A high-strength multiphase hot-rolled wire rod for 2230MPa grade stranded wire and its manufacturing method

Abstract

This invention relates to a high-strength multiphase hot-rolled wire rod for 2230MPa grade stranded wire and its manufacturing method. The method involves using a high-carbon composition containing Nb, rolling the wire rod to a high wire drawing temperature, and then subjecting it to online molten salt mixing and quenching isothermal treatment. The wire rod undergoes a preliminary molten salt treatment with a cooling rate of ≥35℃ / s, transitioning it from an austenitic state to a mixed phase region of bainite and pearlite, promoting the transformation of some austenite into bainite and sorbite. A subsequent molten salt treatment increases the molten salt temperature, promoting the decomposition of untransformed residual austenite into sorbite, followed by isothermal tempering. Finally, the wire rod undergoes slow cooling on a roller conveyor, resulting in a hot-rolled wire rod with a microstructure consisting of tempered bainite and tempered sorbite, which improves draw hardening capacity while maintaining production efficiency. This achieves a tensile strength of 1522~1572MPa and a reduction of area of ​​29%~34%, thus eliminating the need for offline heat treatment and reducing the risk and cost of wire breakage during stranded wire manufacturing.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hot-rolled wire rod, and particularly relates to a 2230MPa-grade high-strength composite-phase hot-rolled wire rod for stranded wire and a manufacturing method thereof. BACKGROUND

[0002] The improvement of the strength grade of steel strand is helpful to material saving, cost reduction and energy consumption reduction of engineering construction projects, so the improvement of the strength grade is still an important development direction in the field of steel strand. As the mother material of steel strand, the base phase of iron-based material is various, but the materials with pearlite and ferrite as the base structure have low work hardening rate. Although the hot-rolled wire rod can be cold-worked in a large deformation mode to obtain high-strength wire material, the large deformation cold working means that the risk of wire breakage in the drawing process is increased, the production efficiency is limited, and even the off-line heat treatment is required to improve the plasticity, resulting in increased production energy consumption and cost, reduced efficiency, and therefore it is necessary to develop a 2230MPa-grade hot-rolled wire rod for stranded wire which can improve the work hardening rate, has higher strength and higher plasticity, and does not require off-line heat treatment, so as to meet the development and market use requirements of the steel industry.

[0003] The high-strength hot-rolled wire rod in the prior art generally adopts high-carbon and high-silicon components, and is combined with Stelmor air cooling line to form sorbite structure. For example, the patent CN120400688A discloses a 2230MPa-grade high-corrosion-resistant steel strand alloy, wire rod, steel strand and method, which adopts C-Si-Mn-Cr-V components combined with Stelmor air cooling line air cooling and off-line salt bath heat treatment to manufacture the wire rod, but there are still the following technical bottlenecks:

[0004] 1. Silicon element can reduce the diffusion activation energy of carbon atoms at the grain boundary. Limited by high-carbon content and the maximum cooling capacity of Stelmor air cooling line, carbides are easily precipitated at the original austenite grain boundary during the cooling process and form a continuous or semi-continuous network structure of defect organization. This network carbide deteriorates the plasticity and toughness of the wire rod, blocks the dislocation movement, causes deformation concentration, increases the wire breakage rate, and affects the service fatigue performance. In order to reduce the level of network carbide as much as possible and promote the refinement of pearlite lamella to improve the work hardening capacity, the air volume is increased after the air volume is increased, the instability of the air volume and air temperature is further increased, the alloy element composition segregation and the hardenability of the wire rod are affected, the wire rod is affected by the difference between the wind and the wind cooling, the brittleness of the strengthening matrix such as bainite and martensite produced on the surface of the wire rod due to the fast cooling speed is very large, and the wire rod is easily broken during transportation and cold working, which affects the yield and subsequent processing. Although the on-line EDC water bath cooling method can improve the cooling speed of the wire rod, a large amount of bubbles are generated during the water cooling process and adhere to the surface of the wire rod, which affects the heat transfer uniformity, and there are still problems of uniform organization and abnormal organization, resulting in large fluctuation of the mechanical properties of the wire rod, high risk of wire breakage and difficulty in off-line heat treatment.

[0005] II. In order to improve the plasticity index and uniformity of the wire rod, although the existing technology discloses the use of online salt bath isothermal treatment, for example: the patent CN120400686A discloses a 2200-2260MPa grade super high stress corrosion resistance pre-stressed steel strand wire rod, steel strand wire and its production method, adopts C-Si-Mn-Cr-V-Nb composition combined with online salt bath isothermal treatment and online aging, and manufactures sorbite structure wire rod, but on the one hand, the initial strength of the wire rod is low, the work hardening rate of the material with sorbite as the matrix structure is relatively insufficient, the addition of multiple alloy components increases the material and smelting cost, and the simplification of the alloy component will further cause the strength loss, thereby increasing the drawing pass and the risk of wire breakage, on the other hand, the use of one-stage salt bath treatment needs to cooperate with a large amount of molten salt circulation to control the cooling of the wire rod and the temperature rise of the molten salt, which increases the production energy consumption, is limited by the anti-softening ability of the alloy component and the structure, and the salt bath treatment time is not too long under the one-stage salt bath treatment, which needs to cooperate with a lower online aging cold speed, and the production efficiency is limited. SUMMARY

[0006] The present application aims to at least solve one of the above technical problems to a certain extent, and provides a 2230MPa grade high-strength complex phase hot-rolled wire rod for strand wire and a manufacturing method thereof, which can simplify the component system, improve the strength and drawing hardening ability, and balance the uniformity of the structure, plasticity and production efficiency, so as to avoid offline heat treatment, reduce the risk of wire breakage and cost in the manufacture of strand wire.

[0007] The technical scheme adopted by the present application to solve the technical problems is:

[0008] A manufacturing method of a 2230MPa grade high-strength complex phase hot-rolled wire rod for strand wire, the manufacturing method comprises:

[0009] A wire rod is produced according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod comprises: C: 0.87%~0.91%, Si: 0.40%~0.60%, Mn: 0.61%~0.81%, Cr: 0.38%~0.57%, Nb: 0.022%~0.042%, P≤0.014%, S≤0.014%, and the rest is Fe and inevitable impurities; after the wire rod is spun into a wire rod according to a spinning temperature of ≥890℃, the wire rod is subjected to online molten salt mixed quenching isothermal treatment, so that the wire rod is first subjected to front segment molten salt treatment and is cooled at a cooling speed of ≥35℃ / s, enters the mixed phase region of bainite and pearlite from the austenite state, promotes the transformation of part of the austenite into bainite and sorbite, is then subjected to rear segment molten salt treatment, the molten salt temperature is increased, the residual austenite that has not been transformed is decomposed and transformed into sorbite and is subjected to isothermal tempering, and finally is subjected to roller slow cooling, so as to be manufactured into a hot-rolled wire rod with a complex phase structure composed of tempered bainite and tempered sorbite.

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

[0011] (1) Carbon: C is a solid solution strengthening element in steel. It is relatively cheaper and can cause significant distortion of the matrix lattice, increase the resistance to dislocation movement, and reduce the starting temperature of bainite and sorbite phase transformation in order to promote bainite nucleation and inhibit its excessive growth. It provides hard phase support for the strength of the multiphase, inhibits pearlite, and promotes the formation of sorbite. In the later stage of molten salt treatment, it can promote the formation of carbides with Cr and Nb, thereby improving the matrix strength and resistance to softening. However, if the C content is too high, it will aggravate the center segregation of high carbon steel billet, increase the risk of rolling cracks, increase the difficulty of controlling network carbides and martensite, accelerate the bainite transformation kinetics, and increase the difficulty of tempering process and the risk of cementite coarsening. Therefore, in order to take into account the high strength requirements of 2230MPa grade strand, reduce the risk of network carbide precipitation, adapt to the multiphase structure and isothermal tempering control, the mass percentage of C is controlled at 0.87%~0.91%.

[0012] (2) Silicon: Si is a solid solution strengthening element. Through solid solution, it can supplement the matrix strength, reduce the diffusion rate of carbon atoms in austenite, significantly delay the nucleation and growth of pearlite, and help refine the bainite structure and stabilize the bainite transformation rate. It will make the growth of sorbite lamellars more uniform. By strongly inhibiting the precipitation and coarsening process of cementite, it can make the residual austenite completely decompose into sorbite, which can refine the precipitated phase and improve the toughness of the material. It can guide the carbides to disperse and precipitate in the sorbite grains, so as to provide a certain resistance to tempering softening. However, excessive silicon will promote decarburization, increase the rolling deformation resistance, affect the rolling efficiency, increase the proportion of bainite and the difficulty of softening, and lead to a decrease in the plasticity and production efficiency of steel. Therefore, in order to adapt to the control of phase transformation structure by online molten salt mixed quenching isothermal treatment and promote the gradual release of internal stress, the Si content should be appropriately reduced. The mass percentage of Si should be controlled at 0.40%~0.60%.

[0013] (3) Manganese: Mn can provide solid solution strengthening and stabilize austenite and inhibit the precipitation of proeutectoid ferrite. By improving the hardenability of high carbon steel, reducing the critical cooling rate and bainite transformation initiation temperature, it slows down the C diffusion rate, so as to refine the bainite structure and sorbite lamellar spacing, inhibit the soft phase in the core, and thus improve the matrix strength. At the same time, it hinders the diffusion of carbon atoms and the merging and growth of cementite, and reduces the strength loss during tempering. However, if the Mn content is too high, it will increase the element segregation during the solidification process of the billet and cause uneven structure. If the bainite ratio is too high and the residual austenite decomposes slowly, it will increase the risk of martensite precipitation and the difficulty of tempering softening, affecting the toughness and structure uniformity of the wire rod and affecting the production efficiency. Therefore, in order to facilitate the control of the multiphase structure of hot-rolled wire rod and control the difficulty of tempering softening, the mass percentage of Mn is controlled at 0.61%~0.81%.

[0014] (4) Chromium: Cr can significantly improve the hardenability of steel, reduce the critical cooling rate and the diffusion coefficient of carbon in austenite, so that the core can be transformed synchronously. At the same time, it refines the bainite structure, makes the sorbite transformation rate moderate, and refines the sorbite lamellar spacing. Chromium is easy to combine with carbon to form chromium-based carbides, which can provide dispersion strengthening, improve the resistance to softening, and make the multiphase structure retain high strength characteristics and pull hardening ability. However, when the Cr content is too high, segregation will occur during solidification, which will increase the uniformity of the structure and the difficulty of controlling martensite. At the same time, it will delay the phase transformation rate, increase the stability of residual austenite and the difficulty of tempering, and affect the rapid production line. Therefore, in order to take into account the control of the multiphase structure of hot-rolled wire rod and appropriately control the resistance to tempering softening of wire rod, the mass percentage of Cr is controlled at 0.38%~0.57%.

[0015] (5) Niobium: As a microalloying element, Nb’s pinning effect can effectively suppress the coarsening of austenite grains during rolling. Fine austenite grains provide a basis for the subsequent phase transformation to form fine bainite and fine sorbite. It also has good high-temperature stability during isothermal toughening and can disperse precipitation to provide strong precipitation strengthening effect, thereby providing a certain resistance to tempering softening and compensating for the strength loss during softening. At the same time, there is no risk of coarsening during service. However, Nb has a high cost, and excessive addition is not conducive to controlling material costs. Therefore, based on the role of Nb and material cost considerations, Nb element is added appropriately, and the mass percentage of Nb is controlled at 0.022%~0.042%.

[0016] (6) Phosphorus and sulfur: P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.014% and S ≤ 0.014%.

[0017] The aforementioned hot-rolled wire rod adopts a high-carbon composition design of C-Si-Mn-Cr-Nb, which can appropriately reduce the Si element content and eliminate the need for V element addition. This simplifies the composition system, reduces smelting difficulty and material costs. Through the optimization of Mn and Cr composition, the initiation temperature of pearlite and bainite phase transformation can be appropriately reduced, the phase transformation temperature range can be expanded, and the hardenability, carbon diffusion rate and tempering softening resistance of the wire rod can be controlled. This provides favorable conditions for promoting the refinement of bainite and sorbite structures, simultaneous phase transformation of mixed structures, and online toughening while maintaining production efficiency. On this basis, a higher wire drawing temperature is selected to promote austenite homogenization and avoid the precipitation of network carbides or proeutectoid ferrite due to excessively low wire drawing temperature. At the same time, it connects the cooling rate and phase transformation range, preparing the microstructure for improving undercooling and promoting the refinement of bainite and sorbite phase transformation. The wire rod undergoes online molten salt mixed quenching isothermal treatment directly without air cooling.

[0018] I. Compared to the limitations of the Stellmore air-cooled line, which suffers from the highest cooling capacity and unstable temperature control, making it difficult to control the transformation of network carbides and mixed structures, the wire rod undergoes rapid cooling during the initial molten salt treatment. This allows it to quickly pass through the sensitive temperature range for network carbide precipitation from a high-temperature austenitic state, inhibiting the precipitation of carbides along austenite grain boundaries to form a network structure and avoiding the adverse effects of network carbides on matrix uniformity and plasticity. Furthermore, the lower temperature of the molten salt during the initial treatment promotes rapid cooling, increasing supercooling and phase transformation driving force, thus suppressing the formation of coarse pearlite. Additionally, unlike air-cooled lines, the wire rod does not suffer from the problems of obstructed airflow at the joints or the temperature difference between the air-receiving and air-receiving surfaces. During molten salt treatment, the molten salt can cover the surface of the wire rod for uniform heat exchange. Compared with water cooling, there is no problem of a large number of bubbles interfering with heat transfer. This avoids the formation of brittle martensite abnormal structure due to local overcooling. The wire rod enters the mixed phase transformation range of bainite and pearlite. With the help of alloying elements, the diffusion rate of carbon in austenite is reduced, which can promote the transformation of some austenite structure into fine bainite and sorbite structure with finer lamellar spacing, so as to achieve a stable transformation of mixed structure. Compared with the high sorbite matrix of existing one-stage salt bath treatment, the first stage of molten salt treatment can improve the matrix strength through fine structure and appropriate bainite matrix, which can make up for the strength loss caused by reducing alloy content and saving alloying elements, and provide a certain pull hardening ability.

[0019] Second, compared to the limitations of the Stellmore air-cooling line, which restricts cooling capacity and continuous operation, making it difficult to control the microstructure toughness online, and resulting in high brittleness and fracture risk in the bainitic matrix, the subsequent molten salt treatment of wire rods can increase the molten salt temperature, accelerate carbon diffusion, and promote the decomposition and transformation of residual austenite from the previous molten salt treatment into sorbite. This further reduces the temperature difference between the wire rod surface and the core, decreasing thermal and phase transformation stresses. Simultaneously, the microstructure is in a high-temperature state after inoculation, rather than a low-temperature state after continuous cooling. During the isothermal process, dislocations can dynamically recover through climb and cross-slip, appropriately reducing dislocation density, improving bainitic toughness, releasing microstructure stress, and thus controlling the online toughening of the multiphase microstructure. This improves the balance between plasticity and strength, avoiding the problems of coiling, transportation, and drawing caused by brittle microstructures. The risk of brittle fracture is eliminated, and offline heat treatment is unnecessary. Compared to the limitations of existing one-stage salt bath treatment, which makes it difficult to balance toughening and production efficiency, wire rods can form a mixed structure after the first stage of molten salt treatment, providing a higher strength base and dislocation density. As the processing time of the second stage of molten salt treatment is extended, the dispersion precipitation of Cr and Nb carbides can compensate for the strength loss, thus balancing strength and uniform structural transformation. This avoids the formation of low-temperature brittle structures due to undecomposed residual austenite during subsequent cooling, while rapidly improving ductility and toughness. The temperature of the wire rod after exiting the molten salt is relatively high, and it can be slowly cooled by roller conveyor to continue the toughening effect of the second stage of molten salt treatment and improve the tempering and softening effect of the wire rod. However, it is not necessary to use excessively long heat treatment, thus balancing production efficiency and optimizing production.

[0020] Before rolling, a higher uniform temperature and an appropriate furnace time can be selected to promote uniform diffusion of components, reduce the segregation effect of carbon and alloying elements, and improve the rollability of the billet. At the same time, excessively high temperature or excessively long furnace time can be avoided to prevent grain coarsening or decarburization. In a preferred embodiment, before rolling, the uniform heating temperature of the heating furnace is controlled at 1183~1233℃ and the furnace time is 140~230min.

[0021] During the rolling process, a higher initial rolling temperature can be used to reduce the resistance of the billet to rolling deformation and the wear on the rolling line. A larger initial rolling reduction promotes the preliminary refinement of the original austenite grains, accumulates strain energy, and improves rolling efficiency. An appropriate final rolling temperature and reduction are selected to induce the precipitation of Nb carbonitrides at pinned grain boundaries, dynamically recrystallize and refine the grains, and connect the wire drawing temperature. In a preferred embodiment, during the rolling process, the initial rolling temperature is controlled at 1032~1085℃, the initial rolling reduction is 37%~43.5%, the final rolling temperature is 900~940℃, and the final rolling reduction is 22%~27%.

[0022] During the wire spinning process, the wire spinning temperature can be further controlled to suppress austenite grain coarsening and provide more nucleation sites for bainite and sorbite. In a preferred embodiment, the wire spinning temperature is controlled to be 890~930℃.

[0023] The lower the molten salt temperature in the initial molten salt treatment, the faster the cooling rate can be, inhibiting the long-distance diffusion of network carbides and carbon, suppressing the formation of coarse pearlite, and promoting the formation of fine bainite and sorbite structures in the wire rod under rapid cooling. With prolonged treatment time, the bainite nucleation rate can be increased, dislocation density and matrix strength can be increased. However, if the molten salt temperature is too low and the treatment time is too long, the shear transformation from austenite to bainite will dominate, increasing the bainite transformation amount, matrix dislocation density, and thermal stress in the microstructure, thereby increasing the softening difficulty of the subsequent molten salt treatment and affecting production energy consumption and efficiency. Conversely, the higher the molten salt temperature, the more effectively carbon diffuses, which is beneficial for uniform phase transformation and reducing the temperature gradient between the wire rod surface and core. With prolonged treatment time... The shorter the processing time, the better it is to reduce dislocation density and thermal stress, and to reduce the difficulty of subsequent isothermal tempering. However, if the molten salt temperature is too high and the processing time is too short, it will be detrimental to suppressing the precipitation of network carbides, microstructure and lamellar refinement, affecting bainite nucleation, resulting in insufficient matrix strength and increased fluctuations in mechanical properties. Therefore, the molten salt temperature and processing time of the first stage of molten salt treatment can be controlled to allow the wire rod to quickly enter the mixed phase region of bainite and pearlite from the high-temperature austenitic state, suppress network carbides and coarse pearlite, and promote the transformation of some austenite into fine bainite and fine lamellar sorbite microstructure, thus preparing the microstructure for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the first stage of molten salt treatment is 459~499℃ and the processing time is 22~52s.

[0024] Since the spinning temperature differs significantly from the molten salt temperature in the preceding molten salt treatment, a larger molten salt circulation rate can be selected to control the molten salt temperature rise and promote microstructure refinement. In the preferred technical solution, the molten salt circulation rate in the preceding molten salt treatment is 350~500t / h, and the molten salt temperature rise is ≤8℃.

[0025] The molten salt temperature in the subsequent molten salt treatment is appropriately increased compared to the temperature in the preceding molten salt treatment. Higher molten salt temperatures enhance carbon diffusion, promote the decomposition of retained austenite into sorbite, and, with prolonged treatment time, facilitate complete microstructural transformation, reducing the risk of brittle microstructure formation during subsequent cooling. Simultaneously, it reduces the temperature gradient between the inside and outside of the wire rod, promoting the full precipitation and uniform dispersion of Cr and Nb alloy carbides in the matrix, thereby releasing structural stress, improving bainitic brittleness, and enhancing microstructure uniformity and ductility. However, excessively high molten salt temperatures are detrimental to suppressing pearlite in the core and controlling the dispersed precipitation of alloy carbides. With prolonged treatment time, the carbides tend to coarsen, further diminishing the strength and ductility. Conversely, lower molten salt temperatures promote the transformation of retained austenite into sorbite with finer lamellar spacing, improving the alloy's... The precipitation of carbides drives dislocation pinning and strengthens grain boundaries, improving matrix strength. Shorter processing times maintain matrix strength and reduce production energy consumption. However, excessively low molten salt temperatures hinder the rapid decomposition of retained austenite. Shorter processing times also result in insufficient thermal dynamics, leading to significant plasticity loss due to inadequate stress release. Failure to fully precipitate alloy carbides further diminishes strength and plasticity, causing fluctuations in mechanical properties. Therefore, controlling the molten salt temperature and processing time in the later stages of molten salt treatment promotes the decomposition of untransformed retained austenite into sorbite. Isothermal tempering of the wire rod structure further promotes the dispersed precipitation of alloy carbides, thus regulating the strength-plasticity balance of the wire rod. In a preferred embodiment, the molten salt temperature in the later stages of molten salt treatment is 552~582℃, and the processing time is 120~180s.

[0026] Since the temperature difference between the wire rod after the first stage of molten salt treatment and the temperature of the molten salt in the second stage of molten salt treatment is small, compared with the single-stage salt bath treatment, the second stage of molten salt treatment can appropriately reduce the molten salt circulation volume, control the molten salt temperature rise, promote the decomposition of residual austenite and isothermal toughening of the microstructure, and reduce production energy consumption. In the preferred technical solution, the molten salt circulation volume of the second stage of molten salt treatment is 250~450t / h, and the molten salt temperature rise is ≤3℃.

[0027] The roller conveyor slow cooling can further control the slow cooling speed of the wire rod, avoiding insufficient plasticity of the wire rod due to insufficient slow cooling. At the same time, slow cooling promotes further toughening of the wire rod structure and improves the tempering and 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 speed of ≤0.65°C / s before winding. To further improve production efficiency, the slow cooling speed can be further preferred to be ≥0.35°C / s.

[0028] In the preferred technical solution, the roller conveyor slow cooling adopts the control of the opening of the heat insulation cover, and blows the hot air of ≥230°C during the online molten salt mixing quenching isothermal treatment into the heat insulation cover. The conveyor roller conveyor transports the wire rod through the heat insulation cover to control the slow cooling of the wire rod, which can further recover and utilize the heat energy of molten salt treatment, reduce production energy consumption, and promote the rapid production of wire rod.

[0029] A high-strength multiphase hot-rolled wire rod for 2230MPa 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 2230MPa grade stranded wire as described in any one of the above-mentioned methods.

[0030] The aforementioned hot-rolled wire rod is designed with a high-carbon composition containing trace amounts of Nb, which can reduce alloy content, simplify the alloy composition system, and appropriately reduce material costs. The hot-rolled wire rod includes a mixed microstructure mainly composed of tempered sorbite with a small amount of tempered bainite. Compared with materials with sorbite as the matrix in existing technologies, it can effectively suppress abnormal microstructures such as network carbides, coarse pearlite, and martensite. Bainite has a higher dislocation density than pearlite, and after tempering, it can retain the fine needle-like structure of bainite. At the same time, the precipitated nano-sized Cr and Nb carbides are evenly distributed, forming a synergistic effect of dislocation and dispersed pinning, which facilitates drawing. When dislocation movement is hindered by carbides, the hardening rate can be improved, the strength can be rapidly increased, and the number of drawing passes can be reduced. This compensates for the strength loss caused by reducing alloy content and eliminating alloying elements. At the same time, the internal stress of tempered bainite is lower and the toughness is better. Compared with the existing bainite matrix, it will not be prone to breakage during coiling, transportation and drawing due to brittleness. While ensuring the yield, the stress of sorbite is released and transformed into tempered sorbite with better plasticity, resulting in higher microstructure stability. Cracks are less likely to occur during drawing, further reducing the risk and cost of wire breakage in stranded wire manufacturing after offline heat treatment, thereby promoting efficient and stable stranded wire production.

[0031] The higher the volume percentage of tempered bainite and the finer the lamellar spacing of tempered sorbite, the higher the matrix strength. The higher the proportion of tempered sorbite, the better the ductility and toughness. In the preferred technical solution, the volume percentage of tempered bainite is 13%~22%, the volume percentage of tempered sorbite is 78%~87%, and the lamellar spacing of tempered sorbite is 70~120nm.

[0032] 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 ≤49MPa. This avoids the adverse effects of network carbides on the uniformity of the structure and the ductility and toughness, improves the strengthening effect of carbon elements and fatigue performance, and the lower mechanical property difference within the same coil helps to make the stress distribution of the wire rod uniform during the drawing process, reduces the wire breakage rate during subsequent stranding, and further improves the yield of stranded wire.

[0033] In the preferred technical solution, the diameter of the hot-rolled wire rod is 10.0~15.0mm, the tensile strength is 1522~1572MPa, and the reduction of area is 29%~34%. The wire rod has high tensile strength, which can be used as a base material to manufacture strands to reduce the number of drawing passes, quickly reach the target strength, and reduce plastic loss during the process. The good reduction of area can reduce the risk of wire breakage during drawing and stranding without offline heat treatment, thereby reducing the cost of stranded wire and increasing the production capacity of stranded wire.

[0034] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0035] (1) To address the shortcomings of existing hot-rolled wire rods for stranded wire, which are limited by the Stellmore air-cooling line or online salt bath isothermal treatment, making it difficult to control abnormal structures such as network carbides and martensite, or to regulate multiphase structure and achieve rapid online toughening, this invention adopts a high-carbon composition design containing trace amounts of Nb, combined with online molten salt mixing quenching isothermal technology. This allows the wire rod to undergo a preliminary molten salt treatment and rapidly transition from the high-temperature austenitic state to the bainite and pearlite mixed phase region, suppressing network carbides and coarse pearlite, and promoting the formation of a mixed phase region of bainite and pearlite. Part of the austenite transforms into bainite and sorbite, and then undergoes a subsequent molten salt treatment to increase the molten salt temperature, promoting the decomposition of untransformed residual austenite into sorbite and isothermal tempering, which promotes the dispersion and precipitation of carbides and avoids the risk of martensite precipitation. Finally, it undergoes slow cooling on a roller table to achieve stable control of the multiphase structure and rapid online toughening. This avoids the brittle fracture caused by bainite during coiling, transportation, and drawing, promotes efficient and stable production of hot-rolled wire rod, controls production energy consumption, and has good industrial adaptability.

[0036] (2) In view of the current situation that existing pearlitic hot-rolled wire rods for stranded wire have low work hardening rate, high alloy content, insufficient strength and plasticity, large fluctuations in mechanical properties, or require offline heat treatment, this invention can reduce alloy content, simplify alloy composition system, and appropriately reduce material cost. The microstructure includes a multiphase structure composed of tempered bainite and tempered sorbite, effectively suppressing abnormal structures such as network carbides, coarse pearlite and martensite. It can utilize the strong and tough tempered bainite, combined with the uniform distribution of precipitated nano-sized Cr and Nb carbides, to improve the quality of wire. With high hardening rate and rapid strength enhancement, it compensates for the strength loss caused by reducing alloy content and eliminating alloying elements. At the same time, the tempering process improves the stability and plasticity of the microstructure, achieving a tensile strength of 1522~1572MPa, a reduction of area of ​​29%~34%, and a mechanical property difference of ≤49MPa between coils. It is used in applications such as manufacturing 2230MPa grade stranded wire to eliminate the need for offline heat treatment, reduce subsequent drawing passes, and lower the risk and cost of wire breakage in stranded wire manufacturing. It has good market application prospects. Attached Figure Description

[0037] 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:

[0038] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention;

[0039] Figure 2 This is a metallographic diagram of Embodiment 2 of the present invention;

[0040] Figure 3 This is a metallographic diagram of Embodiment 3 of the present invention. Detailed Implementation

[0041] 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.

[0042] Example 1:

[0043] A preferred embodiment of the manufacturing method of the 2230MPa grade high-strength multiphase hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.87%, Si: 0.4%, Mn: 0.61%, Cr: 0.57%, Nb: 0.042%, P: 0.014%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0044] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic, promoting the homogenization of the billet composition and avoiding grain coarsening. After exiting the furnace, the billet is rolled into wire rod with a diameter of 10mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce Nb precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1233℃, the furnace time is 140min, and the initial rolling... The temperature is 1085℃, the initial rolling reduction is 43.5%, the final rolling temperature is 940℃, and the final rolling reduction is 27%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing microstructure preparation for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 930℃.

[0045] The online molten salt mixing and quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 35°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, improving... The molten salt temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the dispersed precipitation of Cr and Nb carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 499℃, the treatment time is 22s, the molten salt circulation rate is 350t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 582℃, the treatment time is 120s, the molten salt circulation rate is 250t / h, and the molten salt temperature rise is ≤3℃.

[0046] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥230℃) from above the two salt bath tanks undergoing online molten salt mixing and quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 272℃ at a slow cooling rate of 0.65℃ / s. The coiling process uses a coiling drum to coil the wire rod into coils, which are then packaged and stored to obtain the finished hot-rolled wire rod. Its metallographic structure is shown in the figure below. Figure 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 250 min, an initial rolling temperature of 1000°C, a final rolling temperature of 805°C, and a wire drawing temperature of 800°C. The Steyrmo forced air cooling uses a fan with an air volume of 260,000 m³ / s. 3 At 6:00, fans 1 to 5 are turned on at 65% to cool the wire rod to 674℃ at a cooling rate of 6.6℃ / s. Then, fans 6 to 14 are turned on at 30% to cool the wire rod to 270℃ at a cooling rate of 2.8℃ / 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 1150°C, the furnace time is 240 min, the initial rolling temperature is 1000°C, the final rolling temperature is 810°C, and the wire drawing temperature is 805°C. During the molten salt treatment before the online molten salt mixing and quenching isothermal process, the wire rod is cooled at a rate of 29°C / s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0051] Example 2:

[0052] A preferred embodiment of the manufacturing method of the high-strength multiphase hot-rolled wire rod for 2230MPa 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: 0.60%, Mn: 0.69%, Cr: 0.38%, Nb: 0.035%, P: 0.014%, S: 0.012%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0053] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic, promoting the homogenization of the billet composition and avoiding grain coarsening. After exiting the furnace, the billet is rolled into wire rod with a diameter of 11mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce Nb precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1218℃, and the furnace time is 170min. The rolling temperature is 1070℃, the initial rolling reduction is 40%, the final rolling temperature is 930℃, and the final rolling reduction is 26%. The wire drawing process is used to convert the wire from the rolling line into wire rod through the wire drawing mechanism. The wire rod is distributed on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the wire rod in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing microstructure preparation for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 915℃.

[0054] The online molten salt mixing and quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 36°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, improving... The molten salt temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the dispersed precipitation of Cr and Nb carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 481℃, the treatment time is 31s, the molten salt circulation rate is 395t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 573℃, the treatment time is 145s, the molten salt circulation rate is 295t / h, and the molten salt temperature rise is ≤3℃.

[0055] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥230℃) from above the two sections of the salt bath tank undergoing online molten salt mixing and quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 275℃ at a slow cooling rate of 0.55℃ / 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.

[0056] Comparative Example 3:

[0057] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the online molten salt mixing and quenching isothermal process, the wire rod is cooled at a cooling rate of 40℃ / s, the molten salt temperature of the initial molten salt treatment is 440℃, the treatment time is 45s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0058] Comparative Example 4:

[0059] A method for manufacturing hot-rolled wire rod differs from that in Example 2 in that: during the initial molten salt treatment of the online molten salt mixing and quenching isothermal process, the wire rod is cooled at a cooling rate of 30℃ / s, the molten salt temperature of the initial molten salt treatment is 525℃, the treatment time is 18s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0060] Example 3:

[0061] A preferred embodiment of the manufacturing method of the 2230MPa grade high-strength multiphase hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.90%, Si: 0.53%, Mn: 0.81%, Cr: 0.41%, Nb: 0.033%, P: 0.013%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0062] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic, promoting the homogenization of the billet composition and avoiding grain coarsening. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 13mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce Nb precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1195℃, the furnace time is 200min, and the initial rolling... The temperature is 1045℃, the initial rolling reduction is 38%, the final rolling temperature is 915℃, and the final rolling reduction is 24.5%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing microstructure preparation for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 905℃.

[0063] The online molten salt mixing and quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 37°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for final molten salt treatment, improving... The molten salt temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the dispersed precipitation of Cr and Nb carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 463℃, the treatment time is 46s, the molten salt circulation rate is 460t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 561℃, the treatment time is 160s, the molten salt circulation rate is 385t / h, and the molten salt temperature rise is ≤3℃.

[0064] The roller conveyor slow cooling process employs controlled opening of the insulation cover to blow hot air (≥230℃) from above the two salt bath tanks undergoing online molten salt mixing and quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, promoting further toughening of the wire rod structure and improving the tempering softening effect. Specifically, the wire rod is cooled to 276℃ at a slow cooling rate of 0.45℃ / 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.

[0065] Comparative Example 5: A method for manufacturing hot-rolled wire rod, the difference between the method and Example 3 is that the molten salt temperature of the subsequent molten salt treatment is 590°C, the treatment time is 220s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0066] Comparative Example 6: A method for manufacturing hot-rolled wire rod, the method of which differs from that of Example 3 in that: the molten salt temperature of the subsequent molten salt treatment is 520°C, the treatment time is 100s, and the finished hot-rolled wire rod is obtained after the process is completed.

[0067] Example 4:

[0068] A preferred embodiment of the manufacturing method of the 2230MPa grade high-strength multiphase hot-rolled wire rod for stranded wire according to the present invention, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.91%, Si: 0.47%, Mn: 0.77%, Cr: 0.54%, Nb: 0.022%, P: 0.013%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → wire drawing → online molten salt mixing quenching isothermal → slow cooling on roller table → coiling, specifically:

[0069] The rolling process is used to heat a 220mm×220mm steel billet in a furnace to a high-temperature steel billet that is rollable and plastic, promoting the homogenization of the billet composition and avoiding grain coarsening. After exiting the furnace, the billet is rolled into a wire rod with a diameter of 15mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and induce Nb precipitation to refine the grains. Specifically, the furnace soaking temperature is controlled at 1183℃, and the furnace time is 230 minutes. The rolling temperature is 1032℃, the initial rolling reduction is 37%, the final rolling temperature is 900℃, and the final rolling reduction is 22%. The wire drawing process is used to convert the wire from the rolling line into coils through the wire drawing mechanism. The coils are spread on the roller table and conveyed along the roller table. A higher wire drawing temperature is selected to keep the coils in a high-temperature austenitizing state, suppressing network carbides or proeutectoid ferrite, and providing microstructure preparation for the subsequent nucleation and refinement of bainite and sorbite. Specifically, the wire drawing temperature is controlled at 890℃.

[0070] The online molten salt mixing and quenching isothermal process employs a two-section salt bath with internal molten salt. After coiling, the wire rod is conveyed via roller conveyor through the first section of the salt bath for initial molten salt treatment, causing the wire rod to cool at a rate of 38°C / s. This rapidly transitions the wire rod from a high-temperature austenitic state, bypassing the network carbide region, into the bainite and pearlite mixed phase region, inhibiting the formation of network carbides and coarse pearlite, and promoting the transformation of some austenite into bainite and sorbite. The wire rod is then conveyed via roller conveyor through the second section of the salt bath for subsequent molten salt treatment, improving... The molten salt temperature promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, and promotes the dispersed precipitation of Cr and Nb carbides to regulate the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the first stage of molten salt treatment is 459℃, the treatment time is 52s, the molten salt circulation rate is 500t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the second stage of molten salt treatment is 552℃, the treatment time is 180s, the molten salt circulation rate is 450t / h, and the molten salt temperature rise is ≤3℃.

[0071] The roller conveyor slow cooling process involves adjusting the opening of the insulation cover to blow hot air (≥230℃) from the two salt bath tanks undergoing online molten salt mixing and quenching isothermal treatment into the insulation cover. The wire rod is then conveyed by the conveyor rollers through the insulation cover for slow cooling, which promotes further toughening of the wire rod structure and improves the tempering and softening effect. Specifically, the wire rod is cooled to 279℃ at a slow cooling rate of 0.35℃ / s. The coiling process is used to coil the wire rod into coils using a coiling drum. After packaging and warehousing, the hot-rolled wire rod is obtained as a finished product.

[0072] Comparative Example 7:

[0073] 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 mixing quenching isothermal → air cooling. The air cooling is achieved by opening the insulation cover, conveying the wire rod via a conveyor roller, and controlling the wire rod to cool to 272°C at a slow cooling rate of 1.5°C / s. After the wire rod is removed from the production line, the finished hot-rolled wire rod is obtained.

[0074] 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:

[0075] Table 1. Comparison of microstructure and properties of hot-rolled wire rods with different compositions and manufacturing methods

[0076]

[0077] The comparison between Example 1 and Comparative Example 1 shows that, compared to the high-level network carbides, uncontrollable martensitic abnormal structure, and pearlite core of the wire rod produced on the Stellmore air-cooled line, resulting in insufficient strength and plasticity and large fluctuations in mechanical properties, this invention uses a high-carbon composition design containing trace amounts of Nb combined with online molten salt mixing quenching isothermal technology. This allows the wire rod to undergo a preliminary molten salt treatment and rapidly transition from the high-temperature austenitic state to the bainite and pearlite mixed phase region. This can suppress network carbides and coarse pearlite, and promote the transformation of some austenite into bainite and sorbite. The transformation, followed by subsequent molten salt treatment, promotes the decomposition of untransformed residual austenite into sorbite and isothermal tempering, resulting in dispersed precipitation of carbides. This avoids the risk of martensite precipitation, achieves stable control of the multiphase structure and online toughening, and improves the strength-plasticity matching and hardening capacity of the wire rod. As can be seen from the results of Examples 1-4, hot-rolled wire rod can achieve a tensile strength of 1522-1572 MPa, a reduction of area of ​​29%-34%, and a mechanical property difference of ≤49 MPa within the same coil. This eliminates the need for offline heat treatment and reduces the risk and cost of wire breakage in strand manufacturing.

[0078] As can be seen from the comparison results of Example 1 and Comparative Example 2, selecting a higher wire drawing temperature can promote the homogenization of austenite and avoid the precipitation of network carbides or proeutectoid ferrite due to the wire drawing temperature being too low. At the same time, it connects the cooling rate and the phase transformation range, which prepares the microstructure for increasing the degree of undercooling and promoting the refinement of bainite and sorbite phase transformation.

[0079] As can be seen from the comparison results between Example 2 and Comparative Example 3, the lower the molten salt temperature in the first stage of molten salt treatment, the higher the cooling rate, the more it inhibits the long-distance diffusion of network carbides and carbon, the more it inhibits the formation of coarse pearlite, and the more it promotes the formation of fine bainite and sorbite structures in the wire rod under rapid cooling. With the extension of the treatment time, the bainite nucleation rate can be increased, the dislocation density and matrix strength can be increased. However, if the molten salt temperature is too low and the treatment time is too long, the shear transformation from austenite to bainite will dominate, which will increase the bainite transformation amount, matrix dislocation density and thermal stress in the microstructure, thereby increasing the softening difficulty of the subsequent molten salt treatment and affecting production energy consumption and efficiency.

[0080] As can be seen from the comparison results between Example 2 and Comparative Example 4, the higher the molten salt temperature in the initial molten salt treatment, the more the carbon diffusion ability is appropriately enhanced, which is conducive to uniform phase transformation and reducing the temperature gradient between the wire rod surface and the core. The shorter the treatment time, the more beneficial it is to reduce dislocation density and thermal stress and reduce the difficulty of subsequent isothermal tempering. However, if the molten salt temperature is too high and the treatment time is too short, it is not conducive to suppressing the precipitation of network carbides, the refinement of the microstructure and lamellar structure, and affects the nucleation of bainite, resulting in insufficient matrix strength and increased fluctuations in mechanical properties.

[0081] 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 the diffusion ability of carbon and the better the decomposition of residual austenite into sorbite. With the extension of treatment time, it can promote the full transformation of the microstructure and reduce the risk of forming brittle microstructure during subsequent cooling. At the same time, it can reduce the temperature gradient inside and outside the wire rod, promote the full precipitation and uniform dispersion of Cr and Nb alloy carbides on the matrix, so as to release the microstructure stress, improve the brittleness of bainite, and improve the microstructure uniformity and ductility. However, if the molten salt temperature is too high, it is not conducive to suppressing the pearlite in the core and controlling the diffuse precipitation of alloy carbides. With the longer the treatment time, the carbides tend to coarsen, which will further reduce the strength and ductility.

[0082] As can be seen from the comparison results of Example 3 and Comparative Example 6, the lower the molten salt temperature in the later stage of molten salt treatment, the better it is to promote the transformation of retained austenite into sorbite with finer lamellar spacing, increase the precipitation driving force of alloy carbides, pin dislocations and strengthen grain boundaries, and improve matrix strength. With shorter treatment time, matrix strength can be maintained and production energy consumption can be reduced. However, if the molten salt temperature is too low, it is not conducive to the rapid decomposition of retained austenite. If the treatment time is too short, the thermodynamic force is too low, and the structural stress will not be fully released in time, which will lead to a large loss of plasticity. If alloy carbides are not fully precipitated, the strength and plasticity properties will be further lost. The risk of abnormal phase transformation of untransformed retained austenite during subsequent cooling increases, resulting in fluctuations in mechanical properties.

[0083] As can be seen from the comparison results of Example 4 and Comparative Example 7, the slow cooling of the roller can further control the slow cooling speed of the wire rod, avoid insufficient plasticity of the wire rod due to insufficient slow cooling, and at the same time, slow cooling promotes further toughening of the wire rod structure and improves the tempering and softening effect of the wire rod.

[0084] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of manufacturing high-strength multiphase hot-rolled wire rod for 2230 MPa grade strand, characterized by, The manufacturing method comprises the following steps: The chemical composition of the hot-rolled wire rod comprises the following components in percentage by mass: C: 0.87% to 0.91%, Si: 0.40% to 0.60%, Mn: 0.61% to 0.81%, Cr: 0.38% to 0.57%, Nb: 0.022% to 0.042%, P: ≤0.014%, S: ≤0.014%, and the rest is Fe and inevitable impurities; after the wire rod is drawn at a drawing temperature of ≥890 ℃, the wire rod is subjected to on-line molten salt mixed quenching and isothermal treatment, so that the wire rod is subjected to a front-stage molten salt treatment, and is cooled at a cooling rate of ≥35 ℃ / s, and is transferred from an austenite state to a mixed phase region of bainite and pearlite, so as to promote the transformation of part of the austenite into bainite and sorbite, and then is subjected to a rear-stage molten salt treatment, the molten salt temperature is increased, and the residual untransformed austenite is promoted to decompose and transform into sorbite and is subjected to isothermal tempering, and finally is subjected to roller slow cooling, so as to be manufactured into a hot-rolled wire rod with a microstructure comprising a complex phase structure of tempered bainite and tempered sorbite; the molten salt temperature of the front-stage molten salt treatment is 459 to 499 ℃, and the treatment time is 22 to 52 s; the molten salt temperature of the rear-stage molten salt treatment is 552 to 582 ℃, and the treatment time is 120 to 180 s.

2. The process of producing high strength complex phase hot rolled wire rod for 2230 MPa grade strand as claimed in claim 1, wherein, Before the rolling, the soaking temperature of the heating furnace is controlled to be 1183 to 1233 ℃, and the furnace time is 140 to 230 min.

3. The process of producing high strength complex phase hot rolled wire rod for 2230 MPa grade strand as claimed in claim 1, wherein, During the rolling, the initial rolling temperature is controlled to be 1032 to 1085 ℃, the initial rolling reduction is 37% to 43.5%, the final rolling temperature is 900 to 940 ℃, and the final rolling reduction is 22% to 27%.

4. The process of producing high strength complex phase hot rolled wire rod for 2230 MPa grade strand as claimed in claim 1, wherein, During the drawing, the drawing temperature is controlled to be 890 to 930 ℃.

5. The process of producing high strength complex phase hot rolled wire rod for 2230 MPa grade strand as claimed in claim 1, wherein, The molten salt circulation amount of the front-stage molten salt treatment is 350 to 500 t / h, and the molten salt temperature rise is ≤8 ℃; the molten salt circulation amount of the rear-stage molten salt treatment is 250 to 450 t / h, and the molten salt temperature rise is ≤3 ℃.

6. The process of producing high strength complex phase hot rolled wire rod for 2230 MPa grade strand wire as claimed in claim 1, wherein, The roller slow cooling controls the wire rod to be cooled to below 280 ℃ at a slow cooling speed of ≤0.65 ℃ / s, and is coiled.

7. A high strength multiphase hot rolled wire rod for 2230 MPa grade strand, characterized in that, The hot-rolled wire rod is manufactured by the manufacturing method of the high-strength complex phase hot-rolled wire rod for 2230 MPa grade twisted wire according to any one of claims 1 to 6.

8. The high strength complex phase hot rolled wire rod for 2230 MPa grade strand as claimed in claim 7, wherein, The volume percentage of the tempered bainite is 13% to 22%, the volume percentage of the tempered sorbite is 78% to 87%, the interlamellar spacing of the tempered sorbite is 70 to 120 nm, the reticular carbide level of the hot-rolled wire rod is 0 level, and the mechanical property difference between turns is ≤49 MPa.

9. The high strength complex phase hot rolled wire rod for 2230 MPa grade strand as claimed in claim 7, wherein, The diameter of the hot-rolled wire rod is 10.0 to 15.0 mm, the tensile strength is 1522 to 1572 MPa, and the reduction of area is 29% to 34%.

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