High-strength complex-phase hot-rolled wire rod for 2430mpa grade strand and manufacturing method thereof

Abstract

The application relates to a high-strength complex-phase hot-rolled wire rod for a 2430MPa-grade strand and a manufacturing method thereof. After Mo-Nb-V high-carbon chemical composition rolling is performed on the wire rod, on-line molten salt weak quenching isothermal toughening treatment is performed on the wire rod, the wire rod is first subjected to front-stage molten salt treatment and rapid cooling, the wire rod is in a bainite phase region from an austenite state, partial austenite is promoted to transform into bainite, the wire rod is then subjected to rear-stage molten salt treatment, the molten salt temperature is increased, the molten salt circulation amount is reduced, residual austenite which has not been transformed is promoted to decompose and transform into sorbite and is subjected to isothermal tempering, finally, the wire rod is subjected to roller slow cooling, the hot-rolled wire rod is prepared, the microstructure of the hot-rolled wire rod comprises complex-phase microstructures of tempered bainite and tempered sorbite, abnormal microstructures can be effectively inhibited and complex-phase microstructure regulation can be realized, the tensile strength is 1592-1641MPa, the reduction of area is 27%-32%, the material cost, production efficiency and energy consumption are considered, and therefore, the off-line heat treatment is not needed, and the super-high-strength strand can be produced.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled wire rod, specifically relating to a high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire and its manufacturing method. Background Technology

[0002] With the upgrading of the special steel industry, stranded wire is gradually developing towards ultra-high strength of 2200MPa and higher strength grades. This is conducive to achieving lightweighting of engineering structures, improving the durability and service safety of engineering structures. High-strength stranded wire uses hot-rolled wire rod as the base material and is processed through processes such as drawing and twisting. However, due to the limitations of existing steel mill rolling controlled cooling production lines, the hot-rolled wire rods used for high-strength stranded wire are mostly pearlitic, with high alloy costs and difficulty in controlling abnormal microstructures. The production of ultra-high-strength stranded wire requires an additional heat treatment of the hot-rolled wire rod after hot rolling to meet the usage requirements, which also leads to a sharp increase in user costs and energy consumption. Therefore, it is necessary to develop a 2430MPa grade hot-rolled wire rod for stranded wire and its manufacturing method, which can control the microstructure and toughness online without additional heat treatment, to meet the development needs of the steel industry and market demand.

[0003] Existing high-strength stranded wire rods generally adopt a high-carbon, high-silicon microalloying composition system and are produced using a Steyrmo air-cooling line after rolling. However, the following technical bottlenecks still exist in manufacturing hot-rolled wire rods for 2430MPa grade stranded wire:

[0004] I. Carbon, as a relatively inexpensive and efficient strengthening element, has been further increased in strength to over 0.9% of wire rod. However, this increases the carbon content to over 0.9%. On the one hand, the maximum cooling capacity of the Stellmore air-cooled wire line limits the precipitation of secondary cementite at grain boundaries, forming a network of carbides that affects the uniformity of the microstructure and its ductility and toughness, leading to an increased wire breakage rate during subsequent drawing. On the other hand, to minimize the impact of the network of carbides and increase the drawing temperature and air-cooling intensity, the instability of air-cooling temperature control further increases the difference in cooling rates between overlapping and non-overlapping points, the windward and leeward sides, and the surface and core of the wire rod. Due to the hardenability and segregation of alloying elements, uncontrollable brittle structures such as martensite are easily formed in the locally overcooled areas of the wire rod. These structures remain in the microstructure after phase transformation, increasing the brittleness of the microstructure, increasing the risk of coil breakage, and exacerbating the risk of wire breakage during subsequent wire drawing. Although, as disclosed in patent CN114369760B, there is an ultra-high strength steel strand resistant to stress corrosion. The online water bath cooling and slow cooling process used in the manufacturing of wire rod and steel strand can improve the cooling speed of the wire rod. However, a large number of air bubbles will adhere to the surface of the wire rod during cooling, which will interfere with heat transfer and affect the uniformity of the microstructure and the control of abnormal microstructure. On the other hand, due to the limited length and cooling capacity of the Stellmore air-cooling line under the high silicon content system, the phase transformation incubation period is relatively long. Under continuous cooling, coarse pearlite phase is easy to form in the core of the wire rod, resulting in a loss of wire rod strength. Although microalloying components are added to the wire rod, the wire rod is prone to precipitation of alloy carbides of uneven size under the long high temperature range and unstable temperature control of the air-cooling line, which will affect the binding effect. Adding too much expensive microalloying components will also lead to excessive material cost. At the same time, after the phase transformation incubation, the wire rod is in a low temperature state, and abnormal microstructure and residual stress are left in the microstructure, resulting in large fluctuations in the mechanical properties of the finished product and insufficient plasticity, making it difficult to overcome the production bottleneck of offline heat treatment.

[0005] II. To improve the strength and plasticity of wire rods, existing technologies have proposed the use of online salt bath treatment processes. For example, patent CN120400685A discloses wire rods, wire rods, and production methods for 2400~2460MPa grade stress corrosion resistant prestressed steel strands. These methods use a C-Si-Mn-Cr-V-Nb composition system and combine online salt bath isothermal treatment with online aging to produce sorbitic wire rods. However, due to limitations in wire rod composition, target microstructure, and the temperature of the one-stage salt bath treatment, the treatment time should not be too long. It also requires low-speed online aging treatment, which affects production efficiency and energy consumption. At the same time, there is still a risk of grain boundary cementite and martensite precipitation, which affects the efficient and stable production of hot-rolled wire rods. 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 2430MPa grade stranded wire and its manufacturing method, which can effectively suppress abnormal structure, realize multiphase structure control and online toughening, improve the strength and plasticity of hot-rolled wire rod and the uniformity of structure, and take into account material cost, production efficiency and energy consumption, so as to produce ultra-high strength stranded wire without offline heat treatment.

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

[0008] A method for manufacturing high-strength multiphase hot-rolled wire rod for 2430MPa 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: 0.92%~0.97%, Si: 0.78%~0.98%, Mn: 0.74%~0.92%, Cr: 0.50%~0.70%, Mo: 0.35%~0.55%, Nb: 0.022%~0.042%, V: 0.028%~0.048%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. After being spun into wire rods at a wire drawing temperature of ≥897℃, the wire rods undergo online molten salt weak quenching and isothermal toughening treatment. The wire rods first undergo a preliminary molten salt treatment and are cooled at a cooling rate of ≥32℃ / s, transitioning from the austenitic state to the bainitic phase region, promoting the transformation of some austenite into bainite. Then, a subsequent molten salt treatment is performed, increasing the molten salt temperature and reducing the molten salt circulation rate, promoting the decomposition of untransformed residual austenite into sorbite and isothermal tempering. Finally, the wire rods are slowly cooled on a roller table to produce hot-rolled wire rods with a microstructure consisting of a multiphase structure composed of tempered bainite and tempered sorbite.

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

[0011] (1) Carbon: As an effective strengthening element, C is relatively cheaper and can improve the strength of steel through solid solution strengthening and carbide precipitation strengthening, reduce the critical point of austenite transformation, and expand the bainite transformation range so as to match the bainite transformation temperature of online molten salt front weak quenching, reduce the nucleation rate and growth rate of bainite transformation, prolong the transformation incubation period, and at the same time increase the cementite volume fraction, inhibit the formation of coarse lamellar pearlite, and achieve the strengthening of sorbite phase. However, if the C content is too high, it will aggravate the carbon segregation of steel billet, easily generate brittle phases such as continuous network cementite and high carbon martensite, affect the section shrinkage rate of wire rod, and the work hardening rate is too fast during cold drawing. Therefore, in order to take into account the high strength requirements of 2430MPa grade strand, reduce performance fluctuations, and adapt to online molten salt weak quenching isothermal toughening treatment, the mass percentage of C is controlled at 0.92%~0.97%.

[0012] (2) Silicon: Si can provide solid solution strengthening, reduce the diffusion rate of carbon atoms, delay the nucleation and growth of cementite, broaden the temperature window of bainitic phase transformation, inhibit the premature precipitation of carbides during bainitic phase transformation, increase the carbon content and stability of residual austenite, inhibit the formation of coarse lamellar pearlite during the later stage of molten salt treatment, play a role in refining precipitated phases and improving the toughness of materials, and at the same time improve the tempering stability of steel. However, excessive silicon will promote decarburization, prolong the phase transformation incubation time, and lead to a decrease in the plasticity, toughness and production efficiency of hot-rolled wire rod, and increase the deformation resistance of steel. Therefore, in order to control the multiphase structure, ensure cold working plasticity, and adapt to rapid production, the mass percentage of Si is controlled at 0.78%~0.98%.

[0013] (3) Manganese: Mn can improve the hardenability of steel, reduce the critical temperature of austenitization and the critical point of austenite transformation, improve the stability of austenite, and expand the undercooling range of austenite, so as to reduce the difference in cooling phase transformation between the surface and core of the wire rod, refine the sorbite lamellar spacing and bainite size, and achieve stable control of the bainite transformation ratio in conjunction with Mo. However, when the Mn content is too high, it will increase the element segregation during the solidification process of the billet, induce the risk of martensite precipitation, and at the same time, the stability of residual austenite will increase the efficiency of the subsequent molten salt treatment, resulting in an increase in the deformation resistance of the steel. Therefore, in order to facilitate the control of the multiphase structure of hot-rolled wire rod, improve the uniformity of the entire cross section of the wire rod, and take into account the production efficiency, the mass percentage of Mn is controlled at 0.74%~0.92%.

[0014] (4) Chromium: Cr can improve the hardenability of wire rod, reduce the critical cooling rate of pearlite and bainite phase transformation, so as to promote the synchronous completion of phase transformation in the core and surface of wire rod, refine the bainite size, suppress the growth rate of pearlite, refine the lamellar spacing of sorbite, and at the same time, by reducing the diffusion rate of C, it can form carbide pinning dislocations with C, which can improve the thermal stability and anti-coarsening ability of cementite. However, when the Cr content is too high, segregation will occur during solidification, which will increase the difficulty of controlling martensite and the difficulty of tempering, affecting production efficiency and energy consumption. Therefore, in order to balance the control of multiphase structure of hot-rolled wire rod and rapid production, the anti-tempering softening ability of wire rod should be appropriately controlled, and the mass percentage of Cr should be controlled at 0.50%~0.70%.

[0015] (5) Molybdenum: Mo can strongly inhibit the pearlite transformation, causing the pearlite transformation curve to shift to the right. It can reduce the diffusion ability of carbide-forming elements, thereby delaying the formation of carbide precipitation. At the same time, it can effectively pin dislocations and grain boundaries, hindering dislocation movement and grain coarsening. Together with Nb and V, it can prevent the precipitates from ripening and growing at high temperatures, avoid rapid strength decay, and improve the tempering stability of steel. However, the cost of Mo is relatively high, and excessive addition is not conducive to controlling material costs. Therefore, based on the role of Mo and material cost considerations, the mass percentage of Mo is controlled at 0.35%~0.55%.

[0016] (6) Niobium: As a microalloying element, Nb can strongly pin the austenite grain boundaries during high-temperature hot rolling, inhibit the coarsening of austenite grains, provide more nucleation nuclei for subsequent cooling phase transformation, refine the phase transformation products, and NbC will precipitate in the form of dispersed nanoparticles during isothermal tempering, hindering dislocation slip, rapidly strengthening the matrix, and improving the tempering stability of steel. However, Nb has a high cost, and excessive addition is not conducive to controlling material costs. Therefore, based on the role of Nb and material cost considerations, Nb is added appropriately, and the mass percentage of Nb is controlled at 0.022%~0.042%.

[0017] (7) Vanadium: As a microalloying element, V has a carbide dissolution temperature lower than Nb. It can strongly pin the austenite grain boundaries during high-temperature hot rolling, thereby refining the initial austenite structure and optimizing the phase transformation products. During isothermal tempering, V dissolved in the matrix will precipitate a dispersed nanoscale phase, which can effectively improve the matrix strength and tempering stability of steel through precipitation strengthening mechanism and optimize cold working performance. However, V has a high cost, and excessive addition is not conducive to controlling material cost. 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 0.028%~0.048%.

[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 high-carbon chemical composition design of Mo-Nb-V, with trace additions of Nb and V elements to control material costs. C provides solid solution strengthening and strong carbide precipitation. The synergistic effect of Si, Cr, and Mn elements widens the bainitic transformation temperature range, improves hardenability, and reduces the critical cooling rate of the transformation phase. Combined with Mo, V, and Nb elements, it provides nano-precipitation strengthening and improves tempering stability. This provides favorable conditions for weak quenching to form a small amount of bainitic structure, reduce the difference in microstructure and properties across the entire cross-section of the wire rod, strengthen the matrix, and promote rapid elimination of internal stress during the transformation phase. Furthermore, a higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, allowing elements to fully dissolve in the austenitic matrix and preventing premature precipitation of network carbides along the austenitic grain boundaries due to excessively low temperatures. This provides favorable conditions for increasing the cooling rate of the wire rod and promoting the transformation of austenite to fine bainite. After wire drawing, the wire rod is not air-cooled but undergoes online molten salt weak quenching isothermal toughening treatment.

[0020] Firstly, compared to the limited minimum cooling capacity of Stellmore air-cooled lines or the large cooling fluctuations inherent in water bath cooling, which make it difficult to control abnormal microstructures or complex phase transformations, molten salt offers several advantages. Firstly, molten salt has a higher heat transfer coefficient than air and water, promoting rapid cooling of the wire rod. It quickly bypasses the temperature range where proeutectoid cementite precipitates from the high-temperature austenitic state, suppressing the precipitation of network carbides. It also cools the austenite to the bainitic transformation range, allowing for weak quenching that promotes the transformation of a small portion of austenite into bainite with higher dislocation density and distortion, avoiding the precipitation of coarse alloy carbides due to slow cooling rates and resulting in a loss of strength and plasticity. Secondly, as the wire rod passes through the molten salt, the molten salt covers the surface of the wire rod, achieving rapid and uniform heat exchange. Unlike air-cooled lines, there is no temperature difference between the air-receiving and air-receiving surfaces, and unlike water-cooled lines, there is no temperature difference issue. Bubbles adhering to the surface of the wire rod affect heat transfer, which can prevent localized overcooling and the formation of hard and brittle martensite, thus avoiding increased brittleness and fluctuations in mechanical properties. Under the influence of the wire rod's hardenability, the transformation of some bainite is relatively uniform and controllable. Compared with one-stage salt bath treatment, the molten salt temperature and the molten salt circulation volume of the first stage molten salt treatment are lower. On the one hand, the sufficiently fast cooling rate can effectively suppress the risk of network carbides caused by high carbon or carbon segregation, avoiding them from cutting the matrix and affecting the ductility and toughness properties and the carbon source for phase transformation. On the other hand, under the weak quenching effect of the low-temperature first stage molten salt, the wire rod can suppress coarse lamellar pearlite and obtain a small amount of bainite structure, introduce moderately high-density dislocations to improve the matrix strength, and at the same time, the weak quenching reduces the core-to-surface temperature difference, reducing the difficulty of controlling the molten salt temperature rise during the second stage molten salt treatment, thus taking into account both the uniformity of the structure and the control of production energy consumption.

[0021] II. Compared to the limitations of minimum cooling capacity under the Stellmore air-cooling line or slow water cooling, and the difficulty in controlling and toughening the multiphase microstructure under continuous cooling, on the one hand, the optimal isothermal range of sorbitic phase transformation can be locked by increasing the molten salt temperature during the later stage of molten salt treatment of the wire rod. This can increase the carbon diffusion rate and promote the rapid decomposition of most of the untransformed residual austenite after weak quenching into fine lamellar sorbite. As the wire rod gradually transforms to the same temperature as the molten salt during the later stage of molten salt treatment, it can work with Si to suppress cementite coarsening and form soft pearlite or ferrite, achieving a full phase transformation of the microstructure and promoting the dispersion precipitation of Nb, V, and Mo alloy carbides to strengthen the matrix. On the other hand, after the phase transformation inoculation of the wire rod, isothermal tempering treatment can be carried out at the molten salt temperature, rather than the low-temperature state after continuous cooling. The high temperature can provide continuous thermal power to promote atomic diffusion and release a large amount of residual internal stress generated during the rapid cooling and bainitic and sorbitic phase transformation processes due to the volume change and shear deformation of the phase transformation, thus achieving Online toughening enhances the balance between strength and plasticity. Compared to single-stage salt bath treatment, which carries the risk of abnormal martensite structure and requires low-rate online aging, the higher strength of the bainitic and sorbitic matrix, coupled with the synergistic effect of Mo, Nb, and V to prevent the precipitates from maturing and growing at high temperatures, and the combined effect of Mo and Cr to improve the matrix's resistance to tempering softening, allows for the use of a later-stage molten salt treatment with a higher molten salt temperature. On one hand, this increases the sorbite nucleation rate, and with prolonged treatment time, promotes the completion of residual phase transformation and isothermal tempering on both the surface and core, preventing the continued formation of brittle martensite from retained austenite during subsequent cooling. On the other hand, the strength loss is compensated by the pinning dislocations of alloy carbides, and isothermal tempering allows for rapid online toughening of the structure, reducing the restriction on slow cooling on the roller table. After exiting the molten salt treatment, the softening effect of the later stage of the later-stage molten salt treatment can be continued, further toughening the wire rod structure without the need for excessively low cooling rates, thus promoting efficient and stable production of hot-rolled wire rod.

[0022] Before rolling, selecting a higher heating furnace soaking temperature and an appropriate furnace time can promote the solid solution of alloying elements, uniform diffusion of components, reduce the influence of segregation, reduce the temperature difference between the core and surface of the billet, avoid rolling defects, and prevent excessively high temperature or excessively long furnace time from causing austenite grain coarsening or uncontrolled surface decarburization. In the preferred technical solution, before rolling, the heating furnace soaking temperature is controlled at 1195~1245℃ and the furnace time is controlled at 165~265min.

[0023] The initial rolling temperature can be connected to the heating furnace soaking temperature. Selecting a higher initial rolling temperature reduces rolling deformation resistance and wear on the rolling line, thereby improving rolling efficiency. Combined with a larger initial rolling reduction, it increases the core density of the billet and inhibits grain coarsening. Combined with an appropriate final rolling temperature and final rolling reduction, it promotes the precipitation of NbC and VC at grain boundaries and dynamic recrystallization during final rolling, refining the grains. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1038~1078℃, the initial rolling reduction is 36%~41%, the final rolling temperature is 908~948℃, and the final rolling reduction is 21%~26%.

[0024] During the wire spinning process, the wire spinning temperature can be further controlled to avoid the risk of grain coarsening or decarburization caused by excessively high wire spinning temperature, and to reduce the difficulty of controlling the temperature rise of molten salt. In the preferred technical solution, the wire spinning temperature is controlled at 897~937℃.

[0025] The lower the molten salt temperature in the initial molten salt treatment, the faster the wire rod can pass through the proeutectoid cementite precipitation zone and the pearlite transformation nose temperature, inhibiting the formation of network cementite and coarse lamellar pearlite, resulting in a finer bainite structure and higher dislocation density. With longer treatment time, the proportion of bainite transformation and matrix strength can be increased, reducing the difficulty of controlling the molten salt temperature rise in the subsequent molten salt treatment. However, if the molten salt temperature is too low, the driving force of bainite transformation increases, the transformation rate accelerates, and the wire rod surface cools too quickly, amplifying the differences in microstructure and properties between the core and surface, and even forming abnormal martensitic structures. With excessively long treatment time, excessive bainite transformation leads to excessively high matrix strength and hardness, increasing the difficulty of improving plasticity and affecting production energy consumption and efficiency. Conversely, the higher the molten salt temperature, the smaller the difference in the driving force of phase transformation between the wire rod surface and core, resulting in a gentler driving force for bainite transformation and a more controllable transformation rate. Shorter processing time avoids excessive bainite transformation, resulting in bainite microstructure with lower internal stress during phase transformation. This facilitates stress release and toughening throughout the microstructure, reduces production energy consumption, and promotes rapid production. However, excessively high molten salt temperatures and insufficient wire rod cooling rates are detrimental to suppressing network carbides and coarse cementite. Insufficient driving force for bainite phase transformation makes it difficult to achieve the target proportion of bainite transformation with excessively short processing time. Excessive lag in core temperature drop and phase transformation will affect microstructure uniformity and matrix strength. Therefore, the molten salt temperature and processing time of the initial molten salt treatment can be further controlled to allow the wire rod to quickly transition from a high-temperature austenitic state to the bainite phase region. Weak quenching forms a small portion of bainite microstructure, preparing the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the initial molten salt treatment is 408~438℃, and the processing time is 8~26s.

[0026] The pre-treatment molten salt process can use a higher molten salt circulation rate to control the molten salt temperature rise and improve the consistency of wire rod performance. In the preferred technical solution, the molten salt circulation rate of the pre-treatment molten salt process is 580~880t / h, and the molten salt temperature rise is ≤8℃.

[0027] The higher the molten salt temperature in the subsequent molten salt treatment, the faster the core of the wire rod can reach the target isothermal temperature, increasing the carbon diffusion rate and promoting the transformation of carbon austenite to sorbite and the release of internal stress. With prolonged treatment time, the residual austenite on the wire rod surface and in the core undergoes a complete phase transformation, resulting in a more uniform and complete dispersion of alloy carbides. Simultaneously, it releases phase transformation internal stress and promotes toughening of the quenched structure. However, excessively high molten salt temperatures are detrimental to suppressing sorbite lamellar coarsening and alloy carbide formation, leading to a loss of matrix strength. With prolonged treatment time, excessive tempering of the microstructure causes coarsening and maturation of Nb, V, and Mo alloy carbides, reducing the precipitation strengthening effect and resulting in a loss of strength and plasticity, as well as increased production energy consumption. Conversely, lower molten salt temperatures increase the sorbite nucleation rate, promote microstructure refinement, and allow nano-precipitates to supplement matrix strength through precipitation strengthening. Shorter treatment times suppress the growth rate of precipitates. Obtaining finer nanoscale precipitates avoids strength loss due to excessive tempering and reduces production energy consumption. However, if the molten salt temperature is too low, the incubation time of sorbite phase transformation is prolonged, and the diffusion rate of alloying elements such as Nb and Mo and the isothermal tempering thermodynamics decrease. If the processing time is too short, the matrix dislocation density and internal stress are high, the alloy carbide precipitation is insufficient, and even the formation of abnormal martensite structure during subsequent cooling due to residual austenite leads to a decrease in strength and plasticity. Therefore, the subsequent molten salt treatment can further control the molten salt temperature and processing time, promote the decomposition of untransformed residual austenite into sorbite, and perform isothermal tempering on the quenched structure of bainite and sorbite to promote the full dispersion and precipitation of alloy carbides, so as to regulate the strength and plasticity matching of wire rod and take into account production efficiency. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 573~600℃, and the processing time is 183~293s.

[0028] The downstream molten salt treatment can use an appropriate molten salt circulation rate to control the molten salt temperature rise and production energy consumption. In the preferred technical solution, the molten salt circulation rate of the downstream molten salt treatment is 410~510t / h, and the molten salt temperature rise is ≤3℃.

[0029] The roller conveyor slow cooling can further control the slow cooling speed. Slow cooling promotes further toughening of the wire rod structure and improves the tempering and softening effect of the wire rod, while also taking into account production efficiency. In the preferred technical solution, the roller conveyor slow cooling controls the wire rod to cool to below 280°C at a slow cooling speed of 0.5~0.8°C / s before winding.

[0030] In the preferred technical solution, the slow cooling of the roller conveyor adopts the control of the opening of the heat insulation cover, and blows the hot air at ≥250°C during the online molten salt weak quenching isothermal toughening treatment to the conveyor roller conveyor. The conveyor roller conveyor transports the wire rod through the heat insulation cover. The heat energy of the online molten salt weak quenching isothermal toughening treatment can be recovered and utilized through the hot air, thereby reducing production energy consumption and promoting the rapid production of wire rod.

[0031] A high-strength multiphase hot-rolled wire rod for 2430MPa 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 2430MPa grade stranded wire described in any one of the above-mentioned methods.

[0032] The aforementioned hot-rolled wire rod adopts a C-Si-Mn-Cr-Mo-Nb-V composition design, with trace amounts of Nb and V added to control material costs. The microstructure comprises a multiphase structure mainly composed of tempered sorbite with a small amount of tempered bainite. Compared to hot-rolled wire rods with high sorbitivity obtained from air-cooled lines, it effectively suppresses the precipitation of network carbides and brittle martensite, and inhibits the formation of coarse-laminated soft pearlite. This makes the bainitic phase change, conventionally considered abnormal, controllable. The fine-grain strengthening and dislocation strengthening of bainite can provide higher strength than sorbite. After isothermal tempering to appropriately reduce dislocation density, it retains high strength characteristics and effectively improves brittleness, transforming into tempered bainite that is both strong and tough. Bainite, considered an anomalous structure, is utilized in conjunction with finely spaced sorbite and dispersed nanoscale alloy carbides. This results in a more uniform structure from the surface to the core of the wire rod cross-section. This effectively improves the matrix strength while maintaining uniformity and ductility. After isothermal tempering, the sorbite transforms into an intermediate tempered sorbite that transitions to a spheroidized structure. This effectively coordinates deformation, disperses stress concentration, and inhibits the initiation and propagation of microcracks during drawing. This significantly enhances the plasticity, toughness, and resistance to deformation and cracking of the matrix. Consequently, while considering material cost, production efficiency, and energy consumption, the strength and plasticity of hot-rolled wire rod can be improved, enabling the production of ultra-high strength stranded wire without offline heat treatment.

[0033] The higher the proportion of tempered bainite and the finer the lamellar spacing of tempered sorbite, the higher the matrix strength; the higher the proportion of tempered sorbite, the higher the matrix ductility and toughness; in the preferred technical solution, the volume percentage of tempered bainite is 6%~14%, the volume percentage of tempered sorbite is 86%~94%, and the lamellar spacing of tempered sorbite is 65~110nm.

[0034] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference of ≤53MPa between coils; this avoids the adverse effects of network carbides on the uniformity of the structure and the ductility and toughness, improves the reliability of the stranded wire, and by suppressing abnormal structure and improving the uniformity of the structure, the hot-rolled wire rod has a smaller fluctuation in mechanical properties, which can further improve the stability of subsequent wire drawing and the consistency of the performance of the finished stranded wire.

[0035] In the preferred technical solution, the diameter of the hot-rolled wire rod is 7~15mm, the tensile strength is 1592~1641MPa, and the reduction of area is 27%~32%. The diameter of the hot-rolled wire rod can match the production requirements of different strand specifications. The higher initial tensile strength can reduce the number of drawing passes. Combined with the drawing hardening ability of the hot-rolled wire rod, the target diameter and strength can be quickly achieved, reducing plastic loss during the process. At the same time, the mechanical properties have low difference between the same coil and high reduction of area, which can adapt to large reduction of area drawing without offline heat treatment, reduce the wire breakage rate, ensure the torsion and bending performance of the steel wire after drawing, meet the requirements of strand twisting, and thus promote the efficient and stable production of ultra-high strength strand.

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

[0037] (1) In view of the difficulty in controlling abnormal structure, regulating multiphase structure and online toughening in the production of hot-rolled wire rod for stranding, the present invention adopts Mo-Nb-V chemical composition design combined with online molten salt weak quenching isothermal toughening technology. It can control the wire rod to first pass through the front molten salt treatment, from the austenitic state to the bainite phase region, suppressing the abnormal structure of network carbides and martensite, while promoting the transformation of some austenite to bainite. Then, after the rear molten salt treatment, the molten salt temperature is increased and the molten salt circulation is reduced, which promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering. Finally, after the roller table slow cooling, the regulation and online toughening of the multiphase structure composed of tempered bainite and tempered sorbite are realized, which improves the strength and plasticity of hot-rolled wire rod and the uniformity of structure, reduces the limitation of roller table slow cooling, takes into account material cost, production efficiency and energy consumption, realizes efficient and stable production of hot-rolled wire rod, and has good industrial adaptability.

[0038] (2) In view of the current situation where the cost of existing hot-rolled wire rod materials for stranded wire is high, the strength and plasticity are insufficient and the uniformity of the structure is not uniform, and additional offline heat treatment is required after the wire is produced, which leads to the increase of stranded wire cost and production energy consumption, this invention adopts the Mo-Nb-V chemical composition design, which can control the material cost. The microstructure includes a multiphase structure of tempered sorbite and tempered bainite, which can effectively suppress the precipitation of network carbides, brittle martensite structure and soft pearlite with large lamellar structure, so that the bainite phase change, which is usually considered an abnormal structure, becomes controllable. Through the combination of strong and tough tempered bainite with fine lamellar spacing sorbite structure and dispersed nanoscale alloy carbides, the matrix strength is effectively improved while taking into account the uniformity of structure and plasticity and toughness. The tensile strength can reach 1592~1641MPa and the reduction of area is 27%~32%. It can be used to manufacture 2430MPa grade stranded wire and other application fields, so as to produce ultra-high strength stranded wire without offline heat treatment, 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 2430MPa grade 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: 0.85%, Mn: 0.86%, Cr: 0.70%, Mo: 0.35%, Nb: 0.042%, V: 0.028%, 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 weak quenching isothermal toughening → roller table slow cooling → 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 compositional homogenization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 7mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC and VC precipitation, resulting in grain refinement through grain studs. Specifically, the furnace soaking temperature is controlled at 1195℃, and the furnace dwell time is 200 min. n, the initial rolling temperature is 1038℃, the initial rolling reduction is 41%, the final rolling temperature is 908℃, 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 spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoid the precipitation of network carbides, and provide favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 897℃.

[0047] The online molten salt weak quenching isothermal toughening 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 the initial molten salt treatment, causing the wire rod to cool at a rate of 32℃ / s. This rapidly transitions the wire rod from the high-temperature austenitic state, bypassing the network carbide precipitation temperature range, into the bainite phase region, inhibiting the coarsening of network carbides and cementite lamellars. Weak quenching forms a small amount of bainite structure. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for the final molten salt treatment, increasing the molten salt temperature and reducing the molten salt temperature. The circulation rate promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full dispersion and precipitation of alloy carbides, so as to regulate the strength and plasticity matching of wire rod while taking into account production efficiency. Specifically: the molten salt temperature of the first stage molten salt treatment is 438℃, the treatment time is 8s, the molten salt circulation rate is 580t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage molten salt treatment is 573℃, the treatment time is 293s, the molten salt circulation rate is 410t / 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 (≥250℃) from above the two salt bath tanks undergoing online molten salt weak quenching and isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 279℃ at a slow cooling rate of 0.8℃ / 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 260 min, an initial rolling temperature of 1000°C, a final rolling temperature of 885°C, and a wire drawing temperature of 845°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 85% capacity, fans 1 to 4 are turned on to cool the wire rod to 688℃ at a cooling rate of 8.1℃ / s. Then, fans 5 to 14 are turned on to 25% capacity to cool the wire rod to 263℃ at a cooling rate of 2.9℃ / 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 1165°C, the furnace time is 250 min, the initial rolling temperature is 1010°C, the final rolling temperature is 830°C, and the wire drawing temperature is 825°C. During the initial molten salt treatment, the wire rod is cooled at a rate of 28°C / s, and the finished hot-rolled wire rod is obtained after the wire rod is removed from the production line.

[0053] Example 2:

[0054] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.97%, Si: 0.93%, Mn: 0.74%, Cr: 0.55%, Mo: 0.51%, Nb: 0.022%, V: 0.048%, P: 0.014%, S: 0.012%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt weak quenching isothermal toughening → roller table slow cooling → 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 composition homogenization. After exiting the furnace, the 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 promote the precipitation of NbC and VC, resulting in grain refinement through grain studs. Specifically, the furnace soaking temperature is controlled at 1232℃, and the furnace dwell time is 165 minutes. The initial rolling temperature is 1065℃, the initial rolling reduction is 40%, the final rolling temperature is 938℃, and the final rolling reduction is 24%. 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, avoid the precipitation of network carbides, and provide favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 928℃.

[0056] The online molten salt weak quenching isothermal toughening process employs a two-section salt bath with an internal molten salt compartment. After coiling, 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 34°C / s, rapidly bypassing the network carbide precipitation temperature range from the high-temperature austenitic state and entering the bainite phase region. This suppresses the coarsening of network carbides and cementite lamellars, resulting in a small amount of bainite structure through weak quenching. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The circulation process promotes the decomposition and transformation of untransformed residual austenite into sorbite, followed by isothermal tempering, and facilitates the full dispersion and precipitation of alloy carbides. This helps to control the strength and plasticity of the wire rod while maintaining production efficiency. Specifically, the molten salt temperature for the first stage of molten salt treatment is 427℃, the treatment time is 14s, the molten salt circulation rate is 610t / h, and the molten salt temperature rise is ≤8℃. The molten salt temperature for the second stage of molten salt treatment is 581℃, the treatment time is 240s, the molten salt circulation rate is 430t / h, and the molten salt temperature rise is ≤3℃.

[0057] The roller conveyor slow cooling process employs controlled opening of the insulation hood to blow hot air (≥250℃) from above the two salt bath tanks undergoing online molten salt weak quenching and isothermal toughening treatment into the insulation hood. The wire rod is then conveyed by the roller conveyor through the insulation hood for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 278℃ at a slow cooling rate of 0.7℃ / 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, the wire rod is cooled at a rate of 39°C / s, the molten salt temperature of the initial molten salt treatment is 400°C, the treatment time is 28s, and the finished hot-rolled wire rod is obtained after the process.

[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, the wire rod is cooled at a rate of 29°C / s, the molten salt temperature of the initial molten salt treatment is 470°C, the treatment time is 6s, and the finished hot-rolled wire rod is obtained after the process.

[0062] Example 3:

[0063] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.93%, Si: 0.78%, Mn: 0.92%, Cr: 0.66%, Mo: 0.46%, Nb: 0.034%, V: 0.037%, 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 weak quenching and isothermal toughening → roller table slow cooling → coiling, specifically:

[0064] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic, promoting compositional homogenization. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 12mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC and VC precipitation, resulting in grain refinement through grain studs. Specifically, the furnace soaking temperature is controlled at 1212℃, and the furnace dwell time is 265min. The initial rolling temperature is 1046℃, the initial rolling reduction is 37.5%, the final rolling temperature is 925℃, and the final rolling reduction is 22.5%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, avoid the precipitation of network carbides, and provide favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 910℃.

[0065] The online molten salt weak quenching isothermal toughening process employs a two-section salt bath with an internal molten salt compartment. After coiling, 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 36°C / s, rapidly bypassing the network carbide precipitation temperature range from the high-temperature austenitic state and entering the bainite phase region. This suppresses the coarsening of network carbides and cementite lamellars, resulting in a small amount of bainite structure through weak quenching. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The circulation process promotes the decomposition and transformation of untransformed residual austenite into sorbite, followed by isothermal tempering, and facilitates the full dispersion and precipitation of alloy carbides. This helps to control the strength and plasticity of the wire rod while maintaining production efficiency. Specifically, the molten salt temperature for the first stage of molten salt treatment is 414℃, the treatment time is 20s, the molten salt circulation rate is 740t / h, and the molten salt temperature rise is ≤8℃. The molten salt temperature for the second stage of molten salt treatment is 592℃, the treatment time is 223s, the molten salt circulation rate is 465t / h, and the molten salt temperature rise is ≤3℃.

[0066] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥250℃) from above the two salt bath tanks undergoing online molten salt weak quenching and isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the roller conveyor through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 275℃ at a slow cooling rate of 0.65℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure is shown in the figure below. Figure 3 As shown.

[0067] Comparative Example 5:

[0068] A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that: the molten salt temperature of the subsequent molten salt treatment is 610°C, the treatment time is 320s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0069] Comparative Example 6:

[0070] 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 550°C, the treatment time is 160s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0071] Example 4:

[0072] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.96%, Si: 0.98%, Mn: 0.84%, Cr: 0.5%, Mo: 0.55%, Nb: 0.028%, V: 0.029%, 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 weak quenching isothermal toughening → roller table slow cooling → coiling, specifically:

[0073] 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 composition homogenization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 15mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote NbC and VC precipitation, resulting in grain refinement through grain studs. Specifically, the furnace soaking temperature is controlled at 1245℃, and the furnace dwell time is 235 minutes. The initial rolling temperature is 1078℃, the initial rolling reduction is 36%, the final rolling temperature is 948℃, and the final rolling reduction is 21%. 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, avoid the precipitation of network carbides, and provide favorable conditions for promoting the transformation of austenite into fine bainite. Specifically, the wire drawing temperature is controlled at 937℃.

[0074] The online molten salt weak quenching isothermal toughening process employs a two-section salt bath with an internal molten salt compartment. After coiling, 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 39°C / s, rapidly bypassing the network carbide precipitation temperature range from the high-temperature austenitic state and entering the bainite phase region. This suppresses the coarsening of network carbides and cementite lamellars, resulting in a small amount of bainite structure through weak quenching. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, increasing the molten salt temperature and reducing molten salt circulation. The circulation process promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the full dispersion and precipitation of alloy carbides to regulate the strength and plasticity matching of wire rod while taking into account production efficiency. Specifically: the molten salt temperature of the first stage molten salt treatment is 408℃, the treatment time is 26s, the molten salt circulation rate is 880t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage molten salt treatment is 600℃, the treatment time is 183s, the molten salt circulation rate is 510t / h, and the molten salt temperature rise is ≤3℃.

[0075] The roller conveyor slow cooling process uses an adjustable insulation cover to blow hot air (≥250℃) from the two salt bath tanks of the online molten salt weak quenching isothermal toughening treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, which promotes further toughening of the wire rod structure and improves the tempering softening effect. Specifically, the wire rod is controlled to cool to 273℃ at a slow cooling rate of 0.5℃ / s. The coiling process is used to coil the wire rod into coils through a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0076] Comparative Example 7:

[0077] A method for manufacturing hot-rolled wire rod differs from that in Example 4 in that the method follows a process flow of rolling → wire drawing → online molten salt weak quenching and isothermal toughening → air cooling. The air cooling is achieved by opening the insulation cover, conveying the wire rod via a conveyor roller, and controlling the wire rod to cool to 268°C at a slow cooling rate of 1.4°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0078] 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:

[0079] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0080]

[0081] As can be seen from the comparison results of Example 1 and Comparative Example 1, compared with the Stellmore air-cooled line, the strengthening effect is weakened due to the weak cooling rate during the cooling phase transformation, resulting in abnormal structures such as high-level network carbides and martensite. The present invention adopts Mo-Nb-V chemical composition design combined with online molten salt weak quenching isothermal toughening technology, which can control the wire rod to first undergo molten salt treatment, suppressing abnormal structures of network carbides and martensite while promoting the transformation of a small portion of austenite to bainite. After the subsequent molten salt treatment, the untransformed residual austenite is decomposed into sorbite and isothermally tempered, realizing the control and online toughening of the multiphase structure composed of tempered bainite and tempered sorbite. As can be seen from the results of Examples 1 to 4, the tensile strength can reach 1592~1641MPa and the reduction of area is 27%~32%, which can be used to manufacture 2430MPa grade stranded wire and other application fields, so as to produce ultra-high strength stranded wire without offline heat treatment.

[0082] As can be seen from the comparison between Example 1 and Comparative Example 2, selecting a higher wire drawing temperature can keep the wire rod in a high-temperature austenitizing state, allowing the elements to fully dissolve in the austenite matrix. This avoids the premature precipitation of network carbides along the austenite grain boundaries caused by excessively low temperatures, thus providing favorable conditions for increasing the wire rod cooling rate and promoting the transformation of austenite into fine bainite.

[0083] As can be seen from the comparison results of Example 2 and Comparative Example 3, the lower the molten salt temperature in the first stage of molten salt treatment, the faster the wire rod can pass through the proeutectoid cementite precipitation zone and the pearlite phase transformation nose temperature, inhibiting the formation of network cementite and coarse lamellar pearlite, resulting in a finer bainite structure and a higher dislocation density. As the treatment time increases, the proportion of bainite phase transformation and matrix strength can be increased, reducing the difficulty of controlling the molten salt temperature rise in the second stage of molten salt treatment. However, if the molten salt temperature is too low, the driving force of bainite phase transformation increases, the phase transformation rate accelerates, and the cooling rate of the wire rod surface is too fast, amplifying the differences in structure and properties between the core and the surface. As the treatment time is too long, excessive bainite phase transformation leads to excessively high matrix strength and hardness, increasing the difficulty of improving plasticity, and affecting production energy consumption and efficiency.

[0084] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the smoother the driving force of bainitic phase transformation and the more controllable the phase transformation rate. With a shorter treatment time, excessive bainitic transformation can be avoided, and the generated bainitic microstructure has lower internal stress during phase transformation, making it easier to achieve stress release and toughening of the entire microstructure, reducing production energy consumption and promoting rapid production. However, if the molten salt temperature is too high, the wire rod cooling rate is insufficient, which is not conducive to suppressing network carbides and coarse cementite. The driving force of bainitic phase transformation is insufficient, and with a short treatment time, it is difficult to achieve the target proportion of bainitic transformation. The core temperature drop and excessive phase transformation lag will affect the microstructure uniformity and matrix strength.

[0085] As can be seen from the comparison results between Example 3 and Comparative Example 5, the higher the molten salt temperature in the later stage of molten salt treatment, the more it can promote the transformation of carbon austenite to sorbite and the release of internal stress. With the extension of treatment time, the residual austenite on the surface and core of the wire rod can undergo sufficient phase transformation, allowing the alloy carbides to achieve a more uniform and sufficient dispersion distribution. At the same time, the phase transformation internal stress is released, promoting the toughening of the quenched structure. However, if the molten salt temperature is too high, it is not conducive to suppressing the coarsening of sorbite lamellars and alloy carbides, which will result in a loss of matrix strength. With the excessive treatment time, the structure is over-tempered, and the alloy carbides of Nb, V, and Mo undergo coarsening and ripening, resulting in a decrease in precipitation strengthening effect, leading to a loss of strength and plasticity and an increase in production energy consumption.

[0086] As can be seen from the comparison results of Example 3 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the more refined the microstructure can be. The nano-precipitates can strengthen and supplement the matrix strength through precipitation. With the shortening of the treatment time, smaller nano-precipitates can be obtained, avoiding the strength drop caused by excessive tempering and reducing production energy consumption. However, if the molten salt temperature is too low, the incubation time of sorbite phase transformation is prolonged, and the diffusion rate of alloying elements such as Nb and Mo and the thermal dynamics of isothermal tempering decrease. With the shortest treatment time, the dislocation density and internal stress of the matrix are high, the alloy carbide precipitation is insufficient, and even the abnormal martensitic structure is formed in the subsequent cooling due to the residual austenite, resulting in a decrease in strength and plasticity.

[0087] As can be seen from the comparison results between Example 4 and Comparative Example 7, the slow cooling of the roller table can promote further toughening of the wire rod structure and improve the tempering and softening effect of the wire rod.

[0088] 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 2430MPa grade stranded wire, characterized in that, Its manufacturing methods include: The wire rod is rolled into production line according to the chemical composition of hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.92%~0.97%, Si: 0.78%~0.98%, Mn: 0.74%~0.92%, Cr: 0.50%~0.70%, Mo: 0.35%~0.55%, Nb: 0.022%~0.042%, V: 0.028%~0.048%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a wire drawing temperature of ≥897℃, it undergoes online molten salt weak quenching isothermal toughening treatment. This process involves the wire rod undergoing a preliminary molten salt treatment and cooling at a rate of ≥32℃ / s, causing it to transition from the austenitic state to the bainitic phase region, promoting the partial transformation of austenite into the bainitic phase. The wire rod undergoes bainitic transformation, followed by a subsequent molten salt treatment. This process increases the molten salt temperature and reduces the molten salt circulation rate, promoting the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, it undergoes slow cooling via a roller conveyor to produce a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, forming a multiphase microstructure. The initial molten salt treatment has a molten salt temperature of 408-438℃, a treatment time of 8-26s, a molten salt circulation rate of 580-880t / h, and a molten salt temperature rise ≤8℃. The subsequent molten salt treatment has a molten salt temperature of 573-600℃, a treatment time of 183-293s, a molten salt circulation rate of 410-510t / h, and a molten salt temperature rise ≤3℃. The slow cooling via the roller conveyor controls the wire rod to cool to below 280℃ at a slow cooling rate of 0.5-0.8℃ / s before coiling.

2. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1195~1245℃ and the furnace time is 165~265min.

3. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1038~1078℃, the initial rolling reduction is 36%~41%, the final rolling temperature is 908~948℃, and the final rolling reduction is 21%~26%.

4. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to claim 1, characterized in that, During the spinning process, the spinning temperature is controlled at 897~937℃.

5. A high-strength multiphase hot-rolled wire rod for 2430MPa 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 2430MPa grade strand as described in any one of claims 1 to 4.

6. The high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to claim 5, characterized in that, The volume percentage of the tempered bainite is 6%~14%, the volume percentage of the tempered sorbite is 86%~94%, the lamellar spacing of the tempered sorbite is 65~110nm, the network carbide level of the hot-rolled wire rod is grade 0, and the mechanical property difference between the same ring is ≤53MPa.

7. The high-strength multiphase hot-rolled wire rod for 2430MPa grade stranded wire according to claim 5, characterized in that, The hot-rolled wire rod has a diameter of 7~15mm, a tensile strength of 1592~1641MPa, and a reduction of area of ​​27%~32%.

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