Hot-rolled wire rod for 2160MPa-grade stranded wire and manufacturing method of hot-rolled wire rod

Abstract

The invention relates to a hot-rolled wire rod for a 2160MPa-grade stranded wire and a manufacturing method of the hot-rolled wire rod, after a high-carbon component system containing trace Nb is rolled and spun into a wire rod, the wire rod is subjected to online molten salt sorbite regulation and control treatment, so that the wire rod is firstly subjected to front-section molten salt treatment and quickly enters a sorbite phase region from an austenite state to form a structure taking a sorbite structure as a main structure; the method comprises the following steps: firstly, carrying out hot rolling on unconverted retained austenite, then carrying out rear-section molten salt treatment, increasing the molten salt temperature, reducing the molten salt circulation amount, promoting the unconverted retained austenite to be continuously converted into sorbite, carrying out isothermal tempering, and finally, carrying out roller way slow cooling to prepare a hot-rolled wire rod of which the microscopic structure comprises a mixed structure consisting of tempered sorbite, ferrite and fused sorbite, so that the deterioration of the structure can be inhibited, and the service life of the hot-rolled wire rod is prolonged. The tensile strength of the hot-rolled wire rod is 1461-1496 MPa, the percentage reduction of area is 33%-38%, the difference of the same circle of mechanical properties is smaller than or equal to 42 MPa, meanwhile, the production energy consumption is reduced, and efficient and stable production of the hot-rolled wire rod and the stranded wire is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled wire rod technology, specifically relating to a 2160MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] With the rapid development of prestressed concrete technology, there is a greater demand for improved strength grades of steel strands. Improving the strength grade of steel strands also contributes to energy and material conservation. Therefore, high-strength steel strands have become an important development direction. The improvement of steel strand strength grades is often based on the improvement of the performance of the raw material wire rod. High-strength steel strands are produced after large-volume drawing and subsequent stranding. Improving the performance of the raw material wire rod usually requires the addition of more reinforcing elements such as C, Mn, and Cr. However, the risk of deterioration structures such as network carbides and martensite will increase simultaneously, leading to a higher risk of wire breakage during the drawing and stranding processes in downstream wire drawing plants. Therefore, it is necessary to develop a 2160MPa grade hot-rolled wire rod for strands and its manufacturing method to effectively suppress deterioration structures, reduce the risk of wire breakage, and promote efficient and stable production of hot-rolled wire rods and strands.

[0003] Existing hot-rolled wire rods for high-strength strands employ a high-carbon composition system, combined with post-rolling controlled cooling to achieve a pearlitic microstructure. For example, patent CN119411020A discloses a high-carbon steel wire rod for ultra-high-strength prestressed steel strands and its production method, using a C-Si-Mn-Al composition. The hot-rolled wire rod is produced under low-temperature heating and rolling followed by a post-rolling salt bath, or combined with a V composition under Steyrmo air-cooling conditions. However, the following drawbacks still exist:

[0004] I. To improve steel strength and control material costs, wire rod contains a high carbon content. However, limited by the maximum cooling capacity of the Steyrmore air-cooled line, the austenite in hypereutectoid high-carbon steel will precipitate proeutectoid cementite before the sorbite phase transformation during cooling. Cementite easily precipitates continuously along the austenite grain boundaries, forming a network of hard and brittle carbides, which leads to matrix fracture, decreased ductility and toughness, and affects the service performance of the strand. While efforts to improve air-cooling strength by cooperating with the action of vanadium to promote sorbite lamellar refinement and maximize the strength of the network of carbides have limited improvement effects, they further increase the temperature difference between the air-receiving and air-receiving surfaces of the wire rod, and between the surface and core, leading to localized overcooling, the formation of martensite and other deteriorated structures, increased uneven transformation of the microstructure, and the generation of pearlite in the core. This leads to increased wire rod brittleness, fluctuations in mechanical properties, and a greater risk of coil breakage. Although salt bath cooling lines have higher heat exchange capacity and stability than air-cooled lines, on the one hand, in order to match the wire drawing temperature and promote the sorbite phase transformation, the heating and rolling temperatures are lower, and the rolling mill needs to output greater rolling force to achieve the diameter reduction of the rolled piece. To avoid overloading the rolling mill, the rolling speed must be reduced and the rolling cycle extended, which affects rolling efficiency and increases roll wear. On the other hand, low silicon and low undercooling affect the sorbite phase transformation rate. Due to the influence of local high carbon component segregation, the retained austenite is prone to continue to form martensite structure during subsequent cooling, which leads to increased wire rod brittleness and fluctuations in mechanical properties, increased risk of wire breakage during drawing and stranding, and affects the stable production of hot-rolled wire rod.

[0005] II. To balance the plasticity of the wire rod and improve its grain refinement, the wire rod contains a low amount of silicon and a certain amount of aluminum. However, due to the minimum cooling capacity and continuous cooling limitations of the Stellmore air-cooled line, the hypereutectoid high-carbon steel is already at a low temperature after phase transformation inoculation. The stress caused by the deteriorated microstructure and uneven phase transformation remains in the microstructure, which has limited effect on improving plasticity. Salt bath has better temperature control than air-cooled line, but on the one hand, in order to match the wire drawing temperature and promote the transformation of austenite to sorbite, the molten salt circulation volume required for one-stage processing is large, which increases production energy consumption. On the other hand, due to the high carbon and low silicon composition design, as the processing time in the high-temperature range is extended, the melting and agglomeration of cementite in sorbite accelerates, which will lead to a significant loss of strength and fluctuations in mechanical properties. Coarsened cementite will become the source of fatigue crack initiation. The reduction in the properties of the wire rod base material also requires the increase of drawing passes in stranding processing, which brings the risk of wire breakage and affects the efficiency of stranding production. 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 2160MPa grade and its manufacturing method, which can suppress the deterioration of the microstructure, promote the matching of strength and plasticity of the wire rod, improve the uniformity of the microstructure, reduce production energy consumption, and promote the efficient and stable production of hot-rolled wire rod and stranded wire.

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

[0008] A method for manufacturing hot-rolled wire rod for 2160MPa 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: 0.87%~0.91%, Si: 0.49%~0.66%, Mn: 0.66%~0.86%, Cr: 0.26%~0.37%, Nb: 0.015%~0.035%, 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 ≥925℃ and then processed online. Molten salt sorbitization control treatment involves first subjecting the wire rod to a preliminary molten salt treatment and cooling it at a rate of ≥36℃ / 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, followed by isothermal tempering and promoting partial melting of sorbite lamellars. Finally, the wire rod undergoes slow cooling via a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted sorbite.

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

[0011] (1) Carbon: C is a solid solution strengthening and carbide strengthening element in steel. It is relatively cheaper and can be dissolved in austenite to improve lattice distortion. At the same time, it can stabilize austenite, expand the austenite phase region, and reduce the starting temperature of austenite to sorbite transformation. This is beneficial to improve the phase transformation driving force and promote the precipitation of sufficient fine lamellar sorbite during phase transformation. It can form fine carbides with elements such as Nb and Cr to provide dispersion strengthening. However, if the C content is too high, it will aggravate the center segregation and easily accumulate at the austenite grain boundary and precipitate continuous network carbides. At the same time, the phase transformation incubation period is prolonged, the uniformity of the structure and the difficulty of controlling the martensite deterioration structure are increased, resulting in increased wire rod brittleness and mechanical property fluctuations. Therefore, in order to take into account the high strength requirements of 2160MPa grade stranded wire, improve the uniformity of the structure and promote rapid production, the mass percentage of C is controlled at 0.87%~0.91%.

[0012] (2) Silicon: Si is a solid solution strengthening element. By dissolving in the matrix phase of sorbite, it can hinder dislocation movement and improve tensile strength. During the austenite-to-sorbite phase transformation, it can hinder carbon atom diffusion, inhibit cementite precipitation and coarsening, stabilize the fine structure of sorbite, and at the same time increase the solubility of Nb in austenite and reduce the precipitation temperature of NbC. During isothermal tempering, it can prevent premature spheroidization or coarsening of cementite, resist tempering softening, and avoid insufficient tensile strength due to softening. However, excessive silicon will promote decarburization, increase intragranular segregation, and inhibit cross-slip of dislocations, making isothermal tempering softening more difficult and reducing plasticity. Therefore, in order to adapt to the control of phase transformation structure by online molten salt sorbitization, the resistance to tempering softening should be appropriately increased so as to give full play to the strengthening and toughening effect of Nb. The mass percentage of Si should be controlled at 0.49%~0.66%.

[0013] (3) Manganese: Mn is an austenite stabilizing element that can expand the austenite region, lower the eutectoid temperature, hinder the diffusion of C atoms, suppress non-sorbite phase transformation, significantly improve the hardenability of wire rod, enable the wire rod to fully complete the sorbite transformation in molten salt, improve the uniformity of sorbite structure, enhance the matrix strength through solid solution strengthening and C synergistically, and at the same time increase the recrystallization temperature of austenite. Combined with the rolling deformation before wire drawing, it promotes NbC pinning of austenite grain boundaries, reduces stress concentration during rolling, reduces surface crack rate, and improves tempering stability, suppressing strength loss. However, if the Mn content is too high, it will increase the segregation of alloying elements during the solidification process of the billet, and increase the risk of martensite formation during cooling, leading to increased brittleness and mechanical property fluctuations, affecting the uniformity and toughness of wire rod structure. Therefore, in order to take into account the high strength and plasticity of hot-rolled wire rod and adapt to rolling control, the mass percentage of Mn is controlled at 0.66%~0.86%.

[0014] (4) Chromium: Cr can provide solid solution strengthening effect, while enhancing hardenability, prolonging the stability of austenite, and reducing the transformation temperature of sorbite. During low-temperature transformation, the atomic diffusion rate is slow and the interlamellar spacing of sorbite is finer. At the same time, Cr has a strong affinity for C and can form fine and dispersed Cr-based carbides distributed in the sorbite lamellars, which can continuously pin dislocations, slow down the diffusion rate of cementite, increase the resistance to tempering softening, and enable the wire rod to maintain high strength after tempering. However, excessive Cr content will aggravate the segregation of alloying elements, increase the austenitizing temperature, increase the risk of precipitation of martensite and other deteriorated structures, and reduce the activity of carbon during isothermal tempering, affecting the drawability of steel. Therefore, in order to promote sorbite transformation and provide appropriate resistance to tempering softening, the mass percentage of Cr is controlled at 0.26%~0.37%.

[0015] (5) Niobium: During the hot rolling process, the carbonitrides of Nb can prevent the abnormal growth of austenite grains at high temperatures by pinning grain boundaries, reduce the deformation resistance during hot rolling, reduce the risk of rolling cracks, increase the nucleation energy barrier, delay the pearlite transformation, and refine the sorbite lamellars. Fine grains not only improve strength through fine grain strengthening, but also indirectly improve plasticity and alleviate the brittleness problem of high carbon steel. During the molten salt treatment process, Nb will further precipitate nano-sized NbC. NbC has a high melting point and better stability than VC. It enhances the strengthening effect by pinning dislocations, improves strength and toughness, makes the dislocation distribution more uniform during the drawing process of wire rod, reduces the cold work hardening rate, and hinders the initiation and propagation of fatigue cracks. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling material costs. Therefore, the mass percentage of Nb is controlled at 0.015%~0.035%.

[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 composition design of C-Si-Mn-Cr-Nb, with trace amounts of Nb added. This allows for appropriate control of material costs, the segregation effects of carbon and alloying elements, and the hardenability of the wire rod. It expands the austenite region, lowers the phase transformation temperature, and improves tempering stability. This provides favorable conditions for promoting the refinement of sorbite lamellars, suppressing network carbides and martensite, using Si to suppress cementite coarsening, and strengthening the resistance to tempering softening through carbide dispersion with Cr and Nb under high-temperature rolling and wire drawing. Based on this, a higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitic state, avoiding premature precipitation of proeutectoid ferrite or network carbides. This provides a foundation for the subsequent formation of stable supercooled austenite and the promotion of rapid sorbite phase transformation. After wire drawing, the wire rod is directly fed into molten salt for online molten salt sorbitization control without air cooling.

[0018] Firstly, compared to the limitations imposed by the maximum cooling capacity of the Stellmore air-cooled line, which makes it difficult to control network carbides and martensite, the high heat transfer capacity of the molten salt during the initial molten salt treatment of the wire rod promotes rapid cooling of the wire rod. The wire rod can quickly bypass the precipitation temperature range of secondary cementite, inhibiting the formation of network carbides. At the same time, when the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for uniform heat exchange, eliminating the temperature difference problem between the air-receiving and air-receiving surfaces. This avoids the effects of segregation and localized overcooling that can lead to the formation of martensite. Compared to the risk of martensite abnormalities brought about by high carbon and low silicon composition combined with existing molten salt treatment, on the one hand, the wire rod can quickly enter the sorbite phase region from the high-temperature austenitic state. The high-temperature spinning increases the free energy difference and undercooling between austenite and sorbite. Combined with the grain-refining effect of Nb, this allows for a lower phase transformation temperature. The weakening of atomic diffusion ability and the limitation of carbon diffusion distance to a short range due to rapid cooling can increase the nucleation rate of sorbite, compensating for the adverse effects of increasing Si content on phase transformation inoculation. As the processing time increases, it promotes the transformation of most austenite into sorbite with finer interlamellar spacing. The cementite lamellae of sorbite are difficult to grow after nucleation, and at the same time, it increases the driving force for the dispersion precipitation of NbC, making the dislocation inhibition effect of fine lamellar sorbite and nanoscale carbides stronger, thereby improving the matrix strength. On the other hand, the wire rod that has undergone the first stage of molten salt treatment can be further treated with the second stage of molten salt treatment at a higher molten salt temperature, which can promote the decomposition of untransformed residual austenite to continue to form sorbite, avoiding the effects of segregation, low temperature and austenite residue on the continued formation of brittle martensite structure during subsequent cooling, thereby suppressing martensite structure and improving matrix strength and structure uniformity.

[0019] Second, compared to the limitations of the Stellmore air-cooling line in terms of minimum cooling capacity and continuous cooling, which makes it difficult to control the plasticity and toughness of wire rods online, the higher molten salt temperature in the later stage of molten salt treatment allows for appropriate improvement in atomic diffusion through isothermal tempering, promoting the melting of some cementite lamellars. Internal stress can be slowly released through dislocation rearrangement, enabling online control of the plasticity and toughness of the wire rod. Compared to the high energy consumption and easy over-softening problems caused by high carbon and low silicon composition combined with existing one-stage molten salt treatment, the wire rod can be rapidly cooled after the first stage of molten salt treatment, and the molten salt circulation volume in the later stage of molten salt treatment is lower than that in the first stage, which can reduce the online melting rate. The overall energy consumption of salt sorbitization regulation; on the other hand, the molten salt temperature of the later stage molten salt treatment is higher. While ensuring the phase transformation temperature range and promoting the release of structural stress, it can suppress cementite coarsening by Si and provide a certain resistance to tempering softening by utilizing the dispersion strengthening of fine carbides of Cr and Nb. It can retain high strength characteristics, improve the plasticity and strength matching of wire rod, further reduce the fluctuation of mechanical properties, avoid excessive strength loss and the risk of pull-out fracture caused by coarse cementite. As the roller table slowly cools down the wire rod, it can promote further toughening of the wire rod structure and stabilize the mechanical properties of the hot-rolled wire rod base material.

[0020] Before rolling, a higher furnace temperature and an appropriate furnace time can be selected to promote uniform diffusion of components, reduce component segregation, and ensure that the dissolved Nb is uniformly distributed in austenite, taking into account both rolling plasticity and austenite grain size control, thus preparing for the subsequent rolling microstructure. In the preferred technical solution, before rolling, the heating furnace homogenization temperature is controlled at 1195~1235℃ and the furnace time is controlled at 125~185min.

[0021] Because the wire drawing temperature is relatively high, the limitation on the rolling temperature can be reduced. Using a higher initial rolling temperature reduces the rolling deformation resistance, reduces the load requirements on the rolling line and the impact of wear. Combined with a large initial rolling reduction, the large deformation induces the carbonitride strain precipitation of Nb, providing a large number of nucleation sites for recrystallization. At the same time, the rolling speed and efficiency are improved. Combined with the medium temperature and small reduction in the final rolling, the strain-induced precipitation and grain refinement are controlled, and the microstructure is strengthened and toughened. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1085~1115℃, the initial rolling reduction is 38%~43%, the final rolling temperature is 925~965℃, and the final rolling reduction is 21%~26%.

[0022] During the wire spinning process, the wire spinning temperature can be further controlled to ensure wire spinning formability and suppress the growth of austenite grains in the early stage of controlled cooling after wire spinning. In a preferred embodiment, the wire spinning temperature is controlled at 925~960℃.

[0023] The molten salt temperature in the initial molten salt treatment is within the sorbite phase region. Lower molten salt temperatures facilitate rapid cooling, suppress network carbides and coarse pearlite, increase the driving force for the precipitation of Cr and Nb nano-precipitates, and inhibit cementite growth. With prolonged treatment time, it promotes the transformation of high-temperature austenite to sorbite with finer lamellar spacing, reduces residual austenite content, and improves matrix strength. However, excessively low molten salt temperatures, deviating from the nose region of the sorbite phase transformation, will slow down the transformation driving force of austenite to sorbite and may even lead to the formation of low-temperature brittle structures. Prolonged treatment time will increase production energy consumption. Conversely, higher molten salt temperatures help reduce the temperature difference between the wire rod surface and the core, reducing the risk of low-temperature brittle structure formation. Shorter treatment time can reduce production costs. While energy consumption is high, excessively high molten salt temperatures are detrimental to suppressing network carbides, promoting the refinement of sorbite lamellars, and the precipitation of nano-precipitates. Furthermore, excessively short treatment times slow down sorbite nucleation, and excessive untransformed residual austenite leads to a loss of matrix strength. Therefore, the molten salt temperature and treatment time of the initial molten salt treatment can be controlled to quickly bypass the network carbide region from the high-temperature austenitic state and enter the sorbite phase region, suppressing network carbide formation and forming a microstructure dominated by fine-laminated sorbite, promoting the dispersed precipitation of Cr and Nb carbides, thus balancing production energy consumption and preparing the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the initial molten salt treatment is 510~545℃, and the treatment time is 123~173s.

[0024] The spinning temperature differs significantly from the molten salt temperature of the preceding molten salt treatment. Using a larger molten salt circulation rate can promote rapid cooling of the wire rod, reduce the temperature gradient from the wire rod surface to the core, control the molten salt temperature rise, and reduce phase transformation stress. In the preferred technical solution, the molten salt circulation rate of the preceding molten salt treatment is 355~550t / h, and the molten salt temperature rise is ≤8℃.

[0025] The molten salt temperature in the subsequent molten salt treatment is appropriately increased compared to the preceding molten salt treatment. Higher molten salt temperatures are beneficial for increasing atomic diffusion rates, promoting localized melting of the cementite lamellars within the sorbite at high temperatures. With prolonged treatment time, dislocations within the sorbite rearrange through climb and cross-slip, resulting in a more uniform distribution of carbides, which reduces structural stress and improves the plasticity and fatigue resistance of the wire rod. However, excessively high molten salt temperatures are detrimental to inhibiting the transformation of untransformed retained austenite into sorbite. With prolonged treatment time, excessive melting of sorbite leads to excessive aggregation and growth of cementite and nano-precipitates, which become fatigue crack initiators, resulting in loss of strength and plasticity and increased production energy consumption. Conversely, lower molten salt temperatures promote the decomposition of untransformed retained austenite and inhibit coarsening of the sorbite phase. Shorter treatment times can inhibit... While Cr and Nb carbides can grow and reduce production energy consumption, excessively low molten salt temperatures and short treatment times can affect the melting of sorbite lamellars and stress release, leading to loss of plasticity, fluctuations in mechanical properties, and even the formation of deteriorated structures due to retained austenite during subsequent cooling. Therefore, the molten salt temperature and treatment time can be further controlled to promote the transformation of untransformed retained austenite into fine lamellar sorbite, while simultaneously promoting long-term isothermal tempering of the formed fine lamellar sorbite and causing partial cementite lamellar melting to inhibit carbide growth. This balances production energy consumption, regulates the strength-plasticity matching of wire rods, and suppresses abnormal low-temperature transformation of retained austenite. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 550~565℃, and the treatment time is 120~150s.

[0026] Since the temperature difference between the molten salt in the front-end molten salt treatment and the back-end molten salt treatment is small, the molten salt circulation rate in the back-end molten salt treatment can be appropriately reduced to control the molten salt temperature rise, reduce production energy consumption, and take into account the consistency of wire rod performance. In the preferred technical solution, the molten salt circulation rate in the back-end molten salt treatment is 305~395t / h, and the molten salt temperature rise is ≤3℃.

[0027] The roller conveyor slow cooling can further control the slow cooling rate, maintain the high temperature state of the wire rod after exiting the molten salt, 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 ≤0.78°C / s before winding.

[0028] In the preferred technical solution, the slow cooling of the roller conveyor adopts the control of the opening of the heat insulation cover, and blows the hot air of ≥220°C in the online molten salt sorbitization control treatment to the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the heat insulation cover. The hot air can further reduce the temperature gradient from the surface of the wire rod to the core, reduce the fluctuation of mechanical properties, and at the same time recover and utilize the heat energy of the online molten salt sorbitization control, reduce production energy consumption, and promote the rapid production of wire rod.

[0029] A hot-rolled wire rod for stranded wire of 2160MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method for 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 composition system containing trace amounts of Nb, combined with appropriate Si, Mn, and Cr components, which can control material costs. The microstructure includes a mixed structure mainly composed of tempered sorbite and melted sorbite, with a small amount of ferrite. Compared with air-cooled wire rod or post-rolling salt bath treated pearlitic / sorbitic wire rod, it can effectively suppress the brittleness and mechanical property fluctuations caused by network carbides and martensite. The sorbite has fine lamellar cementite, which has stronger interlaminar bonding force and more uniform strengthening of the matrix by cementite, avoiding the strength loss caused by coarse lamellar pearlite. At the same time, Cr and Nb carbides are distributed in the matrix in the form of dispersed particles, which not only retains a certain strengthening effect, but also releases the structural stress through tempering and improves the plasticity and toughness of lamellar cementite, thereby improving the strength-plasticity matching and structural uniformity, thus reducing the cracking risk in subsequent stranding and improving the fatigue resistance of the strand.

[0031] The higher the volume percentage of tempered sorbite and the finer the lamellar spacing, the higher the matrix strength. The higher the volume percentage of fused sorbite, the better the matrix ductility and toughness. In the preferred technical solution, the volume percentage of tempered sorbite is ≥57%, the lamellar spacing is 65~110nm, the volume percentage of ferrite is ≤5%, and the volume percentage of fused sorbite is ≥28%.

[0032] In the preferred technical solution, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same coil is ≤42MPa. The hot-rolled wire rod effectively inhibits the formation of deteriorating structures such as network carbide and martensite, avoids the formation of coarse pearlite in the core, and promotes the dispersed, fine and uniform distribution of carbides. This can effectively reduce the fluctuation of mechanical properties, promote the efficient and stable production of hot-rolled wire rod, and thus help reduce the risk of wire breakage during drawing and improve the twisting qualification rate and performance stability of strand manufacturing.

[0033] In the preferred technical solution, the diameter of the hot-rolled wire rod is 7~14mm, the tensile strength is 1461~1496MPa, and the reduction of area is 33%~38%. The hot-rolled wire rod has high strength, which can reduce the number of drawing passes, quickly reach the target diameter, and reduce the wire breakage sensitivity after the hot-rolled wire rod is made into steel wire with a thinner diameter. The hot-rolled wire rod also has a high reduction of area, which can further reduce the risk of wire breakage during drawing and twisting, avoid strand cracking, and improve fatigue resistance.

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

[0035] (1) In view of the shortcomings of existing hot-rolled wire rods for high-strength strands, which use high carbon content combined with Stellmore air-cooled wire or post-rolling salt bath treatment, it is difficult to control the deterioration of the microstructure such as network carbides and martensite, which affects rolling efficiency and has high energy consumption in salt bath treatment, this invention adopts a high carbon content design with trace Nb combined with online molten salt sorbitization control technology. It can use high-temperature wire drawing, increase the molten salt temperature through the front-end molten salt treatment, and reduce the molten salt circulation volume to control the wire rod to enter the sorbite phase region from the austenite state, forming a microstructure dominated by sorbite. The process involves a microstructure that inhibits the formation of network carbides and coarse pearlite, promotes the dispersed precipitation of Cr and Nb carbides, and then further transforms untransformed residual austenite into sorbite through subsequent molten salt treatment followed by isothermal tempering. This process inhibits the coarsening of martensite and carbides, promotes the melting of some sorbite lamellars, and finally, slow cooling on a roller table further toughens the microstructure. This process promotes a balance between strength and plasticity in the wire rod, improves the uniformity of the microstructure, reduces production energy consumption, and reduces restrictions on rolling, facilitating rapid rolling and promoting efficient and stable production of hot-rolled wire rod. It also exhibits good industrial adaptability.

[0036] (2) In view of the current situation where the hot-rolled wire rods used for high-strength stranded wire have a high risk of deterioration, insufficient uniformity of structure and strength and plasticity, which leads to an increased risk of wire breakage in the drawing and stranding processes of downstream wire drawing plants, the hot-rolled wire rods of the present invention use a high carbon composition containing trace amounts of Nb to control material costs. The microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can suppress deterioration structures such as network carbides and martensite. It utilizes the stronger interlayer bonding force of fine lamellar sorbite, and the Cr and Nb carbides are distributed in the matrix in the form of dispersed particles, retaining a certain strengthening effect. It can also release the structural stress through tempering, promote the matching of strength and plasticity of wire rods, and improve the uniformity of structure. It can achieve a tensile strength of 1461~1496MPa, a reduction of area of ​​33%~38%, and a mechanical property difference of ≤42MPa between coils. It is used in the manufacturing of 2160MPa grade stranded wire and other application fields, which is conducive to promoting the efficient and stable production of stranded wire 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.

[0042] Example 1:

[0043] A preferred embodiment of the manufacturing method of the 2160MPa 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: 0.91%, Si: 0.52%, Mn: 0.66%, Cr: 0.37%, Nb: 0.035%, P: 0.014%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt sorbitization control treatment → slow cooling on roller table → 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 the full dissolution of alloying elements such as Nb, reduces segregation, and avoids grain coarsening. After exiting the furnace, the billet is rolled into a 9mm diameter wire rod on a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet, improve rolling efficiency, and control strain-induced precipitation and grain refinement. Specifically, the furnace soaking temperature is controlled at 1... The rolling temperature is 210℃, the furnace time is 170 min, the initial rolling temperature is 1095℃, the initial rolling reduction is 39%, the final rolling temperature is 950℃, and the final rolling reduction is 24%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are 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 proeutectoid ferrite or network carbides, and providing a basis for promoting rapid sorbite phase transformation. Specifically, the wire drawing temperature is controlled at 940℃.

[0045] The online molten salt sorbitization control process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 39°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a high-temperature austenitic structure dominated by fine-laminated sorbite. This promotes the precipitation of Cr and Nb carbide nano-precipitates. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, where the molten salt temperature is increased and the molten salt circulation is appropriately reduced. The circulation process promotes the transformation of untransformed retained austenite into fine lamellar interlamellar sorbite, while simultaneously promoting isothermal tempering of the formed fine lamellar interlamellar sorbite. This causes partial melting of cementite lamellars within the sorbite, preventing carbide coarsening and abnormal low-temperature transformation of retained austenite, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 532℃, the treatment time is 141s, the molten salt circulation rate is 415t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 560℃, the treatment time is 140s, the molten salt circulation rate is 325t / 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 (≥220℃) from above the two salt bath tanks undergoing online molten salt martensitization control 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 0.6℃ / s. The coiling process involves coiling 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:

[0048] 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 homogenization temperature is controlled at 1175°C, the furnace time is 200 min, the initial rolling temperature is 1020°C, the initial rolling reduction is 28%, the final rolling temperature is 855°C, the final rolling reduction is 31%, and the wire drawing temperature is 830°C. The Steyrmo forced air cooling uses a fan with an air volume of 200,000 m³ / s. 3 At 90% capacity, fans 1 to 4 are turned on to cool the wire rod to 702℃ at a cooling rate of 8.3℃ / s. Then, fans 5 to 14 are turned on to 30% capacity to cool the wire rod to 265℃ at a cooling rate of 3.1℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0049] Comparative Example 2:

[0050] A method for manufacturing hot-rolled wire rod differs from that in Example 1 in that: the heating furnace homogenization temperature is controlled at 1165℃, the furnace time is 210 min, the initial rolling temperature is 1025℃, the initial rolling reduction is 28%, the final rolling temperature is 850℃, the final rolling reduction is 31%, and the wire drawing temperature is 830℃; during the molten salt treatment before the online molten salt sorbitization control process, the wire rod is cooled at a cooling rate of 29℃ / s, and the finished hot-rolled wire rod is obtained after exiting the line.

[0051] Example 2:

[0052] A preferred embodiment of the manufacturing method of the 2160MPa 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: 0.87%, Si: 0.58%, Mn: 0.86%, Cr: 0.28%, Nb: 0.024%, 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 sorbitization control treatment → slow cooling on roller table → coiling, specifically:

[0053] 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 the full dissolution of alloying elements such as Nb, reduces segregation, and avoids grain coarsening. After exiting the furnace, the billet is rolled into a 12mm diameter wire rod on a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet, improve rolling efficiency, and control strain-induced precipitation and grain refinement. Specifically, the furnace soaking temperature is controlled at 12... The rolling temperature is 25℃, the furnace time is 145 min, the initial rolling temperature is 1105℃, the initial rolling reduction is 41%, the final rolling temperature is 960℃, and the final rolling reduction is 22.5%. 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 conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing a basis for promoting rapid sorbite phase transformation. Specifically, the wire drawing temperature is controlled at 955℃.

[0054] The online molten salt sorbitization control process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 41°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a high-temperature austenitic structure dominated by fine-laminated sorbite. This promotes the precipitation of Cr and Nb carbide nano-precipitates. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, where the molten salt temperature is increased and the molten salt circulation is appropriately reduced. The circulation process promotes the transformation of untransformed retained austenite into fine lamellar interlamellar sorbite, while simultaneously promoting isothermal tempering of the formed fine lamellar interlamellar sorbite. This causes partial melting of cementite lamellars within the sorbite, preventing carbide coarsening and abnormal low-temperature transformation of retained austenite, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 523℃, the treatment time is 155s, the molten salt circulation rate is 485t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 555℃, the treatment time is 130s, the molten salt circulation rate is 372t / h, and the molten salt temperature rise is ≤3℃.

[0055] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥220℃) from above the two salt bath tanks undergoing online molten salt martensitization control 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 0.55℃ / s. The coiling process involves coiling 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.

[0056] Comparative Example 3:

[0057] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the online molten salt sorbitization control process, the wire rod is cooled at a cooling rate of 42°C / s, the molten salt temperature of the initial molten salt treatment is 495°C, the treatment time is 80s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0058] Comparative Example 4:

[0059] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the online molten salt sorbitization control process, the wire rod is cooled at a rate of 34°C / s, the molten salt temperature of the initial molten salt treatment is 570°C, the treatment time is 180s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0060] Example 3:

[0061] A preferred embodiment of the manufacturing method of the 2160MPa 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: 0.91%, Si: 0.49%, Mn: 0.75%, Cr: 0.26%, Nb: 0.015%, P: 0.014%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt sorbitization control treatment → slow cooling on roller table → coiling, specifically:

[0062] 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 the full dissolution of alloying elements such as Nb, reduces segregation, and avoids grain coarsening. After exiting the furnace, the billet is rolled into a 7mm diameter wire rod on a rolling line. Appropriate rolling temperature and reduction are selected to promote rapid rolling deformation of the billet, improve rolling efficiency, and control strain-induced precipitation and grain refinement. Specifically, the furnace soaking temperature is controlled at 1... The rolling temperature is 195℃, the furnace time is 185min, the initial rolling temperature is 1085℃, the initial rolling reduction is 43%, the final rolling temperature is 925℃, and the final rolling reduction is 26%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing a basis for promoting rapid sorbite phase transformation. Specifically, the wire drawing temperature is controlled at 925℃.

[0063] The online molten salt sorbitization process employs a two-stage salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first stage salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 36°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide region into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a predominantly fine-laminated sorbite structure within the high-temperature austenitic microstructure. This promotes the precipitation of Cr and Nb carbide nanoparticles. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, where the molten salt temperature is increased and the molten salt circulation is appropriately reduced. The circulation process promotes the transformation of untransformed retained austenite into fine lamellar interlamellar sorbite, while simultaneously promoting isothermal tempering of the formed fine lamellar interlamellar sorbite. This causes partial melting of cementite lamellars within the sorbite, preventing carbide coarsening and abnormal low-temperature transformation of retained austenite, 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 123s, the molten salt circulation rate is 355t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 565℃, the treatment time is 150s, the molten salt circulation rate is 305t / h, and the molten salt temperature rise is ≤3℃.

[0064] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥220℃) from the two salt bath tanks undergoing online molten salt martensitization control 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 0.78℃ / s. The coiling process involves coiling 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.

[0065] Comparative Example 5:

[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 580°C, the treatment time is 160s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0067] Comparative Example 6:

[0068] A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that: the molten salt temperature of the subsequent molten salt treatment is 500℃, the treatment time is 90s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0069] Example 4:

[0070] A preferred embodiment of the manufacturing method of the 2160MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.89%, Si: 0.66%, Mn: 0.84%, Cr: 0.35%, Nb: 0.032%, 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 sorbitization control treatment → slow cooling on roller table → coiling, specifically:

[0071] 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 the full dissolution of alloying elements such as Nb, reduces segregation, and avoids grain coarsening. 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, improve rolling efficiency, and control strain-induced precipitation and grain refinement. Specifically, the furnace soaking temperature is controlled at 1... The rolling temperature is 235℃, the furnace time is 125 minutes, the initial rolling temperature is 1115℃, the initial rolling reduction is 38%, the final rolling temperature is 965℃, and the final rolling reduction is 21%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing a basis for promoting rapid sorbite phase transformation. Specifically, the wire drawing temperature is controlled at 960℃.

[0072] The online molten salt sorbitization process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first stage salt bath for initial molten salt treatment. This process cools the wire rod at a rate of 42°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a high-temperature austenitic structure dominated by fine-laminated sorbite. This promotes the precipitation of Cr and Nb carbide nanoparticles. The wire rod is then conveyed via rollers through the second stage salt bath for final molten salt treatment, where the molten salt temperature is increased and the molten salt circulation is appropriately reduced. The circulation process promotes the transformation of untransformed retained austenite into fine lamellar interlamellar sorbite, while simultaneously promoting isothermal tempering of the formed fine lamellar interlamellar sorbite. This causes partial melting of cementite lamellars within the sorbite, preventing carbide coarsening and abnormal low-temperature transformation of retained austenite, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 510℃, the treatment time is 173s, the molten salt circulation rate is 550t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 550℃, the treatment time is 120s, the molten salt circulation rate is 395t / h, and the molten salt temperature rise is ≤3℃.

[0073] The slow cooling process of the roller conveyor uses controlled opening of the insulation cover to blow hot air (≥220℃) from the two salt bath tanks of the online molten salt sorbitization control 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 277℃ at a slow cooling rate of 0.35℃ / 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.

[0074] Comparative Example 7:

[0075] A method for manufacturing hot-rolled wire rod differs from that in Example 4 in that the manufacturing method follows a process flow of rolling → wire drawing → online molten salt sorbitization control treatment → air cooling. The air cooling process involves opening the heat insulation cover and controlling the wire rod to cool to 270°C at a slow cooling rate of 1.5°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0076] 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:

[0077] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0078]

[0079] As can be seen from the comparison results of Example 1 and Example 1, compared with the production using Stellmore air-cooled wire, which introduces problems such as network carbides, martensite, and coarse pearlite, resulting in insufficient strength and plasticity, large fluctuations in mechanical properties, and affecting the production and service performance of stranded wire, this invention uses a high-carbon composition containing trace amounts of Nb combined with online molten salt sorbitization control technology. This allows for high-temperature wire spinning, increasing the molten salt temperature through pre-treatment, reducing the molten salt circulation volume, and controlling the wire rod to transition from the austenitic state to the sorbite phase region, forming a structure dominated by sorbite, suppressing network carbides and coarse pearlite, and promoting the dispersed precipitation of Cr and Nb carbides. Then, through... The subsequent molten salt treatment promotes the continued transformation of untransformed residual austenite into sorbite and isothermal tempering, inhibits the coarsening of martensite and carbides, promotes the melting of some sorbite lamellars, and finally, the microstructure is further toughened by slow cooling on a roller table. As can be seen from the results of Examples 1 to 4, the microstructure includes a mixed microstructure composed of tempered sorbite, ferrite and melted sorbite. The hot-rolled wire rod can achieve a tensile strength of 1461~1496MPa, a reduction of area of ​​33%~38%, and a mechanical property difference of ≤42MPa between the same rings. It is used in applications such as manufacturing 2160MPa grade stranded wire, which is conducive to promoting the efficient and stable production of hot-rolled wire rod and stranded wire.

[0080] As can be seen from the comparison results of Example 1 and Example 2, selecting a higher wire drawing temperature keeps the wire rod in a high-temperature austenitic state, avoiding the premature precipitation of proeutectoid ferrite or network carbides, which provides a basis for the subsequent formation of stable supercooled austenite and promotes rapid sorbite phase transformation. At the same time, it can reduce the limitation on rolling temperature. Selecting a higher initial rolling temperature reduces the rolling deformation resistance, reduces the load requirements on the rolling line and the wear effect, and achieves control of strain-induced precipitation and grain refinement.

[0081] As can be seen from the comparison results of Example 2 and Example 3, the lower the molten salt temperature in the initial molten salt treatment, the better it is for rapid cooling, suppression of network carbides and coarse pearlite, improvement of the driving force for the precipitation of Cr and Nb nano-precipitates, and suppression of cementite growth. With the extension of the treatment time, it can promote the transformation of high-temperature austenite to sorbite with finer lamellar spacing, reduce the content of residual austenite, and improve the matrix strength. However, if the molten salt temperature is too low, it will deviate from the nose region of sorbite phase transformation, which will slow down the driving force for the transformation of austenite to sorbite, and may even form a low-temperature brittle structure. With the excessively long treatment time, it will increase the production energy consumption.

[0082] As can be seen from the comparison results of Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the better it is to reduce the temperature difference between the wire rod surface and the core, and reduce the risk of low-temperature brittle structure formation. With the shortening of the treatment time, production energy consumption can be reduced. However, if the molten salt temperature is too high, it is not conducive to suppressing network carbides, promoting the refinement of sorbite lamellars and the precipitation of nano-precipitates. With the treatment time being too short, the nucleation rate of sorbite will be slowed down, and too much untransformed residual austenite will reduce the matrix strength.

[0083] As can be seen from the comparison results of Example 3 and Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the better it is to increase the atomic diffusion rate and promote the local melting of cementite lamellars in sorbite at high temperature. As the treatment time is extended, the dislocations inside the sorbite rearrange through climb and cross-slip, and the carbides are evenly distributed, which can reduce the structural stress and improve the plasticity deterioration and fatigue resistance of the wire rod. However, if the molten salt temperature is too high, it is not conducive to inhibiting the transformation of untransformed residual austenite into sorbite. As the treatment time is too long, too much sorbite melts, and the excessive aggregation and growth of cementite and nano-precipitates will become fatigue crack sources, resulting in loss of strength and plasticity and increased production energy consumption.

[0084] As can be seen from the comparison results of Example 3 and Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the better it is to promote the decomposition of untransformed residual austenite and inhibit the coarsening of sorbite phase transformation. With the shortening of treatment time, the growth of Cr and Nb carbides can be inhibited and the production energy consumption can be reduced. However, if the molten salt temperature is too low and the treatment time is too short, it will affect the melting of sorbite lamellars and stress release, which will result in loss of plasticity, fluctuation of mechanical properties, and even the formation of deteriorated structure during subsequent cooling due to austenite residue.

[0085] As can be seen from the comparison results of Example 4 and Example 7, the slow cooling of the roller can further control the cooling speed, maintain the high temperature state of the wire rod after exiting the molten salt, promote further toughening of the wire rod structure, improve the tempering and softening effect of the wire rod, and at the same time recover and utilize the heat energy of online molten salt sorbitization regulation, reduce production energy consumption, and promote the rapid exit of the wire rod.

[0086] 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 2160MPa grade stranded wire, characterized in that, Its manufacturing methods include: 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: 0.87%~0.91%, Si: 0.49%~0.66%, Mn: 0.66%~0.86%, Cr: 0.26%~0.37%, Nb: 0.015%~0.035%, 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 ≥925℃ and then processed online. Molten salt sorbitization control treatment involves first subjecting the wire rod to a preliminary molten salt treatment and cooling it at a rate of ≥36℃ / 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, followed by isothermal tempering and promoting partial melting of sorbite lamellars. Finally, the wire rod undergoes slow cooling via a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted sorbite.

2. The method for manufacturing hot-rolled wire rod for 2160MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1195~1235℃ and the furnace time is 125~185min.

3. The method for manufacturing hot-rolled wire rod for 2160MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1085~1115℃, the initial rolling reduction is 38%~43%, the final rolling temperature is 925~965℃, and the final rolling reduction is 21%~26%.

4. The method for manufacturing hot-rolled wire rod for 2160MPa grade stranded wire according to claim 1, characterized in that, During the spinning process, the spinning temperature is controlled at 925~960℃.

5. The method for manufacturing hot-rolled wire rod for 2160MPa grade stranded wire according to claim 1, characterized in that, The molten salt temperature of the first stage molten salt treatment is 510~545℃, and the treatment time is 123~173s; the molten salt temperature of the second stage molten salt treatment is 550~565℃, and the treatment time is 120~150s.

6. The method for manufacturing hot-rolled wire rod for 2160MPa grade stranded wire according to claim 5, characterized in that, The molten salt circulation rate of the first-stage molten salt treatment is 355~550t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation rate of the second-stage molten salt treatment is 305~395t / h, and the molten salt temperature rise is ≤3℃.

7. The method for manufacturing hot-rolled wire rod for 2160MPa grade stranded wire according to claim 5, characterized in that, The roller conveyor slow cooling control strip is cooled to below 280°C at a slow cooling rate of ≤0.78°C / s before being wound up.

8. A hot-rolled wire rod for 2160MPa grade stranded wire, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of hot-rolled wire rod for 2160MPa grade strand as described in any one of claims 1 to 7.

9. The hot-rolled wire rod for 2160MPa grade stranded wire according to claim 8, characterized in that, The volume percentage of tempered sorbite is ≥57%, the lamellar spacing is 65~110nm, the volume percentage of ferrite is ≤5%, the volume percentage of melted sorbite is ≥28%, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤42MPa.

10. The hot-rolled wire rod for 2160MPa grade stranded wire according to claim 8, characterized in that, The hot-rolled wire rod has a diameter of 7~14mm, a tensile strength of 1461~1496MPa, and a reduction of area of ​​33%~38%.

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