Low-carbon, energy-saving production method for wire rods for bridge cables with a pressure of ≥ 2060 MPa

Abstract

The present invention relates to a low-carbon, energy-saving production method for bridge cable wire rod of ≧2060Mpa class, with the composition design of “high chromium / medium silicon / no precious alloy”, combined with the process flow of “KR pretreatment→BOF converter→LF refining→RH vacuum degassing→small billet continuous casting→slab finishing→slab heating→controlled rolling→new DP controlled cooling (air cooling+mist cooling)”, which does not require large billet blooming, heat treatment in water bath, salt bath, lead bath, etc. Still, it can obtain a microstructure with high sorbite conversion rate, fine cementite sheet layer spacing, and low net carbon level, and the final steel wire produced has a tensile strength of ≧2060Mpa, a torsion index of ≧30 times, and winding number of ≧8 times, which meets the national requirements for the construction of large span, ultra-high strength bridge cable. This process has successfully replaced twice-heated rolling forming and decomposition heat treatment, and has the advantages of low energy consumption, reduced carbon emissions, environmental friendliness and high production efficiency, which meets the national needs of green and low-carbon development.

Description

[Technical Field]

[0001] The present invention relates to the technical field of metal wire rods and their production and manufacturing, and more particularly to a hot-rolled wire rod for galvanized steel cables of large bridges with high strength of 2060 MPa and large spans, and a production method thereof. [Background technology]

[0002] China's large-scale bridge construction is developing rapidly, with an increasing number of large-span suspension bridges and cable-stayed bridges of over a kilometer in length being built. Improving the strength of bridge cable steel wire can significantly increase the span of a bridge and reduce its dead weight. With technological advances in China's metallurgical enterprises and metal products industry, key technologies such as 2000 MPa-class bridge cable wire are gradually maturing, and development is underway toward high-strength 2060 MPa and 2100 MPa wire rods.

[0003] To ensure the safety and reliability of long-span bridges, stricter requirements are being placed on the tensile strength, number of twists, number of turns, and fatigue life of the bridge cable steel wires of suspension bridges and cable-stayed bridges. Therefore, to obtain bridge cable steel wires with high strength, toughness, and plasticity, steel companies are making innovations in aspects such as chemical composition design, process flow, and annealing heat treatment of wire rods, and are processing them through processes such as drawing, galvanizing, and stabilization.

[0004] At present, the process design for producing galvanized steel wire rods with a pressure of 2000 MPa or higher both at home and abroad is characterized by diversification. The mainstream process uses a high-carbon, high-silicon chemical composition design, and some parts are subjected to large billet blooming, finishing, and two-time hot rolling. The toughening treatment of wire rods with this composition design involves re-austenitizing the wire rod through a decomposition salt bath / lead bath process or an online salt bath treatment process, which results in wire rods with high thermal conductivity, a high sorbite content due to isothermal transformation, fine cementite sheet interlayer spacing, and high toughness. However, the disadvantages are the long process flow, high energy consumption, high carbon emissions and production costs, and environmental pollution, which does not meet the low-carbon, green production process advocated by China. One steel company has successfully developed a high carbon, low silicon + micro-alloy strengthening composition design, and has realized the technology of double heating rolling + EDC water bath online annealing treatment, replacing the salt bath / lead bath process. This technology requires large capital investment and difficult maintenance. In addition, for steels with high alloy content, it is easy to obtain low-temperature brittle structures, and it is necessary to increase the use of different cooling media to reduce thermal conductivity.

[0005] Compared to the general DP (Stelmore air-cooling) process, the low thermal conductivity of air causes a series of problems, such as large differences between the surface and core structures, large overall variations, low sorbite conversion rates, coarse interlayer spacing, and heavy reticulated carbide, especially for large standards and in high summer temperatures. This generally only satisfies the production of galvanized steel wire cable wire of 1960 MPa or less. Furthermore, poor structure makes twist winding performance unstable, making it unsuitable for the production of high-strength cable wire of 2060 MPa or higher. To ensure strength, toughness, and plasticity, a decomposition heat treatment process is used for cable wire of 2060 MPa or higher to obtain a sorbite structure with a high ratio and narrow interlayer spacing of cementite.

[0006] Patent Publication No. CN109468530B relates to a hot-rolled wire rod and production method for galvanized steel wire for bridge cables with a pressure rating of 2000 MPa or higher. The composition is designed using a high-carbon, low-silicon composition system with micro-alloying elements of 0.90-1.10% C, 0.20-0.60% Si, 0.10-0.40% Cr, and 0.02-0.15% V. The process uses a double-heat rolling process for large billets and an online EDC water bath annealing and cooling method. This process requires high capital investment and maintenance costs, increases the amount of cooling medium in the water bath, and is expensive due to the double-heat rolling process and billet processing. The addition of V as a precious alloy strengthens the wire rod, which differs from the composition design, process flow, and toughening process of the wire rod described in this patent.

[0007] The patent with publication number CN112267069A relates to a wire rod for 2100Mpa-class cable steel wire and a manufacturing method thereof. The composition of the wire rod in this patent is 0.96-0.98% C, 1.00-1.09% Si, 0.40-0.49% Mn, ≦0.01-0.10% Ni, ≦0.01-0.10% Cu, ≦0.0005-0.0015% B, and 0.20-0.29% Cr. The composition design in this patent is high carbon, high silicon, and low chromium. This design provides excellent solid solution strengthening and cold work strengthening, but the sorbite content is high and the reticulate carbon level is low, making it difficult to control the structure. Therefore, it is only suitable for toughening treatment using a decomposed salt bath or lead bath, etc., and the wire rod must be reheated and austenitized, which increases decarbonization, coarsens the grains, reduces fatigue performance, increases process costs, reduces production efficiency, and causes environmental pollution.

[0008] The patent with publication number CN107299280A relates to a heat-treated wire rod for 2000 MPa-class cable steel wire and a production method thereof. The wire rod of this patent has the composition of 0.85%-1.0% C, 0.80-1.50% Si, 0.30-0.80% Mn, ≦0.015% P, ≦0.010% S, 0.20-0.80% Cr, and 0.010-0.080% Al. The composition design of this patent is high carbon and high silicon. This high silicon composition design not only plays an important role in solid solution strengthening and cold work strengthening, but also shortens the transformation incubation period, improves the nose temperature, and achieves a high sorbite conversion rate. The low level of reticulated carbide is unfavorable for obtaining the torsion performance of the steel wire, and is only suitable for sorbite treatment in a salt bath or lead bath with high thermal conductivity. The wire rod is then reheated to austenitize and then heat treated in a decomposed salt bath, which increases decarbonization, coarsens the grains, and reduces fatigue performance, resulting in high process costs, low production efficiency, and environmental pollution.

[0009] Patent publication number CN102936688B relates to a wire rod for bridge cables with a tensile strength of ≥ 2000 MPa and a production method thereof, the chemical composition of which is 0.95-1.20% C, 0.10-0.48% Si, 0.10-0.50% Cr, 0.60-1.00% Mn, 0.05-0.15% Al, and 0.0100-0.0300% N, and the composition design of which is high carbon and low silicon, with the appropriate addition of manganese chromium aluminum alloy and the gas element N. The production process involves rolling a continuous cast piece into wire rod, which is then laid on a laying head and cooled using conventional air cooling. This composition design and air-cooling process makes it easy for heavy reticular cementite to form, reducing the torsion index and winding plasticity index. The high manganese content makes it easy for martensite bands to segregate, affecting drawing performance. The high aluminum content makes it easy for casting performance to deteriorate and makes it difficult to control the size of inclusions. The high nitrogen content makes it easy for strain aging to occur, increasing brittleness and adversely affecting the torsion and fatigue properties of the cable.

[0010] The patent with publication number CN112501506A relates to a steel wire rod for bridge cables, which uses a manufacturing method for steel wire rod for bridge cables by using a small billet one-time heating and rolling forming method, omitting subsequent decomposition heat treatment, and the composition design is a general composition design of low silicon, low chromium, and V micro-alloying, and uses a general Stelmor air-cooling process, limited to the production of 1860Mpa-class cable wire rod.

[0011] CN201710764614.0 discloses steel for bridge cables with a tensile strength of ≥ 2300 MPa and a production method thereof, with chemical compositions and weight percentages of C 0.92-0.94%, Si 0.7-1.0%, Mn 0.45-0.55%, P ≤ 0.01%, S ≤ 0.01%, Cr 0.07-0.09%, V 0.16-0.20%, Al 0.16-0.20%, N 0.005-0.008%. , 0.001-0.0015% B, and 0.002-0.005% Zr. The patent uses a high-carbon, high-silicon composition design and a conventional air-cooling process, focusing only on tensile strength and number of twists. The method's low austenite stability and slow cooling rate prevent the achievement of a microstructure with a high sorbite content, fine cementite sheet spacing, and low reticulated carbon content. For bridge steel, in addition to tensile strength, the wire's plasticity index, torsional performance, and fatigue performance directly affect the safety, reliability, and service life of the bridge. The low number of twists and high plasticity index fatigue performance requirements are not met. The addition of a high content of precious alloy V (0.16-0.20%) for precipitation and solid-solution strengthening makes the steel uneconomical. The patent with publication number CN107587071A relates to a steel for bridge cables with a tensile strength of ≥ 2100 MPa and a production method thereof, and the composition design is similar to that of patent CN201710764614.0, but does not include V for alloying.

[0012] CN201710763768.8 relates to steel for bridge cables with a tensile strength of ≥ 2500 MPa and a production method, and is similar to patents with publication numbers CN201710764614.0 and CN107587071A. Both use blooming and double-roll forming. To achieve a strength of ≥ 2500 MPa, the C content is mainly increased to 1.21-1.25% and the Si content to 1.1-1.5%. Si is a non-carbide-forming element that promotes carbon diffusion. If the carbon content is too high and a general air-cooling process is used, the reticulated carbide and sorbite formation rate cannot be controlled. Furthermore, metallurgical theory and practical production data show that the carbon content of hypereutectoid steel is too high, resulting in the development of network carbides, which eventually become acicular, resulting in a rapid decline in the plasticity index and a serious deterioration in the drawability and torsion index of the steel wire. The patent does not disclose the important plasticity index of the steel wire base material. Furthermore, Ti is used for micro-alloying, but the solubility of Ti(NC) in austenite is low. The Ti content is too high, which makes it easy for micron-sized Ti(NC) to precipitate during solidification, posing a greater risk than inclusions and reducing the toughness of the steel. Furthermore, the Al content is 0.16-0.20%, with Al / N>20. The large amount of Al is excessive, and no inclusion modification treatment is used, significantly increasing the casting difficulty during smelting. Therefore, the number of continuous casting furnaces is small, making it easy to form gate nodules. The macroinclusions in the steel result in low cleanliness, and Zr is added to purify the molten steel, improving the smelting difficulty.

[0013] Therefore, when high-strength steel for bridge cables is currently produced using the existing online air-cooling process, it is impossible to obtain a microstructure with a high sorbite content, fine cementite sheet layer spacing, and a low reticulated carbon level, making it difficult to ensure the plasticity index of the wire rod, the torsional performance, and the service performance of the steel wire, which ultimately affects the safety, reliability, and service life of the bridge.

[0014] For these reasons, currently, high-strength (>1960 MPa) cable wire can only be produced by heat treatment in a water bath, salt bath, or lead bath, or by strengthening with the addition of precious alloy elements such as V, Nb, Mo, or Ni, followed by double-heat rolling. Each method has its own drawbacks. Without decomposition / online heat treatment or the addition of precious alloys, the DP process alone cannot produce wire rods or steel wires that meet the high tensile strength requirements, and they cannot meet the microstructural requirements of high toughness and plasticity, a high sorbite content, fine cementite sheet interlaminar spacing, and a low reticulated carbon content. To meet the needs of high-strength, long-span bridges in line with the low-carbon development trend, producing cable wire rods with high toughness and plasticity, simple components, simplified processes, and green, low-carbon properties with high production efficiency is a technical problem that must be solved urgently by those skilled in the art. Summary of the Invention [Problem to be solved by the invention]

[0015] This invention develops a simple, low-carbon, energy-saving hot-rolled wire rod and production method for bridge cables of ≥ 2060 MPa. It provides a new "high chromium / medium silicon / no precious alloy" composition design and a corresponding new DP controlled cooling process, which achieves a microstructure with a high sorbite content, fine cementite sheet interlayer spacing, and a low reticulated carbon level. This breaks the bottleneck of the DP process being unable to produce high-plasticity cable wire rod. It also develops a method of small billet heavy rolling + high-temperature billet heating (surface sprayed with antioxidant paint) + high-temperature controlled rolling to produce high-plasticity wire rod for cables of ≥ 2060 MPa. This replaces the double-heat rolling process and eliminates heat treatments such as water bath, salt bath, and lead bath. The process has the advantages of being green, highly efficient, low-carbon, energy-saving, and economically rational. [Means for solving the problem]

[0016] In the low-carbon, energy-saving hot-rolled wire rod for bridge cables of ≥ 2060 MPa class described in the present invention, the chemical components of the wire rod, calculated in weight percent, are 0.87 to 0.93% C, 0.60 to 0.70% Si, 0.20 to 0.50% Mn, 1.00 to 1.20% Cr, 0.0040 to 0.0070% B, 0.003 to 0.010% Al, P≦0.012%, S≦0.005%, N≦50 ppm, Ca≦3 ppm, TO≦8 ppm, H≦1 ppm, with the balance being Fe and unavoidable residual elements.

[0017] In order to eliminate the need for additional heat treatment, the chemical composition of the present invention is designed by taking into full consideration the differences in thermal conductivity between the online Stelmor air-cooled DP process and those of lead, salt, and water baths. To obtain wire rod with a high sorbite content, toughness, and plasticity, it is necessary to improve the stability of austenite while avoiding harmful structures such as martensite, network carbides, and large inclusions. The components of elements such as Cr, B, Si, C, Mn, and Al are scientifically and rationally designed, and residual elements such as V, Mo, Ni, P, S, Cu, N, and O are all kept as low as possible to improve overall performance. A new DP process has been developed that combines controlled rolling in the high-temperature recrystallization zone with air cooling and mist cooling after the laying head. This improves austenite stability, accelerates the cooling rate, suppresses the precipitation of network cementite, increases the sorbite content, refines the cementite sheet layer spacing, and avoids the appearance of brittle martensite structures.

[0018] Si plays an excellent role in solid solution strengthening and cold working strengthening, significantly improving the strength of steel wire, improving the tempering stability during galvanizing, and reducing the strength loss caused by cementite spheroidization. However, increasing the Si content not only reduces the plasticity of steel, but also shortens the transformation incubation period, promotes the precipitation of cementite, and improves the nose temperature of the CCT curve, which is unfavorable to achieving a high sorbite conversion rate and fine sheet layer spacing through the DP process. In order to meet the high strength requirements of the cable in combination with the DP process, the weight percentage of silicon in the present invention is designed to be 0.60-0.70% of medium silicon, Cr is a strong carbide-forming element, and has excellent solid solution strengthening, improved hardenability, and the role of refining the spacing between cementite sheet layers. This effectively improves strength and plasticity, while also improving the carbon diffusion activation energy and reducing the tendency for decarbonization. Its segregation tendency is lower than that of Mn. Increasing the Cr content helps realize the production of high-strength, high-plasticity cable wire rods through the DP process. If the Cr content is too high, it is easy to form a brittle martensite structure and reduce plasticity. If the Cr content is less than 1.0% and the other components and DP process conditions are not changed, the strength cannot reach the required 2060 MPa, which is unfavorable to the refinement of the sorbite sheet layers and affects plasticity. In the present invention, a high chromium content of 1.00-1.20% is designed, C is the main strengthening element in steel, and improves the tensile strength of steel by forming cementite. However, if the carbon content is too high and there is central carbon segregation, network cementite is likely to precipitate during the cooling process of the DP process, which significantly reduces or worsens the toughness and plasticity, causing the area reduction rate, twisting and winding indexes to fail, which is unfavorable for controlling the microstructure performance under the DP process conditions. If the carbon content is too low, it is difficult to achieve the required strength. Therefore, the weight percentage of carbon in the present invention is designed to be 0.87-0.93%, more preferably 0.88-0.92%. Mn element plays a role in solid solution strengthening and improving the stability of austenite, and is useful for improving strength and sorbite conversion rate. However, it also increases the tendency of grain coarsening and is prone to segregation. If the content is high, it is prone to form martensite band structure during continuous cooling, which deteriorates drawability and mechanical properties. Considering the lack of a subsequent isothermal heat treatment process, the weight percentage of manganese in the present invention is designed to be low manganese, 0.20-0.50%, more preferably 0.25-0.40%. B element is unevenly distributed at the austenite grain boundary, which can further improve the stability of austenite and the rate of sorbite formation in steel. It forms BN with nitrogen element during the cooling process, which plays a role in refining grains and refining the sorbite sheet interlayer spacing. It can also effectively control the strain aging of free nitrogen element, which is beneficial to the structure control of the DP controlled cooling process. If the boron content is too high, borides will precipitate, reducing the toughness and plasticity of the steel. Therefore, the weight percentage of boron in the present invention is designed to be 0.0040~0.0070%. The Al element can reduce the content of inclusions in steel through deoxidation, form AlN through nitrogen fixation, and refine the crystal grains. The deoxidation product Al2O3 is prone to form nodules at the stopper and gate during casting, causing liquid level fluctuations. When large brittle inclusions are present in steel, and inclusion modification treatment makes it difficult to float and remove the liquid inclusions, making it prone to forming large Ds, forming cavities during cold drawing, and reducing torsional and fatigue properties. The weight percentage of aluminum in the present invention is designed to be 0.030-0.010%. N element plays an excellent strengthening role, and free N is prone to strain aging, which significantly deteriorates the toughness and plasticity of steel. Therefore, in order to avoid strain aging caused by N element, it is necessary to add N-binding elements to fix nitrogen, which can refine the grains. The weight percentage of nitrogen in the present invention is designed to be ≦50 ppm, In order to avoid strain aging embrittlement due to free nitrogen atoms that reduces the torsional and winding performance of the steel wire, the composition is preferably designed to satisfy (2B+Al) / N≧2, which fixes nitrogen and forms BN and AlN, thereby refining crystal grains and improving plasticity.

[0019] The calcium element plays a role in modifying inclusions in the steelmaking process, forming low-melting calcium aluminate to improve the castability of molten steel. However, it is difficult to remove low-melting liquid inclusions and is prone to forming Ds inclusions, which will affect the fatigue performance of the cable. Therefore, the calcium content in the steel must be strictly controlled by selecting the alloy and adding the order of addition, and the calcium weight percentage of the present invention must be ≦3 ppm. Therefore, the present invention scientifically designs and rationally blends elements such as Cr, B, Si, C, Mn, and Al, mainly through interface strengthening, transformation strengthening, solid solution strengthening, and fine grain strengthening mechanisms to ensure the strength of the bridge cable wire. Controlled rolling in the high-temperature recrystallization region is combined in cooperation with a new DP process to improve the sorbite conversion rate of the wire, refine the spacing between cementite sheet layers, reduce the network carbon level, and improve the tensile strength and plasticity index of the wire.

[0020] The process flow is "KR → Converter → LF Refining → RH Vacuum Degassing → Small Billet Continuous Casting → Steel Billet Finishing → Steel Billet Heating → Controlled Rolling → New DP Controlled Cooling (Air Cooling + Mist Cooling) → Packaging". (1) In KR pretreatment, the S content of hot metal is controlled to ≦0.002% using a desulfurization agent, but the S content cannot be reduced to 0.005% or less in the low basicity refining process. (2) In the converter, double slag dephosphorization and low-phosphorus, high-carbon tapping are used to prevent overoxidation of molten steel, reduce the cold work embrittlement caused by the high phosphorus content of high-strength steel wire, and control the P content of the converter tapping steel to be ≦0.010%; (3) In LF refining, deoxidation is performed using low-basicity refining slag (CaO:SiO2=1-3) and ferrosilicon alloy, and the deoxidation product is a low-melting-point deformation inclusion. (4) In the RH vacuum degassing, the RH high vacuum degree is ≦133 Pa, and the high vacuum degassing time is ≧15 min. The steel is free of large-sized Al2O3 brittle inclusions and Ds spherical inclusions, improving the drawing performance and torsion performance. (5) In the continuous casting of small billets, the current of the crystallizer and the strong electromagnetic stirring terminal is 300-400A, the secondary cooling is weak cooling ≦0.40L / Kg, the continuous casting drawing speed is consistent with that of the heavy rolling terminal, the solid fraction fs in the rolling area is 0.30-0.75, the single roll rolling amount is 3-6mm, and the total rolling amount X1=(10-20)%×thickness of the billet, and the size of the continuous casting small billet is (160-X1)×(160+X2)mm. 2 and X2 is the spread, Furthermore, the carbon segregation index of the small billet is ≦1.08, and the center shrinkage cavity is ≦0.5 level, thereby improving the low density of the small billet rolled wire rod.

[0021] (6) In the finishing of billets, in order to ensure the surface quality of the wire and to control production costs, magnetic particle inspection is carried out on the billets to an accuracy of 0.3 mm, and spot re-grinding is carried out on defects that do not meet the standards.

[0022] (7) In order to improve the diffusion effect of alloying elements during heating of the steel slabs and reduce oxidation and decarbonization in the furnace, the steel slabs are coated with a heat-resistant oxidation-resistant paint, the high-temperature diffusion temperature is 1220-1270°C, more preferably 1240-1270°C, the total heating time is 130-170 minutes, the high-temperature diffusion heat-retention time is greater than 50% of the total heating time, and the residual oxygen content in each zone in the furnace is 1-2%.

[0023] Furthermore, after shot blasting the steel piece, the paint powder and solvent are mixed uniformly, and then the surface is sprayed evenly with a sprayer and compressed air. After the paint has dried, the steel piece is rotated to control the thickness of the coating film to 0.2-0.6 mm, and the total decarbonization layer of the wire is ≦0.10 mm.

[0024] (8) In controlled rolling, in order to obtain a high sorbite conversion rate and suppress the precipitation of network carbon, controlled rolling in the high-temperature recrystallization region is used to coarsen the grain size, improve the stability of austenite, and shift the CCT curve downward to the right. The temperature of the final rolling after finish rolling + diameter reduction is controlled to 950-990°C, the laying head temperature is controlled to 900-950°C, and the rolling speed is 25-40m / s. Controlled cooling is performed in a 3# tank after finish rolling and in a 6# tank after diameter reduction. Controlled cooling is advantageous for uniformly controlling the annealing of the rolled material and improving the uniformity of the temperature and structure.

[0025] (9) In the new DP controlled cooling (air cooling + mist cooling), the cooling capacity of the stermower after the laying head is 260,000 m of the previous 10 units. 3 / h centrifugal fan, the latter six 200,000 m 3 / h centrifugal fans are used, each corresponding to a 3m roller table. Fans 1 to 6 are turned on at 100%, fans 7 to 12 are turned on at 90 to 100%, fans 13 to 14 are turned on at 70 to 100%, and fans 15 to 16 are turned on at 0 to 100%. Four mist nozzles (every 90°) are specially installed on the dampers on both sides of fans 1 to 10, and the nozzle water flow rate of each fan is set to 0 to 0.5m. 3 / h, compressed air pressure 0.8~1.8Mpa, water temperature is consistent with the ambient temperature, when the fan is turned on, the mist is automatically sucked into the fan, distributed by the Jialing fan, and then blown out from the fan vents in each zone, effectively improving the thermal conductivity of the air cooling, and significantly improving the cooling rate before transformation of the wire rod after use, with an average rate of 15.1~19℃ / s (specification Φ13~15mm). On the one hand, it quickly passes through the two-phase region, inhibits the network precipitation of secondary cementite from the grain boundary, and reduces the network carbon level to ≦1.5 level. On the other hand, based on the high chromium and medium silicon design of the steel grade of this invention and the characteristics of austenite stability after high temperature controlled rolling, the synergistic effect of "air cooling + mist cooling" is achieved. This involves rapidly cooling the austenite to 580-600°C to initiate transformation temperature recovery, avoiding or reducing the amount of pearlite precipitation in the Ar1-650°C range (the pearlite sheets are coarse and the superplasticity is lower than that of sorbite; the lower the ratio, the better the superplasticity of the wire rod). Then, by adjusting the fan air volume, roll speed and temperature in the transformation region, the wire rod is controlled to complete the "isothermal" transformation in the range of 580-620°C, resulting in a wire rod with a high sorbite ratio of 91-96%, fine and uniform cementite sheets, and a sheet thickness of 90-115nm. This indicator determines the high superplasticity of the cable wire rod and has a direct impact on the strength, torsional performance and fatigue life of the cable.

[0026] Furthermore, the roller table speed is set to 55-90m / min, the opening of the jialing fan is set to 20-60%, and when the temperature of the wire reaches ≦580℃, the heat-retaining cover is closed to cool it, and the cooling rate inside the cover is set to 0.3-1℃ / s, which reduces internal stress and prevents martensite transformation.

[0027] Furthermore, the wire rod obtained by the present invention has a tensile strength of 1400 to 1500 MPa, an area reduction rate of ≥ 36%, and a reticulated carbon content of ≤ 1.0 level.

[0028] Furthermore, after the wire is subjected to surface pickling treatment, multiple drawing, hot-dip galvanization (or aluminum magnesium) plating, and stabilization treatment, the galvanized steel wire has a tensile strength of ≧2060 MPa, a torsion index of ≧30 times, and a winding number of ≧8 times. [Effects of the Invention]

[0029] According to the above technical means, the present invention provides a low-carbon, energy-saving wire rod for cables of ≥2060 MPa, with a diameter of Φ13-15 mm, a sorbite content of 91-96%, a cementite sheet layer spacing of 90-115 nm, a tensile strength of 1400-1500 MPa, an area reduction rate of ≥36%, and a reticulated carbon level of ≤1.0. The tensile strength of galvanized steel wire is ≥2060 MPa, a torsion index of ≥30 turns, and windings of ≥8 turns. Both the wire rod and the steel wire have excellent toughness and plasticity indexes, and the key torsion indexes of steel wire far exceed the industry standard, resulting in higher safety, reliability, and fatigue resistance. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a metallographic diagram of the microstructure produced in Example 1. [Figure 2] FIG. 1 is an SEM image of the microstructure produced in Example 1. [Figure 3] FIG. 1 is a metallographic diagram of the microstructure produced in Example 3. [Figure 4] FIG. 1 is an SEM image of the microstructure produced in Example 3. [Figure 5] FIG. 10 is a metallographic diagram of the microstructure produced in Comparative Example 9. [Figure 6] FIG. 10 is an SEM image of the microstructure produced in Comparative Example 9. [Figure 7]FIG. 11 is a metallographic diagram of the microstructure produced in Comparative Example 11. [Figure 8] FIG. 10 is an SEM image of the microstructure produced in Comparative Example 11. [Figure 9] FIG. 10 is a metallographic diagram of the microstructure produced in Comparative Example 13. [Figure 10] FIG. 10 is an SEM image of the microstructure produced in Comparative Example 13. DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention is not limited to the following specific embodiments, and those skilled in the art can implement the present invention using various other specific embodiments based on the disclosure of the present invention, or any changes or modifications made easily using the design structure and concept of the present invention will fall within the protection scope of the present invention.

[0032] The technical means of the present invention will be described in more detail below in combination with the preferred embodiments of the present invention.

[0033] The chemical composition and new DP process are innovative technologies that determine the microstructure and performance of the wire rod of the present invention. As can be seen from the data comparison in Table 5 below, the DP process with added mist cooling plays a major role in the sorbite conversion rate of the wire rod, the spacing between cementite sheet layers, and the torsion of the steel wire. The small billet heavy rolling + billet coating high temperature heating + controlled rolling process also belong to the key technologies of the present invention, so the wire rod for bridge cables of ≥ 2060 MPa of the present invention can be produced without the need for precious alloys, double heating and rolling forming, or heat treatments such as salt baths, lead baths, or water baths.

[0034] Examples and Comparative Examples Chemical composition: The steel grade number at the time of smelting was ZT90SiCr, and the smelting process parameters and smelting chemical composition weight percentages (wt%) of Examples 1* and 2* and Comparative Examples 1*, 2*, and 3* are shown in Tables 1 and 2 below, respectively.

[0035] Smelting Process Parameters [Table 1]

[0036] Chemical composition wt% [Table 2]

[0037] Continuous casting: The crystallizer's strong electromagnetic stirring parameters were 300A / 3Hz, the end electromagnetic stirring was 400A / 6Hz, the drawing speed was 1.95m / min, the No. 1-5 rolls were dynamically adjusted, the initial distribution was 3-5-5-6-6, the center segregation index was 1.04-1.07 on average, and the shrinkage cavity level was 0. The cast size was 135mm x 167mm.

[0038] Steel pieces: Magnetic particle inspection + re-grinding process is carried out to remove surface dents and micro-cracks, and an anti-oxidation coating is sprayed evenly onto the surface of the steel pieces using a paint sprayer + compressed air, with the coating thickness being 0.3~0.5mm for different steel pieces.

[0039] Heating: Using an intelligent combustion heating system, the temperatures of the low temperature zone (preheating zone <900°C, first heating zone 1093~1115°C) and high temperature zone (second heating zone + soaking zone) are 1242~1268°C, the total heating time is 142~165min, the high temperature zone time is 77~89min, and the residual oxygen content of each zone of the heating furnace is 1.0~1.8%.

[0040] Rolling: The temperature for finish rolling and final diameter reduction is controlled at 953~989℃, the laying head temperature is controlled at 910~937℃, and the roller table speed is started at 1.0m / s and gradually increased by 0.01m / s. The rolling speed is 28m / s, and controlled cooling is performed in a 3# tank after finish rolling and in a 6# tank after diameter reduction, which is advantageous for uniformly controlling the annealing of the rolled material.

[0041] The new DP process, air cooling + mist cooling, significantly improves the cooling rate before the wire transformation. Combined with the synergistic effects of the high-chromium, medium-silicon steel design and the austenite stability after high-temperature controlled rolling, it effectively inhibits pearlite transformation during cooling, improving the sorbite content, laminar spacing, and high plasticity of the cable wire. These are crucial factors in the tensile strength and torsion index of the cable steel wire. For the air cooling + mist cooling process parameters at an ambient temperature of 25°C, see the examples and comparative examples in Tables 3 and 4 (nozzle water flow rates for fans 1-10). When these parameters are met, the roller table speed starts at 60 m / min, the Jialing fan opening is set to 20-60%, and after the wire is air-cooled, the insulation cover is closed and the cooling process begins.

[0042] New DP process parameters [Table 3]

[0043] Mist cooling process parameters (water flow rate of nozzle before damper m 3 / h) [Table 4]

[0044] The performance indicators of wire rods produced by Examples A-B and Comparative Examples C-D using the chemical compositions and DP processes shown in Table 2 and Table 3, respectively, are shown in Table 5. These indicators are also shown in Table 5. The results demonstrate that the strength and plasticity of both wire rods and cable wires can be achieved by appropriately combining chemical composition and DP processes. Comparative Examples 1-14 do not meet the technical standard requirements for wire rod strength ≥ 1400-1500 MPa, sorbite content ≥ 90%, reticulated carbon ≤ 2.0, cementite sheet layer spacing 90-150 nm, area reduction ≥ 30%, cable wire strength ≥ 2060 MPa, twist count ≥ 14, and winding count ≥ 8.

[0045] Important performance indicators for wire rods and steel wires [Table 5]

[0046] remarks: 1) The combination of Example 1* of the chemical composition with Examples A and B of the DP process and Comparative Examples C and D corresponds to Examples 1 and 3 and Comparative Examples 1 and 3 in Table 5, respectively. When Example 1* of the chemical composition is combined with Comparative Examples C and D of the DP process, the high plasticity of the wire rod and steel wire does not meet the requirements. 2) The combination of Example 2* of chemical composition with Examples A and B of DP process and Comparative Examples C and D corresponds to Examples 2 and 4 and Comparative Examples 2 and 4 in Table 5, respectively. When Example 2* of chemical composition is combined with Comparative Examples C and D of DP process, the high plasticity of the wire rod and steel wire does not meet the requirements. 3) The combination of Comparative Example 1* of the chemical composition with Examples A and B of the DP process and Comparative Examples C and D corresponds to Comparative Examples 5, 7, 9, and 11 in Table 5, respectively. When Comparative Example 1* of the chemical composition (with reduced Cr) is combined with Comparative Examples A and B of the DP process, the strength of the wire rod and steel wire does not meet the standard requirements. When combined with Comparative Examples C and D of the DP process, the high plasticity of the wire rod and steel wire does not meet the requirements. 4) The combination of Comparative Example 2* of the chemical composition with Examples A and B of the DP process and Comparative Examples C and D corresponds to Comparative Examples 6, 8, 10, and 12 in Table 5, respectively. When Comparative Example 2* of the chemical composition (with reduced Si) is combined with Comparative Examples A and B of the DP process, the strength of the wire rod and steel wire does not meet the standard requirements. When combined with Comparative Examples C and D of the DP process, the high plasticity of the wire rod and steel wire does not meet the requirements. 5) The combination of Comparative Example 3* of chemical composition with Examples A and B of the DP process corresponds to Comparative Examples 13 and 14 in Table 5, respectively. When the Si content is increased based on the composition of Example 1* and designed to be 0.90%, the tensile strength of the wire reaches 1500 MPa or more and the strength of the cable steel wire reaches 2100 MPa or more, but the area reduction rate of the wire, the sorbite conversion rate, the torsion index of the cable, and the winding performance are all significantly reduced, and the requirements for the plasticity index of the wire and steel wire cannot be met.

[0047] In the case of a wire rod having a standard diameter of 13 mm according to the present invention, a cable steel wire having a diameter of ≦5.0 to 6.0 mm is processed accordingly, and in the case of a wire rod having a diameter of 15 mm, a cable steel wire having a diameter of 7.0 mm is processed accordingly.

Claims

1. A method for producing a low-carbon, energy-saving wire rod for bridge cables of ≧2060 MPa class, comprising: The chemical composition of the smelted wire rod to be produced is calculated by mass percent as follows: 0.87-0.93% C, 0.60-0.70% Si, 0.20-0.50% Mn, 1.00-1.20% Cr, 0.0040-0.0070% B, 0.003-0.010% Al, P≦0.012%, S≦0.005%, N≦50 ppm, Ca≦3 ppm, TO≦8 ppm, H≦1 ppm, the balance being Fe and unavoidable residual elements. The process flow for bridge cable wire rod is as follows: KR pretreatment → BOF converter → LF refining → RH vacuum degassing → small billet continuous casting → billet finishing → billet heating → controlled rolling → new DP controlled cooling → packaging. The new DP controlled cooling process involves air cooling + mist cooling the wire rod after the controlled rolling laying head with a stermower, adjusting the air cooling + mist cooling parameters to control the cooling rate before transformation to 15-19 ° C / s, and cooling to 580-600 ° C to start transformation temperature recovery. The wire rod has a sorbite conversion rate of 91 to 96%, a cementite sheet layer spacing of 90 to 115 nm, a tensile strength of 1400 to 1500 MPa, an area reduction rate of ≥ 36%, and a reticulated carbon level of ≤ 1.

0. A production method characterized by:

2. The chemical composition of the wire rod for bridge cables is (2B+Al) / N≧2.

2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.

3. In the KR pretreatment, a desulfurization agent is used to control the sulfur content of the hot metal to ≦0.002%; In the BOF converter, the converter tapping P is controlled to be ≦0.010%; In LF refining, deoxidation is performed using low basicity refining slag and ferrosilicon alloy. In the RH vacuum degassing, the RH high vacuum degree is ≦133 Pa, and the high vacuum degassing time is ≧15 min.

2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.

4. The process conditions for the small billet continuous casting are: the crystallizer and terminal strong electromagnetic stirring current is 300-400A; the secondary cooling is weak cooling ≦0.40L / Kg; the continuous casting drawing speed is consistent with that of terminal heavy rolling; the solid fraction fs in the rolling area is 0.30-0.75; the single roll rolling amount is 3-6mm; and the total rolling amount X １ = (10 to 20)% × thickness of the cast piece, 2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.

5. In the heating of the steel slab, an antioxidant paint is sprayed onto the surface of the steel slab before heating, the high-temperature diffusion temperature is 1220-1270°C, the total heating time is 130-170 min, the high-temperature diffusion heat-keeping time is more than 50% of the total heating time, and the residual oxygen content in each zone in the furnace is 1-2%.

2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.

6. The conditions of the controlled rolling are that the temperature of the finish rolling and the final diameter reducing rolling is controlled to 950 to 990 ° C, the laying head temperature is controlled to 900 to 950 ° C, and the rolling speed is set to 25 to 40 m / s.

2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.

7. The temperature before the transformation temperature recovery is set to 580 to 600°C, and the wire is controlled so that the transformation is completed at 580 to 620°C. When the temperature of the wire reaches 580°C or less, the heat-retaining cover is closed to cool it down.

2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.

8. The torsion index of the bridge cable is ≥ 30 times; 2. The method for producing low-carbon and energy-saving wire rod for bridge cables with a resistance of ≥ 2060 MPa according to claim 1.