Nickel-free annealing-free high-strength weather-resistant steel wire rod for bolt, bolt, and manufacturing method for steel wire rod

Abstract

Disclosed in the present invention is a nickel-free annealing-free high-strength weather-resistant steel wire rod for a bolt. The steel wire rod contains Fe and inevitable impurity elements, and further contains the following chemical elements in percentages by mass: C: 0.05-0.14%, Si: 0.01-2.0%, Mn: 0.3-2.2%, Cr: 2.4-4.5%, Cu: 0.2-0.6%, and Al: 0.01-0.1%, wherein the mass percentage content of Cr and C satisfies: Cr / C > 20; and the steel wire rod does not contain Ni. Correspondingly, further disclosed in the present invention is a manufacturing method for the nickel-free annealing-free high-strength weather-resistant steel wire rod for a bolt. The manufacturing method specifically comprises the following steps: (1) smelting and casting; (2) heating; (3) rolling; and (4) cooling, involving: performing cooling to 900-920ºC at a cooling speed of 4-5ºC / s after rolling, and then continuously performing cooling to room temperature at a cooling speed lower than 0.4ºC / s, so as to obtain a steel wire rod.

Description

Nickel-free, annealing-free, high-strength, weather-resistant wire rod for bolts, bolts, and manufacturing method thereof Technical Field

[0001] The present invention relates to a wire rod, a bolt and a manufacturing method thereof, and in particular to a high-strength and weather-resistant wire rod for bolts, a bolt and a manufacturing method thereof. Background Art

[0002] Weathering steel refers to low-alloy, high-strength steel that has been enhanced with the addition of small amounts of alloying elements to impart excellent atmospheric corrosion resistance. When elements such as phosphorus, copper, and chromium are added to the steel, an amorphous spinel oxide layer approximately 50 to 100 μm thick forms between the rust layer and the base metal. This dense oxide layer adheres well to the base metal, preventing atmospheric oxygen and water from penetrating into the steel matrix. This slows the spread of rust and improves the steel's resistance to atmospheric corrosion. Weathering steel's atmospheric corrosion resistance is 2 to 8 times that of ordinary carbon steel, and its corrosion resistance increases with service life. In addition to its excellent weather resistance, weathering steel also possesses excellent mechanical and welding properties.

[0003] Bolts are important connectors, and rust can seriously compromise the safety of structures. To delay and control bolt corrosion, prevent degradation and deterioration of the bolt material, and ensure reliability, safety, and service life, bolts are typically treated with surface anti-corrosion treatment to improve their corrosion resistance. Surface anti-corrosion treatment involves applying a protective layer to the metal surface, isolating it from the corrosive environment and preventing or inhibiting the onset of corrosion.

[0004] Currently, high-strength bolts are the primary connectors in steel structures such as buildings, bridges, and transmission towers. These bolts are typically treated with coatings such as hot-dip galvanizing, electroplating, and mechanical plating to achieve corrosion resistance. However, high-strength bolts experience high pre-tightening torque, resulting in significant friction between the threads. The coating itself cannot withstand this high friction, and the coating is often damaged during the pre-tightening process, resulting in thinning of the coating. In severe cases, the base material of the bolt may even be exposed to the service environment, reducing the bolt's corrosion resistance.

[0005] Surface coating requires maintenance every three to five years, and re-coating every 10 to 15 years. This maintenance not only increases costs but also poses health risks and environmental pollution. Furthermore, some coating processes introduce hydrogen into the bolts, increasing the susceptibility of high-strength bolts to hydrogen embrittlement, leading to a range of safety hazards such as delayed fracture. Therefore, the development of high-strength, weather-resistant steel for bolts that requires no coating is crucial.

[0006] Summary of the Invention

[0007] One of the purposes of the present invention is to provide a nickel-free, annealing-free, high-strength, weather-resistant wire rod for bolts, which can obtain strong weather resistance without adding Ni elements, and its atmospheric corrosion resistance can meet the requirements of being free of coating.

[0008] In order to achieve the above object, the present invention provides a wire rod, which, in addition to Fe and inevitable impurities, further contains the following chemical elements in the following mass percentages:

[0009] C: 0.05-0.14%, Si: 0.01-2.0%, preferably 0.2-1.6%, Mn: 0.3-2.2%, preferably 0.5-1.7%, Cr: 2.4-4.5%, Cu: 0.2-0.6%, Al: 0.01-0.1%;

[0010] And the mass percentage of Cr and C meets: Cr / C>20;

[0011] The wire rod does not contain Ni.

[0012] Accordingly, the present invention also provides a wire rod, wherein the wire rod has the following chemical elements in percentage by mass:

[0013] C: 0.05-0.14%, Si: 0.01-2.0%, preferably 0.2-1.6%, Mn: 0.3-2.2%, preferably 0.5-1.7%, Cr: 2.4-4.5%, Cu: 0.2-0.6%, Al: 0.01-0.1%, the balance is Fe and other inevitable impurities;

[0014] And the mass percentage of Cr and C satisfies: Cr / C>20.

[0015] In the wire rod of the present invention, the design principles of each chemical element are specifically described as follows:

[0016] C: C is a key element that forms fine dispersed carbides after subsequent quenching and tempering treatment, playing a crucial role in increasing the strength of bolts. However, it is important to note that the C content in steel should not be too high. Excessive C content in steel can affect the material's atmospheric corrosion resistance and increase the strength of the hot-rolled wire rod, preventing it from meeting the annealing-free strength requirements. Therefore, to ensure that the material of the present invention achieves both good atmospheric corrosion resistance and suitable strength, the C content in the steel must be controlled within a reasonable range. Specifically, the mass percentage of C in the present invention is controlled between 0.05% and 0.14%.

[0017] Si: Si has a high solid solubility in steel. Adding an appropriate amount of Si to steel can effectively increase the volume fraction of ferrite and refine the grain size, thereby improving the toughness of the steel. Therefore, to maximize the beneficial effects of Si, the mass percentage of Si in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod described herein is controlled between 0.01% and 2.0%, preferably between 0.2% and 1.6%.

[0018] Mn: Mn has a strong solid solution strengthening effect and is also an important toughening element. Adding an appropriate amount of Mn to steel can significantly lower the steel's phase transition temperature and refine its microstructure. Considering the beneficial effects of Mn, the mass percentage of Mn in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod described herein is controlled between 0.3% and 2.2%, preferably between 0.5% and 1.7%.

[0019] Cr: The presence of Cr element will significantly accelerate the development of electrochemical corrosion products to a thermodynamically stable state. In the analysis of rust layer, Cr element can significantly accelerate (Fe X H Y O Z )→γ-FeOOH→α-FeOOH→α-Fe2O3, which promotes the formation of spinel compounds. At the same time, the Cr element in the steel can partially replace Fe to form chromium iron hydroxide Cr X Fe 1-X OOH, thus making the α-FeOOH rust layer cation-selective and preventing Cl - 、SO4 2- Therefore, in order to maximize the beneficial effects of Cr and ensure a significant improvement in the steel's atmospheric corrosion resistance, the mass percentage of Cr in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod of the present invention is controlled to be between 2.4% and 4.5%.

[0020] Of course, it should be noted that the Cr element will inevitably combine with the C element to form the precipitated phase Cr 23 C6, precipitated Cr 23 C6 does not improve the weather resistance of steel. Therefore, in the present invention, while controlling the mass percentage of a single chemical element, it is also necessary to further control the Cr and C content in the steel to meet the following conditions: Cr / C>20, to ensure that there is enough Cr in the steel to improve weather resistance.

[0021] Cu: Among all alloying elements, Cu has the most significant effect on the weather resistance of materials. Adding an appropriate amount of Cu to steel can effectively delay the anodic dissolution of Fe or reduce the electronic conductivity of the rust layer, thereby reducing the rate at which electrons flow to the cathode area. At the same time, the Cu in steel can also cooperate and form a small amount of insoluble copper hydroxide sulfate, such as Cu4(SO4)(OH) and Cu4(SO4)(OH)4. These compounds can precipitate in the pores of the rust layer and enhance the barrier effect of the corrosion product film. However, it should be noted that the Cu content in steel should not be too high. Too much Cu will cause hot brittleness of steel and increase the difficulty of production. Therefore, in order to give full play to the beneficial effects of Cu, the mass percentage of Cu in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod of the present invention is controlled between 0.2-0.6%.

[0022] Al: Al is a deoxidizer added to steel, which can play a role in deoxidation. In the present invention, the mass percentage of the Al element is controlled between 0.01-0.1%, which is beneficial to grain refinement and improvement of the strength and toughness of the steel.

[0023] Preferably, among the inevitable impurities in the nickel-free annealing-free high-strength weather-resistant bolt wire rod of the present invention, P≤0.012% and S≤0.005%.

[0024] In the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod described in the present invention, P and S are both impurity elements in steel. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the steel should be reduced as much as possible.

[0025] S, P: In the present invention, the presence of the impurity element S will deteriorate the steel's atmospheric corrosion resistance and cause hot brittleness. While P can improve the steel's atmospheric corrosion resistance, excessive P content in the steel significantly reduces its toughness and plasticity and can also cause cold brittleness. Therefore, the P and S content in the steel should be minimized. The present invention utilizes extremely low S and P contents in its design, achieving a P content of ≤ 0.012% by weight and a S content of ≤ 0.005% by weight.

[0026] Preferably, in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod of the present invention, its microstructure is ferrite+pearlite.

[0027] Preferably, the weather resistance index I of the nickel-free annealing-free high-strength weather-resistant bolt wire rod of the present invention is ≥8.5, wherein I=26.01×Cu+3.88×Ni+1.20×Cr+1.49×Si+17.28×P-7.29×Cu×Ni-9.10×Ni×P-33.39×Cu 2; Each chemical element in the formula is substituted into the numerical value before the mass percentage sign of the corresponding element content. For the nickel-free steel of the present invention, the Ni element in the formula is substituted into 0.

[0028] It should be noted that, for steel, the larger the I value, the higher the atmospheric corrosion resistance of the steel. Generally speaking, a material with a weather resistance index I>6 means that it has very good atmospheric corrosion resistance.

[0029] Furthermore, the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod of the present invention has a yield strength of ≤400 MPa, a tensile strength of ≤600 MPa, an elongation of ≥25%, and a cross-sectional shrinkage of ≥60%.

[0030] In addition, another object of the present invention is to disclose a bolt prepared from the above-mentioned nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod. The bolt prepared using the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod not only has mechanical properties of grade 8.8-10.9, but also has very excellent atmospheric corrosion resistance. It can be used in bridge structures, transmission towers, photovoltaic brackets and other fields without painting, and has good application prospects.

[0031] In order to achieve the above-mentioned objectives, the present invention proposes a bolt, which is made using the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod of the present invention. The bolt has a yield strength of ≥720 MPa, preferably ≥820 MPa, a tensile strength of ≥870 MPa, an elongation of ≥16%, and a cross-sectional shrinkage rate of ≥55%.

[0032] The present invention also relates to a method for manufacturing the above-mentioned bolt, characterized in that the method comprises subjecting the wire rod according to any one of claims 1 to 6 to a quenching and tempering treatment.

[0033] Preferably, the quenching and tempering treatment includes: quenching at a temperature of 850°C-880°C, holding for 80 minutes, and then oil quenching; tempering at a temperature of 420-600°C, holding for 1 hour, and then air cooling.

[0034] In the above technical solution of the present invention, based on the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod described in the present invention, when preparing the bolts, downstream users can further perform pickling, phosphorus saponification, drawing, cold heading, tempering treatment, thread rolling and other processes on the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod, so as to process it into high-strength and high-weather-resistant bolts.

[0035] Correspondingly, another object of the present invention is to provide a method for manufacturing the above-mentioned nickel-free, annealing-free, high-strength, weather-resistant wire rod for bolts, which combines component design and suppresses the formation of copper-rich phase by controlling the process parameters in the heating step, and reduces the yield strength and tensile strength of the hot-rolled wire rod by controlling the cooling process, thereby improving the processing performance and producing hot-rolled wire rod that meets the requirements.

[0036] In order to achieve the above object, the present invention proposes a method for manufacturing the above-mentioned nickel-free annealing-free high-strength weather-resistant bolt wire rod, which comprises the steps of:

[0037] (1) smelting and casting to obtain ingots;

[0038] (2) heating the ingot;

[0039] (3) rolling the heated ingot;

[0040] (4) Cooling: After rolling, the steel is cooled to 900-920°C at a cooling rate of 4-5°C / s, and then further cooled to room temperature at a cooling rate lower than 0.4°C / s to obtain wire rod.

[0041] Preferably, in the manufacturing method of the present invention, in step (2), the heating temperature is controlled to be 1050-1100° C., and the time in the furnace is controlled within 120 min, preferably 95-110 min.

[0042] Preferably, in the manufacturing method of the present invention, no annealing is performed after the cooling step.

[0043] In the manufacturing method designed by the present invention, after smelting to obtain molten steel according to the chemical composition designed by the present invention, the molten steel can be cast into steel ingots or steel billets under protective conditions after deoxidation.

[0044] Accordingly, the obtained ingot or billet needs to be further heated, rolled, and cooled. It should be noted that the billet temperature before rolling must be strictly controlled to avoid excessive heating temperature leading to the precipitation of copper-rich phase. Therefore, the specific control process can be as follows: the billet should be heated to a temperature of 1050-1100°C before rolling, and the furnace time should be controlled within 120 minutes, preferably 95-110 minutes.

[0045] The temperature of the wire rod must be strictly controlled during the cooling process to avoid excessive cooling below 900°C, which may lead to excessive strength of the hot-rolled wire rod. Therefore, after rolling, it is cooled to 900-920°C at a cooling rate of 4-5°C / s, and then cooled to room temperature at a cooling rate lower than 0.4°C / s, thereby obtaining the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod of the present invention.

[0046] The nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod, bolt, and manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0047] The nickel-free, anneal-free, high-strength, weather-resistant bolt wire rod described in this invention achieves high strength and weather resistance through rational component matching and process design. Its atmospheric corrosion resistance (weatherability index I ≥ 8.5) meets the requirements for coating-free production. The nickel-free, anneal-free, high-strength, weather-resistant bolt wire rod has a yield strength of ≤400 MPa, a tensile strength of ≤600 MPa, an elongation of ≥25%, and a reduction of area of ​​≥60%.

[0048] Bolts made from nickel-free, anneal-free, high-strength, weather-resistant bolt wire rod according to the present invention can achieve yield strength ≥720 MPa, preferably ≥820 MPa, tensile strength ≥870 MPa, elongation ≥16%, and reduction of area ≥55%. These bolts not only possess high strength but also excellent atmospheric corrosion resistance, enabling them to be used in bridge structures, transmission towers, photovoltaic mounting systems, and other fields without the need for painting. DETAILED DESCRIPTION

[0049] The nickel-free, annealing-free, high-strength, weather-resistant bolt wire rod, bolt, and manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0050] Examples 1-6

[0051] Table 1-1 lists the mass percentages of the chemical elements in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rods of Examples 1-6.

[0052] Table 1-1. (wt%, the balance is Fe and other inevitable impurities except P and S)

[0053] Table 1-2 lists the content relationships among some chemical elements in the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rods of Examples 1-6.

[0054] Table 1-2.

[0055] Note: In the above Table 1-2, the Cr and C in the formula "Cr / C" are substituted with the mass percentage of the corresponding chemical elements; and I = 26.01×Cu+3.88×Ni+1.20×Cr+1.49×Si+17.28×P-7.29×Cu×Ni-9.10×Ni×P-33.39×Cu 2 , and each chemical element in the formula is substituted into the value before the percentage sign of the mass percentage content of the corresponding element.

[0056] In the present invention, the nickel-free annealing-free high-strength weather-resistant bolt wire rods of Examples 1-6 are specifically prepared by the following steps:

[0057] (1) Smelting and casting are carried out according to the chemical composition shown in Table 1-1 and Table 1-2: Smelting is carried out according to the designed composition, and after the molten steel is deoxidized, it is cast into a 142mm×142mm square billet under protective conditions.

[0058] (2) Heating: The 142 mm × 142 mm square billet is transported to a heating furnace for heating, and the heating temperature is controlled to be 1050-1100 ° C, and the time in the furnace is controlled within 120 min.

[0059] (3) Rolling.

[0060] (4) Cooling: After rolling, the steel is cooled to 900-920°C at a cooling rate of 4-5°C / s, and then further cooled to room temperature at a cooling rate lower than 0.4°C / s to obtain the corresponding φ10-26mm nickel-free annealing-free high-strength weather-resistant bolt wire rod.

[0061] In the present invention, 6 heats of steel, namely Examples 1-6, were smelted and prepared according to the chemical compositions designed in Tables 1-1 and 1-2. The chemical element composition and related process design of the nickel-free annealing-free high-strength weathering bolt wire rods of Examples 1-6 met the design specification requirements of the present invention.

[0062] Table 2 lists the specific process parameters of the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rods of Examples 1-6 in the above process steps.

[0063] Table 2

[0064] Samples of the nickel-free, anneal-free, high-strength, weather-resistant bolt wire rods obtained in Examples 1-6 were taken from the wire rod heads and observed using a scanning electron microscope (SEM). The inventors observed that the microstructures of the nickel-free, anneal-free, high-strength, weather-resistant bolt wire rods in Examples 1-6 were all ferrite + pearlite.

[0065] Accordingly, after completing the above observation and analysis, the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rods of the obtained finished product embodiments 1-6 can be further sampled again, and the wire rod samples of each embodiment can be subjected to relevant mechanical property tests. The obtained mechanical property test results are listed in Table 3.

[0066] The relevant mechanical properties test methods are as follows:

[0067] Tensile test: At room temperature, according to GBT228-2002 Metal Material Room Temperature Tensile Test Method, the yield strength, tensile strength, elongation and cross-sectional reduction rate of the nickel-free annealing-free high-strength weathering bolt wire rods of Examples 1-6 were tested.

[0068] Table 3 lists the mechanical property test results of the nickel-free annealing-free high-strength weather-resistant bolt wire rod obtained in finished product Examples 1-6.

[0069] Table 3.

[0070] As shown in Table 3, combined with Tables 1-2, it can be seen that the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rods of Examples 1-6 of the present invention, while having excellent corrosion resistance (weathering resistance index I greater than 8.5), have yield strengths less than 400 MPa and tensile strengths less than 600 MPa, meeting the strength requirements for annealing-free applications. Furthermore, the elongation is greater than 25%, and the reduction of area is greater than 60%. The atmospheric corrosion resistance of the nickel-free, annealing-free, high-strength, weather-resistant bolt wire rods of Examples 1-6 meets the requirements for coating-free applications.

[0071] To further illustrate that the annealing-free high-strength weather-resistant bolt wire rod of the present invention can be used to produce bolts with excellent performance, the annealing-free high-strength weather-resistant bolt wire rods of Examples 1-6 of the present invention can be further subjected to the following steps: pickling, phosphorus saponification, drawing, cold heading, tempering, and thread rolling to obtain high-strength weather-resistant bolts with excellent performance.

[0072] It should be noted that, in some embodiments of the present invention, the specific tempering heat treatment process is: quenching temperature 850℃-880℃, holding for 80 minutes and then oil quenching; tempering temperature 420-600℃, holding for 1 hour and then air cooling.

[0073] Therefore, the inventors conducted mechanical property tests on the high-strength weather-resistant bolts prepared in Examples 1-6. The results are listed in Table 4. The tensile testing method used in the mechanical property tests on the high-strength weather-resistant bolts prepared in Examples 1-6 is the same as that in Table 3 above, and will not be repeated here.

[0074] Table 4 lists the mechanical properties of the high-strength weather-resistant bolts prepared in Examples 1-6.

[0075] Table 4.

[0076] As shown in Table 4 above, the yield strength Rp0.2 of the bolts prepared corresponding to Examples 1-6 are all greater than 720 MPa, the tensile strength is greater than 870 MPa, the elongation is ≥16%, and the cross-sectional shrinkage rate is ≥55%. Their mechanical properties are excellent, and the strength reaches level 8.8-10.9. Therefore, they can be used in bridge structures, transmission towers, photovoltaic brackets and other fields without painting.

[0077] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0078] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A wire rod, which, in addition to Fe and unavoidable impurities, also contains the following chemical elements in the following mass percentages: C: 0.05-0.14%, Si: 0.01-2.0%, preferably 0.2-1.6%, Mn: 0.3-2.2%, preferably 0.5-1.7%, Cr: 2.4-4.5%, Cu: 0.2-0.6%, Al: 0.01-0.1%; And the mass percentage of Cr and C satisfies: Cr / C>20; The wire rod does not contain Ni.

2. The wire rod according to claim 1, characterized in that The wire rod has the following chemical elements in percentage by mass: C: 0.05-0.14%, Si: 0.01-2.0%, preferably 0.2-1.6%, Mn: 0.3-2.2%, preferably 0.5-1.7%, Cr: 2.4-4.5%, Cu: 0.2-0.6%, Al: 0.01-0.1%, the balance is Fe and other inevitable impurities; And the mass percentage of Cr and C satisfies: Cr / C>20.

3. The wire rod according to claim 1 or 2, characterized in that Among the inevitable impurities, P ≤ 0.012%, S ≤ 0.005%.

4. The wire rod according to claim 1 or 2, characterized in that: The microstructure of the wire rod is ferrite+pearlite.

5. The wire rod according to claim 1 or 2, characterized in that: The weather resistance index of the wire rod is I≥8.5, wherein I=26.01×Cu+3.88×Ni+1.20×Cr+1.49×Si+17.28×P-7.29×Cu×Ni-9.10×Ni×P-33.39×Cu 2 ; Substitute the numerical value before the mass percentage sign of the corresponding element content for each chemical element in the formula.

6. The wire rod according to claim 1 or 2, characterized in that: The wire rod has a yield strength of ≤400MPa, a tensile strength of ≤600MPa, an elongation of ≥25%, and a cross-sectional shrinkage of ≥60%.

7. A bolt, made from the wire rod according to any one of claims 1 to 6, wherein the bolt has a yield strength of ≥720 MPa, preferably ≥820 MPa, a tensile strength of ≥870 MPa, an elongation of ≥16%, and a cross-sectional reduction of ≥55%.

8. A method for manufacturing the bolt according to claim 7, characterized in that: The method comprises performing a quenching and tempering treatment on the wire rod according to any one of claims 1 to 6. Preferably, the quenching and tempering treatment comprises: quenching at a temperature of 850° C.-880° C., keeping warm for 80 minutes, and then oil quenching; tempering at a temperature of 420-600° C., keeping warm for 1 hour, and then air cooling.

9. The method for manufacturing a wire rod according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: (1) smelting and casting to obtain ingots; (2) heating the ingot; (3) rolling the heated ingot; (4) Cooling: After rolling, the steel is cooled to 900-920°C at a cooling rate of 4-5°C / s, and then further cooled to room temperature at a cooling rate lower than 0.4°C / s to obtain a wire rod.

10. The manufacturing method according to claim 9, characterized in that: In step (2), the heating temperature is controlled to be 1050-1100°C, and the time in the furnace is controlled within 120 minutes, preferably 95-110 minutes.

11. The manufacturing method according to claim 9 or 10, characterized in that: No annealing was performed after the cooling step.

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