A 1860MPa grade hot-rolled wire rod for bridge cables and a manufacturing method thereof

Abstract

The invention relates to a hot-rolled wire rod for 1860MPa-grade bridge cables and a manufacturing method thereof. After rolling and spinning into a wire rod with C-Si-Mn-Cr components, the wire rod is subjected to an online molten salt isothermal treatment to cool the wire rod at a cooling rate of ≥35°C / s, enter a sorbite phase region from an austenite state, promote sorbite phase transformation, promote isothermal tempering of the organization, and fuse the sorbite lamellae. Finally, the hot-rolled wire rod is subjected to roller slow cooling to obtain a microstructure including a mixed structure consisting of tempered sorbite with a volume percentage of ≥91% and the rest being ferrite and fused sorbite. The hot-rolled wire rod can achieve the strength-plasticity matching of the small-size hot-rolled wire rod, reach a tensile strength of 1250-1300MPa, a cross-sectional shrinkage of 42%-47%, and a mechanical property circle difference of ≤35MPa. The hot-rolled wire rod is used for manufacturing 1860MPa-grade bridge cables and other application fields, and can improve the risk of wire breakage caused by network carbides and abnormal organizations.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled wire rods, and in particular relates to a 1860MPa-grade hot-rolled wire rod for bridge cables and a manufacturing method thereof. Background Art

[0002] As cable-stayed bridges, suspension bridges and cable-stayed bridges can reduce building height and bridge weight, and have a stronger spanning capacity than beam bridges. The higher the strength grade of bridge cables, the lighter the bridge can be, and the shorter the construction period of bridge projects. 1860MPa-grade bridge cables are the main types of bridge cables. They use hot-rolled wire rods as the base material. After pickling, wire drawing, galvanizing and other processes, the strength of the steel wire is improved as the wire drawing reduction rate increases. The steel wire will have a certain strength loss during the galvanizing process, and then it is torsion processed into a finished product. Therefore, the base material of the hot-rolled wire rod for bridge cables determines its strength grade. In order to improve the strength grade of the base material of the hot-rolled wire rod, the 1860MPa-grade hot-rolled wire rod for bridge cables in the prior art is often developed by adding strengthening elements such as C, Cr, and Mn, combined with the post-rolling Stelmor air-cooling and line-controlled cooling process. The following technical difficulties exist in the manufacture of hot-rolled wire rods with small specifications, strong plasticity matching and lower wire breakage risk:

[0003] 1. Due to the insufficient cooling capacity of the hot-rolled wire rod production line, in order to improve the tensile strength and drawing performance of the wire rod by refining the pearlite lamellar spacing at a lower phase transition temperature, for example: Patent CN112501506B discloses a steel wire rod for bridge cables. The addition of strengthening elements such as C and Mn in the wire rod is relatively large, which will aggravate the difficulty of segregation control during continuous casting, resulting in an increased risk of precipitation of network carbides and abnormal tissues, causing increased fluctuations in the mechanical properties of the wire rod, and increasing the risk of wire breakage during the wire making process; and in order to reduce the impact of segregation, after reducing the C content, for example: Patent CN116590624A discloses a wire rod for hot-dip galvanized steel wire for bridge cables, using The C-Si-Mn-Cr-V composition design is combined with the fast-then-slow process after spinning to obtain wire rod. However, on the one hand, the addition of V and other components will increase the material cost. For the purpose of reducing costs, omitting the V alloy will lead to a large loss of strength. It is necessary to increase the area reduction rate to meet the torsion processing after galvanizing. The plasticity loss in the process will bring the risk of wire breakage. On the other hand, when the carbon content exceeds 0.77%, there will still be segregation. When the air cooling rate is increased, the wire rod still takes a long time to pass through the network carbide precipitation temperature range, and the improvement of the network carbide is limited. It will have an adverse effect on the mechanical properties of the wire rod, especially the plasticity and organizational uniformity of the wire rod, and then cause wire breakage during drawing.

[0004] Second, in order to minimize the precipitation level of network carbides and promote the nucleation of sorbite in wire rods at a faster cooling rate, the cooling rate of wire rods is accelerated by increasing the air volume of the fan after rolling. However, in addition to limited improvement of network carbides, on the one hand, the increase in the air volume of the fan increases the uncontrollability of wind speed and wind temperature, and the temperature difference between the wind side and the leeward side of the wire rod increases. Accompanied by the high hardenability and segregation of the wire rod, some austenite structures that are cooled too quickly are prone to phase transformation to form abnormal martensite and bainite structures that are unfavorable to the drawing performance. The abnormal structure will lead to a rapid increase in the risk of wire breakage in the subsequent wire making process; on the other hand, the maximum cooling rate of the wire rod during the air cooling process is 10℃ / s, and the cooling capacity is insufficient. When the spinning temperature is increased, the wire rod will be online for a long time, thereby affecting the processing efficiency. The insufficient temperature gradient will coarsen the sorbite lamellae of the wire rod and lose the strength of the wire rod.

[0005] 3. Although in order to maximize the troostite content in the organization, the cooling rate of the wire rod is reduced by increasing the spinning temperature, reducing the air volume during phase transformation, turning off the fan or adding a heat preservation cover to extend the phase transformation time, on the one hand, the phase transformation time of the wire rod is still limited after continuous cooling, which will limit the troostite content in the organization and cause a certain strength loss; on the other hand, due to the coarse pearlite or troostite lamellae, with the continuous cooling phase transformation, the organization is already in a low temperature state after incubation, which will comprehensively hinder the softening of the troostite organization, resulting in large stress and dislocation density of the obtained troostite organization, resulting in insufficient final plasticity of the wire rod. In the wire drawing process of small-sized hot-rolled wire rods, as the drawing hardening increases, the risk of wire breakage during drawing and torsion increases. Summary of the invention

[0006] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 1860MPa grade hot-rolled wire rod for bridge cables and a manufacturing method thereof, which can achieve the strength and plasticity matching of small-size hot-rolled wire rods and can be used for the manufacture of 1860MPa grade bridge cables, and can improve the risk of wire breakage caused by network carbides and abnormal tissues.

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

[0008] A method for manufacturing a 1860MPa grade hot-rolled wire rod for bridge cables, the manufacturing method comprising:

[0009] The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.80%~0.84%, Si: 0.30%~0.40%, Mn: 0.45%~0.52%, Cr: 0.32%~0.43%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥900°C, it is subjected to an online molten salt isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥35°C / s, and enters the sorbite phase region from the austenite state, promotes the sorbite phase transformation, promotes isothermal tempering of the organization, and melts the sorbite lamellae, and finally passes through a roller to slowly cool, so as to be a hot-rolled wire rod with a microstructure including tempered sorbite with a volume percentage of ≥91%, and the rest is a mixed organization composed of ferrite and melted sorbite.

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

[0011] (1) Carbon: Carbon is the main strengthening element in steel. It can improve the tensile strength of wire rod by solid solution strengthening and precipitation of compounds with carbide-forming elements. However, if the carbon content is too high, it will make it more difficult to control the central segregation, decarburization and network carbides of high-carbon steel alloy billets. Therefore, in order to take into account the high strength requirements of 1860MPa bridge cables and reduce the difficulty of controlling the uniformity of the structure, it is more conducive to refining the troostite lamellae during online molten salt isothermal treatment to reduce the softening difficulty. The carbon content is appropriately reduced, and the mass percentage of C is controlled to be 0.80%~0.84%.

[0012] (2) Silicon: The Si element is often used as a deoxidizer during the smelting process. It has a strong solid solution strengthening ability, which is beneficial to obtaining a finer troostite structure. At the same time, it can inhibit the coarsening of cementite and accelerate the diffusion rate of carbon, which is beneficial to promoting the melting of troostite, improving the stability of cementite, and reducing the strength loss in the subsequent hot-dip galvanizing process. However, too much silicon will promote C desolvation and prolong the phase change time, which is not conducive to rapid phase change and tempering in the online molten salt isothermal process. Therefore, the mass percentage of Si is controlled to be 0.30%~0.40%.

[0013] (3) Manganese: Mn can be added as a deoxidizer during the steelmaking process. It can be dissolved in austenite to strengthen the matrix, increase the stability of austenite and the hardenability of wire rod, reduce the phase transformation temperature, and promote the refinement of the interlamellar spacing of the sorbite structure to improve the matrix strength. However, when the Mn content is too high, it will increase the central alloy element segregation of the steel billet, and it is easy to form a low-temperature martensite structure during the cooling process, which increases the risk of abnormal tissue precipitation and material cost. At the same time, it reduces the diffusion activity of carbon, which is not conducive to the online molten salt isothermal promotion of sorbite rapid melting and tempering softening, thereby reducing the plasticity of the wire rod. Therefore, in order to take into account the high strength and plasticity of the hot-rolled wire rod and reduce the difficulty of controlling the uniformity of the structure, it is chosen to reduce the Mn content. The mass percentage of Mn is controlled to be 0.45%~0.52%.

[0014] (4) Chromium: Cr is a medium-strong carbide-forming element in high carbon steel. It can improve the stability of austenite, prevent the growth of grains during rolling, and shift the continuous cooling transformation curve of steel to the right. At the same cooling rate, it can refine the spacing between troostite lamellae, thereby compensating for the strength loss of wire rod while reducing the Mn content, improving the work hardening rate of wire rod during wire drawing, and reducing the strength loss during subsequent hot-dip galvanizing of steel wire. However, high Cr content will aggravate segregation, increase the risk of abnormal microstructure precipitation and material cost, reduce the activity of carbon in steel, and be unfavorable for the rapid melting and tempering softening of troostite, resulting in increased difficulty in improving plasticity. Therefore, the Cr content is appropriately increased, and the mass percentage of Cr is controlled to be 0.32%~0.43%.

[0015] (5) Phosphorus and sulfur: P and S are impurity elements. The lower the better. Therefore, P is controlled to be ≤ 0.015% and S is ≤ 0.015%.

[0016] The hot-rolled wire rod adopts C-Si-Mn-Cr composition design, with relatively low C and Mn contents to reduce the influence of alloy element segregation in the billet, and does not contain alloy components such as V and Ni, so the material cost is relatively low. The wire rod strength and the peak precipitation temperature of sorbite are regulated by combining appropriate amounts of Si and Cr, which provides favorable conditions for controlling network carbides and abnormal structures, refining the spacing between sorbite layers, and reducing the difficulty of short-time tempering toughening. Furthermore, a higher spinning temperature is selected in the manufacturing method to avoid the precipitation of network carbides in the spinning stage, and at the same time, austenitizing the structure provides favorable conditions for the subsequent formation of a large temperature gradient and thus promoting the rapid nucleation and refinement of sorbite. After spinning, online molten salt isothermal treatment is directly adopted:

[0017] 1. Compared with the limited cooling capacity of the Stelmor air-cooled line, the heat exchange capacity of the molten salt is stronger, which can quickly cool the wire rod from the spinning temperature. On the one hand, it can effectively reduce the time for the wire rod to pass through the secondary cementite and tertiary cementite precipitation temperature range of 700~800℃, combined with the reduction of segregation, thereby avoiding the formation of network carbides that degrade the uniformity and plasticity of the organization, and improving the utilization value and organizational uniformity of carbon. On the other hand, through the composition design, the phase transition temperature of the wire rod is appropriately reduced. Combined with a faster cooling rate, the wire rod can quickly enter the sorbite phase region from the high-temperature austenite state, forming a greater degree of supercooling, thereby promoting the rapid nucleation of the sorbite organization, the refinement of the sorbite organization lamellae, and then improve the matrix strength, providing favorable conditions for the melting and toughening of the sorbite lamellae, and at the same time, there is no need to consider the adverse effects of high-temperature spinning that will excessively extend the online time and thus lose processing efficiency.

[0018] 2. Compared with the Stelmor air-cooled line, the risk of abnormal tissue precipitation will be aggravated by increasing the air volume. When the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for more uniform and rapid heat exchange, and there is no problem of windward side and leeward side. As the segregation decreases, the wire rod can maintain the same temperature as the molten salt after entering the troostite phase region, thereby avoiding the wire rod from entering the bainite or martensite phase region due to excessive cooling at some positions, and forming hard and brittle phase martensite or bainite with degraded drawing performance. While effectively avoiding abnormal tissue, the temperature difference from the edge to the core of the wire rod cross section is smaller, which can form a troostite structure dominated by fine lamellar spacing, thereby improving the uniformity of the structure and further reducing the fluctuation of mechanical properties in the same circle.

[0019] 3. Compared with the Stelmore air cooling line, which has limited sorbite incubation time and excessive loss of temperature due to continuous cooling, the wire rod maintains an isothermal state for a longer time during the online molten salt isotherm, rather than continuous cooling, which promotes the refinement of the sorbite lamellae while maintaining the wire rod at the sorbite peak precipitation temperature, promoting the full transformation of high-temperature austenite to sorbite, thereby reducing the ferrite content, combining the dispersion precipitation of Cr to improve the matrix strength, making up for the strength loss caused by reducing C and Mn, and avoiding the formation of abnormal organization by austenite residues during the subsequent roller slow cooling process. On the other hand, the refinement of the sorbite lamellae can reduce Difficulty of melting. When the wire rod maintains the same temperature as the molten salt in the troostite phase region, the temperature of the troostite phase region is higher than that of continuous cooling or the bainite phase region, which can keep the wire rod in a high temperature state for a longer time and provide more thermal power. Then, under short-time high-temperature isothermal treatment, the troostite and ferrite can be tempered, the troostite can be melted, the matrix can be strengthened and toughened, and the high strength and plasticity matching of the wire rod can be improved. It can also be quickly produced and the production capacity can be increased through short-time treatment. Finally, after slow cooling on the roller, the wire rod can be slowly cooled from the troostite phase region, continuing the toughening and softening effect of the later stage of online molten salt isothermal treatment, thereby realizing the organization and strength and plasticity regulation of the wire rod.

[0020] Since the spinning temperature is relatively high, there is no need to reduce the rolling temperature excessively. Before the rolling, a higher heating furnace soaking temperature can be selected to promote the homogenization of the components, and the furnace time can be controlled to be relatively short to avoid decarburization and burning. In a preferred embodiment, before the rolling, the heating furnace soaking temperature is controlled to be 1125-1180°C, and the furnace time is 2-4h.

[0021] Due to the high spinning temperature, a higher initial rolling temperature can be selected to reduce the deformation resistance of the steel billet and the rolling line load to improve the rolling speed and efficiency. In a preferred embodiment, during the rolling, the initial rolling temperature is controlled to be 1095~1135℃, and the initial rolling reduction is 29%~34%.

[0022] During rolling, appropriate rolling temperature and deformation are selected to promote dynamic recrystallization and grain refinement during the final rolling process to improve the matrix strength. In a preferred embodiment, during the rolling, the final rolling temperature is controlled to be ≥920°C and the final rolling reduction is 25%~32%.

[0023] In a preferred embodiment, the molten salt temperature of the online molten salt isothermal treatment is 505-540°C, and the treatment time is 40-110s. The molten salt temperature is in the bainite phase region of the hot-rolled wire rod. The lower the molten salt temperature, the more conducive it is to promoting the nucleation of the bainite structure with fine lamellar spacing, which is beneficial to improving the strength of the wire rod. However, if the molten salt temperature is too low, the thermal power that can be provided is reduced, which affects the short-term tempering softening effect of the structure, is detrimental to the plasticity of the wire rod, and even enters the bainite phase region to produce abnormal structure; on the contrary, the higher the molten salt temperature, the more it can promote the melting of the bainite lamellae, improve the softening effect and the plasticity of the wire rod, but if the molten salt temperature is too high, it will affect the cooling rate of the wire rod, which is not conducive to controlling the network carbide and forming the bainite structure with fine lamellar spacing as the main component. The sorbite structure is unfavorable, and the formation of coarse pearlite will lose the strength of the wire rod, increase the difficulty of softening the wire rod, and cause the loss of strength and plasticity; the longer the treatment time, the more complete the transformation of the sorbite structure, the better the tempering and softening effect of sorbite and ferrite, and the plasticity of the wire rod increases. However, if the treatment time is too long, the wire rod will lose strength due to excessive softening, and the precipitation and coarsening of Cr will lose strength and plasticity; conversely, the shorter the treatment time, the lower the toughening and softening effect, and the plasticity of the wire rod decreases. If the treatment time is too short, the sorbite of the wire rod is not fully inoculated and the softening is insufficient, which will increase the fluctuation of mechanical properties and affect the plasticity of the wire rod. Therefore, the molten salt temperature and treatment time of the online molten salt isothermal treatment can be further controlled to regulate the organizational state and strength and plasticity matching.

[0024] In a preferred embodiment, the online molten salt isothermal treatment is divided into a front-stage treatment and a back-stage treatment. The molten salt circulation volume of the front-stage treatment is greater than that of the back-stage treatment. The processing time of the front-stage treatment is 10-20s, and the processing time of the back-stage treatment is 20-90s. Since the wire rod enters the sorbite phase region from the high-temperature austenite state during the front-stage treatment, the temperature difference is large. Selecting a larger molten salt circulation volume can promote the wire rod to form a sorbite structure dominated by fine lamellar spacing. When the back-stage treatment is carried out, the untransformed high-temperature austenite in the wire rod is controlled to be transformed into sorbite and a small amount of ferrite, and the structure is short-term tempered to improve the strength-plasticity matching of the wire rod, reduce the difficulty of controlling the molten salt temperature rise, and select a lower molten salt circulation volume to reduce production energy consumption.

[0025] In a preferred embodiment, the molten salt circulation rate of the front-stage treatment is 550-680 t / h, and the molten salt temperature rise is ≤10°C.

[0026] In a preferred embodiment, the molten salt circulation rate of the post-processing is 350-460 t / h, and the molten salt temperature rise is ≤4°C.

[0027] In a preferred embodiment, the roller slow cooling controls the wire rod to be cooled to below 350°C at a slow cooling rate of ≤1.0°C / s for coiling, thereby avoiding the reduction of plasticity of the wire rod due to rapid cooling. At the same time, slow cooling promotes further toughening of the wire rod structure and improves the softening effect of the wire rod.

[0028] A 1860MPa grade hot-rolled wire rod for bridge cables, the hot-rolled wire rod is manufactured by any one of the above-mentioned methods for manufacturing 1860MPa grade hot-rolled wire rod for bridge cables.

[0029] The composition of the hot-rolled wire rod is relatively simple, which can reduce the influence of segregation. At the same time, it is different from the traditional wire rod structure of bridge cable with troostite + ferrite. The microstructure of the hot-rolled wire rod includes a high content of tempered troostite, a small amount of ferrite and melted troostite. The interlamellar spacing of troostite is finer than that of pearlite, which can improve the matrix strength and drawing performance. Troostite and ferrite are transformed into tempered troostite and ferrite after short-time tempering. After part of the troostite lamellae are melted and transformed into melted troostite, the carbides in the tempered troostite are more dispersed, which can further reduce the organization stress. force and dislocation density, improve organizational plasticity, increase the proportion of tempered troostite and lamellar troostite, reduce the proportion of ferrite, use short-time tempering to avoid the loss of wire rod strength, improve the strengthening effect of carbon elements, make up for the strength loss caused by reducing the C and Mn content, use the tempered state to regulate the toughening of the wire rod organization, obtain better comprehensive mechanical properties, do not contain abnormal organizations such as martensite and bainite, improve the overall strength and plasticity of the wire rod, which is beneficial to be used as a base material to meet the performance level of 1860-grade bridge cables, while reducing the risk of wire breakage during wire drawing and torsion.

[0030] The finer the tempered troostite lamellae spacing, the higher the wire rod strength. However, as the molten salt temperature decreases, the short-time tempering softening effect of the wire rod decreases, causing the wire rod plasticity loss. In a preferred embodiment, the tempered troostite lamellae spacing is 70~125nm.

[0031] In a preferred embodiment, the network carbide level of the hot-rolled wire rod is level 0, which can effectively avoid the adverse effects of the network carbide on the mechanical properties of the wire rod.

[0032] The hot-rolled wire rod can effectively avoid the risk of abnormal tissues such as network carbides and martensite produced by the C element, improve tissue uniformity, and thus reduce fluctuations in mechanical properties. In a preferred embodiment, the same-circle difference in mechanical properties of the hot-rolled wire rod is ≤35MPa.

[0033] In a preferred embodiment, the diameter of the hot-rolled wire rod is 7.0~13.0mm, the tensile strength is 1250~1300MPa, and the cross-sectional shrinkage rate is 42%~47%. The diameter specification of the hot-rolled wire rod is small, which is conducive to quickly reaching the target diameter of the bridge cable steel wire, thereby reducing the drawing passes and improving processing efficiency. The wire rod has high tensile strength and cross-sectional shrinkage rate, which can avoid excessive loss of wire rod plasticity during wire drawing and cause wire breakage. At the same time, it can still meet the 1860 grade bridge cable performance grade requirements after hot-dip galvanizing treatment.

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

[0035] (1) In view of the fact that the strength grade of the existing 1860MPa grade hot-rolled wire rod for bridge cables is often improved by adding strengthening elements such as C, Cr, and Mn, but the risk of wire breakage in the wire making process is rapidly increased due to network carbides and abnormal structures, the manufacturing method of the present invention combines C-Si-Mn-Cr chemical composition design with online molten salt isothermal technology to appropriately reduce C and Mn to reduce the influence of segregation, which can promote the wire rod to quickly enter the troostite phase region from the high-temperature austenite state, skip the network carbide precipitation zone, form a troostite structure with fine lamellar spacing as the main structure, and perform short-term tempering on the structure to compensate for the strength loss caused by reducing C and Mn, improve the strength-plasticity matching of the wire rod, and finally slowly cool to promote further toughening of the wire rod structure, improve the softening effect of the wire rod, avoid abnormal structures, realize wire rod structure regulation and rapid production, improve structure uniformity and strength-plasticity properties, enable efficient production, and have good industrial adaptability.

[0036] (2) In view of the current situation that the hot-rolled wire rods for 1860MPa bridge cables have insufficient plasticity and large fluctuations in mechanical properties, resulting in a high risk of wire breakage, the hot-rolled wire rods of the present invention have a mixed microstructure of tempered troostite, ferrite, and fused troostite, which can avoid network carbides and abnormal structures, improve the utilization value of carbon, increase the proportion of tempered troostite and lamellar troostite, and reduce the proportion of ferrite. In combination with Cr precipitation strengthening, the strength of small-sized wire rods is improved, and the toughening of the wire rod structure is regulated by the tempering state to reduce the structural stress and dislocation density. The tensile strength can reach 1250~1300MPa, the cross-sectional shrinkage rate is 42%~47%, and the mechanical property circle difference is ≤35MPa. It is used in the manufacture of 1860MPa bridge cables and other application fields, can reduce the risk of wire breakage during deep drawing and torsion, and improve the yield rate and production efficiency of bridge cables. It has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0038] Figure 1 is a metallographic structure diagram of Example 1 of the present invention;

[0039] Figure 2 is the metallographic structure diagram of Example 2 of the present invention;

[0040] Figure 3 It is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0041] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention, so as to propose the best way to implement the present invention, and are intended to be used to explain the present invention and are sufficient to enable those skilled in the art to implement the present invention, but cannot be understood as any limitation on the scope of the present invention, which is limited only by the appended claims; the organization and performance testing of the hot-rolled wire rods obtained in the following embodiments and comparative examples include: the tensile test is carried out using "GB-T 228.1-2021 Metallic Material Tensile Test Part 1: Room Temperature Test Method" to obtain the tensile strength and cross-sectional reduction rate; the organization test is carried out according to the metal microstructure detection method of GB / T13298 standard; the mechanical property same circle difference test method: take 2 circles of wire rod 5m away from the end of the coil, take the overlap area position as the base point, divide each circle of wire rod into 8 sections on average, take 1 tensile specimen on each section, and the strength extreme difference of the tensile specimen after the tensile test is the mechanical property same circle difference. Embodiment 1:

[0042] A preferred embodiment of the manufacturing method of the 1860MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.81%, Si: 0.38%, Mn: 0.52%, Cr: 0.35%, P: 0.014%, S: 0.012%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically:

[0043] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet that can be rolled into plasticity, control the parameters of the heating furnace to promote the uniformity of the steel billet composition, avoid the risk of decarburization and burning, and roll the steel billet into a wire with a diameter of 13mm through a rolling line after the steel billet leaves the heating furnace. Appropriate rolling temperature and reduction amount are selected to promote rapid rolling, and dynamic recrystallization and grain refinement during the final rolling process are controlled. Specifically: the heating furnace is heated to 1180°C, and the steel billet is heated to 1180°C. The furnace time is 2h, the initial rolling temperature is 1135℃, the initial rolling reduction is 29%, the final rolling temperature is 940℃, and the final rolling reduction is 25%; the wire-spinning process is used to make the wire rods out of the rolling line into wire rods through a wire-spinning mechanism, and the wire rods are spread on the rollers and transported along the rollers. The wire rods are austenitized by the higher wire-spinning temperature, and the precipitation of network carbides in the wire-spinning stage is prevented, which provides favorable conditions for the subsequent formation of a larger degree of undercooling and thus refinement of the troostite lamella spacing. Specifically: the wire-spinning temperature is controlled to be 915℃.

[0044] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 36°C / s, and quickly passes through the network carbide interval from the high-temperature austenite state to enter the sorbite phase area, inhibiting the formation of network carbides, promoting the austenite phase transformation to form a sorbite structure with fine lamellar spacing as the main component, and then conveyed by a roller through the second stage salt bath tank for back-end treatment, reducing the molten salt circulation amount, and controlling The untransformed high-temperature austenite of the wire rod is transformed into troostite and a small amount of ferrite, the troostite lamellar layer is melted, and the structure is short-term isothermal tempering to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 540°C, and the total processing time is 68s, of which the processing time of the front-end processing is 15s, the molten salt circulation volume is 600t / h, and the molten salt temperature rise is ≤10°C; the processing time of the back-end processing is 53s, the molten salt circulation volume is 420t / h, and the molten salt temperature rise is ≤4°C.

[0045] The roller slow cooling process uses an open insulation cover, and the wire rod that has passed through the second salt bath tank is transported by the conveyor roller into the insulation cover to promote further toughening of the wire rod structure and improve the tempering and softening effect of the wire rod. Specifically: the wire rod is controlled to be cooled to 333°C at a slow cooling rate of 0.87°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0046] Comparative Example 1:

[0047] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 1 in that: the manufacturing method is manufactured according to the process flow of rolling → spinning → Stelmor air cooling, specifically: the heating furnace soaking temperature is controlled to be 1120°C, the furnace time is 3h, the initial rolling temperature is 1090°C, the final rolling temperature is 910°C, and the spinning temperature is 890°C; the Stelmor strong air cooling adopts an air volume of 260,000 m per fan 3 / h, open 1~5# fans at 85%, cool the wire rod to 670℃ at a cooling rate of 8.5℃ / s, and then open 6~13# fans at 35%, cool the wire rod to 295℃ at a cooling rate of 3.8℃ / s, and obtain the hot-rolled wire rod product after it comes off the line.

[0048] Comparative Example 2:

[0049] A method for manufacturing a hot-rolled wire rod, which differs from Example 1 in that: the heating furnace soaking temperature is controlled to be 1115°C, the furnace time is 3h, the initial rolling temperature is 1085°C, the final rolling temperature is 905°C, the spinning temperature is 885°C, and during the front section of the online molten salt isothermal process, the wire rod is cooled at a cooling rate of 33°C / s. Embodiment 2:

[0050] A preferred embodiment of the manufacturing method of the 1860MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.84%, Si: 0.4%, Mn: 0.49%, Cr: 0.32%, P: 0.013%, S: 0.01%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically:

[0051] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet that can be rolled into plasticity, control the parameters of the heating furnace to promote the uniformity of the steel billet composition, avoid the risk of decarburization and burning, and roll the steel billet into a wire with a diameter of 11mm through a rolling line after the steel billet leaves the heating furnace. Select appropriate rolling temperature and reduction to promote rapid rolling, and control the dynamic recrystallization and grain refinement during the final rolling process. Specifically: control the heating furnace soaking temperature to 1165°C, and in the furnace The time is 2.5h, the initial rolling temperature is 1125℃, the initial rolling reduction is 31%, the final rolling temperature is 935℃, and the final rolling reduction is 27%; the wire-spinning process is used to make the wire rods out of the rolling line into wire rods through the wire-spinning mechanism, and the wire rods are spread on the rollers and transported along the rollers. The higher wire-spinning temperature makes the wire rods austenitized, and prevents the precipitation of network carbides in the wire-spinning stage, which provides favorable conditions for the subsequent formation of a larger degree of supercooling and thus refinement of the troostite lamellae spacing. Specifically: the wire-spinning temperature is controlled to be 910℃.

[0052] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 38°C / s, and quickly passes through the network carbide interval from the high-temperature austenite state to enter the sorbite phase area, inhibiting the formation of network carbides, promoting the austenite phase transformation to form a sorbite structure with fine lamellar spacing as the main component, and then conveyed by a roller through the second stage salt bath tank for back-end treatment, reducing the molten salt circulation amount, and controlling The untransformed high-temperature austenite of the wire rod is transformed into troostite and a small amount of ferrite, the troostite lamellae are melted, and the structure is short-term isothermal tempering to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 505°C, and the total processing time is 85s, of which the processing time of the front treatment is 10s, the molten salt circulation volume is 680t / h, and the molten salt temperature rise is ≤10°C; the processing time of the back treatment is 75s, the molten salt circulation volume is 370t / h, and the molten salt temperature rise is ≤4°C.

[0053] The roller slow cooling process uses an open insulation cover, and the wire rod that has passed through the second salt bath tank is transported by the conveyor roller into the insulation cover to promote further toughening of the wire rod structure and improve the tempering and softening effect of the wire rod. Specifically: the wire rod is controlled to be cooled to 340°C at a slow cooling rate of 0.7°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0054] Comparative Example 3:

[0055] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which differs from that of Example 2 in that: the molten salt temperature is 550°C, during the front-end treatment, the wire rod is cooled at a cooling rate of 34°C / s, and a hot-rolled wire rod finished product is obtained after it comes off the line.

[0056] Comparative Example 4:

[0057] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which differs from that of Example 2 in that: the molten salt temperature is 490°C, during the front-end treatment, the wire rod is cooled at a cooling rate of 39°C / s, and a hot-rolled wire rod finished product is obtained after it comes off the line. Embodiment 3:

[0058] A preferred embodiment of the manufacturing method of the 1860MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.8%, Si: 0.32%, Mn: 0.45%, Cr: 0.41%, P: 0.014%, S: 0.013%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically:

[0059] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to obtain a high-temperature steel billet with rollable plasticity, control the parameters of the heating furnace to promote the uniformity of the steel billet composition and avoid the risk of decarburization and burning, and roll the steel billet into a wire with a diameter of 7mm through a rolling line after the steel billet leaves the heating furnace, select appropriate rolling temperature and reduction, promote rapid rolling, and control the dynamic recrystallization and grain refinement during the final rolling process. Specifically: control the heating furnace soaking temperature to 1125°C, and The furnace time is 4h, the initial rolling temperature is 1095℃, the initial rolling reduction is 34%, the final rolling temperature is 920℃, and the final rolling reduction is 32%; the wire-spinning process is used to convert the wire rods coming out of the rolling line into wire rods through a wire-spinning mechanism, and the wire rods are spread on the rollers and transported along the rollers. The higher wire-spinning temperature makes the wire rods austenitized, and prevents the precipitation of network carbides in the wire-spinning stage, which provides favorable conditions for the subsequent formation of a larger degree of undercooling and thus refinement of the troostite lamellae spacing. Specifically, the wire-spinning temperature is controlled to be 900℃.

[0060] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 35°C / s, and quickly passes through the network carbide interval from the high-temperature austenite state to the sorbite phase area, inhibiting the formation of network carbides, promoting the austenite phase transformation to form a sorbite structure with fine lamellar spacing as the main component, and then conveyed by a roller through the second stage salt bath tank for back-end treatment, reducing the molten salt circulation amount, and controlling The untransformed high-temperature austenite of the wire rod is transformed into troostite and a small amount of ferrite, the troostite lamellae are melted, and the structure is short-term isothermal tempering to improve the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature is 534°C, and the total processing time is 110s, of which the processing time of the front-end processing is 20s, the molten salt circulation volume is 550t / h, and the molten salt temperature rise is ≤10°C; the processing time of the back-end processing is 90s, the molten salt circulation volume is 350t / h, and the molten salt temperature rise is ≤4°C.

[0061] The roller slow cooling process uses an open insulation cover, and the wire rod that has passed through the second salt bath tank is transported by the conveyor roller into the insulation cover to promote further toughening of the wire rod structure and improve the tempering and softening effect of the wire rod. Specifically: the wire rod is controlled to be cooled to 325°C at a slow cooling rate of 1°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 3 shown.

[0062] Comparative Example 5:

[0063] A method for manufacturing a hot-rolled wire rod, which differs from Example 3 in that the total processing time of the online molten salt isothermal process is 150 s, the processing time of the front-end processing is 20 s, and the processing time of the back-end processing is 130 s, and the hot-rolled wire rod finished product is obtained after going offline.

[0064] Comparative Example 6:

[0065] A method for manufacturing a hot-rolled wire rod, which differs from Example 3 in that the total processing time of the online molten salt isothermal process is 30 seconds, the processing time of the front-end processing is 20 seconds, and the processing time of the back-end processing is 10 seconds, and the hot-rolled wire rod finished product is obtained after going offline. Embodiment 4:

[0066] A preferred embodiment of the manufacturing method of the 1860MPa grade hot-rolled wire rod for bridge cables of the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.82%, Si: 0.3%, Mn: 0.47%, Cr: 0.43%, P: 0.015%, S: 0.015%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically:

[0067] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet with rollable plasticity, control the parameters of the heating furnace to promote the uniformity of the steel billet composition and avoid the risk of decarburization and burning, and roll the steel billet into a wire with a diameter of 10mm through a rolling line after the steel billet leaves the heating furnace, select appropriate rolling temperature and reduction, promote rapid rolling, and control the dynamic recrystallization and grain refinement during the final rolling process. Specifically: control the heating furnace soaking temperature to 1140°C, and The furnace time is 3h, the initial rolling temperature is 1115℃, the initial rolling reduction is 32%, the final rolling temperature is 930℃, and the final rolling reduction is 30%; the wire-spinning process is used to make the wire rods out of the rolling line into wire rods through the wire-spinning mechanism, and the wire rods are spread on the rollers and transported along the rollers. The higher wire-spinning temperature makes the wire rods austenitized, and prevents the precipitation of network carbides in the wire-spinning stage, which provides favorable conditions for the subsequent formation of a larger degree of undercooling and thus refinement of the troostite lamellae spacing. Specifically: the wire-spinning temperature is controlled to be 905℃.

[0068] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 37°C / s, and quickly passes through the network carbide interval from the high-temperature austenite state to enter the sorbite phase area, inhibiting the formation of network carbides, promoting the austenite phase transformation to form a sorbite structure with fine lamellar spacing as the main component, and then conveyed by a roller through the second stage salt bath tank for back-end treatment, reducing the molten salt circulation amount, and controlling The untransformed high-temperature austenite of the wire rod is transformed into troostite and a small amount of ferrite, the troostite lamellae are melted, and the structure is short-term isothermal tempering to improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 516°C, and the total processing time is 40s, of which the processing time of the front treatment is 20s, the molten salt circulation volume is 630t / h, and the molten salt temperature rise is ≤10°C; the processing time of the back treatment is 20s, the molten salt circulation volume is 460t / h, and the molten salt temperature rise is ≤4°C.

[0069] The roller slow cooling process uses an open insulation cover, and the wire rod that has passed through the second salt bath tank is transported by the conveyor roller into the insulation cover, which promotes further toughening of the wire rod structure and improves the tempering and softening effect of the wire rod. Specifically: the wire rod is controlled to be cooled to 345°C at a slow cooling rate of 0.75°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage.

[0070] Comparative Example 7:

[0071] A method for manufacturing a hot-rolled wire rod, which differs from Example 4 in that: the method is manufactured according to a process flow of rolling → spinning → online molten salt isothermal → air cooling, and the air cooling process is performed by opening a heat preservation cover, and conveying the wire rod through a second salt bath tank by a conveyor roller, and controlling the wire rod to be cooled to 340°C at a slow cooling rate of 1.8°C / s, and obtaining a hot-rolled wire rod finished product after it comes off the line.

[0072] The hot rolled wire rods obtained in the above examples 1 to 4 and comparative examples 1 to 7 were subjected to microstructure and performance testing, and the comparative results obtained are shown in Table 1 below:

[0073] Table 1. Comparison of microstructure and performance of hot rolled wire rods with different compositions and manufacturing methods

[0074]

[0075] From the comparison results of Example 1 and Comparative Example 1, it can be seen that compared with the C-Si-Mn-Cr composition design, the Stelmor air-cooling line adopts a fast-first-then-slow air-cooling cooling process, which still produces a higher level of network carbides, and the resulting pearlite structure is coarse and low in content, with insufficient strength and plasticity, and large fluctuations in mechanical properties. The present invention, combined with online molten salt isothermal technology, can promote the wire rod to quickly skip the network carbide precipitation interval from the high-temperature austenite state and enter the sorbite phase region, forming a sorbite structure dominated by fine lamella spacing and short-term tempering the structure, improving the strength and plasticity matching of the wire rod, compensating for the strength loss caused by reducing C and Mn, and finally slow cooling to promote further toughening of the wire rod structure. From the results of Examples 1 to 4, it can be seen that the present invention can achieve a tensile strength of 1250 to 1300 MPa and a cross-sectional shrinkage of 42% to 47%, which is used to manufacture 1860 MPa-level bridge cables and other application fields, which is conducive to reducing the risk of wire breakage during deep drawing and torsion.

[0076] From the comparison results of Example 1 and Comparative Example 2, it can be seen that the selection of a higher spinning temperature can avoid the precipitation of network carbides in the spinning stage, and at the same time, austenitize the structure, so as to form a larger temperature gradient for the subsequent online molten salt isothermal treatment, thereby providing favorable conditions for promoting the rapid nucleation and refinement of troostite.

[0077] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the higher the molten salt temperature of the online molten salt isothermal treatment, the more it can promote the melting of the troostite lamellae, improve the softening effect and the plasticity of the wire rod, but if the molten salt temperature is too high, it will affect the cooling rate of the wire rod, which is not conducive to controlling the network carbides and forming a troostite structure dominated by fine lamellae spacing, and will lead to insufficient strength of the wire rod.

[0078] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the lower the molten salt temperature of the online molten salt isothermal treatment, the more conducive it is to promoting the nucleation of the troostite structure with fine lamellar spacing, which is beneficial to improving the strength of the wire rod. However, if the molten salt temperature is too low, the thermal power that can be provided is reduced, which affects the short-time tempering softening effect of the structure and is detrimental to the plasticity of the wire rod.

[0079] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the longer the treatment time of the online molten salt isothermal treatment, the more complete the transformation of the troostite structure, the better the tempering and softening effect of troostite and ferrite, and the increased plasticity of the wire rod. However, if the treatment time is too long, the wire rod will soften too much and lose strength, and the precipitation and coarsening of Cr will lose the strength-plasticity properties.

[0080] From the comparison results of Example 3 and Comparative Example 6, it can be seen that the shorter the treatment time of the online molten salt isothermal treatment, the lower the toughening and softening effects, and the plasticity of the wire rod decreases. If the reverse treatment time is too short, the troostite of the wire rod is not fully inoculated and the softening is insufficient, which will affect the plasticity of the wire rod.

[0081] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow roller cooling can avoid the reduction of plasticity of the wire rod due to rapid cooling. At the same time, slow cooling promotes further toughening of the wire rod structure and improves the softening effect of the wire rod.

[0082] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the scope of protection of the present invention. For example, the steel billet is obtained by processing according to the converter smelting, refining, and continuous casting processes. All equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing a 1860MPa grade hot-rolled wire rod for bridge cables, characterized in that: The manufacturing method thereof comprises: The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.80%~0.84%, Si: 0.30%~0.40%, Mn: 0.45%~0.52%, Cr: 0.32%~0.43%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥900°C, it is subjected to an online molten salt isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥35°C / s, and enters the sorbite phase region from the austenite state, thereby promoting the sorbite phase transformation and promoting the organization Isothermal tempering and melting of troostite lamellae are carried out, and finally, slow cooling is performed on a roller to produce a hot-rolled wire rod having a microstructure including tempered troostite with a volume percentage of ≥91% and the rest being a mixed structure of ferrite and melted troostite; the molten salt temperature of the online molten salt isothermal treatment is 505-540°C, the treatment time is 40-110s, the roller slow cooling controls the wire rod to be cooled to below 350°C at a slow cooling rate of ≤1.0°C / s for coiling, the interlamellar spacing of the tempered troostite is 70-125nm, the network carbide grade of the hot-rolled wire rod is grade 0, and the mechanical property same-circle difference is ≤35MPa.

2. The method for manufacturing the 1860MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: Before the rolling, the heating furnace is controlled to have a soaking temperature of 1125-1180° C. and a furnace time of 2-4 hours.

3. The method for manufacturing the 1860MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: During the rolling, the initial rolling temperature is controlled to be 1095-1135° C., the initial rolling reduction is 29%-34%, the final rolling temperature is ≥920° C., and the final rolling reduction is 25%-32%.

4. The method for manufacturing the 1860MPa grade hot rolled wire rod for bridge cables according to claim 1, characterized in that: The online molten salt isothermal treatment is divided into a front-end treatment and a back-end treatment. The molten salt circulation volume of the front-end treatment is greater than that of the back-end treatment. The treatment time of the front-end treatment is 10 to 20 seconds, and the treatment time of the back-end treatment is 20 to 90 seconds.

5. The method for manufacturing the 1860MPa grade hot rolled wire rod for bridge cables according to claim 4, characterized in that: The molten salt circulation volume of the front-stage treatment is 550-680 t / h, and the molten salt temperature rise is ≤10°C; the molten salt circulation volume of the back-stage treatment is 350-460 t / h, and the molten salt temperature rise is ≤4°C.

6. A 1860MPa grade hot-rolled wire rod for bridge cables, characterized in that: The hot-rolled wire rod is manufactured by the method for manufacturing 1860MPa grade hot-rolled wire rod for bridge cables as described in any one of claims 1 to 5.

7. The 1860MPa grade hot rolled wire rod for bridge cables according to claim 6, characterized in that: The hot-rolled wire rod has a diameter of 7.0-13.0 mm, a tensile strength of 1250-1300 MPa, and a cross-sectional shrinkage of 42%-47%.

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