the sorbite phase region. The lower the molten salt temperature, the better it is to increase the degree of undercooling, promote the rapid cooling of the wire rod, inhibit the formation of network carbides, and reduce the diffusion rate of carbon atoms, thus promoting the nucleation and refinement of sorbite lamellars. With the extension of the treatment time, it is beneficial to promote the full transformation of austenite to sorbite and improve the uniformity of the microstructure across the entire cross section. However, if the molten salt temperature is too low, it will increase the accumulation of internal stress and the brittleness of the wire rod. With the extension of the treatment time, it will increase the energy consumption of production. Moreover, if the treatment time is too long, the higher molten salt temperature in the first stage of molten salt treatment will accelerate the softening of the wire rod and bring about a certain loss of strength.

[0078] 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 lower the temperature difference and stress accumulation from the wire rod surface to the core, and the lower the risk of brittle abnormal structure precipitation, so as to soften the subsequent structure. With the shortening of the treatment time, the production energy consumption can be reduced and the structure can retain a small amount of residual austenite. However, if the molten salt temperature is too high, it is not conducive to suppressing network carbides and grain boundary embrittlement. At the same time, the atomic diffusion rate is accelerated, making the lamellar structure more prone to coarsening and losing strength performance. If the treatment time is too short, the carbon atom diffusion is insufficient, which will increase the risk of coarsening of the core structure, causing fluctuations in mechanical properties and a decrease in strength.

[0079] As can be seen from the comparison results between Example 3 and Comparative Example 5, the molten salt temperature in the later stage of molten salt treatment is appropriately lower than that in the earlier stage. The higher the molten salt temperature and the longer the treatment time, the greater the atomic diffusion ability, which can provide more thermal power and promote the melting of cementite lamellars in sorbite and their transformation into spheroidization. This not only releases structural stress and improves the plasticity and toughness of wire rod, but also strengthens the matrix and improves the uniformity of the structure by melting and dispersing the sorbite. However, if the molten salt temperature is too high, it is not conducive to controlling the transformation of untransformed residual austenite into refined sorbite. At the same time, the increased atomic diffusion ability will drive the melted structure to grow along the grain boundaries to form coarse carbides as the treatment time is extended. Excessive softening reduces strength, and coarse carbides will become stress concentration points, which will reduce plasticity.

[0080] As can be seen from the comparison results between 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 promote the transformation of residual austenite to sorbite, reduce the proportion of ferrite and the atomic diffusion rate. With the shortening of the treatment time, production energy consumption can be reduced and excessive softening and loss of strength can be avoided. However, if the molten salt temperature is too low and the treatment time is too short, it will affect the melting of cementite lamellars, and the residual stress in the structure will affect the plasticity of the wire rod and increase the risk of deformation and cracking during drawing.

[0081] As can be seen from the comparison results between Example 4 and Comparative Example 7, the slow cooling of the roller can further control the cooling speed, promote the further toughening of the wire rod structure, and improve the tempering and softening effect of the wire rod.

[0082] 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 1960MPa 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: 0.79%~0.83%, Si: 0.20%~0.40%, Mn: 0.55%~0.65%, Cr: 0.20%~0.40%, B: 0.001%~0.008%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a spinning temperature of ≥885℃, it undergoes rapid online molten salt isothermal treatment. The wire rod first undergoes a pre-treatment molten salt treatment, which cools the wire rod at a cooling rate of ≥32℃ / s. The wire rod transitions from an austenitic state to a sorbite phase, forming a microstructure dominated by sorbite. The wire rod then undergoes a subsequent molten salt treatment to lower the molten salt temperature, promoting the transformation of untransformed residual austenite into sorbite. Simultaneously, it undergoes isothermal tempering and partial melting of sorbite lamellars. Finally, it is slowly cooled 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. The molten salt temperature of the initial molten salt treatment is 490–540°C, and the treatment time is 35–75 seconds. The molten salt temperature of the subsequent molten salt treatment is 400–445°C, and the treatment time is 35–95 seconds.

2. The method for manufacturing hot-rolled wire rod for 1960MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace is heated to a temperature of 1150-1200℃ and the time spent in the furnace is 200-280 minutes.

3. The method for manufacturing hot-rolled wire rod for 1960MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1040~1070℃, the final rolling temperature is controlled at 910~940℃, and the final rolling reduction is controlled at 25%~30%.

4. The method for manufacturing hot-rolled wire rod for 1960MPa grade stranded wire according to claim 1, characterized in that, During the silk-spinning process, the silk-spinning temperature is controlled at 885~920℃.

5. The method for manufacturing hot-rolled wire rod for 1960MPa grade stranded wire according to claim 1, characterized in that, The molten salt circulation rate of the front-end molten salt treatment is 500~650t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation rate of the rear-end molten salt treatment is 240~350t / h, and the molten salt temperature rise is ≤3℃.

6. The method for manufacturing hot-rolled wire rod for 1960MPa grade stranded wire according to claim 1, characterized in that, The roller conveyor slow cooling control strip is cooled to below 280°C at a slow cooling rate of ≤0.5°C / s before being wound up.

7. A hot-rolled wire rod for stranded wire of grade 1960MPa, characterized in that, The hot-rolled wire rod is manufactured by the method for manufacturing 1960MPa grade stranded wire rod as described in any one of claims 1 to 6.

8. The hot-rolled wire rod for 1960MPa grade stranded wire according to claim 7, characterized in that, The volume percentage of tempered sorbite is ≥82%, the lamellar spacing is 75~120nm, the volume percentage of ferrite is ≤8%, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤32MPa.

9. The hot-rolled wire rod for 1960MPa grade stranded wire according to claim 7, characterized in that, The hot-rolled wire rod has a diameter of 8.0~16.0mm, a tensile strength of 1305~1355MPa, and a reduction of area of ​​37%~42%.

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