High-strength complex-phase hot-rolled wire rod for 2100mpa class strand and manufacturing method thereof

Abstract

The application relates to a high-strength complex-phase hot-rolled wire rod for a 2100MPa-grade strand and a manufacturing method thereof. After a high-carbon component system of C-Si-Mn-Cr-Mo is adopted to roll and spin the wire rod, the wire rod is subjected to on-line molten salt quenching isothermal treatment, the wire rod is subjected to front-stage molten salt treatment and is cooled at a cooling speed of greater than or equal to 33 DEG C / s, the wire rod is cooled from an austenite state into a bainite phase region, the transformation from austenite to bainite is promoted, the wire rod is subjected to rear-stage molten salt treatment, a bainite-based structure is formed, the structure is isothermal tempered, part of untransformed residual austenite is reserved, the wire rod is finally subjected to roller slow cooling, the untransformed residual austenite is transformed into martensite and is tempered, the hot-rolled wire rod with a microstructure composed of complex-phase structures of tempered bainite and tempered martensite is prepared, the tensile strength of the hot-rolled wire rod can reach 1435-1485MPa, the reduction of area is 31%-36%, the hot-rolled wire rod can be free of subsequent processing of off-line heat treatment and the risk of wire breaking is reduced.

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 2100MPa grade stranded wire and its manufacturing method. Background Technology

[0002] Pearlitic hot-rolled wire rods are widely used in the steel strand industry due to their excellent cold-drawing properties. With the development of infrastructure construction and the large-scale construction of high-speed railways in recent years, higher requirements have been placed on the prestressed steel strand industry to further reduce traditional energy consumption, among which improving material strength is an important technological direction. Traditional hot-rolled steel strands are usually cooled in a Stellmore air-cooled line. The hot-rolled bainite and martensite produced in this process are extremely brittle, easily causing wire breakage during drawing. The strength grade of the steel strand is limited to 1960 MPa. Further improvement requires offline heat treatment for toughening, which is not conducive to reducing traditional energy consumption.

[0003] Although existing technologies have proposed 2100MPa grade ultra-high strength steel strand wire rods, such as the ultra-high strength vanadium-titanium composite microalloyed high-carbon steel wire rod and its preparation method disclosed in patent CN102352469B, which uses a C-Si-Mn-V-Ti-Cr-Cu-Ni composition design and combines Stellmore rapid cooling, heat preservation and slow cooling processes to produce sorbitic wire rods, the following technical bottlenecks still exist:

[0004] To suppress austenite grain growth during hot rolling and promote the refinement of pearlite lamellar spacing during air cooling to improve the original wire rod strength, the wire rod contains high alloy content and various microalloying systems. However, this also brings problems such as high material cost and difficulty in smelting. At the same time, the pearlite / sorbite lamellar structure has a relatively weak hindering effect on dislocation slip, so although the material has a certain plasticity, the initial strength of the wire rod is relatively low. In order to reduce material cost and smelting difficulty, the alloying components are omitted, which will eliminate the carbonitride pinning effect, reduce the deformation nucleation sites of the sorbite phase, and affect the chemical stability of austenite.

[0005] On the one hand, in order to make up for the adverse effects of omitting alloying elements on phase transformation as much as possible, low-temperature rolling is adopted. However, due to the limitation of the maximum cooling capacity of the Stellmore air-cooled line, before the supercooled austenite undergoes pearlite or sorbite phase transformation in the high-temperature region, carbon atoms at the grain boundaries precipitate first due to their strong diffusion ability and form cementite, which is continuously distributed along the grain boundaries. The network carbides not only cut the matrix and cause a decrease in plasticity and toughness, but also consume free carbon, resulting in insufficient carbon source and increased lamellar spacing for subsequent sorbite phase transformation. At the same time, the slow cooling of the core of the wire rod will also form coarse pearlite, leading to a further decrease in final strength and toughness.

[0006] On the other hand, in order to minimize the adverse effects of omitting alloying elements on phase transformation, reduce the level of network carbides, and improve the air-cooling strength after wire drawing, the temperature difference between the air-receiving and air-receiving surfaces of the wire rod, as well as between the overlapping and non-overlapping areas, will also increase. Local overcooling of the wire rod can easily form uncontrollable bainite and martensite structures. During the continuous processing of the air-cooling process, the dislocation density of the brittle phase is high and the internal stress is concentrated, which will lead to a further increase in the brittleness of the wire rod and a greater fluctuation in mechanical properties. When under stress, it will be unable to disperse stress through slippage, which will bring the risk of wire breakage and make it impossible to break through the production bottleneck of eliminating offline heat treatment. Summary of the Invention

[0007] 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 2100MPa grade stranded wire and its manufacturing method, which can simplify the composition system, realize the control of multiphase structure, and improve the overall strength and plasticity matching of the wire rod, so as to eliminate the need for offline heat treatment in subsequent processing and reduce the risk of wire breakage.

[0008] The technical solution adopted by this invention to solve its technical problem is:

[0009] A method for manufacturing high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire, the method comprising:

[0010] 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: 0.86%~0.91%, Si: 0.42%~0.62%, Mn: 0.45%~0.65%, Cr: 0.36%~0.56%, Mo: 0.2%~0.4%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a spinning temperature of ≥900℃, it undergoes online molten salt tempering. Quenching isothermal treatment involves first subjecting the wire rod to a molten salt treatment at a cooling rate of ≥33℃ / s, which transforms it from austenite to bainite, promoting the transformation of austenite to bainite. Then, a subsequent molten salt treatment is performed to increase the molten salt temperature, forming a bainite-dominant microstructure. The microstructure is then isothermally tempered, retaining some untransformed residual austenite. Finally, the wire rod undergoes slow cooling on a roller conveyor to promote the transformation of the untransformed residual austenite to martensite, followed by tempering. This process produces a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered martensite, forming a multiphase microstructure.

[0011] The chemical composition and mass percentage of the above-mentioned hot-rolled wire rods are designed based on the following:

[0012] (1) Carbon: C is an effective strengthening element in steel and is relatively cheaper. Carbon forms interstitial solid solution in austenite, which increases the stability and diffusion resistance of austenite. It can delay the transformation kinetics of austenite to pearlite, reduce the starting temperature of bainite phase transformation, accelerate the preferential formation of bainite quenching during the first stage of molten salt treatment through short-range diffusion, and make the ferrite needles in bainite finer. At the same time, it can improve the lattice distortion of martensite through solid solution strengthening, thereby improving the matrix strength. However, if the C content is too high, it will make it more difficult to control the center segregation, network carbides and carbide coarsening of high carbon steel billet. At the same time, it will cause the martensitic phase transformation stress to surge, which will increase the difficulty of isothermal tempering, resulting in a decrease in toughness and affecting the efficiency of the wire. Therefore, in order to take into account the high strength requirements of 2100MPa grade strand, adapt to the control of multiphase structure, and control the difficulty of isothermal tempering, the mass percentage of C is controlled at 0.86%~0.91%.

[0013] (2) Silicon: Si can slow down the transformation kinetics of pearlite by hindering the diffusion of carbon atoms into cementite, which helps to promote the transformation of bainite, avoid coarse cementite from cutting the matrix, and provide sufficient lattice distortion effect to improve strength. At the same time, it will reduce the diffusion rate of carbon in austenite, inhibit the enrichment of carbon into retained austenite, reduce its stability, and indirectly promote the transformation of retained austenite into martensite. However, excessive silicon will promote decarburization, and the degree of lattice distortion will be aggravated. Excessive resistance to dislocation movement will increase the difficulty of softening, resulting in a decrease in the plasticity and production efficiency of steel, and then fracture in subsequent wire drawing, twisting and other processing stages. Therefore, in order to adapt to the control of phase transformation structure and the difficulty of isothermal tempering in online molten salt strong quenching isothermal treatment, the mass percentage of Si is controlled at 0.42%~0.62%.

[0014] (3) Manganese: Mn is a strong austenite stabilizing element. During the initial molten salt treatment, it can prevent austenite from decomposing into pearlite too early during cooling, reduce the transformation temperature of austenite to bainite and the diffusion rate of carbon atoms, slow down the decomposition rate of austenite in the medium temperature range, and allow bainite to nucleate and grow for a sufficient time. At the same time, it can prevent excessive delay of martensitic phase transformation, induce lattice distortion through substitution solid solution, and improve the matrix strength. However, if the Mn content is too high, it will increase element segregation during the solidification process of the billet, require greater rolling force during rolling, and easily lead to increased wear of the rolls and excessive rolling load. At the same time, it will increase the shear resistance of austenite, making the shear deformation of martensitic phase transformation more likely to be carried out in a twinning manner, increasing the difficulty of tempering and softening, and affecting the toughness and efficiency of the wire rod. Therefore, in order to take into account the control of the multiphase structure of hot-rolled wire rod, reduce the difficulty of tempering control, and promote rapid production, the mass percentage of Mn is controlled at 0.45%~0.65%.

[0015] (4) Chromium: Cr is a strong carbide-forming element in steel. The fine carbides formed by Cr and C pin the austenite grain boundaries, which can prevent the grains from becoming coarse during heating, increase the stability of austenite, reduce the diffusion coefficient of carbon atoms in austenite, delay the diffusion-type pearlite phase transformation, and improve the hardenability of wire rod with Mn, which increases the driving force for bainite nucleation at low temperature, refines the quenched structure dominated by bainite, and improves the tempering stability. At the same time, the slower phase transformation rate provides conditions for more fine martensite nuclei to form and grow at the same time. However, when the Cr content is too high, segregation will occur during solidification, which will increase the difficulty of controlling the uniformity of the structure. At the same time, persistent lattice distortion will occur in the martensite lattice, which will hinder dislocation movement, increase the internal stress of phase transformation and the difficulty of tempering, and affect the rapid production line. Therefore, in order to take into account the control of the multiphase structure of hot-rolled wire rod, reduce the difficulty of tempering control, and promote rapid production, the mass percentage of Cr is controlled at 0.36%~0.56%.

[0016] (4) Molybdenum: The addition of Mo strongly inhibits the pearlite transformation, causing the pearlite transformation curve to shift to the right. At the same time, it improves the hardenability of steel, providing favorable conditions for bainite and martensite phase transformation. During isothermal tempering, it can reduce the diffusion ability of carbide-forming elements, thereby delaying the formation of carbide precipitation and inhibiting the precipitation of related second-phase particles. This can prevent the precipitated phase from ripening and growing at high temperatures. However, Mo is expensive, and excessive addition is not conducive to controlling material costs. At the same time, excessive addition can easily cause dendritic segregation during the solidification of steel billets, reducing hot plasticity and causing fluctuations in mechanical properties. Therefore, in order to balance the control of the multiphase structure of hot-rolled wire rod and improve tempering stability, the mass percentage of Mo is controlled at 0.01%~0.03%.

[0017] (5) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015%.

[0018] The aforementioned hot-rolled wire rod adopts a high-carbon composition system of C-Si-Mn-Cr-Mo, without alloying elements such as V, Ni, and Ti. This simplifies the composition system, controls material costs, and simultaneously regulates the hardenability of the wire rod, lowers the bainite phase temperature, and provides favorable conditions for hindering the transformation of austenite to pearlite, promoting preferential and rapid bainite nucleation, avoiding excessive delay in martensitic transformation, controlling tempering difficulty, and promoting rapid production of the wire rod. Furthermore, a higher wire-drawing temperature is selected to keep the wire rod in a high-temperature austenitic state, preventing the formation of network carbides or proeutectoid ferrite due to excessively low temperatures during the wire-drawing stage. This prepares the microstructure for increasing undercooling, promoting preferential bainite transformation, and decomposing retained austenite. After wire drawing, the wire rod undergoes online molten salt quenching isothermal treatment without air cooling.

[0019] Firstly, compared to the simplified composition system, which is limited by the maximum cooling capacity of the Stellmore air-cooled line and cannot control network carbides and coarse pearlite, the high heat transfer capacity of molten salt can be used to promote rapid cooling of the wire rod, quickly bypassing the precipitation temperature range of secondary cementite from the high-temperature austenitic state, thus inhibiting the formation of network carbides and proeutectoid ferrite, and preventing network carbides from cutting the matrix and affecting the carbon source of bainite phase transformation. Secondly, rapid cooling can also avoid the pearlite transformation nose, thus preventing the formation of coarse pearlite or sorbite. The martensite structure is formed by short-term high-intensity quenching in the bainitic phase region to promote short-range diffusion of carbon in the iron matrix. Some austenite transforms into bainite, while some untransformed austenite cannot fully nucleate and grow due to insufficient diffusion time. The enrichment of carbon also lowers the martensite transformation initiation temperature, preventing it from transforming into martensite during molten salt treatment. Instead, it is retained as residual austenite, thus compensating for the strength loss caused by omitting alloying elements, avoiding the reduction of the overall strength of the wire rod due to the formation of low-strength pearlite, and preparing the microstructure for subsequent molten salt treatment.

[0020] Second, compared to the limitations of the minimum cooling capacity and continuous cooling of the Stellmore air-cooling line, which prevents control over the bainitic and martensitic phase transformations and results in higher brittleness of the wire rod, the advantages of molten salt treatment are twofold. Firstly, when the wire rod passes through molten salt, the molten salt covers the surface for uniform heat exchange, eliminating the temperature difference issues between the air-receiving and air-receiving surfaces, and between overlapping and non-overlapping areas. While the increased molten salt temperature in the later stages of molten salt treatment does not lead to the pearlite phase region, it can appropriately increase the diffusion coefficient of carbon in the retained austenite, providing conditions for the decomposition and transformation of some retained austenite into bainite. This results in a more uniform and controllable quenched microstructure dominated by bainite, exhibiting high strength characteristics. Secondly, because the molten salt temperature in the later stages of molten salt treatment is relatively high, the wire rod temperature gradually transitions to match the molten salt temperature as the treatment time increases. This extends the time the wire rod remains in the high-temperature range, unlike continuous cooling followed by a low-temperature treatment, thus providing better heat transfer. Thermodynamics reduces lattice distortion and internal stress in the quenched structure. Combined with Mo, it can suppress the coarsening of carbides during tempering, avoiding excessive strength loss. At the same time, it retains some untransformed retained austenite, reducing the carbon concentration and stability inside the retained austenite, alleviating the hindrance of carbon enrichment to phase transformation. Finally, it undergoes slow cooling on a roller conveyor to meet the requirements for martensitic phase transformation, thereby promoting the transformation of a small amount of untransformed retained austenite into martensite. Since the temperature of the wire rod exiting the molten salt is relatively high rather than the low temperature state after air cooling, the slow cooling on the roller conveyor can prolong the tempering effect, promote further toughening of the wire rod structure, effectively alleviate the brittleness of the structure, and avoid brittle fracture during subsequent wire drawing or even coiling and transportation. This allows bainite and martensite, which are conventionally considered abnormal structures, to be utilized, enhancing the strengthening effect of carbon elements. It can achieve multiphase structure control and regulate the strength and plasticity matching of the wire rod.

[0021] Before rolling, selecting a higher heating furnace temperature and an appropriate furnace dwell time can promote uniform diffusion of alloying elements, suppress segregation, and reduce rolling deformation resistance. At the same time, it avoids excessive furnace dwell time, which can lead to coarsening or decarburization of austenite grains. In the preferred technical solution, before rolling, the heating furnace homogenization temperature is controlled at 1180~1230℃, and the furnace dwell time is 150~240min.

[0022] Because the wire drawing temperature is relatively high, the limitation on the rolling temperature can be reduced. Using a higher initial rolling temperature reduces the rolling deformation resistance and wear on the rolling line, improves the rolling speed and efficiency, and initially refines the grains. With appropriate final rolling temperature and final rolling reduction, dynamic recrystallization can be promoted during the final rolling process, refining the grains and retaining an appropriate dislocation density. In the preferred technical solution, during the rolling process, the initial rolling temperature is controlled at 1040~1090℃, the final rolling temperature is 890~930℃, and the final rolling reduction is 25%~30%.

[0023] During the wire spinning process, the spinning temperature can be further controlled to avoid excessively high temperatures that could lead to abnormal growth of austenite grains. In a preferred embodiment, the spinning temperature is controlled to be 900~940℃.

[0024] The molten salt temperature in the initial molten salt treatment is within the bainitic phase region. Lower molten salt temperatures facilitate rapid reduction of the wire rod temperature, inhibiting network carbides and pearlite structures. Strong quenching promotes preferential nucleation of bainite over pearlite, increasing the bainite nucleation rate. With prolonged treatment time, bainite can remain fine during slow growth, forming an initial structure dominated by fine bainite, thus improving matrix strength. However, excessively low molten salt temperatures and prolonged treatment times reduce retained austenite or cause excessively high carbon content in retained austenite, increasing softening difficulty and production energy consumption, and affecting subsequent bainitic and martensitic phase transformations. Conversely, higher molten salt temperatures and shorter treatment times help reduce dislocation density and softening during bainite transformation. To reduce the difficulty of bainite transformation, a certain amount of residual austenite is retained for subsequent molten salt treatment to strengthen the wire rod and reduce fluctuations in mechanical properties. However, if the molten salt temperature is too high or the treatment time is too short, the driving force for bainite transformation will weaken, affecting the suppression of pearlite nucleation and resulting in a loss of matrix strength. Therefore, the molten salt temperature and treatment time of the initial molten salt treatment can be controlled to control the wire rod to quickly enter the bainite phase region from the high-temperature austenitic state, suppress the network carbide and pearlite structure, promote the transformation of fine bainite, provide the initial strength foundation for the wire rod, and retain an appropriate amount of residual austenite to prepare the microstructure for subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the initial molten salt treatment is 470~500℃, and the treatment time is 20~40s.

[0025] Since the spinning temperature differs significantly from the molten salt temperature in the preceding treatment, a larger molten salt circulation rate can control the molten salt temperature rise, promote rapid cooling of the wire rod, inhibit pearlite formation, and improve the cooling uniformity of the wire rod cross section. In the preferred technical solution, the molten salt circulation rate in the preceding molten salt treatment is 420~620 t / h, and the molten salt temperature rise is ≤10℃.

[0026] The molten salt temperature in the subsequent molten salt treatment is appropriately higher than that in the preceding molten salt treatment. Higher molten salt temperatures are beneficial for increasing the carbon diffusion coefficient. With prolonged treatment time, this promotes bainite tempering, allowing supersaturated carbon within the bainite to precipitate more fully as fine carbides, thus improving the wire rod's ductility and toughness. However, excessively high molten salt temperatures, approaching the pearlite transformation temperature, will result in insufficient undercooling, affecting the bainite transformation. Furthermore, prolonged treatment time accelerates carbide diffusion and aggregation, weakening the grain boundary strengthening effect. Coarsened carbides become crack initiation sources, leading to a loss of strength and ductility, while also increasing production energy consumption. Conversely, lower molten salt temperatures are beneficial for increasing undercooling and promoting the bainite transformation. Shorter treatment times help suppress excessive diffusion, retain fine microstructure, and maintain higher strength and production efficiency. However, excessively low molten salt temperatures... If the processing time is too short, the tempering thermal dynamics will be insufficient, the lattice distortion will not be relieved, and internal stress will remain. At the same time, as the processing time decreases, the amount of untransformed retained austenite increases, which will induce excessive martensitic transformation during subsequent cooling, leading to increased tempering difficulty, increased brittleness, increased performance fluctuations, and easy fracture at stress concentration points during drawing. Therefore, the subsequent molten salt treatment can control the molten salt temperature and processing time, promote the quenching decomposition of some untransformed retained austenite after the initial molten salt treatment into bainite, form a bainite-dominated structure, and isothermal temper the structure, retaining some untransformed retained austenite to regulate the strength-plasticity matching of the wire rod, and prepare the structure for slow cooling on the roller table. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 500~530℃, and the processing time is 100~240s.

[0027] Since the temperature difference between the molten salt in the first stage and the second stage is small, the second stage of molten salt treatment can appropriately reduce the molten salt circulation volume, reduce production energy consumption, and at the same time control the molten salt temperature rise to improve the consistency of the bar structure. In the preferred technical solution, the molten salt circulation volume of the second stage of molten salt treatment is 320~420t / h, and the molten salt temperature rise is ≤3℃.

[0028] The roller conveyor slow cooling can promote the transformation of untransformed residual austenite to martensite and prolong the softening effect on the microstructure in the later stage of molten salt treatment. It can further control the slow cooling rate, alleviate the lattice distortion of martensite, promote further toughening of the wire rod microstructure, and improve the tempering softening effect of the wire rod. In the preferred technical solution, the roller conveyor slow cooling controls the wire rod to cool to below 280°C at a slow cooling rate of ≤0.4°C / s before winding.

[0029] In the preferred technical solution, the roller conveyor slow cooling adopts a closed heat insulation cover, and the hot air at ≥250℃ during the online molten salt strong quenching isothermal treatment is blown 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 the online molten salt strong quenching isothermal treatment, reduce production energy consumption, and promote the rapid production of wire rod.

[0030] A high-strength multiphase hot-rolled wire rod for 2100MPa 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 2100MPa grade stranded wire described in any one of the above-mentioned methods.

[0031] The aforementioned wire rods utilize a high-carbon composition system containing Mo, which simplifies the composition system, controls material costs, mitigates the risk of precious metal price fluctuations, and reduces smelting difficulty. Simultaneously, the microstructure comprises a multiphase structure dominated by tempered bainite and containing a small amount of tempered martensite. Compared to traditional high-strength stranded wire rods with a pearlitic microstructure, this avoids stress concentration and strength loss caused by pearlite in the core. Furthermore, the hindering effect of pearlite lamellar structure on dislocation slip is relatively weaker. The bainitic cementite is distributed in a fine-grained manner, resulting in a significant fine-grain strengthening effect. After tempering, the cementite further… One-step dispersion can improve brittleness, significantly increase grain boundary area, and hinder fatigue crack initiation. The dislocation density in the martensite is extremely high, which can form dislocation strengthening. After tempering, the dislocation density is appropriately reduced, and the high strength characteristics can still be maintained, but the ductility and toughness can be greatly improved. It can effectively make up for the strength loss caused by omitting alloying elements. It is suitable for stranded wire to withstand alternating loads for a long time during service. At the same time, by controlling the tempering state of the microstructure, the plasticity and strength matching can be improved, so as to break through the production bottleneck of offline heat treatment and reduce the strength loss in subsequent drawing and twisting processes.

[0032] The higher the proportion of tempered bainite in the microstructure, the better the plasticity and toughness of the wire rod, and the higher the strength of the wire rod. In the preferred technical solution, the volume percentage of tempered bainite is 68%~78%, and the volume percentage of tempered martensite is 22%~32%.

[0033] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0 and a mechanical property difference within the same coil of ≤43MPa; the absence of network carbides avoids grain boundary embrittlement, enhances the strengthening effect of carbon, improves the toughness and fatigue resistance of the stranded steel, and results in higher wire rod microstructure uniformity and smaller mechanical property difference within the same coil, which can avoid local performance degradation, improve the stability of subsequent processing and the stability of stranded wire performance, and reduce the risk of overall stranded wire failure.

[0034] In the preferred technical solution, the hot-rolled wire rod has a diameter of 5.0~15.0mm, a tensile strength of 1435~1485MPa, and a section reduction rate of 31%~36%. The hot-rolled wire rod has high tensile strength, which is beneficial to reduce the number of drawing passes and processing cycles, quickly achieve the target strength of high-strength stranded wire, realize the lightweighting of stranded wire, and cope with load fluctuations in engineering service. It also has a high section reduction rate, which can improve the anti-fracture ability in drawing and twisting processing, and the anti-brittle fracture and anti-fatigue ability in stranded wire service.

[0035] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0036] (1) Traditional hot-rolled steel strands are usually cooled by Stellmore air-cooling line, which results in complex alloy composition and extremely brittle bainite and martensite in the hot-rolled state, which easily causes wire breakage during drawing. This invention, through the design of Mo-containing chemical composition and the combination of online molten salt strong quenching isothermal technology, can control the wire to enter the bainite phase region from the high-temperature austenite state, suppress network carbides and pearlite, promote the transformation of austenite to bainite through strong quenching, and then appropriately increase the molten salt temperature to form a bainite-dominant structure and isothermally temper the structure, retaining a small amount of untransformed residual austenite. Finally, the roller table slow cooling promotes the transformation of untransformed residual austenite to martensite and tempering. This can simplify the composition system, realize the control of multiphase structure, control the structure and toughness online, improve the overall strength and plasticity matching of the wire, and has good industrial adaptability.

[0037] (2) In view of the fact that traditional hot-rolled steel strand has complex alloy composition, insufficient strength and plasticity or high risk of brittleness, and requires offline heat treatment, resulting in high risk of wire breakage, low production efficiency, high energy consumption and high cost in subsequent processing, this invention adopts a high carbon composition system containing Mo, which can simplify the composition system, control material costs, avoid the risk of precious metal price fluctuations and reduce smelting difficulty. At the same time, the microstructure includes a multiphase structure of tempered bainite and tempered martensite, which can avoid grain boundary embrittlement caused by network carbides and stress concentration and strength loss caused by pearlite in the core. By utilizing the high strength characteristics of bainite and martensite, the plastic and tough properties are further improved after tempering, and the strengthening effect of carbon elements is enhanced, which can effectively make up for the strength loss caused by omitting alloy elements, achieving a tensile strength of 1435~1485MPa and a section reduction rate of 31%~36%. It can be used to manufacture 2100MPa grade stranded wire and other application fields without offline heat treatment and reduce the risk of wire breakage, and has good market application prospects. Attached Figure Description

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

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

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

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

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

[0043] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2100MPa grade 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: 0.42%, Mn: 0.59%, Cr: 0.36%, Mo: 0.25%, P: 0.013%, S: 0.016%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt strong quenching isothermal → roller table slow cooling → coiling, specifically:

[0044] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity, reducing component segregation and avoiding decarburization. After exiting the furnace, the steel billet is rolled into wire rod with a diameter of 5mm through a rolling line. Appropriate rolling temperature and reduction are selected to promote dynamic recrystallization and refine grains in the final rolling. Specifically, the furnace soaking temperature is controlled at 1180℃, the furnace time is 240min, the initial rolling temperature is 1040℃, the final rolling temperature is 890℃, and the final rolling reduction is 30%. The wire drawing process is used to convert the wire rod exiting the rolling line into wire rod through a wire drawing machine. The wire rod is distributed on the roller conveyor and conveyed along the roller conveyor. An appropriate wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding the precipitation of network carbides during the wire drawing stage, and preparing the microstructure for subsequent preferential bainite transformation and residual austenite decomposition. Specifically, the wire drawing temperature is controlled at 900℃.

[0045] The online molten salt 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. This causes the wire rod to cool at a rate of 33°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide and pearlite phase regions into the bainite phase region. This inhibits the formation of network carbides and pearlite, partially hardens the high-temperature austenitic phase to form bainite, and retains an appropriate amount of residual austenite. 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 the molten salt circulation rate. This process promotes the further decomposition of some retained austenite into bainite, forming a bainite-dominated structure. The structure is then isothermally tempered, retaining a small amount of untransformed retained austenite to prevent carbide coarsening, thereby controlling the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 470℃, the treatment time is 40s, the molten salt circulation rate is 420t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage of molten salt treatment is 500℃, the treatment time is 240s, the molten salt circulation rate is 320t / h, and the molten salt temperature rise is ≤3℃.

[0046] The roller conveyor slow cooling process employs a closed insulation hood, blowing hot air (≥250℃) from above the two salt bath tanks undergoing online molten salt quenching and isothermal treatment into the insulation hood. The wire rod is then conveyed through the insulation hood by the conveyor rollers, promoting the transformation of untransformed residual austenite to martensite and subsequent tempering and toughening, thus improving the wire rod's tempering softening effect. Specifically, the wire rod is cooled to 279℃ 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 1 As shown.

[0047] Comparative Example 1:

[0048] 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 1145°C, with a furnace time of 265 minutes. The initial rolling temperature is 1020°C, the final rolling temperature is 820°C, and the wire drawing temperature is 810°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 60% capacity, fans 1 to 5 are turned on to cool the wire rod to 670℃ at a cooling rate of 6.5℃ / s. Then, fans 6 to 14 are turned on to 30% capacity to cool the wire rod to 272℃ at a cooling rate of 2.9℃ / s. After cooling, the hot-rolled wire rod is obtained.

[0049] Comparative Example 2:

[0050] 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 1145°C, the furnace time is 265 min, the initial rolling temperature is 1025°C, the final rolling temperature is 830°C, and the wire drawing temperature is 820°C. During the molten salt treatment before the online molten salt 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 the process is completed. Example 2:

[0051] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.86%, Si: 0.55%, Mn: 0.45%, Cr: 0.50%, Mo: 0.20%, 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 strong quenching isothermal → roller table slow cooling → coiling, specifically:

[0052] The rolling process is used to heat a 180mm×180mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic, reducing component segregation and avoiding decarburization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 9mm through a rolling line. Appropriate rolling temperature and reduction are selected to promote dynamic recrystallization and grain refinement during final rolling. Specifically, the furnace soaking temperature is controlled at 1200℃, the furnace time at 220min, the initial rolling temperature at 1065℃, the final rolling temperature at 910℃, and the final rolling reduction at 28.5%. The wire drawing process is used to convert the wire rod exiting the rolling line into coil through a wire drawing machine. The coil is distributed on a roller conveyor and transported along the roller conveyor. An appropriate wire drawing temperature is selected to keep the coil in a high-temperature austenitizing state, preventing the precipitation of network carbides during the wire drawing stage. This prepares the microstructure for subsequent preferential bainite transformation and residual austenite decomposition. Specifically, the wire drawing temperature is controlled at 910℃.

[0053] The online molten salt 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. This causes the wire rod to cool at a rate of 33°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide and pearlite phase regions into the bainite phase region. This inhibits the formation of network carbides and pearlite, partially hardens the high-temperature austenitic phase to form bainite, and retains an appropriate amount of residual austenite. 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 the molten salt circulation rate. This process promotes the further decomposition of some retained austenite into bainite, forming a bainite-dominated structure. The structure is then isothermally tempered, retaining a small amount of untransformed retained austenite to prevent carbide coarsening, thereby controlling the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 478℃, the treatment time is 32s, the molten salt circulation rate is 500t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage of molten salt treatment is 514℃, the treatment time is 193s, the molten salt circulation rate is 360t / h, and the molten salt temperature rise is ≤3℃.

[0054] The roller conveyor slow cooling process employs a closed insulation hood, blowing hot air (≥250℃) from above the two salt bath tanks undergoing online molten salt quenching and isothermal treatment into the insulation hood. The wire rod is then conveyed through the insulation hood by the conveyor rollers, promoting the transformation of untransformed residual austenite to martensite and subsequent tempering and toughening, thus improving the wire rod's tempering softening effect. Specifically, the wire rod is cooled to 275℃ at a slow cooling rate of 0.3℃ / 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.

[0055] Comparative Example 3:

[0056] 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 quenching isothermal process, the wire rod is cooled at a rate of 37°C / s, the molten salt temperature of the initial molten salt treatment is 460°C, the treatment time is 50s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0057] Comparative Example 4:

[0058] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the online molten salt quenching isothermal process, the wire rod is cooled at a rate of 31°C / s, the molten salt temperature of the initial molten salt treatment is 540°C, the treatment time is 15s, and the finished hot-rolled wire rod is obtained after the process is completed. Example 3:

[0059] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.89%, Si: 0.49%, Mn: 0.65%, Cr: 0.45%, Mo: 0.35%, 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 strong quenching isothermal → roller table slow cooling → coiling, specifically:

[0060] 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, reducing component segregation and avoiding decarburization. After exiting the furnace, the billet is rolled into wire rod with a diameter of 12.5mm through a rolling line. Appropriate rolling temperatures and reductions are selected to promote dynamic recrystallization and grain refinement during final rolling. Specifically, the furnace soaking temperature is controlled at 1215℃, the furnace time at 185min, the initial rolling temperature at 1080℃, the final rolling temperature at 930℃, and the final rolling reduction at 26%. The wire rod drawing process is used to draw the wire rod from the rolling line into coil through a wire rod drawing machine. The coil is distributed on a roller conveyor and transported along the roller conveyor. An appropriate wire rod drawing temperature is selected to keep the coil in a high-temperature austenitizing state, preventing the precipitation of network carbides during the drawing stage. This prepares the microstructure for subsequent preferential bainite transformation and residual austenite decomposition. Specifically, the wire rod drawing temperature is controlled at 925℃.

[0061] The online molten salt 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. This causes the wire rod to cool at a rate of 33°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide and pearlite phase regions into the bainite phase region. This inhibits the formation of network carbides and pearlite, partially hardens the high-temperature austenitic phase to form bainite, and retains an appropriate amount of residual austenite. 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 the molten salt circulation rate. This process promotes the further decomposition of some retained austenite into bainite, forming a bainite-dominated structure. The structure is then isothermally tempered, retaining a small amount of untransformed retained austenite to prevent carbide coarsening, thereby controlling the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 491℃, the treatment time is 27s, the molten salt circulation rate is 580t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage of molten salt treatment is 522℃, the treatment time is 153s, the molten salt circulation rate is 400t / h, and the molten salt temperature rise is ≤3℃.

[0062] The roller conveyor slow cooling process employs a closed insulation hood, blowing hot air (≥250℃) from above the two salt bath tanks undergoing online molten salt quenching and isothermal treatment into the insulation hood. The wire rod is then conveyed through the insulation hood by the conveyor rollers, promoting the transformation of untransformed residual austenite to martensite and subsequent tempering and toughening, thus improving the wire rod's tempering softening effect. Specifically, the wire rod is cooled to 270℃ at a slow cooling rate of 0.2℃ / 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.

[0063] Comparative Example 5:

[0064] 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 250s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0065] Comparative Example 6:

[0066] 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 480℃, the treatment time is 90s, and the finished hot-rolled wire rod is obtained after the process is completed. Example 4:

[0067] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.91%, Si: 0.62%, Mn: 0.53%, Cr: 0.56%, Mo: 0.4%, P: 0.015%, 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 strong quenching isothermal → roller table slow cooling → coiling, specifically:

[0068] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that can be rolled into plasticity, reducing component segregation and avoiding decarburization. After exiting the furnace, the steel billet is rolled into wire rod with a diameter of 15mm through a rolling line. Appropriate rolling temperature and reduction are selected to promote dynamic recrystallization and refine grains in the final rolling. Specifically, the furnace soaking temperature is controlled at 1230℃, the furnace time is 150min, the initial rolling temperature is 1090℃, the final rolling temperature is 930℃, and the final rolling reduction is 25%. The wire drawing process is used to convert the wire rod exiting the rolling line into wire rod through a wire drawing machine. The wire rod is distributed on the roller conveyor and conveyed along the roller conveyor. An appropriate wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, avoiding the precipitation of network carbides during the wire drawing stage, and preparing the microstructure for subsequent preferential bainite transformation and residual austenite decomposition. Specifically, the wire drawing temperature is controlled at 940℃.

[0069] The online molten salt 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. This causes the wire rod to cool at a rate of 35°C / s, rapidly transitioning from a high-temperature austenitic state through the network carbide and pearlite phase regions to the bainite phase region. This inhibits the formation of network carbides and pearlite, partially hardens the high-temperature austenitic phase to form bainite, and retains an appropriate amount of residual austenite. 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 the molten salt circulation rate. This process promotes the further decomposition of some retained austenite into bainite, forming a bainite-dominated structure. The structure is then isothermally tempered, retaining a small amount of untransformed retained austenite to prevent carbide coarsening, thereby controlling the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 500℃, the treatment time is 20s, the molten salt circulation rate is 620t / h, and the molten salt temperature rise is ≤10℃; the molten salt temperature of the second stage of molten salt treatment is 530℃, the treatment time is 100s, the molten salt circulation rate is 420t / h, and the molten salt temperature rise is ≤3℃.

[0070] The slow cooling process of the roller conveyor adopts a closed heat preservation cover, and hot air at ≥250°C above the two salt bath tanks of the online molten salt strong quenching isothermal treatment is blown into the heat preservation cover. The wire rod is conveyed by the conveyor roller conveyor through the heat preservation cover, which promotes the transformation of untransformed residual austenite to martensite and tempering, further toughening, and improving the tempering and softening effect of the wire rod. Specifically, the wire rod is controlled to be cooled to 268°C at a slow cooling rate of 0.15°C / s. The coiling process is used to coil the wire rod into coils through the coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0071] Comparative Example 7:

[0072] 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 quenching and isothermal treatment → heat preservation and cooling. The heat preservation and cooling process involves closing the heat preservation cover and controlling the wire rod to cool to 270°C at a slow cooling rate of 0.9°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0073] 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:

[0074] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0075]

[0076] As can be seen from the comparison results between Example 1 and Comparative Example 1, compared with the simplified composition system, under the air-cooling process, a higher level of network carbides is generated. The slow cooling of the core of the wire rod leads to the formation of coarse pearlite, weakening the strengthening effect of carbon and resulting in a further decrease in final strength and toughness. Local overcooling of the wire rod easily leads to the formation of uncontrollable martensite, resulting in a further increase in the brittleness of the wire rod and a greater fluctuation in mechanical properties. The present invention, through the design of a Mo-containing chemical composition combined with online molten salt strong quenching isothermal technology, can control the wire rod to enter the bainite phase region from the high-temperature austenitic state, suppressing the formation of network carbides and pearlite. The process involves a light-body structure, followed by strong quenching to promote the transformation of austenite to bainite. Then, the molten salt temperature is appropriately increased to form a bainite-dominated microstructure. The microstructure is then isothermally tempered, retaining a small amount of untransformed residual austenite. Finally, slow cooling via roller conveyor promotes the transformation of the untransformed residual austenite to martensite, followed by tempering. This process achieves multiphase microstructure control and online regulation of microstructure toughness. As seen in Examples 1-4, the tensile strength can reach 1435-1485 MPa, and the reduction of area is 31%-36%. This allows for applications such as manufacturing 2100 MPa grade stranded wire, eliminating the need for offline heat treatment and reducing the risk of wire breakage.

[0077] 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 avoid the formation of network carbides or proeutectoid ferrite due to excessively low temperature during the wire drawing stage. This prepares the microstructure for increasing undercooling, promoting preferential bainite transformation, and decomposing residual austenite. However, if the wire drawing temperature is too low, it will affect the rolling temperature and efficiency, and will also be detrimental to suppressing network carbides. Excessive nucleation transformation of bainite will affect the martensitic transformation, resulting in loss of strong plasticity and mechanical fluctuations.

[0078] 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 better it is to quickly reduce the wire rod temperature, suppress the network carbide and pearlite structure, and promote the nucleation of bainite preferentially over pearlite by strong quenching, thereby increasing the bainite nucleation rate. With the extension of the treatment time, the bainite can remain fine during the slow growth process, forming an initial structure dominated by fine bainite, which improves the matrix strength. However, if the molten salt temperature is too low or the treatment time is too long, the residual austenite will decrease, or the carbon content of the residual austenite will be excessively increased, which will increase the softening difficulty and production energy consumption, and affect the subsequent bainite and martensite phase transformation.

[0079] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature and the shorter the treatment time in the first stage of molten salt treatment, the better it is to reduce the dislocation density and softening difficulty of bainite transformation, retain a certain amount of residual austenite for subsequent molten salt treatment to strengthen it, and reduce the fluctuation of mechanical properties. However, if the molten salt temperature is too high and the treatment time is too short, the driving force of bainite transformation will be weakened, which will affect the suppression of pearlite nucleation and reduce the strength of the matrix.

[0080] 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 better it is to improve the diffusion coefficient of carbon. With the extension of treatment time, it can promote the tempering of bainite, so that the supersaturated carbon in bainite can be more fully precipitated into fine carbides, thereby improving the plasticity and toughness of wire rod. However, if the molten salt temperature is too high, close to the pearlite phase transformation temperature, the insufficient undercooling will affect the bainite phase transformation. With the longer treatment time, the diffusion and aggregation of carbides will be accelerated, the grain boundary strengthening effect will be weakened, and the coarsened carbides will become crack initiation sources, resulting in the loss of plasticity and increasing production energy consumption.

[0081] As can be seen from the comparison results between Example 3 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the better it is to increase the supercooling and promote the bainitic phase transformation. With the shortening of the treatment time, it is better to suppress excessive diffusion, retain fine structure, and retain higher strength and production efficiency. However, if the molten salt temperature is too low and the treatment time is too short, the tempering thermal power will be insufficient, the lattice distortion will not be relieved, and the internal stress will remain. At the same time, with the shortening of the treatment time, the amount of untransformed residual austenite increases, which will induce too much martensitic phase transformation in subsequent cooling, leading to increased tempering difficulty, increased brittleness, and increased performance fluctuations.

[0082] As can be seen from the comparison results between Example 4 and Comparative Example 7, the slow cooling of the roller conveyor can promote the transformation of untransformed residual austenite to martensite and prolong the softening effect of the microstructure in the later stage of the molten salt treatment, alleviate the lattice distortion of martensite, promote further toughening of the wire rod microstructure, and improve the tempering softening effect of the wire rod. The hot air at ≥250°C during the online molten salt strong quenching isothermal treatment is blown into the heat preservation cover, and the wire rod is transported by the conveyor roller conveyor through the heat preservation cover to control the slow cooling of the wire rod. This can further recover and utilize the heat energy of the online molten salt strong quenching isothermal treatment, reduce production energy consumption, and promote the rapid production of the wire rod.

[0083] 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 2100MPa 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: 0.86%~0.91%, Si: 0.42%~0.62%, Mn: 0.45%~0.65%, Cr: 0.36%~0.56%, Mo: 0.2%~0.4%, P≤0.015%, S≤0.015%, with the remainder being Fe and unavoidable impurities. After the wire rod is spun into wire rod at a wire drawing temperature of ≥900℃, it undergoes online molten salt quenching isothermal treatment. This process involves first undergoing a preliminary molten salt treatment and then cooling at a rate of ≥33℃ / s, causing the wire rod to transition from the austenitic state to the bainitic phase, thus promoting the transformation of austenite into bainite. The wire rod undergoes a phase transformation, followed by a subsequent molten salt treatment to increase the molten salt temperature, forming a microstructure dominated by bainite. This microstructure is then isothermally tempered, retaining some untransformed residual austenite. Finally, it undergoes slow cooling via a roller conveyor to promote the transformation of the untransformed residual austenite into martensite, followed by tempering. This process produces a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered martensite, forming a multiphase microstructure. The molten salt temperature of the initial molten salt treatment is 470~500℃, and the treatment time is 20~40s. The molten salt temperature of the subsequent molten salt treatment is 500~530℃, and the treatment time is 100~240s. The slow cooling via the roller conveyor controls the wire rod to cool to below 280℃ at a slow cooling rate of ≤0.4℃ / s before coiling.

2. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to claim 1, characterized in that, Before rolling, the heating furnace temperature is controlled at 1180~1230℃ and the furnace time is 150~240min.

3. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to claim 1, characterized in that, During the rolling process, the initial rolling temperature is controlled at 1040~1090℃, the final rolling temperature is controlled at 890~930℃, and the final rolling reduction is 25%~30%.

4. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to claim 1, characterized in that, During the silk-spinning process, the silk-spinning temperature is controlled at 900~940℃.

5. The method for manufacturing high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to claim 1, characterized in that, The molten salt circulation rate of the first-stage molten salt treatment is 420~620t / h, and the molten salt temperature rise is ≤10℃; the molten salt circulation rate of the second-stage molten salt treatment is 320~420t / h, and the molten salt temperature rise is ≤3℃.

6. A high-strength multiphase hot-rolled wire rod for 2100MPa 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 2100MPa grade strand as described in any one of claims 1 to 5.

7. The high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to claim 6, characterized in that, The volume percentage of tempered bainite is 68%~78%, the volume percentage of tempered martensite is 22%~32%, the network carbide grade of the hot-rolled wire rod is 0, and the mechanical property difference between the same ring is ≤43MPa.

8. The high-strength multiphase hot-rolled wire rod for 2100MPa grade stranded wire according to claim 6, characterized in that, The hot-rolled wire rod has a diameter of 5.0~15.0mm, a tensile strength of 1435~1485MPa, and a reduction of area of ​​31%~36%.

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