High-strength complex-phase hot-rolled wire rod for 2260MPa-grade stranded wire and manufacturing method of high-strength complex-phase hot-rolled wire rod

Abstract

The invention relates to a high-strength complex-phase hot-rolled wire rod for a 2260MPa-grade stranded wire and a manufacturing method of the high-strength complex-phase hot-rolled wire rod, after the high-carbon component design of C-Si-Mn-Cr-V-Nb-V is rolled and spun into a wire rod, the wire rod is subjected to online molten salt mixed quenching isothermal treatment, so that the wire rod is subjected to front-section molten salt treatment to be quickly cooled, enters a bainite and pearlite mixed phase region from an austenite state, and is subjected to hot rolling and hot rolling to form the high-strength complex-phase hot-rolled wire rod. The method comprises the following steps: carrying out first-stage molten salt treatment on austenite to promote transformation of part of austenite to bainite and sorbite, then carrying out later-stage molten salt treatment to increase the molten salt temperature, promoting decomposition and transformation of untransformed retained austenite into sorbite, carrying out isothermal tempering, and finally carrying out slow cooling through a roller way, a hot-rolled wire rod with a microscopic structure comprising a complex-phase structure composed of tempered bainite, tempered sorbite and ferrite is manufactured, the tensile strength is 1547-1597 MPa, and the percentage reduction of area is 27%-32%, so that when a high-strength stranded wire is manufactured, off-line heat treatment is not needed, the drawing pass number is reduced, and the wire breaking risk is reduced.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled wire rod technology, specifically relating to a high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire and its manufacturing method. Background Technology

[0002] Steel strand, as a key component for load bearing and transmission, is widely used in bridges, road transportation, power, energy, and construction. With the rapid development of industry, the trend towards ultra-high strength steel strand is becoming increasingly apparent, and the ordinary high-strength 1960MPa grade is increasingly unable to meet the rapidly developing market demand. Hot-rolled wire rod, the raw material for steel strand, is usually produced by controlled cooling on a Stellmore air-cooled line. However, the limited cooling capacity of the air-cooled line makes it difficult for the microstructure and properties of the hot-rolled wire rod to meet the production requirements of ultra-high strength strand. At the same time, the microstructure is mainly pearlitic, requiring offline heat treatment. Other microstructures are difficult to toughen and control, and offline heat treatment also brings problems such as increased processes, energy consumption, and costs for downstream users. Therefore, it is necessary to develop a 2260MPa grade hot-rolled wire rod and its manufacturing method so that offline heat treatment can be eliminated, meeting the development and market needs of the steel industry.

[0003] Existing high-strength hot-rolled wire rods for stranded wire are generally sorbitic wire rods with a high carbon content. Manufacturing higher-strength hot-rolled wire rods that do not require offline heat treatment still faces the following technical bottlenecks:

[0004] I. To improve wire rod strength, existing technologies generally increase the C, Si, and Mn content, combined with low-temperature rolling and strong air cooling on the Steyrmo air-cooling line after wire drawing, to obtain a sorbite structure with fine lamellar spacing. However, on the one hand, high-carbon systems are prone to central carbon segregation during continuous casting. Due to the limitation of the maximum cooling capacity of the Steyrmo air-cooling line, the carbon concentration in the segregation zone becomes supersaturated, preferentially precipitating secondary cementite at the austenite grain boundaries to form a continuous network structure. This network carbide disrupts the continuity of the matrix, leading to stress concentration, decreased plasticity, and in severe cases, brittle fracture during cold drawing of the wire rod. To minimize the level of network carbides and refine the sorbite lamellar spacing, strong air cooling further increases temperature control instability, further increasing the temperature difference between the air-receiving and air-receiving surfaces of the wire rod, and between the surface and the core. During cooling, uncontrollable bainite or martensite is easily formed due to local overcooling. The martensite structure leads to significant brittleness and fluctuations in mechanical properties. On the other hand, although the wire rod is refined with microalloying elements such as V, Nb, and Ti to increase the strength and toughness of the matrix, the unstable temperature control of the strong air cooling and the limitation of continuous cooling result in uneven precipitation, making it difficult to fully exert the strengthening effect and limiting the strengthening improvement. Excessive addition of microalloying elements will also increase the material cost. At the same time, due to the limitations of the length, minimum cooling capacity, and continuous cooling of the Steyrmo air cooling line, the wire rod is already in a low-temperature state after phase transformation inoculation. The brittle structures such as bainite or martensite formed have high dislocation density, are brittle, and are difficult to toughen. The residual structural stress in the structure causes dislocation movement to be hindered by residual stress and brittle structure during cold drawing, which easily leads to stress concentration and wire breakage, or even brittle breakage during transportation or coiling, making it difficult to achieve stable production and requiring offline heat treatment.

[0005] II. To achieve high strength and ductility, existing technologies have proposed online salt bath treatment processes. For example, patent CN120400686A discloses wire rods, steel strands, and their production methods for 2200-2260MPa grade ultra-high stress corrosion resistant prestressed steel strands. These methods employ a C-Si-Mn-Cr-V-Nb composition design, combined with low-temperature heating rolling, online salt bath isothermal treatment, and online aging to improve the sorbitization rate. However, the strength of sorbite depends on lamellar refinement. Although V / Nb microalloying can improve strength, the initial... Insufficient strength and work hardening rate lead to more drawing passes during stranding, resulting in greater plasticity loss and a risk of wire breakage. Furthermore, low-temperature rolling is required to suppress cementite growth, limiting rolling efficiency and exacerbating rolling line wear. On the other hand, limitations imposed by phase transformation control and single-stage salt bath treatment necessitate a higher molten salt circulation rate, leading to higher energy consumption. Additionally, to prevent cementite coarsening, the single-stage salt bath treatment time should not be too long to reduce softening, requiring slow, long-term online aging at low cooling rates, 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 2260MPa grade stranded wire and its manufacturing method, which can suppress abnormal structure, realize multiphase structure control and online control of structure toughness, and take into account production efficiency, so that when manufacturing high-strength stranded wire, offline heat treatment is not required, the number of drawing passes is reduced, and the risk of wire breakage is reduced.

[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 2260MPa 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: 0.58%~0.78%, Mn: 0.55%~0.75%, Cr: 0.42%~0.62%, Al: 0.20%~0.40%, Nb: 0.015%~0.030%, V: 0.015%~0.030%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is produced at a drawing temperature of ≥895℃. After being rolled into wire rod, the wire rod undergoes an online molten salt mixing quenching isothermal treatment. The wire rod first undergoes a preliminary molten salt treatment and is cooled at a rate of ≥35℃ / s, transitioning from the austenitic state to the mixed phase region of bainite and pearlite. This promotes the transformation of some austenite into bainite and sorbite, forming a quenched structure dominated by bainite. Then, it undergoes a subsequent molten salt treatment, which increases the molten salt temperature and promotes the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, it undergoes slow cooling on a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of tempered bainite, tempered sorbite, and ferrite, forming a multiphase microstructure.

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

[0011] (1) Carbon: C is a solid solution strengthening element in steel. It is relatively cheaper and can cause significant distortion of the matrix lattice, increase the resistance to dislocation movement, and lower the starting temperature of bainite and sorbite phase transformation, so as to promote most of the austenite to form bainite by shear mechanism, providing hard phase support for the strength of the multiphase. The diffusion ability of C increases with temperature, which can promote the refinement of the sorbite lamellar spacing. At the same time, it combines with Cr, Nb and V to form nanoscale alloy carbides, thereby improving the matrix strength and resistance to softening. However, if the C content is too high, the billet is prone to central carbon segregation, and the control of network carbides and martensite will be more difficult, increasing the risk of internal stress accumulation and cementite coarsening, causing performance fluctuations. Therefore, in order to take into account the high strength requirements of 2260MPa grade strand, reduce performance fluctuations, adapt to multiphase structure and isothermal tempering control, the mass percentage of C is controlled at 0.88%~0.92%.

[0012] (2) Silicon: Si is a solid solution strengthening element. Through solid solution, it can supplement the matrix strength, reduce the diffusion rate of carbon atoms in austenite, delay the nucleation and growth of pearlite, shift the bainite transformation range upward, which is conducive to refining the bainite structure and retaining a high dislocation density, laying the foundation for subsequent tempering strengthening. At the same time, it slows down the coarsening rate of carbides during tempering so that the residual austenite can be completely decomposed into sorbite, which plays a role in refining the precipitated phase and improving the toughness of the material. It also allows the nano carbides to remain dispersed at high temperatures for a long time, so as to provide a certain resistance to tempering softening. However, excessive silicon will promote decarburization, increase the rolling deformation resistance, affect the rolling efficiency, increase the softening difficulty, and lead to a decrease in the cold working plasticity and production efficiency of steel. Therefore, in order to control the phase transformation structure and ensure the cold working plasticity, the mass percentage of Si is controlled at 0.58%~0.78%.

[0013] (3) Manganese: Mn can enhance the matrix strength through interstitial solid solution and is also a strong austenite stabilizing element. It can significantly reduce the critical cooling rate and bainite transformation initiation temperature of steel, inhibit ferrite precipitation, and refine the bainite structure and sorbite lamellar spacing by reducing the diffusion coefficient of C, thereby suppressing the soft phase in the core and reducing the strength loss during tempering. However, if the Mn content is too high, it will increase element segregation during the solidification process of the billet, cause uneven structure, increase the risk of martensite precipitation and the difficulty of tempering softening, and affect the toughness and structure uniformity of the wire rod. Therefore, in order to facilitate the control of the multiphase structure of hot-rolled wire rod and reduce the fluctuation of mechanical properties, the mass percentage of Mn is controlled at 0.55%~0.75%.

[0014] (4) Chromium: Cr stabilizes austenite, prolongs the phase transformation incubation period, and works synergistically with Mn to optimize hardenability, reduce the critical cooling rate and the diffusion coefficient of carbon in austenite, so that the core can transform synchronously. At the same time, it refines the bainite structure and the lamellar spacing of sorbite. Chromium is also easy to combine with carbon to form chromium-based carbides, which can provide dispersion strengthening and improve the resistance to softening. This allows the multiphase structure to retain high strength characteristics and pull-hardening ability. However, when the Cr content is too high, segregation will occur during solidification, which will increase the uniformity of the structure and the difficulty of controlling martensite. At the same time, it will slow down the phase transformation rate, increase the stability of residual austenite and the difficulty of tempering, and affect the rapid production line. Therefore, in order to take into account the control of the multiphase structure of hot-rolled wire rod, the resistance to tempering softening of wire rod should be appropriately controlled. The mass percentage of Cr is controlled at 0.42%~0.62%.

[0015] (5) Aluminum: Al is a solid solution strengthening element of ferrite. It can reduce the interfacial energy between carbides and the matrix, inhibit the formation of coarse carbides during cooling, strongly inhibit the precipitation and coarsening process of cementite, and thus refine the sorbite lamellars, playing the role of refining the precipitated phase and improving the toughness of the material. However, if the Al content is too high, it will promote the excessive precipitation of ferrite. During the solidification of the billet, the inclusions will adhere to the inner wall of the nozzle, causing unstable steel flow and defects. Therefore, in order to adapt to the online toughening of the multiphase structure by online molten salt quenching isothermal, the mass percentage of Al is controlled at 0.20%~0.40%.

[0016] (6) Niobium: As a microalloying element, Nb’s pinning effect can effectively suppress the coarsening of austenite grains during rolling, so as to promote the uniform nucleation of bainite and refine the sorbite lamellae. It also has good high-temperature stability during isothermal toughening and can disperse precipitation to provide strong precipitation strengthening effect, thereby providing a certain resistance to tempering softening and compensating for the strength loss during softening. At the same time, there is no risk of coarsening during service. However, Nb has a high cost, and excessive addition is not conducive to controlling material cost. Therefore, based on the role of Nb and material cost considerations, Nb element is added appropriately, and the mass percentage of Nb is controlled at 0.015%~0.030%.

[0017] (7) Vanadium: Nb and V are microalloying elements. Their composite precipitation can effectively inhibit the coarsening of wire rod grains during the rolling process, so as to refine the bainite structure, inhibit the coarsening of sorbite, stabilize the residual austenite, and disperse precipitation during the isothermal toughening process to form VC nano carbides, providing a strong precipitation strengthening effect. However, V has a high cost, and excessive addition is not conducive to controlling material cost and has the risk of coarsening. Therefore, based on the role of V and material cost considerations, V element is added appropriately, and the mass percentage of V is controlled at 0.015%~0.030%.

[0018] (8) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0019] The aforementioned hot-rolled wire rod adopts a C-Si-Mn-Cr-V-Nb-V composition design. The Si and Mn contents are relatively low, while Nb and V are added in trace amounts, which can appropriately control material costs. The solid solution strengthening effect is optimized through C, Si, and Mn; C forms multi-component nanoscale alloy carbides with Cr, Nb, and V to provide dispersion strengthening; Mn and Cr regulate the hardenability of the wire rod and expand the bainitic transformation range; Si and Al inhibit cementite coarsening and provide appropriate resistance to tempering softening, thus contributing to the formation of most of the... This process provides favorable conditions for quenching bainite, refining bainite and sorbite structures, and rapid tempering softening under isothermal treatment. Based on this, a higher wire-drawing temperature is selected to keep the wire rod in a high-temperature austenitic state, promoting the solid solution of microalloying elements and preventing premature precipitation of proeutectoid ferrite or network carbides due to excessively low wire-drawing temperatures. This creates favorable conditions for achieving greater undercooling, promoting phase transformation towards bainite and refining the sorbite structure. After wire drawing, the wire rod is not air-cooled but directly undergoes online salt bath quenching isothermal treatment.

[0020] Firstly, compared to the limitations of the Stellmore air-cooled line, which is constrained by its maximum cooling capacity and unstable temperature control, and the existing online salt bath treatment, which is limited by its one-stage processing and thus struggles to control network carbides and mixed phase transformations, molten salt offers several advantages. Firstly, molten salt has a higher heat transfer coefficient than air cooling, and the temperature of the bainite-pearlite mixed phase region is lower. The wire rod undergoes rapid cooling after the initial molten salt treatment, allowing it to quickly pass through the precipitation temperature range of secondary cementite from high-temperature austenite, suppressing the risk of network carbides caused by high-carbon segregation, and avoiding impacts on microstructure uniformity and the carbon source for subsequent phase transformations. Secondly, compared to the existing one-stage salt bath treatment, the wire rod enters a lower-temperature mixed phase region after the initial molten salt treatment, allowing for faster cooling and reducing the carbon diffusion coefficient. This process promotes the transformation of most austenite into fine bainite and fine lamellar sorbite in the low-temperature cooling phase region, thereby suppressing cementite coarsening, microalloyed carbide coarsening precipitation, and coarse pearlite lamellae. This achieves multiphase microstructure control, provides high-density dislocations and matrix strength, and provides a high-strength microstructure basis for isothermal tempering. At the same time, when the wire rod passes through molten salt, the molten salt can cover the surface of the wire rod for uniform heat exchange, eliminating the temperature difference problem between the air-cooled and air-received surfaces. Compared with air-cooled lines, this can reduce the temperature difference between the wire rod surface and the core, avoiding local overcooling that leads to the formation of brittle martensite. Combined with the hardenability of the wire rod, the formation of bainite and sorbite is more uniform and controllable, which can reduce mechanical property fluctuations and reduce restrictions on rolling.

[0021] Second, compared to the Stellmore air-cooled line, which is limited by minimum cooling capacity and continuous cooling, making it difficult to control the full phase transformation and online toughening of brittle structures, and compared to the existing online salt bath treatment, which is limited by one-stage processing and makes it difficult to balance structural toughening and production efficiency, the subsequent molten salt treatment of the wire rod can, on the one hand, increase the molten salt temperature, which can accelerate the decomposition of residual austenite compared to continuous cooling, promote the full phase transformation of the structure to continue forming sorbite, and reduce ferrite, thereby improving the matrix strength and preventing the residual austenite from continuing to form martensite and other low-temperature brittle structures during subsequent cooling; on the other hand, the wire rod has a high density of dislocations after the initial molten salt treatment, which, combined with the compositional inhibition of cementite precipitation and coarsening, can appropriately extend the life of the wire rod after the subsequent molten salt treatment. The processing time further reduces the temperature difference between the wire rod surface and the core, promoting the combination of C with Cr, Nb, and V to form nanoscale alloy carbides, which are uniformly distributed in the tempered bainite and sorbite matrix. Through the dislocation pinning effect of the carbides, the strength loss during the toughening process is compensated, the toughening effect of the alloy is fully utilized, and a certain resistance to tempering softening is provided. Then, during the isothermal tempering process, through dislocation rearrangement and full release of structural stress, the rapid online toughening of the multiphase structure is achieved, effectively improving the brittleness of the bainite matrix. At the same time, it can reduce the restriction on the slow cooling of the roller table. The wire rod has a high temperature after exiting the molten salt, and the slow cooling of the roller table can promote the further toughening of the wire rod structure, improve the tempering softening effect, control the strength and plasticity matching of the wire rod, and take into account the production efficiency.

[0022] Before rolling, a higher homogenization temperature and an appropriate furnace time can be selected to promote uniform diffusion of alloy components, reduce the influence of segregation, and avoid excessively high homogenization temperature and excessively long furnace time, which may lead to grain coarsening or oxidation burn-off. In the preferred technical solution, before rolling, the homogenization temperature of the heating furnace is controlled at 1188~1238℃ and the furnace time is controlled at 145~235min.

[0023] Since online molten salt mixing quenching isothermal can reduce the limitations on rolling, a higher initial rolling temperature can be selected during rolling, reducing the deformation resistance of the rolled piece, increasing rolling speed and efficiency. Combined with a larger initial rolling reduction, it can eliminate original structural defects and refine the grain structure. By selecting an appropriate final rolling temperature and final rolling reduction, the precipitation of NbC and VC pinning grain boundaries can be induced. Dynamic recrystallization during the final rolling process accumulates strain energy storage and refines the grain structure, strengthening and toughening the matrix. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1036~1086℃, the initial rolling reduction is 36.5%~42%, the final rolling temperature is 905~945℃, and the final rolling reduction is 23%~28%.

[0024] During the wire spinning process, the wire spinning temperature can be further controlled to suppress grain coarsening. In a preferred embodiment, the wire spinning temperature is controlled to be 895~935℃.

[0025] The molten salt temperature in the initial molten salt treatment is located in the mixed phase region of bainite and pearlite. Lower molten salt temperatures promote rapid cooling of the wire rod, inhibiting the coarsening and precipitation of network carbides, coarse lamellar pearlite, and alloy carbides, reducing carbon diffusion, and promoting bainitic phase transformation. This fosters the formation of fine bainite and fine-lamellar sorbite structures. With prolonged treatment time, the proportion of bainite and dislocation density in the microstructure increases, strengthening the matrix. However, excessively low molten salt temperatures are detrimental to suppressing abnormal martensitic structures, increasing the temperature gradient from the wire rod surface to the core, leading to fluctuations in mechanical properties and significant thermal stress. With prolonged treatment time, the cumulative thermal stress increases softening difficulty and production energy consumption. Conversely, higher molten salt temperatures reduce the temperature difference from the wire rod surface to the core, improving microstructure uniformity and reducing... The thermal stress and tempering difficulty can be reduced by shortening the processing time, which can decrease the proportion of bainite in the microstructure, reserve an appropriate amount of residual austenite, and reduce production energy consumption. However, if the molten salt temperature is too high, it will be detrimental to suppressing network carbides and controlling the refinement of bainite phase transformation. If the processing time is too short, the bainite phase transformation will be insufficient, affecting the strength of the matrix. Therefore, the molten salt temperature and processing time of the first stage of molten salt treatment can be controlled to control the wire rod to quickly enter the bainite and pearlite mixed phase region from the high-temperature austenite state, suppress the formation of network carbides, and form a quenched microstructure dominated by fine bainite and sorbite. Bainite accounts for the majority of the quenched microstructure, which prepares the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the first stage of molten salt treatment is 415~455℃, and the processing time is 125~220s.

[0026] Since the spinning temperature differs significantly from the molten salt temperature of the preceding molten salt treatment, a higher molten salt circulation rate can control the molten salt temperature rise and promote rapid cooling of the wire rod to suppress abnormal structures. In the preferred technical solution, the molten salt circulation rate of the preceding molten salt treatment is 380~600t / h, and the molten salt temperature rise is ≤8℃.

[0027] The molten salt temperature in the subsequent molten salt treatment is appropriately increased compared to the preceding molten salt treatment. Higher molten salt temperatures improve the carbon diffusion coefficient, promoting the rapid transformation of untransformed retained austenite into sorbite. With prolonged treatment time, it promotes dislocation climb in the formed bainite structure, releasing structural stress and facilitating the full precipitation and uniform distribution of alloy carbides, thus improving strength and toughness. However, excessively high molten salt temperatures are detrimental to suppressing lamellar coarsening and alloy carbide aggregation and growth. With prolonged treatment time, carbide coarsening occurs, and the dispersion strengthening effect decreases, leading to a loss of strength and plasticity while increasing production energy consumption. Conversely, lower molten salt temperatures result in slower carbon diffusion, suppressing sorbite lamellar coarsening, promoting the dispersion precipitation of alloy carbides, and enhancing the anchoring effect. Shorter treatment times further reduce the risk of carbide coarsening and accelerate production. While improving production efficiency and reducing energy consumption, excessively low molten salt temperatures are detrimental to dislocation rearrangement, stress release, and uniform diffusion of carbides in the matrix. This leads to a slower decomposition and transformation rate of retained austenite. Furthermore, shorter treatment times result in reduced softening and stress release effects, incomplete precipitation of alloy carbides, and even the continued formation of brittle structures from retained austenite during subsequent cooling, affecting the wire rod's plasticity and structural uniformity. Therefore, further control of the molten salt temperature and treatment time in the later stages of molten salt treatment is necessary to promote the transformation of untransformed retained austenite into sorbite. Isothermal tempering of the quenched structure formed by bainite and sorbite further promotes the dispersed precipitation of alloy carbides, thereby regulating the strength-plasticity balance of the wire rod. In a preferred embodiment, the molten salt temperature in the later stages of molten salt treatment is 547–587°C, and the treatment time is 160–245 seconds.

[0028] Since the temperature difference between the molten salt in the first stage and the second stage of molten salt treatment is small, the second stage of molten salt treatment can appropriately reduce the molten salt circulation volume, control the molten salt temperature rise, and promote the uniform treatment of the wire rod structure. At the same time, it can reduce production energy consumption compared with one-stage salt bath treatment. In the preferred technical solution, the molten salt circulation volume of the second stage of molten salt treatment is 350~550t / h, and the molten salt temperature rise is ≤3℃.

[0029] Since the wire rod has undergone sufficient phase transformation and rapid toughening through online salt bath mixed quenching isothermal treatment, the restriction on slow cooling on the roller table can be reduced, the slow cooling rate can be further controlled, the wire rod structure can be further toughened, the tempering softening effect can be improved, and production efficiency can be taken into account. In the preferred technical solution, the slow cooling on the roller table controls the wire rod to cool to below 290°C at a slow cooling rate of ≤0.6°C / s before winding.

[0030] In the preferred technical solution, the roller conveyor slow cooling adopts the control of the opening of the heat insulation cover, and blows the hot air of ≥220°C during the online molten salt mixing quenching isothermal treatment into the heat insulation cover. The conveyor roller conveyor transports the wire rod through the heat insulation cover to control the slow cooling of the wire rod, which can further recover and utilize the heat energy of molten salt treatment, reduce production energy consumption, and promote the rapid production of wire rod.

[0031] A high-strength multiphase hot-rolled wire rod for 2260MPa 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 2260MPa grade stranded wire described in any one of the above-mentioned methods.

[0032] The aforementioned hot-rolled wire rods are designed with a high carbon content of Al-Si, with trace amounts of V and Nb added to control material costs. The microstructure comprises a multiphase structure consisting mostly of tempered bainite, a small portion of tempered sorbite, and a small amount of ferrite. Compared to air-cooled wire rods with a filamentous structure, this effectively suppresses defects in the network cementite, martensite, and coarse pearlite core, achieving stable control of the multiphase structure. Compared to existing sorbite wire rods obtained through online salt bath treatment, the bainite has a high dislocation density and higher strength, providing a high-strength skeleton and appropriate... The work hardening capacity, after tempering and regulation, is appropriately reduced in dislocation density. Combined with the dispersed precipitation of alloy carbides of Cr, Nb, and V, it increases dislocation resistance and inhibits the coarsening of sorbite and cementite. This enhances the strengthening effect of carbon and alloys, thereby increasing the initial strength of the wire rod and providing a high strength reserve for subsequent drawing. At the same time, the toughness of the tempered bainite is improved. Combined with the tempered sorbite structure after stress relief, the wire rod has both good plasticity and toughness, which can delay crack initiation, reduce stress concentration, and thus eliminate the need for offline heat treatment, reducing the risk of wire breakage during drawing and twisting.

[0033] The higher the volume percentage of tempered bainite and the finer the lamellar spacing of tempered sorbite, the higher the matrix strength. The higher the proportion of tempered sorbite, the better the ductility and toughness of the matrix. In the preferred technical solution, the volume percentage of tempered bainite is 65%~75%, the volume percentage of tempered sorbite is ≥17%, the lamellar spacing of tempered sorbite is 65~115nm, and the volume percentage of ferrite is ≤8%.

[0034] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference of ≤50MPa within the same coil. This can effectively avoid the cutting effect of the network carbide and avoid stress concentration. At the same time, the wire rod has better structural uniformity and smaller mechanical property difference within the same coil, which is conducive to improving processing stability and the consistency of stranded products, and thus is suitable for long-term service scenarios.

[0035] In the preferred technical solution, the hot-rolled wire rod has a diameter of 8.0~15.0mm, a tensile strength of 1547~1597MPa, and a reduction of area of ​​27%~32%. The hot-rolled wire rod has high initial strength and work hardening ability in its structure. As a base material for manufacturing stranded wire, it can reduce the number of drawing passes, reduce plastic loss and wire breakage risk during the process, and improve production efficiency. With a high reduction of area, offline heat treatment is not required, which can reduce the cost and energy consumption of downstream users and promote the efficient and stable production of stranded wire.

[0036] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0037] (1) In view of the current situation where hot-rolled wire rods for steel strands are limited by the process, abnormal structures are difficult to control, and online toughening of structures is difficult to balance with production efficiency, this invention adopts a high carbon composition design of Al-Si, with trace addition of V and Nb, combined with online molten salt quenching isothermal technology, which can control the wire rod to quickly enter the mixed phase region of bainite and pearlite from the high temperature austenite state, suppress the formation of network cementite and coarse pearlite, promote the transformation of some austenite into bainite and sorbite, form a quenched structure dominated by bainite, and then after the subsequent molten salt treatment, promote the decomposition of the untransformed residual austenite into sorbite and isothermal tempering, suppress the abnormal martensite structure, realize the control of multiphase structure and online control of structure toughness, improve the strength and plasticity matching of wire rods, and at the same time reduce the restrictions on rolling and slow cooling of roller table, balance production efficiency, promote the efficient and stable production of hot-rolled wire rods, and have good industrial adaptability.

[0038] (2) In view of the current situation that hot-rolled wire rods for steel strands are mainly made of pearlite-type materials, the material cost is high, the initial strength or microstructure is insufficient and offline heat treatment is required. This invention can control the material cost. The microstructure includes a multiphase structure composed of tempered bainite, tempered sorbite and ferrite, which can effectively suppress the defects of network cementite, martensite and coarse pearlite in the core. By utilizing the dispersed precipitation of alloy carbides of bainite with Cr, Nb and V, a high-strength skeleton and appropriate work hardening ability are provided. With the microstructure control of the tempered state, the brittleness is fully improved and the microstructure is toughened. It can delay crack initiation and reduce stress concentration, and achieve a tensile strength of 1547~1597MPa and a reduction of area of ​​27%~32%. It can be used to manufacture 2260MPa grade strands and other application fields without offline heat treatment. It is beneficial to reduce the number of drawing passes, reduce the risk of wire breakage, meet the high-efficiency and low-cost production of ultra-high strength strands, and has good market application prospects. Attached Figure Description

[0039] 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:

[0040] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention;

[0041] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention;

[0042] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0043] 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.

[0044] Example 1:

[0045] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.90%, Si: 0.58%, Mn: 0.69%, Cr: 0.61%, Al: 0.20%, Nb: 0.03%, V: 0.03%, 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 mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0046] 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 the homogenization of alloy composition and avoiding grain coarsening. After exiting the furnace, the steel billet is rolled into wire rod with a diameter of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC and VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1238℃, and the furnace dwell time is 145min. The initial rolling temperature is 1086℃, the initial rolling reduction is 42%, the final rolling temperature is 945℃, and the final rolling reduction is 28%. 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, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for promoting phase transformation towards bainite and refining the sorbite structure. Specifically, the wire drawing temperature is controlled at 935℃.

[0047] The online molten salt mixing quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 35°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region. This inhibits the formation of network carbides and coarse pearlite, promoting the transformation of some austenite into bainite and sorbite, resulting in a quenched microstructure dominated by fine bainite and sorbite. The quenched microstructure has a high proportion of bainite. The wire rod is then conveyed via roller conveyor through the second section... The first stage of molten salt treatment involves a salt bath to increase the molten salt temperature, promote the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promote the dispersed precipitation of Cr, Nb, and V carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 455℃, the treatment time is 125s, the molten salt circulation rate is 380t / 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 245s, the molten salt circulation rate is 350t / h, and the molten salt temperature rise is ≤3℃.

[0048] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥220℃) from above the two sections of the online molten salt mixing quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 282℃ at a slow cooling rate of 0.6℃ / 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.

[0049] Comparative Example 1:

[0050] 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 1150°C, with a furnace time of 255 min, an initial rolling temperature of 1005°C, a final rolling temperature of 815°C, and a wire drawing temperature of 800°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 75% speed, fans 1 to 4 are turned on to cool the wire rod to 694℃ at a cooling rate of 7.1℃ / s. Then, fans 5 to 14 are turned on to 35% speed to cool the wire rod to 280℃ at a cooling rate of 3.3℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0051] Comparative Example 2:

[0052] 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 1155°C, the furnace time is 240 min, the initial rolling temperature is 1010°C, the final rolling temperature is 830°C, and the wire drawing temperature is 810°C. During the molten salt treatment before the online molten salt mixing and quenching isothermal process, the wire rod is cooled at a rate of 30°C / s, and the finished hot-rolled wire rod is obtained after leaving the production line.

[0053] Example 2:

[0054] A preferred embodiment of the manufacturing method of the 2260MPa grade high-strength multiphase hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.88%, Si: 0.77%, Mn: 0.55%, Cr: 0.62%, Al: 0.22%, Nb: 0.026%, V: 0.015%, 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 mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0055] 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 the homogenization of alloy composition and avoiding grain coarsening. After exiting the furnace, the steel billet is rolled into wire rod with a diameter of 10mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC and VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1195℃, and the furnace dwell time is 200min. The initial rolling temperature is 1046℃, the initial rolling reduction is 40%, the final rolling temperature is 920℃, 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, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for promoting phase transformation towards bainite and refining the sorbite structure. Specifically, the wire drawing temperature is controlled at 905℃.

[0056] The online molten salt mixing quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 36℃ / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region. This inhibits the formation of network carbides and coarse pearlite, and promotes the transformation of some austenite into bainite and sorbite, forming a quenched structure dominated by fine bainite and sorbite. The quenched structure has a high proportion of bainite. The wire rod is then conveyed via roller conveyor through the second section... The first stage of molten salt treatment involves a salt bath to increase the molten salt temperature, promote the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promote the dispersed precipitation of Cr, Nb, and V carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 443℃, the treatment time is 166s, the molten salt circulation rate is 440t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 558℃, the treatment time is 210s, the molten salt circulation rate is 400t / h, and the molten salt temperature rise is ≤3℃.

[0057] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥220℃) from above the two salt bath tanks undergoing online molten salt mixing and quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers 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 284℃ at a slow cooling rate of 0.5℃ / 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.

[0058] Comparative Example 3:

[0059] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the online molten salt mixing and quenching isothermal process, the wire rod is cooled at a cooling rate of 40℃ / s, the molten salt temperature of the initial molten salt treatment is 405℃, the treatment time is 230s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0060] Comparative Example 4:

[0061] 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 mixing and quenching isothermal process, the wire rod is cooled at a cooling rate of 31°C / s, the molten salt temperature of the initial molten salt treatment is 475°C, the treatment time is 100s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0062] Example 3:

[0063] A preferred embodiment of the manufacturing method of the 2260MPa grade high-strength multiphase hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.92%, Si: 0.64%, Mn: 0.75%, Cr: 0.42%, Al: 0.40%, Nb: 0.03%, V: 0.025%, 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 mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0064] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature state that allows for rolling plasticity, promoting alloy composition homogenization and preventing grain coarsening. After exiting the furnace, the billet is rolled into a 12.5mm diameter wire rod via a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce NbC and VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1215℃, and the furnace dwell time is 175 minutes. n, the initial rolling temperature is 1066℃, the initial rolling reduction is 37%, the final rolling temperature is 930℃, 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 higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for promoting phase transformation towards bainite and refining the sorbite structure. Specifically, the wire drawing temperature is controlled at 925℃.

[0065] The online molten salt mixing quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 39°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region. This inhibits the formation of network carbides and coarse pearlite, promoting the transformation of some austenite into bainite and sorbite, resulting in a quenched microstructure dominated by fine bainite and sorbite. The quenched microstructure has a high proportion of bainite. The wire rod is then conveyed via roller conveyor through the second section... The first stage of molten salt treatment involves a salt bath to increase the molten salt temperature, promote the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promote the dispersed precipitation of Cr, Nb, and V carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 427℃, the treatment time is 190s, the molten salt circulation rate is 510t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 572℃, the treatment time is 175s, the molten salt circulation rate is 450t / h, and the molten salt temperature rise is ≤3℃.

[0066] The roller conveyor slow cooling process employs controlled opening of the insulation hood to blow hot air (≥220℃) from above the two salt bath tanks undergoing online molten salt mixing and quenching isothermal 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 285℃ at a slow cooling rate of 0.4℃ / 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.

[0067] 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 590°C, the treatment time is 250s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0068] 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 100s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0069] Example 4:

[0070] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.90%, Si: 0.78%, Mn: 0.59%, Cr: 0.57%, Al: 0.31%, Nb: 0.015%, V: 0.024%, 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 mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0071] The rolling process is used to heat a 220mm × 220mm steel billet in a furnace to a high-temperature, rollable form, promoting alloy homogenization and preventing grain coarsening. After exiting the furnace, the billet is rolled into a 15mm diameter wire rod on a rolling line. Appropriate rolling temperatures and reductions are selected to improve rolling efficiency and induce NbC and VC precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1188℃, and the furnace dwell time is 235 minutes. The initial rolling temperature is 1036℃, the initial rolling reduction is 36.5%, the final rolling temperature is 905℃, and the final rolling reduction is 23%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are distributed on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing favorable conditions for promoting phase transformation towards bainite and refining the sorbite structure. Specifically, the wire drawing temperature is controlled at 895℃.

[0072] The online molten salt mixing quenching isothermal process employs a two-section salt bath with internal molten salt. After wire drawing, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment. This causes the wire rod to cool at a rate of 37°C / s, rapidly transitioning it from a high-temperature austenitic state through the network carbide region into a mixed phase region of bainite and pearlite. This process inhibits the formation of network carbides and coarse pearlite, promotes the transformation of some austenite into bainite and sorbite, and forms a quenched microstructure dominated by fine bainite and sorbite. The quenched microstructure has a high proportion of bainite. The wire rod is then conveyed via roller conveyor through the second section... The first stage of molten salt treatment involves a salt bath to increase the molten salt temperature, promote the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promote the dispersed precipitation of Cr, Nb, and V carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 415℃, the treatment time is 550s, 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 587℃, the treatment time is 160s, the molten salt circulation rate is 550t / h, and the molten salt temperature rise is ≤3℃.

[0073] The roller conveyor slow cooling process uses an adjustable heat insulation cover to blow hot air (≥220℃) from the two salt bath tanks undergoing online molten salt mixing and quenching isothermal treatment into the heat insulation cover. The wire rod is then conveyed by the conveyor rollers through the heat insulation cover for slow cooling, which promotes further toughening of the wire rod structure and improves the tempering and softening effect. Specifically, the wire rod is controlled to cool to 287℃ at a slow cooling rate of 0.3℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0074] Comparative Example 7:

[0075] A method for manufacturing hot-rolled wire rod differs from that in Example 4 in that the manufacturing method follows a process flow of rolling → wire drawing → online molten salt mixing quenching isothermal → air cooling. The air cooling is achieved by opening the heat insulation cover, conveying the wire rod by a conveyor roller, and controlling the wire rod to cool to 275°C at a slow cooling rate of 1.7°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0076] The microstructure and properties of the hot-rolled wire rods obtained in Examples 1-4 and Comparative Examples 1-7 were tested, and the comparative results are shown in Table 1 below:

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

[0078]

[0079] The comparison between Example 1 and Comparative Example 1 shows that under the strong air cooling treatment of the Stellmore air-cooling line, the wire rod not only contains abnormal structures such as network carbides and martensite, but also has a high proportion of ferrite and a pearlite core, affecting the strengthening effect of carbon and microalloying elements. This invention employs a high-carbon Al-Si design with trace amounts of V and Nb, combined with online molten salt quenching isothermal technology, which can control the wire rod to rapidly transition from a high-temperature austenitic state to a mixed phase region of bainite and pearlite, inhibiting the formation of network cementite and coarse pearlite, promoting the transformation of some austenite into bainite and sorbite, and forming a quenching phase dominated by bainite. The wire rod undergoes a fire-treated microstructure, followed by a molten salt treatment to promote the decomposition of untransformed residual austenite into sorbite and isothermal tempering. This process suppresses abnormal martensite structures, enabling multiphase microstructure control and online regulation of microstructure toughness. This improves the strength-plasticity matching of the wire rod. As seen in the results of Examples 1-4, the microstructure consists of a multiphase microstructure composed of tempered bainite, tempered sorbite, and ferrite, achieving a tensile strength of 1547-1597 MPa and a reduction of area of ​​27%-32%. This material is suitable for manufacturing 2260 MPa grade stranded wire and other applications. It does not require offline heat treatment, which helps reduce the number of drawing passes and lower the risk of wire breakage.

[0080] As can be seen from the comparison between Example 1 and Comparative Example 2, selecting a higher wire drawing temperature, so that the wire rod is in a high-temperature austenitic state, can promote the solid solution of microalloying elements, avoid premature precipitation of proeutectoid ferrite or network carbides due to excessively low wire drawing temperature, and provide favorable conditions for forming a larger degree of undercooling, promoting phase transformation towards bainite and refining sorbite structure.

[0081] As can be seen from the comparison results between Example 2 and Comparative Example 3, the lower the molten salt temperature in the initial molten salt treatment, the faster the wire rod cools down, the more it inhibits the coarsening precipitation of network carbides, coarse lamellar pearlite and alloy carbides, and the reduction of carbon diffusion. Bainite phase transformation becomes the dominant process, promoting the formation of fine bainite structure and fine lamellar sorbite structure. With the extension of treatment time, the proportion of bainite and dislocation density in the structure can be increased, strengthening the matrix. However, if the molten salt temperature is too low, the temperature gradient from the wire rod surface to the core will increase, which will lead to fluctuations in mechanical properties and large thermal stress. With the excessively long treatment time, the thermal stress will accumulate, increasing the softening difficulty and production energy consumption.

[0082] As can be seen from the comparison results of Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the lower the temperature difference between the wire rod surface and the core, the better the uniformity of the structure, the lower the thermal stress and the difficulty of tempering. As the treatment time is shortened, the proportion of bainite in the structure can be reduced, an appropriate amount of residual austenite can be reserved, and 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 controlling the refinement of bainite phase transformation. If the treatment time is too short, the bainite phase transformation is insufficient and the matrix strength is affected.

[0083] As can be seen from the comparison results between Example 3 and Comparative Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the higher the diffusion coefficient of carbon, which promotes the rapid transformation of untransformed residual austenite into sorbite. With the extension of treatment time, it can promote the dislocation climb of the formed bainite structure, release the structural stress, promote the full precipitation and uniform distribution of alloy carbides, and improve the strength and toughness. However, if the molten salt temperature is too high, it is not conducive to suppressing lamellar coarsening and alloy carbide agglomeration and growth. With the longer treatment time, the carbides coarsen and the dispersion strengthening effect decreases, which will lead to the loss of strength and plasticity, and at the same time increase production energy consumption.

[0084] As can be seen from the comparison results between Example 3 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the slower the carbon diffusion, which can inhibit the coarsening of sorbite lamellars, promote the dispersed precipitation of alloy carbides, and enhance the pinning effect. With the shortening of the treatment time, the risk of carbide coarsening can be further reduced, the production pace can be accelerated, and the production energy consumption can be reduced. However, if the molten salt temperature is too low, it is not conducive to the rearrangement of matrix dislocations, stress release, and uniform diffusion of carbides. The decomposition transformation rate of retained austenite decreases. With the treatment time being too short, the tempering softening and stress release effects decrease, the alloy carbides are not fully precipitated, and even the retained austenite continues to form brittle structures during subsequent cooling, affecting the plasticity and structural uniformity of the wire rod.

[0085] As can be seen from the comparison results of Example 4 and Comparative Example 7, the wire rod has undergone sufficient phase transformation and rapid toughening after online salt bath mixed quenching isothermal treatment, which can reduce the limitation on the slow cooling of the roller table. The slow cooling of the roller table can further control the slow cooling speed, promote the further toughening of the wire rod structure, improve the tempering softening effect, and take into account the production efficiency.

[0086] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire, characterized in that, Its manufacturing methods include: 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: 0.58%~0.78%, Mn: 0.55%~0.75%, Cr: 0.42%~0.62%, Al: 0.20%~0.40%, Nb: 0.015%~0.030%, V: 0.015%~0.030%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is produced at a drawing temperature of ≥895℃. After being rolled into wire rod, the wire rod undergoes an online molten salt mixing quenching isothermal treatment. The wire rod first undergoes a preliminary molten salt treatment and is cooled at a rate of ≥35℃ / s, transitioning from the austenitic state to the mixed phase region of bainite and pearlite. This promotes the transformation of some austenite into bainite and sorbite, forming a quenched structure dominated by bainite. Then, it undergoes a subsequent molten salt treatment, which increases the molten salt temperature and promotes the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, it undergoes slow cooling on a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of tempered bainite, tempered sorbite, and ferrite, forming a multiphase microstructure.

2. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1188~1238℃ and the furnace time is 145~235min.

3. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1036~1086℃, the initial rolling reduction is 36.5%~42%, the final rolling temperature is 905~945℃, and the final rolling reduction is 23%~28%.

4. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 1, characterized in that, During the spinning process, the spinning temperature is controlled at 895~935℃.

5. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 1, characterized in that, The molten salt temperature of the first stage of molten salt treatment is 415~455℃, and the treatment time is 125~220s; the molten salt temperature of the second stage of molten salt treatment is 547~587℃, and the treatment time is 160~245s.

6. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 5, characterized in that, The molten salt circulation rate of the first-stage molten salt treatment is 380~600t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation rate of the second-stage molten salt treatment is 350~550t / h, and the molten salt temperature rise is ≤3℃.

7. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 5, characterized in that, The roller conveyor slow cooling control strip is cooled to below 290°C at a slow cooling rate of ≤0.6°C / s before being wound up.

8. A high-strength multiphase hot-rolled wire rod for 2260MPa 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 2260MPa grade strand as described in any one of claims 1 to 7.

9. The high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 8, characterized in that, The volume percentage of tempered bainite is 65%~75%, the volume percentage of tempered sorbite is ≥17%, the lamellar spacing of tempered sorbite is 65~115nm, the volume percentage of ferrite is ≤8%, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤50MPa.

10. The high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire according to claim 8, characterized in that, The hot-rolled wire rod has a diameter of 8.0~15.0mm, a tensile strength of 1547~1597MPa, and a reduction of area of ​​27%~32%.

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