A high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire and its manufacturing method. The method involves designing and rolling the wire rod with a high hardenability composition, followed by online molten salt quenching and isothermal treatment in the mixed phase region. This process first involves the wire rod undergoing a pre-treatment molten salt treatment, transitioning it from an austenitic state to a bainite and pearlite phase region, promoting the transformation of some austenite into bainite and sorbite. A subsequent molten salt treatment, with increased molten salt temperature and reduced molten salt circulation, further promotes the decomposition of untransformed residual austenite into bainite and sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, comprising a multiphase microstructure. This method reduces material costs, allows for online control of the multiphase microstructure and toughness, achieving a tensile strength of 1505~1555MPa and a reduction of area of ​​32%~37%, balancing efficiency and energy consumption, eliminating the need for offline heat treatment, and promoting efficient stranded wire production.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled wire rod technology, specifically relating to a high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire and its manufacturing method. Background Technology

[0002] Prestressed technology is mainly applied to railway concrete bridges and concrete sleepers, and 1960MPa grade prestressed steel strands have been successfully used. With the upgrading of prestressed technology towards higher load-bearing capacity and lighter weight, it is necessary to develop higher strength strands. Hot-rolled wire rod, as the base material for strands, directly affects the strand production process and performance grade. Conventional pearlitic hot-rolled wire rod is the mainstream raw material for steel strands, with good cold-drawing performance. However, due to insufficient matrix strength, high material cost, and the layered structure and low defect density of sorbite limiting its hardening effect, further increasing the strength grade faces significant challenges. While hot-rolled bainitic wire rod has high strength and better draw hardening ability, it is extremely brittle after drawing, which is not conducive to the drawing and twisting of strands by downstream users. Therefore, it is necessary to develop a 2200MPa grade high-strength hot-rolled wire rod for strands, balancing material cost and wire rod strength and plasticity to meet the development needs of the steel industry and market demand.

[0003] The hot-rolled wire rods for stranded wire, which have lower material costs and better strength and plasticity, still face the following technical bottlenecks:

[0004] I. To improve the strength of wire rod, existing technologies combine microalloying and Steyrmore air-cooled wire rod controlled rolling and cooling production. For example, patent CN113652610A discloses an ultra-high strength stranded wire rod for railway bridges and its production process, which adopts a C-Si-Mn-Cr-V composition design and combines small billet rolling and fan-cooled controlled cooling to produce sorbitic wire rod. On the one hand, the initial strength of hot-rolled wire rod is insufficient, and further increasing the alloy content will increase the material cost. At the same time, due to the influence of carbon segregation caused by high carbon content and the limitation of the maximum cooling capacity of the Steyrmore air-cooled wire rod, a high level of network carbides will form in the wire rod, which will cause the matrix to be fractured and affect the plasticity and toughness of the wire rod. In order to minimize the influence of network carbides and promote the refinement of pearlite lamellars to form sorbitic wire rods, the following measures are taken. After increasing the strength of air cooling, the temperature difference between the air-exposed and air-received surfaces of the wire rod will be further amplified. Due to the hardenability of the high alloy content, local overcooling is prone to form uncontrollable brittle martensite, further increasing the brittleness of the structure and the fluctuation of mechanical properties. On the other hand, due to the limitations of the maximum cooling capacity and unstable temperature control of the wire rod, it is difficult to stably control the multiphase structure on the Steyrmo air-cooling line. The phase transformation temperature of bainite is relatively low. Under the phase transformation incubation of continuous cooling, large distortion and structural stress will be generated, resulting in extremely brittle wire rod after the line is cut off. Stress concentration at hard points during cold drawing increases the risk of cracking during wire drawing and torsion for downstream users. Long-term heat treatment has limited improvement on ductility and toughness, and may also affect production efficiency or require offline heat treatment downstream, causing production inconvenience.

[0005] II. To improve the strength and plasticity of wire rods, existing technologies combine molten salt treatment to produce sorbitic wire rods. For example, patent CN120400686A discloses 2200-2260MPa grade ultra-high stress corrosion resistant prestressed steel strand wire rods, steel strands, and their production method. This method uses a C-Si-Mn-Cr-V-Nb composition design and combines low-temperature heating for initial rolling and a one-stage molten salt treatment after wire drawing to produce wire rods. However, on the one hand, the addition of V and Nb increases material costs, and the initial strength of the wire rods is insufficient due to the lack of dislocation movement resistance in the sorbitic structure. The one-stage treatment requires a large molten salt circulation volume to control the molten salt temperature rise, resulting in high overall production energy consumption. On the other hand, the wire rods have limited resistance to softening due to the thermal instability of the layered cementite in the sorbitic structure. The salt bath treatment time should not be too long, and low-temperature heating rolling and low-cooling-rate long-term aging are needed to supplement plasticity and toughness, which prolongs the entire production cycle and affects production capacity. 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 high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire and its manufacturing method, which can reduce material costs, control multiphase structure and structure toughness online, improve the matching of strength and plasticity, and take into account production efficiency and energy consumption, so as to eliminate offline heat treatment and promote efficient production of stranded wire.

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

[0008] A method for manufacturing high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire, the method comprising:

[0009] The wire rod is rolled into production wire according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.87%~0.91%, Si: 0.73%~0.88%, Mn: 0.70%~0.90%, Cr: 0.44%~0.53%, Al: 0.05%~0.12%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a spinning temperature of ≥900℃, it undergoes online molten salt treatment. The mixed-phase region quenching isothermal treatment involves first passing the wire rod through a front-end molten salt treatment and cooling it at a rate of ≥34℃ / s, transforming it from an austenitic state into a bainite and pearlite phase region, promoting the transformation of some austenite into bainite and sorbite. Then, it undergoes a rear-end molten salt treatment, increasing the molten salt temperature and reducing the molten salt circulation rate, promoting the decomposition of untransformed residual austenite into bainite and sorbite, followed by isothermal tempering. Finally, it undergoes slow cooling on a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, forming a multiphase microstructure.

[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 element in steel. It is relatively cheaper and can hinder dislocation movement through interstitial solid solution, improve the stability of austenite, reduce the starting temperature of bainite and sorbite phase transformation, affect the driving force and diffusion rate of bainite phase transformation, and ensure the fine lamellar structure of sorbite. In the later stage of molten salt treatment, it can promote the dispersed precipitation of Cr carbides and the stable decomposition of residual austenite, thereby improving the matrix strength and resistance to softening. However, if the C content is too high, it will cause the center segregation of high carbon steel billet, increase the risk of rolling cracks, increase the difficulty of controlling network carbides and martensite, and increase the carbon supersaturation during bainite phase transformation, which will prolong the phase transformation time, increase the difficulty of tempering process and increase the risk of cementite coarsening. Therefore, in order to take into account the high strength requirements of 2200MPa grade strand, reduce the risk of network carbide precipitation, adapt to the multiphase structure and isothermal tempering control, the mass percentage of C is controlled at 0.87%~0.91%.

[0012] (2) Silicon: Si can provide solid solution strengthening, reduce the diffusion rate of carbon at the interface, improve the stability of austenite in the medium and low temperature range, inhibit the transformation of austenite to proeutectoid ferrite, increase the shear resistance of bainite phase transformation, refine bainite, refine the lamellar spacing of sorbite by hindering ferrite growth, avoid local carbon accumulation during sorbite formation by reducing the coarsening rate of cementite, and improve the resistance to tempering softening by long-term stable pinning of dislocations by carbides, so as to promote the gradual release of internal stress. However, excessive silicon will promote decarburization, increase rolling deformation resistance, affect rolling efficiency and bainite transformation rate, prolong the phase transformation incubation period, and lead to a decrease in the plasticity and production efficiency of steel and an increase in strength fluctuation. Therefore, in order to adapt to the control of phase transformation structure by online molten salt mixed phase region quenching isothermal treatment and provide a certain resistance to tempering softening, the mass percentage of Si is controlled at 0.73%~0.88%.

[0013] (3) Manganese: Mn can improve the hardenability of austenite, reduce the critical cooling rate of austenite, reduce the phase transformation time difference between the surface and core of the wire rod by slowing down the decomposition of austenite, create temperature conditions for the multiphase transformation by regulating the stability of austenite, improve the transformation and stability of bainite by lattice distortion and hindering dislocation movement, reduce the diffusion activation energy of carbon in austenite, refine the interlamellar spacing of sorbite, and thus improve the matrix strength. At the same time, it hinders the diffusion of carbon atoms and the merging and growth of cementite, and reduces the strength loss during tempering. However, when the content of Mn is too high, it will increase the element segregation during the solidification process of the billet, cause uneven structure, and slow decomposition of residual austenite will increase the risk of martensite precipitation and the difficulty of tempering softening, affect the toughness and structure uniformity of the wire rod, and reduce the drawing performance. Therefore, in order to facilitate the control of the multiphase structure of hot-rolled wire rod and control the difficulty of tempering softening, the mass percentage of Mn is controlled at 0.70%~0.90%.

[0014] (4) Chromium: Cr can improve the hardenability of austenite, reduce the activation energy of bainite phase transformation, shorten the incubation period, refine the bainite structure, reduce the diffusion coefficient of carbon in austenite, and the weak stabilizing effect of chromium in the medium temperature range makes the transformation rate of sorbite moderate and refines the lamellar spacing of sorbite. At the same time, chromium is easy to combine with carbon to form chromium-based carbides, which can provide dispersion strengthening, improve the resistance to softening, and make the multiphase structure retain high strength characteristics and pull hardening ability. However, when the Cr content is too high, segregation will occur during solidification, which will increase the uniformity of the structure and the difficulty of controlling martensite. At the same time, it will increase the internal stress of phase transformation, dislocation density and tempering difficulty, affecting the rapid production line. Therefore, in order to take into account the control of multiphase structure of hot-rolled wire rod, the resistance to tempering softening of wire rod should be appropriately controlled, and the mass percentage of Cr should be controlled at 0.44%~0.53%.

[0015] (5) Aluminum: Al can slow down the carbon diffusion rate, increase the bainite nucleation sites through fine austenite grains, avoid carbon from accumulating too quickly to form coarse carbides during bainite phase transformation, refine the bainite structure, and at the same time inhibit the abnormal transformation of austenite to coarse pearlite or ferrite, so that carbon diffuses evenly during sorbite transformation, reduce the segregation of the structure, and help stabilize carbides during isothermal tempering, improve the resistance to softening, so as to help release internal stress and improve machinability. However, if the Al content is too high, it will cause continuous casting defects. If the carbon diffusion rate is too slow, it will prolong the phase transformation incubation period. Therefore, in order to control the phase transformation rate and suppress the risk of cementite coarsening during isothermal tempering, Al should be added appropriately. The mass percentage of Al should be controlled at 0.05%~0.12%.

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

[0017] The aforementioned wire rods are designed with a high carbon composition of C-Si-Mn-Cr-Al, without the addition of precious alloying elements such as V and Nb. This can appropriately reduce material costs, control austenite stability, phase transformation temperature range, and transformation kinetics. The high carbon content lowers the phase transformation temperature, while the optimized content of Mn, Cr, and Si expands the overlap between the bainite and pearlite phase regions. Si and Al synergistically suppress cementite coarsening, providing favorable conditions for controlling the mixed phase transformation of bainite and sorbite, reducing production energy consumption, resisting tempering softening, and promoting rapid production. Furthermore, a higher wire drawing temperature is selected to promote the full dissolution of carbides and the uniform distribution of carbon concentration in austenite, avoiding premature precipitation of network carbides or proeutectoid ferrite at excessively low wire drawing temperatures. This provides favorable conditions for rapid cooling during the initial molten salt treatment, promoting the nucleation and refinement of bainite and sorbite. After wire drawing, the wire rods undergo online molten salt mixed phase region quenching isothermal treatment.

[0018] Firstly, compared to the limitations of the Stellmore air-cooled line, which suffers from the highest cooling capacity and unstable temperature control, making it difficult to suppress network carbides and martensite structures and regulate the transformation of multiphase structures, the wire rod, after undergoing a pre-treatment with molten salt, enters a low-temperature mixed phase region of bainite and pearlite. On the one hand, the high heat transfer capacity of the molten salt can be utilized to promote rapid cooling of the wire rod, quickly bypassing the precipitation temperature range of secondary cementite from the high-temperature austenitic state, thus avoiding the precipitation of carbon along grain boundaries to form network carbides and avoiding the adverse effects of network carbides on microstructure uniformity and ductility. On the other hand, when the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for uniform heat exchange, eliminating the temperature difference problem between the air-cooled and air-received surfaces compared to the air-cooled line. In uniform and rapid cooling, the formation of martensite structure due to localized overcooling of the wire rod can be avoided. Compared with the single-stage molten salt treatment, which is limited by the controlled cooling route and requires the addition of microalloying and low-temperature rolling and wire drawing, the phase region temperature is lower when the wire rod passes through the first stage of molten salt treatment. This can improve the degree of undercooling, inhibit the decomposition of austenite into coarse pearlite, and reduce the restrictions on alloy composition, rolling and wire drawing temperature. At medium and low temperatures, due to the stabilizing effect of C and Mn, the phase transformation driving force is concentrated in the shear mechanism of bainite. The fine lamellar interlayer spacing of sorbite is initially inoculated, which can realize the control of the multiphase matrix structure to compensate for the strength loss and phase deformation nucleation disadvantage caused by omitting alloying elements, and improve the matrix strength and pull hardening ability.

[0019] Second, compared to the limitations of the Stellmore air-cooling line, which restricts the control of microstructure brittleness and causes significant fluctuations in mechanical properties, the wire rod undergoes a post-treatment with molten salt to increase the molten salt temperature. On one hand, this utilizes the upper-middle overlapping region of bainite and pearlite phases to enhance carbon diffusion, driving the primary transformation of retained austenite into sorbite. Simultaneously, incompletely transformed austenite contributes to the formation of bainite. Combined with compositional hardenability, this synergistically ensures core phase transformation, stabilizing the mixed microstructure matrix. The high-temperature treatment and sufficient austenite transformation also prevent the formation of martensite from retained austenite during subsequent cooling. On the other hand, as the treatment time increases, the temperature difference between the wire rod surface and core decreases. Isothermal tempering enhances thermal dynamics, while the continuous cooling of the air-cooling line at the low temperature after phase transformation allows dislocations to more easily rearrange through climb and annihilation. This reduces residual stress after tempering, thereby mitigating the brittleness caused by bainite and achieving online toughening. Compared to the single-stage molten salt treatment, which is limited by the controlled cooling route, resulting in higher energy consumption and requiring low-rate aging to affect production efficiency, the first stage of molten salt treatment allows for rapid cooling of the wire rod, while the second stage can appropriately reduce the molten salt circulation volume, thus lowering overall production energy consumption. Furthermore, after the phase transformation in the mixed-phase region, the wire rod contains some highly distorted bainite. The second stage of molten salt treatment promotes the dispersion and precipitation of Cr carbides at nailed grain boundaries, and Si and Al synergistically inhibit carbide coarsening, improving resistance to tempering softening and preserving the strength characteristics of the mixed structure. As the isothermal tempering time is extended, dislocations are fully rearranged, releasing internal stress and significantly improving the uniformity of the structure and cold-drawing workability, enhancing process stability. After exiting the molten salt treatment, the wire rod can be further toughened by slow cooling on a roller conveyor, without the need for excessively low cooling rates and long processing times. This improves the matching of strength and plasticity of the wire rod while ensuring efficient production.

[0020] During the rolling process, selecting a higher heating furnace homogenization temperature and an appropriate furnace dwell time can promote uniform austenitization of the billet, improve the billet's ductility and toughness, and ensure the billet's thermal permeability. This allows the billet to better deform during subsequent rolling processes and avoids oxidation burn-off and grain coarsening caused by excessively high homogenization temperature and excessively long furnace dwell time. In the preferred technical solution, before rolling, the heating furnace homogenization temperature is controlled at 1170~1220℃, and the furnace dwell time is controlled at 160~250min.

[0021] Because the wire drawing temperature is relatively high, the limitations on rolling can be reduced. A higher initial rolling temperature can be selected during rolling to balance plasticity and grain refinement, thereby reducing deformation resistance, avoiding equipment overload, and improving rolling speed and efficiency. This lays the foundation for a uniform microstructure in the final rolling. With appropriate final rolling temperature and final rolling reduction, deformation energy storage and grain breakage are controlled, promoting dynamic recrystallization and grain refinement during the final rolling process, and connecting the wire drawing temperature. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1045~1095℃, the final rolling temperature at 895~935℃, and the final rolling reduction at 26%~31%.

[0022] During the wire spinning process, the wire spinning temperature can be further controlled to reduce the risk of austenite grain coarsening and avoid subsequent excessive cooling of austenite. In a preferred embodiment, the wire spinning temperature is controlled at 900~940℃.

[0023] The lower the molten salt temperature in the initial molten salt treatment, the faster the wire rod cools down, reducing the rate of carbon diffusion to the interface, suppressing network carbides and coarse pearlite. A greater free energy difference between austenite and bainite increases the driving force for bainite transformation, promoting rapid nucleation of bainite via shear. Extended treatment time increases the amount of bainite transformation, promotes refinement of the interlamellar spacing of trace sorbite, and improves matrix strength. However, excessively low molten salt temperature and excessively long treatment time will increase the bainite transformation rate and internal stress, reduce the amount of retained austenite, increase softening difficulty, and affect the final ductility and toughness. Conversely, higher molten salt temperature accelerates carbon diffusion, reduces the temperature difference between the wire rod surface and core, lowers phase transformation stress, and shortens the treatment time, slowing down bainite formation. While improving the phase transformation rate and reducing production energy consumption, excessively high molten salt temperatures can affect the control of network carbides, the amount of bainite phase change, and the refinement of the microstructure. Insufficient processing time leads to insufficient wire rod temperature drop and bainite transformation driving force, resulting in excessive residual untransformed austenite, increasing the risk of sorbite coarsening and mechanical property fluctuations, and causing strength loss. Therefore, the molten salt temperature and processing time of the initial molten salt treatment can be controlled to allow the wire rod to quickly transition from a high-temperature austenitic state to the bainite-pearlite mixed phase region, suppressing network carbides and promoting the transformation of some austenite into bainite and sorbite microstructures, thus preparing the microstructure for the subsequent molten salt treatment. In a preferred embodiment, the molten salt temperature of the initial molten salt treatment is 465~506℃, and the processing time is 25~40s.

[0024] Since the spinning temperature differs significantly from the molten salt temperature of the preceding molten salt treatment, a higher molten salt circulation rate can control the molten salt temperature rise, promote rapid cooling of the wire rod, and refine its structure. In the preferred technical solution, the molten salt circulation rate of the preceding molten salt treatment is 510~650t / h, and the molten salt temperature rise is ≤10℃.

[0025] The molten salt treatment in the latter stage has a higher molten salt temperature than the former. Higher molten salt temperatures improve the carbon diffusion coefficient, promote the decomposition of retained austenite, and drive the sorbite phase transformation. With extended treatment time, this ensures complete decomposition of retained austenite, preventing it from forming a low-temperature brittle structure during subsequent cooling. It also promotes stress release in the formed bainite and sorbite, resulting in a more uniform microstructure, improved ductility and toughness, and better cold-drawing workability. However, excessively high molten salt temperatures are detrimental to controlling bainite phase transformation and lamellar refinement, inhibiting cementite coarsening, and hindering the dispersed precipitation of Cr carbides. Prolonged treatment time reduces dislocation movement and causes carbide aggregation and coarsening, leading to a loss of strength and plasticity, increased energy consumption, and reduced production efficiency. Conversely, lower molten salt temperatures promote bainite transformation and carbide dispersion, reduce sorbite lamellar spacing, prevent core microstructure deterioration, and improve interface properties. The strengthening effect can reduce production energy consumption by shortening the processing time. However, if the molten salt temperature is too low, the diffusion activity of carbon will be affected, which will affect the precipitation of carbides, phase transformation inoculation and isothermal tempering efficiency. If the processing time is too short, the carbon diffusion rate in the core will be insufficient, the sorbite transformation will be delayed, the microstructure will have large residual stress, less carbide precipitation, and reduced pinning dislocation effect, which will lead to loss of strength and plasticity. In fact, the retained austenite may even form a brittle microstructure during subsequent cooling, increasing the fluctuation of mechanical properties. Therefore, the subsequent molten salt treatment can control the molten salt temperature and processing time to promote the decomposition of untransformed retained austenite into bainite and sorbite, and perform isothermal tempering on the mixed microstructure to promote the dispersed precipitation of carbides and pinning dislocations, release the microstructure stress, and regulate the strength and plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 545~575℃ and the processing time is 255~440s.

[0026] Since the temperature difference between the molten salt in the first stage and the second stage of molten salt treatment is small, appropriately reducing the molten salt circulation rate in the second stage of molten salt treatment can control the molten salt temperature rise, promote mixed phase transformation and isothermal tempering, and reduce production energy consumption. In the preferred technical solution, the molten salt circulation rate in the second stage of molten salt treatment is 250~440t / h, and the molten salt temperature rise is ≤3℃.

[0027] Since the subsequent molten salt treatment has carried out sufficient phase transformation and isothermal tempering, it suppresses the sudden martensitic phase transformation of the residual austenite and avoids hard and brittle defects. This reduces the restriction on the slow cooling of the roller table. The slow cooling of the roller table can further control the slow cooling rate, continue the isothermal tempering effect of the subsequent molten salt treatment, promote further toughening of the wire rod structure, and improve the softening effect. In the preferred technical solution, the slow cooling of the roller table controls the wire rod to cool to below 290°C at a slow cooling rate of ≤0.9°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 at ≥255°C during the online molten salt mixed phase quenching isothermal treatment into the heat insulation cover. The conveyor roller conveyor transports the wire rod through the heat insulation cover to control the slow cooling of the wire rod, which can further recover and utilize the heat energy of molten salt treatment, reduce production energy consumption, and promote the rapid production of wire rod.

[0029] A high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire, wherein the hot-rolled wire rod is manufactured by the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2200MPa grade stranded wire described in any one of the above-mentioned methods.

[0030] The aforementioned wire rods are designed with high hardenability, and their microstructure consists of a multiphase structure dominated by tempered sorbite and containing a small amount of tempered bainite. Compared to existing high-strength strand wire rods with a sorbitic microstructure, this design eliminates the need for Nb and V, thus reducing material costs. It also effectively suppresses the network carbide problem caused by high carbon content. Compared to air-cooled hot-rolled wire rods, this design effectively suppresses the brittle martensitic structure, achieving stable control and online toughening of the multiphase structure, reducing fluctuations in mechanical properties. Compared to existing salt-bath treated hot-rolled wire rods, this design also contains a small amount of bainite. Compared to sorbite, it has a higher dislocation density and better draw hardening ability. After isothermal tempering, the carbide pinning dislocations of Cr can inhibit cementite coarsening, retain strength characteristics, and fully release structural stress, resulting in tempered bainite and tempered sorbite that balance strength and toughness. This improves the matrix strength, eliminates coarse carbides or martensite hard spots, and has a low stress concentration factor, thus optimizing the strength and toughness balance. This eliminates the need for offline heat treatment during subsequent strand manufacturing, reduces the risk of wire breakage during drawing and twisting, and makes the strand less prone to brittle fracture under high tension.

[0031] The higher the volume percentage of tempered bainite and the finer the lamellar spacing of tempered sorbite, the higher the matrix strength. The higher the proportion of tempered sorbite, the higher the ductility and toughness of the matrix. In the preferred embodiment, the volume percentage of tempered bainite is 8% to 14%, the volume percentage of tempered sorbite is 86% to 92%, and the lamellar spacing of tempered sorbite is 85 to 125 nm.

[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 coils is ≤47MPa. This avoids the adverse effects of network carbides cutting the matrix, inducing cold drawing wire breakage, and deteriorating fatigue performance. At the same time, by effectively avoiding abnormal structures such as martensite and coarse pearlite, the wire rod surface to the core undergoes sufficient phase transformation, which can reduce the fluctuation of mechanical properties. The smaller mechanical property difference between coils can improve the strand performance and the uniformity of cold drawing deformation, further reducing the risk of cracking during strand manufacturing and service, and extending the service life of the strand.

[0033] In the preferred technical solution, the diameter of the hot-rolled wire rod is 5.0~15.0mm, the tensile strength is 1505~1555MPa, and the reduction of area is 32%~37%. The tempered bainite appropriately improves the draw hardening ability of the hot-rolled wire rod. Combined with the high initial tensile strength, it can reduce the number of drawing passes, reduce the diameter reduction rate during drawing, quickly reach the target strength, and improve the production efficiency of stranded wire. The high reduction of area indicates that the hot-rolled wire rod has high plasticity, avoids local stress concentration, further reduces the risk of wire breakage during drawing and twisting cracking, improves the fatigue resistance and brittle fracture resistance during service, and extends the service life of stranded wire.

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

[0035] (1) In view of the defects of existing hot-rolled wire rod manufacturing methods for stranded wire, such as high brittleness risk, high energy consumption or insufficient production efficiency, the present invention adopts a high hardenability composition combined with online molten salt mixed phase region quenching isothermal treatment, so that the wire rod first enters the bainite and pearlite phase region through the first stage molten salt treatment, promotes the transformation of some austenite into bainite and sorbite, and inhibits network carbides. Then, through the second stage molten salt treatment, the molten salt temperature is increased, the molten salt circulation volume is reduced, the production energy consumption is reduced, the untransformed residual austenite is decomposed into bainite and sorbite and isothermal tempering is performed, the abnormal structure of martensite and coarse pearlite is inhibited, the multiphase structure and the structure toughness are controlled online, the strength and plasticity performance are improved, the mechanical property fluctuation is reduced, and the restrictions on rolling and slow cooling of the roller table are reduced, the rolling and slow cooling efficiency of the roller table is improved, and the production efficiency and energy consumption are balanced, which has good industrial adaptability.

[0036] (2) In view of the shortcomings of existing hot-rolled wire rod materials of pearlitic type for stranded wire, such as high cost, insufficient strength, plasticity and pull hardening ability, and extreme brittleness of bainitic wire rod, this invention adopts a high hardenability composition, without relying on precious alloy components such as V and Nb, which can appropriately reduce material cost. The microstructure includes a multiphase structure composed of tempered bainite and tempered sorbite. Bainite has a higher dislocation density and better pull hardening ability than sorbite. Combined with Cr carbide pinning dislocations and inhibiting cementite coarsening, it can retain strength characteristics and fully release structural stress, thereby improving matrix strength. There are no coarse carbides or martensite hard spots. It takes into account the balance of strength and toughness, and can achieve a tensile strength of 1505~1555MPa and a section reduction rate of 32%~37% for hot-rolled wire rod. It can be used in applications such as 2200MPa grade stranded wire to avoid offline heat treatment, promote efficient 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 according to the metal microstructure testing method of GB / T13298 standard; mechanical property same-ring difference test method: take 2 rings of wire rod 5m from the end of the coil, and divide each ring of wire rod into 8 equal segments with the overlap area as the base point. Take 1 tensile specimen from each segment. The strength difference of the tensile specimens after tensile testing is the mechanical property same-ring difference.

[0042] Example 1:

[0043] A preferred embodiment of the manufacturing method of the 2200MPa grade high-strength multiphase 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: 0.75%, Mn: 0.77%, Cr: 0.47%, Al: 0.1%, P: 0.015%, 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 mixed phase region quenching isothermal → 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 state suitable for rolling plasticity, promoting uniform austenitization and preventing grain coarsening. After exiting the furnace, the billet is rolled into a 5mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote dynamic recrystallization and grain refinement during the final rolling process. Specifically, the furnace soaking temperature is controlled at 1185℃. The rolling time is 220 min, the initial rolling temperature is 1065℃, the final rolling temperature is 925℃, and the final rolling reduction is 31%. The wire drawing process is used to convert the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is distributed on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 915℃.

[0045] The online molten salt mixed-phase quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 34°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The process involves increasing the amount of molten salt to promote the decomposition of untransformed residual austenite into bainite and sorbite, followed by isothermal tempering. This promotes the dispersion and precipitation of Cr carbides for strengthening, thereby controlling the strength-plasticity matching of the wire rod and reducing overall production energy consumption. Specifically, the molten salt temperature for the first stage of molten salt treatment is 506℃, the treatment time is 25s, the molten salt circulation rate is 510t / h, and the molten salt temperature rise is ≤10℃. The molten salt temperature for the second stage of molten salt treatment is 545℃, the treatment time is 440s, the molten salt circulation rate is 250t / h, and the molten salt temperature rise is ≤3℃.

[0046] The slow cooling process on the roller conveyor involves adjusting the opening of the insulation cover to blow hot air (≥255℃) from the two sections of the salt bath tank undergoing online molten salt mixed phase quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 282℃ 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 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 is controlled at a uniform heating temperature of 1135°C, with a furnace time of 255 min, an initial rolling temperature of 1020°C, a final rolling temperature of 815°C, and a wire drawing temperature of 800°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 60% capacity, fans 1 to 5 are turned on to cool the wire rod to 670℃ at a cooling rate of 5.7℃ / s. Then, fans 6 to 14 are turned on to 25% capacity to cool the wire rod to 281℃ at a cooling rate of 2.5℃ / 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 is heated to a uniform temperature of 1120°C, the furnace time is 260 min, the initial rolling temperature is 1000°C, the final rolling temperature is 820°C, and the wire drawing temperature is 815°C. During the molten salt treatment before the online molten salt mixed phase quenching isothermal process, the wire rod is cooled at a cooling rate of 29°C / s, and the finished hot-rolled wire rod is obtained after leaving the production line.

[0051] Example 2:

[0052] A preferred embodiment of the manufacturing method of the 2200MPa grade high-strength multiphase 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: 0.73%, Mn: 0.7%, Cr: 0.44%, Al: 0.12%, P: 0.014%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt mixed phase region quenching isothermal → slow cooling on roller table → coiling, specifically:

[0053] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high temperature that allows for rolling plasticity, promoting uniform austenitization and preventing grain coarsening. After exiting the furnace, the billet is rolled into wire rod with a diameter of 8.5mm via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote dynamic recrystallization and grain refinement during the final rolling process. Specifically, the furnace soaking temperature is controlled at 1170℃. The rolling time is 250 minutes, the initial rolling temperature is 1045℃, the final rolling temperature is 895℃, and the final rolling reduction is 29%. The wire drawing process is used to convert the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 900℃.

[0054] The online molten salt mixed-phase quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 35°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The process involves increasing the amount of molten salt to promote the decomposition of untransformed residual austenite into bainite and sorbite, followed by isothermal tempering. This promotes the dispersion and precipitation of Cr carbides for strengthening, thereby controlling the strength-plasticity matching of the wire rod and reducing overall production energy consumption. Specifically, the molten salt temperature for the first stage of molten salt treatment is 487℃, the treatment time is 30s, the molten salt circulation rate is 570t / h, and the molten salt temperature rise is ≤10℃. The molten salt temperature for the second stage of molten salt treatment is 553℃, the treatment time is 350s, the molten salt circulation rate is 295t / h, and the molten salt temperature rise is ≤3℃.

[0055] The slow cooling process on the roller conveyor involves adjusting the opening of the insulation cover to blow hot air (≥255℃) from the two sections of the salt bath tank undergoing online molten salt mixed phase quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 282℃ 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 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 mixed phase quenching isothermal process, the wire rod is cooled at a cooling rate of 38°C / s, the molten salt temperature of the initial molten salt treatment is 450°C, the treatment time is 45s, 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 mixed phase quenching isothermal process, the wire rod is cooled at a cooling rate of 30℃ / s, the molten salt temperature of the initial molten salt treatment is 530℃, the treatment time is 20s, 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 2200MPa grade high-strength multiphase 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: 0.83%, Mn: 0.9%, Cr: 0.49%, Al: 0.08%, 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 mixed phase region quenching isothermal → slow cooling on roller table → coiling, specifically:

[0062] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature state suitable for rolling plasticity, promoting uniform austenitization and preventing grain coarsening. After exiting the furnace, the billet is rolled into a 12mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote dynamic recrystallization and grain refinement during the final rolling process. Specifically, the furnace soaking temperature is controlled at 1195℃. The rolling time is 180 minutes, the initial rolling temperature is 1075℃, the final rolling temperature is 930℃, and the final rolling reduction is 27.5%. The wire drawing process is used to convert the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is distributed on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 930℃.

[0063] The online molten salt mixed-phase quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 38°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The process involves increasing the amount of molten salt to promote the decomposition of untransformed residual austenite into bainite and sorbite, followed by isothermal tempering. This promotes the dispersion and precipitation of Cr carbides for strengthening, thereby controlling the strength-plasticity matching of the wire rod and reducing overall production energy consumption. Specifically, the molten salt temperature for the first stage of molten salt treatment is 473℃, the treatment time is 35s, the molten salt circulation rate is 620t / h, and the molten salt temperature rise is ≤10℃. The molten salt temperature for the second stage of molten salt treatment is 562℃, the treatment time is 290s, the molten salt circulation rate is 355t / h, and the molten salt temperature rise is ≤3℃.

[0064] The slow cooling process on the roller conveyor involves adjusting the opening of the insulation cover to blow hot air (≥255℃) from the two sections of the salt bath tank undergoing online molten salt mixed phase quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 286℃ at a slow cooling rate of 0.75℃ / 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 450s, 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 510℃, the treatment time is 150s, 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 2200MPa grade high-strength multiphase 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: 0.88%, Mn: 0.86%, Cr: 0.53%, Al: 0.05%, P: 0.013%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt mixed phase region quenching isothermal → 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 that allows for rolling plasticity, promoting uniform austenitization and preventing grain coarsening. After exiting the furnace, the billet is rolled into a 15mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote dynamic recrystallization and grain refinement during the final rolling process. Specifically, the furnace soaking temperature is controlled at 1200℃. The rolling time is 160 minutes, the initial rolling temperature is 1095℃, the final rolling temperature is 935℃, and the final rolling reduction is 26%. The wire drawing process is used to convert the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is distributed on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 940℃.

[0072] The online molten salt mixed-phase quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 41℃ / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The process involves increasing the amount of molten salt to promote the decomposition of untransformed residual austenite into bainite and sorbite, followed by isothermal tempering. This promotes the dispersion and precipitation of Cr carbides for strengthening, thereby controlling the strength-plasticity matching of the wire rod and reducing overall production energy consumption. Specifically, the molten salt temperature for the first stage of molten salt treatment is 465℃, the treatment time is 40s, the molten salt circulation rate is 650t / h, and the molten salt temperature rise is ≤10℃. The molten salt temperature for the second stage of molten salt treatment is 575℃, the treatment time is 255s, the molten salt circulation rate is 440t / h, and the molten salt temperature rise is ≤3℃.

[0073] The slow cooling process of the roller conveyor uses an adjustable opening of the insulation cover to blow hot air (≥255℃) from the two sections of the salt bath tank undergoing online molten salt mixed phase quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor roller conveyor through the insulation cover for slow cooling, which promotes further toughening of the wire rod structure and improves the tempering softening effect. Specifically, the wire rod is controlled to cool to 287℃ at a slow cooling rate of 0.67℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0074] Comparative Example 7:

[0075] A method for manufacturing hot-rolled wire rod differs from that in Example 4 in that the method follows a process flow of rolling → wire drawing → online quenching and isothermal treatment in the molten salt mixed phase region → air cooling. The air cooling is achieved by opening the insulation cover, conveying the wire rod via a conveyor roller, and controlling the wire rod to cool to 280°C at a slow cooling rate of 1.7°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] The comparison between Example 1 and Comparative Example 1 shows that, compared to the Stellmore air-cooled linear controlled rolling process, the high carbon and high alloy content leads to higher levels of network carbides and brittle martensite due to reduced hardenability, resulting in insufficient strength and plasticity of the wire rod and significant fluctuations in mechanical properties. This invention employs a high hardenability composition combined with online molten salt mixing phase region quenching isothermal treatment. This allows the wire rod to first pass through the initial molten salt treatment to enter the bainite and pearlite phase region, promoting the transformation of some austenite into bainite and sorbite, suppressing network carbides, and then undergoing subsequent... The molten salt treatment increases the molten salt temperature, reduces the molten salt circulation volume, lowers production energy consumption, promotes the decomposition of untransformed residual austenite into bainite and sorbite followed by isothermal tempering, suppresses abnormal structures of martensite and coarse pearlite, and regulates the multiphase structure and structural toughness online. As can be seen from the results of Examples 1 to 4, the tensile strength of hot-rolled wire rod can reach 1505~1555MPa, and the section reduction rate is 32%~37%. It can be used in applications such as 2200MPa grade stranded wire to avoid offline heat treatment and promote efficient production of stranded wire.

[0080] As can be seen from the comparison results between Example 1 and Comparative Example 2, selecting a higher spinning temperature can promote the full dissolution of carbides and the uniform distribution of carbon concentration in austenite, and avoid the premature precipitation of network carbides or proeutectoid ferrite due to excessively low spinning temperature. This provides favorable conditions for rapid cooling of the front-end molten salt treatment and promotes the nucleation and refinement of bainite and sorbite.

[0081] As can be seen from the comparison results of Example 2 and Comparative Example 3, the lower the molten salt temperature in the initial molten salt treatment, the better it is for the wire rod to cool down quickly, reduce the rate of carbon diffusion to the interface, suppress network carbides and coarse pearlite, and the greater the free energy difference between austenite and bainite, the more it can increase the driving force of bainite transformation and drive bainite to nucleate rapidly in a shear manner. With the extension of the treatment time, the amount of bainite transformation can be increased, the interlamellar spacing of trace sorbite can be refined, and the matrix strength can be improved. However, if the molten salt temperature is too low and the treatment time is too long, the bainite transformation rate and internal stress will be increased, the amount of residual austenite will be reduced, the softening difficulty will be increased, and the final ductility and toughness properties will be affected.

[0082] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the faster the carbon diffusion and the smaller the temperature difference from the wire rod surface to the core, which can reduce the phase transformation stress. As the treatment time is shortened, the bainitic phase transformation rate can be slowed down and the production energy consumption can be reduced. However, if the molten salt temperature is too high, it will affect the control of network carbides, the amount of bainitic phase change and the refinement of the microstructure. If the treatment time is too short, the wire rod temperature drop and the driving force for bainitic transformation will be insufficient, and too much untransformed austenite will remain, which will increase the risk of sorbite coarsening and fluctuations in mechanical properties, causing strength loss.

[0083] As can be seen from the comparison results between Example 3 and Comparative Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the better it is to increase the diffusion coefficient of carbon, promote the decomposition of retained austenite, and drive the sorbite phase transformation. With the extension of treatment time, it can ensure that the retained austenite is fully decomposed, avoid the retained austenite from continuing to form low-temperature brittle structure in subsequent cooling, promote the stress release of the formed bainite and sorbite, homogenize the structure, improve the ductility and toughness, and improve the cold drawing workability. However, if the molten salt temperature is too high, it is not conducive to controlling the bainite phase transformation and lamellar refinement, and it is not conducive to inhibiting cementite coarsening and Cr carbide dispersion precipitation. With the longer treatment time, the dislocation movement structure decreases and the carbide aggregates and coarsens, which will lead to the loss of strong plasticity, while increasing production energy consumption and affecting production efficiency.

[0084] 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 bainite transformation and carbide dispersion precipitation, reduce the sorbite lamellar spacing, avoid core structure deterioration, and enhance the interface strengthening effect. With the shortening of the treatment time, production energy consumption can be reduced. However, if the molten salt temperature is too low, the carbon diffusion activity will be affected, which will affect the carbide precipitation, phase transformation inoculation and isothermal tempering efficiency. If the treatment time is too short, the core carbon diffusion rate will be insufficient, the sorbite transformation will be delayed, the structure will have large residual stress, less carbide precipitation, and reduced pinning dislocation effect, which will lead to loss of strength and plasticity. In fact, the residual austenite will form a brittle structure during subsequent cooling, increasing the fluctuation of mechanical properties.

[0085] 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 slow cooling speed, continue the isothermal tempering effect of the subsequent molten salt treatment, and promote further toughening of the wire rod structure and improve the softening effect.

[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 of manufacturing high-strength multiphase hot-rolled wire rod for 2200 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The manufacturing method of the high-strength complex-phase hot-rolled wire rod for 2200MPa-grade twisted wire comprises the following steps: rolling a wire rod according to the chemical composition of a hot-rolled coil, wherein the chemical composition of the hot-rolled coil and the mass percentage thereof comprise C: 0.87% to 0.91%, Si: 0.73% to 0.88%, Mn: 0.70% to 0.90%, Cr: 0.44% to 0.53%, Al: 0.05% to 0.12%, P: ≤0.015%, S: ≤0.015%, and the rest is Fe and inevitable impurities; after the wire rod is spun into a coil according to a spinning temperature of ≥900℃, the wire rod is subjected to on-line molten salt mixed-phase zone quenching and isothermal treatment, so that the coil is first subjected to front-stage molten salt treatment and is cooled at a cooling rate of ≥34℃ / s, and then is subjected to rear-stage molten salt treatment, so that the coil is cooled at a cooling rate of ≤0.9℃ / s to ≤290℃ and is collected.

2. The method of producing high strength complex phase hot rolled wire rod for 2200 MPa class strand wire according to claim 1, characterized by, Before the rolling, the soaking temperature of a heating furnace is controlled to be 1170-1220℃, and the furnace time is 160-250 min.

3. The method of producing high strength complex phase hot-rolled wire rod for 2200 MPa grade strand wire according to claim 1, characterized by, During the rolling, the initial rolling temperature is controlled to be 1045-1095℃, the final rolling temperature is controlled to be 895-935℃, and the final rolling reduction is 26%-31%.

4. The method of producing high strength complex phase hot-rolled wire rod for 2200 MPa grade strand wire according to claim 1, characterized by, During the spinning, the spinning temperature is controlled to be 900-940℃.

5. The method of producing high strength complex phase hot rolled wire rod for 2200 MPa class strand wire according to claim 1, characterized by, The molten salt circulation amount of the front-stage molten salt treatment is 510-650 t / h, and the molten salt temperature rise is ≤10℃; the molten salt circulation amount of the rear-stage molten salt treatment is 250-440 t / h, and the molten salt temperature rise is ≤3℃.

6. The method of producing high strength complex phase hot rolled wire rod for 2200 MPa class strand wire according to claim 1, characterized by, The coil is cooled at a slow cooling rate of ≤0.9℃ / s to ≤290℃ and is collected.

7. A high strength multiphase hot-rolled wire rod for 2200 MPa class strand, characterized by, The hot-rolled coil is manufactured by the manufacturing method of the high-strength complex-phase hot-rolled wire rod for 2200MPa-grade twisted wire.

8. The high strength complex phase hot rolled wire rod for 2200 MPa class strand as claimed in claim 7, wherein, The volume percentage of the tempered bainite is 8%-14%, the volume percentage of the tempered sorbite is 86%-92%, the interlamellar spacing of the tempered sorbite is 85-125 nm, the reticular carbide level of the hot-rolled coil is 0 level, and the mechanical property difference between turns is ≤47MPa.

9. The high strength complex phase hot rolled wire rod for 2200 MPa class strand as claimed in claim 7, wherein, The diameter of the hot-rolled coil is 5.0-15.0 mm, the tensile strength is 1505-1555MPa, and the reduction of area is 32%-37%.

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