1250MPa-grade high-strength tool steel wire rod and manufacturing method thereof

Abstract

The invention relates to a 1250MPa-grade high-strength tool steel wire rod and a manufacturing method thereof.The manufacturing method includes the steps that after medium-carbon alloy components containing trace Nb and V are designed, rolled and spun into a wire rod, online molten salt quenching strong isothermal treatment is conducted, the wire rod is made to pass through front-section molten salt firstly, the wire rod is controlled to enter a bainite phase region from a high-temperature austenite state at the cooling speed larger than or equal to 35 DEG C / s, and then the high-strength tool steel wire rod is manufactured; the preparation method comprises the following steps: firstly, quenching bainite is taken as a main raw material to form a quenching structure taking the quenched bainite as a main raw material, then, the steel wire rod with a microscopic structure comprising tempered bainite and ferrite is prepared through rear-section molten salt heating, isothermal tempering and toughening destressing treatment on the quenching structure, carbide precipitation control and roller way slow cooling finally, and the strength and plasticity matching of the steel wire rod can be improved by adjusting the structure state, so that the strength and plasticity of the steel wire rod are improved. The tensile strength is 1175-1225 MPa, the percentage reduction of area is 58%-63%, and the steel can be directly machined into parts after cold machining forming, so that the production energy consumption of the tool steel is reduced, the cost is reduced, and the efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the field of hot-rolled wire rods, and particularly relates to a 1250 MPa grade high-strength tool steel wire rod and a manufacturing method thereof. Background Art

[0002] Tools are basic implements used in production. The production process mainly involves hot-rolled wire rods being pickled and cold-worked into shape, and then matching a suitable heat treatment process to achieve appropriate mechanical properties. Therefore, improving the base metal properties of hot-rolled wire rods is of great significance for reducing the processing difficulty of tool steel and maintaining good strength, toughness, stiffness, and durability of tools. The conventional hot-rolled wire rod has a microstructure composed of pearlite and ferrite, which has relatively high cold-working performance. However, for 1250 MPa grade high-strength tool steel, the development of its wire rod is limited by the controlled cooling ability of the Stelmor air-cooling line after wire laying. It mainly relies on alloy design with hardenability elements and requires a suitable heat treatment process after cold working to adjust the microstructure state to achieve the service performance. Subsequent heat treatment will also lead to increased energy consumption, cost, and reduced efficiency in the tool production process. Therefore, it is necessary to develop a 1250 MPa grade high-strength tool steel wire rod and a manufacturing method thereof, so that it can be directly processed into parts after cold working into shape, reducing the production energy consumption, cost, and improving the efficiency of tool steel.

[0003] In order to reduce the risk of low-temperature abnormal microstructures such as brittle martensite caused by segregation of hardenability elements such as Mn and Cr and unstable temperature control of the air-cooling line, and to take into account the strength performance of the wire rod, the existing alloy tool steel wire rods will adjust the microstructure to bainite or contain bainite microstructure, but there are still the following defects: (1) In order to increase the martensite transformation temperature and reduce the precipitation tendency of martensite during the cooling process after phase transformation, the content of elements such as C and Si in the microstructure is relatively high. For example, a kind of alloy tool steel wire rod and preparation method and tool steel without martensite microstructure disclosed in Patent CN114752858B adopt a C-Si-Mn-Cr-Mo-V-Ni composition design and combine fast-then-slow cooling of the air-cooling line after low-temperature wire laying to obtain pearlite, ferrite, and a small amount of bainite microstructure. On the one hand, due to the limited maximum cooling ability of the Stelmor air-cooling line, the residence time of the wire rod in the pearlite transformation zone is relatively long, promoting the lamellar precipitation of ferrite to form pearlite. The soft phases of pearlite and ferrite account for a relatively large proportion, and the strength of the mixed microstructure is relatively low, resulting in the need to add a relatively high content of C, Si, or expensive alloys such as Mo and V in the composition, leading to a relatively high material cost. On the other hand, the relatively high content of C and Si increases the stability of austenite and delays the start time of pearlite transformation, but at the same time reduces the bainite transformation temperature, making the bainite transformation need to occur at a lower temperature, and the kinetic conditions are more severe. After the continuous cooling phase transformation of the wire rod, it is already in a low-temperature state, and there will be relatively large residual tissue stress, resulting in limited improvement in the plasticity and toughness of the wire rod, and it is difficult to meet the requirement of omitting heat treatment after cold working and directly processing into parts.

[0004] (2) To avoid brittle fracture of martensite structure and improve the service and fatigue performance of tool steel, bainite structure is obtained by large deformation rolling and strong air cooling treatment. For example, a production process of alloy tool steel wire rod for obtaining fully bainite structure disclosed in Patent CN111690801B adopts the composition design of C-Si-Mn-Cr-Ni-Al-Mo-V-Nb, combines large deformation rolling, strong air cooling after low-temperature wire laying and cooling with a heat preservation cover to obtain fully bainite structure. However, on the one hand, low-temperature rolling and wire laying require a large load on the rolling line and will exacerbate the wear of the rolling line. The material cost of the composition system with medium-high carbon and Mo hardenability elements is relatively high, which will delay the bainite transformation kinetics. Moreover, the maximum cooling capacity of the Stelmor air cooling line is limited. Even with strong air cooling treatment, it can only cool down to the upper bainite phase region. The bainite ferrite laths are relatively thick and unevenly distributed, resulting in poor strength, toughness and wear resistance. On the other hand, with the increase of the temperature difference and uncontrollability between the windward surface and the leeward surface during the strong air cooling process, the risk of martensite formation due to surface supercooling of the wire rod will increase, and it is also difficult to control the dispersion strengthening of microalloy carbides. And the minimum cooling capacity of the phase transformation cooling control of the heat preservation cover is limited. After the phase transformation incubation of the wire rod, it will be in a low-temperature state, resulting in higher bainite distortion and residual large tissue stress. Especially for small-sized wire rods, due to the fast temperature drop rate and concentrated deformation resistance, the cracking risk in subsequent cold processing is increased. Summary of the Invention

[0005] The present invention aims to solve at least one of the above technical problems to some extent. The present invention provides a 1250 MPa grade high-strength tool steel wire rod and its manufacturing method, which can improve the strength-plasticity matching of the wire rod by adjusting the tissue state, is beneficial to be directly processed into parts after cold processing forming, and promotes the reduction of energy consumption, cost and improvement of efficiency in tool steel production.

[0006] The technical solution adopted by the present invention to solve its technical problems is: A manufacturing method of a 1250 MPa grade high-strength tool steel wire rod, the manufacturing method includes: After the billet is rolled and spun into wire rod at a spinning temperature of ≥905℃, it is subjected to online molten salt quenching strong isothermal treatment, so that the wire rod first passes through the front section of molten salt, and the wire rod is controlled to enter the bainite phase region from the high temperature austenite state at a cooling rate of ≥35℃ / s to form a quenched structure mainly composed of quenched bainite, and then the molten salt is heated in the back section, and the quenched structure is subjected to isothermal tempering, toughening and stress relief treatment, and the precipitation of carbides is controlled. Finally, it is slowly cooled through a roller to form a microstructure including tempering. The wire rod of fire bainite and ferrite comprises the following chemical compositions and mass percentages: C: 0.45%-0.50%, Si: 0.25%-0.35%, Mn: 0.50%-0.65%, Cr: 0.55%-0.65%, Nb: 0.027%-0.035%, V: 0.045%-0.055%, P≤0.015%, S≤0.015%, and the rest is Fe and unavoidable impurities.

[0007] The design basis of the chemical composition and mass percentage of the above wire rod includes: (1) Carbon: The C element improves the strength of the matrix by solid solution strengthening and forming carbides. It has a relatively low price. As the carbon content increases, it can increase the stability of austenite, delay the transformation of pearlite, provide the necessary carbon source for the formation of quenched bainite in the front molten salt, promote the transformation of bainite, and give the tool steel wire rod high strength and wear resistance. However, if the carbon content is too high, it is easy to form coarse cementite or martensite, resulting in a decrease in toughness and a decrease in the bainite transformation temperature, which makes isothermal tempering more difficult during the subsequent molten salt treatment. Therefore, in order to meet the strength and wear resistance requirements of tool steel and control material costs, and at the same time facilitate short-term quenching treatment of wire rods and improve strength-toughness matching, a medium carbon content is used to balance strength and toughness, and the mass percentage of C is controlled to be 0.45%~0.50%.

[0008] (2) Silicon: The Si element can improve the purity of molten steel, hinder the diffusion of carbon in austenite, inhibit the grain coarsening and cementite precipitation during the previous molten salt treatment, delay the pearlite transformation, and promote the formation of a structure dominated by quenched bainite after a short treatment of the previous molten salt, thereby improving the strength of the steel. However, too high a silicon content will lead to excessive solid solution strengthening of silicon, aggravate lattice distortion, reduce the toughness of the steel, affect plasticity control, and lead to a decrease in plasticity during cold working. Therefore, in order to reduce the difficulty of tempering and adapt to the strong isothermal control of the structure by online molten salt quenching, the mass percentage of Si is controlled to be 0.25%~0.35%.

[0009] (3) Manganese: As a strong austenite stabilizing element, Mn can reduce the diffusion coefficient of carbon in austenite, delay the pearlite transformation, increase the hardenability of the wire rod, expand the austenite region, promote the shear formation of quenched bainite during the previous molten salt treatment, enable the bainite phase transformation to be triggered at a higher temperature and shorten the phase transformation time, improve the strength and hardness of the wire rod. However, when the content of Mn is too high, the austenite grains are prone to coarsening during heating, higher energy is required for stress release, the tempering softening process is delayed, and the toughness and plasticity of the steel are reduced. Therefore, in order to enable the rapid formation of quenched bainite during short-time quenching and facilitate the control of tempering softening, the mass percentage of Mn is controlled to be 0.50% - 0.65%.

[0010] (4) Chromium: As a carbide-forming element, Cr can form high-hardness alloy carbides, significantly improve the anti-wear ability of the tool, increase the hardenability of the steel, reduce the diffusion coefficient of carbon in austenite, expand the bainite transformation range, and cooperate with Mn to further inhibit the precipitation of pearlite, making quenched bainite the main transformation product during the previous treatment, which is beneficial to the refinement of bainite ferrite lamellae and improve the matrix strength. However, too high Cr content will exacerbate composition segregation, increase the difficulty of controlling tissue uniformity and the difficulty of improving plasticity, and reduce cold workability. Therefore, in order to balance the wear resistance requirements of tool steel, enable the rapid formation of quenched bainite during short-time quenching and avoid excessive strength loss during tempering, the Cr content is appropriately increased, and the mass percentage of Cr is controlled to be 0.55% - 0.65%.

[0011] (5) Niobium: The Nb element can pin the grain boundaries, inhibit the growth of austenite grains, obtain a fine-grained structure, which is beneficial to shortening the incubation period of bainite transformation, promoting the uniform formation of quenched bainite. At the same time, it precipitates dispersively in the high-temperature isothermal tempering temperature range, improving the strength and toughness of the wire rod. However, the cost of Nb is relatively high, and there is a risk of precipitation, aggregation and growth to form coarse particles under high-temperature treatment, which will instead reduce the toughness. Therefore, based on the role, cost and manufacturing control considerations of the Nb element, the mass percentage of Nb is controlled to be 0.027% - 0.035%.

[0012] (6) Vanadium: As an alloying element, the V element pins the grain boundaries to inhibit the growth of austenite grains, can refine the original structure, promote the formation of high-density dislocations and twins in bainite ferrite, and can form nano-scale VC carbides during the isothermal process, which are uniformly distributed in the matrix, improving the wear resistance of tool steel, and at the same time compensating for the strength loss caused by the tempering softening of quenched bainite. However, the cost of the V element is relatively high, and excessive addition is not conducive to controlling the cost of the wire rod. The vanadium-containing carbides aggregate to form coarse carbides, which will cut the matrix and deteriorate the impact resistance of the tool. Therefore, based on the role, cost and manufacturing control considerations of the V element, the mass percentage of V is controlled to be 0.045% - 0.055%.

[0013] (7) Phosphorus and sulfur: Elements P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% are controlled.

[0014] The above wire rod adopts a medium-carbon alloy composition design containing trace amounts of Nb and V. By optimizing the contents of Si, Mn, and Cr, the austenite region is expanded, providing favorable conditions for the rapid formation of quenched bainite by short-time quenching at a higher temperature during the front-section molten salt treatment, avoiding excessive tempering loss of strength performance, and giving full play to the dispersion strengthening effect of microalloys. On this basis, a relatively high spinning temperature, i.e., the quenching temperature, is selected to prepare for a large supercooling degree after the front-section molten salt treatment and promote the short-time quenching of the wire rod to form quenched bainite. The wire rod after spinning is directly subjected to on-line molten salt quenching and strong isothermal treatment without air cooling: I. Compared with the limited maximum cooling capacity and unstable temperature control of the Stelmor air-cooling line, it is difficult to suppress soft-phase tissues and abnormally brittle tissues, resulting in insufficiently strong and tough upper bainite tissues. On the one hand, after the front-section molten salt treatment of the wire rod, the strong heat transfer capacity of the molten salt can be utilized to promote the rapid cooling of the wire rod, enabling the wire rod to quickly pass through the pearlite transformation region from the high-temperature austenite state into the bainite phase region, avoiding the formation of pearlite tissues. At the same time, due to the increased supercooling degree and the lower molten salt temperature, the long-range diffusion of carbon in austenite can be inhibited, accelerating the nucleation rate of quenched bainite and forming needle-like quenched bainite instead of feathery upper bainite tissues. The carbide size is small and the dislocation density is high, improving the matrix strength and making up for the strength loss caused by the reduction of carbon, silicon, and microalloy contents. On the other hand, the thermal conductivity of the molten salt is higher than that of air. When the wire rod passes through the front-section molten salt, the molten salt covers the surface of the wire rod for uniform heat transfer, eliminating the temperature difference problem between the windward and leeward surfaces of the wire rod in the air-cooling line. The temperature uniformity from the surface to the core of the wire rod is better, which can promote the uniform transformation of the wire rod in the bainite phase region and avoid the formation of martensite tissues due to local supercooling or austenite remnants during subsequent cooling.

[0015] II. Compared with the limited minimum cooling capacity and continuous cooling of the Stelmor air-cooling line with a heat preservation cover, it is difficult to regulate the tissue state and carbide precipitation. On the one hand, after the rear-section molten salt treatment of the wire rod, it can be heated from the lower-temperature bainite phase region to the high-temperature isothermal temperature range, and the wire rod can be transformed to be isothermally treated at the same temperature as the molten salt, which can extend the time of the wire rod in the high-temperature phase region, promote the quenched tissue to reduce the dislocation density and release stress, reduce the temperature difference from the surface to the core of the wire rod, and improve the overall softening effect. On the other hand, it can extend the time of the wire rod in the temperature range of Nb and V carbide dispersion precipitation, avoiding diffusion difficulties caused by too low temperature or too short treatment time, ensuring the kinetic conditions for the nucleation of the precipitation phase, and making up for the strength loss of isothermal tempering through the large-scale dispersion precipitation of microalloy carbides, providing a strengthening and toughening effect. After that, the wire rod is slowly cooled through the roller table, which can continue the softening effect of the rear-section molten salt, promote the further toughening of the wire rod tissue, and realize the regulation of the tissue state and the improvement of the strength-plasticity performance matching of the wire rod.

[0016] Before rolling, selecting a relatively high soaking temperature of the heating furnace and an appropriate residence time in the furnace can promote the homogenization of alloy components, reduce the influence of segregation, and avoid overburning. In a preferred technical solution, before rolling, the soaking temperature of the heating furnace is controlled at 1160 - 1200 °C, and the residence time in the furnace is 140 - 185 min.

[0017] Due to the relatively high spinning temperature, the limitation on the rolling temperature can be reduced. Selecting a relatively high initial rolling temperature can improve the plasticity of the billet, reduce the mill load, avoid cracking, and increase the rolling speed and efficiency. The microalloying elements Nb and V can precipitate and pin the grain boundaries during the initial rolling stage. Selecting an appropriate finishing rolling temperature and finishing rolling reduction can promote dynamic recrystallization during the finishing rolling process, refine the grains. In a preferred technical solution, during rolling, the initial rolling temperature is controlled at 1040 - 1090 °C, the finishing rolling temperature is 910 - 940 °C, and the finishing rolling reduction is 23% - 28%.

[0018] During spinning, the spinning temperature can be further controlled to avoid the risk of coarse austenite grains caused by too high temperature. In a preferred technical solution, during spinning, the spinning temperature is controlled at 905 - 930 °C.

[0019] The front-section molten salt is in the bainite phase region. The lower the molten salt temperature and the longer the treatment time, the more beneficial it is to increase the cooling rate of the wire rod, inhibit the formation of pearlite, promote the transformation of the high-temperature austenite structure into needle-like quenched bainite, form a quenched structure mainly composed of quenched bainite, and improve the matrix strength. However, if the molten salt temperature is too low, there is a risk of forming abnormal martensite structure. At the same time, as the treatment time prolongs, the residual stress increases, which will increase the difficulty of isothermal tempering and affect carbide regulation; on the contrary, the higher the molten salt temperature and the shorter the treatment time, the more beneficial it is to reduce the dislocation density and stress of the structure, reduce the difficulty of isothermal tempering. However, if the molten salt temperature is too high, it is not conducive to the transformation of austenite structure into needle-like quenched bainite and the inhibition of upper bainite formation. At the same time, if the treatment time is too short, it will affect the transformation of austenite structure into quenched bainite, increase the austenite residue, and affect tissue regulation and matrix strength. Therefore, the molten salt temperature and treatment time of the front-section molten salt can be controlled to enable the wire rod to quickly enter the bainite phase region from the high-temperature austenite state, form a structure mainly composed of quenched bainite, and make organizational preparations for the subsequent-section molten salt treatment. In a preferred technical solution, the molten salt temperature of the front-section molten salt is 325 - 355 °C, and the treatment time is 15 - 25 s.

[0020] Due to the large temperature difference between the spinning temperature and the bainite phase region, selecting a relatively high molten salt circulation rate can reduce the temperature rise of the molten salt and improve the through-bar tissue uniformity. In a preferred technical solution, the molten salt circulation rate of the front-section molten salt is 550 - 750 t / h, and the temperature rise of the molten salt ≤ 8 °C.

[0021] The molten salt temperature of the latter-stage molten salt is in the high-temperature range. The higher the molten salt temperature and the longer the treatment time, the more beneficial it is to provide more thermal power, eliminate quenching stress through tempering, reduce lattice distortion, and transform the matrix from the high brittleness in the quenched state to the strong toughness in the tempered state. At the same time, the high temperature provides the driving force for atomic diffusion, accelerating the rate of carbide formation by the combination of Nb and C, increasing the precipitation amount, and increasing the number of strengthening phases. However, if the molten salt temperature is too high and the treatment time is too long, over-tempering softening will occur, the Nb / V carbides will coarsen excessively, the precipitation strengthening effect will weaken, and even the strength and plasticity will decrease due to the too large size of the carbides, and the production energy consumption will also increase. On the contrary, the lower the molten salt temperature, the more beneficial it is to promote the fine and dispersed precipitation of vanadium-containing carbides and reduce the softening rate. With the shortening of the treatment time, it is beneficial to reduce the production energy consumption. However, if the molten salt temperature is too low and the treatment time is too short, insufficient tempering of the quenched structure will lead to high residual stress and tissue brittleness, and the precipitation amount of Nb / V carbides is insufficient, resulting in the loss of the precipitation strengthening effect. Therefore, an appropriate molten salt temperature and treatment time can be selected to control the high-temperature isothermal tempering of the quenched structure in the high-temperature isothermal range of the wire rod, toughen and remove stress, control the carbide precipitation, and improve the strength-plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the latter-stage molten salt is 580-600 °C, and the treatment time is 300-550 s.

[0022] Due to a certain temperature difference between the former-stage molten salt and the latter-stage molten salt, selecting an appropriate molten salt circulation rate can reduce the molten salt temperature rise and improve the tissue uniformity of the same wire rod. In the preferred technical solution, the molten salt circulation rate of the latter-stage molten salt is 450-650 t / h.

[0023] In the preferred technical solution, the roller table slow cooling controls the wire rod to cool slowly at a cooling rate of 0.35-0.95 °C / s to below 260 °C, which can prevent the wire rod from increasing stress due to too fast cooling rate during the cooling process and promote further toughening of the wire rod tissue, improving the softening effect of the wire rod.

[0024] In the preferred technical solution, the roller table slow cooling controls the closing of the heat preservation cover or controls the opening degree of the heat preservation cover, and the wire rod is transported by the conveying roller table through the heat preservation cover to control the cooling of the wire rod, which can promote the rapid offline of the wire rod.

[0025] A 1250 MPa grade high-strength tool steel wire rod, which is manufactured by the manufacturing method of the 1250 MPa grade high-strength tool steel wire rod described in any one of the above.

[0026] The above-mentioned wire rod adopts a C-Si-Mn-Cr-Nb-V composition design. The contents of C and Si are relatively low, and trace amounts of Nb and V components are added. It does not contain precious metal alloy elements such as Ni and Mo, so the material cost is relatively low. At the same time, the microstructure includes a mixed structure mainly composed of tempered bainite and a small amount of ferrite. Compared with the existing pearlite wire rod for tool air-cooled lines, it can avoid the risk of martensite brittle structure caused by the content of high hardenability components. Using the bainite low-temperature transformation zone quenching to form a needle-like quenched bainite matrix, the pearlite soft phase can significantly improve the matrix strength. After tempering, the strength characteristics are retained. Combining the precipitation and fine grain strengthening of Nb and V can make up for the strength loss caused by reducing the contents of C and alloy components. It is more wear-resistant than pearlite and is suitable for wire rod cold forming and tool service scenarios. Compared with the existing bainite wire rod for tool air-cooled lines, it can avoid the loss of toughness caused by the formation of feathery upper bainite in the bainite medium-temperature transformation zone, and can form a quenched bainite structure with better strength and toughness. At the same time, using the quenched bainite structure for tempering, the ferrite remains in a needle-like shape, but the internal dislocation density decreases, and the carbide distribution is more uniform. Combining the carbide dispersion strengthening effect of alloy elements such as Nb and V, the strength decreases slowly, and a strong and tough tissue state is obtained, thus meeting the requirements of good cold working performance, impact resistance, and no need for heat treatment to adjust the tissue performance after cold working of the tool.

[0027] The higher the proportion of tempered bainite in the wire rod, the higher the strength and wear resistance of the wire rod can be improved, and a certain toughness and plasticity can be maintained, making the material not easily brittle fracture. In the preferred technical solution, the volume percentage of the tempered bainite ≥ 97%.

[0028] In the preferred technical solution, the diameter of the wire rod is 6 - 10 mm, the tensile strength is 1175 - 1225 MPa, the reduction of area is 58% - 63%, and the mechanical property difference within the same coil ≤ 37 MPa. Medium and small-sized wire rods can be cold formed to manufacture tool blanks, wrenches and other tool products. The wire rod reaches a relatively high strength level and plastic index, and the longitudinal property uniformity is good. It can withstand large plastic deformation before fracture and is not easily cracked during direct cold heading, cold drawing and other cold working processes. It is suitable for forming complex-shaped parts and is not easily brittle fracture under dynamic loads, meeting the tool anti-fatigue requirements. Therefore, it can be used as a tool product after cold forming.

[0029] Compared with the prior art, the beneficial effects of the present invention are at least as follows: (1)In view of the fact that the existing pearlite / bainite tool steel wire rods are limited by the controlled cooling capacity of the Stelmor air cooling line and it is difficult to regulate the tissue properties, the manufacturing method of the present invention combines Nb-V chemical composition design with on-line molten salt quenching and strong isothermal technology, and performs on-line molten salt quenching and strong isothermal treatment at a relatively high quenching temperature to control the wire rod to quickly enter the bainite phase region from the high-temperature austenite state, forming a quenched structure mainly composed of quenched bainite, which can avoid the formation of pearlite soft phase, upper bainite or martensite brittle phase. Then, the wire rod is controlled to perform high-temperature isothermal tempering on the quenched structure in the high-temperature isothermal range for toughening and stress relief treatment, and the precipitation of carbides can be controlled. Finally, slow cooling through the roller table promotes further toughening of the wire rod structure, which can effectively adjust the tissue state, improve the strength-plasticity matching of the wire rod, and has good industrial adaptability.

[0030] (2)In view of the current situation that the existing pearlite / bainite tool steel wire rods mainly adjust the tissue properties through the alloy design of hardenability elements combined with the heat treatment process after cold processing, resulting in increased energy consumption, cost and reduced efficiency in the tool production process, the C and Si contents of the wire rods of the present invention are relatively low, and trace amounts of Nb and V components are added, without precious metal alloy elements such as Ni and Mo. The material cost is relatively low. At the same time, the microstructure includes a mixed structure mainly composed of tempered bainite and a small amount of ferrite. By avoiding soft phase and martensite brittle structure, a quenched bainite structure with better strength and toughness can be formed, and the carbide dispersion strengthening effect of alloy elements such as Nb and V is utilized through the tempering of quenched bainite to obtain a strong and tough tissue state, which can achieve a tensile strength of 1175-1225 MPa and a reduction of area of 58%-63%. It is used in application fields such as 1250 MPa grade high-strength tool steel. After cold processing and forming, it can be directly processed into parts, realizing the reduction of production energy consumption, cost and efficiency improvement, and having good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, wherein: Figure 1 is the metallographic structure diagram of Embodiment 1 of the present invention; Figure 2 is the metallographic structure diagram of Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] The embodiments described below with reference to the accompanying drawings are exemplary, solely for illustrative purposes and do not limit the description of the features and characteristics of the present invention. They are intended to present the best mode of implementing the present invention, for the purpose of explaining the present invention and being sufficient to enable those skilled in the art to implement the present invention. It should not be construed as any limitation on the scope of the present invention, which is only defined by the appended claims. The detection of the structure and properties of the wire rods obtained from the following examples and comparative examples includes: the tensile test is carried out in accordance with "GB-T 228.1-2021 Metallic materials-Tensile testing-Part 1: Method of test at room temperature" to obtain the tensile strength and reduction of area; the structure detection is carried out in accordance with the metallic microstructure detection method of the GB / T13298 standard; the method for testing the difference in mechanical properties within the same coil: take 2 coils of wire rods at a distance of 5 m from the end of the coil. Taking the lap area position as the base point, each coil of wire rod is evenly divided into 8 segments on average, and 1 tensile specimen is taken from each segment. The strength difference after the tensile test of the taken tensile specimens is the difference in mechanical properties within the same coil. Example 1:

[0033] A preferred embodiment of the manufacturing method of the 1250 MPa grade high-strength tool steel wire rod of the present invention. The chemical composition and mass percentage of the wire rod include C: 0.47%, Si: 0.25%, Mn: 0.50%, Cr: 0.59%, Nb: 0.027%, V: 0.045%, P: 0.014%, S: 0.014%, and the rest are Fe and inevitable impurities. Its manufacturing method is carried out according to the technological process of rolling → wire laying → on-line molten salt quenching and strong isothermal treatment → slow cooling on the roller table → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, reducing the influence of segregation. The heating furnace is controlled according to the three-stage heating program of the preheating section, heating section, and soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 6 mm through the rolling line. Appropriate rolling temperature and reduction ratio are selected to increase the rolling speed, induce the precipitation of microalloys, promote dynamic recrystallization during the finishing rolling process, and refine the grains. Specifically: control the soaking temperature of the heating furnace to be 1160 °C, the residence time in the furnace to be 185 min, the initial rolling temperature to be 1040 °C, the finishing rolling temperature to be 910 °C, and the finishing rolling reduction ratio to be 28%; the wire laying process is used to make the wire rod coming out of the rolling line into a coil through a wire laying machine. The coil is scattered on the roller table and conveyed along the roller table, making the coil in the high-temperature austenite state to prepare for quenching the structure at a higher quenching temperature. Specifically: control the wire laying temperature to be 905 °C.

[0034] The online molten salt quenching and strong isothermal treatment process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 37 °C / s, quickly crosses the pearlite phase region from the high-temperature austenite state, enters the bainite phase region, inhibits the formation of pearlite, and forms a quenched structure mainly composed of quenched bainite. Then, the wire rod is transported through the second-stage salt bath tank by a roller table for the rear-stage molten salt treatment, controlling the wire rod to perform high-temperature strong isothermal tempering on the quenched structure in the high-temperature isothermal range, toughening and stress-relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature for the front-stage molten salt treatment is 341 °C, the treatment time is 19 s, the molten salt circulation rate is 550 t / h, and the molten salt temperature rise ≤ 8 °C; the molten salt temperature for the rear-stage molten salt treatment is 598 °C, the treatment time is 300 s, and the molten salt circulation rate is 450 t / h.

[0035] In the roller table slow cooling process, the heat preservation cover is closed, and the wire rod passing through the second-stage salt bath tank is slowly cooled by the conveying roller table through the heat preservation cover, preventing the wire rod from having too fast a cooling rate during the cooling process, resulting in an increase in stress, and promoting further toughening of the wire rod structure, improving the softening effect of the wire rod, until coiling. Specifically: controlling the wire rod to slowly cool to 259 °C at a cooling rate of 0.35 °C / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and after packaging and warehousing, the finished wire rod is obtained, and its metallographic structure diagram is as Figure 1 shown.

[0036] Comparative Example 1: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 1 lies in: its manufacturing method is carried out according to the technological process of rolling → wire laying → Stelmor air cooling line → coiling. Specifically: in the rolling process, the soaking temperature of the heating furnace is controlled at 1085 °C, the time in the furnace is 225 min, the initial rolling temperature is 955 °C, the final rolling temperature is 845 °C, the wire laying temperature is controlled at 820 °C. In the Stelmor air cooling line, the front 1-6# heat preservation covers are opened, and the fan is turned on to control the wire rod to cool at a speed of 3.6 °C / s to 672 °C. Then, the heat preservation cover is closed, the wire rod enters the heat preservation cover and cools at a speed of 2.5 °C / s to 260 °C, and is collected by a coiling drum to obtain the finished wire rod.

[0037] Comparative Example 2: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 1 lies in: controlling the soaking temperature of the heating furnace at 1085 °C, the time in the furnace at 225 min, the initial rolling temperature at 955 °C, the final rolling temperature at 845 °C, controlling the wire laying temperature at 820 °C, and the wire rod undergoes front-stage molten salt treatment and cools down at a cooling rate of 31 °C / s to obtain the finished wire rod. Example 2:

[0038] A preferred embodiment of the manufacturing method of the 1250 MPa grade high-strength tool steel wire rod. The chemical composition and mass percentage of the wire rod include C: 0.47%, Si: 0.30%, Mn: 0.65%, Cr: 0.65%, Nb: 0.029%, V: 0.049%, P: 0.014%, S: 0.015%, and the rest are Fe and inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt quenching and strong isothermal treatment → slow cooling on the roller table → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 220 mm × 220 mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, reducing the influence of segregation. The heating furnace is controlled according to the three-stage heating program of the preheating section, heating section, and soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 10 mm through the rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, induce the precipitation of microalloying, promote dynamic recrystallization during the finishing rolling process, and refine the grains. Specifically: control the soaking temperature of the heating furnace to be 1200 °C, the residence time in the furnace to be 140 min, the initial rolling temperature to be 1090 °C, the finishing rolling temperature to be 940 °C, and the finishing rolling reduction to be 23%; The wire laying process is used to make the wire rod coming out of the rolling line into a wire rod through a wire laying machine. The wire rod is scattered on the roller table and transported along the roller table, making the wire rod in the high-temperature austenite state to prepare for quenching the structure at a higher quenching temperature. Specifically: control the wire laying temperature to be 930 °C.

[0039] The on-line molten salt quenching and strong isothermal treatment process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is transported through the first-stage salt bath tank by the roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 40 °C / s, quickly crosses the pearlite phase region from the high-temperature austenite state, enters the bainite phase region, inhibits the formation of pearlite, and forms a quenched structure mainly composed of quenched bainite. Then the wire rod is transported through the second-stage salt bath tank by the roller table for the rear-stage molten salt treatment, controlling the wire rod to perform high-temperature strong isothermal tempering on the quenched structure in the high-temperature isothermal interval, toughening and stress relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 326 °C, the treatment time is 25 s, the molten salt circulation volume is 750 t / h, and the molten salt temperature rise ≤ 8 °C; the molten salt temperature of the rear-stage molten salt treatment is 580 °C, the treatment time is 550 s, and the molten salt circulation volume is 650 t / h.

[0040] In the roller table slow cooling process, the opening degree of the heat preservation cover is adjusted, and the wire rod passing through the second salt bath tank is slowly cooled by the conveying roller table through the heat preservation cover, preventing the wire rod from cooling too quickly during the cooling process, which may cause an increase in stress, and promoting further toughening of the wire rod structure, improving the softening effect of the wire rod until coiling. Specifically: control the wire rod to be slowly cooled at a cooling rate of 0.9 °C / s to 253 °C; the coiling process is used to coil the wire rod into a coil by a coiling drum, and after packaging and warehousing, the finished wire rod is obtained. Its metallographic structure diagram is as shown in Figure 2 shown.

[0041] Comparative Example 3: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 2 is that: the wire rod is subjected to front-section molten salt treatment and cooled at a cooling rate of 37 °C / s. The molten salt temperature of the front-section molten salt treatment is 365 °C, and the treatment time is 10 s to obtain the finished wire rod.

[0042] Comparative Example 4: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 2 is that: the wire rod is subjected to front-section molten salt treatment and cooled at a cooling rate of 41 °C / s. The molten salt temperature of the front-section molten salt treatment is 290 °C, and the treatment time is 35 s to obtain the finished wire rod. Example 3:

[0043] A preferred implementation manner of the manufacturing method of the 1250 MPa grade high-strength tool steel wire rod of the present invention. The chemical composition and mass percentage of the wire rod include C: 0.45%, Si: 0.35%, Mn: 0.62%, Cr: 0.60%, Nb: 0.031%, V: 0.055%, P: 0.015%, S: 0.012%, and the rest are Fe and unavoidable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt quenching and strong isothermal treatment → roller table slow cooling → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace, reducing the influence of segregation. The heating furnace is controlled according to the three-stage temperature rising program of the preheating section, heating section, and soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 9 mm through the rolling line. Select appropriate rolling temperature and reduction ratio, increase the rolling speed, induce the precipitation of microalloying elements, promote dynamic recrystallization during the finishing rolling process, and refine the grains. Specifically: control the soaking temperature of the heating furnace to be 1185 °C, the residence time in the furnace to be 165 min, the initial rolling temperature to be 1085 °C, the finishing rolling temperature to be 935 °C, and the finishing rolling reduction ratio to be 24%; the wire laying process is used to make the wire rod coming out of the rolling line into a wire rod through a wire laying machine. The wire rod is scattered on the roller table and conveyed along the roller table, so that the wire rod is in a high-temperature austenite state, preparing for quenching with a relatively high quenching temperature. Specifically: control the wire laying temperature to be 925 °C.

[0044] The online molten salt quenching and strong isothermal treatment process uses a two-stage salt bath tank with internal molten salt. The wire rod after wire laying is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 39 °C / s, quickly crosses the pearlite phase region from the high-temperature austenite state, enters the bainite phase region, inhibits the formation of pearlite, and forms a quenched structure mainly composed of quenched bainite. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the rear-stage molten salt treatment, controlling the wire rod to perform high-temperature strong isothermal tempering on the quenched structure in the high-temperature isothermal range, toughening and stress relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature for the front-stage molten salt treatment is 335 °C, the treatment time is 22 s, the molten salt circulation rate is 670 t / h, and the molten salt temperature rise ≤ 8 °C; the molten salt temperature for the rear-stage molten salt treatment is 586 °C, the treatment time is 465 s, and the molten salt circulation rate is 580 t / h.

[0045] The roller table slow cooling process adopts adjusting the opening degree of the heat preservation cover. The wire rod transported by the conveying roller table after passing through the second-stage salt bath tank is slowly cooled through the heat preservation cover, preventing the wire rod from having too fast a cooling rate during the cooling process, resulting in increased stress, and promoting further toughening of the wire rod structure, improving the softening effect of the wire rod until coiling. Specifically: controlling the wire rod to slowly cool to 254 °C at a cooling rate of 0.8 °C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and after packaging and warehousing, the finished wire rod is obtained.

[0046] Comparative Example 5: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 3 lies in that: the molten salt temperature for the rear-stage molten salt treatment is 615 °C, the treatment time is 600 s, and the finished wire rod is obtained.

[0047] Comparative Example 6: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 3 lies in that: the molten salt temperature for the rear-stage molten salt treatment is 520 °C, the treatment time is 250 s, and the finished wire rod is obtained. Example 4:

[0048] A preferred embodiment of the manufacturing method of the 1250 MPa grade high-strength tool steel wire rod of the present invention. The chemical composition and mass percentage of the wire rod include C: 0.50%, Si: 0.33%, Mn: 0.54%, Cr: 0.55%, Nb: 0.035%, V: 0.053%, P: 0.014%, S: 0.012%, and the rest are Fe and inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt quenching and strong isothermal treatment → roller table slow cooling → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 180mm×180mm into a high-temperature steel billet that reaches the rollable plasticity through a heating furnace, reducing the influence of segregation. The heating furnace is controlled according to a three-stage temperature-rising program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into wire rods with a diameter specification of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, induce the precipitation of microalloys, promote dynamic recrystallization during the finishing rolling process, and refine the grains. Specifically: control the soaking temperature of the heating furnace at 1170°C, the residence time in the furnace at 155min, the initial rolling temperature at 1065°C, the finishing rolling temperature at 925°C, and the finishing rolling reduction at 26%; The wire laying process is used to make the wire rods exiting the rolling line into coiled bars through a wire laying machine. The coiled bars are scattered on the roller table and transported along the roller table, keeping the coiled bars in a high-temperature austenite state to prepare for quenching the structure at a relatively high quenching temperature. Specifically: control the wire laying temperature at 915°C.

[0049] The on-line molten salt quenching and strong isothermal treatment process uses a two-stage salt bath tank with molten salt inside. The coiled bars after wire laying are transported through the first-stage salt bath tank by the roller table for the front-stage molten salt treatment, cooling the coiled bars at a cooling rate of 35°C / s, quickly crossing the pearlite phase region from the high-temperature austenite state and entering the bainite phase region, inhibiting the formation of pearlite, and forming a quenched structure mainly composed of quenched bainite. Then the coiled bars are transported through the second-stage salt bath tank by the roller table for the back-stage molten salt treatment, controlling the coiled bars to perform high-temperature strong isothermal tempering on the quenched structure in the high-temperature isothermal interval, toughening and stress-relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the coiled bars. Specifically: the molten salt temperature for the front-stage molten salt treatment is 355°C, the treatment time is 15s, the molten salt circulation volume is 630t / h, and the molten salt temperature rise ≤ 8°C; the molten salt temperature for the back-stage molten salt treatment is 592°C, the treatment time is 395s, and the molten salt circulation volume is 490t / h.

[0050] The roller table slow cooling process adjusts the opening degree of the heat preservation cover, and the coiled bars transported by the conveying roller table through the second-stage salt bath tank are slowly cooled through the heat preservation cover, preventing the coiled bars from having too fast a cooling rate during the cooling process, which may lead to an increase in stress, and promoting further toughening of the coiled bar structure, improving the softening effect of the coiled bars until coiling. Specifically: control the coiled bars to slowly cool at a cooling rate of 0.55°C / s to 256°C; The coiling process is used to coil the coiled bars into coils through a coiling drum, and the finished coiled bar products are obtained after packaging and warehousing.

[0051] Comparative Example 7: A manufacturing method of coiled bars, the difference between its manufacturing method and that of Example 4 lies in: its manufacturing method is carried out according to the technological process of rolling → wire laying → on-line molten salt quenching and strong isothermal → air cooling → coiling. Specifically: the air cooling process uses the coiled bars transported by the conveying roller table through the second-stage salt bath tank to be naturally cooled in the air, and the coiled bars are cooled at a cooling rate of 1.6°C / s to 257°C to obtain coiled bars.

[0052] The wire rods obtained from the above Examples 1-4 and Comparative Examples 1-7 were subjected to microstructure and property tests, and the comparison results obtained are shown in Table 1 below: Table 1. Comparison results of microstructure and properties of different wire rod compositions and manufacturing methods

[0053] From the comparison results between Example 1 and Comparative Example 1, it can be seen that C and Cr often weaken the strengthening effect due to the uncontrollable cooling rate during the cooling phase transformation, and abnormal structures such as martensite are generated, which reduces the content of C and Cr and the air-cooling intensity. Although it is beneficial to control the martensite structure, it will also affect the nucleation and lamellar refinement of the pearlite structure, resulting in large strength losses and mechanical property fluctuations. At the same time, the plasticity is still insufficient. However, through the Nb-V chemical composition design combined with the online molten salt quenching and strong isothermal technology in the present invention, the wire rod can be controlled to quickly enter the bainite low-temperature transformation zone from the high-temperature austenite state, avoiding the formation of pearlite soft phase, upper bainite or martensite brittle phase. Then, by controlling the wire rod to perform high-temperature isothermal tempering on the quenched structure in the high-temperature isothermal range and controlling the precipitation of carbides, a mixed structure mainly composed of tempered bainite and a small amount of ferrite is formed, improving the strength-plasticity matching of the wire rod, achieving a tensile strength of 1175-1225 MPa and a reduction of area of 58%-63%. After cold processing and forming, it can be directly processed into parts.

[0054] From the comparison results between Example 1 and Comparative Example 2, it can be seen that selecting a higher spinning temperature, i.e., the quenching temperature, can provide favorable conditions for forming a large supercooling degree after the front-section molten salt treatment and promoting the short-time quenching of the wire rod to form quenched bainite structure. At the same time, it can reduce the limitation on the rolling temperature, reduce the rolling mill load, and improve the rolling speed and efficiency.

[0055] From the comparison results between Example 2 and Comparative Example 3, it can be seen that when the front-section molten salt is in the bainite phase region, the higher the molten salt temperature and the shorter the treatment time, the more beneficial it is to reduce the dislocation density and stress of the structure and the difficulty of isothermal tempering. However, if the molten salt temperature is too high, it is not conducive to the transformation of austenite structure into needle-shaped quenched bainite and the inhibition of the formation of upper bainite. At the same time, if the treatment time is too short, it will affect the transformation of austenite structure into quenched bainite, increase the austenite residue, and cause pearlite structure in the subsequent molten salt treatment, affecting the structure regulation and matrix strength.

[0056] From the comparison results between Example 2 and Comparative Example 4, it can be seen that the lower the molten salt temperature and the longer the treatment time of the front-section molten salt, the more beneficial it is to inhibit the formation of pearlite, promote the transformation of high-temperature austenite structure into needle-shaped quenched bainite, and form a quenched structure mainly composed of quenched bainite, improving the matrix strength. However, if the molten salt temperature is too low, there is a risk of forming abnormal martensite structure. At the same time, as the treatment time prolongs, the residual stress increases, which will increase the difficulty of isothermal tempering and affect the carbide regulation.

[0057] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the molten salt temperature in the latter stage of the molten salt is in the high-temperature range. The higher the molten salt temperature and the longer the treatment time, the more beneficial it is to provide more thermal power, eliminate quenching stress through tempering, reduce lattice distortion, and transform the matrix from the high brittleness of the quenched state to the high toughness of the tempered state. At the same time, by providing atomic diffusion power at high temperature, the rate of combination of Nb and C to form carbides is accelerated, the precipitation amount increases, and the number of strengthening phases increases. However, if the molten salt temperature is too high and the treatment time is too long, over-tempering softening will occur, the Nb / V carbides will coarsen excessively, the precipitation strengthening effect will weaken, and even the strength and plasticity will decrease due to the too large size of the carbides, and the production energy consumption will also increase.

[0058] From the comparison results of Example 3 and Comparative Example 6, it can be seen that the lower the molten salt temperature in the latter stage of the molten salt, the more beneficial it is to promote the fine and dispersed precipitation of vanadium-containing carbides and reduce the softening rate. With the shortening of the treatment time, it is beneficial to reduce the production energy consumption. However, if the molten salt temperature is too low and the treatment time is too short, insufficient tempering of the quenched structure will lead to higher residual stress and tissue brittleness, and the precipitation strengthening effect will be lost due to insufficient precipitation amount of Nb / V carbides.

[0059] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow cooling on the roller table can prevent the stress from increasing due to too fast cooling rate during the cooling process of the wire rod, and promote the further toughening of the wire rod structure and improve the softening effect of the wire rod.

[0060] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A manufacturing method of a 1250MPa grade high-strength tool steel wire rod, characterized in that, Its manufacturing method includes: After subjecting the steel billet to rolling and then spinning it into wire rods at a spinning temperature of ≥905°C, it undergoes online molten salt quenching and strong isothermal treatment. The wire rods first pass through the front-section molten salt, and the cooling rate of the wire rods is controlled to be ≥35°C / s to enter the bainite phase region from the high-temperature austenite state, forming a quenched structure mainly composed of quenched bainite. Then, the molten salt in the rear section is heated up, and the quenched structure is subjected to isothermal tempering and toughening stress relief treatment to control the precipitation of carbides. Finally, it is slowly cooled on the roller table to produce wire rods with a microstructure including tempered bainite and ferrite. The chemical composition and mass percentage of the wire rods are as follows: C: 0.45% - 0.50%, Si: 0.25% - 0.35%, Mn: 0.50% - 0.65%, Cr: 0.55% - 0.65%, Nb: 0.027% - 0.035%, V: 0.045% - 0.055%, P ≤ 0.015%, S ≤ 0.015%, and the rest is Fe and unavoidable impurities.

2. The manufacturing method of the 1250 MPa grade high-strength tool steel wire rod according to claim 1, characterized in that, Before the rolling, the soaking temperature of the heating furnace is controlled to be 1160 - 1200°C, and the residence time in the furnace is 140 - 185 min.

3. The manufacturing method of the 1250 MPa grade high-strength tool steel wire rod according to claim 1, characterized in that, During the rolling, the initial rolling temperature is controlled to be 1040 - 1090°C, the final rolling temperature is 910 - 940°C, and the reduction in the final rolling is 23% - 28%.

4. The manufacturing method of the 1250MPa grade high-strength tool steel wire rod according to claim 1, characterized in that, During the spinning, the spinning temperature is controlled to be 905 - 930°C.

5. The manufacturing method of the 1250MPa grade high-strength tool steel wire rod according to claim 1, characterized in that, The molten salt temperature of the front-section molten salt is 325 - 355°C, and the treatment time is 15 - 25 s. The molten salt temperature of the rear-section molten salt is 580 - 600°C, and the treatment time is 300 - 550 s.

6. The manufacturing method of the 1250MPa grade high-strength tool steel wire rod according to claim 5, characterized in that, The molten salt circulation rate of the front-section molten salt is 550 - 750 t / h, and the temperature rise of the molten salt is ≤8°C. The molten salt circulation rate of the rear-section molten salt is 450 - 650 t / h.

7. The manufacturing method of the 1250 MPa grade high-strength tool steel wire rod according to claim 5, characterized in that, The slow cooling on the roller table controls the wire rods to be slowly cooled at a cooling rate of 0.35 - 0.95°C / s to below 260°C.

8. A 1250 MPa grade high-strength tool steel wire rod, characterized in that, The wire rods are manufactured by the manufacturing method of the 1250 MPa grade high-strength tool steel wire rods described in any one of claims 1 - 7.

9. The wire rod of 1250 MPa grade high-strength tool steel according to claim 8, characterized in that, The volume percentage of the tempered bainite is ≥97%.

10. The wire rod of 1250MPa grade high-strength tool steel according to claim 8, characterized in that, The diameter of the wire rods is 6 - 10 mm, the tensile strength is 1175 - 1225 MPa, the reduction of area is 58% - 63%, and the difference in mechanical properties within the same coil is ≤37 MPa.

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