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

Abstract

The present invention relates to a 1250MPa grade high-strength tool steel wire rod and a manufacturing method thereof. After being rolled and spun into a wire rod using a medium-carbon alloy composition containing trace amounts of Nb and V, the wire rod is subjected to an online molten salt quenching and strong isothermal treatment. The wire rod first passes through a front-end molten salt, and the wire rod is controlled to enter a bainite phase region from a high-temperature austenite state at a cooling rate of ≥35°C / s to form a quenched structure mainly composed of quenched bainite. The wire rod is then heated through a rear-end molten salt, and the quenched structure is isothermally tempered and toughened to relieve stress, thereby controlling carbide precipitation. Finally, the wire rod is slowly cooled through a roller to form a wire rod having a microstructure including tempered bainite and ferrite. The strength-plasticity matching of the wire rod can be improved by adjusting the microstructure state, achieving a tensile strength of 1175-1225MPa and a cross-sectional reduction rate of 58%-63%. This is conducive to being directly processed into parts after cold working, thereby promoting energy consumption and cost reduction and efficiency improvement in tool steel production.

Description

Technical Field

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

[0002] Tools are fundamental instruments used in production. Their production process primarily involves pickling and cold-working hot-rolled wire rod, followed by a suitable heat treatment process to achieve appropriate mechanical properties. Therefore, improving the parent material properties of hot-rolled wire rod is crucial for reducing the difficulty of tool steel processing and maintaining the strength, toughness, rigidity, and durability of tools. Conventional hot-rolled wire rods have a microstructure composed of pearlite and ferrite, offering high cold-working performance. However, the development of 1250MPa-grade high-strength tool steels is limited by the post-rolling Stelmor air-cooling capacity. Optimizing performance requires alloying with hardenability elements and then cold-working followed by a suitable heat treatment process to adjust the microstructure. However, subsequent heat treatment increases energy consumption, costs, and efficiency during tool production. Therefore, it is necessary to develop a 1250MPa-grade high-strength tool steel wire rod and its manufacturing method, enabling direct processing into parts after cold working, thereby reducing energy consumption, costs, and improving efficiency in tool steel production.

[0003] To reduce the risk of abnormal low-temperature structures such as hardenability martensite caused by segregation of hardenability elements such as Mn and Cr and unstable temperature control in the air cooling line, while also taking into account the strength performance of the wire rod, the existing alloy tool steel wire rod is controlled to have bainite or contain bainite structure. However, it still has the following defects:

[0004] (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 structure is relatively high. For example, patent CN114752858B discloses an alloy tool steel wire rod without martensite structure and its preparation method and tool steel, which adopts C-Si-Mn-Cr-Mo-V-Ni composition design, combined with the air cooling line after low-temperature spinning and first fast and then slow cooling, to obtain pearlite, ferrite and a small amount of bainite structure. However, on the one hand, due to the limited maximum cooling capacity of the Stelmor air cooling line, the wire rod stays in the pearlite transformation zone for a long time, which promotes the lamellar precipitation of ferrite. To form pearlite, the soft phases of pearlite and ferrite account for a large proportion, and the strength of the mixed structure is relatively low, which requires the addition of higher contents of C, Si or precious alloys such as Mo and V in the composition, resulting in higher material costs; on the other hand, higher contents of C, Si, etc. improve the stability of austenite and delay the start of pearlite transformation, but at the same time reduce the bainite transformation temperature, so that the bainite transformation needs to be carried out at a lower temperature, and the kinetic conditions are more stringent. As the wire rod is continuously cooled and phase transformed, it is in a low-temperature state, and large structural stress will remain, resulting in limited improvement in the plasticity and toughness of the wire rod, making it difficult to eliminate heat treatment after cold forming and directly process it into parts.

[0005] (2) In order to avoid brittle fracture of martensitic structure and improve the use and fatigue performance of tool steel, large deformation rolling and strong air cooling treatment are used to obtain bainitic structure. For example, patent CN111690801B discloses a production process for alloy tool steel wire rods to obtain full bainitic structure, which adopts C-Si-Mn-Cr-Ni-Al-Mo-V-Nb composition design, combines large deformation rolling, strong air cooling after low-temperature wire drawing and heat preservation cover cooling to obtain full bainitic structure. However, on the one hand, low-temperature wire drawing requires a large load on the rolling line and will aggravate the wear of the rolling line. The material cost of the composition system of medium-high carbon and Mo-containing hardenability elements is high, which will delay the bainite transformation dynamics, and The maximum cooling capacity of the Stelmor air-cooling line is limited. Even with strong air cooling treatment, the temperature can only be reduced to the upper bainite phase region. The bainitic ferrite laths are relatively coarse and unevenly distributed, resulting in poor strength, toughness and wear resistance. On the other hand, as the temperature difference and uncontrollability between the windward and windward surfaces increase during strong air cooling, the risk of overcooling and forming martensite on the wire rod surface will increase, and it will also be difficult to control the dispersion strengthening of microalloy carbides. The minimum cooling capacity of the insulation cover phase transformation cooling control is limited. After phase transformation incubation, the wire rod will be in a low temperature state, resulting in higher bainite distortion and larger residual tissue stress. In particular, small-sized wire rods have a fast temperature drop rate and concentrated deformation resistance, which increases the risk of cracking in subsequent cold working treatments. Summary of the Invention

[0006] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 1250MPa grade high-strength tool steel wire rod and a manufacturing method thereof, which can improve the strength-plasticity matching of the wire rod by adjusting the organizational state, and is conducive to being directly processed into parts after cold forming, so as to promote energy consumption, cost reduction and efficiency improvement in tool steel production.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] A method for manufacturing a 1250MPa grade high-strength tool steel wire rod, the manufacturing method comprising:

[0009] The steel billet is rolled and spun into wire rod at a spinning temperature of ≥905℃, and then undergoes online molten salt quenching and strong isothermal treatment. The wire rod first passes through the front section of molten salt, and the wire rod is controlled to cool from the high temperature austenite state to the bainite phase region at a cooling rate of ≥35℃ / s to form a quenched structure mainly composed of quenched bainite. Then, the molten salt is heated in the back section, and the quenched structure is isothermally tempered, toughened and stress-relieved to control the precipitation of carbides. Finally, it is slowly cooled by rollers to form a microstructure including tempering. The invention relates to a wire rod of fire bainite and ferrite, wherein the chemical composition and mass percentage of the wire rod include: 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 are Fe and unavoidable impurities.

[0010] The chemical composition and mass percentage of the above-mentioned wire rod are designed based on the following:

[0011] (1) Carbon: The C element improves the strength of the matrix through solid solution strengthening and the formation of carbides. It is relatively cheaper. 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-time 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%.

[0012] (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 mainly composed of 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, aggravated lattice distortion, reduced toughness of steel, affected plasticity control, and caused 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 during online molten salt quenching, the mass percentage of Si is controlled to be 0.25%~0.35%.

[0013] (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 zone, and promote the shear formation of quenched bainite during the previous molten salt treatment, so that the bainite phase transformation can be triggered at a higher temperature, the phase transformation time is shortened, and the strength and hardness of the wire rod are improved. However, when the Mn content is too high, the austenite grains will be easily coarsened during heating, and higher energy will be required for stress release, which will delay the tempering softening process and reduce the toughness and plasticity of the steel. Therefore, in order to enable short-time quenching to quickly form quenched bainite and facilitate tempering softening control, the mass percentage of Mn is controlled to 0.50%~0.65%.

[0014] (4) Chromium: Cr, as a carbide-forming element, can form high-hardness alloy carbides, significantly improving the wear resistance of tools, while improving the hardenability of steel, reducing the diffusion coefficient of carbon in austenite, and expanding the bainite transformation range. Combined with Mn, it can further inhibit the precipitation of pearlite, making the quenched bainite the main transformation product during the front-end treatment, which is beneficial to the refinement of the bainitic ferrite lamellar layer and improving the matrix strength. However, too high Cr content will aggravate the composition segregation, increase the difficulty of controlling the uniformity of the structure and the difficulty of improving the plasticity, and reduce the cold workability. Therefore, in order to take into account the wear resistance requirements of tool steel, the Cr content should be appropriately increased so that short-time quenching can quickly form quenched bainite and avoid excessive loss of strength properties during tempering. The mass percentage of Cr is controlled to be 0.55%~0.65%.

[0015] (5) Niobium: Nb can pin grain boundaries, inhibit the growth of austenite grains, obtain fine-grained structure, shorten the incubation period of bainite transformation, promote the uniform formation of quenched bainite, and disperse and precipitate in the high-temperature isothermal tempering temperature range to improve the strength and toughness of the wire rod. However, the cost of Nb is relatively high. Under high-temperature treatment, there is a risk of precipitation, aggregation and growth to form coarse particles, which reduces the toughness. Therefore, based on the role of Nb element, cost and manufacturing control considerations, the mass percentage of Nb is controlled to be 0.027%~0.035%.

[0016] (6) Vanadium: As an alloying element, V pins grain boundaries to inhibit the growth of austenite grains, refines the original structure, and promotes the formation of high-density dislocations and twins in bainitic ferrite. During the isothermal process, it can form nano-scale VC carbides that are evenly distributed in the matrix, which can improve the wear resistance of tool steel and compensate for the strength loss caused by tempering and softening of quenched bainite. However, the cost of V is relatively high, and excessive addition is not conducive to controlling the cost of wire rods. Vanadium-containing carbides aggregate to form coarse carbides, which will split the matrix and cause the impact resistance of the tool to deteriorate. Therefore, based on the role of V, cost, and manufacturing control, the mass percentage of V is controlled to be 0.045%~0.055%.

[0017] (7) Phosphorus and sulfur: P and S are impurity elements. The lower the better. Therefore, P is controlled to be ≤ 0.015% and S is controlled to be ≤ 0.015%.

[0018] The above-mentioned wire rod is designed with a medium-carbon alloy composition containing trace amounts of Nb and V. By optimizing the Si, Mn, and Cr contents, the austenite region is expanded, providing favorable conditions for rapid formation of quenched bainite by short-term quenching at a higher temperature during the previous molten salt treatment, avoiding excessive loss of strength during tempering, and giving full play to the dispersion strengthening effect of microalloys. On this basis, a higher wire drawing temperature, i.e., the quenching temperature, is selected to form a large degree of undercooling after the previous molten salt treatment, thereby preparing the structure for promoting short-term quenching of the wire rod to form quenched bainite. After drawing, the wire rod is directly subjected to online molten salt quenching and strong isothermal treatment without air cooling:

[0019] 1. Compared with the Stelmor air cooling line, which has limited maximum cooling capacity and unstable temperature control, it is difficult to suppress the soft phase structure and abnormal brittle structure, forming an upper bainite structure with insufficient strength and toughness. On the one hand, after the wire rod is treated with molten salt in the front section, the strong heat exchange capacity of the molten salt can be used to promote the rapid cooling of the wire rod, so that the wire rod can quickly pass through the pearlite transformation zone from the high-temperature austenite state to the bainite phase zone, avoiding the formation of pearlite structure. At the same time, due to the increase in supercooling, the molten salt temperature is low, which can inhibit the long-range diffusion of carbon in the austenite, accelerate the nucleation rate of the quenched bainite, and form a needle structure. The steel wire rod has a quenched bainite structure rather than a feather-like upper bainite structure, with small carbide size and high dislocation density, which improves the matrix strength and compensates for the strength loss caused by reducing the carbon, silicon and microalloy content. On the other hand, the thermal conductivity of molten salt is higher than that of air. When the wire rod passes through the front section of molten salt, the molten salt covers the surface of the wire rod for uniform heat exchange, and there is no temperature difference problem between the wind-receiving and wind-receiving sides of the wire rod in the air-cooled 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, avoid local overcooling or austenite residue and form martensite structure in the subsequent cooling process.

[0020] 2. Compared with the Stelmore air-cooled line, the minimum cooling capacity of the insulation cover cooling is limited and continuous cooling, and it is difficult to control the organizational state and carbide precipitation. On the one hand, the wire rod can be heated from the lower temperature bainite phase region to the high temperature isothermal temperature range after the subsequent molten salt treatment, and the wire rod can be transformed to the same temperature as the molten salt for isothermal treatment, which can extend the time the wire rod is in the high temperature phase region, promote the quenching organization to reduce 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 the wire rod is in the Nb and V carbide dispersion precipitation temperature range, avoid diffusion difficulties caused by too low temperature or too short treatment time, ensure the kinetic conditions for the nucleation of the precipitation phase, and compensate for the strength loss of isothermal tempering through the large-scale dispersion and precipitation of microalloy carbides, providing strengthening and toughening effects. After that, the wire rod is subjected to roller slow cooling treatment, which can continue the softening effect of the subsequent molten salt, promote further toughening of the wire rod organization, realize organizational state control, and improve the matching of the strength and plasticity of the wire rod.

[0021] Before the rolling, a higher soaking temperature of the heating furnace and an appropriate time in the furnace are selected to promote the homogenization of the alloy composition, reduce the influence of segregation and avoid overburning. In the preferred technical solution, before the rolling, the soaking temperature of the heating furnace is controlled to be 1160~1200℃ and the time in the furnace is 140~185min.

[0022] Due to the high spinning temperature, the restriction on rolling temperature can be reduced. A higher initial rolling temperature is selected to improve the plasticity of the steel billet, reduce the rolling mill load and avoid cracking, and improve the rolling speed and efficiency. Nb and V microalloying elements can precipitate and pin the grain boundaries in the initial rolling stage. Appropriate final rolling temperature and final rolling reduction are selected to promote dynamic recrystallization and grain refinement during the final rolling process. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1040~1090℃, the final rolling temperature is controlled to be 910~940℃, and the final rolling reduction is controlled to be 23%~28%.

[0023] During the spinning, the spinning temperature can be further controlled to avoid the risk of coarsening austenite grains due to excessively high temperature. In a preferred technical solution, during the spinning, the spinning temperature is controlled to be 905-930°C.

[0024] The front-stage molten salt is in the bainite phase region. The lower the molten salt temperature and the longer the treatment time, the more favorable it is to increase the cooling rate of the wire rod, inhibit the formation of pearlite, promote the transformation of high-temperature austenite structure to needle-shaped quenched bainite, form a quenched structure dominated by quenched bainite, and improve the strength of the matrix. 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 is prolonged, the residual stress increases, which will increase the difficulty of isothermal tempering and affect the control of carbides. On the contrary, the higher the molten salt temperature and the shorter the treatment time, the more favorable it is to reduce the dislocation density and stress of the structure and reduce the difficulty of isothermal tempering. However, if the molten salt temperature is too low, the risk of forming abnormal martensite structure will increase. At the same time, as the treatment time is prolonged, the residual stress increases, which will increase the difficulty of isothermal tempering and affect the control of carbides. On the contrary, the higher the molten salt temperature and the shorter the treatment time, the more favorable it is to reduce the dislocation density and stress of the structure and reduce the difficulty of isothermal tempering. If the temperature is too high, it will be detrimental to the transformation of the 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 the austenite structure into quenched bainite, increase the austenite residue, and affect the tissue control and matrix strength. Therefore, the molten salt temperature and treatment time of the front-stage molten salt can be controlled to control the wire rod to quickly enter the bainite phase region from the high-temperature austenite state, forming a structure mainly composed of quenched bainite, and making organizational preparations for the rear-stage molten salt treatment. In the preferred technical solution, the molten salt temperature of the front-stage molten salt is 325~355℃, and the treatment time is 15~25s.

[0025] Since the temperature difference between the spinning temperature and the bainite phase region is large, selecting a higher molten salt circulation rate can reduce the molten salt temperature rise and improve the uniformity of the wire rod structure. In the preferred technical solution, the molten salt circulation rate of the front-stage molten salt is 550~750t / h, and the molten salt temperature rise is ≤8°C.

[0026] The molten salt temperature of the latter section is in the high temperature range. The higher the molten salt temperature and the longer the treatment time, the more thermal power is provided, and the quenching stress is eliminated by tempering, the lattice distortion is reduced, and the matrix is ​​transformed from the high brittleness of the quenched state to the strong toughness of the tempered state. At the same time, the atomic diffusion power is provided by the high temperature, so that the rate of Nb and C combining to form carbides is accelerated, the precipitation amount is increased, and the number of strengthening phases is increased. However, if the molten salt temperature is too high and the treatment time is too long, excessive tempering softening will occur, the Nb / V carbides will be excessively coarsened, the precipitation strengthening effect will be weakened, and even the strength and plasticity will be reduced due to the excessive size of the carbides, and the production energy consumption will increase. On the contrary, the lower the molten salt temperature, the more conducive it is to promote the fine dispersion precipitation of vanadium-containing carbides and reduce the softening rate. As the treatment time is shortened, it is conducive to reducing 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 structural brittleness, and Nb / V Insufficient carbide precipitation results in loss of precipitation strengthening effect. Therefore, appropriate molten salt temperature and treatment time can be selected to control the wire rod to undergo high-temperature isothermal tempering of the quenched structure in the high-temperature isothermal range, toughening and stress relief treatment, control 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℃, and the treatment time is 300~550s.

[0027] Since there is a certain temperature difference between the molten salt in the front section and the molten salt in the back section, selecting an appropriate molten salt circulation rate can reduce the temperature rise of the molten salt and improve the uniformity of the wire rod structure. In the preferred technical solution, the molten salt circulation rate of the back section molten salt is 450~650t / h.

[0028] In the preferred technical solution, the roller slow cooling controls the wire rod to slowly cool to below 260°C at a cooling rate of 0.35~0.95°C / s, which can prevent the wire rod from cooling too fast during the cooling process, resulting in increased stress, and promote further toughening of the wire rod structure, thereby improving the softening effect of the wire rod.

[0029] In the preferred technical solution, the roller slow cooling controls the heat preservation cover to be closed or controls the opening of the heat preservation cover, and the wire rod is conveyed by the conveying roller through the heat preservation cover to control the cooling of the wire rod, which can promote the wire rod to be quickly taken off the line.

[0030] A 1250MPa grade high-strength tool steel wire rod is manufactured by any of the above-mentioned methods for manufacturing 1250MPa grade high-strength tool steel wire rods.

[0031] The above-mentioned wire rod adopts the C-Si-Mn-Cr-Nb-V composition design, with relatively low C and Si content, trace addition of Nb and V components, and does not contain precious metal alloying 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. Compared with the existing tool air-cooled pearlite wire rod, it can avoid the risk of martensite brittle structure caused by high hardenability component content, and utilizes the quenching of bainite in the low temperature transformation zone to obtain a needle-shaped quenched bainite matrix, which can significantly improve the matrix strength through the pearlite soft phase. , it retains strength characteristics after tempering, and combined with the precipitation fine grain strengthening of Nb and V, it can make up for the strength loss caused by reducing the content of C and alloy components. It is more wear-resistant than pearlite and suitable for wire rod cold processing and tool service scenarios; compared with the existing tool air-cooled bainite wire rod, it can avoid the formation of feather-shaped upper bainite in the bainite medium temperature transformation zone and the loss of toughness, and can form a quenched bainite structure with better toughness. At the same time, by tempering the quenched bainite structure, the ferrite maintains a needle-shaped shape, but the internal dislocation density is reduced, and the carbide distribution is more uniform. Combined with the carbide dispersion strengthening effect of alloying elements such as Nb and V, the strength decreases slowly, and a strong and tough organizational state is obtained, thereby meeting the needs of the tool for good cold processing performance, impact resistance, and no need for heat treatment to adjust the organizational properties after cold processing.

[0032] The higher the proportion of tempered bainite in the wire rod, the higher the strength and wear resistance of the wire rod can be, and a certain toughness and plasticity can be maintained, making the material less prone to brittle fracture. In the preferred technical solution, the volume percentage of the tempered bainite is ≥97%.

[0033] In the preferred technical solution, the diameter of the wire rod is 6~10mm, the tensile strength is 1175~1225MPa, the cross-sectional shrinkage rate is 58%~63%, and the mechanical property difference is ≤37MPa. Small and medium-sized wire rods can be cold-formed to manufacture tool blanks, wrenches and other tool products. The wire rod reaches a high strength level and plasticity index, has good longitudinal performance uniformity, can withstand large plastic deformation before breaking, is not easy to crack during direct cold heading, cold drawing and other cold processing, and is suitable for the forming of complex-shaped parts. At the same time, it is not easy to undergo brittle fracture under dynamic loads, which meets the tool fatigue resistance requirements, so it can be used as a tool product after cold forming.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) The existing pearlite / bainite tool steel wire rod is limited by the cooling capacity of the Stelmor air cooling line, making it difficult to control the microstructure and performance. The manufacturing method of the present invention combines the Nb-V chemical composition design with the online molten salt quenching strong isothermal technology, performs online molten salt quenching strong isothermal treatment at a higher quenching temperature, controls the wire rod to quickly enter the bainite phase region from the high-temperature austenite state, and forms 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, toughening and stress relief treatment, which can control the precipitation of carbides. Finally, the wire rod structure is further toughened by roller slow cooling, which can effectively adjust the microstructure state and improve the strength-plasticity matching of the wire rod, and has good industrial adaptability.

[0036] (2) In view of the current situation that the existing pearlite / bainite tool steel wire rod is mainly designed by alloying hardenability elements and adjusting the microstructure and properties in combination with the heat treatment process after cold working, which leads to increased energy consumption, cost and reduced efficiency in the tool production process, the wire rod of the present invention has relatively low C and Si contents, trace amounts of Nb and V components, and does not contain precious metal alloying 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 the soft phase and martensite brittle structure, a quenched bainite structure with better toughness is formed, and the carbide dispersion strengthening effect of alloying elements such as Nb and V is utilized during the tempering of the quenched bainite to obtain a strong and tough microstructure. The tensile strength can reach 1175~1225MPa and the cross-sectional shrinkage rate can reach 58%~63%. It is used in application fields such as 1250MPa grade high-strength tool steel. After cold working, it can be directly processed into parts, achieving reduced production energy consumption, cost and improved efficiency, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0038] Figure 1 is a metallographic structure diagram of Example 1 of the present invention;

[0039] Figure 2 This is the metallographic structure diagram of Example 2 of the present invention. DETAILED DESCRIPTION

[0040] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention. They are intended to propose the best way to implement the present invention, are intended to explain the present invention, and are sufficient to enable those skilled in the art to practice the present invention, but should not be understood as limiting the scope of the present invention, which is defined solely by the appended claims. The wire rods obtained in the following embodiments and comparative examples are subjected to microstructure and performance testing, including: tensile testing using "GB-T 228.1-2021 Metallic Materials Tensile Testing Part 1: Room Temperature Test Method" to obtain tensile strength and cross-sectional reduction rate; microstructure testing is performed in accordance with the metal microstructure testing method of GB / T13298 standard; mechanical property same-turn difference test method: 2 turns of wire rod are taken 5m away from the end of the coil, and each turn of wire rod is divided into 8 equal sections with the overlap area as the base point. 1 tensile specimen is taken from each section, and the extreme difference in strength of the tensile specimens after the tensile test is the mechanical property same-turn difference. Example 1:

[0041] A preferred embodiment of the method for manufacturing 1250MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: 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 remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt quenching and strong isothermal treatment → roller slow cooling → coiling, specifically:

[0042] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, reduce the influence of segregation, and control the heating furnace according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 6mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, induce microalloy precipitation, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1160°C, the furnace time is 185min, the initial rolling temperature is 1040°C, the final rolling temperature is 910°C, and the final rolling reduction is 28%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for structural quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 905°C.

[0043] The online molten salt quenching strong isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 37°C / s, and quickly passes through the pearlite phase region from the high-temperature austenite state and enters the bainite phase region, inhibiting the formation of pearlite, and forming a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, and the wire rod is controlled to perform high-temperature strong isothermal tempering of the quenching structure in the high-temperature isothermal range, toughening and stress relief treatment, control carbide precipitation, and improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 341°C, the treatment time is 19s, the molten salt circulation volume is 550t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 598°C, the treatment time is 300s, and the molten salt circulation volume is 450t / h.

[0044] The roller slow cooling process adopts the method of closing the heat preservation cover, and the wire rod conveyed by the conveyor roller through the second salt bath tank is slowly cooled through the heat preservation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to an increase in stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled 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 through a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0045] Comparative Example 1:

[0046] A method for manufacturing wire rod, which differs from that of Example 1 in that: the manufacturing method is manufactured according to the process flow of rolling → spinning → Stelmor air cooling line → coiling, specifically: in the rolling process, the heating furnace soaking temperature is controlled to 1085°C, the furnace time is 225min, the initial rolling temperature is 955°C, the final rolling temperature is 845°C, and the spinning temperature is controlled to 820°C. The Stelmor air cooling line adopts the front 1~6# insulation covers to be opened, and the fan is turned on to control the wire rod to be cooled to 672°C at a rate of 3.6°C / s. Thereafter, the insulation cover is closed, the wire rod enters the insulation cover and is cooled to 260°C at a rate of 2.5°C / s, and is collected by the coiling drum to obtain a finished wire rod.

[0047] Comparative Example 2:

[0048] A method for manufacturing a wire rod, which differs from Example 1 in that: the soaking temperature of the heating furnace is controlled to 1085°C, the time in the furnace is 225 minutes, the initial rolling temperature is 955°C, the final rolling temperature is 845°C, the spinning temperature is controlled to be 820°C, the wire rod is treated with molten salt in the front section and cooled at a cooling rate of 31°C / s to obtain a finished wire rod. Example 2:

[0049] A preferred embodiment of the method for manufacturing 1250MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: 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 remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt quenching and strong isothermal treatment → roller slow cooling → coiling, specifically:

[0050] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, reduce the influence of segregation, and control the heating furnace according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 10mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, induce microalloy precipitation, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1200°C, the furnace time is 140min, the initial rolling temperature is 1090°C, the final rolling temperature is 940°C, and the final rolling reduction is 23%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for tissue quenching at a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 930°C.

[0051] The online molten salt quenching strong isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 40°C / s, and quickly passes through the pearlite phase region from the high-temperature austenite state and enters the bainite phase region, inhibiting the formation of pearlite, and forming a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, and the wire rod is controlled to perform high-temperature strong isothermal tempering of the quenching structure in the high-temperature isothermal range, toughening and stress relief treatment, control carbide precipitation, and improve the strength and 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 25s, the molten salt circulation volume is 750t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 580°C, the treatment time is 550s, and the molten salt circulation volume is 650t / h.

[0052] The roller slow cooling process adopts the method of adjusting the opening of the insulation cover, and the wire rod conveyed by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to an increase in stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 253°C at a cooling rate of 0.9°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0053] Comparative Example 3:

[0054] A method for manufacturing wire rod, which differs from Example 2 in that: the wire rod is subjected to a front-stage molten salt treatment and cooled at a cooling rate of 37°C / s, the molten salt temperature of the front-stage molten salt treatment is 365°C, and the treatment time is 10s to obtain a finished wire rod.

[0055] Comparative Example 4:

[0056] A method for manufacturing wire rod, which differs from Example 2 in that: the wire rod is subjected to a front-stage molten salt treatment and cooled at a cooling rate of 41°C / s, the molten salt temperature of the front-stage molten salt treatment is 290°C, and the treatment time is 35s to obtain a finished wire rod. Example 3:

[0057] A preferred embodiment of the method for manufacturing 1250MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: 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 remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt quenching and strong isothermal treatment → roller slow cooling → coiling, specifically:

[0058] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, reduce the influence of segregation, and control the heating furnace according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 9mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, induce microalloy precipitation, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1185°C, the furnace time is 165min, the initial rolling temperature is 1085°C, the final rolling temperature is 935°C, and the final rolling reduction is 24%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for structural quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 925°C.

[0059] The online molten salt quenching strong isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 39°C / s, and quickly passes through the pearlite phase region from the high-temperature austenite state and enters the bainite phase region, inhibiting the formation of pearlite, and forming a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, and the wire rod is controlled to perform high-temperature strong isothermal tempering of the quenching structure in the high-temperature isothermal range, toughening and stress relief treatment, control carbide precipitation, and improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 335°C, the treatment time is 22s, the molten salt circulation volume is 670t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 586°C, the treatment time is 465s, and the molten salt circulation volume is 580t / h.

[0060] The roller slow cooling process adopts adjusting the opening of the insulation cover, and the wire rod transported by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, resulting in increased stress, and promote further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until coiling. Specifically: the wire rod is controlled 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 the finished wire rod is obtained after packaging and storage.

[0061] Comparative Example 5:

[0062] A method for manufacturing a wire rod, which differs from Example 3 in that the molten salt temperature of the latter molten salt treatment is 615° C., the treatment time is 600 s, and the finished wire rod is obtained.

[0063] Comparative Example 6:

[0064] A method for manufacturing a wire rod, which differs from Example 3 in that the molten salt temperature of the latter molten salt treatment is 520° C., the treatment time is 250 s, and the finished wire rod is obtained. Example 4:

[0065] A preferred embodiment of the method for manufacturing 1250MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: 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 remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt quenching and strong isothermal treatment → roller slow cooling → coiling, specifically:

[0066] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, reduce the influence of segregation, and control the heating furnace according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, induce microalloy precipitation, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1170°C, the furnace time is 155min, the initial rolling temperature is 1065°C, the final rolling temperature is 925°C, and the final rolling reduction is 26%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for structural quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 915°C.

[0067] The online molten salt quenching strong isothermal treatment process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 35°C / s, and quickly passes through the pearlite phase region from the high-temperature austenite state and enters the bainite phase region, inhibiting the formation of pearlite, and forming a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, and the wire rod is controlled to perform high-temperature strong isothermal tempering of the quenching structure in the high-temperature isothermal range, toughening and stress relief treatment, control carbide precipitation, and improve the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of 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 is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 592°C, the treatment time is 395s, and the molten salt circulation volume is 490t / h.

[0068] The roller slow cooling process adopts adjusting the opening of the insulation cover, and the wire rod transported by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, resulting in increased stress, and promote further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until coiling. Specifically: the wire rod is controlled to slowly cool to 256°C at a cooling rate of 0.55°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage.

[0069] Comparative Example 7:

[0070] A method for manufacturing a wire rod, which differs from Example 4 in that: the manufacturing method is manufactured according to a process flow of rolling → spinning → online molten salt quenching strong isothermal → air cooling → coiling. Specifically: the air cooling process uses a wire rod transported by a conveyor roller through a second salt bath tank to naturally cool in the air. The wire rod is cooled to 257°C at a cooling rate of 1.6°C / s to obtain the wire rod.

[0071] The structure and performance of the wire rods obtained in Examples 1 to 4 and Comparative Examples 1 to 7 were tested, and the comparative results are shown in Table 1 below:

[0072] Table 1. Comparison of microstructure and properties of different wire rod compositions and manufacturing methods

[0073]

[0074] From the comparison results of 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 process, and lead to the appearance of abnormal structures such as martensite. Although reducing the C and Cr content and air-cooling strength is beneficial to controlling the martensite structure, it will also affect the pearlite structure incubation and lamellar refinement, resulting in a large strength loss and mechanical property fluctuations. At the same time, the plasticity is still insufficient. The present invention controls the wire rod to quickly enter the bainite low-temperature transformation zone from the high-temperature austenite state through the Nb-V chemical composition design combined with the online molten salt quenching strong isothermal technology, which can avoid the formation of pearlite soft phase, upper bainite or martensite brittle phase, and then controls the wire rod to perform high-temperature isothermal tempering on the quenched structure in the high-temperature isothermal range to control the precipitation of carbides to form a mixed structure including tempered bainite as the main component and a small amount of ferrite, thereby improving the strength-plasticity matching of the wire rod, and can achieve a tensile strength of 1175~1225MPa and a cross-sectional shrinkage rate of 58%~63%. After cold working, it can be directly processed into parts.

[0075] From the comparison results of Example 1 and Comparative Example 2, it can be seen that the selection of a higher spinning temperature, i.e., the quenching temperature, can provide favorable conditions for forming a larger degree of undercooling after the previous molten salt treatment and promoting the short-time quenching of the wire rod to form quenched bainite as the microstructure. At the same time, it can reduce the restriction on the rolling temperature, reduce the rolling mill load, and improve the rolling speed and efficiency.

[0076] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the molten salt in the front stage is in the bainite phase region. The higher the molten salt temperature and the shorter the treatment time, the more conducive it is to reducing the dislocation density and stress of the tissue and reducing the difficulty of isothermal tempering. However, if the molten salt temperature is too high, it will be detrimental to the transformation of the austenite structure to 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 the austenite structure to quenched bainite, increase the austenite residue, and cause the subsequent molten salt treatment to produce pearlite structure, affecting the tissue control and matrix strength.

[0077] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the lower the molten salt temperature of the front-stage molten salt and the longer the treatment time, the more conducive it is to inhibiting the formation of pearlite, promoting the transformation of high-temperature austenite structure to needle-shaped quenched bainite, forming a quenched structure mainly composed of quenched bainite, and 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 increases, the residual stress increases, which will increase the difficulty of isothermal tempering and affect the carbide control.

[0078] From the comparison results of Example 3 and Comparative Example 5, it can be seen that 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 thermal power is provided. The quenching stress is eliminated by tempering, the lattice distortion is reduced, and the matrix is ​​transformed from the high brittleness of the quenched state to the strong toughness of the tempered state. At the same time, the high temperature provides atomic diffusion power, which accelerates the rate at which Nb and C combine to form carbides, increases the precipitation amount, and increases the number of strengthening phases. However, if the molten salt temperature is too high and the treatment time is too long, excessive tempering and softening will occur, the Nb / V carbides will be excessively coarsened, the precipitation strengthening effect will be weakened, and even the strength and plasticity will decrease due to the excessive size of the carbides, and the production energy consumption will increase.

[0079] From the comparison results of Example 3 and Comparative Example 6, it can be seen that the lower the molten salt temperature of the latter stage, the more conducive it is to promoting the fine dispersion and precipitation of vanadium-containing carbides and reducing the softening rate. As the treatment time is shortened, it is beneficial to reduce 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 structural brittleness, and the amount of Nb / V carbide precipitation is insufficient, resulting in a loss of precipitation strengthening effect.

[0080] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow roller cooling can prevent the wire rod from cooling too fast during the cooling process, which leads to increased stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod.

[0081] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing 1250MPa grade high strength tool steel wire rod, characterized in that: The manufacturing method includes: The steel billet is rolled and spun into a wire rod at a spinning temperature of ≥905°C, and then subjected to an online molten salt quenching strong isothermal treatment, wherein the wire rod first passes through a front-stage molten salt, and the wire rod is controlled to enter a bainite phase region from a high-temperature austenite state at a cooling rate of ≥35°C / s to form a quenched structure mainly composed of quenched bainite. The steel billet is then heated in a rear-stage molten salt, and the quenched structure is isothermally tempered and toughened to relieve stress, and carbide precipitation is controlled. Finally, the steel billet is slowly cooled through a roller to form a wire rod having a microstructure including tempered bainite and ferrite. The chemical composition and mass percentage of the wire rod include: 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 are Fe and unavoidable impurities; the molten salt temperature of the front section molten salt is 325~355℃, and the processing time is 15~25s, the molten salt temperature of the rear section molten salt is 580~600℃, and the processing time is 300~550s, and the roller slow cooling controls the wire rod to slowly cool to below 260℃ at a cooling rate of 0.35~0.95℃ / s.

2. The method for manufacturing 1250MPa 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 soaking time in the furnace is 140-185 minutes.

3. The method for manufacturing 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 controlled to be 910-940° C., and the final rolling reduction is controlled to be 23%-28%.

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

5. The method for manufacturing 1250 MPa grade high strength tool steel wire rod according to claim 1, characterized in that: The molten salt circulation rate of the front-stage molten salt is 550-750 t / h, the molten salt temperature rise is ≤8°C, and the molten salt circulation rate of the rear-stage molten salt is 450-650 t / h.

6. A 1250MPa grade high strength tool steel wire rod, characterized in that: The wire rod is manufactured by the manufacturing method of 1250MPa grade high strength tool steel wire rod according to any one of claims 1 to 5.

7. The 1250 MPa grade high strength tool steel wire rod according to claim 6, characterized in that: The volume percentage of the tempered bainite is ≥97%.

8. The 1250 MPa grade high strength tool steel wire rod according to claim 6, characterized in that: The wire rod has a diameter of 6-10 mm, a tensile strength of 1175-1225 MPa, a cross-sectional shrinkage rate of 58%-63%, and a mechanical property difference of ≤37 MPa.

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