A high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a high-strength multiphase hot-rolled wire rod for 2260MPa grade stranded wire and its manufacturing method. The method involves designing and rolling the high-carbon C-Si-Mn-Cr-V-Nb-V wire rod, followed by online molten salt mixing and isothermal treatment. This process rapidly cools the wire rod from an austenitic state to a mixed phase region of bainite and pearlite, promoting the transformation of some austenite into bainite and sorbite. A subsequent molten salt treatment increases the molten salt temperature, further promoting the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered bainite, tempered sorbite, and ferrite, achieving a tensile strength of 1547~1597MPa and a reduction of area of ​​27%~32%. This eliminates the need for offline heat treatment, reduces drawing passes, and lowers the risk of wire breakage when manufacturing high-strength stranded wire.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hot-rolled wire rod, and particularly relates to a 2260MPa-grade high-strength multiphase hot-rolled wire rod for stranded wire and a manufacturing method thereof. BACKGROUND

[0002] As a key component for load bearing and transmission, steel strand is widely used in the fields of bridge, road traffic, power, energy, building, etc. With the rapid development of industry, the trend of steel strand towards ultra-high strength is becoming increasingly obvious. The ordinary high-strength 1960MPa-grade strength is increasingly difficult to meet the market demand of rapid development. The hot-rolled wire rod as the raw material of steel strand is usually produced by controlled cooling in a Stelmor air cooling line, but it is also limited by the controlled cooling capacity of the air cooling line, resulting in that the microstructure and performance of the hot-rolled wire rod are difficult to meet the production of ultra-high strength stranded wire, and the microstructure state is mainly pearlite type material, which needs to be combined with offline heat treatment, and other types of microstructure are difficult to control toughness, and offline heat treatment also brings the problems of process, energy consumption and cost increase of downstream users, so it is necessary to develop a 2260MPa-grade stranded wire hot-rolled wire rod and a manufacturing method thereof, so as to meet the development of the steel industry and the market use demand without offline heat treatment.

[0003] The existing high-strength stranded wire hot-rolled wire rod is generally a high-carbon component system sorbite microstructure wire rod, and there are still the following technical bottlenecks in manufacturing a high-strength stranded wire hot-rolled wire rod without offline heat treatment.

[0004] I. To improve the strength of wire rod, the prior art generally increases the content of C, Si and Mn, combined with low temperature rolling and strong air cooling after wire drawing Stelmor air cooling line to obtain fine lamellar spacing of sorbite structure, but on the one hand, due to the high carbon component system is easy to form center carbon segregation in continuous casting process, limited by the highest cooling capacity of Stelmor air cooling line, the carbon concentration of the segregation zone is supersaturated, which will preferentially precipitate secondary cementite in the austenite grain boundary to form continuous network structure. This network carbide cuts off the continuity of the matrix, leading to stress concentration, plasticity decrease and even brittle fracture during cold drawing of wire rod. In order to reduce the level of network carbide and refine the sorbite lamellar spacing, the temperature control instability is further increased after strong air cooling, and the temperature difference between the wind surface and the wind surface of the wire rod and the surface to the core is further increased. During cooling, uncontrollable bainite or martensite structure is easily formed due to local supercooling, resulting in large brittleness and mechanical property fluctuation. On the other hand, although the wire rod is refined by microalloying elements such as V, Nb and Ti to increase the strength and toughness of the matrix, the precipitation is uneven due to the limitation of unstable temperature control and continuous cooling during strong air cooling, which makes it difficult to fully play the strengthening effect, and the strengthening effect is limited. The excessive addition of microalloying elements also increases the cost of materials. At the same time, limited by the length of Stelmor air cooling line, the minimum cooling capacity and continuous cooling, the wire rod is in a low temperature state after phase transformation incubation, and the brittle structure such as bainite or martensite is difficult to be toughened due to high dislocation density, large brittleness and large residual stress in the structure, which hinders the dislocation movement during cold drawing, leading to stress concentration and easy fracture. Even in the process of transportation or coiling, brittle fracture occurs, which is difficult to produce stably and needs to be combined with offline heat treatment.

[0005] II. In order to obtain high strength and plasticity, although the prior art proposes an online salt bath treatment process, for example: patent CN120400686A discloses a 2200-2260MPa grade ultra-high stress corrosion resistant pre-stressed steel strand wire rod, steel strand and its production method, which adopts 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 sorbitizing rate, but on the one hand, the strength of sorbite depends on the refinement of lamellar, although the strength is improved by V / Nb microalloying, but the initial strength and work hardening rate are insufficient, leading to more drawing passes during strand processing, and large plasticity loss, which brings the risk of wire breakage. At the same time, it needs to be combined with low temperature rolling to inhibit the growth of cementite, which limits the rolling efficiency and aggravates the wear of rolling line. On the other hand, due to the limitation of phase transformation regulation and one-stage salt bath treatment, a large amount of molten salt circulation is needed, which leads to large energy consumption. In order to avoid the coarsening of cementite, the treatment time of one-stage salt bath treatment should not be too long, which reduces the softening rhythm and needs to be combined with low cooling rate and long time online aging slow treatment, which affects the production efficiency. SUMMARY

[0006] The application aims to solve at least one of the above technical problems, and provides a high-strength complex-phase hot-rolled wire rod for 2260MPa-grade stranded wire and a manufacturing method thereof, which can inhibit abnormal structure, realize complex-phase structure regulation and online regulation of structure toughness, and balance production efficiency, so as to manufacture high-strength stranded wire without offline heat treatment, reduce drawing passes, and reduce the risk of wire breakage.

[0007] The application adopts the technical scheme of solving the technical problems:

[0008] The manufacturing method of the high-strength complex-phase hot-rolled wire rod for 2260MPa-grade stranded wire comprises the following steps:

[0009] The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod comprises: C: 0.88% to 0.92%, Si: 0.58% to 0.78%, Mn: 0.55% to 0.75%, Cr: 0.42% to 0.62%, Al: 0.20% to 0.40%, Nb: 0.015% to 0.030%, V: 0.015% to 0.030%, P≤0.014%, S≤0.014%, and the rest is Fe and inevitable impurities; after the wire rod is produced by wire drawing according to a wire drawing temperature of ≥895℃, the wire rod is subjected to online molten salt mixed quenching and isothermal treatment, so that the wire rod is first subjected to front molten salt treatment and is cooled at a cooling rate of ≥35℃ / s, enters a mixed phase region of bainite and pearlite from an austenite state, promotes partial austenite to transform into bainite and sorbite, forms a quenched structure mainly composed of bainite, is subjected to rear molten salt treatment, the molten salt temperature is increased, residual austenite is promoted to decompose and transform into sorbite and is subjected to isothermal tempering, and finally is subjected to roller slow cooling, so as to be manufactured into a hot-rolled wire rod with a complex-phase structure composed of tempered bainite, tempered sorbite and ferrite.

[0010] The design basis of the chemical composition and mass percentage of the above hot-rolled wire rod comprises:

[0011] (1) Carbon: C is a solid solution strengthening element in steel, has a relatively lower price, can cause significant lattice distortion of the matrix, increase the resistance to dislocation movement, and reduce the starting temperature of bainite and sorbite phase transformation, so as to promote most of the austenite to form bainite in a shear mechanism, provide hard phase support for complex-phase strength, the diffusion ability of C is enhanced with the increase of temperature, can promote the sorbite lamellar spacing to be refined, and can form nanoscale alloy carbides in combination with Cr, Nb and V, so as to improve the matrix strength and softening resistance, but if the content of C is too high, the billet is prone to central carbon segregation, the control of network carbide and martensite is difficult, the accumulation of internal stress is increased, the risk of cementite coarsening is increased, and the performance fluctuation is caused, therefore, in order to balance the high-strength requirement of 2260MPa-grade stranded wire, reduce the performance fluctuation, adapt to complex-phase structure and isothermal tempering regulation, the mass percentage of C is controlled to be 0.88% to 0.92%.

[0012] (2) Silicon: Si element is a solid solution strengthening element, which can supplement the matrix strength, reduce the diffusion rate of carbon atoms in austenite, delay the nucleation and growth of pearlite, move the bainite transformation interval upwards, benefit the refinement of bainite structure, retain high dislocation density, lay the foundation for subsequent tempering strengthening, slow down the coarsening rate of carbide in the tempering process, so as to make the residual austenite completely decompose into sorbite, refine the precipitated phase and improve the toughness of the material, and the nanocarbide still maintains a dispersed distribution under long-term high temperature, so as to provide certain anti-tempering softening capacity, but too high silicon will promote decarburization, increase the rolling deformation resistance, affect the rolling efficiency, increase the softening difficulty, and lead to the decrease of cold working plasticity and production efficiency of the steel, therefore, in order to regulate the phase transformation structure and ensure the cold working plasticity, the mass percentage of Si is controlled to be 0.58%~0.78%.

[0013] (3) Manganese: Mn element can improve the matrix strength through interstitial solid solution, and is a strong austenite stabilizing element, which can significantly reduce the critical cooling rate and the bainite transformation start temperature of the steel, inhibit the precipitation of ferrite, refine the bainite structure and sorbite lamellar spacing, and reduce the strength loss in the tempering process, but too high Mn content will increase the element segregation during the solidification of the billet, cause uneven structure, increase the risk of martensite precipitation and tempering softening difficulty, affect the toughness and structure uniformity of the wire rod, therefore, in order to regulate the complex phase structure of the hot-rolled wire rod and reduce the fluctuation of mechanical properties, the mass percentage of Mn is controlled to be 0.55%~0.75%.

[0014] (4) Chromium: Cr element can stabilize austenite, prolong the phase transformation incubation period, optimize the hardenability with Mn, reduce the critical cooling rate and the diffusion coefficient of carbon in austenite, so that the core can be transformed synchronously, and the bainite structure and sorbite lamellar spacing are refined, and chromium is easy to combine with carbon to form chromium-based carbide, which can provide dispersion strengthening and improve the anti-softening capacity, so that the complex phase structure retains high strength characteristics and drawing hardening capacity, but too high Cr content will cause segregation during solidification, increase the difficulty of controlling the structure uniformity and martensite, delay the phase transformation rate, increase the stability of residual austenite and the difficulty of tempering, affect the rapid offline, therefore, in order to regulate the complex phase structure of the hot-rolled wire rod and appropriately control the anti-tempering softening capacity of the wire rod, the mass percentage of Cr is controlled to be 0.42%~0.62%.

[0015] (5) Aluminum: Al is a solid solution strengthening element of ferrite, which can reduce the interface energy between carbide and matrix, inhibit the formation of coarse carbide during cooling, strongly inhibit the precipitation and coarsening process of cementite, and further refine the sorbite lamella, play the role of refining precipitates and improving material toughness, but too high Al content will promote excessive precipitation of ferrite, and inclusions will adhere to the inner wall of the nozzle during the solidification process of the steel billet, resulting in unstable steel flow and defects. Therefore, in order to adapt to the online toughening of the isothermal quenching of molten salt on the complex phase structure, the mass percentage of Al is controlled to be 0.20%~0.40%.

[0016] (6) Niobium: Nb as a micro-alloying element, its pinning effect can effectively inhibit the austenite grain coarsening during rolling, so as to promote the uniform nucleation of bainite and refine the sorbite lamella, and has good high temperature stability during isothermal toughening, can provide strong precipitation strengthening effect by dispersed precipitation, and further provide certain anti-tempering softening capacity to make up for the strength loss in the softening process, and has no risk of coarsening during service, but the cost of Nb is high, and excessive addition is not conducive to controlling the material cost. Therefore, based on the effect of Nb and the material cost, Nb element is appropriately added, and the mass percentage of Nb is controlled to be 0.015%~0.030%.

[0017] (7) Vanadium: Nb and V elements as micro-alloying elements, their combined precipitation can effectively inhibit the wire rod grain coarsening during rolling, so as to refine the bainite structure, inhibit the sorbite coarsening, stabilize the residual austenite, and form VC nanocarbide by dispersed precipitation during isothermal toughening, providing strong precipitation strengthening effect, but the cost of V is high, and excessive addition is not conducive to controlling the material cost, and there is a risk of coarsening. Therefore, based on the effect of V and the material cost, V element is appropriately added, and the mass percentage of V is controlled to be 0.015%~0.030%.

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

[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 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 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 of manufacturing high strength multiphase hot-rolled wire rod for 2260 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The manufacturing method of the high-strength complex-phase hot-rolled wire rod for 2260MPa-grade twisted wire comprises the following steps: rolling a wire rod according to the chemical composition of a hot-rolled coil, wherein the chemical composition of the hot-rolled coil comprises the following components in percentage by mass: 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%, and the rest is Fe and inevitable impurities; after the wire rod is spun into a coil at a spinning temperature of ≥895℃, the wire rod is subjected to on-line molten salt mixed quenching and isothermal treatment, so that the coil is first subjected to front-stage molten salt treatment and is cooled at a cooling rate of ≥35℃ / s, and then enters a mixed phase region of bainite and pearlite from an austenite state, promotes partial austenite to transform into bainite and sorbite, and forms a quenched structure mainly composed of bainite, and then the coil is subjected to rear-stage molten salt treatment, the molten salt temperature is increased, and the residual untransformed austenite is promoted to decompose and transform into sorbite and is subjected to isothermal tempering, and finally the coil is subjected to roller slow cooling to form a hot-rolled coil with a microstructure composed of complex phases including tempered bainite, tempered sorbite and ferrite; the molten salt temperature of the front-stage molten salt treatment is 415-455℃, and the treatment time is 125-220s; the molten salt temperature of the rear-stage molten salt treatment is 547-587℃, and the treatment time is 160-245s.

2. The process of producing high strength complex phase hot rolled wire rod for 2260 MPa grade strand as claimed in claim 1, wherein, Before the rolling, the soaking temperature of a heating furnace is controlled to be 1188-1238℃, and the furnace time is 145-235min.

3. The process of producing high strength complex phase hot rolled wire rod for 2260 MPa grade strand as claimed in claim 1, wherein, During the rolling, the initial rolling temperature is controlled to be 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 process of producing high strength complex phase hot rolled wire rod for 2260 MPa grade strand wire as claimed in claim 1, wherein, During the spinning, the spinning temperature is controlled to be 895-935℃.

5. The process of producing high strength complex hot rolled wire rod for 2260 MPa grade strand as claimed in claim 1, wherein, The molten salt circulation amount of the front-stage molten salt treatment is 380-600t / h, and the molten salt temperature rise is ≤8℃; the molten salt circulation amount of the rear-stage molten salt treatment is 350-550t / h, and the molten salt temperature rise is ≤3℃.

6. The process of producing high strength complex hot rolled wire rod for 2260 MPa grade strand wire as claimed in claim 1, wherein, The roller slow cooling controls the coil to be cooled to below 290℃ at a slow cooling rate of ≤0.6℃ / s for coiling.

7. A high strength multiphase hot rolled wire rod for 2260 MPa grade strand, characterized in that, The hot-rolled coil is manufactured by the manufacturing method of the high-strength complex-phase hot-rolled wire rod for 2260MPa-grade twisted wire according to any one of claims 1-6.

8. The high strength complex rod for 2260 MPa grade strand wire as claimed in claim 7, wherein, The volume percentage of the tempered bainite is 65%-75%, the volume percentage of the tempered sorbite is ≥17%, the lamellar spacing of the tempered sorbite is 65-115nm, the volume percentage of the ferrite is ≤8%, the reticular carbide level of the hot-rolled coil is 0 level, and the mechanical property difference between turns is ≤50MPa.

9. The high strength complex rod for 2260 MPa grade strand wire as claimed in claim 7, wherein, The diameter of the hot-rolled coil is 8.0-15.0mm, the tensile strength is 1547-1597MPa, and the reduction of area is 27%-32%.

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