High-strength complex-phase hot-rolled wire rod for 2400mpa grade strand and manufacturing method thereof

Abstract

The application relates to a high-strength complex-phase hot-rolled wire rod for a 2400MPa-grade strand and a manufacturing method thereof. After a high-carbon component design of a trace amount of Mo-Nb is adopted to roll and spin the wire rod, the wire rod is subjected to on-line molten salt short quenching isothermal toughening treatment, so that the wire rod is first subjected to front-stage molten salt treatment and enters a bainite phase region from an austenite state, partial austenite is promoted to transform into bainite, then the wire rod is subjected to rear-stage molten salt treatment, the molten salt temperature is increased, residual austenite which has not been transformed is promoted to decompose and transform into sorbite and is subjected to isothermal tempering, finally the wire rod is subjected to roller slow cooling, and the hot-rolled wire rod with a microstructure composed of complex-phase structures of tempered bainite and tempered sorbite is prepared, the component system can be simplified, complex-phase structure regulation and on-line toughening can be realized, high-strength plasticity and plasticity matching can be improved, the tensile strength is 1588-1638MPa, the reduction of area is 30%-36%, and the hot-rolled wire rod is promoted to be produced efficiently and stably, so that the strand can be manufactured without off-line heat treatment.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire and its manufacturing method. Background Technology

[0002] Prestressed steel strands, as high-stress load-bearing tools, are widely used in railway sleepers, highways, bridges, elevated structures, urban overpasses, and construction. Improving the strength grade of prestressed steel strands not only saves materials and reduces costs but also greatly facilitates construction. Existing pearlitic hot-rolled wire rods have excellent cold-drawing properties, but due to their low microstructure strength, large amounts of cold drawing are required to achieve a significant strength increase. Once the strength grade reaches 2000MPa, further increases through cold drawing alone are insufficient; offline heat treatment is necessary to adjust the microstructure, leading to increased production processes and costs for users. Therefore, there is a need to develop a 2400MPa grade hot-rolled wire rod for strands and its manufacturing method, which eliminates the need for offline heat treatment and enables efficient and stable production.

[0003] Existing high-carbon pearlitic hot-rolled wire rods are manufactured by adding various microalloying components and combining them with Stellmore air-cooled lines, water-cooled lines, or salt bath processes. However, the following technical bottlenecks still exist in manufacturing hot-rolled wire rods for 2400MPa grade stranded wire:

[0004] I. To improve the strength and sorbitization rate of wire rods, existing stranded wire rods often have increased C, Si, and V content, combined with post-coiling forced air cooling or water bath cooling. For example, patent CN114369760B discloses a high-carbon design of C-Si-Mn-Cr-VB for stress corrosion resistant ultra-high strength steel strands, combined with online water bath cooling and slow cooling after coiling, to obtain sorbitic wire rods. However, due to the high carbon content and carbon segregation, online cooling on the Stellmore air-cooling line after coiling makes it difficult to suppress the formation of network carbides. While water bath cooling can increase the cooling rate of the wire rod, it also forms a large number of bubbles that adhere to the surface of the wire rod, affecting the heat transfer uniformity and exacerbating the cooling rate differences between different parts of the wire rod, thus affecting the stable control of network carbides. This is also affected by the hardenability of the wire rod and the segregation of alloying elements. Localized overcooling can easily lead to the formation of abnormal structures such as martensite. Although bainitic and martensitic structures have high strength, they are also brittle and can degrade the plasticity, toughness, and uniformity of the wire rod. On the other hand, slow cooling regions can lead to the formation of coarse alloy carbides or coarse pearlite, which limits the strengthening effect of alloying elements. The addition of expensive alloys such as V is relatively large, resulting in higher material costs and greater fluctuations in mechanical properties, affecting the final strength of hot-rolled wire rod and increasing the risk of wire breakage during subsequent processing. On the other hand, due to the influence of high silicon content, the incubation period of sorbite phase transformation is prolonged. On the Steyrmo air-cooled line or under slow water cooling, the minimum cooling capacity is limited. After the phase transformation incubation, the wire rod is in a low-temperature state, making it difficult to toughen abnormal structures. This results in large residual stress in the wire rod, ultimately leading to insufficient plasticity and toughness or even greater brittleness, further increasing the risk of wire breakage during subsequent processing. It is difficult to achieve stable production of hot-rolled wire rod or to avoid offline heat treatment.

[0005] II. To improve the strength and plasticity of hot-rolled wire rods for stranding, existing technologies, such as the 2400~2460MPa grade stress corrosion resistant prestressed steel strand wire rod, steel strand, and production method disclosed in patent CN120400685A, use a C-Si-Mn-Cr-V-Nb composition design combined with online salt bath isothermal treatment and slow cooling at a rate not exceeding 0.2℃ / s to produce sorbite wire rods, have limited strengthening due to the lamellar structure of the sorbite. This necessitates the use of multiple V and Nb alloys, increasing material costs and smelting difficulty. At low initial rolling temperatures, the deformation resistance of the billet increases significantly, limiting rolling efficiency. Furthermore, due to phase transformation and one-stage online salt bath isothermal treatment, there is still a risk of network carbide precipitation. To reduce strength loss and improve wire rod plasticity, online aging with low-speed slow cooling in an insulated corridor is also required, further impacting 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 high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire and its manufacturing method, which can simplify the composition system, realize the control of multiphase structure and online toughening, improve the matching of high strength and plasticity, promote the efficient and stable production of hot-rolled wire rod, so as to manufacture stranded wire without offline heat treatment.

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

[0008] A method for manufacturing high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire, the method comprising:

[0009] The wire rod is rolled into a production line based on the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%~0.98%, Si: 0.73%~0.93%, Mn: 0.75%~0.95%, Cr: 0.46%~0.64%, Nb: 0.030%~0.050%, Mo: 0.10%~0.30%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities; the wire rod is manufactured according to ≥89 After being spun into wire rods at a wire rod temperature of 0℃, the wire rods undergo online molten salt short-quench isothermal toughening treatment. This process involves first undergoing a preliminary molten salt treatment and cooling at a rate of ≥30℃ / s, causing the wire rods to transition from the austenitic state to the bainitic phase region, promoting the transformation of some austenite into bainite. Then, a subsequent molten salt treatment is performed, increasing the molten salt temperature to promote the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rods are slowly cooled on a roller conveyor to produce hot-rolled wire rods with a microstructure consisting of a multiphase structure composed of tempered bainite and tempered 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: As an effective strengthening element, C is relatively cheaper and can improve the strength of the matrix through solid solution strengthening, reduce the transformation temperature of austenite to bainite and sorbite, and expand the transformation range of bainite. In the past, it was combined with the first stage of molten salt treatment to promote the preferential transformation of austenite to bainite, and at the same time improve the stability of the residual austenite, creating conditions for the residual austenite in the later stage of molten salt to decompose into sorbite and temper. Through the synergistic effect of high carbon with Cr, Nb and Mo, it can pin dislocations and refine grains, so that the wire rod has a moderate work hardening rate. However, if the C content is too high, it will aggravate the carbon segregation of the billet, increase the difficulty of controlling network carbides and martensite, accelerate crack initiation, and the isothermal tempering time of the later stage of molten salt treatment needs to be greatly extended. Therefore, in order to take into account the high strength requirements of 2400MPa grade strand, reduce performance fluctuations, and adapt to online molten salt short quenching isothermal toughening treatment, the mass percentage of C is controlled at 0.95%~0.98%.

[0012] (2) Silicon: Si can enhance the solid solution strengthening effect, delay the nucleation and growth of cementite, inhibit the transformation of austenite to pearlite during the first stage of molten salt treatment, and hinder the coarsening of sorbite lamellar cementite during the second stage of molten salt treatment. It plays a role in refining the precipitated phase and improving the toughness of the material, thereby improving the wire rod's resistance to tempering softening and the strength of the multiphase structure. However, excessive silicon increases the deformation resistance of steel and prolongs the phase transformation incubation time, leading to a decrease in the plasticity, toughness and production efficiency of hot-rolled wire rod. Therefore, in order to control the multiphase structure, ensure the plasticity of cold working and adapt to rapid production, the mass percentage of Si is controlled at 0.73%~0.93%.

[0013] (3) Manganese: Mn can improve the hardenability of steel, enhance the stability of austenite, reduce the critical cooling rate and the bainite transformation initiation temperature, so as to inhibit the precipitation of coarse lamellar pearlite and proeutectoid ferrite, promote the uniform transformation of bainite and sorbite, avoid the weakening of the relative strength of soft materials, and hinder dislocation movement through solid solution strengthening. However, when the Mn content is too high, it will increase the element segregation during the solidification process of the billet, induce the risk of martensite precipitation, and at the same time, the stability of residual austenite will increase the efficiency of the subsequent molten salt treatment and reduce the low temperature toughness. Therefore, in order to facilitate the control of the multiphase structure of hot-rolled wire rod, improve the uniformity of the entire cross section of the wire rod, and take into account the production efficiency, the mass percentage of Mn is controlled at 0.75%~0.95%.

[0014] (4) Chromium: Cr can improve the hardenability of wire rod and significantly reduce the critical cooling rate of steel in order to promote the transformation of bainite and promote the uniform transformation of residual austenite into fine lamellar sorbite. At the same time, by reducing the diffusion rate of C, it can form carbide pinning dislocations with C, which can inhibit the growth of cementite during tempering in the later stage of molten salt treatment. However, if 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. It will also increase the difficulty of tempering, affecting production efficiency and energy consumption. Therefore, in order to take into account the control of the multiphase structure of hot-rolled wire rod and appropriately control the resistance to tempering softening of wire rod, the mass percentage of Cr is controlled at 0.46%~0.64%.

[0015] (5) Niobium: As a microalloying element, Nb can strongly pin the austenite grain boundaries during high-temperature hot rolling, thereby refining the grains. The fine austenite grains provide more nucleation sites for the subsequent transformation of bainite and sorbite. During isothermal tempering, NbC will precipitate in the form of dispersed nanoparticles, refining the distribution of Cr-based carbides, hindering dislocation slip, rapidly strengthening the matrix, and improving the tempering stability of steel. However, Nb is expensive, and excessive addition is not conducive to controlling material costs. Therefore, based on the role of Nb and material cost considerations, Nb is added appropriately, and the mass percentage of Nb is controlled at 0.030%~0.050%.

[0016] (6) Molybdenum: Mo can strongly inhibit the pearlite transformation, causing the pearlite transformation curve to shift to the right. It can reduce the diffusion ability of carbide-forming elements, thereby delaying the formation of carbide precipitation. At the same time, it can effectively pin dislocations and grain boundaries, hindering dislocation movement and grain coarsening. Together with Nb and Cr, it can prevent the precipitate phase from ripening and growing at high temperature, avoid rapid strength decay, and prevent the multiphase structure from over-tempering and softening. However, the cost of Mo is relatively high, and excessive addition is not conducive to controlling material cost. Therefore, based on the role of Mo and material cost considerations, the mass percentage of Mo is controlled at 0.10%~0.30%.

[0017] (7) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% should be controlled.

[0018] The aforementioned hot-rolled wire rod adopts a high-carbon composition design of C-Si-Mn-Cr-Nb-Mo, eliminating the need for added elements such as V and B, thus simplifying the composition system. The trace addition of Nb and Mo controls material costs. Simultaneously, C and Si provide a solid solution strengthening basis, Si inhibits cementite coarsening, Mn and Cr regulate the wire rod's hardenability, reduce the critical cooling rate of the steel, and expand the bainite phase region. Cr, Nb, and Mo improve the stability of carbide precipitation strengthening, thereby promoting preferential bainite transformation under short quenching and controlling martensite formation. This process provides favorable conditions for abnormal microstructure, refinement of sorbite microstructure, appropriate tempering and softening of multiphase microstructure, and adaptation to rapid production. Based on this, the wire drawing temperature is ≥890℃, ensuring the wire rod is in a high-temperature austenitizing state, with sufficient solid solution of alloying elements and uniform carbon concentration distribution. This avoids premature precipitation of network carbides or insufficient austenite recrystallization caused by excessively low temperatures. It also provides favorable conditions for increasing the wire rod cooling rate and promoting the transformation of austenite to fine bainite. After wire drawing, the wire rod does not undergo air cooling but instead undergoes online molten salt short-quench isothermal toughening treatment.

[0019] Firstly, compared to the limited maximum cooling capacity of the Stellmore air-cooled line and the large cooling fluctuations inherent in water bath cooling, which make it difficult to suppress network carbides or martensite and control the preferential bainite transformation, molten salt offers several advantages. Firstly, molten salt has a higher heat transfer coefficient than air and water, promoting rapid cooling of the wire rods. This allows them to quickly bypass the temperature range where network carbides precipitate from the high-temperature austenitic state, suppressing network carbide precipitation and directly entering the bainite transformation zone, thus promoting the preferential formation of bainite with higher dislocation density and distortion. Secondly, as the wire rods pass through the molten salt, the molten salt covers the surface of the wire rods for uniform heat transfer. Unlike air-cooled lines, there is no temperature difference between the air-receiving and air-receiving surfaces. Unlike water-cooled lines, heat transfer is not affected by air bubbles adhering to the wire rod surface. Furthermore, short-term treatment avoids localized overcooling that could lead to abnormal martensite structures and increased brittleness. Fluctuations in mechanical properties are avoided by preventing the precipitation of coarse alloy carbides due to excessively slow cooling rates, which would result in a loss of strength and ductility. Under the influence of wire rod hardenability, the partial bainite transformation is relatively uniform and controllable. Compared to a single-stage salt bath treatment, the molten salt temperature in the first-stage molten salt treatment is lower. On the one hand, carbon atom diffusion is suppressed under rapid cooling of the wire rod, which can reduce segregation and effectively suppress the risk of network carbides caused by high carbon or carbon segregation, preventing them from cutting the matrix and affecting ductility and toughness, as well as the carbon source for phase transformation. On the other hand, under the short quenching effect of the low-temperature first-stage molten salt, the wire rod can form a partial bainite matrix. The bainite transformation introduces a large number of high-density dislocations, which can improve the matrix strength and reduce its dependence on alloy composition. At the same time, the rapid phase transformation reduces residual austenite, reduces the phase transformation pressure of the subsequent molten salt treatment, and compensates for the adverse effects of high silicon and other factors that prolong the phase transformation incubation period, so as to facilitate rapid production.

[0020] Second, compared to the limitations of minimum cooling capacity under the Stellmore air-cooling line or slow water cooling, and the difficulty in controlling and toughening the multiphase microstructure under continuous cooling, the wire rod undergoes a different approach. Firstly, the higher molten salt temperature during the later stage of molten salt treatment promotes the decomposition of untransformed residual austenite after short quenching into fine lamellar sorbite. Simultaneously, the wire rod gradually transforms to the same temperature as the molten salt during this later stage, promoting a full transformation of the core microstructure. Combined with Si's ability to suppress cementite coarsening and form soft pearlite or ferrite, this achieves a controllable phase transformation from bainite to sorbite and promotes the dispersed precipitation of alloy carbides from Cr, Nb, and Mo to strengthen the matrix. Secondly, after the phase transformation inoculation, the wire rod can undergo isothermal tempering at the molten salt temperature. The high temperature provides continuous thermal kinetics to promote atomic diffusion, appropriately reducing the bainite dislocation density, effectively improving bainite brittleness, reducing microstructural stress, and achieving bainite formation. Online toughening of sorbite structure utilizes bainite, which is conventionally considered an abnormal structure, to improve the strength-ductility balance. Compared to single-stage salt bath treatment, which carries the risk of martensitic abnormal structure and requires low-rate online aging, sorbite can rapidly undergo phase transformation inoculation, resulting in higher strength in both bainite and sorbite matrix. The dispersed distribution of alloy carbides in the matrix improves the matrix's resistance to tempering softening. As the treatment time increases, the full phase transformation of the structure can prevent residual austenite from continuing to form brittle martensitic structure during subsequent cooling. Furthermore, after phase transformation inoculation, the structure can be rapidly toughened online through isothermal tempering in the subsequent molten salt treatment, reducing the restriction on slow cooling on the roller table. After the wire rod exits the molten salt, the softening effect of the later stage of the subsequent molten salt treatment can be continued, promoting further toughening of the wire rod structure without the need for excessively low cooling rates, thus promoting efficient and stable production of hot-rolled wire rod.

[0021] Before rolling, selecting a higher heating furnace homogenization temperature and an appropriate furnace time can promote the solid solution of alloying elements, reduce the influence of segregation, ensure thorough heating of the billet to avoid uneven rolling deformation, and prevent excessively high temperatures or excessively long furnace times from causing grain coarsening or overheating. In the preferred technical solution, before rolling, the heating furnace homogenization temperature is controlled at 1170~1220℃, and the furnace time is 160~300min.

[0022] The initial rolling temperature can be connected to the heating furnace soaking temperature. Selecting a higher initial rolling temperature reduces rolling deformation resistance and wear on the rolling line. A larger initial rolling reduction can promote rapid rolling and provide deformation energy storage. With appropriate final rolling temperature and final rolling reduction, NbC precipitation of studded grain boundaries and final rolling dynamic recrystallization refine the grains. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1050~1100℃, the initial rolling reduction is 37%~42%, the final rolling temperature is 900~940℃, and the final rolling reduction is 22%~27%.

[0023] During the wire spinning process, the wire spinning temperature can be further controlled to avoid the risk of grain coarsening caused by excessively high wire spinning temperature. In a preferred technical solution, the wire spinning temperature is controlled at 890~930℃.

[0024] The lower the molten salt temperature in the initial molten salt treatment, the higher the supercooling, which inhibits the formation of network carbides and pearlite, increases the free energy difference between austenite and bainite, thereby increasing the bainite nucleation rate and promoting bainite microstructure refinement. With prolonged treatment time, the proportion and uniformity of bainite transformation can be increased, improving matrix strength and reducing the difficulty of temperature rise control in the subsequent molten salt treatment. However, if the molten salt temperature is too low, the bainite dislocation density increases, potentially even producing abnormal martensite microstructure, increasing the difficulty of molten salt temperature rise control and energy consumption. With excessively long treatment times, bainite transformation increases, internal stress increases, and residual austenite is insufficient, increasing the difficulty of isothermal tempering softening in the subsequent molten salt treatment, leading to decreased ductility and toughness, increased mechanical fluctuations, and impacted production efficiency. Conversely, higher molten salt temperatures can reduce the difference in cooling rate between the wire rod surface and core, improving... To improve microstructure uniformity, shorter processing times allow for control of bainite transformation, reducing the difficulty of isothermal tempering in subsequent molten salt treatment and increasing production efficiency. However, excessively high molten salt temperatures result in insufficient bainite nucleation driving force, even falling below the critical cooling rate for pearlite to form coarse lamellar pearlite. Insufficient bainite transformation and excessive retained austenite, due to shorter processing times, negatively impact matrix strength. Furthermore, the increased difficulty in controlling temperature rise and accelerated softening in subsequent molten salt treatment leads to greater fluctuations in mechanical properties. Therefore, the initial molten salt treatment allows for control of molten salt temperature and processing time, suppressing network carbides and martensite structures. Short quenching promotes the transformation of some high-temperature austenite into bainite, preparing the microstructure for subsequent molten salt treatment. In the preferred embodiment, the molten salt temperature for the initial treatment is 472~515℃, and the processing time is 5~24s.

[0025] The pre-treatment of molten salt can be controlled by selecting an appropriate molten salt circulation rate to improve the consistency of wire rod performance while taking into account production energy consumption. In the preferred technical solution, the molten salt circulation rate of the pre-treatment is 500~600t / h, and the molten salt temperature rise is ≤10℃.

[0026] The higher the molten salt temperature in the subsequent molten salt treatment, the better the diffusion ability of carbon and alloying elements, promoting the decomposition and transformation of residual austenite, promoting the slip of high-density dislocations in bainite, and reducing the difficulty of controlling the molten salt temperature rise. With prolonged treatment time, it can promote the full phase transformation of austenite, alleviate lattice distortion and release structural stress, and allow alloy carbides to fully precipitate and diffuse, improving ductility and toughness. However, excessively high molten salt temperatures are detrimental to suppressing cementite coarsening. With prolonged treatment time, the dispersed nanoscale carbides aggregate and grow, losing their ability to pin dislocations, accelerating tempering softening, and resulting in a loss of strong plasticity due to excessively low dislocation density, while also increasing production energy consumption. Conversely, lower molten salt temperatures are beneficial for suppressing cementite coarsening, refining the interlamellar spacing of sorbite, promoting the dispersed precipitation of alloy carbides, and slowly releasing structural stress. Shorter treatment times can reduce production energy consumption and maintain stability. While maintaining the strengthening effect of nanoprecipitation and retaining high strength characteristics, if the molten salt temperature is too low, the decomposition rate of retained austenite and the release rate of internal stress decrease. If the treatment time is too short, insufficient bainite toughening and residual internal stress will exacerbate stress concentration when the wire rod is under stress. Insufficient precipitation of alloy carbides will result in a loss of strength and plasticity. Furthermore, incomplete phase transformation of retained austenite will increase the risk of abnormal precipitation of martensite, leading to fluctuations in mechanical properties. Therefore, the subsequent molten salt treatment can further control the molten salt temperature and treatment time, promote the decomposition of untransformed retained austenite into sorbite, and perform isothermal tempering on the quenched structure of bainite and sorbite to promote the full dispersion and precipitation of alloy carbides, thereby regulating 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 532~572℃, and the treatment time is 200~350s.

[0027] The downstream molten salt treatment can use an appropriate molten salt circulation rate to control the molten salt temperature rise and production energy consumption. In the preferred technical solution, the molten salt circulation rate of the downstream molten salt treatment is 400~600t / h, and the molten salt temperature rise is ≤3℃.

[0028] The roller conveyor slow cooling can further control the slow cooling speed. Slow cooling promotes further toughening of the wire rod structure and improves the tempering and softening effect of the wire rod, while also taking into account production efficiency. In the preferred technical solution, the roller conveyor slow cooling controls the wire rod to cool to below 280°C at a slow cooling speed of 0.45~0.75°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 at ≥260°C during the online molten salt short quench isothermal toughening treatment to the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the heat insulation cover. The heat energy of the online molten salt short quench isothermal toughening treatment can be recovered and utilized through the hot air, thereby reducing production energy consumption and promoting the rapid production of wire rod.

[0030] A high-strength multiphase hot-rolled wire rod for 2400MPa 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 2400MPa grade stranded wire described in any one of the above-mentioned methods.

[0031] The aforementioned hot-rolled wire rods are designed with a high carbon content of trace amounts of Mo-Nb, which simplifies the composition system and controls material costs. The microstructure comprises a multiphase structure of tempered bainite and tempered sorbite. Compared to sorbite wire rods obtained from air-cooled, water-cooled, or one-stage salt bath treatment, this effectively suppresses network carbides, abnormal martensite structures, and core cementite coarsening, improving microstructure uniformity. Furthermore, bainite possesses a high dislocation density, effectively hindering dislocation slip under external forces, resulting in higher strength than sorbite. After isothermal tempering to transform into tempered bainite, it retains high strength characteristics and effectively improves brittleness, thus addressing the conventionally considered anomalous... The bainite structure, with its numerous proliferating dislocations, can enhance the draw hardening capacity of hot-rolled wire rods. Sorbite, with its finer lamellar spacing and higher strength compared to pearlite, transforms into intermediate tempered sorbite after sufficient phase transformation inoculation and isothermal tempering. This further alleviates stress concentration, provides good plastic deformation capacity, and prevents crack initiation and propagation caused by stress concentration. The nanoscale carbides formed by the combination of carbon and alloying elements are uniformly distributed within the multiphase structure, which can pin dislocation movement and further improve strength, toughness, and draw hardening uniformity, thus eliminating the need for offline heat treatment and reducing the risk of wire breakage during strand manufacturing.

[0032] The higher the proportion 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 matrix ductility and toughness; in the preferred technical solution, the volume percentage of tempered bainite is 42%~54%, the volume percentage of tempered sorbite is 46%~58%, and the lamellar spacing of tempered sorbite is 75~120nm.

[0033] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference of ≤53MPa within the same coil. This avoids the adverse effects of network carbides on the uniformity of the structure and the ductility and toughness, and improves the reliability of the stranded wire. By suppressing abnormal structures and improving the uniformity of the structure, the hot-rolled wire rod has smaller fluctuations in mechanical properties, which can further improve the stability of subsequent wire drawing and the consistency of the finished stranded wire performance.

[0034] In the preferred technical solution, the hot-rolled wire rod has a diameter of 5.5~16mm, a tensile strength of 1588~1638MPa, and a reduction of area of ​​30%~36%. The hot-rolled wire rod can be used to manufacture strands of different specifications. The high initial tensile strength and microstructure result in better draw hardening ability, which can reduce the number of drawing passes, quickly reach the target diameter and strength, and reduce the risk of wire breakage due to plastic loss during the process. The high reduction of area can withstand large deformation drawing, so as to avoid offline heat treatment and reduce the risk of wire breakage during drawing and twisting.

[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 the manufacturing method of hot-rolled wire rod for stranded wire has high material cost, difficulty in effectively controlling abnormal structure and strength and plasticity, and limited production efficiency, the present invention adopts Mo-Nb chemical composition design combined with online molten salt short quenching isothermal toughening technology. The wire rod after wire drawing first undergoes a front-end molten salt treatment and enters the bainite phase region from the austenite state. This can effectively inhibit the coarsening of network carbides and cementite, and promote the transformation of some austenite to bainite. Then, after the rear-end molten salt treatment, the molten salt temperature is increased, which promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full precipitation and dispersion of alloy carbides. Finally, after slow cooling by roller table, the composition system can be simplified, the multiphase structure can be controlled and online toughening can be achieved, the high strength and plasticity matching can be improved, and the efficient and stable production of hot-rolled wire rod can be promoted. It has good industrial adaptability.

[0037] (2) In view of the current situation that the cost of existing pearlitic wire rod materials for stranded wire is high and offline processing is required, while bainitic and martensitic structures are brittle and have a high risk of wire breakage, this invention adopts Mo-Nb chemical composition design, which does not require the addition of V, B and other elements, which can simplify the composition system and control the material cost. The microstructure includes a multiphase structure composed of tempered bainite and tempered sorbite, which can effectively suppress the coarsening of network carbides, abnormal martensite structure and core cementite, and improve the uniformity of the structure. At the same time, tempered bainite can retain high strength characteristics and effectively improve brittleness, and tempered sorbite has good plastic deformation ability. The nanoscale carbides formed by the combination of carbon and alloying elements are uniformly distributed in the multiphase structure, which can improve the strength and plasticity matching of hot rolled wire rod, achieving a tensile strength of 1588~1638MPa and a section reduction rate of 30%~36%, so as to avoid offline heat treatment and reduce the risk of wire breakage during drawing and twisting. 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 high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%, Si: 0.73%, Mn: 0.79%, Cr: 0.49%, Nb: 0.05%, Mo: 0.3%, P: 0.013%, 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 short quenching isothermal toughening → roller table slow cooling → coiling, specifically:

[0045] 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, promoting compositional homogenization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 5.5mm via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC precipitation, grain boundary formation, and dynamic recrystallization to refine the grains. Specifically, the furnace soaking temperature is controlled at 1220℃, and the furnace dwell time is 160 minutes. The initial rolling temperature is 1100℃, the initial rolling reduction is 42%, the final rolling temperature is 940℃, and the final rolling reduction is 27%. 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 higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 930℃.

[0046] The online molten salt short-quench isothermal toughening 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, causing the wire rod to cool at a rate of 30°C / s. This rapidly transitions the wire rod from the high-temperature austenitic state, bypassing the network carbide precipitation temperature range, into the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and promotes the transformation of some austenite into bainite. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, improving the molten salt content. Temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full precipitation of microalloyed carbides to regulate the strength and plasticity matching of wire rods. Specifically: the molten salt temperature of the first stage molten salt treatment is 515℃, the treatment time is 5s, the molten salt circulation rate is 500t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage molten salt treatment is 572℃, the treatment time is 200s, the molten salt circulation rate is 400t / h, and the molten salt temperature rise is ≤3℃.

[0047] The roller conveyor slow cooling process employs controlled opening of the insulation hood to blow hot air (≥260℃) from above the two salt bath tanks undergoing online molten salt short-quench isothermal toughening treatment into the insulation hood. The wire rod is then conveyed by the roller conveyor through the insulation hood for slow cooling, 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 0.75℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure 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 is controlled at a uniform heating temperature of 1175°C, with a furnace time of 250 min, an initial rolling temperature of 1000°C, a final rolling temperature of 860°C, and a wire drawing temperature of 840°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 80% capacity, fans 1 to 4 are turned on to cool the wire rod to 694℃ at a cooling rate of 7.7℃ / s. Then, fans 5 to 14 are turned on to 20% capacity to cool the wire rod to 260℃ 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 is heated to a uniform temperature of 1160°C, the furnace time is 260 min, the initial rolling temperature is 995°C, the final rolling temperature is 830°C, and the wire drawing temperature is 820°C. During the molten salt treatment before the online molten salt short quenching isothermal toughening process, the wire rod is cooled at a rate of 28°C / s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0052] Example 2:

[0053] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.98%, Si: 0.93%, Mn: 0.79%, Cr: 0.46%, Nb: 0.03%, Mo: 0.11%, P: 0.015%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt short quenching isothermal toughening → roller table slow cooling → 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, promoting composition homogenization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 9mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC precipitation, grain boundary formation, and dynamic recrystallization to refine the grains. Specifically, the furnace soaking temperature is controlled at 1200℃, and the furnace dwell time is 190 minutes. The initial rolling temperature is 1080℃, the initial rolling reduction is 40%, 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 coils through the wire drawing mechanism. The coils are distributed on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for promoting the transformation of austenite to fine bainite. Specifically, the wire drawing temperature is controlled at 925℃.

[0055] The online molten salt short-quench isothermal toughening 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, causing the wire rod to cool at a rate of 31°C / s. This rapidly transitions the wire rod from the high-temperature austenitic state, bypassing the network carbide precipitation temperature range, into the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and promotes the transformation of some austenite into bainite. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, improving the molten salt content. Temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full precipitation of microalloyed carbides to regulate the strength and plasticity matching of wire rods. Specifically: the molten salt temperature of the first stage molten salt treatment is 503℃, the treatment time is 10s, the molten salt circulation rate is 530t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage molten salt treatment is 560℃, the treatment time is 265s, the molten salt circulation rate is 430t / h, and the molten salt temperature rise is ≤3℃.

[0056] The roller conveyor slow cooling process employs controlled opening of the insulation hood to blow hot air (≥260℃) from above the two salt bath tanks undergoing online molten salt short-quench isothermal toughening treatment into the insulation hood. The wire rod is then conveyed by the roller conveyor through the insulation hood for slow cooling, 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 uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure 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 short quenching isothermal toughening process, the wire rod is cooled at a cooling rate of 36°C / s, the molten salt temperature of the initial molten salt treatment is 460°C, the treatment time is 28s, 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 short quenching isothermal toughening process, the wire rod is cooled at a cooling rate of 29°C / s, the molten salt temperature of the initial molten salt treatment is 520°C, the treatment time is 5s, 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 high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%, Si: 0.88%, Mn: 0.75%, Cr: 0.52%, Nb: 0.039%, Mo: 0.25%, 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 short quenching isothermal toughening → roller table slow cooling → 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, promoting compositional homogenization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 13mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC precipitation, grain boundary anchoring, and dynamic recrystallization to refine the grains. Specifically, the furnace soaking temperature is controlled at 1190℃, and the furnace dwell time is 240 minutes. n, the initial rolling temperature is 1060℃, the initial rolling reduction is 38%, the final rolling temperature is 920℃, and the final rolling reduction is 23.5%; the wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 910℃.

[0064] The online molten salt short-quench isothermal toughening 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, causing the wire rod to cool at a rate of 33°C / s. This rapidly transitions the wire rod from the high-temperature austenitic state, bypassing the network carbide precipitation temperature range, into the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and promotes the transformation of some austenite into bainite. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, improving the molten salt content. Temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full precipitation of microalloyed carbides to regulate the strength and plasticity matching of wire rods. Specifically: the molten salt temperature of the first stage molten salt treatment is 486℃, the treatment time is 18s, the molten salt circulation rate is 575t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage molten salt treatment is 546℃, the treatment time is 310s, the molten salt circulation rate is 520t / h, and the molten salt temperature rise is ≤3℃.

[0065] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥260℃) from above the two salt bath tanks undergoing online molten salt short-quench isothermal toughening 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 275℃ at a slow cooling rate of 0.55℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure 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 590°C, the treatment time is 360s, 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 525°C, the treatment time is 180s, 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 high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.96%, Si: 0.79%, Mn: 0.95%, Cr: 0.64%, Nb: 0.044%, Mo: 0.24%, 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 short quenching isothermal toughening → roller table slow cooling → 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, promoting compositional homogenization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 16mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC precipitation, grain boundary formation, and dynamic recrystallization to refine the grains. Specifically, the furnace soaking temperature is controlled at 1170℃, and the furnace dwell time is 300 minutes. The initial rolling temperature is 1050℃, the initial rolling reduction is 37%, the final rolling temperature is 900℃, and the final rolling reduction is 22%. 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 higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding the precipitation of network carbides and providing favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 890℃.

[0073] The online molten salt short-quench isothermal toughening 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, causing the wire rod to cool at a rate of 34°C / s. This rapidly transitions the wire rod from the high-temperature austenitic state, bypassing the network carbide precipitation temperature range, into the bainite phase region. This inhibits the coarsening of network carbides and cementite lamellars, and promotes the transformation of some austenite into bainite. The wire rod is then conveyed via roller conveyor through the second stage salt bath for subsequent molten salt treatment, improving the molten salt content. Temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full precipitation of microalloyed carbides to regulate the strength and plasticity matching of wire rods. Specifically: the molten salt temperature of the first stage molten salt treatment is 472℃, the treatment time is 24s, the molten salt circulation rate is 600t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage molten salt treatment is 532℃, the treatment time is 350s, the molten salt circulation rate is 600t / h, and the molten salt temperature rise is ≤3℃.

[0074] The roller conveyor slow cooling process uses an adjustable insulation cover to blow hot air (≥260℃) from the two salt bath tanks of the online molten salt short quenching isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers 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 278℃ at a slow cooling rate of 0.45℃ / 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 method follows a process flow of rolling → wire drawing → online molten salt short quenching and isothermal toughening → 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 268°C at a slow cooling rate of 1.4°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[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 results of Example 1 and Comparative Example 1, compared with the problems of higher-level network carbides, abnormal martensite structure and core pearlite generated on the Stellmore air-cooled line, resulting in insufficient strength and plasticity and poor microstructure uniformity of hot-rolled wire rod, the present invention adopts Mo-Nb chemical composition design combined with online molten salt short quenching isothermal toughening technology. After wire drawing, the wire rod first undergoes a first-stage molten salt treatment and enters the bainite phase region from the austenite state, which can effectively inhibit the coarsening of network carbides and cementite, and promote the transformation of some austenite to bainite. Then, after the second-stage molten salt treatment, the untransformed residual austenite is decomposed into sorbite and isothermally tempered, which promotes the full precipitation and dispersion of alloy carbides, realizing the control of multiphase structure and online toughening. As can be seen from the results of Examples 1 to 4, the hot-rolled wire rod can achieve a tensile strength of 1588~1638MPa and a section reduction rate of 30%~36%, so as to avoid offline heat treatment and reduce the risk of wire breakage during drawing and twisting.

[0081] As can be seen from the comparison results of Example 1 and Comparative Example 2, a wire drawing temperature ≥890℃ can keep the wire rod in a high-temperature austenitizing state, ensure sufficient solid solution of alloying elements, and achieve uniform carbon concentration distribution. This avoids insufficient austenite recrystallization caused by premature precipitation of network carbides due to excessively low temperature, and provides favorable conditions for increasing the cooling rate of the wire rod and promoting the transformation of austenite to fine bainite.

[0082] As can be seen from the comparison results between Example 2 and Comparative Example 3, the lower the molten salt temperature in the first stage of molten salt treatment, the more it can inhibit the formation of network carbides and pearlite, increase the nucleation rate of bainite, and promote the refinement of bainite structure. With the extension of treatment time, the proportion and distribution uniformity of bainite transformation can be increased, and the matrix strength can be improved. However, if the molten salt temperature is too low, the bainite dislocation density will increase, and the difficulty of controlling the molten salt temperature rise and energy consumption will increase. With the treatment time being too long, the bainite transformation will increase, the internal stress will increase, and the residual austenite will be insufficient, which will increase the difficulty of isothermal tempering and softening in the second stage of molten salt treatment, causing a decrease in plasticity and toughness, an increase in mechanical fluctuations, and affecting production efficiency.

[0083] The comparison results between Example 2 and Comparative Example 4 show that a higher molten salt temperature in the initial molten salt treatment can reduce the difference in cooling rates between the wire rod surface and core, improving the uniformity of the microstructure. With a shorter treatment time, the amount of bainite transformation can be controlled, reducing the difficulty of isothermal tempering in the subsequent molten salt treatment and improving production efficiency. However, if the molten salt temperature is too high, the driving force for bainite nucleation is insufficient, even falling below the critical cooling rate for pearlite, resulting in coarse lamellar pearlite. With a shorter treatment time, insufficient bainite transformation and excessive retained austenite will affect the matrix strength. Simultaneously, the difficulty of controlling the temperature rise in the subsequent molten salt treatment increases, and softening accelerates, leading to greater fluctuations in mechanical properties.

[0084] 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 more it can promote the decomposition and phase transformation of residual austenite, promote the slip of high-density dislocations in bainite, and reduce the difficulty of controlling the temperature rise of molten salt. With the extension of treatment time, it can promote the full phase transformation of austenite structure, alleviate lattice distortion and release structural stress, and allow alloy carbides to fully precipitate and diffuse, thereby improving plasticity and toughness. However, if the molten salt temperature is too high, it will not be conducive to suppressing cementite coarsening. With the treatment time being too long, the dispersed nanoscale carbides will aggregate and grow, lose their ability to pin dislocations, and tempering softening will be accelerated. If the dislocation density is too low, it will lead to a loss of strong plasticity and increase production energy consumption.

[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 suppress cementite coarsening, refine the sorbite lamellar spacing, promote the dispersed precipitation of alloy carbides, and slowly release structural stress. With the shortening of treatment time, production energy consumption can be reduced, the nano-precipitation strengthening effect can be maintained, and high strength characteristics can be retained. However, if the molten salt temperature is too low, the decomposition rate of residual austenite and the internal stress release rate will decrease. With the treatment time being too short, insufficient bainite toughening and residual internal stress will aggravate stress concentration when the wire rod is under stress. Insufficient precipitation of alloy carbides will result in loss of strong and ductile properties. In fact, incomplete phase transformation of residual austenite will increase the risk of abnormal precipitation of martensite, leading to fluctuations in mechanical properties.

[0086] As can be seen from the comparison results of Example 4 and Comparative Example 7, the slow cooling of the wire rod on the roller table after exiting the molten salt can prolong the softening effect of the wire rod in the later stage of the molten salt treatment, and promote the further toughening of the wire rod structure. However, it is not necessary to use excessively low cooling rates, which can promote the efficient and stable production of hot-rolled 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 for manufacturing high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire, characterized in that, Its manufacturing methods include: The wire rod is rolled into production line according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%~0.98%, Si: 0.73%~0.93%, Mn: 0.75%~0.95%, Cr: 0.46%~0.64%, Nb: 0.030%~0.050%, Mo: 0.10%~0.30%, 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 ≥890℃, it undergoes online molten salt short-quench isothermal toughening treatment. This process involves the wire rod undergoing a preliminary molten salt treatment and cooling at a rate of ≥30℃ / s to achieve the desired temperature. The austenite enters the bainite phase region, promoting the transformation of some austenite into bainite. After a subsequent molten salt treatment, the molten salt temperature is increased, promoting the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling via a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, forming a multiphase microstructure. The molten salt temperature of the initial molten salt treatment is 472~515℃, and the treatment time is 5~24s. The molten salt temperature of the subsequent molten salt treatment is 532~572℃, and the treatment time is 200~350s. The slow cooling via the roller conveyor controls the wire rod to cool to below 280℃ at a slow cooling rate of 0.45~0.75℃ / s before coiling.

2. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1170~1220℃ and the furnace time is 160~300min.

3. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1050~1100℃, the initial rolling reduction is 37%~42%, the final rolling temperature is 900~940℃, and the final rolling reduction is 22%~27%.

4. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to claim 1, characterized in that, During the spinning process, the spinning temperature is controlled at 890~930℃.

5. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to claim 1, characterized in that, The molten salt circulation rate of the first-stage molten salt treatment is 500~600t / h, and the molten salt temperature rise is ≤10℃; the molten salt circulation rate of the second-stage molten salt treatment is 400~600t / h, and the molten salt temperature rise is ≤3℃.

6. A high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of high-strength multiphase hot-rolled wire rod for 2400MPa grade strand as described in any one of claims 1 to 5.

7. The high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to claim 6, characterized in that, The volume percentage of the tempered bainite is 42%~54%, the volume percentage of the tempered sorbite is 46%~58%, the lamellar spacing of the tempered sorbite is 75~120nm, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤53MPa.

8. The high-strength multiphase hot-rolled wire rod for 2400MPa grade stranded wire according to claim 6, characterized in that, The hot-rolled wire rod has a diameter of 5.5~16mm, a tensile strength of 1588~1638MPa, and a reduction of area of ​​30%~36%.

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