A hot-rolled wire rod for 2230MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a hot-rolled wire rod for stranded wire of grade 2230MPa and its manufacturing method. The method involves designing and rolling a high-carbon Cr-Nb-Al composition into wire rod, followed by online molten salt isothermal toughening treatment. This process first involves the wire rod undergoing a pre-treatment molten salt treatment, transitioning it from an austenitic state to a sorbite phase, forming a microstructure dominated by sorbite. A subsequent molten salt treatment reduces the molten salt circulation rate, promoting the further transformation of untransformed residual austenite into sorbite, followed by isothermal tempering and partial melting of sorbite lamellars. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered sorbite, ferrite, and melted sorbite. This method controls material costs, achieving a tensile strength of 1513~1563MPa and a reduction of area of ​​31%~36%, while balancing microstructure uniformity and production efficiency. This eliminates the need for offline heat treatment during stranded wire manufacturing and reduces the number of drawing passes and the risk of cracking.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a 2230MPa grade hot-rolled wire rod for stranded wire and its manufacturing method. Background Technology

[0002] With the rapid development of the construction, transportation, and bridge industries, and the gradual improvement of prestressed technology, steel strand, as an important stress-bearing structure, is also constantly evolving. Conventional 1960MPa-grade high-strength strand can no longer meet the rapidly growing market demand. After the strength grade of steel strand was increased to 2200MPa, conventional raw material wire rod production lines could no longer meet the production of high-strength, high-toughness hot-rolled wire rod. At the same time, the risk of wire breakage during downstream wire forming and stranding processes increased rapidly, often requiring additional high-energy-consuming heat treatment methods such as offline lead baths. This led to a significant increase in production costs and energy consumption, and a significant decrease in production efficiency. Therefore, it is necessary to develop a hot-rolled wire rod for strand with a high strength and plasticity match and uniform structure to eliminate the need for offline heat treatment and for the production of 2230MPa-grade strands.

[0003] Existing high-strength hot-rolled wire rods for stranded wire use a high-carbon composition system. However, the following technical bottlenecks remain in manufacturing hot-rolled wire rods for stranded wire with a high balance of strength and plasticity and uniform microstructure:

[0004] I. To improve the strength of wire rod, existing technologies often employ high-carbon, high-silicon compositions combined with Stellmor air-cooled wire production. For example, patent CN120400688A discloses an alloy, wire rod, steel strand, and method for 2230MPa grade high corrosion-resistant steel strand. This method uses a C-Si-Mn-Cr-V composition, combined with strong air cooling after billet rolling and wire drawing to produce sorbitic wire rods, which are then manufactured into strands using an offline salt bath. However, due to the high carbon content and the maximum cooling capacity of the Stellmor air-cooled wire, carbon atoms tend to segregate along austenite grain boundaries, forming network carbides. These network carbides can disrupt the microstructure, affect the wire rod's ductility and toughness, increase stress concentration during subsequent drawing, lower the fatigue crack initiation threshold, and shorten the strand's service life. While efforts are made to minimize the level of network carbides and promote lamellar refinement, increasing the air-cooling strength only provides limited improvement to the network carbide problem and can further exacerbate the temperature differences between the air-receiving and air-receiving surfaces of the wire rod, as well as between the surface and the core. While existing technologies employ water bath cooling to enhance cooling intensity—for example, patent CN119162504A discloses a high-strength prestressed steel strand wire rod and its preparation method, which combines continuous rolling and online EDC water bath cooling—water bath cooling generates numerous bubbles that adhere to the wire rod surface, leading to uneven heat transfer. Due to the wire rod's hardenability, localized overcooling can easily form a brittle martensitic structure, while slow cooling in the core or localized areas will result in the formation of coarse pearlite, causing plasticity loss and significant fluctuations in mechanical properties, exacerbating the risk of wire breakage during subsequent drawing. On the other hand, rolling can extend the process chain, and high silicon content can significantly improve austenite stability. Due to the minimum cooling capacity and length limitations of the Stellmore air-cooled line, the wire rod is already at a low temperature after phase transformation inoculation, leaving significant residual phase transformation stress in the structure. Alternatively, residual austenite may easily transform into martensite during subsequent cooling, leading to increased fluctuations in the wire rod's mechanical properties and increased susceptibility to cracking due to uneven tension during strand twisting.

[0005] II. To improve the sorbitization rate and strength and plasticity of wire rods, existing technologies incorporate microalloying components in production. For example, patent CN120400686A discloses wire rods, wire rods, and their production methods for 2200-2260MPa grade ultra-high stress corrosion resistant prestressed steel strands. These materials employ a C-Si-Mn-V-Nb composition design, appropriately reducing the carbon content, and combining post-rolling isothermal salt bath treatment to obtain the wire rods. However, the addition of multiple microalloying components increases material costs. On the one hand, the high initial rolling temperature limits rolling efficiency due to the limitation of single-stage salt bath treatment. The phase transformation temperature and microstructure control also lead to a higher molten salt circulation volume and increased production energy consumption. On the other hand, the strength loss caused by cementite coarsening means that the salt bath treatment time should not be too long, requiring low-speed cooling aging to compensate for plasticity control, which affects production efficiency. At the same time, the insufficient initial strength of the wire rod also increases the number of subsequent drawing passes and the reduction rate, affecting strand production efficiency, or leading to the risk of wire breakage due to significant plasticity loss during the process. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the above-mentioned technical problems. The present invention provides a hot-rolled wire rod for stranded wire of 2230MPa grade and its manufacturing method, which can control material costs, improve the strength and plasticity matching of hot-rolled wire rod, and take into account the uniformity of structure and production efficiency, so as to eliminate offline heat treatment during stranded wire manufacturing and reduce the number of drawing passes and cracking risk.

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

[0008] A method for manufacturing hot-rolled wire rod for 2230MPa 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: 0.88%~0.92%, Si: 0.63%~0.81%, Mn: 0.65%~0.85%, Cr: 0.58%~0.68%, Nb: 0.005%~0.020%, Al: 0.16%~0.26%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is produced at a spinning temperature of ≥880℃. After being rolled into wire rod, the wire rod undergoes online molten salt isothermal toughening treatment. The wire rod first undergoes a preliminary molten salt treatment and is cooled at a rate of ≥33℃ / s, transitioning from the austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. Then, it undergoes a subsequent molten salt treatment, reducing the molten salt circulation rate to promote the continued transformation of untransformed residual austenite into sorbite and isothermal tempering, and promoting the melting of some sorbite lamellars. Finally, it undergoes slow cooling on a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, ferrite, and melted sorbite.

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

[0011] (1) Carbon: The price of C is relatively lower. It can be dissolved in austenite to improve lattice distortion. At the same time, it can regulate the stability of austenite and reduce the starting temperature of the transformation from austenite to sorbite. This is beneficial to improve the driving force of phase transformation, promote the refinement of sorbite lamellars during phase transformation, and improve the nucleation rate. It can also improve the strength stability after tempering by forming dispersed carbides in synergy with Cr and Nb. However, if the C content is too high, it will aggravate the central segregation and easily enrich and precipitate continuous network carbides at the austenite grain boundaries. At the same time, the phase transformation incubation period will be prolonged, and the difficulty of controlling the martensite deterioration structure will increase. The driving force for the growth of cementite lamellars will be enhanced during the later molten salt treatment, which will lead to increased brittleness and mechanical property fluctuations in the wire rod. Therefore, in order to take into account the high strength requirements of 2230MPa grade stranded wire, improve the uniformity of the structure, and promote rapid production, the mass percentage of C is controlled at 0.88%~0.92%.

[0012] (2) Silicon: Si is a solid solution strengthening element and a ferrite forming element. Through solid solution strengthening, it can hinder dislocation slip and improve tensile strength. In the austenite to sorbite phase transformation process, it can hinder the diffusion of carbon atoms into cementite and slow down the formation rate of pearlite, so that fine lamellar layers can be formed quickly after sorbite nucleation. In the isothermal tempering process, it can promote the enrichment of carbon into the retained austenite, improve the high temperature stability of cementite, resist tempering softening, and avoid insufficient drawing strength due to softening. However, excessive silicon will promote decarburization, increase intragranular segregation, and cause performance fluctuations due to excessive retained austenite, which will increase the difficulty of isothermal tempering softening and reduce production efficiency. Therefore, in order to adapt to the control of phase transformation structure by online molten salt isothermal toughening, appropriately improve the resistance to tempering softening and take into account production efficiency, the mass percentage of Si is controlled at 0.63%~0.81%.

[0013] (3) Manganese: Mn is an austenite stabilizing element that can reduce rolling deformation resistance, reduce cracking risk, and provide substitution solid solution strengthening and improve wire rod hardenability. It can reduce the critical cooling rate of austenite to pearlite transformation and the diffusion coefficient of carbon in austenite, so as to promote the uniformity of the wire rod structure across the entire cross section, avoid pearlite in the core, and at the same time hinder the diffusion of C atoms to cementite particles, improve the strength stability and work hardening rate after tempering. However, if the Mn content is too high, it will increase the risk of alloy element segregation and grain coarsening during the solidification process of the billet. The risk of forming bainite and martensite during cooling will be aggravated, leading to increased brittleness and mechanical property fluctuations, affecting the uniformity of wire rod structure. Therefore, in order to make hot-rolled wire rod have a high strength-plasticity combination and take into account the uniformity of structure, the mass percentage of Mn is controlled at 0.65%~0.85%.

[0014] (4) Chromium: Cr can provide solid solution strengthening effect, while significantly improving hardenability, increasing the recrystallization temperature of austenite, reducing the critical cooling rate of austenite to pearlite transformation, prolonging the incubation period of austenite to sorbite transformation, stabilizing sorbite lamellars, and forming nanoscale carbides in synergy with C. It can achieve dispersion strengthening by pinning dislocations, slow down the diffusion rate of cementite, increase the resistance to tempering softening, and enable the wire rod to maintain high strength after tempering. However, excessive Cr content will aggravate the segregation of alloying elements, increase the risk of precipitation of martensite and other deteriorated structures, and reduce the activity of carbon during isothermal tempering. Excessive carbide nucleation density will increase the risk of coarsening and affect the drawability of steel. Therefore, in order to control the sorbite transformation and microstructure uniformity and provide appropriate resistance to tempering softening, the mass percentage of Cr is controlled at 0.58%~0.68%.

[0015] (5) Niobium: During the hot rolling process, Nb can pin the austenite grain boundaries by forming high-temperature stable carbonitrides, inhibiting the growth of austenite grains at high temperatures, reducing the risk of rolling cracks, and enabling rapid rolling. At the same time, the high dislocation density of the deformed austenite can increase the nucleation rate of sorbite, indirectly inhibiting the formation of pearlite and reducing the fluctuation of the structure by refining the austenite grains. During the isothermal tempering process, Nb will further precipitate nanoscale NbC. NbC has a high melting point and better stability than VC. It can fill the strength gap by pinning dislocations, inhibit the coarsening of Cr carbides at the grain boundaries, and improve strength, toughness and service toughness. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling the material cost. Therefore, the mass percentage of Nb is controlled at 0.005%~0.020%.

[0016] (6) Aluminum: Al can reduce harmful inclusions, improve the purity of steel, form aluminum nitride to pin grain boundaries and inhibit austenite grain coarsening. The solid solution strengthening of Al has a smaller effect on plasticity than that of Si. At the same time, Al can hinder carbon diffusion through solid solution, reduce the growth rate of cementite lamellars, inhibit the formation of coarse carbides during cooling and the coarsening of cementite during isothermal tempering, and play a role in refining precipitates and improving the toughness of materials. However, excessive Al content will increase the risk of coarse inclusions and billet defects. Therefore, in order to achieve the synergy of deoxidation purification and microalloying strengthening and strongly inhibit the precipitation and coarsening process of cementite, the mass percentage of Al is controlled at 0.16%~0.26%.

[0017] (7) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0018] The hot-rolled wire rods described above adopt a C-Si-Mn-Cr-Nb-Al composition design, which reduces the amount of microalloying components and appropriately controls material costs. Simultaneously, it lowers the phase transformation temperature range, regulating the wire rod's hardenability, solid solution strengthening, dispersion strengthening, and grain refinement. This provides favorable conditions for increasing nucleation sites, improving sorbite nucleation rate, inhibiting the growth of harmful phases and cementite, promoting uniform microstructure transformation, and stress release. Furthermore, selecting a higher wire drawing temperature promotes uniform austenite composition and prevents premature precipitation of network carbides or proeutectoid ferrite during the wire drawing stage. It also provides favorable conditions for controlling undercooling, promoting sorbite phase deformation nucleation, and reducing phase transformation stress. After wire drawing, the wire rods undergo online molten salt isothermal toughening without air cooling.

[0019] Firstly, compared to the difficulty in controlling network carbides due to the maximum cooling capacity of the Stellmore air-cooled line, the molten salt treatment of the wire rod allows for rapid cooling of the wire rod by utilizing the higher heat transfer coefficient of molten salt compared to air. This bypasses the secondary cementite precipitation temperature range, inhibiting the formation of network carbides and preventing them from disrupting the microstructure and losing the carbon source for phase transformation. Secondly, compared to the difficulty in controlling abnormal structures such as martensite and coarse pearlite due to the unstable temperature control of the Stellmore air-cooled line and water cooling, the molten salt covers the wire rod for uniform heat exchange. There are no issues with the temperature difference between the air-cooled and air-received surfaces or the impact of water-cooled air bubbles on heat transfer. This avoids the formation of brittle martensite due to alloy element eccentricity and localized overcooling, allowing the wire rod to transition from high temperature to high temperature. The austenitic phase rapidly transitions into the sorbite phase region, and the presence of Si and Al inhibits cementite lamellar coarsening. With prolonged treatment time, it promotes the transformation of austenite into sorbite with finer lamellar spacing and inhibits the formation of coarse pearlite. Compared to single-stage salt bath treatment, which struggles to balance phase transformation and production energy consumption, the pre-treatment molten salt circulation is larger, promoting wire rod cooling, inhibiting abnormal structures, and simultaneously promoting sorbite nucleation and dispersed precipitation of alloy carbides. With prolonged treatment time, it further reduces the temperature gradient from the wire rod surface to the core, ensuring core cooling rate meets standards, adapting to the phase transformation incubation period, and reducing phase transformation stress and thermal stress. The subsequent molten salt treatment reduces molten salt circulation, thereby lowering overall production energy consumption.

[0020] II. Compared to the limitations of the Stellmore air-cooling line's minimum cooling capacity and length, which makes it difficult to control the full phase transformation and online toughening of the microstructure, the wire rod, after undergoing a subsequent molten salt treatment, can maintain the temperature range within the sorbite phase region. This promotes the continued transformation of untransformed retained austenite into a fine lamellar sorbite microstructure, thereby improving the matrix strength. Simultaneously, it prevents the retained austenite from forming a low-temperature brittle microstructure during subsequent cooling. As the treatment time extends, the wire rod undergoes relatively high-temperature isothermal tempering, providing thermal kinetics to promote the melting of some cementite lamellars within the sorbite, releasing structural stress and online regulating the balance between toughness and strength. Compared to the limitations imposed by the steel alloy composition and the controlled cooling of a single-stage salt bath treatment, which makes it difficult to balance strength, toughness, and production efficiency, the wire rod, after undergoing a subsequent molten salt treatment... During the subsequent molten salt treatment, as the diffusion of C is appropriately suppressed, Si and Al inhibit the coarsening of cementite, and the dispersed precipitation of Cr and Nb carbides hinders dislocation movement, thus maintaining interfacial strengthening and reducing strength loss. As the treatment time is extended, some of the cementite lamellars in the sorbite melt, which can quickly release structural stress and further improve the uniformity and stability of the structure. After exiting the molten salt, the wire rod undergoes slow cooling on the roller conveyor. The high temperature state of the wire rod after exiting the molten salt can be used to promote further toughening of the wire rod structure and improve the tempering and softening effect of the wire rod. However, since the wire rod can retain its strength characteristics and undergo rapid toughening after online molten salt isothermal toughening, it is not necessary to use excessively low cooling rates for long-term slow cooling treatment. This can balance production efficiency and promote efficient and stable production of wire rod.

[0021] Before rolling, selecting an appropriate heating furnace soaking temperature and furnace time can promote uniform diffusion of alloy components, reduce segregation and rolling deformation resistance, release casting stress of continuous casting billets, and avoid rolling cracking. At the same time, it can avoid mixed crystal defects and decarburization caused by excessively high temperature and excessively long furnace time. In the preferred technical solution, before rolling, the heating furnace soaking temperature is controlled at 1190~1230℃ and the furnace time is controlled at 155~225min.

[0022] During the rolling process, a higher initial rolling temperature can be used to connect with the heating furnace soaking temperature, reduce rolling deformation resistance and wear on the rolling line, and a larger initial rolling reduction can be used to eliminate casting defects, accumulate deformation energy, and improve rolling efficiency. An appropriate final rolling temperature and final rolling reduction can be selected to induce the precipitation of Nb carbonitrides to refine the grains and retain appropriate deformation energy. At the same time, the rolling temperature can be connected with the wire drawing temperature. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1075~1095℃, the initial rolling reduction is 38%~42.5%, the final rolling temperature is 884~914℃, and the final rolling reduction is 22%~27%.

[0023] During the wire spinning process, the wire spinning temperature can be further controlled to suppress austenite grain coarsening, improve the fine grain strengthening effect and pinning effect. In the preferred technical solution, the wire spinning temperature is controlled at 880~905℃.

[0024] The initial molten salt treatment is performed in the sorbite phase region. Lower molten salt temperatures increase the cooling rate of the wire rod, suppress network carbides, and, in conjunction with the effects of Si and Al, reduce the atomic diffusion rate. Increased undercooling restricts cementite lamellar growth, preventing the formation of coarse lamellars. Simultaneously, it allows dissolved Cr and Nb to disperse and precipitate, forming denser, finer carbides. Extended treatment time promotes austenite-to-sorbite transformation and enhances the dispersion strengthening effect of fine alloy carbides. However, excessively low molten salt temperatures result in slow atomic diffusion, prolonging the sorbite transformation incubation period, increasing the temperature gradient from the wire rod surface to the core, increasing transformation stress, and requiring more molten salt circulation. Longer treatment times also increase production energy consumption. Conversely, higher molten salt temperatures approach the nose temperature of the sorbite transformation, accelerate carbon diffusion, balance the phase transformation delay effect caused by Si and Al, promote rapid sorbite nucleation, and accelerate the formation of Cr and Nb crystals. The carbide precipitation rate of b is reduced, thermal stress is decreased, and production energy consumption can be reduced as the processing time is shortened. However, if the molten salt temperature is too high, it is not conducive to suppressing network carbides and coarse pearlite, affecting the strength and toughness of the matrix. At the same time, if the processing time is too short, the temperature gradient between the wire rod surface and the core will increase, and too much residual untransformed sorbite will lead to a prolonged incubation period for subsequent phase transformation, which will bring about fluctuations in mechanical properties and the risk of abnormal structure. Therefore, the pre-treatment of molten salt can control the molten salt temperature and processing time, promote the wire rod to quickly enter the sorbite phase region from the high-temperature austenitic state, bypass the network carbide region, suppress the formation of network carbides and form a structure dominated by fine lamellar sorbite, promote the dispersed precipitation of alloy carbides, reduce phase transformation stress, and prepare the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the pre-treatment is 552~592℃, and the processing time is 120~170s.

[0025] Since the wire drawing temperature differs significantly from the molten salt temperature of the preceding molten salt treatment, using a higher molten salt circulation rate can promote rapid cooling of the wire rod, control the molten salt temperature rise, suppress abnormal structures, and reduce phase transformation stress. In the preferred technical solution, the molten salt circulation rate of the preceding molten salt treatment is 500~600t / h, and the molten salt temperature rise is ≤8℃.

[0026] The higher the molten salt temperature in the subsequent molten salt treatment, the greater the driving force for the transformation of retained austenite, the shorter the phase transformation incubation period, the faster carbon atom diffusion, and the supplementary strength through the dispersed precipitation of alloy carbides. With prolonged treatment time, it can promote the full transformation of austenite, preventing the retained austenite from continuing to induce martensitic transformation during subsequent cooling, further reducing the temperature difference between the wire rod surface and the core, allowing for sufficient diffusion of alloy carbides, and promoting online toughening of the microstructure through high-temperature isothermal treatment, alleviating stress concentration, and improving the softening effect. However, excessively high molten salt temperatures are detrimental to inhibiting the aggregation and growth of alloy carbides. With prolonged treatment time, the dispersion strengthening effect weakens, and excessive melting of sorbite lamellars leads to losses in strength, plasticity, and energy consumption. Conversely, lower molten salt temperatures are beneficial for reducing carbon diffusion and precipitation, inhibiting the coarsening of cementite lamellars in sorbite, and suppressing the growth of alloy carbides. With prolonged treatment time... Shortening the processing time allows carbides to remain in a fine and dispersed state, maintaining strength and reducing production energy consumption. However, if the molten salt temperature is too low, it will affect the transformation of retained austenite and the toughening of the microstructure. If the processing time is too short, the precipitation of alloy carbides will be insufficient and the diffusion will be uneven. The stress concentration in the microstructure will not be released in time, resulting in a loss of strength and plasticity and fluctuations in mechanical properties. Therefore, the subsequent molten salt treatment can control the molten salt temperature and processing time, promote the continued transformation of untransformed retained austenite into fine lamellar interlamellar sorbite, and promote the formation of fine lamellar interlamellar sorbite for a long time isothermal tempering. It also causes some cementite lamellars to melt and inhibits the agglomeration and growth of alloy carbides, so as to balance production energy consumption, control the strength and plasticity matching of wire rod, and improve the uniformity of microstructure. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 539~569℃ and the processing time is 80~140s.

[0027] Since the temperature difference between the wire rod temperature after the first stage of molten salt treatment and the molten salt temperature after the second stage of molten salt treatment is small, a smaller molten salt circulation rate can control the molten salt temperature rise, improve the consistency of isothermal toughening of the wire rod, and reduce production energy consumption. In the preferred technical solution, the molten salt circulation rate of the second stage of molten salt treatment is 200~300t / h, and the molten salt temperature rise is ≤3℃.

[0028] Since the wire rod has undergone sufficient phase transformation and its temperature after exiting the molten salt is relatively high, the slow cooling rate can be further controlled to promote further toughening of the wire rod structure. In the preferred technical solution, the roller conveyor controls the wire rod to cool to below 270°C at a slow cooling rate of ≤0.75°C / s before winding. To further improve the efficiency of the line exit, the slow cooling rate is preferably ≥0.5°C / s.

[0029] In the preferred technical solution, the slow cooling of the roller conveyor adopts the control of the opening of the heat insulation cover, and blows the hot air of ≥200℃ in the online molten salt 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 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 hot-rolled wire rod for stranded wire of 2230MPa grade, wherein the hot-rolled wire rod is manufactured by the manufacturing method for hot-rolled wire rod for stranded wire of any one of the above-mentioned methods.

[0031] The aforementioned hot-rolled wire rods are designed with a high carbon content of Cr, Nb, and Al, which reduces the addition of expensive alloys and appropriately controls material costs. Simultaneously, the microstructure is mainly tempered sorbite and melted sorbite, containing a small amount of ferrite. Compared to air-cooled / water-cooled wire rods with sorbitic microstructure, this effectively suppresses abnormal structures such as network carbides, martensite, and coarse pearlite, reduces ferrite content, and improves microstructure uniformity. The sorbite microstructure has finer lamellar spacing and a greater hindering effect on dislocation movement compared to pearlite. Furthermore, compared to… Existing salt bath treated stranded steel sorbitic wire rods, after isothermal toughening, partially melt and fracture the sorbitic cementite to form fused sorbite and intermediate tempered sorbite transitioning to spheroidized structure, reducing stress concentration points. Fine and dispersed alloy carbides are distributed on the matrix, which can enhance the strengthening effect of carbon elements and retain strength characteristics through the dislocation pinning effect of the dispersed carbides by interface strengthening. This achieves online toughening of the microstructure, so that offline heat treatment is not required in the manufacture of high-strength stranded wire, reducing the production costs and energy consumption of downstream users.

[0032] In the microstructure, the higher the volume percentage of tempered sorbite and the finer the lamellar spacing, the higher the matrix strength. The higher the volume percentage of fused sorbite, the higher the matrix ductility and toughness, and the lower the resistance to cold drawing deformation. In the preferred technical solution, the volume percentage of tempered sorbite is ≥59%, the lamellar spacing is 85~135nm, the volume percentage of ferrite is ≤5%, and the volume percentage of fused sorbite is ≥26%.

[0033] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference of ≤46MPa within the same coil. This can effectively avoid the adverse effects of network carbides on the matrix and ductility, improve the drawing stability and service life of the strand, and effectively reduce mechanical property fluctuations by suppressing abnormal structures, releasing structural stress, and improving structural uniformity. This can improve the efficiency of subsequent drawing processes, the stability of strand quality, and meet the performance consistency requirements of high-strength strands.

[0034] In the preferred technical solution, the diameter of the hot-rolled wire rod is 7~15mm, the tensile strength is 1513~1563MPa, and the reduction of area is 31%~36%. The higher initial tensile strength can reduce the number of drawing passes, quickly reach the target strength, reduce plastic loss during the process, and improve the production efficiency of stranded wire. The higher reduction of area can improve the crack resistance and alleviate stress concentration under fatigue load without offline heat treatment, so as to promote the stable production of high-strength stranded wire.

[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 existing wire rod production process is difficult to meet the production of high-strength and high-toughness hot-rolled wire rod and the production efficiency is limited, the present invention combines the high carbon composition design of Cr-Nb-Al with online molten salt isothermal toughening technology. The wire rod first undergoes a front-end molten salt treatment and quickly enters the sorbite phase region from the high-temperature austenitic state, suppressing network carbides and forming a structure dominated by sorbite, promoting the dispersion precipitation of alloy carbides. Then, after the back-end molten salt treatment, the amount of molten salt circulation and production energy consumption are reduced, promoting the transformation of untransformed residual austenite into sorbite and isothermal tempering, promoting the melting of some sorbite lamellars, suppressing the growth of alloy carbides and abnormal martensite structure. Finally, it is further toughened by slow cooling on the roller table. This can control material costs, improve the strength and plasticity matching of hot-rolled wire rod, take into account the uniformity of structure and production energy consumption, reduce the restrictions on rolling and slow cooling on the roller table, improve production efficiency, promote the efficient and stable production of high-strength and high-toughness hot-rolled wire rod, and has good industrial adaptability.

[0037] (2) In view of the current situation where the cost of existing stranded wire rod materials is high, the strength and plasticity are not good, and the performance is limited by offline heat treatment, the production of downstream users is restricted. This invention adopts a high carbon composition design of Cr, Nb and Al, which can reduce the addition of expensive alloys and appropriately control the material cost. At the same time, the microstructure includes a mixed structure composed of tempered sorbite, ferrite and melted sorbite, which can effectively suppress abnormal structures such as network carbides, martensite and coarse pearlite, reduce ferrite content, improve the uniformity of structure, and enhance the strengthening effect of carbon elements and retain strength characteristics through the dislocation pinning effect of interface strengthening superimposed on dispersed carbides, thereby achieving online toughening of the structure. The tensile strength can reach 1513~1563MPa and the reduction of area is 31%~36%. It can be used to manufacture 2230MPa grade stranded wire and other application fields, so that downstream users can avoid offline heat treatment when manufacturing stranded wire, and reduce the number of drawing passes and cracking risk, which has good market application prospects. Attached Figure Description

[0038] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

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

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

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

[0042] The embodiments described below with reference to the accompanying drawings are exemplary and are merely for illustrative purposes and do not limit the description of the features and characteristics of the invention. They are intended to provide the best mode for carrying out the invention, to explain the invention, and to enable those skilled in the art to practice the invention. However, they should not be construed as limiting the scope of the invention in any way, which is defined only by the appended claims. The microstructure and performance testing of the hot-rolled wire rods obtained in the following embodiments and comparative examples includes: tensile testing using GB-T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, to obtain tensile strength and reduction of area; microstructure testing using the metal microstructure testing method of GB / T13298 standard; and mechanical property same-coil difference test method: two coils of wire rod are taken 5m from the end of the coil. Using the overlap area as the base point, each coil of wire rod is divided into 8 equal segments. One tensile specimen is taken from each segment. The difference in strength of the tensile specimens after tensile testing is the mechanical property same-coil difference.

[0043] Example 1:

[0044] A preferred embodiment of the manufacturing method of the 2230MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.92%, Si: 0.63%, Mn: 0.69%, Cr: 0.58%, Nb: 0.011%, Al: 0.16%, P: 0.013%, S: 0.012%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → slow cooling on roller table → 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. Controlling the furnace parameters promotes uniform diffusion of alloying elements and avoids mixed-crystal defects. After exiting the furnace, the billet is rolled into a 7mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency, induce Nb carbonitride precipitation to refine grains, and retain appropriate deformation energy storage. Specifically, the furnace soaking temperature is controlled at 1210℃, and the furnace dwell time is... The rolling process takes 195 minutes, with an initial rolling temperature of 1080℃ and an initial rolling reduction of 42.5%, and a final rolling temperature of 898℃ and a final rolling reduction of 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 spread on the roller conveyor and conveyed along the roller conveyor. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing favorable conditions for promoting the deformation nucleation of sorbite phase and reducing phase transformation stress. Specifically, the wire drawing temperature is controlled at 895℃.

[0046] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 33°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a predominantly fine-laminated sorbite structure within the high-temperature austenitic microstructure. This promotes the dispersed precipitation of Cr and Nb alloy carbides and reduces phase transformation stress. The wire rod is then conveyed via rollers through the second salt bath for final molten salt treatment, further reducing... Low molten salt circulation promotes the transformation of untransformed residual austenite into fine lamellar sorbite, while simultaneously promoting isothermal tempering of the formed sorbite, causing partial melting of cementite lamellars and inhibiting the aggregation and growth of alloy carbides, thereby controlling the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 592℃, the treatment time is 120s, the molten salt circulation rate is 500t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 569℃, the treatment time is 140s, the molten salt circulation rate is 200t / h, and the molten salt temperature rise is ≤3℃.

[0047] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 263℃ at a slow cooling rate of 0.75℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product, the metallographic structure of which is shown in the figure below. Figure 1 As shown.

[0048] Comparative Example 1: A method for manufacturing hot-rolled wire rod, the difference between this method and Example 1 is that the method follows a process flow of rolling → wire drawing → Steyrmo air cooling. Specifically, the heating furnace homogenization temperature is controlled at 1160℃, the furnace time is 250 min, the initial rolling temperature is 1030℃, the initial rolling reduction is 25.5%, the final rolling temperature is 845℃, the final rolling reduction is 32.5%, and the wire drawing temperature is 800℃; the Steyrmo forced air cooling uses an air volume of 200,000 m³ / s per fan. 3 At 75% capacity, fans 1 to 4 are turned on to cool the wire rod to 700℃ at a cooling rate of 7.4℃ / s. Then, fans 5 to 14 are turned on to 25% capacity to cool the wire rod to 265℃ at a cooling rate of 2.9℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0049] Comparative Example 2: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 1 is that: the heating furnace homogenization temperature is controlled at 1150℃, the furnace time is 255min, the initial rolling temperature is 1010℃, the initial rolling reduction is 27%, the final rolling temperature is 845℃, the final rolling reduction is 31%, and the wire drawing temperature is 810℃; during the molten salt treatment before the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 29℃ / s, and the finished hot-rolled wire rod is obtained after leaving the line.

[0050] Example 2:

[0051] A preferred embodiment of the manufacturing method of the 2230MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.91%, Si: 0.66%, Mn: 0.65%, Cr: 0.60%, Nb: 0.005%, Al: 0.20%, 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 isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0052] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and avoids mixed-crystal defects. After exiting the furnace, the billet is rolled into a 10mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency, induce Nb carbonitride precipitation to refine grains, and retain appropriate deformation energy storage. Specifically, the furnace soaking temperature is controlled at 1190℃. The rolling time is 225 minutes, the initial rolling temperature is 1075℃, the initial rolling reduction is 41%, the final rolling temperature is 884℃, and the final rolling reduction is 25%. 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 suitable wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing favorable conditions for promoting the deformation nucleation of sorbite phase and reducing phase transformation stress. Specifically, the wire drawing temperature is controlled at 880℃.

[0053] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 33°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a predominantly fine-laminated sorbite structure within the high-temperature austenitic microstructure. This promotes the dispersed precipitation of Cr and Nb alloy carbides and reduces phase transformation stress. The wire rod is then conveyed via rollers through the second salt bath for final molten salt treatment, further reducing... Low molten salt circulation promotes the transformation of untransformed residual austenite into fine lamellar sorbite, while simultaneously promoting isothermal tempering of the formed sorbite, causing partial melting of cementite lamellars and inhibiting the aggregation and growth of alloy carbides, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 579℃, the treatment time is 140s, the molten salt circulation rate is 530t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 554℃, the treatment time is 120s, the molten salt circulation rate is 230t / h, and the molten salt temperature rise is ≤3℃.

[0054] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 265℃ at a slow cooling rate of 0.6℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product, the metallographic structure of which is shown in the figure below. Figure 2 As shown.

[0055] Comparative Example 3: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the first stage of the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 38°C / s, the molten salt temperature of the first stage of the molten salt treatment is 520°C, the treatment time is 180s, and the finished hot-rolled wire rod is obtained after going offline.

[0056] Comparative Example 4: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 2 is that: during the first stage of the online molten salt isothermal toughening process, the wire rod is cooled at a cooling rate of 30℃ / s, the molten salt temperature of the first stage of the molten salt treatment is 600℃, the treatment time is 100s, and the finished hot-rolled wire rod is obtained after going offline.

[0057] Example 3:

[0058] A preferred embodiment of the manufacturing method of the 2230MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.88%, Si: 0.81%, Mn: 0.76%, Cr: 0.68%, Nb: 0.02%, Al: 0.23%, P: 0.012%, 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 isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0059] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and avoids mixed-crystal defects. After exiting the furnace, the billet is rolled into a 13mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency, induce Nb carbonitride precipitation to refine grains, and retain appropriate deformation energy storage. Specifically, the furnace soaking temperature is controlled at 1230℃. The rolling time is 180 minutes, the initial rolling temperature is 1095℃, the initial rolling reduction is 39%, the final rolling temperature is 914℃, and the final rolling reduction is 24%. 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 suitable wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing favorable conditions for promoting the deformation nucleation of sorbite phase and reducing phase transformation stress. Specifically, the wire drawing temperature is controlled at 905℃.

[0060] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a predominantly fine-laminated sorbite structure within the high-temperature austenitic microstructure. This promotes the dispersed precipitation of Cr and Nb alloy carbides and reduces phase transformation stress. The wire rod is then conveyed via rollers through the second salt bath for final molten salt treatment, further reducing... Low molten salt circulation promotes the transformation of untransformed residual austenite into fine lamellar sorbite, while simultaneously promoting isothermal tempering of the formed sorbite, causing partial melting of cementite lamellars and inhibiting the aggregation and growth of alloy carbides, thereby controlling the strength-ductility balance of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 563℃, the treatment time is 150s, the molten salt circulation rate is 570t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 547℃, the treatment time is 100s, the molten salt circulation rate is 260t / h, and the molten salt temperature rise is ≤3℃.

[0061] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) from above the two salt bath tanks in the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 267℃ at a slow cooling rate of 0.55℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product, the metallographic structure of which is shown in the figure below. Figure 3 As shown.

[0062] Comparative Example 5: 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 595°C, the treatment time is 150s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0063] Comparative Example 6: 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 515°C, the treatment time is 70s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0064] Example 4:

[0065] A preferred embodiment of the manufacturing method of the 2230MPa grade hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.89%, Si: 0.78%, Mn: 0.85%, Cr: 0.64%, Nb: 0.16%, Al: 0.26%, P: 0.014%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → slow cooling on roller table → coiling, specifically:

[0066] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic. Controlling the furnace parameters promotes uniform diffusion of alloying elements and avoids mixed-crystal defects. 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, induce Nb carbonitride precipitation to refine grains, and retain appropriate deformation energy storage. Specifically, the furnace soaking temperature is controlled at 1220℃. The rolling time is 155 minutes, the initial rolling temperature is 1090℃, the initial rolling reduction is 38%, the final rolling temperature is 906℃, 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 spread on the roller table and conveyed along the roller table. A suitable wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoiding premature precipitation of proeutectoid ferrite or network carbides, and providing favorable conditions for promoting the deformation nucleation of sorbite phase and reducing phase transformation stress. Specifically, the wire drawing temperature is controlled at 900℃.

[0067] The online molten salt isothermal toughening process employs a two-stage salt bath with an internal molten salt compartment. After coiling, the wire rod is conveyed via rollers through the first salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the sorbite phase region. This inhibits the formation of network carbides and coarse pearlite, resulting in a predominantly fine-laminated sorbite structure within the high-temperature austenitic microstructure. This promotes the dispersed precipitation of Cr and Nb alloy carbides and reduces phase transformation stress. The wire rod is then conveyed via rollers through the second salt bath for final molten salt treatment, further reducing... Low molten salt circulation promotes the transformation of untransformed residual austenite into fine lamellar sorbite, while simultaneously promoting isothermal tempering of the formed sorbite, causing partial melting of cementite lamellars and inhibiting the aggregation and growth of alloy carbides, thereby controlling the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 552℃, the treatment time is 170s, the molten salt circulation rate is 600t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 539℃, the treatment time is 80s, the molten salt circulation rate is 300t / h, and the molten salt temperature rise is ≤3℃.

[0068] The slow cooling process of the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥200℃) from the two salt bath tanks of the online molten salt isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover, promoting further toughening of the wire rod structure and improving the tempering and softening effect of the wire rod. Specifically, the wire rod is cooled to 269℃ at a slow cooling rate of 0.5℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0069] Comparative Example 7: A method for manufacturing hot-rolled wire rod, the difference between which is that the manufacturing method is according to the process flow of rolling → wire drawing → online molten salt isothermal toughening treatment → air cooling. The air cooling process is carried out by opening the heat insulation cover and controlling the wire rod to cool to 265°C at a 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.

[0070] 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:

[0071] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0072]

[0073] The comparison between Example 1 and Comparative Example 1 shows that, compared to using a high-carbon composition system, production on the Steyrmo air-cooled line produces a higher level of abnormal microstructure, including network carbides, martensite, and coarse pearlite, resulting in insufficient strength and plasticity and significant fluctuations in mechanical properties. This invention addresses this by combining a high-carbon composition design (Cr-Nb-Al) with online molten salt isothermal toughening technology. This allows the wire rod to undergo a preliminary molten salt treatment, rapidly transitioning from a high-temperature austenitic state to the sorbite phase region, forming a microstructure dominated by sorbite. A subsequent molten salt treatment further promotes the complete phase transition of the microstructure. As can be seen from the results of Examples 1-4, the microstructure of hot-rolled wire rod, after variable and isothermal tempering, consists of a mixed structure composed of tempered sorbite, ferrite, and melted sorbite. This effectively suppresses abnormal structures such as network carbides, martensite, and coarse pearlite, reduces ferrite content, and improves the uniformity of the structure. It can achieve a tensile strength of 1513-1563 MPa and a reduction of area of ​​31%-36%, which can be used to manufacture 2230 MPa grade stranded wire and other applications. This allows downstream users to avoid offline heat treatment during stranded wire manufacturing, promoting efficient and stable stranded wire production.

[0074] As can be seen from the comparison results between Example 1 and Comparative Example 2, selecting a higher wire drawing temperature can promote the uniformity of austenite composition, avoid the premature precipitation of network carbides or proeutectoid ferrite during the wire drawing stage, provide favorable conditions for controlling undercooling, promoting the deformation nucleation of sorbite phase, and reducing phase transformation stress, while reducing the restrictions on rolling to facilitate rapid rolling.

[0075] The comparison results between Example 2 and Comparative Example 3 show that the lower the molten salt temperature in the initial molten salt treatment, the higher the cooling rate of the wire rod, the more suppressed the network carbides, and the lower the atomic diffusion rate in conjunction with the effects of Si and Al. By increasing the undercooling, the growth of cementite lamellars is restricted, preventing the formation of coarse lamellars. At the same time, the dissolved Cr and Nb are dispersed and precipitated, forming denser fine carbides. With the extension of the treatment time, the transformation of austenite to sorbite can be promoted, and the dispersion strengthening effect of fine alloy carbides can be improved. However, if the molten salt temperature is too low, the atomic diffusion will be too slow, which will prolong the incubation period of the sorbite phase transformation, increase the temperature gradient from the wire rod surface to the core, increase the phase transformation stress and the demand for molten salt circulation. With the excessively long treatment time, the production energy consumption will increase.

[0076] 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 closer it can be to the nose temperature of sorbite transformation, the faster carbon diffusion, the balance of the phase transformation delay effect caused by Si and Al, the faster generation of sorbite nuclei, the faster precipitation rate of Cr and Nb carbides, and the reduction of thermal stress. With the shortening of the treatment time, the production energy consumption can be reduced. However, if the molten salt temperature is too high, it is not conducive to suppressing network carbides and coarse pearlite, and affects the strength and toughness of the matrix. At the same time, if the treatment time is too short, the temperature gradient between the wire rod surface and the core will increase, and too much residual untransformed sorbite will lead to a prolonged incubation period for subsequent phase transformation, which will bring about fluctuations in mechanical properties and the risk of abnormal structure.

[0077] 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 greater the driving force for the transformation of retained austenite, the shorter the phase transformation incubation period, the faster the diffusion of C atoms, and the supplementary strength through the dispersion precipitation of alloy carbides. With the extension of treatment time, it can promote the full transformation of austenite structure, avoid the continued induction of martensite transformation by retained austenite in subsequent cooling, further reduce the temperature difference from the wire rod surface to the core, and ensure the full diffusion of alloy carbides. The online toughening of the structure is promoted through high temperature isothermal treatment, which can alleviate stress concentration and improve the softening effect. However, if the molten salt temperature is too high, it is not conducive to inhibiting the aggregation and growth of alloy carbides. With the excessive treatment time, the dispersion strengthening effect weakens, and the excessive melting of sorbite lamellars will lead to loss of strength, plasticity and energy consumption.

[0078] 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 reduce the diffusion and precipitation of carbon, suppress the coarsening of cementite lamellars in sorbite and the growth of alloy carbides. As the treatment time is shortened, the carbides are kept in a fine and dispersed state, which can maintain strength and reduce production energy consumption. However, if the molten salt temperature is too low, it will affect the transformation of retained austenite and the toughening of the microstructure. If the treatment time is too short, the precipitation of alloy carbides will be insufficient and the diffusion will be uneven. The stress concentration in the microstructure will not be released in time, which will lead to loss of strength and plasticity and fluctuation of mechanical properties.

[0079] 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 and promote further toughening of the wire rod structure.

[0080] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of manufacturing a hot-rolled wire rod for a 2230 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The chemical composition of the hot-rolled wire rod comprises the following components in percentage by mass: C: 0.88%-0.92%, Si: 0.63%-0.81%, Mn: 0.65%-0.85%, Cr: 0.58%-0.68%, Nb: 0.005%-0.020%, Al: 0.16%-0.26%, P≤0.014%, S≤0.014%, and the rest is Fe and inevitable impurities; after the wire rod is spun at a spinning temperature of ≥880 ℃, the wire rod is subjected to on-line molten salt isothermal toughening treatment, the wire rod is first subjected to front-stage molten salt treatment and is cooled at a cooling rate of ≥33 ℃ / s, and the wire rod is changed from an austenite state into a sorbite phase region to form a structure mainly composed of sorbite structure, then the wire rod is subjected to rear-stage molten salt treatment, the molten salt circulation amount is reduced, the untransformed residual austenite is continuously transformed into sorbite and is isothermally tempered, and part of the sorbite lamellas are melted and broken, and finally the wire rod is subjected to roller slow cooling to be made into a hot-rolled wire rod with a microstructure composed of a mixed structure of tempered sorbite, ferrite and melted and broken sorbite; the molten salt temperature of the front-stage molten salt treatment is 552-592 ℃, and the treatment time is 120-170 s; the molten salt temperature of the rear-stage molten salt treatment is 539-569 ℃, and the treatment time is 80-140 s.

2. The method of producing a hot rolled wire rod for a 2230 MPa grade strand according to claim 1, characterized by, Before the rolling, the soaking temperature of the heating furnace is controlled to be 1190-1230 ℃, and the furnace time is 155-225 min.

3. The method of producing a hot rolled wire rod for 2230 MPa grade strand wire according to claim 1, characterized by, During the rolling, the initial rolling temperature is controlled to be 1075-1095 ℃, the initial rolling reduction is 38%-42.5%, the final rolling temperature is 884-914 ℃, and the final rolling reduction is 22%-27%.

4. The method of producing a hot-rolled wire rod for a 2230 MPa grade strand according to claim 1, characterized by, During the spinning, the spinning temperature is controlled to be 880-905 ℃.

5. The method of producing a hot rolled wire rod for 2230 MPa grade strand wire according to claim 1, characterized by, The molten salt circulation amount of the front-stage molten salt treatment is 500-600 t / h, and the molten salt temperature rise is ≤8 ℃; the molten salt circulation amount of the rear-stage molten salt treatment is 200-300 t / h, and the molten salt temperature rise is ≤3 ℃.

6. The method of producing a hot-rolled wire rod for a 2230 MPa grade strand according to claim 1, characterized by, The roller slow cooling controls the wire rod to be cooled to below 270 ℃ at a slow cooling speed of ≤0.75 ℃ / s to be coiled.

7. A hot rolled wire rod for 2230 MPa grade strand, characterized in that, The hot-rolled wire rod is obtained by the manufacturing method of the hot-rolled wire rod for a 2230 MPa grade twisted wire rod according to any one of claims 1-6.

8. The hot rolled wire rod for 2230 MPa grade strand according to claim 7, characterized by, The volume percentage of the tempered sorbite is ≥59%, the lamellar spacing is 85-135 nm, the volume percentage of the ferrite is ≤5%, the volume percentage of the melted and broken sorbite is ≥26%, the reticular carbide level of the hot-rolled wire rod is 0 level, and the mechanical property same circle difference is ≤46 MPa.

9. The hot rolled wire rod for 2230 MPa grade strand as claimed in claim 7, wherein, The diameter of the hot-rolled wire rod is 7-15 mm, the tensile strength is 1513-1563 MPa, and the reduction of area is 31%-36%.

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