A steel for 1700mpa high-strength bolts and a manufacturing method thereof

Abstract

The application discloses a kind of 1700MPa high-strength bolt steel, in addition to Fe and inevitable impurities, it also contains the following chemical elements with mass percentage as follows: C: 0.30-0.50wt.%, Mn: 0.30-2.00wt.%, Si: 0.55-2.00wt.%, Ni: 0.50-2.00wt.%, Cr: 0.30-3.00wt.%, Mo: 0.50-2.00wt.%, V: 0.03-0.50wt.%, Ti: 0.01-0.50wt.%; it also satisfies formula: [C]+0.19[Mn]+0.09[Si]+0.42[Ni] +0.26[Cr]+0.38[Mo]≥1.05, wherein [C], [Mn], [Si], [Ni], [Cr], [Mo] are respectively substituted into the numerical value before each chemical element mass percentage. Furthermore, the application also discloses the manufacturing method of the above-mentioned 1700MPa high-strength bolt steel, which comprises the following steps: (1) smelting and casting; (2) rolling into a rod; (3) spheroidizing annealing; (4) part shape processing; (5) isothermal heat treatment: heated to austenitizing temperature 860-940 DEG C, keeps for 30-90min, then cools to 250-400 DEG C, keeps for 60-180min.

Description

Technical Field

[0001] This invention relates to a metallic material and a method for manufacturing the same, and more particularly to a high-strength steel and a method for manufacturing the same. Background Technology

[0002] Currently, the manufacturing sectors of vehicles, ships, construction engineering, energy, and bridge transportation have an increasingly urgent need for bolts with long service life and high strength. In practical applications, high-strength bolts can reduce weight and increase installation space by reducing their size. Therefore, using high-strength bolts allows for functional and volume optimization of connected components, thereby achieving the goal of overall weight reduction and performance optimization in equipment.

[0003] Bolts are the most common and widely used connecting parts in modern mechanical devices. Although their connection method seems simple, their tightening process is quite complex. Currently, there are three main tightening techniques: torque method, torque-angle method, and yield point tightening method.

[0004] The torque method is a common tightening method that utilizes the linear relationship between torque and preload. Tightening stops when the specified torque is reached. Usually, the preload is in the elastic strain zone of the bolt. This method has low control accuracy of axial preload and does not fully utilize the service strength of the material.

[0005] The torque-angle method involves first tightening the bolt or nut to a specified torque to ensure it is tightened to the mating surface and to overcome factors such as uneven surfaces and friction coefficients. Then, it is tightened to a specified angle. Typically, the preload is in the plastic strain zone of the bolt, above the yield strength, thus achieving both full utilization of material strength and high-precision tightening control.

[0006] The yield point tightening method determines the yield point by measuring the relationship between torque and rotation angle, i.e., the change in torque rate. This method requires real-time monitoring of the slope of the torque-rotation angle curve during tightening. When the slope drops significantly, it indicates that the yield point has been reached, and tightening is stopped. The advantage of this method is that it can achieve a large tightening force with a relatively concentrated distribution, fully utilizing the material's potential, and represents the future direction of tightening technology.

[0007] In practical applications, whether using the torque-angle method or the yield point tightening method to tighten bolts, the plastic deformation zone of the bolt material must be as large as possible. Therefore, when designing bolt materials, in addition to improving bolt strength, it is also necessary to reduce the yield strength ratio to increase the plastic deformation zone, in order to meet the needs of future assembly processes.

[0008] In current technologies, the strengthening and toughening mechanisms of bolt materials mainly include grain refinement strengthening, solid solution strengthening, and precipitation strengthening. Among them, grain refinement strengthening is a means to improve both strength and effectively optimize ductility and toughness. It is mainly achieved by adding microalloying elements such as Ti and V, as well as controlled rolling and controlled cooling techniques. Solid solution strengthening utilizes point defects (interstitial atoms and substitutional atoms) within the metal material to strengthen the metal matrix. Its main strengthening elements include Mn, Si, Ni, and Mo. Precipitation strengthening increases strength by generating a stress field through the precipitation of second-phase particles in the metal matrix and hindering the movement of dislocations. The more dispersed the distribution of second-phase particles and the larger their volume fraction, the better the strengthening effect.

[0009] All three of these strengthening mechanisms share a problem: while increasing tensile strength, they also increase the yield strength ratio, leading to a reduction in the plastic deformation zone, which is detrimental to the future application of high-strength bolts. Therefore, there is an urgent need for a bolt steel manufacturing method that can both improve bolt strength and reduce the yield strength ratio.

[0010] Chinese patent document CN1900343A, published on January 24, 2007, entitled "Manufacturing Method of Steel with Excellent Delayed Fracture Resistance and Tensile Strength of 1600 MPa or Above and its Formed Products," discloses a method for manufacturing steel with excellent delayed fracture resistance and a tensile strength of 1600 MPa or above and its formed products. This method improves the steel's strength by adding 3.0–10.0% Mo to precipitate carbides and induce significant secondary hardening. While the steel produced by this method achieves a tensile strength of 1600 MPa, the alloy composition design is unreasonable, with an excessively high Mo content, and the yield strength and yield ratio are not mentioned.

[0011] Chinese patent document CN1900344A, published on January 24, 2007, entitled "High-strength bolt with excellent resistance to delayed fracture and its manufacturing method", discloses a high-strength bolt with excellent resistance to delayed fracture and its manufacturing method. The bolt head and shaft are formed from steel, then heated to 900-1100℃ and quenched, and then tempered at 580℃ or above. The maximum strength can reach 1864MPa, but the yield strength of the material is not mentioned and the yield strength ratio is not emphasized.

[0012] Chinese patent document CN110527908A, published on December 3, 2019, entitled "A Medium-Carbon Micro-Nano Structure Bainitic Steel and Its Heat Treatment Method," discloses a medium-carbon micro-nano structure bainitic steel and its heat treatment method. The method involves austenitizing the steel at Ac3+(20~120)℃ for 0.25~0.75h, followed by bainitic transformation at Bf+(10~60)℃ for 1.0~6.0h, and then water quenching to room temperature. The tensile strength of the medium-carbon micro-nano structure bainitic steel treated with this method is 1500~2000MPa. This method adds 1.0~3.0% Al, which differs from the chemical composition design of this patent.

[0013] In summary, existing technologies mainly focus on the tensile strength of materials, while paying insufficient attention to the yield strength ratio, resulting in materials that are difficult to meet the tightening process requirements of high-end bolts. Summary of the Invention

[0014] One of the objectives of this invention is to provide a 1700MPa high-strength bolt steel, which employs a reasonable chemical composition design, optimizing the content of C, Si, Mn, Ni, Cr, and Mo elements in the steel to ensure that the steel obtains excellent mechanical properties.

[0015] This 1700MPa high-strength bolt steel possesses both high strength and a low yield strength ratio. Its tensile strength is ≥1700MPa, yield strength ratio is <0.9, and elongation after fracture is ≥10%. It can be effectively applied to bolts and related products in fields such as vehicles, ships, construction engineering, energy, bridges, and transportation, meeting the urgent needs of the market and possessing excellent prospects for promotion and application value.

[0016] To achieve the above objectives, the present invention provides a 1700MPa high-strength bolt steel, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0017] C: 0.30~0.50wt.%, Mn: 0.30~2.00wt.%, Si: 0.55~2.00wt.%, Ni: 0.50~2.00wt.%, Cr: 0.30~3.00wt.%, Mo: 0.50~2.00wt.%, V: 0.03~0.50wt.%, Ti: 0.01~0.50wt.%;

[0018] It also satisfies the formula: [C]+0.19[Mn]+0.09[Si]+0.42[Ni]+0.26[Cr]+0.38[Mo]≥1.05, where [C], [Mn], [Si], [Ni], [Cr], and [Mo] are respectively substituted with the values ​​before the mass percentage sign of each chemical element.

[0019] Furthermore, in the 1700MPa high-strength bolt steel described in this invention, the mass percentage content of each chemical element is as follows:

[0020] C: 0.30–0.50 wt.%, Mn: 0.30–2.00 wt.%, Si: 0.55–2.00 wt.%, Ni: 0.50–2.00 wt.%, Cr: 0.30–3.00 wt.%, Mo: 0.50–2.00 wt.%, V: 0.03–0.50 wt.%, Ti: 0.01–0.50 wt.%; balance Fe and unavoidable impurities.

[0021] It also satisfies the formula: [C]+0.19[Mn]+0.09[Si]+0.42[Ni]+0.26[Cr]+0.38[Mo]≥1.05, where [C], [Mn], [Si], [Ni], [Cr], and [Mo] are respectively substituted with the values ​​before the mass percentage sign of each chemical element.

[0022] The design principles of each chemical element in the 1700MPa high-strength bolt steel described in this invention are as follows:

[0023] C: In the 1700MPa high-strength bolt steel described in this invention, C is an effective element for achieving strength. Adding an appropriate amount of C to the steel is beneficial to its strength. However, it should be noted that the C content in the steel should not be too high. When the C content in the steel is too high, it will adversely affect the cold heading properties, toughness, and resistance to delayed fracture of the steel. Based on this, considering the strength, cold heading properties, toughness, and resistance to delayed fracture of the material, in this invention, the mass percentage of C is controlled between 0.30 and 0.50 wt.%.

[0024] Mn: In the 1700MPa high-strength bolt steel described in this invention, Mn is an important element for stabilizing the austenite phase. Simultaneously, the added Mn element can also fix sulfur in the steel by forming MnS, thereby preventing hot brittleness. However, it should be noted that the Mn content in the steel should not be too high. If the Mn content is too high, it can easily cause grain boundary segregation, reducing grain boundary strength. Therefore, in the 1700MPa high-strength bolt steel described in this invention, the mass percentage of Mn element is controlled between 0.30 and 2.00 wt.%.

[0025] Si: In the 1700MPa high-strength bolt steel described in this invention, Si not only inhibits the precipitation of cementite in bainitic steel, but also dissolves in ferrite, thereby playing a role in solid solution strengthening and improving the strength and hardness of the steel plate. However, the Si content in the steel should not be too high. When the Si content in the steel is too high, it will significantly reduce the plasticity and toughness of the steel. Based on this, in the 1700MPa high-strength bolt steel described in this invention, the mass percentage content of Si is controlled between 0.55 and 2.00 wt.%.

[0026] Ni: In the 1700MPa high-strength bolt steel described in this invention, Ni can stabilize austenite, increase hardenability, and improve the low-temperature toughness of the steel. In addition, Ni can improve the iron oxide layer structure, increase density, thereby improving the corrosion resistance of the steel and inhibiting hydrogen adsorption, which has a positive effect on the steel's resistance to delayed fracture. Therefore, in order to maximize the beneficial effects of Ni and comprehensively consider cost factors, the mass percentage of Ni in the 1700MPa high-strength bolt steel described in this invention is controlled between 0.50 and 2.00 wt.%.

[0027] Cr: In the 1700MPa high-strength bolt steel described in this invention, Cr improves the hardenability and corrosion resistance of the steel. During tempering, it precipitates carbides, enhancing the steel's resistance to temper softening, thus playing an effective role in the steel's strength and resistance to delayed fracture. However, it is important to note that the Cr content in the steel should not be too high. Excessive Cr content leads to reduced cold working properties and increased production costs. Therefore, in the 1700MPa high-strength bolt steel described in this invention, the Cr content is controlled between 0.30 and 3.00 wt.%.

[0028] Mo: In the 1700MPa high-strength bolt steel described in this invention, Mo mainly precipitates Mo-containing carbides during the tempering stage, thereby producing significant secondary strengthening and improving the steel's resistance to temper softening. However, it should be noted that excessive Mo should not be added to the steel, as this will increase material costs. Therefore, considering cost factors, the mass percentage of Mo element is controlled between 0.50 and 2.00 wt.% in this invention.

[0029] V and Ti: In the 1700MPa high-strength bolt steel described in this invention, V and Ti elements can form precipitates with C and N in the steel. These precipitates can refine the austenite grains during the heating stage of the steel, thereby improving the strength of the steel plate. However, excessive V and Ti will form coarse inclusions, which will be detrimental to the performance of the steel. Therefore, in order to maximize the beneficial effects of V and Ti elements, in this invention, the mass percentage of V element is controlled between 0.03 and 0.50 wt.%, and the mass percentage of Ti element is controlled between 0.01 and 0.50 wt.%.

[0030] It should be noted that in the 1700MPa high-strength bolt steel described in this invention, if only the range of chemical elements is adjusted by designing the elemental composition as described above, the purpose of this invention cannot still be achieved.

[0031] Therefore, while controlling the mass percentage of the aforementioned single chemical element, the inventors further controlled the coordination relationships between the elements, which also needed to satisfy the following relationship:

[0032] [C]+0.19[Mn]+0.09[Si]+0.42[Ni]+0.26[Cr]+0.38[Mo]≥1.05.

[0033] Where [C], [Mn], [Si], [Ni], [Cr], and [Mo] are respectively replaced with the values ​​before the mass percentage sign corresponding to the mass percentage content of each chemical element.

[0034] In this invention, the above-mentioned relationship is a calculation formula for determining the relationship between the martensitic transformation start temperature and the chemical composition content. The design that satisfies the above-mentioned relationship can ensure that the martensitic transformation start temperature of the steel is below 300°C, so as to avoid the formation of martensitic structure during heat treatment and thus deteriorate the mechanical properties of the steel.

[0035] When the value calculated by ([C]+0.19[Mn]+0.09[Si]+0.42[Ni]+0.26[Cr]+0.38[Mo]) is called the M value, when the M value is lower than 1.05, the martensite transformation temperature is higher than 300℃. When the isothermal heat treatment temperature is low, martensite structure may be generated during heat treatment, which will lead to an excessively high yield strength ratio of the steel and deteriorate the mechanical properties of the steel.

[0036] The 1700MPa high-strength bolt steel of the present invention is used to process bolts and related products. In actual preparation, the steel can obtain bainitic structure through isothermal heat treatment. Under the same tensile strength grade, this structure has a lower yield strength ratio and good plasticity than conventional martensitic structure, which can meet the tightening process requirements of high-end bolts.

[0037] Furthermore, in the 1700MPa high-strength bolt steel described in this invention, among other unavoidable impurities: P ≤ 0.015wt.%, and / or S ≤ 0.015wt.%.

[0038] In the above-described technical solution of the present invention, both P and S elements are impurity elements in the 1700MPa high-strength bolt steel of the present invention. 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.

[0039] It should be noted that impurity elements P and S are prone to segregation at grain boundaries, reducing the toughness of steel and significantly affecting its cold working properties. Therefore, the content of P and S in steel must be strictly controlled, with P controlled to P≤0.015wt.% and S controlled to S≤0.015wt.%.

[0040] Furthermore, in the 1700MPa high-strength bolt steel described in this invention, its microstructure is bainite.

[0041] Furthermore, in the 1700MPa high-strength bolt steel described in this invention, its tensile strength is ≥ 1700MPa, yield strength ratio is <0.9, and elongation after fracture is ≥10%.

[0042] Accordingly, another objective of the present invention is to provide a method for manufacturing 1700MPa high-strength bolt steel. This method is simple to operate, and the 1700MPa high-strength bolt steel obtained by this method has excellent mechanical properties. It can be effectively applied to bolts and related products in fields such as vehicles, ships, construction engineering, energy, bridges and transportation, and has very good prospects for promotion and application value.

[0043] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned 1700MPa high-strength bolt steel, comprising the following steps:

[0044] (1) Smelting and casting;

[0045] (2) Rolled into wire rod;

[0046] (3) Isothermal spheroidizing annealing;

[0047] (4) Machining of part shape;

[0048] (5) Isothermal heat treatment: Heat to the austenitizing temperature of 860-940℃, hold for 30-90 min, then cool to 250-400℃ and hold for 60-180 min.

[0049] In the manufacturing method described above in this invention, the invention is based on a scientific and reasonable alloy composition design. The wire rod is produced by smelting, secondary refining, continuous casting, and rolling. The microstructure and properties of the wire rod are optimized by isothermal spheroidizing annealing treatment, which can improve the plasticity of the material and prevent quality problems such as cracking and surface roughness during the part processing.

[0050] It should be noted that in this invention, after the part is shaped and machined, an optimized isothermal heat treatment is required to obtain 1700MPa high-strength bolt steel with tensile strength ≥1700MPa, yield strength ratio <0.9, and elongation after fracture ≥10%.

[0051] In step (2) above, in some embodiments, after the wire rod is rolled into a wire rod, a slow cooling measure can be taken when it passes through the Steyrmo air-cooling line. The specific operation can be as follows: when passing through the Steyrmo air-cooling line, all fans are turned off and an insulation cover is placed on top to reduce the cooling rate of the wire rod. This operation is because the wire rod has high hardenability, and if the cooling rate is too fast, abnormal martensitic structure is easily generated.

[0052] Accordingly, in the isothermal heat treatment process of step (5) above, in some preferred embodiments, it is also necessary to control the cooling rate and ensure precise control of the isothermal heat treatment temperature. Among them, molten salt solution is preferred as the isothermal medium.

[0053] Furthermore, in the manufacturing method described in this invention, in step (2), the rolled wire rod is slowly cooled, and the cooling rate is controlled to be <5℃ / s.

[0054] Furthermore, in the manufacturing method described in this invention, in step (5), the isothermal medium is a molten salt solution.

[0055] Furthermore, in the manufacturing method described in this invention, in step (5), the cooling rate is controlled to be greater than 5°C / s.

[0056] The 1700MPa high-strength bolt steel and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0057] This invention, through the rational design of chemical composition, especially the precise control of the content of chemical elements such as C, Mn, Si, Ni, Cr, and Mo, combined with an optimized isothermal heat treatment process, can obtain a high-strength bolt steel with a tensile strength ≥1700MPa, a yield strength ratio <0.9, and an elongation after fracture ≥10%.

[0058] Compared to existing bolt steels, the 1700MPa high-strength bolt steel described in this invention has significant advantages in chemical composition design and mechanical property control, exhibiting lower alloy costs and excellent strength and plasticity. This steel, while possessing a tensile strength ≥1700MPa, also meets the requirement of a yield strength ratio <0.9, exhibiting a wider plastic deformation range, which is beneficial for controlling tightening force during bolt assembly.

[0059] The 1700MPa high-strength bolt steel described in this invention has excellent mechanical properties and can be effectively applied to bolts and related products in fields such as vehicles, ships, construction engineering, energy, bridges and transportation. It has a very good prospect for promotion and application value. Detailed Implementation

[0060] The following will provide a further explanation and description of the 1700MPa high-strength bolt steel and its manufacturing method described in this invention, with reference to specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0061] Examples 1-6

[0062] The 1700MPa high-strength bolt steel described in Examples 1-6 of this invention is all prepared using the following steps:

[0063] (1) Smelting, secondary refining and casting are carried out according to the chemical composition shown in Table 1.

[0064] (2) Rolled into wire rod: The wire rod specifications are φ5.5~20mm. When the rolled wire rod passes through the Stellmore air-cooling line, slow cooling measures are taken. When passing through the Stellmore air-cooling line, all fans are turned off and covered with heat insulation cover to reduce the cooling rate of the wire rod and carry out slow cooling. The cooling rate of slow cooling is controlled to be <5℃ / s.

[0065] (3) Spheroidizing annealing.

[0066] (4) The annealed wire rod is processed by drawing, cold heading, turning, milling and other processes to shape the parts.

[0067] (5) Isothermal heat treatment: First, heat to the austenitizing temperature of 860-940℃ and hold for 30-90 minutes, then cool to 250-400℃ at a rate of >5℃ / s and hold for 60-180 minutes. In the above cooling and isothermal operations, the isothermal medium is molten salt solution to ensure rapid cooling and precise temperature control of the parts.

[0068] It should be noted that, in this invention, the chemical composition design and related processes of the 1700MPa high-strength bolt steel in Examples 1-6 all meet the design specifications of this invention.

[0069] Table 1 lists the mass percentage of each chemical element in the 1700MPa high-strength bolt steel of Examples 1-6.

[0070] Table 1. (wt%, balance Fe and other unavoidable impurities besides P and S)

[0071]

[0072] Note: M value = [C] + 0.19[Mn] + 0.09[Si] + 0.42[Ni] + 0.26[Cr] + 0.38[Mo], where [C], [Mn], [Si], [Ni], [Cr], and [Mo] are respectively replaced with the values ​​before the mass percentage sign of the corresponding chemical element.

[0073] Table 2 lists the specific process parameters of the 1700MPa high-strength bolt steel of Examples 1-6 in the above manufacturing method.

[0074] Table 2.

[0075]

[0076]

[0077] Samples of the 1700MPa high-strength bolt steel of Examples 1-6 obtained through the above process steps were taken, and mechanical properties of the 1700MPa high-strength bolt steel of each example were tested. The results of the mechanical property tests are listed in Table 3.

[0078] The specific testing methods are as follows:

[0079] Tensile testing: The test was conducted according to the national standard GB / T 228.1-2010 "Metallic materials, tensile testing—Part 1: Test at room temperature". The specimens were fabricated into ASTM M14 threaded tensile test specimens, and the testing environment was controlled at room temperature (10–35°C). Based on this tensile test, the yield strength, tensile strength, yield ratio, and elongation after fracture of the 1700MPa high-strength bolt steel used in Examples 1-6 can be effectively measured.

[0080] Table 3 lists the relevant performance test results of the 1700MPa high-strength bolt steel in Examples 1-6.

[0081] Table 3.

[0082]

[0083] As can be seen from Table 3, the 1700MPa high-strength bolt steels of Examples 1-6 of this invention have significant advantages in terms of mechanical properties. The yield strength of the 1700MPa high-strength bolt steels of Examples 1-6 is between 1360 and 1530 MPa, the tensile strength is between 1703 and 1780 MPa, the elongation after fracture is between 11 and 12.5%, and the yield strength ratio is between 0.80 and 0.86.

[0084] In summary, it can be seen that by rationally designing the chemical composition, especially by precisely controlling the content of chemical elements such as C, Mn, Si, Ni, Cr, and Mo, and by using an optimized isothermal heat treatment process, this invention can obtain a 1700MPa high-strength bolt steel with excellent mechanical properties.

[0085] Compared with existing bolt steel, the 1700MPa high-strength bolt steel of this invention has significant advantages in chemical composition design and mechanical property control. It can be effectively applied to bolts and related products in fields such as vehicles, ships, construction engineering, energy, bridges and transportation, and has a very good prospect for promotion and application value.

[0086] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0087] 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 thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high-strength bolt steel with a strength of 1700MPa, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.30~0.50 wt.%, Mn: 0.30~2.00 wt.%, Si: 0.55~2.00 wt.%, Ni: 0.50~2.00 wt.%, Cr: 0.30~3.00 wt.%, Mo: 0.50~2.00 wt.%, V: 0.03~0.50 wt.%, Ti: 0.01~0.50 wt.%; the balance is Fe and unavoidable impurities. It also satisfies the formula: [C]+0.19[Mn]+0.09[Si]+0.42[Ni]+0.26[Cr]+0.38[Mo]≥1.05, where [C], [Mn], [Si], [Ni], [Cr], and [Mo] are respectively substituted with the values ​​before the mass percentage sign of each chemical element; The steel used for the 1700MPa high-strength bolts has a tensile strength ≥1700MPa, a yield strength ratio <0.9 with a lower limit of 0.8, and an elongation after fracture ≥10%. The microstructure of the steel used for the 1700MPa high-strength bolts is bainite.

2. The 1700MPa high-strength bolt steel as described in claim 1, characterized in that, In other unavoidable impurities: P ≤ 0.015 wt.%, and / or S ≤ 0.015 wt.%.

3. The method for manufacturing 1700MPa high-strength bolt steel as described in any one of claims 1-2, characterized in that, It includes the following steps: (1) Smelting and casting; (2) Rolled into wire rod; (3) Spheroidizing annealing; (4) Machining of part shape; (5) Isothermal heat treatment: Heat to the austenitizing temperature of 860-940℃, hold for 30-90 min, then cool to 250-400℃ and hold for 60-180 min.

4. The manufacturing method as described in claim 3, characterized in that, In step (2), the rolled wire rod is slowly cooled, and the cooling rate is controlled to be <5℃ / s.

5. The manufacturing method as described in claim 3, characterized in that, In step (5), the isothermal medium is a molten salt solution.

6. The manufacturing method according to any one of claims 3-5, characterized in that, In step (5), the cooling rate is controlled to be greater than 5°C / s.

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