A hot-rolled wire rod for ultra-high-strength bridge cables and a production method thereof

Abstract

The application discloses a kind of super-high-strength bridge cable hot-rolled wire rod and its production method, sequentially including smelting, continuous casting, rolling, salt bath isothermal treatment, collecting volume, slow cooling treatment step, by smelting and continuous casting step obtains the continuous casting billet of required composition and proportion, wire rod is wiredrawing as wire rod at 880~920 DEG C temperature, wire rod after wiredrawing is directly immersed in 200-300 DEG C salt bath and isothermal treatment is carried out, obtain medium carbon martensite and untransformed austenite, wire rod after salt bath treatment is immediately collected volume, with not less than 180 DEG C temperature enters 380-420 DEG C constant temperature annealing furnace and is slowly cooled, so that carbon element in martensite diffuses to untransformed austenite, obtain a small amount of bainite and residual austenite.The wire rod of the application is used to manufacture bridge cable steel wire, only needs to carry out surface treatment to wire rod, after 1-2 pass drawing, galvanizing / aluminum plating, the strength of steel wire reaches 2000-2100MPa, and the torsion frequency of finished product steel wire is not less than 12 times.

Description

Technical Field

[0001] This invention relates to the field of metal wire manufacturing technology, specifically to a hot-rolled wire rod for ultra-high strength bridge cables and its production method. Background Technology

[0002] Galvanized steel wire for bridges, as a core engineering material for the construction of long-span suspension bridges and cable-stayed bridges, has always been a key research focus for steel companies and research institutes. Based on the requirements for the overall structural rigidity, service life, and safety of bridges, high-strength and ultra-high-strength galvanized steel wire for bridges has become the development direction. In recent years, key projects such as the Lingdingyang Bridge, Changtai Bridge, Chaoma Bridge, and Zhangjinggao Bridge have used ultra-high-strength bridge cable steel wires with strength levels continuously increased to 2060MPa, 2100MPa, and 2200MPa. The most critical indicator for the research and development of hot-dip galvanized steel wire for ultra-high-strength bridge cables is the torsional performance of the wire. The national standard GB / T17101-2019 "Hot-dip Galvanized Zinc-Aluminum Alloy Steel Wire for Bridge Cables," promulgated in 2020, stipulates that the torsional performance of galvanized or zinc-aluminum alloy steel wire should not be less than 8 cycles, while in practical applications, the standard has been further raised to more than 12 cycles.

[0003] Currently, bridge cable steel wire is manufactured using high-carbon pearlitic wire rod, with a processing technology of hot-rolled wire rod - multi-pass drawing - hot-dip galvanizing (aluminum) - stabilization - finished steel wire. The strength of bridge cable steel wire is the initial strength of the wire rod + cold work hardening - strength loss from galvanizing (aluminum) treatment. Measures to improve tensile strength mainly involve increasing the carbon content of the wire rod to increase the original tensile strength, and increasing the diameter of the original wire rod to increase the deformation amount and enhance cold work strengthening. However, these measures all lead to a reduction in the plasticity of the steel wire, making it impossible to meet the requirements for another key performance indicator of bridge cable steel wire: the number of torsion cycles. In fact, the biggest challenge in manufacturing ultra-high strength bridge cable steel wire currently lies in the difficulty of simultaneously improving strength and meeting the torsion performance requirements.

[0004] Numerous studies have shown that the main reason for the variation in the torsional properties of bridge cable wires lies in the changes in the cementite phase within the pearlite during drawing and hot-dip galvanizing processes. During wire drawing, the cementite lamellae in the pearlite microstructure gradually evolve into an amorphous structure due to the large deformation. At this point, it exhibits good deformation consistency with the violently deformed ferrite phase, resulting in good torsional properties of the wire before galvanizing. However, during subsequent hot-dip galvanizing (aluminum galvanizing), the amorphous cementite reverts to a crystalline structure due to heat. If not properly controlled, this leads to a sharp decline in the torsional properties of the finished galvanized (aluminum galvanized) wire. To suppress this change, the current technical approach for developing high-strength bridge cable wires involves significantly increasing the carbon content in the chemical composition and adding large amounts of alloying elements such as Si, Cr, and V. This increases production difficulty and makes balancing strength and torsional properties more challenging. Therefore, a completely new chemical composition and microstructure design are necessary to solve the technical problem of poor torsional properties in ultra-high-strength bridge cable wires. Summary of the Invention

[0005] The purpose of this invention is to provide a hot-rolled wire rod for ultra-high strength bridge cables and its production method, so as to solve the problem that there is currently no segregation evaluation method for medium and low carbon steel (C<0.4%) in the industry.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A hot-rolled wire rod for ultra-high strength bridge cables has the following chemical composition by weight: C: 0.24-0.36 wt.%; Si: 1.50-2.50 wt.%; Mn: 1.50-2.50 wt.%; Cr: 0.2-1.30 wt.%; Mo: 0.15-0.45 wt.%; P: ≤0.020 wt.%; S: ≤0.010 wt.%; O: ≤0.0015 wt.%; N: ≤0.0030 wt.%; the remainder being Fe and unavoidable impurities.

[0008] Furthermore, the wire rod has a diameter of 7mm or 9mm, a tensile strength of 1700-1800MPa, and a reduction of area of ​​48-58%.

[0009] Furthermore, the wire rod contains 75-85% martensite, 10-15% austenite, and 0-10% bainite.

[0010] C is the most important strengthening element in steel. This invention mainly utilizes phase transformation strengthening and grain refinement strengthening to enable wire rods to obtain high strength and plasticity.

[0011] Si is a deoxidizing and strengthening element in steel, mainly distributed in ferrite, playing a role in solid solution strengthening. Most importantly, Si also inhibits carbide formation, promoting the diffusion of C atoms into untransformed austenite during phase transformation, improving its stability, and thus obtaining retained austenite.

[0012] Mn is a strengthening element in steel, which can improve the strength and hardenability of steel and ensure good uniformity of cross-sectional microstructure; it also has the effect of stabilizing austenite.

[0013] Cr effectively improves hardenability and delays bainitic transformation, refines microstructure, and enhances steel toughness. Furthermore, it helps reduce the tendency for surface decarburization during wire rod manufacturing, resulting in high fatigue resistance. It is also an austenite stabilizing element, increasing the stability and content of retained austenite.

[0014] Al is a major deoxidizing element. Maintaining a certain amount of Al in steel helps to refine the austenite grains and improve the strength and toughness of the steel.

[0015] AlN, formed by the combination of nitrogen and al, can refine austenite grains, but excessive nitrogen dissolved in the matrix will reduce the plasticity of steel.

[0016] The oxygen element is controlled at a certain level to ensure the purity of molten steel and improve the fatigue performance of the material.

[0017] A method for producing hot-rolled wire rod for ultra-high strength bridge cables includes the following steps: smelting, continuous casting, rolling, isothermal salt bath treatment, coiling, and slow cooling. The smelting and continuous casting steps yield a continuously cast billet with the composition and proportions described in claim 1. The wire is spun into wire rod at a temperature of 880–920°C. The spun wire rod is then directly immersed in a salt bath at 200–300°C for isothermal treatment to obtain medium-carbon martensite and untransformed austenite. The salt-treated wire rod is immediately coiled and slowly cooled in a constant-temperature annealing furnace at 380–420°C at a temperature not lower than 180°C, allowing carbon elements in the martensite to diffuse into the untransformed austenite, resulting in a small amount of bainite and retained austenite. The wire rod ultimately obtains a composite microstructure of low-carbon martensite, a small amount of bainite, and retained austenite.

[0018] Preferably, the isothermal treatment time in the salt bath is 30-90 seconds.

[0019] Preferably, the slow cooling process takes no less than 30 minutes.

[0020] Preferably, after the continuously cast billet is heated in the continuous casting process, it is first rough rolled and then finish rolled. The heating temperature is 1040-1150℃, the starting rolling temperature of the rough rolling is 980-1020℃, and the entry temperature of the finish rolling is 880-920℃.

[0021] Preferably, the continuous casting process yields a rectangular billet with a cross-sectional dimension of not less than 240mm × 180mm, to ensure that the wire rod has a sufficient compression ratio after rolling to guarantee the homogeneity of the wire rod.

[0022] Preferably, in the continuous casting process, the superheat of the molten steel is controlled at 30-45°C, the electromagnetic stirring current at the end of solidification is 240±25A, the stirring frequency of the crystallizer is 5.5±0.5Hz, the casting speed during continuous casting is 1.05±0.05m / min, and the specific water content during continuous casting is 0.30±0.01L / kg.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] This invention relates to a wire rod design featuring a low-to-medium carbon alloy composition and a post-rolling residual heat treatment process. First, isothermal quenching in a salt bath yields medium-carbon martensite and untransformed austenite. Immediately following, online tempering diffuses carbon from the martensite into the untransformed austenite. During tempering and subsequent cooling, a small amount of bainite and retained austenite are obtained. The wire rod ultimately achieves a composite microstructure of low-carbon martensite, a small amount of bainite, and retained austenite, resulting in wire rods with diameters of 7mm and 9mm exhibiting a tensile strength of 1700-1800MPa and a reduction of area of ​​4%. Performance of 8-58%; When manufacturing bridge cable steel wire using the wire rod of this invention, only surface treatment of the wire rod is required, followed by 1-2 draw passes and galvanizing (aluminizing) to obtain finished steel wire with a diameter of 5-7mm and a strength of 2000-2100MPa. Due to the adoption of a brand-new microstructure design and a significant reduction in the drawing amount, the deterioration of the steel wire torsion index caused by carbide changes during the drawing and galvanizing (aluminizing) process of the prior art is avoided. The finished steel wire produced by the wire rod of this invention has a torsion count of no less than 12 times. Attached Figure Description

[0025] Figure 1 This is a microstructure diagram of the wire rod in Embodiment 1 of the present invention;

[0026] Figure 2 This is a microstructure diagram of the drawn steel wire from Embodiment 1 of the present invention;

[0027] Figure 3 This is a microstructure diagram of the galvanized aluminum steel wire of Example 1 of the present invention;

[0028] Figure 4 This is a microstructure diagram of the galvanized aluminum steel wire after twisting in Example 1 of the present invention;

[0029] Figure 5 This is a microstructure diagram of the wire rod in Embodiment 2 of the present invention;

[0030] Figure 6 This is a microstructure diagram of the drawn steel wire from Embodiment 2 of the present invention;

[0031] Figure 7 This is a microstructure diagram of the galvanized aluminum steel wire of Example 2 of the present invention;

[0032] Figure 8 This is a microstructure diagram of the galvanized aluminum steel wire after twisting in Example 2 of the present invention;

[0033] Figure 9 This is a microstructure diagram of the wire rod in Embodiment 3 of the present invention;

[0034] Figure 10 This is a microstructure diagram of the drawn steel wire from Embodiment 3 of the present invention;

[0035] Figure 11This is a microstructure diagram of the galvanized aluminum steel wire of Example 3 of the present invention;

[0036] Figure 12 This is a microstructure diagram of the galvanized aluminum steel wire after twisting in Example 3 of the present invention. Detailed Implementation

[0037] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below through examples.

[0038] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0039] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0040] Example 1:

[0041] Please see Figure 1 This embodiment provides a hot-rolled wire rod for ultra-high strength bridge cables, having the following chemical composition by weight: C: 0.30 wt.%; Si: 1.50 wt.%; Mn: 2.50 wt.%; Cr: 0.85 wt.%; Mo: 0.30 wt.%; P: 0.010 wt.%; S: 0.010 wt.%; O: 0.0012 wt.%; N: 0.0030 wt.%; the remainder being Fe and unavoidable impurities.

[0042] This embodiment also provides a production method for the aforementioned hot-rolled wire rod for ultra-high strength bridge cables. The process involves sequentially passing blast furnace hot metal → KR desulfurization → top and bottom blowing converter steelmaking → LF refining → RH refining → continuous casting to obtain a rectangular billet of 180*240mm. After KR desulfurization, the sulfur content of the blast furnace hot metal entering the converter is ≤0.0055%. The converter uses a double-slag blowing method, which reduces the content of impurity elements in the molten steel. At the converter endpoint, the phosphorus content is ≤0.015%, and the sulfur content is ≤0.010%. The LF furnace uses aluminum deoxidation and a high-basicity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25min, controlling the oxygen content in the steel to ≤15ppm and the nitrogen content to ≤30ppm. Continuous casting is performed under full protective conditions to avoid secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billet. During continuous casting, the superheat of molten steel is controlled at 35-40℃, and electromagnetic stirring is used to control the compositional segregation of the continuously cast billet. The electromagnetic stirring current at the end of solidification is 240A, the stirring frequency of the crystallizer is 5.5Hz, the casting speed during continuous casting is 1.05m / min, the specific water content during continuous casting is 0.30L / kg, and the segregation index of the continuously cast billet is 1.05.

[0043] After shot blasting and flaw detection, the continuously cast billets are rolled. Billets that fail the flaw detection need to be ground to prevent surface defects from affecting the surface quality of the wire rod. The billets are rolled into finished products using a one-fire rolling process. The continuously cast billet is heated to 1040-1070℃, with the preheating section temperature not lower than 1040℃ and the heating and soaking sections temperatures not lower than 1060℃, and the heating and holding time not lower than 150min. After the continuously cast billet is taken out of the furnace, it is descaled by high-pressure water to ensure that the iron oxide scale is removed, and then rolled into wire rod with a diameter of 7mm. The rolling process temperature and the wire drawing temperature are controlled at 880-900℃. After the wire rod is drawn, it is rolled in a loose and flat manner and isothermal transformation is carried out in a constant temperature salt bath. The wire rod must be completely immersed in the salt bath at a temperature of 295±2℃ for 90s. The wire rod after salt bath treatment is immediately collected on a vertical core rack and tempered in a constant temperature annealing furnace at 380℃. The furnace entry temperature is 280℃ and the tempering time is 50min.

[0044] The microstructure of the wire rod produced in this embodiment is shown in the figure. Figure 1 The wire rod is composed of 85% low-carbon martensite, 10% retained austenite, and 5% lower bainite. The wire rod has a diameter of 7mm, a tensile strength of 1785 MPa, a reduction of area of ​​56%, and an elongation after fracture of 11%. After pickling, phosphating, and two-pass drawing to a diameter of 5.58mm, the wire rod is then hot-dip galvanized to aluminum alloy, resulting in a bridge cable wire with a diameter of 5.61mm, a tensile strength of 2115 MPa, and a torsion resistance of 12 cycles. Microscopic images of the cold-drawn and galvanized wires before and after torsion are shown below. Figure 2 , Figure 3 and Figure 4It can be seen that due to the small amount of drawing deformation and the fact that the wire rod has undergone medium-temperature tempering, the microstructure is well-stable. Therefore, the microstructure of the steel wire at different stages is relatively stable. Figure 1 The microstructure of the wire rod shown did not show significant changes, therefore its torsion index was good.

[0045] Example 2:

[0046] Please see Figure 5 A hot-rolled wire rod for ultra-high strength bridge cables has the following chemical composition by weight: C: 0.24 wt.%; Si: 2.00 wt.%; Mn: 1.50 wt.%; Cr: 0.2 wt.%; Mo: 0.45 wt.%; P: 0.020 wt.%; S: 0.009 wt.%; O: 0.0015 wt.%; N: 0.0028 wt.%; the remainder being Fe and unavoidable impurities.

[0047] This embodiment also provides a production method for the aforementioned hot-rolled wire rod for ultra-high strength bridge cables. The method sequentially includes blast furnace hot metal → KR desulfurization → top and bottom blowing converter steelmaking → LF refining → RH refining → continuous casting, resulting in a rectangular billet of 180*240mm. After KR desulfurization, the sulfur content of the blast furnace hot metal entering the converter is ≤0.0055%. The converter uses a double-slag blowing method, which can reduce the content of impurity elements in the molten steel. The final P content in the converter is ≤0.015%, and the S content is ≤0.010%. The LF furnace uses aluminum deoxidation and a high-basicity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25min, controlling the O content in the steel to ≤15ppm and the N content to ≤30ppm. The continuous casting process is protected during casting to avoid secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billet. During continuous casting, the superheat of molten steel is controlled at 30-35℃, and electromagnetic stirring is used to control the compositional segregation of the continuously cast billet. The electromagnetic stirring current at the end of solidification is 215A, the stirring frequency of the crystallizer is 5Hz, the casting speed during continuous casting is 1.00m / min, the specific water content during continuous casting is 0.29L / kg, and the segregation index of the continuously cast billet is 1.05.

[0048] After shot blasting and flaw detection, the continuously cast billets are rolled. Billets that fail the flaw detection need to be ground to prevent surface defects from affecting the surface quality of the wire rod. The billets are rolled into finished products using a one-fire rolling process. The continuously cast billet is heated to 1070-1110℃, with the preheating section temperature not lower than 1040℃ and the heating and soaking sections not lower than 1100℃, and the heating and holding time not lower than 150min. After the continuously cast billet is taken out of the furnace, it is descaled by high-pressure water to ensure that the iron oxide scale is removed, and then rolled into wire rod with a diameter of 9mm. The rolling process temperature and the wire drawing temperature are controlled at 900-920℃. After the wire rod is drawn, it is rolled in a loose and flat manner and isothermal transformation is carried out in a constant temperature salt bath. The wire rod must be completely immersed in the salt bath. The salt bath temperature is 205±2℃ and the isothermal time is 30s. The wire rod after the salt bath treatment is immediately collected on the vertical core rack and put into a constant temperature annealing furnace at 420℃ for tempering treatment. The furnace temperature is 190℃ and the tempering time is 30min.

[0049] The microstructure of the wire rod produced in this embodiment is 75% low-carbon martensite + 15% retained austenite + 10% lower bainite. The wire rod has a diameter of 9mm, a tensile strength of 1715MPa, a reduction of area of ​​48%, and an elongation after fracture of 11%. The wire rod is pickled, phosphated, and drawn twice to a diameter of 6.98mm, then hot-dip galvanized with aluminum alloy to obtain a bridge cable wire with a diameter of 7.00mm. The wire has a tensile strength of 2040MPa and a torsion resistance of 12 times. Microstructure photographs of the cold-drawn wire and the galvanized wire before and after torsion are shown below. Figure 6 , Figure 7 and Figure 8 It can be seen that due to the small amount of drawing deformation and the fact that the wire rod has undergone medium-temperature tempering, the microstructure is well-stable. Therefore, the microstructure of the steel wire at different stages is relatively stable. Figure 5 The microstructure of the wire rod shown did not show significant changes, therefore its torsion index was good.

[0050] Example 3

[0051] Please see Figure 9 A hot-rolled wire rod for ultra-high strength bridge cables has the following chemical composition by weight: C: 0.36 wt.%; Si: 2.50 wt.%; Mn: 2.00 wt.%; Cr: 1.30 wt.%; Mo: 0.15 wt.%; P: 0.015 wt.%; S: 0.009 wt.%; O: 0.0013 wt.%; N: 0.0027 wt.%; the remainder being Fe and unavoidable impurities.

[0052] This embodiment also provides a production method for the aforementioned hot-rolled wire rod for ultra-high strength bridge cables. The method sequentially includes blast furnace hot metal → KR desulfurization → top and bottom blowing converter steelmaking → LF refining → RH refining → continuous casting, resulting in a rectangular billet of 180*240mm. After KR desulfurization, the sulfur content of the blast furnace hot metal entering the converter is ≤0.0050%. The converter uses a double-slag blowing method, which can reduce the content of impurity elements in the molten steel. The final P content at the converter end is ≤0.015%, and the S content is ≤0.010%. The LF furnace uses aluminum deoxidation and a high-basicity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25min, controlling the O content in the steel to ≤15ppm and the N content to ≤30ppm. The continuous casting process is protected during casting to avoid secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billet. During continuous casting, the superheat of molten steel is controlled at 40-45℃, and electromagnetic stirring is used to control the compositional segregation of the continuously cast billet. The electromagnetic stirring current at the end of solidification is 265A, the stirring frequency of the crystallizer is 6Hz, the casting speed during continuous casting is 1.10m / min, the specific water content of continuous casting is 0.31L / kg, and the segregation index of the continuously cast billet is 1.05.

[0053] After shot blasting and flaw detection, the continuously cast billets are rolled. Billets that fail the flaw detection need to be ground to prevent surface defects from affecting the surface quality of the wire rod. The billets are rolled into finished products using a one-fire rolling process. The continuously cast billet is heated to 1100-1150℃, with the preheating section temperature not lower than 1040℃ and the heating and soaking sections not lower than 1140℃, and the heating and holding time not lower than 150min. After the continuously cast billet is taken out of the furnace, it is descaled by high-pressure water to ensure that the iron oxide scale is removed, and then rolled into wire rod with a diameter of 7mm. The rolling process temperature and the wire drawing temperature are controlled at 890-910℃. After the wire rod is drawn, it is rolled in a loose and flat manner and isothermal transformation is carried out in a constant temperature salt bath. The wire rod must be completely immersed in the salt bath at a temperature of 265±2℃ for 70s. The wire rod after salt bath treatment is immediately collected on a vertical core rack and tempered in a constant temperature annealing furnace at 410℃. The furnace temperature is 240℃ and the tempering time is 45min.

[0054] The microstructure of the wire rod produced in this embodiment is 82% low-carbon martensite + 14% retained austenite + 4% lower bainite. The wire rod has a diameter of 7mm, a tensile strength of 1800MPa, a reduction of area of ​​53%, and an elongation after fracture of 11%. The wire rod is pickled, phosphated, and drawn twice to a diameter of 5.58mm, then hot-dip galvanized with aluminum alloy to obtain a bridge cable wire with a diameter of 5.60mm. The wire has a tensile strength of 2135MPa and a torsion resistance of 12 times. Microstructure photographs of the cold-drawn wire and the galvanized wire before and after torsion are shown below. Figure 10 , Figure 11 and Figure 12It can be seen that due to the small amount of drawing deformation and the fact that the wire rod has undergone medium-temperature tempering, the microstructure is well-stable. Therefore, the microstructure of the steel wire at different stages is relatively stable. Figure 9 The microstructure of the wire rod shown did not show significant changes, therefore its torsion index was good.

[0055] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for producing hot-rolled wire rod for ultra-high strength bridge cables, characterized in that: The hot-rolled wire rod for ultra-high strength bridge cables has the following chemical composition by weight: C: 0.24-0.36 wt.%; Si: 1.50-2.50 wt.%; Mn: 1.50-2.50 wt.%; Cr: 0.2-1.30 wt.%; Mo: 0.15-0.45 wt.%; P: ≤0.020 wt.%; S: ≤0.010wt.%; O: ≤0.0015wt.%; N: ≤0.0030 wt.%; the remainder is Fe and unavoidable impurities; The production method of the hot-rolled wire rod for ultra-high strength bridge cables includes the following steps: smelting, continuous casting, rolling, isothermal treatment in a salt bath, coiling, and slow cooling. The continuous casting billet with the specified composition and proportion is obtained through the smelting and continuous casting steps. The wire rod is spun into wire rod at a temperature of 880-920℃. The spun wire rod is directly immersed in a salt bath at 200-300℃ for isothermal treatment to obtain medium-carbon martensite and untransformed austenite. The wire rod after salt bath treatment is immediately coiled and slowly cooled in a constant-temperature annealing furnace at a temperature of not less than 180℃ at 380-420℃. This allows the carbon element in the martensite to diffuse into the untransformed austenite, resulting in a small amount of bainite and retained austenite. The wire rod ultimately obtains a composite microstructure of low-carbon martensite, a small amount of bainite, and retained austenite.

2. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, The wire rod has a diameter of 7mm or 9mm, a tensile strength of 1700-1800MPa, and a reduction of area of ​​48-58%.

3. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, The wire rod contains 75-85% martensite, 0-10% bainite, and 10-15% austenite.

4. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, The isothermal treatment in the salt bath lasts for 30-90 seconds.

5. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, The slow cooling process shall take no less than 30 minutes.

6. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, After the continuously cast billet is heated, it is first rough rolled and then finish rolled. The heating temperature is 1040-1150℃, the rough rolling start temperature is 980-1020℃, and the finish rolling inlet temperature is 880-920℃.

7. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, The continuous casting process yields a rectangular billet with a cross-sectional dimension of not less than 240mm × 180mm, ensuring that the wire rod has a sufficient compression ratio after rolling to guarantee its homogeneity.

8. The method for producing hot-rolled wire rod for ultra-high strength bridge cables according to claim 1, characterized in that, In the continuous casting process, the superheat of the molten steel is controlled at 30-45℃, the electromagnetic stirring current at the end of solidification is 240±25A, the stirring frequency of the crystallizer is 5.5±0.5Hz, the casting speed during continuous casting is 1.05±0.05m / min, and the specific water content during continuous casting is 0.30±0.01L / kg.

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