A hot-rolled wire rod for 2130MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a hot-rolled wire rod for 2130MPa grade stranded wire and its manufacturing method. The method involves rolling the wire rod into wire using a high-carbon C-Si-Mn-Cr-Nb composition system, followed by online molten salt sorbitization treatment. This process cools the wire rod at a rate of ≥35℃ / s, transitioning it from the austenitic state to the sorbite phase region, forming a sorbite-dominant microstructure. The wire rod is then isothermally tempered to promote partial melting of the sorbite lamellars. Finally, it undergoes slow cooling on a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of tempered sorbite, ferrite, and melted sorbite. This method simplifies the composition system, improves the strength and plasticity matching and microstructure uniformity of the wire rod, achieving a tensile strength of 1443~1491MPa and a reduction of area of ​​35%~40%, while also considering production efficiency. This method is suitable for manufacturing 2130MPa grade stranded wire and other applications, eliminating the need for offline heat treatment and promoting stable stranded wire production.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a 2130MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] Steel strand, as a stress-bearing structure, is widely used in bridges, power, water conservancy, roads, and other fields. With the advancement of industry, the strength grade of steel strand is constantly improving, and the requirements for high strength and lightweight are gradually increasing. As the strength grade of steel strand increases, the production of raw wire rod and the manufacturing of downstream drawn strands become increasingly difficult. In particular, after the strength grade of steel strand is increased to the ultra-high strength grade of 2130MPa, the production of raw wire rod requires additional offline heat treatment to improve the uniformity of the microstructure, which is necessary to ensure the manufacturing of downstream drawn strands. However, this significantly increases the production cost and energy consumption. Therefore, it is necessary to develop a hot-rolled wire rod for 2130MPa grade strand and its manufacturing method to eliminate the need for offline heat treatment, promote stable strand production, and meet the development needs of the steel industry and market demand.

[0003] Existing high-strength hot-rolled wire rods for stranded wire generally employ a high-carbon alloy composition system, combined with Steyrmo air-cooling after wire drawing. For example, patent CN102352469B discloses an ultra-high-strength vanadium-titanium composite microalloyed high-carbon steel wire rod and its preparation method. This method uses a C-Si-Mn-V-Ti-Cr-Cu-Ni composition design, combined with a Steyrmo air-cooling process involving rapid cooling, heat preservation, and slow cooling to produce a sorbitic wire rod, achieving a tensile strength of 1330–1410 MPa and a reduction of area ≥32%. However, it still has the following drawbacks:

[0004] I. To refine austenite grains, delay pearlite transformation, and lower phase transformation temperature to improve strengthening effects, wire rods contain high Si content and various alloying elements. However, this also increases smelting difficulty and alloy cost. Furthermore, the high resistance to rolling deformation increases the risk of stress concentration and affects rolling efficiency. To reduce material costs and smelting / rolling difficulty, simplifying the alloy composition and reducing alloy content leads to several issues. Firstly, macroscopic carbon segregation intensifies during solidification. Limited by the maximum cooling capacity of the Steyrmo air-cooling line, carbides are more likely to accumulate at austenite grain boundaries and form a network structure during cooling. These coarse network carbides can further exacerbate the problem. This not only breaks the matrix, affects the uniformity of the microstructure, and significantly degrades the ductility and toughness, but also loses the carbon source for the sorbite phase transformation and weakens the strengthening effect of carbon. On the other hand, due to the uncontrolled growth of austenite grains and the decrease in austenite stability, the pearlite transformation temperature range shifts upward. Limited by the maximum cooling capacity of the Stellmore air-cooled wire, the wire rod slowly passes through the high-temperature section. At high temperatures, atomic diffusion is fast, and pearlite lamellae are prone to grow rapidly and coarsen, further aggravating the microstructure coarsening, resulting in greater microstructure stress and loss of strong plasticity. If the initial strength of the wire rod is insufficient, more drawing passes are required to reach the target strength, which increases the risk of wire breakage and affects the yield of stranded wire.

[0005] II. To minimize network carbides and promote the refinement of pearlite lamellar structure, low-temperature rolling combined with strong air cooling after wire drawing is employed. However, low-temperature rolling increases wear on the rolling line and affects rolling efficiency. Furthermore, it is limited by the unstable temperature control of the air cooling line. On the one hand, reducing the content of alloys such as Si decreases the stability of austenite. Combined with the omission of alloying elements such as Ni, the martensitic transformation initiation temperature increases. Influenced by the segregation of alloying elements, strong air cooling further increases the temperature difference between the air-receiving and air-receiving surfaces of the wire rod, and between overlapping and non-overlapping areas. The wire rod is prone to forming brittle abnormal structures such as martensite due to localized overcooling, while the core cools too slowly, forming a pearlite structure. Uniformity is affected, and the hardness difference between martensite and pearlite is extremely large, which can easily lead to stress concentration in the microstructure, a sharp decrease in plasticity and toughness, and the risk of wire breakage. On the other hand, the hardenability of the wire rod is reduced, and the continuous cooling of the air-cooling line can easily cause local phase transformation lag and large differences in phase transformation products. The length of the air-cooling line and the slow cooling stage are limited. Due to the uneven microstructure transformation and the inability to slowly release the volume expansion generated by the core phase transformation, the final wire rod has a large residual internal stress, which can easily crack during subsequent drawing and twisting. It is impossible to break through the production bottleneck of offline heat treatment. Excessively reducing the wire rod transportation speed and extending the heat preservation time will also lead to a longer offline time and affect production efficiency. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the above-mentioned technical problems. The present invention provides a hot-rolled wire rod for stranded wire of 2130MPa grade and its manufacturing method, which can simplify the composition system, improve the matching of strength and plasticity properties and the uniformity of structure of the wire rod, and take into account the production efficiency, so as to eliminate offline heat treatment and promote stable production of 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 2130MPa grade stranded wire, the method comprising:

[0009] The hot-rolled wire rod is rolled into wire rod according to its chemical composition. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.86%~0.90%, Si: 0.35%~0.48%, Mn: 0.75%~0.95%, Cr: 0.12%~0.25%, Nb: 0.01%~0.03%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a spinning temperature of ≥920℃, it undergoes online molten salt sorbitization control treatment, which cools the wire rod at a cooling rate of ≥35℃ / s, allowing it to transition from the austenitic state to the sorbite phase region, forming a structure dominated by sorbite. It is then isothermally tempered to promote the melting of some sorbite lamellars. Finally, it undergoes slow cooling on a roller table 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 a solid solution strengthening and carbide strengthening element in steel. It is relatively cheaper and can be dissolved in ferrite to improve the strength of the matrix. It forms fine carbides with elements such as Nb and Cr to provide dispersion strengthening. At the same time, it can expand the austenite phase region, improve the stability of austenite, and lower the starting temperature of the transformation from austenite to sorbite. This is conducive to promoting the rapid transformation and refinement of sorbite. However, if the C content is too high, it will aggravate the center segregation, easily enrich and precipitate continuous network carbides at the austenite grain boundary, and at the same time, the phase transformation incubation period will be prolonged, the uniformity of the structure and the difficulty of controlling the abnormal structure of martensite will increase, resulting in increased brittleness and mechanical property fluctuations of wire rod. Therefore, in order to take into account the high strength requirements of 2130MPa grade strand, improve the uniformity of the structure and promote rapid production, the mass percentage of C is controlled at 0.86%~0.90%.

[0012] (2) Silicon: Si is a solid solution strengthening element. By dissolving in the sorbite matrix phase, it can hinder dislocation movement and improve matrix strength. During the austenite-to-sorbite phase transformation, it can reduce the pearlite phase transformation temperature and delay the pearlite transformation so as to form a structure dominated by sorbite. At the same time, it can increase the solubility of Nb in austenite and reduce the precipitation temperature of NbC. During isothermal tempering, it can hinder the premature spheroidization or coarsening of cementite, improve tempering stability, and avoid insufficient drawing strength due to softening. It can also refine the precipitated phase and improve the toughness of the material. However, excessive silicon will promote decarburization, prolong the phase transformation incubation period, increase the resistance to dislocation movement, increase the difficulty of isothermal tempering softening, reduce plasticity index, and cause fracture during drawing or twisting. Therefore, in order to adapt to the control of phase transformation structure by online molten salt sorbitization control and control the resistance to tempering softening, the mass percentage of Si is controlled at 0.35%~0.48%.

[0013] (3) Manganese: Mn is an austenite stabilizing element that can delay the transformation of austenite to pearlite, reduce the critical cooling rate of sorbite phase transformation, slow down the precipitation temperature of NbC, promote the refinement of sorbite lamellars, and improve the hardenability of wire rod, so that the wire rod can fully complete the sorbite transformation in molten salt. During isothermal tempering, it can also hinder the diffusion and migration of carbon atoms, thereby inhibiting the coarsening of carbides and keeping the wire rod with high strength. However, if the Mn content is too high, it will increase the segregation of alloying elements during the solidification process of the billet, increase the resistance to hot rolling deformation, and form martensite or bainite hard spots during cooling, resulting in increased brittleness and mechanical property fluctuations, affecting the uniformity and toughness of the wire rod structure. Therefore, in order to take into account the high strength and plasticity of hot-rolled wire rod and control the difficulty of controlling the uniformity of the structure, the mass percentage of Mn is controlled at 0.75%~0.95%.

[0014] (4) Chromium: Cr is a strong carbide-forming element. It can form carbides and distribute them in the sorbite lamellae, which can disperse and strengthen the matrix by hindering dislocation movement and improving the matrix strength. At the same time, it can enhance hardenability and reduce the sorbite phase transformation temperature. During the low-temperature phase transformation, the atomic diffusion rate is slow and the sorbite lamellae spacing is finer. It also inhibits the growth and spheroidization of carbides in the sorbite lamellae, increases the resistance to tempering softening, and enables the wire rod to maintain high strength after tempering. However, if the Cr content is too high, it will aggravate the segregation of alloying elements, increase the risk of abnormal structures such as martensite precipitation, and increase the high-temperature deformation resistance of steel during the rolling stage. It will also reduce the activity of carbon during isothermal tempering, which is not conducive to sorbite melting and tempering softening, and affects the drawing plasticity of steel. Therefore, in order to control the difficulty of the uniformity of the structure and reduce the difficulty of tempering softening, the Cr content should be appropriately reduced. The mass percentage of Cr should be controlled at 0.12%~0.25%.

[0015] (5) Niobium: Nb precipitates fine particles in the high-temperature austenite region, which strongly inhibits austenite grain growth through the pinning grain boundary effect, providing a fine-grained matrix for subsequent sorbitization. Fine grains not only improve strength through fine grain strengthening, but also improve plasticity and alleviate the brittleness problem of high carbon steel. During the molten salt treatment process, Nb will further precipitate nano-sized NbC, which can hinder dislocation movement, improve strength and toughness, make the dislocation distribution more uniform during the drawing process of wire rod, and reduce the cold work hardening rate. 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 0.01%~0.03%.

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

[0017] The aforementioned wire rod adopts a high-carbon composition system of C-Si-Mn-Cr-Nb, which does not contain elements such as V and Ni. The Si and Cr contents are relatively low. Through solid solution strengthening by Si and Mn, precipitation strengthening by Cr and Nb, and fine grain strengthening dominated by Nb with trace amounts of Nb, the composition can be simplified, material costs can be appropriately reduced, and rolling deformation resistance can be improved. At the same time, the hardenability of the wire rod can be controlled, and the sorbite phase transformation temperature can be lowered. This provides favorable conditions for delaying the pearlite phase transformation, promoting the refinement and full transformation of sorbite lamellars, reducing the uniformity of the microstructure and the difficulty of melting sorbite lamellars, suppressing excessive softening, and promoting rapid production. On this basis, a higher wire drawing temperature is selected to ensure that the wire rod is in a high-temperature austenitizing state, avoiding the formation of network carbides due to low-temperature wire drawing. At the same time, it prepares the microstructure for increasing the undercooling and promoting sorbite nucleation. After wire drawing, the wire rod does not undergo air cooling and is directly put into molten salt for online molten salt sorbitization control.

[0018] Firstly, compared to simplifying the alloy composition and reducing the alloy content, which limits the maximum cooling capacity of the Steyr air-cooled line and makes it impossible to control the network carbide and coarse pearlite structure, the advantages of molten salt can be improved. On the one hand, the significantly higher thermal conductivity of molten salt compared to air can be used to promote the rapid transition of the wire rod from the high-temperature austenitic state to the precipitation temperature range of secondary cementite, preventing carbides from agglomerating at the austenitic grain boundaries to form a network structure. This avoids the adverse effects of network carbides on the uniformity and plasticity of the matrix. On the other hand, the wire rod can directly enter the lower temperature range of sorbite phase transformation. Combined with the grain-refining effect of Nb and the large supercooling, the precipitation of proeutectoid ferrite and coarse pearlite can be suppressed. The slow atomic diffusion rate during low-temperature phase transformation can promote the refinement of sorbite lamellars and enhance the strengthening effect of carbon, thereby compensating for the strength loss caused by simplifying the alloy composition and reducing the alloy content.

[0019] Second, compared to the limitations of the minimum cooling capacity and continuous cooling of the Stellmore air-cooled line, which makes it impossible to control abnormal structures such as martensite, and results in insufficient uniformity and strength, the molten salt treatment offers several advantages. Firstly, when the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for uniform heat exchange. Unlike air cooling, there is no temperature difference between the air-receiving and air-receiving surfaces, or between overlapping and non-overlapping areas. This avoids localized overcooling and the formation of brittle structures such as martensite. Furthermore, as the processing time increases, the temperature difference between the edge and core of the wire rod can be further reduced, promoting a full transformation of austenite to sorbite, increasing precipitation kinetics, and promoting the formation of fine carbides from Cr and Nb to provide dispersion strengthening. This prevents the formation of coarse pearlite in the core due to slow cooling, which increases phase transformation stress. Combined with phase transformation inoculation, this helps prevent austenite formation in the core. The residual material continues to form low-temperature brittle structures such as martensite during the subsequent slow cooling process on the roller conveyor, avoiding the loss of wire rod plasticity. On the other hand, during the phase transformation, the wire rod can gradually transform to the same temperature as the molten salt, allowing for isothermal tempering instead of continuous cooling on the air-cooling line to the low-temperature state after the phase transformation. As the processing time is extended, it can promote the melting of some cementite lamellars in the sorbite, further rapidly releasing the phase transformation stress and improving the uniformity of the structure, while avoiding premature spheroidization or coarsening of cementite that would result in a loss of strength. Finally, the slow cooling on the roller conveyor further extends the isothermal softening effect of the later stage of online molten salt sorbitization control, promoting further toughening of the wire rod structure and achieving a matching control of the structure state and strength and plasticity, so as to avoid offline heat treatment and reduce the risk of cracking in subsequent processing.

[0020] Before rolling, a higher heating furnace temperature and an appropriate furnace time can be selected to promote the full dissolution of alloying elements such as Nb, alleviate the effect of segregation, reduce rolling deformation resistance and avoid overheating. In the preferred technical solution, before rolling, the heating furnace temperature is controlled at 1185~1225℃ and the furnace time is 115~175min.

[0021] Because the alloy composition is simplified, the heating furnace soaking temperature and wire drawing temperature are higher. During the rolling process, a larger initial rolling temperature can be used to reduce the mill load, reduce roll wear on the rolling line, and improve rolling speed and efficiency. Combined with a larger initial rolling reduction, coarse grains and columnar grains in the microstructure are broken down, dislocation density is increased, and a more uniform and fine microstructure is provided for subsequent final rolling. Further control of the final rolling temperature and final rolling reduction promotes Nb precipitation and pinning of grain boundaries, which can refine austenite grains and offset the risk of grain growth that may be caused by high temperature. Fine austenite grains provide more sites for sorbite nucleation, which is conducive to promoting rapid sorbite nucleation and refinement in the subsequent process. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1040~1080℃, the initial rolling reduction is 35%~40%, the final rolling temperature is 925~965℃, and the final rolling reduction is 20%~25%.

[0022] During the wire spinning process, the spinning temperature can be further controlled to avoid excessive high-temperature residence time leading to austenite grain growth. In a preferred embodiment, the spinning temperature is controlled at 920~950℃.

[0023] In a preferred technical solution, the online molten salt sorbitization control process is divided into a pre-stage molten salt treatment and a post-stage molten salt treatment. The molten salt circulation volume in the post-stage molten salt treatment is less than that in the pre-stage molten salt treatment. By appropriately reducing the molten salt circulation volume, production energy consumption can be further reduced.

[0024] The molten salt temperature in the initial molten salt treatment is within the sorbite phase region. Lower molten salt temperatures promote rapid cooling of the wire rod, increase undercooling, suppress network carbides, coarse pearlite, and proeutectoid ferrite, and promote the refinement of sorbite lamellars. Simultaneously, the atomic diffusion rate decreases, and the NbC nucleation rate increases, but growth is limited. With prolonged treatment time, the temperature difference between the wire rod surface and core is reduced, promoting the full transformation of austenite to sorbite and the dispersed precipitation of carbides, thus improving matrix strength. However, excessively low molten salt temperatures can increase structural stress and even induce brittle structures such as martensite, affecting the uniformity of precipitate distribution. Longer treatment times also increase production energy consumption. Conversely, higher molten salt temperatures reduce the risk of brittle structures such as martensite, lower structural stress, and reduce softening difficulty. Shorter treatment times allow for... While reducing production energy consumption and limiting the growth of NbC microalloyed carbides, excessively high molten salt temperatures accelerate the uniform diffusion of carbon, affecting the control of network carbides and pearlite, and hindering the suppression of carbide coarsening. Furthermore, excessively short treatment times increase the temperature difference between the wire rod surface and the core, affecting the control of microstructure uniformity. Therefore, the initial molten salt treatment allows for control of the molten salt temperature and treatment time, rapidly transitioning from the high-temperature austenitic state through the network carbide region to the sorbite phase region. This suppresses the formation of network carbides and fosters a microstructure dominated by fine-laminated interlamellar sorbite, promoting the dispersed precipitation of microalloyed carbides and improving microstructure uniformity. This prepares the microstructure for the subsequent molten salt treatment. In the preferred embodiment, the molten salt temperature for the initial molten salt treatment is 533~563℃, and the treatment time is 78~118s.

[0025] Since the coiling temperature differs significantly from the molten salt temperature in the preceding molten salt treatment, using a larger molten salt circulation rate can promote rapid cooling of the wire rod, control the molten salt temperature rise, and improve the consistency of wire rod performance. In the preferred technical solution, the molten salt circulation rate in the preceding molten salt treatment is 380~545t / h, and the molten salt temperature rise is ≤8℃.

[0026] Higher molten salt temperatures and longer treatment times in the subsequent molten salt treatment are beneficial for driving the cementite lamellars of the sorbite to melt and fracture, resulting in a more even stress distribution across the entire cross-section of the wire rod. This provides sufficient time for the short-range diffusion of Nb atoms, improving the matrix's ductility and toughness and reducing the risk of processing cracks. However, excessively high molten salt temperatures weaken the driving force for the transformation of austenite to sorbite. With prolonged treatment times, this accelerates the diffusion-type coarsening of cementite or microalloyed carbides. Larger cementite or carbides become stress concentration sources, and the grown NbC cannot impede dislocation movement, leading to a loss of strength and ductility, and affecting fatigue life under long-term stress. Conversely, lower molten salt temperatures promote the continued transformation of untransformed residual austenite into a fine-lamellar sorbite structure. With longer treatment times... Short processing time is beneficial for reducing production energy consumption and suppressing excessive coarsening of cementite and carbides. However, if the molten salt temperature is too low and the processing time is too short, thermal dynamics will be lost, affecting the melting of sorbite lamellars, stress release, and microstructure uniformity, resulting in loss of plasticity and toughness and fluctuations in mechanical properties. Therefore, the subsequent molten salt treatment can further control the molten salt temperature and processing time to promote the continued transformation of untransformed residual austenite into fine lamellar sorbite. At the same time, it promotes the formation of fine lamellar sorbite for a long time of isothermal tempering, causing some cementite lamellars to melt and suppressing the coarsening of microalloyed carbides, so as to regulate the strength and plasticity matching of wire rod and promote rapid production. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 513~538℃ and the processing time is 100~155s.

[0027] After the initial molten salt treatment, the temperature difference between the wire rod temperature and the molten salt temperature in the subsequent molten salt treatment is small. This allows for a suitable reduction in the molten salt circulation rate, control of the molten salt temperature rise, and reduction of production energy consumption. In the preferred technical solution, the molten salt circulation rate in the subsequent molten salt treatment is 280~380t / h, and the molten salt temperature rise is ≤3℃.

[0028] Since the wire rod has already been rapidly isothermally tempered and softened after the subsequent molten salt treatment, the slow cooling on the roller table does not require a long period of heat preservation. The cooling rate can be further controlled to promote further toughening of the wire rod structure, improve the tempering and softening effect of the wire rod, and avoid affecting the production line speed and efficiency. In the preferred technical solution, the slow cooling on the roller table controls the wire rod to cool to below 275°C at a slow cooling rate of 0.5~0.9°C / s before winding.

[0029] 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 ≥200℃ 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, thereby reducing production energy consumption.

[0030] A hot-rolled wire rod for stranded wire of 2130MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method of the hot-rolled wire rod for stranded wire of 2130MPa grade as described in any one of the above-mentioned methods.

[0031] The aforementioned wire rods are designed with a high carbon content containing trace amounts of Nb, which simplifies the composition, controls material costs, and reduces the difficulty of smelting and rolling. The microstructure consists of a mixed structure mainly composed of tempered sorbite and melted sorbite, with a small amount of ferrite. This effectively suppresses abnormal structures such as network carbides and martensite. The sorbite has finer lamellar spacing, stronger dislocation movement resistance, and higher strength than pearlite, which can suppress the strength loss caused by pearlite and excessive ferrite soft phases. After isothermal tempering and partial cementite lamellar melting, the sorbite transforms into a spheroidized structure. The intermediate transition state of tempered sorbite and melted sorbite during the microstructure transformation, combined with Cr and Nb to form fine carbides, provides dispersion strengthening and enhances the strengthening effect of carbon. While retaining the high strength of sorbite, it can further release structural stress, compensate for the strength and toughness loss caused by simplifying alloy composition and reducing alloy content, effectively improve the microstructure uniformity and strength-plasticity matching. Therefore, in the manufacturing of high-strength stranded wire, it can reduce the risk of wire breakage and cracking during drawing and twisting, and improve the yield of stranded wire, without the need for subsequent offline heat treatment.

[0032] The higher the volume percentage of tempered sorbite and the finer the lamellar spacing in the microstructure, the higher the wire rod strength; the higher the proportion of fused sorbite, the better the ductility and toughness. In the preferred technical solution, the volume percentage of tempered sorbite is ≥68%, the lamellar spacing is 70~120nm, the volume percentage of ferrite is ≤6%, and the volume percentage of fused sorbite is ≥15%.

[0033] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference of ≤39MPa between coils. This can effectively avoid the adverse effects of network carbides on the continuity and ductility of the matrix, prevent early fatigue fracture of the strand during service caused by network carbides, and effectively suppress coarse pearlite and martensite, thereby further improving the uniformity of the structure, reducing the fluctuation of mechanical properties, and avoiding stress concentration cracking under subsequent stress.

[0034] In the preferred technical solution, the hot-rolled wire rod has a diameter of 8~15mm, a tensile strength of 1443~1491MPa, and a section reduction rate of 35%~40%. The hot-rolled wire rod base material has high tensile strength, which can reduce the number of drawing passes, promote the strand to quickly reach the target strength, meet the load-bearing requirements of the strand, reduce plastic loss during the process, and improve production efficiency. At the same time, it has a high section reduction rate, which can eliminate the need for offline heat treatment, ensure the ability to withstand large deformation during drawing, reduce the risk of surface cracks, core splitting and wire breakage during drawing and twisting, promote stable production of strand, and improve the yield.

[0035] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0036] (1) In view of the current situation that hot-rolled wire rods for stranded wire are produced by Stellmore air-cooling line, resulting in high material cost and difficulty in achieving both strength, plasticity and uniformity of structure, this invention combines Nb-containing composition design with online molten salt sorbitization control technology to control the wire rod to quickly bypass the network carbide region from the high-temperature austenitic state and enter the sorbite phase region, suppressing the formation of network carbides and forming a structure dominated by fine lamellar sorbite structure. At the same time, it promotes the formation of fine lamellar sorbite structure to undergo long-term isothermal tempering and causes some sorbite lamellars to melt. Finally, the slow cooling of the roller table promotes further toughening of the wire rod structure. This invention simplifies the composition system, improves the matching of strength and plasticity of wire rod and the uniformity of structure, and reduces the restrictions on rolling and slow cooling of the roller table, taking into account production efficiency and having good industrial adaptability.

[0037] (2) In view of the current situation that hot-rolled wire rods for stranded wire have high alloy content and many components, resulting in high material cost, increased difficulty in stranded wire manufacturing, and the need for additional offline heat treatment, this invention adopts a high carbon composition design with trace amounts of Nb, which can simplify the composition, control material cost, and reduce the difficulty of smelting and rolling. The microstructure includes tempered sorbite, melted sorbite and ferrite, which can effectively suppress abnormal structures such as network carbides and martensite, suppress the strength loss caused by pearlite, and form fine carbides with Cr and Nb to provide dispersion strengthening, thereby improving the strengthening effect of carbon elements. By controlling the microstructure through isothermal tempering, the strength and plasticity of the wire rod can be improved, and the tensile strength can reach 1443~1491MPa, with a section reduction rate of 35%~40%. It can be used to manufacture 2130MPa grade stranded wire and other application fields. It eliminates the need for subsequent offline heat treatment, reduces the risk of wire breakage and cracking during drawing and twisting, improves the yield of stranded wire, and has good market application prospects. Attached Figure Description

[0038] 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:

[0039] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention;

[0040] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention;

[0041] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0042] 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.

[0043] Example 1:

[0044] A preferred embodiment of the manufacturing method of the 2130MPa 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: 0.48%, Mn: 0.75%, Cr: 0.18%, Nb: 0.019%, 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:

[0045] 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 the full dissolution of alloying elements such as Nb, reduce segregation, and prevent overheating. 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 1... The rolling temperature is 200℃, the furnace time is 160 min, the initial rolling temperature is 1055℃, the initial rolling reduction is 38%, the final rolling temperature is 940℃, and the final rolling reduction is 23%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller table and conveyed along the roller table. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding the precipitation of network carbides during the wire drawing stage, and providing favorable conditions for the subsequent sorbitic phase transformation. Specifically, the wire drawing temperature is controlled at 935℃.

[0046] The online molten salt martensitization control 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 38°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, promoting the dispersed precipitation of NbC microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for final molten salt treatment, with a reduced molten salt circulation rate. The process promotes the transformation of untransformed residual 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, resulting in a more even stress distribution and preventing carbide coarsening. This helps to control the strength-ductility balance of the wire rod. Specifically: the molten salt temperature for the first stage of the molten salt treatment is 552℃, the treatment time is 85s, the molten salt circulation rate is 435t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature for the second stage of the molten salt treatment is 529℃, the treatment time is 142s, the molten salt circulation rate is 310t / h, and the molten salt temperature rise is ≤3℃.

[0047] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) 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 272℃ at a slow cooling rate of 0.8℃ / 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.

[0048] Comparative Example 1:

[0049] 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 1170°C, the furnace time is 205 min, the initial rolling temperature is 1000°C, the initial rolling reduction is 25%, the final rolling temperature is 850°C, the final rolling reduction is 30%, and the wire drawing temperature is 825°C. The Steyrmo forced air cooling uses a fan with an air volume of 200,000 m³ / s. 3 At 70% capacity, fans 1 to 5 are turned on to cool the wire rod to 690℃ at a cooling rate of 6.7℃ / s. Then, fans 6 to 14 are turned on to 35% capacity to cool the wire rod to 270℃ at a cooling rate of 2.6℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0050] Comparative Example 2:

[0051] A method for manufacturing hot-rolled wire rod differs from that in Example 1 in that: the heating furnace homogenization temperature is controlled at 1170℃, the furnace time is 205min, the initial rolling temperature is 1000℃, the initial rolling reduction is 25%, the final rolling temperature is 850℃, the final rolling reduction is 30%, and the wire drawing temperature is 825℃; 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.

[0052] Example 2:

[0053] A preferred embodiment of the manufacturing method of the 2130MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.90%, Si: 0.45%, Mn: 0.78%, Cr: 0.12%, Nb: 0.026%, 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:

[0054] 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. The furnace parameters are controlled to promote the full dissolution of alloying elements such as Nb, reduce segregation, and prevent overheating. After exiting the furnace, the billet is rolled into wire rod with a diameter of 8mm 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 185℃, the furnace time is 175min, the initial rolling temperature is 1040℃, the initial rolling reduction is 40%, the final rolling temperature is 925℃, 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 subsequent sorbitic phase transformation. Specifically, the wire drawing temperature is controlled at 920℃.

[0055] The online molten salt martensitization control 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 35°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 dispersed precipitation of NbC microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for final molten salt treatment, with a reduced molten salt circulation rate. The process promotes the transformation of untransformed residual 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, resulting in a more even stress distribution and preventing carbide coarsening. This helps to regulate the strength-ductility balance of the wire rod. Specifically: the molten salt temperature for the first stage of the molten salt treatment is 563℃, the treatment time is 78s, the molten salt circulation rate is 380t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature for the second stage of the molten salt treatment is 538℃, the treatment time is 155s, the molten salt circulation rate is 280t / h, and the molten salt temperature rise is ≤3℃.

[0056] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) 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 270℃ at a slow cooling rate of 0.9℃ / 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.

[0057] Comparative Example 3:

[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 online molten salt sorbitization control process, the wire rod is cooled at a rate of 40°C / s, the molten salt temperature of the initial molten salt treatment is 500°C, the treatment time is 130s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0059] Comparative Example 4:

[0060] 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 30°C / s, the molten salt temperature of the initial molten salt treatment is 585°C, the treatment time is 50s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0061] Example 3:

[0062] A preferred embodiment of the manufacturing method of the 2130MPa 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: 0.86%, Si: 0.35%, Mn: 0.83%, Cr: 0.25%, Nb: 0.03%, 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:

[0063] 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 the full dissolution of alloying elements such as Nb, reduce segregation, and prevent overheating. After exiting the furnace, the billet is rolled into a 13mm diameter wire rod on a rolling line. Appropriate rolling temperatures and reductions 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 205℃, the furnace time is 140 min, the initial rolling temperature is 1065℃, the initial rolling reduction is 37%, the final rolling temperature is 950℃, 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 the precipitation of network carbides during the wire drawing stage, and providing favorable conditions for the subsequent sorbitic phase transformation. Specifically, the wire drawing temperature is controlled at 940℃.

[0064] The online molten salt martensitization control 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 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 predominantly fine-laminated sorbite structure within the high-temperature austenitic microstructure. This promotes the dispersed precipitation of NbC microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, with a slightly reduced molten salt circulation rate. The untransformed residual austenite continues to transform into fine lamellar interlamellar sorbite, while the formed fine lamellar interlamellar sorbite undergoes isothermal tempering, causing some of the cementite lamellars to melt and fracture, resulting in a more even stress distribution and preventing carbide coarsening. This helps to control the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 541℃, the treatment time is 102s, the molten salt circulation rate is 500t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 519℃, the treatment time is 127s, the molten salt circulation rate is 350t / h, and the molten salt temperature rise is ≤3℃.

[0065] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) 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 273℃ at a slow cooling rate of 0.65℃ / 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.

[0066] Comparative Example 5:

[0067] 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 570°C, the treatment time is 195s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0068] Comparative Example 6:

[0069] 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 490℃, the treatment time is 60s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0070] Example 4:

[0071] A preferred embodiment of the manufacturing method of the 2130MPa 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: 0.88%, Si: 0.46%, Mn: 0.95%, Cr: 0.24%, Nb: 0.01%, P: 0.013%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt sorbitization control treatment → slow cooling on roller table → coiling, specifically:

[0072] 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 the full dissolution of alloying elements such as Nb, reduce segregation, and prevent overheating. After exiting the furnace, the billet is rolled into a 15mm diameter wire rod on a rolling line. Appropriate rolling temperatures and reductions 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 225℃, the furnace time is 115 minutes, the initial rolling temperature is 1080℃, the initial rolling reduction is 35%, the final rolling temperature is 965℃, and the final rolling reduction is 20%. 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 subsequent sorbitic phase transformation. Specifically, the wire drawing temperature is controlled at 950℃.

[0073] The online molten salt martensitization control 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 40°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 dispersed precipitation of NbC microalloyed carbides. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, with a slightly reduced molten salt circulation rate. The untransformed residual austenite continues to transform into fine lamellar interlamellar sorbite, while the formed fine lamellar interlamellar sorbite undergoes isothermal tempering, causing some of the cementite lamellars to melt and fracture, resulting in a more even stress distribution and preventing carbide coarsening. This helps to control the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 533℃, the treatment time is 118s, the molten salt circulation rate is 545t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 513℃, the treatment time is 100s, the molten salt circulation rate is 380t / h, and the molten salt temperature rise is ≤3℃.

[0074] The slow cooling process of the roller conveyor uses controlled opening of the insulation cover to blow hot air (≥200℃) 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 274℃ at a slow cooling rate of 0.5℃ / 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.

[0075] Comparative Example 7:

[0076] 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 260°C at a slow cooling rate of 1.6°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0077] 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:

[0078] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0079]

[0080] As can be seen from the comparison between Example 1 and Comparative Example 1, compared with simplifying the alloy composition and reducing the alloy content to reduce material costs and smelting and rolling difficulties, the cooling capacity of the Steyrmo air-cooled line limits the formation of higher-level network carbides, increasing the risk of coarse microstructure, affecting rolling efficiency and microstructure uniformity, resulting in greater microstructure stress and loss of strength and plasticity, and larger fluctuations in mechanical properties. This invention, through Nb-containing composition design combined with online molten salt sorbitization control technology, suppresses the formation of network carbides and forms sorbite with fine lamellar spacing. The predominantly microstructure promotes the formation of fine lamellar interlayer sorbite structure, which undergoes prolonged isothermal tempering, improving the strength-plasticity matching and microstructure uniformity of the wire rod. This also reduces limitations on rolling and slow cooling on the roller table, balancing production efficiency. As seen in Examples 1-4, hot-rolled wire rods can achieve tensile strengths of 1443-1491 MPa and a reduction of area of ​​35%-40%. This allows for applications such as manufacturing 2130 MPa grade stranded wire, eliminating the need for subsequent offline heat treatment and reducing the risk of wire breakage and cracking during drawing and twisting.

[0081] As can be seen from the comparison results of Example 1 and Comparative Example 2, selecting a higher wire drawing temperature can ensure that the wire rod is in a high-temperature austenitizing state, avoid the formation of network carbides due to low-temperature wire drawing, and at the same time prepare the microstructure for increasing the supercooling and promoting sorbite nucleation.

[0082] The comparison results between Example 2 and Comparative Example 3 show that a lower molten salt temperature in the initial molten salt treatment is beneficial for promoting rapid cooling of the wire rod, increasing undercooling, suppressing network carbides, coarse pearlite and proeutectoid ferrite, promoting the refinement of sorbite lamellars, and reducing the atomic diffusion rate. At the same time, the nucleation rate of NbC increases, but growth is limited. As the treatment time is extended, it is beneficial for reducing the temperature difference between the wire rod surface and the core, promoting the full transformation of austenite to sorbite and the diffuse precipitation of carbides, and improving the matrix strength. However, if the molten salt temperature is too low, it is easy to increase the structural stress and affect the uniformity of precipitate distribution. As the treatment time is too long, it will increase production energy consumption.

[0083] 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 better it is to reduce the risk of brittle structures such as martensite, reduce structural stress and softening difficulty. The shorter the treatment time, the lower the production energy consumption and the more it can limit the growth of NbC microalloyed carbides. However, if the molten salt temperature is too high, it will accelerate the uniform diffusion of carbon, affect the control of network carbides and pearlite, and be detrimental to suppressing carbide coarsening. If the treatment time is too short, it will increase the temperature difference from the surface of the wire rod to the core, affecting the control of structural uniformity.

[0084] As can be seen from the comparison results between Example 3 and Comparative Example 5, the higher the molten salt temperature and the longer the treatment time in the later stage of molten salt treatment, the better it is to drive the cementite lamellars of sorbite to melt and fracture, so that the stress distribution of the entire cross section of the wire rod tends to be gentle, providing sufficient time for the short-range diffusion of Nb atoms, improving the plasticity and toughness of the matrix and reducing the risk of processing cracks. However, if the molten salt temperature is too high, the driving force for the transformation of austenite to sorbite will be weakened. As the treatment time is too long, it will accelerate the diffusion-type coarsening of cementite or microalloyed carbides. The coarse cementite or carbides will become stress concentration sources. The grown NbC cannot hinder the movement of dislocations, resulting in a loss of strong plasticity.

[0085] 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 continued transformation of untransformed residual austenite into fine lamellar sorbite. The shorter the treatment time, the better it is to reduce production energy consumption and suppress excessive coarsening of cementite and carbides. However, if the molten salt temperature is too low and the treatment time is too short, thermal dynamics will be lost, affecting the melting of sorbite lamellars, stress release and microstructure uniformity, resulting in loss of plasticity and toughness and fluctuations in mechanical properties.

[0086] 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.

[0087] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of manufacturing a hot-rolled wire rod for a 2130 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The chemical composition of the hot-rolled wire rod comprises the following components in percentage by mass: C: 0.86% to 0.90%, Si: 0.35% to 0.48%, Mn: 0.75% to 0.95%, Cr: 0.12% to 0.25%, Nb: 0.01% to 0.03%, P: ≤0.014%, S: ≤0.014%, and the rest is Fe and inevitable impurities; after the wire rod is drawn into a wire rod at a drawing temperature of ≥920℃, the wire rod is cooled at a cooling rate of ≥35℃ / s from an austenite state into a sorbite phase region, a microstructure mainly composed of sorbite is formed, isothermal tempering is performed, partial sorbite layers are melted and broken, and finally the hot-rolled wire rod is prepared by slow cooling on a roller bed, and the microstructure of the hot-rolled wire rod comprises a mixed microstructure of tempered sorbite, ferrite and melted and broken sorbite; the on-line molten salt sorbitizing regulation and control treatment is divided into a front-stage molten salt treatment and a rear-stage molten salt treatment, the molten salt temperature of the front-stage molten salt treatment is 533 to 563℃, and the treatment time is 78 to 118s; the molten salt temperature of the rear-stage molten salt treatment is 513 to 538℃, and the treatment time is 100 to 155s.

2. The method of producing a hot rolled wire rod for a 2130 MPa grade strand according to claim 1, characterized by, Before the rolling, the soaking temperature of a heating furnace is controlled to be 1185 to 1225℃, and the furnace time is 115 to 175min.

3. The method of producing a hot rolled wire rod for 2130 MPa grade strand wire according to claim 1, characterized by, During the rolling, the initial rolling temperature is controlled to be 1040 to 1080℃, the initial rolling reduction is 35% to 40%, the final rolling temperature is 925 to 965℃, and the final rolling reduction is 20% to 25%; during the drawing, the drawing temperature is controlled to be 920 to 950℃.

4. The method of producing a hot rolled wire rod for a 2130 MPa grade strand according to claim 1, characterized by, The molten salt circulation amount of the rear-stage molten salt treatment is less than that of the front-stage molten salt treatment.

5. The method of producing a hot rolled wire rod for a 2130 MPa grade strand according to claim 1, characterized by, The molten salt circulation amount of the front-stage molten salt treatment is 380 to 545t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation amount of the rear-stage molten salt treatment is 280 to 380t / h, and the molten salt temperature rise is ≤3℃.

6. The method of producing a hot rolled wire rod for a 2130 MPa grade strand according to claim 1, characterized by, The roller bed slow cooling controls the wire rod to be cooled to below 275℃ at a slow cooling rate of 0.5 to 0.9℃ / s and then be coiled.

7. A hot rolled wire rod for 2130 MPa grade strand, characterized in that, The hot-rolled wire rod is obtained by the manufacturing method of the hot-rolled wire rod for a 2130MPa-grade twisted wire rod according to any one of claims 1 to 6.

8. The hot rolled wire rod for 2130 MPa grade strand according to claim 7, characterized by, The volume percentage of the tempered sorbite is ≥68%, the interlamellar spacing is 70 to 120nm, the volume percentage of the ferrite is ≤6%, the volume percentage of the melted and broken sorbite is ≥15%, the reticular carbide level of the hot-rolled wire rod is 0 level, and the mechanical property difference between turns is ≤39MPa.

9. The hot rolled wire rod for 2130 MPa grade strand as claimed in claim 7, wherein, The diameter of the hot-rolled wire rod is 8 to 15mm, the tensile strength is 1443 to 1491MPa, and the reduction of area is 35% to 40%.

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