A high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables and its manufacturing method

Abstract

The present invention relates to a high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables and a manufacturing method thereof. After hot-rolling and coiling into wire rods with a high-carbon composition of Nb-V-Mo, the wire rods are subjected to on-line molten salt end quenching and toughening treatment. The wire rods first pass through the front-section molten salt and cool down at a cooling rate of ≥38 °C / s, causing partial austenite structure to transform into quenched martensite. Then, they pass through the rear-section molten salt and are heated to the temperature in the sorbite phase region, controlling the untransformed austenite to transform into sorbite and isothermal tempering, controlling the precipitation and growth of carbides. Finally, they are slowly cooled on a roller table to produce hot-rolled wire rods with a duplex structure composed of tempered martensite and tempered sorbite in the microstructure, which can achieve a tensile strength of 1710-1750 MPa and an area reduction of 25%-30%, control the material cost, improve the production efficiency, be applicable to the production of 2500 MPa-class bridge cables, and help reduce the risk of wire breakage during downstream cold drawing and subsequent processing.

Description

Technical Field

[0001] The present invention belongs to the technical field of hot-rolled wire rods, and particularly relates to a high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables and a manufacturing method thereof. Background Art

[0002] Ferrite matrix cold-drawn pearlitic steel is widely used in fields such as wire ropes, bridge cables, stranded wires, and piano wires due to its good toughness and suitability for large-deformation drawing. With the rapid development of long-span bridges, the strength grade of the cold-drawn pearlitic hot-rolled wire rod used as the base material for bridge cables has also been continuously improved. However, since cold-drawn pearlitic steel has a ferrite matrix, its strength level is often lower than that of steels with shear transformation phase structures. In order to meet the requirements of bridge cable steel for bearing loads and lightweight in long-span bridge structures, it is necessary to develop a high-strength hot-rolled wire rod to adapt to the production of 2500 MPa-class bridge cables.

[0003] The hot-rolled wire rods for high-strength bridge cables in the prior art mainly adopt high-carbon cold-drawn pearlitic steel controlled-cooled by a Stelmor air-cooling line. There are still the following technical difficulties in manufacturing hot-rolled wire rods for 2500 MPa-class bridge cables with high strength and good plasticity matching:

[0004] 1. The improvement of the strength grade of the hot-rolled wire rod base material often involves increasing the carbon and alloy element contents. Cold-drawn pearlitic steel has lower strength compared to steel with a shear transformation phase change structure, and more carbon and alloy elements are required. However, more carbon and alloy elements also increase the risk of deteriorating the control of the wire rod microstructure. The main reasons are as follows: On the one hand, when the carbon content of cold-drawn pearlitic steel exceeds the eutectoid point of 0.77%, due to the limited maximum cooling capacity of the Stelmor air-cooling line, the wire rod spends a relatively long time in the temperature range of secondary cementite precipitation. With the increase in carbon content and segregation, the range of network carbide precipitation continuously increases, resulting in a certain degree of deterioration of the strength and plasticity of cold-drawn pearlitic steel. Although carbon is an effective strengthening element, improper utilization and control can also cause wire breakage problems during downstream cold drawing and subsequent processing of the steel. On the other hand, in order to minimize the network carbide grade, strong air-cooling treatment is adopted after wire laying. However, due to the influence of the air-cooling line's cooling control ability and uncontrollability, the temperature difference between the surface and the core of the wire rod will be further increased. With the increase in the alloy element content of the wire rod, the hardenability of the wire rod and the influence of alloy element segregation increase, and uncontrollable brittle structures such as bainite and martensite are more likely to form at some over-cooled positions. Reducing the carbon content requires further increasing the alloy element content. For example, a high-toughness bridge cable steel with a tensile strength ≥ 2500 Mpa and its preparation method disclosed in Patent CN110055392B adopts a low-carbon composition design of C-Mn-Al-V, with a relatively high alloy element content. This not only brings the problem of high material cost but also further increases the difficulty of controlling the deteriorating microstructure. The deteriorating microstructure will increase the tissue brittleness and mechanical property fluctuations. As the wire rod undergoes continuous cooling phase transformation, after the phase transformation incubation period, the wire rod is already in a low-temperature state, and there are large stresses in the wire rod microstructure, affecting the plasticity and tissue uniformity of the wire rod, and increasing the risk of wire breakage during downstream cold drawing and subsequent processing.

[0005] 2. To improve the strength and drawing performance of the wire rod, existing wire rods for high-strength bridge cables often choose to increase the carbon and alloy element contents to improve the hardenability of the steel and reduce the critical cooling rate of the steel, obtaining a fine pearlite structure, namely a sorbite structure. However, on the one hand, the minimum cooling capacity of the Stelmor air-cooling line is limited, and the wire rod spends a short time in the phase transformation range, which will affect the full phase transformation of sorbite, thereby affecting the matrix strength. Residual austenite is also prone to form abnormal structures during subsequent cooling. Moreover, increasing the wire laying temperature will significantly extend the in-line time of the wire rod, affecting production efficiency. On the other hand, after the wire rod undergoes continuous cooling phase transformation, it is already in a low-temperature state, and there are large tissue stresses in the wire rod, which will increase mechanical property fluctuations, affect the plasticity of the wire rod, and further increase the risk of wire breakage during subsequent drawing.

[0006] III. Although the wire rod can be strengthened by micro-alloying elements such as Nb, V, and Ti, the cost of micro-alloying elements is relatively high. Excessive addition is not beneficial to the material cost. At the same time, limited by the controlled cooling capacity of the Stelmor air-cooled line, the wire rod is affected by the temperature difference between the surface and the core and continuous cooling, and the precipitation driving force is small, resulting in a reduction in the number of precipitation phases and an increase in size, weakening the contribution to strength, and even affecting the material properties due to the formation of relatively large-sized precipitation phases. Summary of the Invention

[0007] The present invention aims to solve at least one of the above technical problems to a certain extent. The present invention provides a high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables and a manufacturing method thereof, which can improve the strength-plasticity matching and tissue uniformity of the wire rod, control the material cost, improve the production efficiency, be applicable to the production of 2500 MPa-class bridge cables, and is beneficial to reducing the wire breakage risk in the downstream cold drawing and subsequent processing processes.

[0008] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0009] A manufacturing method of a high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables, the manufacturing method comprising:

[0010] Rolling wire rods according to the chemical composition of the hot-rolled wire rod. The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.95% - 0.98%, Si: 1.05% - 1.25%, Mn: 0.65% - 0.85%, Cr: 0.40% - 0.60%, Nb: 0.04% - 0.048%, V: 0.035% - 0.045%, Mo: 0.40% - 0.50%, P ≤ 0.015%, S ≤ 0.015%, and the rest are Fe and inevitable impurities. After the wire rods are spun into wire rods at a spinning temperature of ≥ 925 °C, they are subjected to online molten salt end quenching and toughening treatment. The wire rods first pass through the front-section molten salt and cool down at a cooling rate of ≥ 38 °C / s, and part of the austenite structure transforms into quenched martensite. Then, they pass through the rear-section molten salt and are heated to the temperature in the sorbite phase region, controlling the untransformed austenite to transform into sorbite and isothermal tempering, controlling the precipitation and growth of carbides. Finally, they are slowly cooled through the roller path to produce hot-rolled wire rods with a duplex structure composed of tempered martensite and tempered sorbite in the microstructure.

[0011] The design basis for the chemical composition and mass percentage of the above hot-rolled wire rod includes:

[0012] (1) Carbon: Element C is an effective carbide strengthening element and austenite forming element with a relatively lower price. With the increase of carbon content, it can produce solid solution strengthening, reduce the austenite transformation temperature, lower the martensite transformation temperature, promote short-time quenching by rapid cooling during the front-stage molten salt treatment, increase the content of tempered martensite in the structure, affect the morphology and distribution of cementite in sorbite, contribute to obtaining a fine and uniform sorbite structure during the back-stage molten salt treatment, and improve the material strength. However, excessive carbon content will increase the carbon segregation tendency during the solidification of steel billets, increase the difficulty of controlling network carbide and isothermal tempering, and affect the plastic and toughness properties of the material and the control of carbide precipitation. Therefore, in order to improve the strength of the hot-rolled wire rod base material, control the material cost, and facilitate the control of duplex structure and carbide precipitation, the mass percentage of C is controlled at 0.95% - 0.98%.

[0013] (2) Silicon: Element Si is the main deoxidizing element in steel. It can inhibit the grain coarsening during the front-stage molten salt treatment, promote the uniform transformation of quenched martensite. During the sorbite phase transformation process in the back-stage molten salt treatment, it makes the cementite lamellae in sorbite finer and more uniform, and keeps the tempered martensite with higher hardness and strength. However, too high silicon content will increase the surface decarburization tendency of steel, prolong the time required for phase transformation incubation, reduce the toughness of steel, and is not conducive to the control of carbide precipitation and rapid production. Therefore, in order to refine grains, improve the uniformity and strength of the duplex structure, and facilitate rapid production, the mass percentage of Si is controlled at 1.05% - 1.25%.

[0014] (3) Manganese: As an austenite forming element, Mn can increase the hardenability of the wire rod, reduce the critical cooling rate of steel, make it easier to form martensite during the cooling process of steel, promote the sorbite phase transformation, and improve the strength of sorbite, thus being beneficial to improving the tensile strength of the wire rod. However, when the content of Mn is too high, it will exacerbate the segregation of alloying elements, reduce the activity of carbon, increase the difficulty of stress relief during isothermal tempering, and is not conducive to the control of carbide precipitation during the back-stage molten salt treatment, reducing the toughness and plasticity of steel. Therefore, in order to facilitate the regulation of the duplex structure of hot-rolled wire rod, reduce the difficulty of controlling tissue uniformity and short-time tempering, and facilitate rapid production, the mass percentage of Mn is controlled at 0.65% - 0.85%.

[0015] (4) Chromium: The Cr element can improve the hardenability of steel, enabling the steel to rapidly form a high proportion of quenched martensite during short-time quenching in the front-section molten salt. At the same time, it can enhance the stability of austenite. During the subsequent molten salt treatment process, it can lower the phase transformation temperature, refine pearlite colonies and cementite lamellae, increase the strength of sorbite, and reduce the strength loss during the subsequent hot-dip galvanizing process of wire rope production. However, if the Cr content is too high, it will exacerbate composition segregation, increase the control difficulty of tissue uniformity and plasticity improvement, and the coarsening of carbides will reduce the toughness of the steel, thereby affecting the drawing and torsion properties of the wire. Therefore, for the convenience of duplex tissue, plasticity regulation, and rapid production, the mass percentage of Cr is controlled at 0.40% - 0.60%.

[0016] (5) Niobium: As a microalloying element, the Nb element can effectively inhibit the coarsening of rod grains during rolling when combined with V, prevent the growth of austenite grains, improve the strength and toughness of the matrix, and there are more grain boundaries available as the nucleation sites of martensite, making the martensite transformation easier. The carbonitrides of niobium will precipitate dispersively during the online molten salt end-quenching toughening process, producing a precipitation strengthening effect, which can further increase the strength of the rod. However, the cost of the Nb element is relatively high. Considering the role and cost of the Nb element, the content of the Nb element is controlled at 0.04% - 0.048%.

[0017] (6) Vanadium: As a microalloying element, the V element can precipitate dispersively during the online molten salt end-quenching toughening process, providing a strong precipitation strengthening effect, thereby increasing the strength level of hot-rolled rods without reducing plasticity. However, the cost of the V element is relatively high. Excessive addition is not conducive to controlling the cost of rods and there is a risk of coarsening. Considering the role and cost of the V element, the content of the V element is controlled at 0.035% - 0.045%.

[0018] (7) Molybdenum: The Mo element can improve the hardenability of steel, promote the short-time acquisition of most quenched martensite during the front-section molten salt treatment process, and at the same time can effectively inhibit the coarsening of strengthening phases such as cementite, Cr, Nb, and V precipitation phases, preventing the coarsening of precipitation phases during the online molten salt end-quenching toughening process and reducing the plasticity of rods. However, the cost of the Mo element is relatively high. Excessive addition is not conducive to controlling the cost of rods. Considering the role and cost of the Mo element, the content of the Mo element is controlled at 0.40% - 0.50%.

[0019] (8) Phosphorus and sulfur: The P element and S element belong to impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% are controlled.

[0020] The above hot-rolled wire rods adopt a high-carbon composition design of Nb-V-Mo, optimize the ratios of Si, Mn, and Cr, and regulate the hardenability of the wire rods, as well as the transformation temperature ranges of martensite and sorbite, providing favorable conditions for obtaining fine quenched martensite during the short-time transformation of martensite, for the transformation of sorbite and the tempering of the duplex structure while being able to control the precipitation strengthening of carbides in a dispersed manner and enabling rapid offline. On this basis, an appropriate spinning temperature is selected to avoid the limitation of rolling control due to too low a spinning temperature and the formation of network carbides during the spinning stage, promoting the uniformity and fineness of austenite, and providing favorable conditions for promoting the short-time transformation of martensite by molten salt end quenching at a higher temperature after spinning. The wire rods after spinning pass through the molten salt instead of being air-cooled and are subjected to on-line molten salt end quenching toughening treatment:

[0021] 1. After further increasing the contents of carbon and alloying elements, the current situation that it is difficult to control the deteriorated structures such as network carbides and martensite on the Stelmor air-cooling line. When the wire rods pass through the molten salt, the molten salt can cover the surface of the wire rods for uniform heat exchange, and the heat exchange capacity of the molten salt is stronger, which can promote the rapid cooling of the wire rods. On the one hand, the temperature of the front-section molten salt is relatively lower than that of the rear-section molten salt. When the wire rods are treated by the front-section molten salt, they can quickly pass through the secondary cementite precipitation temperature range of 700 - 800 °C, avoiding the formation of network carbides that affect the tissue uniformity and plastic and toughness properties, and improving the strengthening effect of carbon elements. On the other hand, the wire rods can quickly enter the region below the sorbite phase from the high-temperature austenite state through ultra-high cooling rates, forming a certain amount of retained austenite with a large degree of undercooling, undergoing short-time martensite transformation, and rapidly obtaining partial quenched martensite. The formation of quenched martensite is more controllable and uniform, thereby further increasing the strength of the steel compared with the cold-drawn pearlitic steel with a ferrite matrix, and also facilitating the rapid transformation incubation of sorbite, reducing the transformation time, and improving production efficiency.

[0022] 2. Due to the limited cooling control capacity of the Stelmor air cooling line and the continuous cooling of the wire rod, it is difficult for the troostite to undergo a full phase transformation, and the organizational stress and mechanical properties fluctuate greatly. After the wire rod is treated with the molten salt in the front section, a small amount of untransformed austenite remains in the wire rod. After passing through the molten salt in the rear section, the temperature can be raised to the troostite phase region, and the wire rod temperature is kept consistent with the molten salt temperature of the rear section. On the one hand, isothermal phase transformation can be carried out to reduce the temperature difference from the surface to the core of the wire rod, control the transformation of the untransformed high-temperature austenite in the wire rod into a troostite structure with finer interlamellar spacing, prolong the time that the wire rod is at the peak precipitation temperature of the troostite, promote the full phase transformation of the troostite structure, and increase the carbon element. It can also prevent the residual austenite from continuing to form low-temperature brittle tissue that is unfavorable to the plasticity of the tissue during the subsequent cooling process; on the other hand, the molten salt temperature of the rear-stage molten salt is higher than that of the front-stage molten salt, which can prolong the time that the wire rod is in the high temperature range, provide more thermal power for the isothermal tempering of the tissue, and promote the short-term tempering and toughening of the quenched martensite and troostite tissue formed by phase transformation, and transform the quenched martensite into tempered martensite with both strong and plastic properties, reduce the stress of the tempered troostite tissue, and then improve the matching of the strength and plasticity of the wire rod, so that the quenched martensite, which is conventionally regarded as an abnormal tissue, can be better utilized, which is beneficial to rapid offline and improved production efficiency.

[0023] 3. Due to the limited cooling control capacity of the Stelmor air-cooling line, it is difficult to bring into play the role of Nb and V elements. On the one hand, the high spinning temperature can reduce the restrictions on rolling and facilitate the induction of Nb precipitation in the rolling stage. After the wire rod is quenched and toughened at the end of the online molten salt, the wire rod is quickly cooled and the temperature difference from the surface to the core of the wire rod can be reduced. During the isothermal process of the wire rod in the later molten salt treatment, the atomic diffusion rate slows down, the nucleation rate of Nb increases, and V can form a fine and dispersed carbonitride precipitation phase, which is evenly distributed in the matrix, effectively hindering dislocation movement and improving the strength of the steel; on the other hand, the complex phase structure formed by the wire rod after isothermal tempering control undergoes a certain degree of short-term tempering. Combined with the role of Mo, the risk of carbide coarsening caused by too long a treatment time can be avoided, thereby giving full play to the strengthening and toughening role of Nb and V elements and avoiding excessive addition of micro-alloy elements to affect material costs.

[0024] The temperature of the wire rod is relatively high after the molten salt treatment in the latter stage. After slow cooling on a roller, the wire rod can be cooled too fast during the cooling process, resulting in excessive stress inside the wire rod, and promoting further toughening of the wire rod structure, thereby improving the softening effect of the wire rod, enhancing the high-strength plasticity matching of the wire rod, and reducing fluctuations in mechanical properties.

[0025] Selecting appropriate heating furnace soaking temperature and furnace time before rolling can promote the homogenization of alloy components, reduce the influence of segregation, and increase the dissolution amount of Nb and V, so that the carbonitrides of niobium and vanadium can be finely dispersed and precipitated during rolling. In the preferred technical solution, before rolling, the heating furnace soaking temperature is controlled to be 1200-1250°C, and the furnace time is 160-200min.

[0026] During rolling, an appropriate initial rolling temperature is selected to make Nb evenly distributed in the steel, so that the austenite grains are not easy to grow during rolling. The combined initial rolling reduction causes strong deformation of austenite, increases the stored energy of deformation, provides more nucleation sites for the carbonitrides of Nb, and exerts the fine grain strengthening and precipitation strengthening effects of Nb. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1040 - 1090 °C, and the initial rolling reduction is 15% - 20%.

[0027] During rolling, an appropriate finish rolling temperature and finish rolling reduction are selected to promote dynamic recrystallization during finish rolling, inhibit the coarsening of the wire rod grains during rolling through the combined precipitation of Nb and V elements, and refine the grains. In the preferred technical solution, during rolling, the finish rolling temperature is controlled at 960 - 1000 °C, and the finish rolling reduction is 23.5% - 28.5%.

[0028] In the preferred technical solution, the molten salt temperature of the front - stage molten salt is 455 - 480 °C, and the treatment time is 5 - 15 s. The molten salt temperature of the front - stage molten salt is below the temperature of the sorbite phase region. The lower the molten salt temperature and the longer the treatment time, the more the transformation of high - temperature austenite to quenched martensite, and the increase in the proportion of tempered martensite in the structure, which improves the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, the untransformed high - temperature austenite is small, which will affect the sorbite transformation and isothermal tempering effect, and the plasticity of the wire rod will be lost; on the contrary, the higher the molten salt temperature and the shorter the treatment time, the less conducive to the transformation of quenched martensite, and more austenite will transform into sorbite structure, and the matrix plasticity increases. However, if the molten salt temperature is too high and the treatment time is too short, the transformation of quenched martensite is too little, and the strength of the wire rod will be significantly lost. Therefore, the molten salt temperature and treatment time of the front - stage molten salt can be controlled to form a certain amount of retained austenite with a large degree of supercooling, promote a large part of austenite to transform into quenched martensite through short - time phase transformation, and make organizational preparations for subsequent duplex structure regulation and carbide control.

[0029] Due to the large temperature difference between the wire - laying temperature and the molten salt temperature of the front - stage molten salt, selecting a larger molten salt circulation rate can reduce the molten salt temperature rise and promote the uniform phase transformation of the wire rod. At the same time, the larger the specification, the larger the molten salt circulation rate per unit time. In the preferred technical solution, the molten salt circulation rate of the front - stage molten salt is 400 - 800 t / h, and the molten salt temperature rise ≤ 5 °C.

[0030] In a preferred technical solution, the molten salt temperature of the latter-stage molten salt is 545 - 570 °C, and the treatment time is 160 - 200 s. The molten salt temperature of the latter-stage molten salt is in the sorbite phase region and the temperature range for the dispersed precipitation of V-containing carbides. The lower the molten salt temperature, the more beneficial it is for the lamination of sorbite lamellae, and it can provide more driving force for the dispersed precipitation of Nb and V carbides to improve the matrix strength. However, if the molten salt temperature is too low, it is not conducive to the short-time tempering of the duplex structure and will result in a loss of plastic properties. On the contrary, the higher the molten salt temperature, the more thermal driving force it can provide for the isothermal tempering of quenched martensite and sorbite, promoting the improvement of the plasticity of the wire rod. However, if the molten salt temperature is too high, the sorbite lamellar spacing is larger, the softening of the wire rod accelerates, and there is a risk of coarsening of the carbide precipitation, which is not conducive to the control of strength and plasticity. The longer the treatment time of the latter-stage molten salt, the more conducive it is to the full precipitation of Nb and V carbides, improving the softening effect of the duplex structure, and enhancing the strength-plasticity matching of the wire rod. However, if the treatment time is too long, there is a risk of coarsening of the carbide precipitation, and at the same time, the production energy consumption increases, which is not conducive to the rapid offline production and the strength-plasticity performance of the wire rod. On the contrary, the shorter the treatment time, the worse the isothermal tempering effect, the decrease in the plasticity of the wire rod, and the reduction of energy consumption. However, if the treatment time is too short, there will be greater brittleness and stress in the wire rod, and the Nb and V carbides will not have enough time to fully precipitate, resulting in a loss of wire rod strength and significantly affecting the plasticity of the wire rod. Therefore, the molten salt temperature and treatment time of the latter-stage molten salt can be further controlled to control the transformation of the untransformed high-temperature austenite in the wire rod into sorbite. At the same time, after isothermal tempering, the formed duplex structure is controlled to undergo a certain degree of short-time tempering, controlling the precipitation and growth of carbides, and enhancing the strength-plasticity matching of the wire rod.

[0031] After the wire rod is rapidly cooled after the front-stage molten salt treatment, a lower molten salt circulation rate is selected for the latter-stage molten salt to control the molten salt temperature rise and reduce the production energy consumption. In a preferred technical solution, the molten salt circulation rate of the latter-stage molten salt is 200 - 450 t / h, and the molten salt temperature rise ≤ 3 °C.

[0032] The slow cooling on the roller table can control the slow cooling of the wire rod at an appropriate cooling rate to avoid an increase in wire rod stress caused by too fast a cooling rate, promoting the further toughening of the wire rod structure, and avoiding too slow a cooling rate from affecting the offline time of the wire rod and promoting the rapid production of the wire rod. In a preferred technical solution, the slow cooling on the roller table controls the wire rod to cool slowly at a cooling rate of 0.5 - 0.75 °C / s to below 250 °C.

[0033] A high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables, wherein the hot-rolled wire rod is obtained by manufacturing with the manufacturing method of the high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables described in any one of the above.

[0034] The above hot-rolled wire rods are designed with Nb-V-Mo chemical composition and combined with the online molten salt end quenching toughening technology. The microstructure formed is different from that of ferrite matrix cold-drawn pearlitic steel, but a duplex structure including tempered martensite and tempered sorbite. The network carbide is effectively controlled. During the solid-state phase transformation process, quenched martensite structure is formed by short-time quenching in a shear mode. The quenched martensite has the characteristic of high strength compared with the pearlite structure. After isothermal tempering, the distortion and dislocation density are improved, and the plasticity increases, and it is transformed into a strong and tough tempered martensite structure; the lamellar spacing of the sorbite structure is finer than that of the pearlite structure, and the strength and drawing hardening ability are better. After isothermal tempering, the tissue stress is greatly improved, and it is transformed into a tempered sorbite structure with better plasticity. The elements Nb and V are used to provide a strong precipitation strengthening effect, and the element Mo is used to inhibit the coarsening of the strengthening phase, so that the hot-rolled wire rods with duplex structure have high strength and plasticity and better tissue uniformity.

[0035] The more the proportion of the tempered martensite, the higher the matrix strength. In the preferred technical solution, the volume percentage of the tempered martensite is 68% - 73%.

[0036] The finer the lamellar spacing of the tempered sorbite, the higher the matrix strength. In the preferred technical solution, the lamellar spacing of the tempered sorbite is 70 - 100 nm.

[0037] In the preferred technical solution, the network carbide grade of the hot-rolled wire rod is grade 0, which can make the continuity of the matrix structure better, there is no weak link caused by network carbide, and when subjected to external force impact, it can better absorb energy, show higher toughness, and reduce the risk of brittle fracture during the subsequent drawing process.

[0038] Due to the effective improvement of the network carbide of the wire rod, the austenite structure is fully transformed uniformly during the phase transformation, and the carbide strengthening phase is precipitated dispersedly, which can further reduce the mechanical property fluctuation of the wire rod and reduce the risk of wire breakage. In the preferred technical solution, the mechanical property difference within the same coil of the hot-rolled wire rod ≤ 50 MPa.

[0039] In the preferred technical solution, the diameter of the hot-rolled wire rod is 9.0 - 16.0 mm, the tensile strength is 1710 - 1750 MPa, and the reduction of area is 25% - 30%. The hot-rolled wire rod has high strength and good plasticity, which is beneficial to improving the strength grade of the bridge cable and reducing the risk of wire breakage during drawing and torsion.

[0040] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0041] (1)In view of the current situation that it is difficult to control the deteriorated structure of wire rods and the insufficient strength and plasticity after the improvement of carbon and alloy elements in cold-drawn pearlitic steel, the present invention designs a high-carbon chemical composition of Nb-V-Mo and combines it with an on-line molten salt end quenching toughening technology to control the wire rods to rapidly enter the region below the sorbite phase from the high-temperature austenite state at an ultra-high cooling rate, forming a certain amount of retained austenite with a large degree of supercooling, and undergoing a short-time martensite phase transformation. Then, the wire rods are controlled to enter the sorbite phase region for isothermal phase transformation, controlling the untransformed high-temperature austenite of the wire rods to transform into sorbite. At the same time, after isothermal tempering, the formed duplex structure is controlled to undergo a certain degree of short-time tempering, controlling the precipitation of carbides. Finally, slow cooling on the roller table promotes further toughening of the wire rod structure, realizing the regulation of the duplex structure, improving the strength-plasticity matching of the wire rods, reducing the mechanical property fluctuation of the wire rods, being beneficial to controlling the material cost, improving the production efficiency, and having good industrial adaptability.

[0042] (2)In view of the current situation that the base material of the hot-rolled wire rods for existing bridge cables has insufficient strength and plasticity and poor tissue uniformity, resulting in an increased risk of wire breakage during downstream cold drawing and subsequent processing, the microscopic structure formed by the hot-rolled wire rods of the present invention includes a duplex structure of tempered martensite and tempered sorbite, which can effectively prevent the formation of network carbides by C elements, transform quenched martensite into tempered martensite with both strength and plasticity, maximize the strengthening effect of carbon elements, utilize the strong precipitation strengthening effect provided by Nb and V elements, and utilize Mo elements to prevent the coarsening of precipitation phases, improving the overall strength and plasticity of the wire rods. The product can reach a tensile strength of 1710 - 1750 MPa and an area reduction of 25% - 30%, and is used in application fields such as manufacturing 2500 MPa grade ultra-high strength bridge cables, which is beneficial to reducing the risk of wire breakage and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, wherein:

[0044] Figure 1 is the metallographic structure diagram of Embodiment 1 of the present invention;

[0045] Figure 2 is the metallographic structure diagram of Embodiment 2 of the present invention;

[0046] Figure 3 is the metallographic structure diagram of Embodiment 3 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] The embodiments described below with reference to the accompanying drawings are exemplary, merely for illustration purposes and do not limit the description of the features and characteristics of the present invention. They are intended to present the best mode of implementing the present invention, for the purpose of explaining the present invention and enabling those skilled in the art to implement the present invention. It should not be construed as any limitation on the scope of the present invention, which is only defined by the appended claims. The organization and performance testing of the hot-rolled wire rods obtained from the following examples and comparative examples include: the tensile test is carried out in accordance with "GB-T 228.1-2021 Metallic materials - Tensile testing - Part 1: Method of test at room temperature" to obtain the tensile strength and reduction of area; the microstructure is detected according to the metallic microstructure detection method of the GB / T13298 standard; the same-coil difference test method for mechanical properties: take 2 coils of wire rods at a distance of 5 m from the end of the coil, with the lap area as the base point, evenly divide each coil of wire rods into 8 segments on average, take 1 tensile specimen on each segment, and the strength difference after the tensile test of the taken tensile specimens is the same-coil difference of mechanical properties. Example 1:

[0048] A preferred embodiment of the manufacturing method of the 2500MPa grade high-strength duplex hot-rolled wire rod for bridge cables according to the present invention. The chemical composition and mass percentage of the hot-rolled wire rod include C: 0.98%, Si: 1.1%, Mn: 0.75%, Cr: 0.49%, Nb: 0.04%, V: 0.045%, Mo: 0.44%, P: 0.011%, S: 0.015%, and the rest is Fe and inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt end quenching and toughening treatment → slow cooling on the roller table → coiling. Specifically:

[0049] The rolling process is used to heat the steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the plastic state for rolling, promote the homogenization of alloy components, reduce segregation, and increase the dissolution amount of Nb and V. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 12mm through the rolling line. Appropriate rolling temperature and reduction ratio are selected to induce the precipitation of Nb, promote dynamic recrystallization and grain refinement during the finishing rolling process, and strengthen and toughen the matrix. Specifically: control the soaking temperature of the heating furnace to be 1220°C, the residence time in the furnace to be 185min, the initial rolling temperature to be 1060°C, the initial rolling reduction ratio to be 18.5%, the finishing rolling temperature to be 970°C, and the finishing rolling reduction ratio to be 27%. The wire laying process is used to make the wire rod exiting the rolling line into a coil through the wire laying machine. The coil is scattered on the roller table and conveyed along the roller table, making the coil in a high-temperature austenite state, promoting the uniformity and fineness of austenite, and providing favorable conditions for the short-time rapid phase transformation of martensite. Specifically: control the wire laying temperature to be 940°C.

[0050] The online molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with internal molten salt. The wire rod after wire laying is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 39 °C / s, quickly skips the reticulated carbide precipitation range from the high-temperature austenite state and enters the pearlite phase region or below, forming a certain amount of retained austenite with a large degree of supercooling, undergoing short-time martensitic transformation, promoting the transformation of most of the austenite structure into quenched martensite. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the back-stage molten salt treatment, controlling the wire rod to enter the pearlite phase region for isothermal transformation, controlling the untransformed high-temperature austenite to transform into pearlite, promoting the massive dispersion precipitation of niobium- and vanadium-containing carbides, inhibiting the growth of carbide strengthening phases. At the same time, after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering, improving the strength-ductility matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 461 °C, the treatment time is 12 s, the molten salt circulation rate is 580 t / h, and the molten salt temperature rise ≤ 5 °C; the molten salt temperature of the back-stage molten salt is 551 °C, the treatment time is 185 s, the molten salt circulation rate is 310 t / h, and the molten salt temperature rise ≤ 3 °C.

[0051] The roller table slow cooling process adopts not fully closing the heat preservation cover. The wire rod transported by the conveying roller table through the second-stage salt bath tank enters the heat preservation cover, preventing the wire rod from having too fast a cooling rate during cooling, which may lead to an increase in stress, promoting the further toughening of the wire rod structure, and improving the softening effect of the wire rod. Specifically: controlling the wire rod to slowly cool at a cooling rate of 0.53 °C / s to 241 °C; the coiling process is used to coil the wire rod into a coil through a coiling drum, and after packaging and warehousing, the finished hot-rolled wire rod is obtained, and its metallographic structure diagram is as Figure 1 shown.

[0052] Comparative Example 1:

[0053] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 1 lies in: controlling the soaking temperature of the heating furnace to be 1150 °C, the time in the furnace to be 230 min, the initial rolling temperature to be 1000 °C, the final rolling temperature to be 880 °C, the wire laying temperature to be 850 °C, and the wire rod to cool down at a cooling rate of 31 °C / s during the front-stage molten salt treatment, and obtaining the hot-rolled wire rod after offline. Example 2:

[0054] A preferred implementation method of the manufacturing method of the 2500 MPa grade high-strength duplex hot-rolled wire rod for bridge cables described in the present invention. The chemical composition and mass percentage of the hot-rolled wire rod include C: 0.97%, Si: 1.25%, Mn: 0.65%, Cr: 0.4%, Nb: 0.042%, V: 0.045%, Mo: 0.44%, P: 0.013%, S: 0.012%, and the rest are Fe and unavoidable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt end quenching and toughening treatment → roller table slow cooling → coiling. Specifically:

[0055] The rolling process is used to heat a steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, promoting the homogenization of alloy components, reducing segregation, increasing the dissolution amount of Nb and V. After the steel billet exits the heating furnace, it is rolled into wire rods with a diameter specification of 9 mm through a rolling line. Appropriate rolling temperature and deformation amount are selected to induce the precipitation of Nb, promote dynamic recrystallization during the finish rolling process, refine grains, and strengthen and toughen the matrix. Specifically: control the soaking temperature of the heating furnace to be 1200 °C, the residence time in the furnace to be 200 min, the rough rolling temperature to be 1040 °C, the rough rolling reduction to be 20%, the finish rolling temperature to be 960 °C, and the finish rolling reduction to be 28.5%; The wire laying process is used to make the wire rods exiting the rolling line into coiled rods through a wire laying machine. The coiled rods are scattered on the roller table and transported along the roller table, making the coiled rods in a high-temperature austenite state, promoting the uniformity and fineness of austenite, and providing favorable conditions for the short-time rapid phase transformation of martensite in the future. Specifically: control the wire laying temperature to be 925 °C.

[0056] The on-line molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with molten salt inside. The coiled rods after wire laying are transported through the first-stage salt bath tank by the roller table for the front-stage molten salt treatment, making the coiled rods cool down at a cooling rate of 38 °C / s, quickly passing through the reticular carbide precipitation range from the high-temperature austenite state into the sorbite phase region, forming a certain amount of retained austenite with a large degree of supercooling, undergoing short-time martensite phase transformation, and promoting the transformation of most of the austenite structure into quenched martensite. Then the coiled rods are transported through the second-stage salt bath tank by the roller table for the back-stage molten salt treatment, controlling the coiled rods to enter the sorbite phase region for isothermal phase transformation, controlling the untransformed high-temperature austenite to transform into sorbite, promoting the massive dispersion precipitation of niobium- and vanadium-containing carbides, inhibiting the growth of carbide strengthening phases, and at the same time, after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering to improve the strength-plasticity matching of the coiled rods. Specifically: the molten salt temperature of the front-stage molten salt is 480 °C, the treatment time is 5 s, the molten salt circulation volume is 400 t / h, and the molten salt temperature rise ≤ 5 °C; the molten salt temperature of the back-stage molten salt is 570 °C, the treatment time is 160 s, the molten salt circulation volume is 200 t / h, and the molten salt temperature rise ≤ 3 °C.

[0057] The slow cooling process of the roller table uses an incompletely closed heat preservation cover. The coiled rods transported by the conveying roller table after passing through the second-stage salt bath tank enter the heat preservation cover to prevent the coiled rods from cooling too fast during the cooling process, resulting in increased stress, promoting the further toughening of the coiled rod structure, and improving the softening effect of the coiled rods. Specifically: control the coiled rods to cool slowly at a cooling rate of 0.75 °C / s to 229 °C; The coiling process is used to coil the coiled rods into coils through a coiling drum, and after packaging and warehousing, hot-rolled coil rod products are obtained, and its metallographic structure diagram is as Figure 2 shown.

[0058] Comparative Example 2:

[0059] A manufacturing method of hot-rolled wire rod, the difference between the manufacturing method and that of Example 2 lies in that: during the front-section molten salt treatment, the wire rod is cooled at a cooling rate of 37 °C / s, the molten salt temperature of the front-section molten salt is 505 °C, and the hot-rolled wire rod is obtained after being taken off the production line.

[0060] Comparative Example 3:

[0061] A manufacturing method of hot-rolled wire rod, the difference between the manufacturing method and that of Example 2 lies in that: during the front-section molten salt treatment, the wire rod is cooled at a cooling rate of 39 °C / s, the molten salt temperature of the front-section molten salt is 450 °C, the treatment time is 20 s, and the hot-rolled wire rod is obtained after being taken off the production line. Example 3:

[0062] A preferred implementation manner of the manufacturing method of the 2500 MPa grade high-strength duplex hot-rolled wire rod for bridge cables according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include C: 0.97%, Si: 1.23%, Mn: 0.85%, Cr: 0.55%, Nb: 0.044%, V: 0.036%, Mo: 0.4%, P: 0.015%, S: 0.012%, and the rest are Fe and inevitable impurities; the manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt end quenching and toughening treatment → slow cooling on the roller table → coiling, specifically:

[0063] The rolling process is used to heat a steel billet with a specification of 220 mm × 220 mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, promote the homogenization of alloy components, reduce segregation, and increase the dissolution amount of Nb and V. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 16 mm through a rolling line. Appropriate rolling temperature and deformation amount are selected to induce the precipitation of Nb, promote dynamic recrystallization and grain refinement during the finishing rolling process, and strengthen and toughen the matrix. Specifically: control the soaking temperature of the heating furnace to be 1250 °C, the residence time in the furnace to be 160 min, the rough rolling temperature to be 1090 °C, the rough rolling reduction to be 15%, the finishing rolling temperature to be 1000 °C, and the finishing rolling reduction to be 23.5%; the wire laying process is used to make the wire rod exiting the rolling line into a wire rod through a wire laying machine, and the wire rod is scattered on the roller table and conveyed along the roller table, so that the wire rod is in a high-temperature austenite state, promoting the uniformity and fineness of austenite, and providing favorable conditions for the short-time rapid phase transformation of martensite. Specifically: control the wire laying temperature to be 970 °C.

[0064] The online molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with internal molten salt. The wire rod after wire drawing is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 40 °C / s, quickly skips the reticular carbide precipitation range from the high-temperature austenite state and enters the pearlite phase region or below, forming a certain amount of retained austenite with a large degree of supercooling, undergoing short-time martensitic transformation, promoting the transformation of most of the austenite structure into quenched martensite. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the back-stage molten salt treatment, controlling the wire rod to enter the pearlite phase region for isothermal transformation, controlling the untransformed high-temperature austenite to transform into pearlite, promoting the massive dispersion precipitation of niobium- and vanadium-containing carbides, inhibiting the growth of carbide strengthening phases. At the same time, after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering, improving the strength-ductility matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 473 °C, the treatment time is 9 s, the molten salt circulation rate is 800 t / h, and the molten salt temperature rise ≤ 5 °C; the molten salt temperature of the back-stage molten salt is 562 °C, the treatment time is 175 s, the molten salt circulation rate is 450 t / h, and the molten salt temperature rise ≤ 3 °C.

[0065] The roller table slow cooling process adopts not fully closing the heat preservation cover. The wire rod transported by the conveying roller table after passing through the second-stage salt bath tank enters the heat preservation cover to prevent the wire rod from cooling too fast during the cooling process, resulting in increased stress, promoting the further toughening of the wire rod structure, and improving the softening effect of the wire rod. Specifically: controlling the wire rod to slowly cool to 237 °C at a cooling rate of 0.62 °C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and after packaging and warehousing, the finished hot-rolled wire rod is obtained, and its metallographic structure diagram is as Figure 3 shown.

[0066] Comparative Example 4:

[0067] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 3 is that: the molten salt temperature of the back-stage molten salt is 585 °C, the treatment time is 230 s, and the hot-rolled wire rod is obtained after being taken off the production line.

[0068] Comparative Example 5:

[0069] A manufacturing method of a hot-rolled wire rod, the difference between its manufacturing method and that of Example 3 is that: the molten salt temperature of the back-stage molten salt is 525 °C, the treatment time is 100 s, and the hot-rolled wire rod is obtained after being taken off the production line. Example 4:

[0070] A preferred embodiment of the manufacturing method of the high-strength duplex hot-rolled wire rod for 2500MPa-class bridge cables of the present invention. The chemical composition and mass percentage of the hot-rolled wire rod include C: 0.95%, Si: 1.05%, Mn: 0.82%, Cr: 0.6%, Nb: 0.048%, V: 0.035%, Mo: 0.5%, P: 0.013%, S: 0.012%, and the rest are Fe and inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt end quenching and toughening treatment → slow cooling on the roller table → coiling. Specifically:

[0071] The rolling process is used to heat the steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the plastic state for rolling, promoting the homogenization of alloy components, reducing segregation, and increasing the dissolution amount of Nb and V. After the steel billet exits the heating furnace, the steel billet is rolled into a wire rod with a diameter specification of 15mm through the rolling line. Appropriate rolling temperature and reduction ratio are selected to induce the precipitation of Nb, promote dynamic recrystallization and grain refinement during the finishing rolling process, and strengthen and toughen the matrix. Specifically: control the soaking temperature of the heating furnace to be 1235°C, the residence time in the furnace to be 175min, the initial rolling temperature to be 1075°C, the initial rolling reduction to be 16%, the finishing rolling temperature to be 985°C, and the finishing rolling reduction to be 25%; The wire laying process is used to make the wire rod exiting the rolling line into a wire coil through the wire laying machine. The wire coil is scattered on the roller table and transported along the roller table, making the wire coil in the high-temperature austenite state, promoting the uniform fineness of austenite, and providing favorable conditions for the short-time rapid phase transformation of martensite in the future. Specifically: control the wire laying temperature to be 955°C.

[0072] The on-line molten salt end quenching and toughening treatment process uses a two-stage salt bath tank with molten salt inside. The wire coil after wire laying is transported through the roller table and passes through the first-stage salt bath tank for the front-stage molten salt treatment, making the wire coil cool down at a cooling rate of 41°C / s, quickly passing through the reticular carbide precipitation range from the high-temperature austenite state into the sorbite phase region or below, forming a certain amount of retained austenite with a large degree of supercooling, and undergoing short-time martensite phase transformation, promoting the transformation of most of the austenite structure into quenched martensite. Then the wire coil is transported through the roller table and passes through the second-stage salt bath tank for the rear-stage molten salt treatment, controlling the wire coil to enter the sorbite phase region for isothermal phase transformation, controlling the untransformed high-temperature austenite to transform into sorbite, promoting the massive dispersion precipitation of niobium- and vanadium-containing carbides, inhibiting the growth of carbide strengthening phases, and at the same time, after isothermal tempering, controlling the formed duplex structure to undergo a certain degree of short-time tempering, improving the strength-plasticity matching of the wire coil. Specifically: the molten salt temperature of the front-stage molten salt is 455°C, the treatment time is 15s, the molten salt circulation volume is 710t / h, and the molten salt temperature rise ≤ 5°C; the molten salt temperature of the rear-stage molten salt is 545°C, the treatment time is 200s, the molten salt circulation volume is 400t / h, and the molten salt temperature rise ≤ 3°C.

[0073] For the roller table slow cooling process, the heat preservation cover is not fully closed. The wire rods passing through the second salt bath tank are conveyed by the conveying roller table into the heat preservation cover to prevent the wire rods from cooling too fast during the cooling process, which may lead to an increase in stress, promote further toughening of the wire rod structure, and improve the softening effect of the wire rods. Specifically: the wire rods are slowly cooled at a cooling rate of 0.5 °C / s to 245 °C; the coiling process is used to coil the wire rods into coils through a coiling drum, and the finished hot-rolled wire rods are obtained after packaging and warehousing.

[0074] Comparative Example 5:

[0075] A manufacturing method of hot-rolled wire rods, the difference between its manufacturing method and that of Example 4 lies in: its manufacturing method is carried out according to the technological process of rolling → spinning → on-line molten salt end quenching and toughening treatment → air cooling → coiling. For the air cooling, the heat preservation cover is opened, and the wire rods passing through the second salt bath tank are conveyed by the conveying roller table. The wire rods are controlled to be slowly cooled at a cooling rate of 1.3 °C / s to 240 °C. After being taken off the production line, the hot-rolled wire rods are obtained. The tensile strength of the hot-rolled wire rods is 1761 MPa, the reduction of area is 23%, and the mechanical property difference within the same coil is 57 MPa.

[0076] The hot-rolled wire rods obtained from the above Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to microstructure and property tests, and the comparison results are shown in Table 1 below:

[0077] Table 1. Comparison results of microstructure and properties of different hot-rolled wire rod compositions and manufacturing methods

[0078]

[0079] From the results of Examples 1 to 4, it can be seen that by adopting the Nb-V-Mo chemical composition design combined with the on-line molten salt end quenching and toughening technology design, controlling the wire rods to quickly enter the sub-Sorbite phase region from the high-temperature austenite state at an ultra-high cooling rate for short-time martensite phase transformation, controlling the wire rods to enter the Sorbite phase region for isothermal phase transformation and short-time tempering, and roller table slow cooling toughening, a duplex structure composed of tempered sorbite and tempered martensite is formed in the microstructure, which can improve the strength-plasticity matching of the wire rods, effectively control the deteriorated structure of the wire rods, and achieve a product tensile strength of 1710 - 1750 MPa and a reduction of area of 25% - 30%.

[0080] From the comparison results of Example 1 and Comparative Example 1, it can be seen that selecting an appropriate spinning temperature can avoid the limitation of rolling control due to too low spinning temperature and the formation of network carbide during the spinning stage, and provide favorable conditions for promoting short-time martensite phase transformation in the subsequent molten salt end quenching at a higher temperature.

[0081] It can be seen from the comparison results between Example 2 and Comparative Example 2 that the higher the molten salt temperature and the shorter the treatment time in the front-stage molten salt, the less conducive it is to quenched martensite transformation. More austenite will transform into sorbite structure, and the matrix plasticity increases. However, if the molten salt temperature is too high and the treatment time is too short, the quenched martensite transformation is too little, and the strength of the wire rod will be significantly lost.

[0082] It can be seen from the comparison results between Example 2 and Comparative Example 3 that the lower the molten salt temperature and the longer the treatment time in the front-stage molten salt, the more the transformation of high-temperature austenite into quenched martensite. The proportion of tempered martensite in the structure increases, improving the matrix strength. However, if the molten salt temperature is low and the treatment time is too long, the untransformed high-temperature austenite is small, which will affect the sorbite transformation and isothermal tempering effect, and the plasticity of the wire rod will be lost.

[0083] It can be seen from the comparison results between Example 3 and Comparative Example 4 that the higher the molten salt temperature and the longer the treatment time in the rear-stage molten salt, the more thermal power can be provided for the isothermal tempering of quenched martensite and sorbite, promoting the improvement of the plasticity of the wire rod. However, if the molten salt temperature is too high and the treatment time is too long, the sorbite lamellar spacing is large, the wire rod softens quickly, and there is a risk of coarsening of carbide precipitation, which is not conducive to the control of strength and plasticity.

[0084] It can be seen from the comparison results between Example 3 and Comparative Example 5 that the lower the molten salt temperature in the rear-stage molten salt, the more beneficial it is to the lamination of sorbite lamellae, and more driving force can be provided for the dispersion precipitation of Nb and V carbides to improve the matrix strength. However, if the molten salt temperature is too low and the treatment time is too short, it is not conducive to the short-time tempering of the duplex structure. There is a large brittleness and stress left in the wire rod, and the Nb and V carbides do not have enough time to precipitate fully, resulting in the loss of wire rod strength and significantly affecting the plasticity of the wire rod.

[0085] It can be seen from the comparison results between Example 4 and Comparative Example 6 that by using an appropriate cooling rate in the roller table slow cooling to control the slow cooling of the wire rod, it is possible to avoid the increase in wire rod stress caused by too fast cooling rate and promote the further toughening of the wire rod structure.

[0086] 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 protection scope of the present invention. Any equivalent embodiments or changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A manufacturing method of high-strength dual-phase hot-rolled wire rod for 2500MPa-class bridge cables, characterized in that, The manufacturing method includes: Rolling wire rods according to the chemical composition of hot-rolled wire rods. The chemical composition and mass percentage of the hot-rolled wire rods include: C: 0.95% - 0.98%, Si: 1.05% - 1.25%, Mn: 0.65% - 0.85%, Cr: 0.40% - 0.60%, Nb: 0.04% - 0.048%, V: 0.035% - 0.045%, Mo: 0.40% - 0.50%, P ≤ 0.015%, S ≤ 0.015%, and the rest is Fe and inevitable impurities. After the wire rods are spun into coils at a spinning temperature of ≥925°C, they are subjected to on-line molten salt end quenching and toughening treatment. The coils first pass through the front-section molten salt and cool down at a cooling rate of ≥38°C / s, and part of the austenite structure transforms into quenched martensite. Then, they pass through the rear-section molten salt and are heated to the temperature of the sorbite phase region, controlling the untransformed austenite to transform into sorbite and isothermal tempering, controlling the precipitation and growth of carbides. Finally, they are slowly cooled on a roller table to produce hot-rolled wire rods with a duplex structure composed of tempered martensite and tempered sorbite. The molten salt temperature of the front-section molten salt is 455 - 480°C, and the treatment time is 5 - 15 s. The molten salt temperature of the rear-section molten salt is 545 - 570°C, and the treatment time is 160 - 200 s.

2. The manufacturing method of the high-strength duplex hot-rolled wire rod for 2500 MPa class bridge cables according to claim 1, characterized in that, Before rolling, control the soaking temperature of the heating furnace to be 1200 - 1250°C, and the time in the furnace to be 160 - 200 min.

3. The manufacturing method of the high-strength dual-phase hot-rolled wire rod for 2500 MPa class bridge cables according to claim 1, characterized in that, During rolling, control the initial rolling temperature to be 1040 - 1090°C, the initial rolling reduction to be 15% - 20%, the final rolling temperature to be 960 - 1000°C, and the final rolling reduction to be 23.5% - 28.5%.

4. The manufacturing method of the high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables according to claim 1, characterized in that, The molten salt circulation rate of the front-section molten salt is 400 - 800 t / h, and the molten salt temperature rise ≤ 5°C; the molten salt circulation rate of the rear-section molten salt is 200 - 450 t / h, and the molten salt temperature rise ≤ 3°C.

5. The manufacturing method of the high-strength dual-phase hot-rolled wire rod for 2500 MPa-class bridge cables according to claim 1, characterized in that, The slow cooling on the roller table controls the coils to be slowly cooled at a cooling rate of 0.5 - 0.75°C / s to below 250°C.

6. A high-strength duplex hot-rolled wire rod for 2500 MPa class bridge cables, characterized in that, The hot-rolled wire rods are obtained by the manufacturing method of the high-strength duplex hot-rolled wire rods for 2500 MPa-class bridge cables described in any one of claims 1 - 5.

7. The high-strength dual-phase hot-rolled wire rod for 2500 MPa-class bridge cables according to claim 6, wherein The volume percentage of the tempered martensite is 68% - 73%, and the lamellar spacing of the tempered sorbite is 70 - 100 nm.

8. The high-strength dual-phase hot-rolled wire rod for 2500 MPa-class bridge cables according to claim 6, wherein, The reticular carbide grade of the hot-rolled wire rods is grade 0, and the difference in mechanical properties within the same coil ≤ 50 MPa.

9. The high-strength duplex hot-rolled wire rod for 2500 MPa-class bridge cables according to claim 6, wherein The diameter of the hot-rolled wire rods is 9.0 - 16.0 mm, the tensile strength is 1710 - 1750 MPa, and the cross-sectional shrinkage rate is 25% - 30%.

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