Hot-rolled wire rod for ultrahigh-strength bridge cable and production method of hot-rolled wire rod

Abstract

The production method sequentially comprises the steps of smelting, continuous casting, rolling, salt bath isothermal treatment, coil collection and slow cooling treatment, continuous casting billets with required components and proportions are obtained through the smelting and continuous casting steps, a wire rod is spun into the wire rod at the temperature of 880-920 DEG C, and the wire rod is subjected to hot rolling at the temperature of 880-920 DEG C; a wire rod obtained after spinning is directly immersed in a salt bath of 200-300 DEG C for isothermal treatment, medium-carbon martensite and untransformed austenite are obtained, the wire rod obtained after salt bath treatment is immediately coiled and enters a constant-temperature annealing furnace of 380-420 DEG C at the temperature not lower than 180 DEG C for slow cooling treatment, the carbon element in the martensite is diffused into the untransformed austenite, and the medium-carbon martensite and the untransformed austenite are subjected to annealing treatment at the constant temperature of 380-420 DEG C at the temperature not lower than 180 DEG C; and a small amount of bainite and retained austenite are obtained. When the steel wire rod is used for manufacturing the bridge cable steel wire, the steel wire rod only needs to be subjected to surface treatment and zinc / aluminum plating after 1-2 passes of drawing, the strength of the steel wire reaches 2000-2100 MPa, and the number of times of torsion of the finished steel wire is not less than 12.

Description

Technical Field

[0001] The present invention relates to the technical field of metal wire manufacturing, and in particular to a hot-rolled wire rod for ultra-high-strength bridge cables and a production method thereof. Background Art

[0002] As a core engineering material for the construction of long-span suspension bridges and cable-stayed bridges, galvanized steel wire for bridges has always been a focus of research and development for steel companies and research institutes. Based on the requirements for overall structural rigidity, service life, and safety of bridges, high-strength and ultra-high-strength galvanized steel wire for bridges have become the development direction. In recent years, the strength levels of ultra-high-strength bridge cable steel wire used in key projects such as the Lingdingyang Bridge, Changtai Bridge, Chaoma Bridge, and Zhangjinggao Bridge have been 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 steel 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 must not be less than 8 times, and in actual application, the standard is further raised to more than 12 times.

[0003] At present, high-carbon pearlite wire rods are used in the manufacture of bridge cable steel wires, and the processing technology is hot-rolled wire rods-multiple 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 working hardening-strength loss of galvanizing (aluminum) treatment. The measures to improve tensile strength are mainly to increase the carbon content of the wire rod to increase the tensile strength of the original wire rod, and to increase the diameter of the original wire rod to increase the deformation and increase cold working strengthening. However, these measures have led to a decrease in the plasticity of the steel wire, making the number of torsions, another key performance indicator of the bridge cable steel wire, unable to meet the requirements. In fact, the biggest difficulty in the current manufacture of ultra-high-strength bridge cable steel wire is that the torsion index is difficult to meet while the strength is increased.

[0004] Numerous studies have shown that the primary factor affecting the torsion performance of bridge cable steel wires is the change in the cementite phase within the pearlite during the drawing and hot-dip galvanizing processes. During wire rod drawing, the cementite flakes within the pearlite structure gradually transform into an amorphous structure due to the large deformation. At this point, they exhibit good deformation consistency with the intensely deformed ferrite phase, resulting in excellent torsion performance before galvanizing. However, during the subsequent hot-dip galvanizing (aluminum) treatment, the amorphous cementite transforms back into crystals due to the heat. Poor control can lead to a sharp drop in the torsion performance of the finished galvanized (aluminum) steel wire. To mitigate this change, the current technological approach to high-strength bridge cable steel wire development involves significantly increasing the carbon content in the chemical composition and adding significant amounts of alloying elements such as Si, Cr, and V. This increases production complexity and further complicates the trade-off between strength and torsion. Therefore, a novel chemical composition and microstructure design must be employed to address the technical challenges of the poor torsion performance of ultra-high-strength bridge cable steel wires. Summary of the Invention

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

[0006] To achieve the above object, the present invention provides the following technical solutions:

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

[0008] Furthermore, the diameter of the wire rod is 7 mm or 9 mm, the tensile strength is 1700-1800 MPa, and the cross-sectional shrinkage is 48-58%.

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

[0010] C is the most important strengthening element in steel. The present invention mainly utilizes phase transformation strengthening and fine grain strengthening to make the wire rod obtain high strength and plasticity.

[0011] Si is a deoxidizing and strengthening element in steel, primarily distributed in ferrite to provide solid solution strengthening. Most importantly, Si inhibits carbide formation, promoting the diffusion of carbon atoms into untransformed austenite during phase transformation, improving its stability and resulting in 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 and performance; it also has the effect of stabilizing austenite.

[0013] Cr effectively improves hardenability and delays bainite transformation. It also refines the microstructure and improves the toughness of the steel. It also helps reduce the tendency of surface decarburization during the manufacturing process, thereby achieving high fatigue resistance. It is also an austenite stabilizing element, improving the stability and content of retained austenite.

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

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

[0016] The O element is controlled below a certain level to ensure the purity of the 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 comprises, in sequence, the steps of smelting, continuous casting, rolling, isothermal treatment in a salt bath, coiling, and slow cooling treatment. A continuous casting billet having the composition and proportion described in claim 1 is obtained through the smelting and continuous casting steps. The wire rod is spun into a wire rod at a temperature of 880-920°C. The wire rod after spinning is directly immersed in a salt bath of 200-300°C for isothermal treatment to obtain medium-carbon martensite and untransformed austenite. The wire rod after salt bath treatment is immediately coiled and placed in a constant temperature annealing furnace of 380-420°C for slow cooling treatment at a temperature of not less than 180°C, so that the carbon element in the martensite diffuses into the untransformed austenite to obtain a small amount of bainite and residual austenite. The wire rod ultimately obtains a composite microstructure of low-carbon martensite, a small amount of bainite, and residual austenite.

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

[0019] Preferably, the slow cooling treatment time is not less than 30 minutes.

[0020] Preferably, the continuous casting process obtains continuous casting billets which are heated and then subjected to rough rolling and finish rolling, the heating temperature is 1040-1150°C, the start rolling temperature of rough rolling is 980-1020°C, and the inlet temperature of finish rolling is 880-920°C.

[0021] Preferably, the continuous casting process obtains a rectangular billet with a cross-sectional size of not less than 240 mm×180 mm, so as to ensure that after being rolled into wire rod, there is a sufficient compression ratio to ensure the homogeneity of the wire rod.

[0022] Preferably, in the continuous casting process, the superheat of the molten steel is controlled to be 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 pulling speed during continuous casting is 1.05±0.05m / min, and the continuous casting water content is 0.30±0.01L / kg.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The wire rod of the present invention is designed with a medium-low carbon alloy composition and a post-rolling residual heat treatment process. First, medium-carbon martensite and untransformed austenite are obtained by salt bath austempering, followed by online tempering to diffuse the carbon element in the martensite into the untransformed austenite. A small amount of bainite and residual austenite are obtained during the tempering and subsequent cooling process. The wire rod finally obtains a composite microstructure of low-carbon martensite, a small amount of bainite and residual austenite, so that the wire rods with diameters of 7mm and 9mm respectively have a tensile strength of 1700-1800MPa and a cross-sectional reduction rate of 4 The performance of the present invention is 8-58%; when the wire rod of the present invention is used to manufacture bridge cable steel wire, it is only necessary to perform surface treatment on the wire rod, draw it through 1-2 passes, and then galvanize (aluminize) it to obtain a finished steel wire with a diameter of 5-7 mm. The strength of the steel wire reaches 2000-2100 MPa. Due to the use of a new microstructure design and a significant reduction in the drawing amount, the deterioration of the steel wire torsion index caused by the change of carbides in the drawing and galvanizing (aluminum plating) process of the prior art is avoided. The finished steel wire made of the wire rod of the present invention has a torsion frequency of not less than 12 times. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a microstructure diagram of the wire rod according to Example 1 of the present invention;

[0026] Figure 2 This is a microstructure diagram of the drawn steel wire according to Example 1 of the present invention;

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

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

[0029] Figure 5 This is a microstructure diagram of the wire rod according to Example 2 of the present invention;

[0030] Figure 6 This is a microstructure diagram of the drawn steel wire according to Example 2 of the present invention;

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

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

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

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

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

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

[0037] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below through examples.

[0038] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0039] Secondly, the term "one embodiment" or "embodiment" 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 various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0040] Example 1:

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

[0042] This embodiment also provides a method for producing the aforementioned hot-rolled wire rod for ultra-high-strength bridge cables, which sequentially undergoes the following steps: blast furnace molten iron → KR desulfurization → top-bottom double-blowing converter steelmaking → LF refining → RH refining → continuous casting, resulting in a 180*240mm rectangular billet. After KR desulfurization, the molten iron entering the converter has a S content of ≤0.0055%. The converter uses a double-slag blowing process to reduce the content of impurity elements in the molten steel, with the P content at the converter end being ≤0.015% and the S content ≤0.010%. The LF furnace uses aluminum deoxidation and a high-alkalinity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25 minutes, controlling the O content in the steel to ≤15ppm and the N content to ≤30ppm. Protective casting is employed throughout the continuous casting process to prevent secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billets. During the continuous casting process, the superheat of the molten steel is controlled at 35-40°C, and electromagnetic stirring is used to control the segregation of the continuous casting billet composition. The electromagnetic stirring current at the end of solidification is 240A, the stirring frequency of the crystallizer is 5.5Hz, the pulling speed during continuous casting is 1.05m / min, the continuous casting water content is 0.30L / kg, and the continuous casting billet segregation index is 1.05.

[0043] The continuous casting billets are shot blasted and flaw detected before rolling. Billets that fail the flaw detection need to be ground to prevent the surface defects of the continuous casting billets from affecting the surface quality of the wire rod. The billets are rolled into products using a one-fire process. The continuous casting billet is heated to 1040-1070℃, with the temperature of the preheating section not lower than 1040℃, the temperature of the heating section and the soaking section not lower than 1060℃, and the heating and holding time not less than 150min. After the continuous casting billet is discharged from the furnace, it is dephosphorized with high-pressure water to ensure that the iron oxide scale is removed, and then rolled into a wire rod with a diameter of 7mm. The rolling process temperature and the spinning temperature are controlled at 880-900℃. After the wire rod is spun, it is isothermally transformed in a constant temperature salt bath in a loose coil-laying manner. The wire rod needs to be completely immersed in the salt bath. The salt bath temperature is 295±2℃ and the isothermal time is 90s. The wire rod after salt bath treatment is immediately collected on a vertical coil core rack and enters a 380℃ constant temperature annealing furnace for tempering treatment. 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 FIG. Figure 1 , which is 85% low carbon martensite + 10% retained austenite + 5% lower bainite. The wire rod specification is 7mm, the tensile strength is 1785Mpa, the cross-sectional shrinkage rate is 56%, and the elongation after fracture is 11%. The wire rod is pickled, phosphating, and drawn in two passes to a steel wire with a diameter of 5.58mm. Then, it is hot-dip galvanized with aluminum alloy to obtain a bridge cable steel wire with a diameter of 5.61mm. The tensile strength of the steel wire is 2115Mpa and the number of torsion times is 12 times. The microstructure photos of the cold-drawn steel wire and the galvanized steel wire before and after torsion are shown in the following table. 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 been tempered at medium temperature, the microstructure stability is good, so the microstructure of the steel wire at different stages is relatively stable. Figure 1 The wire rod shown has no significant changes in its microstructure, so its torsion indicators are good.

[0045] Example 2:

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

[0047] This embodiment also provides a method for producing the aforementioned hot-rolled wire rod for ultra-high-strength bridge cables, which comprises, in sequence, blast furnace molten iron → KR desulfurization → top-bottom double-blowing converter steelmaking → LF refining → RH refining → continuous casting, to produce rectangular billets measuring 180*240mm. After KR desulfurization, the molten iron entering the converter has a S content of ≤0.0055%. The converter uses a double-slag blowing process to reduce the content of impurity elements in the molten steel, with the P content at the converter end being ≤0.015% and the S content ≤0.010%. The LF furnace uses aluminum deoxidation and a high-alkalinity slag system to reduce the equilibrium oxygen content in the molten steel. The RH vacuum treatment time is ≥25 minutes, controlling the O content in the steel to ≤15ppm and the N content to ≤30ppm. Protective casting is employed throughout the continuous casting process to prevent secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billets. During the continuous casting process, the superheat of the molten steel is controlled at 30-35°C, and electromagnetic stirring is used to control the composition segregation of the continuous casting billet. The electromagnetic stirring current at the end of solidification is 215A, the stirring frequency of the crystallizer is 5Hz, the pulling speed during continuous casting is 1.00m / min, the continuous casting water content is 0.29L / kg, and the continuous casting billet segregation index is 1.05.

[0048] The continuous casting billets are shot blasted and flaw detected before rolling. Billets that fail the flaw detection need to be ground to prevent the surface defects of the continuous casting billets from affecting the surface quality of the wire rod. The billets are rolled into products using a one-fire process. The continuous casting billet is heated to 1070-1110℃, with the temperature of the preheating section not lower than 1040℃, the temperature of the heating section and the soaking section not lower than 1100℃, and the heating and holding time not less than 150min. After the continuous casting billet is discharged from the furnace, it is dephosphorized with high-pressure water to ensure that the iron oxide scale is removed, and then rolled into a wire rod with a diameter of 9mm. The rolling process temperature and the spinning temperature are controlled at 900-920℃. After the wire rod is spun, it is isothermally transformed in a constant temperature salt bath in a loose coiled and flat manner. The wire rod needs to be completely immersed in the salt bath. The salt bath temperature is 205±2℃ and the isothermal time is 30s. The wire rod after salt bath treatment is immediately collected on a vertical coil core rack and enters a 420℃ constant temperature annealing furnace for tempering treatment. The furnace entry 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 specification is 9mm, the tensile strength is 1715Mpa, the cross-sectional shrinkage rate is 48%, and the elongation after fracture is 11%. The wire rod is pickled, phosphating, and drawn in two passes to a steel wire with a diameter of 6.98mm. Then, it is hot-dip galvanized with aluminum alloy to obtain a bridge cable steel wire with a diameter of 7.00mm. The tensile strength of the steel wire is 2040Mpa and the number of torsion times is 12 times. The microstructure photos of the cold-drawn steel wire and the galvanized steel wire before and after torsion are shown in the figure 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 been tempered at medium temperature, the microstructure stability is good, so the microstructure of the steel wire at different stages is relatively stable. Figure 5 The wire rod shown has no significant changes in its microstructure, so its torsion indicators are good.

[0050] Example 3

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

[0052] This embodiment also provides a method for producing the aforementioned hot-rolled wire rod for ultra-high-strength bridge cables, which sequentially comprises blast furnace molten iron → KR desulfurization → top-bottom combined blowing converter steelmaking → LF refining → RH refining → continuous casting, to produce rectangular billets measuring 180*240mm. After KR desulfurization, the molten iron entering the converter has a S content of ≤0.0050%. The converter uses a double slag blowing process to reduce the content of impurity elements in the molten steel, with the P content at the converter end point being ≤0.015% and the S content ≤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 ≥25 minutes, controlling the O content in the steel to ≤15ppm and the N content to ≤30ppm. Protective casting is employed throughout the continuous casting process to prevent secondary oxidation of the molten steel, ultimately reducing the number of inclusions in the continuously cast billets. During the continuous casting process, the superheat of the molten steel is controlled at 40-45°C, and electromagnetic stirring is used to control the composition segregation of the continuous casting billet. The electromagnetic stirring current at the end of solidification is 265A, the stirring frequency of the crystallizer is 6Hz, the pulling speed during continuous casting is 1.10m / min, the continuous casting water content is 0.31L / kg, and the continuous casting billet segregation index is 1.05.

[0053] The continuous casting billets are shot blasted and flaw detected before rolling. Billets that fail the flaw detection need to be ground to prevent the surface defects of the continuous casting billets from affecting the surface quality of the wire rod. The billets are rolled into products using a one-fire process. The continuous casting billet is heated to 1100-1150℃, of which the temperature of the preheating section is not less than 1040℃, the temperature of the heating section and the soaking section is not less than 1140℃, and the heating and holding time is not less than 150min; after the continuous casting billet is discharged from the furnace, it is subjected to high-pressure water dephosphorization to ensure that the iron oxide scale is removed, and then rolled into a wire rod with a diameter of 7mm; the rolling process temperature and the spinning temperature are controlled at 890-910℃, and after the wire rod is spun, it is laid flat in a loose coil and is isothermally transformed in a constant temperature salt bath. The wire rod must be completely immersed in the salt bath, the salt bath temperature is 265±2℃, and the isothermal time is 70s; the wire rod after salt bath treatment is immediately collected on a vertical coil core rack and enters a 410℃ constant temperature annealing furnace for tempering treatment, with the furnace entry temperature at 240℃ and the tempering time being 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 specification is 7mm, the tensile strength is 1800Mpa, the cross-sectional shrinkage rate is 53%, and the elongation after fracture is 11%. The wire rod is pickled, phosphating, and drawn in two passes to a steel wire with a diameter of 5.58mm. Then, it is hot-dip galvanized with aluminum alloy to obtain a bridge cable steel wire with a diameter of 5.60mm. The tensile strength of the steel wire is 2135Mpa and the number of torsion times is 12 times. The microstructure photos of the cold-drawn steel wire and the galvanized steel wire before and after torsion are shown in the figure 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 been tempered at medium temperature, the microstructure stability is good, so the microstructure of the steel wire at different stages is relatively stable. Figure 9 The wire rod shown has no significant changes in its microstructure, so its torsion indicators are 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 the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A hot-rolled wire rod for ultra-high-strength bridge cables, characterized in that: The chemical composition has the following weight percentages: 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: ≤0.0030wt.%; the rest is Fe and unavoidable impurities.

2. The hot-rolled wire rod for ultra-high-strength bridge cables according to claim 1, characterized in that: The wire rod has a diameter of 7 mm or 9 mm, a tensile strength of 1700-1800 MPa, and a cross-sectional shrinkage rate of 48-58%.

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

4. A method for producing hot-rolled wire rod for ultra-high-strength bridge cables, characterized in that: The method comprises the steps of smelting, continuous casting, rolling, isothermal treatment in a salt bath, coiling and slow cooling treatment in sequence. The continuous casting billet with the composition and proportion described in claim 1 is obtained through the smelting and continuous casting steps. The wire rod is spun into a wire rod at a temperature of 880-920°C. The wire rod after spinning is directly immersed in a salt bath of 200-300°C for isothermal treatment to obtain medium-carbon martensite and untransformed austenite. The wire rod after salt bath treatment is immediately coiled and enters a constant temperature annealing furnace of 380-420°C for slow cooling treatment at a temperature of not less than 180°C, so that the carbon element in the martensite diffuses into the untransformed austenite to obtain a small amount of bainite and residual austenite. The wire rod finally obtains a composite microstructure of low-carbon martensite, a small amount of bainite and residual austenite.

5. The method for producing hot-rolled wire rod for ultra-high-strength bridge cables according to claim 4, characterized in that: The time of the salt bath isothermal treatment is 30-90S.

6. The method for producing hot-rolled wire rod for ultra-high-strength bridge cables according to claim 4, characterized in that: The time of the slow cooling treatment is not less than 30 minutes.

7. The method for producing hot-rolled wire rod for ultra-high-strength bridge cables according to claim 4, characterized in that: The continuous casting process obtains continuous casting billets which are heated and then subjected to rough rolling and then to finish rolling. The heating temperature is 1040-1150°C, the starting temperature of rough rolling is 980-1020°C, and the inlet temperature of finish rolling is 880-920°C.

8. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 4, characterized in that: The continuous casting process obtains a rectangular billet with a cross-sectional size of not less than 240 mm×180 mm, so as to ensure that after being rolled into wire rod, a sufficient compression ratio is obtained to ensure the homogeneity of the wire rod.

9. The method for evaluating center segregation of medium and low carbon steel wire rod according to claim 4, characterized in that: 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 pulling speed during continuous casting is 1.05±0.05m / min, and the continuous casting water content is 0.30±0.01L / kg.

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