A molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire and its manufacturing method

Abstract

This invention relates to a molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire and its manufacturing method. The hot-rolled wire rod is designed with an Nb-V-Mo chemical composition. After being rolled into wire rod, it undergoes online molten salt high-temperature isothermal toughening treatment. The wire rod first undergoes a molten salt treatment to form a microstructure mainly composed of fine lamellar sorbite, and then undergoes a subsequent molten salt treatment to fully transform the phase and isothermal tempering to induce partial melting of cementite lamellars. Finally, it undergoes slow cooling on a roller table for further toughening, resulting in a hot-rolled wire rod with a microstructure consisting of tempered sorbite, tempered ferrite, and melted sorbite. This method can suppress abnormal microstructure, achieve efficient and stable production, reduce production energy consumption, and improve the strength and plasticity matching of the hot-rolled wire rod. The hot-rolled wire rod achieves a tensile strength of 1636~1686MPa and a reduction of area of ​​34%~39%, which is suitable for the production of ultra-high strength stranded wires such as 2560MPa grade. It eliminates the need for offline heat treatment and reduces the risk of wire breakage.

Description

Technical Field

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

[0002] Stranded wire plays a role in load transfer and support, and is widely used in energy, power, transportation, roads, bridges and other fields. The improvement of the strength level of steel strand can not only save materials and reduce costs, but also bring great convenience to the construction of engineering projects. Therefore, ultra-high strength steel strand is the future development trend. However, the current rolling cooling capacity of steel mills is limited, making it difficult to effectively control the microstructure and properties of hot-rolled wire rods, which means that the wire rods need to undergo an offline heat treatment after they come off the production line before they can be transferred to the stranding plant for processing. This is inefficient. There is a need to develop a 2560MPa grade hot-rolled wire rod for stranding and its manufacturing method that does not require offline heat treatment, so as to achieve the purpose of energy and material saving, and meet the development and market demand of the steel industry.

[0003] Existing hot-rolled wire rods for high-strength stranded wire generally use high-carbon components. The following technical bottlenecks remain in developing efficient and stable production of 2560MPa grade ultra-high-strength hot-rolled wire rods for stranded wire:

[0004] I. Existing hot-rolled wire rods for stranded steel are generally produced using a Steyrmore air-cooled wire line. For example, patent CN112176258B discloses a 2500MPa grade steel wire rod for strand and its manufacturing method, which uses a C-Si-Mn-Cr-Al-V-Ti composition system. After combining the processes of billet preparation, hot rolling, Steyrmore cooling, coiling, and salt bath cooling, a sorbitic wire rod is obtained. If it is necessary to further improve the strength of the wire rod, on the one hand, the maximum cooling capacity of the Steyrmore air-cooled wire line is limited. Under high carbon content, carbides tend to accumulate at grain boundaries and form a continuous network. The network carbides become a defect in the hypereutectoid high-carbon steel structure, which will break the grain boundaries and cause intergranular brittleness during wire rod drawing. Furthermore, the segregation of elements such as Cr and Mn increases local hardenability. To minimize the influence of network carbides and promote lamellar refinement to improve wire rod strength, increasing the air-cooling intensity will further increase the temperature difference between the air-receiving and air-exposed surfaces, easily forming a surface martensite layer, resulting in higher brittleness. The uneven precipitation of alloy carbides during this process will further increase the fluctuation of wire rod mechanical properties. On the other hand, to reduce the influence of segregation, it is necessary to combine large billet opening and two-heat forming. After continuous cooling, the sorbitization rate in the microstructure is not high, and there are coarse pearlite in the core. Ultimately, the wire rod has insufficient plasticity and toughness. After coiling, it is necessary to unwind and reheat offline heat treatment, resulting in more processes, higher energy consumption, and higher costs.

[0005] Second, although existing technologies disclose online salt bath isothermal treatment processes, such as the 2500~2560MPa grade stress corrosion resistant prestressed steel strand wire rod, steel strand and production method disclosed in patent CN120400683A, which uses C-Si-Mn-Cr-V-Nb composition and combines online salt bath isothermal and online annealing to obtain sorbite wire rod, the risk of abnormal structures such as grain boundary cementite and martensite still exists due to the influence of wire rod composition and the molten salt temperature of the one-stage salt bath treatment. It cannot quickly improve the matrix stress and still needs to be combined with long-term online annealing treatment, which affects production efficiency and stable production. 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 molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire and its manufacturing method, which can suppress abnormal structure, achieve efficient and stable production, reduce production energy consumption, and improve the strength and plasticity matching of hot-rolled wire rod, so as to be used for the production of ultra-high strength stranded wire such as 2560MPa grade, without the need for offline heat treatment.

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

[0008] The first aspect of this invention is to provide a method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire, the method comprising:

[0009] The hot-rolled wire rod is rolled into finished products according to its chemical composition. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%~1.00%, Si: 1.06%~1.25%, Mn: 0.94%~1.14%, Cr: 0.45%~0.65%, Nb: 0.025%~0.040%, V: 0.025%~0.040%, Mo: 0.35%~0.55%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities.

[0010] The wire rod is spun into coils at a spinning temperature of ≥898℃, and then undergoes online molten salt high-temperature isothermal toughening treatment. The coils first undergo a preliminary molten salt treatment with a cooling rate of ≥32℃ / s, where the molten salt temperature of the preliminary molten salt treatment is 548~578℃ and the treatment time is 88~128s. Then, they undergo a subsequent molten salt treatment with a molten salt temperature of 560~578℃ and a treatment time of 50~95s, where the molten salt circulation rate of the subsequent molten salt treatment is lower than that of the preliminary molten salt treatment. Finally, the coils undergo slow cooling on a roller conveyor, where the roller conveyor controls the coils to cool slowly to below 280℃ at a cooling rate of 0.35~0.8℃ / s, resulting in a hot-rolled coil with a microstructure consisting of a mixture of tempered sorbite, tempered ferrite, and melted sorbite.

[0011] In the preferred technical solution, a rectangular steel billet with a side length of 180mm~220mm is manufactured by continuous casting before rolling. The current of the electromagnetic stirring in the crystallizer is controlled to be 220~270A and the frequency is 2~4Hz. The current of the electromagnetic stirring at the end is controlled to be 440~490A and the frequency is 4~6Hz. The casting speed is 0.66~0.86m / min.

[0012] In a preferred embodiment, a rectangular steel billet with a side length of 180mm to 220mm is heated before rolling. During heating, the uniform heating temperature of the heating furnace is controlled at 1197 to 1257℃, and the furnace time is 180 to 240 minutes.

[0013] In the preferred technical solution, the initial rolling temperature is controlled at 1042~1092℃, the final rolling temperature is controlled at 908~958℃, and the final rolling reduction is 22%~27%.

[0014] In the preferred technical solution, the spinning temperature is 898~938℃; the wire is spun into coils and then conveyed to the salt tank by roller conveyors with a speed of 0.88~1.08m / s.

[0015] In a preferred embodiment, the roller speed of the front-end molten salt treatment is 0.63~0.83m / s.

[0016] In the preferred technical solution, the molten salt circulation rate of the front-end molten salt treatment is 540~635t / h, and the molten salt temperature rise is ≤8℃.

[0017] In a preferred embodiment, the roller speed of the downstream molten salt treatment is 0.43~0.63m / s.

[0018] In the preferred technical solution, the molten salt circulation rate of the downstream molten salt treatment is 350~445t / h, and the molten salt temperature rise is ≤3℃.

[0019] In a preferred embodiment, during the slow cooling process of the roller conveyor, the wire rod enters the conveyor roller conveyor after exiting the salt tank, and the roller conveyor speed is 0.18~0.38m / s.

[0020] In a preferred embodiment, during the slow cooling of the roller conveyor, hot air at ≥240°C from the salt bath undergoing online molten salt high-temperature isothermal toughening treatment is blown onto the conveyor roller conveyor.

[0021] A second aspect of the present invention is to provide a hot-rolled molybdenum-containing wire rod for ultra-high strength stranded wire, wherein the hot-rolled wire rod is manufactured by the manufacturing method of any one of the above-described methods for manufacturing hot-rolled molybdenum-containing wire rod for ultra-high strength stranded wire.

[0022] In the preferred embodiment, the diameter of the hot-rolled wire rod is 5.5~15mm, the volume ratio of tempered sorbite and melted sorbite is ≥97%, and the lamellar spacing of the tempered sorbite is 50~70nm.

[0023] In the preferred technical solution, the hot-rolled wire rod has a network carbide level of 0, is free of martensite, and has a mechanical property difference of ≤51MPa between the same rings.

[0024] In the preferred embodiment, the hot-rolled wire rod has a tensile strength of 1636~1686MPa and a reduction of area of ​​34%~39%.

[0025] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0026] (1) In view of the current situation that existing methods are difficult to control the microstructure and properties of hot-rolled wire rod for steel strand, and offline heat treatment is required, which makes it difficult to balance production efficiency and energy consumption, this invention adopts Nb-V-Mo chemical composition design combined with online molten salt high-temperature isothermal toughening technology. Through the first stage of molten salt treatment, the wire rod is controlled to quickly bypass the network carbide region from the high-temperature austenitic state and enter the sorbite phase region, which inhibits the formation of network carbides and martensite and forms a microstructure dominated by fine lamellar interlayer sorbite, promoting the dispersion precipitation of alloy carbides. The second stage of molten salt treatment reduces molten salt circulation. The process involves increasing the circulation volume to promote the transformation of untransformed residual austenite into fine lamellar interlamellar sorbite, while simultaneously promoting the formation of this fine lamellar interlamellar sorbite through prolonged isothermal tempering. This process also induces partial melting of cementite lamellars and uniform distribution of alloy carbides, thereby controlling the strength-plasticity matching and microstructure uniformity of the wire rod. Finally, the wire rod undergoes slow cooling via a roller conveyor, utilizing its residual heat to enhance the tempering and softening effect. This process can suppress abnormal microstructures, achieve more efficient and stable production compared to single-stage salt bath treatment, reduce production energy consumption, and improve the strength-plasticity matching of hot-rolled wire rods, demonstrating excellent industrial adaptability.

[0027] (2) In view of the problem of insufficient strength, plasticity or microstructure uniformity of existing hot-rolled wire rods for ultra-high strength stranded wire, this invention uses Mo to prevent the precipitates from maturing and growing at high temperature, so as to give full play to the role of Nb and V in refining grains, increasing the phase deformation nucleation rate and refining precipitation, and rapidly improving the strength of the matrix. The microstructure includes a mixed structure composed of tempered sorbite, tempered ferrite and melted sorbite, which improves the matching of strength and plasticity and microstructure uniformity of hot-rolled wire rods. The network carbide level is 0, there is no martensite, the mechanical property difference between the same ring is ≤51MPa, the tensile strength is 1636~1686MPa, and the reduction of area is 34%~39%, so as to be used for the production of ultra-high strength stranded wires such as 2560MPa grade. No offline heat treatment is required, the risk of wire breakage is reduced, and it has good market application prospects. Attached Figure Description

[0028] 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:

[0029] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention.

[0030] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention.

[0031] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0032] 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.

[0033] To address the limitations of existing Stellmore air-cooled production lines in effectively controlling the microstructure and properties of hot-rolled wire rods used for steel strand, which necessitates offline heat treatment after production, or the impact of single-stage salt bath treatment on production efficiency and stability, this invention addresses the issue of utilizing the residual heat of the wire rods. Through Nb-V-Mo chemical composition design combined with online molten salt high-temperature isothermal toughening technology, a 2560MPa grade hot-rolled wire rod for strand is developed, eliminating the need for offline heat treatment. This achieves energy and material savings, and high-efficiency production. A preferred embodiment of this invention is described below. The chemical composition and mass percentage of the hot-rolled wire rod are as follows: C: 0.95%~1.00%, Si: 1.06%~1.25%, Mn: 0.94%~1.14%, Cr: 0.45%~0.65%, Nb: 0.025%~0.040%, V: 0.025%~0.040%, Mo: 0.35%~0.55%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities; the microstructure comprises a mixed structure consisting of tempered sorbite, tempered ferrite, and melted sorbite.

[0034] The chemical composition and mass percentage of the above-mentioned hot-rolled wire rods are designed based on the following:

[0035] (1) Carbon: C is a strengthening element with a relatively low price. It can hinder dislocation slip movement through solid solution strengthening, increase the volume fraction of cementite in sorbite structure, and regulate the thermodynamic conditions of sorbite phase transformation with Mn and Cr elements, promote the formation of fine lamellar sorbite structure, and improve work hardening efficiency in subsequent cold drawing and large deformation process. However, excessive C content will increase carbon segregation. The segregation zone is very easy to form continuous network carbides, which increases the difficulty of controlling the martensite deterioration structure and the uniformity of structure. At the same time, the incubation period of sorbite transformation is prolonged, which leads to a decrease in production efficiency and toughness. Therefore, in order to balance the high strength and structural uniformity control of 2560MPa grade strand, the mass percentage of C is controlled at 0.95%~1.00%.

[0036] (2) Silicon: Si is a solid solution strengthening element of ferrite. It can inhibit the formation of coarse carbides during cooling and improve the tempering stability of steel. By reducing the diffusion rate of carbon atoms at the interface between ferrite and cementite, it can delay the aggregation, growth and spheroidization process of cementite during high temperature isothermal process, so as to form melt-fractured sorbite, which can refine the precipitated phase and improve the toughness of the material. However, excessive silicon will increase the surface decarburization tendency, increase the high temperature rolling deformation resistance of steel, affect production efficiency and the section shrinkage rate of hot rolled wire rod, and improve the deformation resistance of cold drawing. Therefore, in order to balance the wire rod strength and the high temperature isothermal rapid toughening treatment of molten salt, the mass percentage of Si is controlled at 1.06%~1.25%.

[0037] (3) Manganese: Mn is an austenite stabilizing element that can expand the austenite phase region, reduce the critical cooling rate of sorbite phase transformation, increase the undercooling of phase transformation, improve the nucleation rate of sorbite, refine the lamellar structure, enhance the interface strengthening effect, and synergistically regulate hardenability with Cr to avoid the formation of coarse pearlite structure in the core due to insufficient cooling rate, ensure the uniformity of cross-sectional structure, and improve the overall tensile uniformity and torsional performance of finished steel strands. However, when the Mn content is too high, it will increase the component segregation behavior during the solidification process of the billet, and increase the risk of martensite formation during cooling, leading to increased brittleness and mechanical property fluctuations. Therefore, in order to refine the sorbite lamellars and take into account both the uniformity of structure and the control of abnormal structure, the mass percentage of Mn is controlled at 0.94%~1.14%.

[0038] (4) Chromium: Cr is a strong hardenability and carbide stabilizing element. It can significantly improve the overall hardenability of hypereutectoid steel, promote the complete sorbite transformation in the core of large-diameter wire rod, refine the sorbite lamellar spacing, and enhance the matrix interface strengthening effect. Chromium can be dissolved in cementite to form alloy carbides, improve the structural stability and strength of cementite, and stabilize the fine lamellar structure of sorbite. However, excessive Cr content will exacerbate the risk of element segregation, increase the risk of precipitation of martensite and other deteriorated structures, increase the difficulty of tempering softening, and affect the efficiency of the line. Therefore, in order to control the sorbite phase transformation behavior and improve the resistance to tempering softening, the mass percentage of Cr is controlled at 0.45%~0.65%.

[0039] (5) Niobium: As a microalloying element, Nb can be precipitated in the high-temperature hot rolling process to pin grain boundaries and refine grains, increase sorbite nucleation sites, and refine sorbite lamellar structure. Excess Nb can be dispersed and precipitated in the isothermal process, and work with V to rapidly strengthen the matrix, make up for the strength loss in the isothermal process, and improve the uniformity of plastic deformation of wire rod and the ability to resist crack propagation. However, the cost of Nb is relatively high, and excessive addition is not conducive to controlling material cost and improving the high-temperature deformation resistance of steel. Therefore, the mass percentage of Nb is controlled at 0.025%~0.040%.

[0040] (6) Vanadium: As a microalloying element, V can be precipitated in the high-temperature hot rolling process to pin the austenite grain boundaries, inhibit the high-temperature coarsening of austenite grains, and increase the deformation nucleus density of sorbite phase. Excess V can be dispersed and precipitated in the medium-temperature isothermal process, and precipitation strengthening can be achieved by pinning dislocations and hindering dislocation movement, which meets the requirements of ultra-high strength strand drawing and twisting. However, V is relatively expensive, and excessive addition is not conducive to controlling material costs. At the same time, it increases the risk of coarsening and may become a crack source in the matrix. Therefore, the mass percentage of V is controlled at 0.025%~0.040%.

[0041] (7) Molybdenum: The addition of Mo strongly inhibits 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 and inhibiting the precipitation of related second-phase particles. This can prevent the precipitated phase from maturing and growing at high temperatures, so as to stably generate melt-resistant sorbite structure during high-temperature isothermal processes and weaken the segregation effect of continuous casting components. However, the cost of Mo is relatively high, and excessive addition is not conducive to controlling material costs. At the same time, it prolongs the complete transformation cycle of austenite, which is not conducive to rapid production. Therefore, the mass percentage of Mo is controlled at 0.35%~0.55%.

[0042] (8) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0043] The aforementioned hot-rolled wire rod adopts a high-carbon, high-silicon, and high-manganese composition of Nb-V-Mo. C, Si, Mn, and Cr are used to regulate the hardenability of the wire rod, suppress cementite coarsening, and Mo synergistically regulates the thermodynamic and kinetic conditions of phase transformation. Simultaneously, Mo prevents the precipitates from maturing and growing at high temperatures, allowing Nb and V to refine grains, increase the phase deformation nucleation rate, refine precipitation, and rapidly improve matrix strength. This provides favorable conditions for online molten salt high-temperature isothermal toughening treatment to promote the full transformation of fine-lamellar interlayer sorbite structure and rapid online toughening. Compared to existing sorbite structures... In hot-rolled wire rod, on the one hand, abnormal structures such as network carbides and martensite are effectively suppressed, and the uniformity of the structure and the strength of the matrix are improved by suppressing the coarse lamellar pearlite in the core. On the other hand, the austenite fully transforms into sorbite. After high-temperature isothermal tempering, some cementite lamellars melt and transform into intermediate tempered sorbite and melted sorbite that transition to spheroidized structure. This avoids stress concentration. Combined with the dispersed distribution of Nb and V carbides, it effectively improves the matching of strength and plasticity, so that offline heat treatment is not required when producing ultra-high strength strands, and the requirements for drawing and twisting are met.

[0044] The manufacturing method of the above-mentioned hot-rolled wire rod includes: rolling the wire rod into a production line according to the chemical composition of the hot-rolled wire rod; after the wire rod is spun into wire rod at a spun temperature of ≥898℃, it undergoes online molten salt high-temperature isothermal toughening treatment, in which the wire rod first undergoes a first-stage molten salt treatment and is cooled at a cooling rate of ≥32℃ / s, the molten salt temperature of the first-stage molten salt treatment is 548~578℃, and the treatment time is 88~128s; then it undergoes a second-stage molten salt treatment, the molten salt temperature of the second-stage molten salt treatment is 560~578℃, and the treatment time is 50~95s, the molten salt circulation rate of the second-stage molten salt treatment is lower than that of the first-stage molten salt treatment; finally, it undergoes slow cooling on a roller table, in which the wire rod is slowly cooled to below 280℃ at a cooling rate of 0.35~0.8℃ / s, resulting in a hot-rolled wire rod with a microstructure consisting of a mixture of tempered sorbite, tempered ferrite, and melted sorbite.

[0045] The above manufacturing method uses a high wire drawing temperature to ensure that the wire rod is in a high-temperature austenitizing state and to prevent premature precipitation of network carbides. Based on this, the wire rod after wire drawing is not air-cooled and is directly subjected to online molten salt high-temperature isothermal toughening treatment:

[0046] I. Compared to the Stellmore air-cooled line, which has limited maximum cooling capacity and unstable temperature control, making it difficult to simultaneously address network carbides, martensite structure, alloy carbide precipitation, and sorbite phase transformation, molten salt offers several advantages. Firstly, molten salt has a higher heat transfer coefficient than air. The wire rods, after undergoing initial molten salt treatment, can cool rapidly, allowing them to quickly pass through the proeutectoid carbide precipitation temperature range. Carbon atoms do not have enough time to diffuse to the grain boundaries, preventing the formation of a continuous network structure. Simultaneously, as the wire rods pass through the molten salt, the molten salt covers the surface for uniform heat exchange. Compared to air cooling, there are no issues with the airflow surface or fan fluctuations, avoiding the formation of brittle martensite due to element segregation or localized processes. It also prevents uneven alloy carbide precipitation caused by insufficient cooling capacity or temperature control fluctuations. Compared to single-stage salt bath treatment, the pre-treatment molten salt circulation volume is higher, which can further control the consistency of phase transformation of the wire rod and reduce the risk of abnormal structures such as grain boundary cementite and martensite under high carbon content. On the other hand, the pre-treatment molten salt treatment can control the wire rod to quickly enter the sorbite phase region from the high-temperature austenitic state. The large undercooling caused by the rapid cooling of molten salt increases the nucleation rate of sorbite and promotes the dispersion precipitation of alloy carbides. As the treatment time is extended, the wire rod gradually transforms to the molten salt temperature isothermal treatment, which inhibits the formation of network carbides and forms a structure dominated by fine lamellar sorbite. In addition, Mo inhibits the pearlite transformation and reduces the diffusion ability of carbide-forming elements. The resulting sorbite lamellar spacing is small and uniform, thereby improving the matrix strength.

[0047] Second, compared to the continuous cooling of the Stellmore air-cooled line, its limited minimum cooling capacity, and the difficulty in rapidly controlling the microstructure and toughness while balancing production energy consumption and efficiency under a single-stage salt bath treatment, the subsequent molten salt treatment can use a higher molten salt temperature. On the one hand, a higher molten salt temperature can promote the continued transformation of untransformed retained austenite into fine lamellar interlamellar sorbite, increasing the sorbitization rate and preventing retained austenite from continuing to form martensite and other low-temperature brittle structures during subsequent cooling. This compensates for the adverse effects of high Si content on phase transformation inoculation, improving phase transformation inoculation efficiency and microstructure uniformity. On the other hand, the subsequent molten salt treatment can use a lower molten salt circulation rate. As the treatment time increases, the wire rod gradually transforms to the isothermal state of the molten salt temperature. This process can reduce production energy consumption and slow down the aggregation, growth, and spheroidization of cementite during high-temperature isothermal processing. It can promote the formation of fine lamellar sorbite structure for long-term isothermal tempering and cause some cementite lamellars to melt, thereby rapidly improving the ductility and toughness of the structure. At the same time, Mo inhibits the curing and growth of precipitates at high temperatures, promotes the dispersed precipitation of alloy carbides, and rapidly improves the matrix strength, thereby controlling the strength and plasticity matching of the wire rod. After the wire rod exits the salt bath, it is at a relatively high temperature. Using roller table slow cooling can utilize its own residual heat to further toughen the wire rod structure and improve the tempering softening effect. However, it is not necessary to use excessively low slow cooling rates. This allows the hot-rolled molybdenum-containing wire rod to achieve both strength and plasticity and structural uniformity while producing efficiently and stably.

[0048] Furthermore, before rolling, a rectangular steel billet with a side length of 180mm~220mm is manufactured using a continuous casting process. Protective casting is employed to prevent secondary oxidation of the molten steel. The current of the electromagnetic stirring in the crystallizer is controlled at 220~270A, and the frequency at 2~4Hz; the current of the electromagnetic stirring at the end is controlled at 440~490A, and the frequency at 4~6Hz, with a casting speed of 0.66~0.86m / min. Since online molten salt high-temperature isothermal toughening can effectively control abnormal microstructures, it is unnecessary to use a large square billet for two-stage steel production. In the continuous casting process, an appropriate rectangular steel billet can be used as the billet to balance the internal quality of the billet and the rolling compression ratio. By controlling the electromagnetic stirring of the crystallizer, the surface composition segregation is reduced, the equiaxed crystal zone of the billet is refined, and the growth of columnar crystals is suppressed. Controlling the electromagnetic stirring at the end of solidification improves the uniformity of the core composition of the billet. With appropriate casting speed, surface and internal defects are avoided. Too slow a casting speed will affect production efficiency or cause surface depressions and slag inclusions. Too fast a casting speed will cause surface cracks and the risk of steel leakage. This provides a foundation for subsequent large deformation rolling to refine the grains.

[0049] Furthermore, before rolling, the rectangular steel billet with a side length of 180mm~220mm is heated. During heating, the heating furnace temperature is controlled at 1197~1257℃ and the furnace time is 180~240min, so as to control the homogenization of austenite composition, further improve the compositional segregation of the rectangular steel billet, and at the same time prevent the abnormal growth of austenite grains caused by excessive temperature and excessive furnace time.

[0050] Furthermore, during the rolling process, the initial rolling temperature is controlled at 1042~1092℃, the final rolling temperature at 908~958℃, and the final rolling reduction at 22%~27%. This avoids excessive rolling deformation resistance due to excessively low initial rolling temperature, which could increase the mill load or cause rolling cracks, and avoids grain coarsening due to excessively high initial rolling temperature. Selecting appropriate rolling temperature and deformation amount can drive strain-induced precipitation of solid solution Nb and V, promote dynamic recrystallization during the final rolling process, refine the grains, and provide nucleation sites for subsequent sorbite phase transformation.

[0051] Furthermore, the wire-spinning temperature is 898~938℃, which can control the wire rod to be in a high-temperature austenitic state before entering the molten salt, avoiding premature precipitation of proeutectoid carbides due to excessively low temperature, and avoiding abnormal growth of austenitic grains due to excessively high temperature.

[0052] Furthermore, after the wire is spun into coils, it is conveyed to the salt tank via roller conveyors with a speed of 0.88~1.08m / s. Controlling this roller conveyor speed can control the conveying time of the coils from the spinning machine to the salt tank, avoiding excessive cooling time of the coils in the air and excessive temperature drop of the coils during conveying, which would lead to the precipitation of proeutectoid carbides. It also avoids unstable coil conveying or tangled wires caused by excessively fast roller conveyor speed.

[0053] Furthermore, the roller speed of the front-end molten salt treatment is 0.63~0.83m / s. Controlling this roller speed can control the processing time of the wire rod through the front-end molten salt treatment, avoiding a decrease in the uniformity of the structure due to excessively fast roller speed and short processing time, and avoiding an increase in production energy consumption due to excessively slow roller speed and long processing time.

[0054] Furthermore, the molten salt circulation rate of the front-end molten salt treatment is 540~635t / h, and the molten salt temperature rise is ≤8℃. The use of a larger molten salt circulation rate in the front-end molten salt treatment can control the molten salt temperature rise, provide sufficient cooling intensity to suppress the precipitation of network carbides and refine the sorbite lamellars, and avoid excessive energy consumption caused by excessive molten salt circulation rate.

[0055] Furthermore, the roller speed of the subsequent molten salt treatment is 0.43~0.63m / s. Controlling this roller speed can control the processing time of the wire rod through the subsequent molten salt treatment, avoiding insufficient plasticity and toughness due to excessively fast roller speed and short processing time, and avoiding excessively slow roller speed and long processing time, which can lead to an increase in melt-broken sorbite and a decrease in wire rod strength.

[0056] Furthermore, the molten salt circulation rate of the subsequent molten salt treatment is 350~445t / h, and the molten salt temperature rise is ≤3℃. The use of a smaller molten salt circulation rate in the subsequent molten salt treatment can control the molten salt temperature rise, promote uniform melting and fracture of cementite sheets, and reduce production energy consumption.

[0057] Furthermore, during the slow cooling process on the roller conveyor, the wire rod enters the conveyor roller conveyor after exiting the salt tank. The roller conveyor speed is 0.18~0.38m / s. Controlling this roller conveyor speed can be adapted to the slow cooling rate, slowly releasing the residual stress caused by deformation, avoiding stress increase caused by excessively fast roller conveyor speed or excessively slow cooling rate, and avoiding production efficiency being affected by excessively slow roller conveyor speed.

[0058] Furthermore, during the slow cooling of the roller conveyor, hot air at ≥240°C from the salt bath undergoing online molten salt high-temperature isothermal toughening treatment is blown onto the conveyor roller conveyor, which can recover and utilize the waste heat of the molten salt and reduce production energy consumption.

[0059] To further illustrate the present invention, a preferred embodiment of the molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire is further described using Example 1 as an example. The hot-rolled wire rod manufacturing method of Example 1 follows the process flow of continuous casting → heating → rolling → wire drawing → online molten salt high-temperature isothermal toughening treatment → slow cooling on roller table → coiling. Specifically: The continuous casting is used to continuously cast molten steel with a chemical composition conforming to hot-rolled wire rod through a continuous casting machine to produce steel billets with a length × width of 220mm × 220mm. During the continuous casting process, protective pouring is adopted to prevent secondary oxidation of the molten steel. The current of the electromagnetic stirring in the crystallizer is controlled at 270A and the frequency at 4Hz; the current of the electromagnetic stirring at the end is controlled at 460A and the frequency at 6Hz, and the casting speed is 0.86m / min, so as to improve the uniformity of the composition of the billet core.

[0060] The heating process is used to feed the steel billet obtained from continuous casting into the heating furnace and heat it to a high temperature steel billet that can be rolled into plasticity. The heating furnace is heated in sequence according to the preheating section, the first heating section, the second heating section and the soaking section. The soaking temperature of the heating furnace is controlled at 1231℃ and the time in the furnace is 180min.

[0061] The rolling process uses a rolling line to roll high-temperature steel billets into wire rods with a diameter of 14mm. During rolling, the initial rolling temperature is controlled at 1092℃, the final rolling temperature is 915℃, and the final rolling reduction is 22%.

[0062] The spinning process is used to turn the wire from the rolling line into coils through the spinning mechanism. The spinning temperature is 938°C. The coils are spread on the roller conveyor and conveyed to the salt tank along the closely spaced roller conveyor. The roller conveyor speed is 1.08 m / s.

[0063] The online molten salt high-temperature isothermal toughening process employs a two-section salt tank with internal molten salt. After spinning, the wire rod is conveyed by rollers through the first section of the salt tank for the initial molten salt treatment. The wire rod is conveyed at a roller speed of 0.63 m / s during the initial molten salt treatment, allowing it to cool down at a rate of 40℃ / s. The molten salt temperature in the initial molten salt treatment is 578℃, the treatment time is 97s, the molten salt circulation rate is 540t / h, and the molten salt temperature rise is ≤8℃. Then, the wire rod is conveyed by rollers through the second section of the salt tank for the final molten salt treatment. The wire rod is conveyed at a roller speed of 0.55 m / s during the final molten salt treatment, the molten salt temperature in the final molten salt treatment is 578℃, the treatment time is 95s, the molten salt circulation rate is 420t / h, and the molten salt temperature rise is ≤3℃.

[0064] The slow cooling process on the roller conveyor involves controlling the opening of the insulation cover to blow hot air (≥240℃) from above the two salt baths in the online molten salt high-temperature isothermal toughening treatment onto the conveyor roller conveyor. The conveyor roller conveyor then transports the wire rod through the insulation cover at a speed of 0.18 m / s, controlling the wire rod to cool to 268℃ at a slow cooling rate of 0.8℃ / s. The coiling process involves coiling the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product, the metallographic structure of which is shown in the figure below. Figure 1 As shown.

[0065] The chemical composition of each embodiment is shown in Table 1 below, with the remainder being Fe and unavoidable impurities:

[0066] Table 1. Chemical composition of each embodiment

[0067] Comparative Examples 1 and 2 have the same chemical composition as Example 1; Comparative Examples 3 and 4 have the same chemical composition as Example 2; Comparative Example 5 has the same chemical composition as Example 3; The process parameters of the manufacturing methods of each example and comparative example are shown in Table 2 below.

[0068] Table 2. Process parameters for each embodiment and comparative example

[0069] The microstructure and properties of the hot-rolled wire rods obtained in the above embodiments and comparative examples were tested, and the comparison results are shown in Table 3 below:

[0070] Table 3. Microstructure and property test results of different hot-rolled wire rods

[0071] As can be seen from the results in the table above, compared to the difficulty in suppressing the coarsening of network carbides, pearlite, and alloy carbides during the Stellmore air-cooling phase transformation of high-carbon steel, which weakens its strengthening effect and produces abnormal martensitic structure and structural stress, requiring offline heat treatment, this invention uses Nb-V-Mo chemical composition design combined with online molten salt high-temperature isothermal toughening technology. The first stage of molten salt treatment controls the wire rod to rapidly bypass the network carbide region from the high-temperature austenitic state and enter the sorbite phase region, suppressing the formation of network carbides and martensite and forming a structure dominated by fine lamellar interlamellar sorbite, promoting the dispersed precipitation of alloy carbides. The second stage of molten salt treatment promotes the continued transformation of untransformed residual austenite into fine lamellar interlamellar sorbite, while simultaneously promoting the long-term formation of the formed fine lamellar interlamellar sorbite structure. Isothermal tempering promotes the melting of some cementite lamellars and the uniform distribution of alloy carbides to regulate the strength-plasticity matching of the wire rod. Finally, slow cooling on a roller conveyor improves the softening effect of the wire rod during tempering. This process can suppress abnormal structures, achieve more efficient and stable production compared to single-stage salt bath treatment, reduce production energy consumption, and improve the strength-plasticity matching and structural uniformity of hot-rolled wire rods. The volume ratio of tempered sorbite and melted sorbite is ≥97%, the lamellar spacing of tempered sorbite is 50~70nm, the network carbide level is 0, there is no martensite, the mechanical property difference between the same coil is ≤51MPa, the tensile strength is 1636~1686MPa, and the reduction of area is 34%~39%. This allows it to be used in the production of ultra-high strength stranded wires such as 2560MPa grade. Offline heat treatment is not required, simplifying the process and reducing the risk of wire breakage.

[0072] The comparison results between Comparative Examples 1 and 2 and Example 1 show that if the molten salt temperature in the initial molten salt treatment is too high or too low, or the molten salt circulation rate is too low, abnormal structures such as martensite and network carbides are easily generated. Martensite and network carbides increase the brittleness of the matrix, resulting in poor wire rod performance and increased fluctuations in mechanical properties. In addition, if the initial molten salt treatment temperature is too low or the treatment time is too long, it will increase production energy consumption. If the molten salt temperature is too high or the treatment time is too short, it will lead to insufficient sorbite transformation, coarsening of sorbite lamellars, and insufficient precipitation of alloy carbides, affecting the matrix strength. Therefore, the preferred parameters for the initial molten salt treatment are: molten salt temperature 548~578℃, treatment time 88~128s, so that the wire rod cools down at a cooling rate of ≥32℃ / s when passing through the initial molten salt treatment.

[0073] Comparison results of Comparative Examples 3 and 4 with Example 2 show that the present invention can reduce the molten salt circulation rate through the subsequent molten salt treatment, promoting the transformation of untransformed residual austenite into fine lamellar interlamellar sorbite. Simultaneously, it can perform prolonged isothermal tempering on the already formed fine lamellar sorbite, causing some cementite lamellars to melt and allowing for uniform distribution of alloy carbides, thereby controlling the strength-plasticity matching and microstructure uniformity of the wire rod. However, if the molten salt circulation rate in the initial molten salt treatment is too low, abnormal microstructure precipitation and microstructure inhomogeneity may occur. With excessively long subsequent molten salt treatment times, the matrix will coarsen, causing the wire rod's area shrinkage performance to decrease instead of increase. Conversely, if the subsequent molten salt treatment time is too short, it will hinder microstructure tempering and stress release, affecting the complete precipitation of alloy carbides. Therefore, to balance production energy consumption and microstructure uniformity, the preferred molten salt temperature for the subsequent molten salt treatment is 560~578℃, the treatment time is 50~95s, and the molten salt circulation rate for the subsequent molten salt treatment is lower than that for the initial molten salt treatment.

[0074] As can be seen from the comparison results between Comparative Example 5 and Example 3, if the molten salt circulation volume in the front-end molten salt treatment is too low, it will lead to an excessive difference in the same coil of wire rod; if the roller speed is too fast, the cooling rate in the slow cooling section will be too high, which will further increase the stress; by controlling the slow cooling process of the roller, the residual stress of deformation can be released slowly; therefore, in order to balance production efficiency, the present invention preferably controls the wire rod to be slowly cooled to below 280°C at a cooling rate of 0.35~0.8°C / s through the slow cooling of the roller, and the final hot-rolled wire rod microstructure is a mixed structure composed of tempered sorbite, tempered ferrite and melted sorbite.

[0075] 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 molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire, characterized in that, Its manufacturing methods include: The hot-rolled wire rod is rolled into finished products according to its chemical composition. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95%~1.00%, Si: 1.06%~1.25%, Mn: 0.94%~1.14%, Cr: 0.45%~0.65%, Nb: 0.025%~0.040%, V: 0.025%~0.040%, Mo: 0.35%~0.55%, P≤0.014%, S≤0.014%, with the remainder being Fe and unavoidable impurities. The wire rod is spun into coils at a spinning temperature of ≥898℃, and then undergoes online molten salt high-temperature isothermal toughening treatment. The coils first undergo a preliminary molten salt treatment with a cooling rate of ≥32℃ / s, where the molten salt temperature of the preliminary molten salt treatment is 548~578℃ and the treatment time is 88~128s. Then, they undergo a subsequent molten salt treatment with a molten salt temperature of 560~578℃ and a treatment time of 50~95s, where the molten salt circulation rate of the subsequent molten salt treatment is lower than that of the preliminary molten salt treatment. Finally, the coils undergo slow cooling on a roller conveyor, where the roller conveyor controls the coils to cool slowly to below 280℃ at a cooling rate of 0.35~0.8℃ / s, resulting in a hot-rolled coil with a microstructure consisting of a mixture of tempered sorbite, tempered ferrite, and melted sorbite.

2. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, Before rolling, a rectangular steel billet with a side length of 180mm~220mm is manufactured by continuous casting. The current of the electromagnetic stirring in the crystallizer is controlled at 220~270A and the frequency is 2~4Hz. The current of the electromagnetic stirring at the end is controlled at 440~490A and the frequency is 4~6Hz. The casting speed is 0.66~0.86m / min.

3. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, Before rolling, a rectangular steel billet with a side length of 180mm to 220mm is heated. During heating, the uniform heating temperature of the heating furnace is controlled at 1197 to 1257℃, and the time in the furnace is 180 to 240 minutes.

4. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, The initial rolling temperature is controlled at 1042~1092℃, the final rolling temperature at 908~958℃, and the final rolling reduction at 22%~27%.

5. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, The spinning temperature is 898~938℃; the wire is spun into coils and then conveyed to the salt tank by roller conveyors with a speed of 0.88~1.08m / s.

6. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, The roller speed of the front-end molten salt treatment is 0.63~0.83m / s, the molten salt circulation rate is 540~635t / h, and the molten salt temperature rise is ≤8℃.

7. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, The roller speed of the downstream molten salt treatment is 0.43~0.63m / s, the molten salt circulation rate is 350~445t / h, and the molten salt temperature rise is ≤3℃.

8. The method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 1, characterized in that, During the slow cooling process of the roller conveyor, the wire rod enters the conveyor roller conveyor after exiting the salt tank. The roller conveyor speed is 0.18~0.38m / s. Hot air at ≥240℃ from the salt tank undergoing online molten salt high-temperature isothermal toughening treatment is blown onto the conveyor roller conveyor.

9. A molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire, characterized in that, The hot-rolled wire rod is manufactured by the method for manufacturing molybdenum-containing hot-rolled wire rod for ultra-high strength strand as described in any one of claims 1 to 8.

10. The molybdenum-containing hot-rolled wire rod for ultra-high strength stranded wire according to claim 9, characterized in that, The hot-rolled wire rod has a diameter of 5.5~15mm, a volume ratio of tempered sorbite and melted sorbite of ≥97%, a lamellar spacing of 50~70nm in tempered sorbite, a network carbide grade of 0, no martensite, a mechanical property difference of ≤51MPa between rings, a tensile strength of 1636~1686MPa, and a reduction of area of ​​34%~39%.

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