1500MPa-grade high-strength tool steel wire rod and manufacturing method thereof

Abstract

The invention relates to a 1500MPa-grade high-strength tool steel wire rod and a manufacturing method thereof.The method includes the steps that after Nb-V chemical component design is adopted for rolling and spinning to form a wire rod, online molten salt subcooling quenching isothermal treatment is conducted, molten salt of the wire rod at the front section is controlled to enter a martensite phase region from a high-temperature austenite state at the cooling speed larger than or equal to 37 DEG C / s, part of austenite is promoted to be converted into a martensite structure, and then the steel wire rod is rolled and spun; the method comprises the following steps of: heating to a sorbite phase region through a rear-section molten salt, promoting the transformation of austenite residues to a sorbite structure, carrying out isothermal tempering toughening destressing treatment on the structure, and finally slowly cooling through a roller way to prepare a wire rod of which the microscopic structure comprises tempered sorbite and tempered martensite, the material cost can be properly controlled, the phase change of the wire rod is powerfully regulated and controlled, the strength and plasticity matching of the wire rod is improved, the tensile strength is 1470-1520 MPa, and the percentage reduction of area is 52%-57%, so that additional heat treatment after cold machining is omitted, the production energy consumption and cost of the tool steel are reduced, and the efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field related to hot-rolled wire rods, and particularly relates to a 1500MPa grade high-strength tool steel wire rod and a manufacturing method thereof. Background Art

[0002] Alloy tool steel can be used to manufacture cutting tools, measuring tools, impact tools, hand tools, etc. With the development of new energy, electronic equipment, mechanical industry and other industries, tool steel as a whole is also showing an upward trend. However, although the current output of tool steel is constantly increasing, most of them are still mid- and low-end products. There are relatively few high-end products and the manufacturing cost is high. Due to the limitations of the Stelmor air-cooled line for the production of hot-rolled wire rods, tool steel contains high contents of elements such as carbon, silicon, manganese, chromium, and precious elements such as nickel, molybdenum, and vanadium. High-strength tool steel uses hot-rolled wire rods as the base material. After cold processing such as drawing and cold heading, it is necessary to cooperate with heat treatment to release the material properties in order to achieve the performance of the tool steel. However, it also brings about the problems of increased manufacturing costs and energy consumption, and decreased production efficiency. Therefore, in order to meet the development and application needs of high-end tool steel products, reducing production processes as an economical and effective way is to improve the comprehensive performance of the base material wire rods, control the cost and yield rate of tool steel wire rods.

[0003] The factors that restrict the improvement of comprehensive performance and cost control of existing tool steel wire rods include: (1) In order to take into account the hardness of the finished product and improve the plasticity of the tool steel wire rod so that it can be directly drawn without annealing, the wire rod will adopt a medium-high carbon and high silicon component system, and be made into a bainite structure wire rod with a Stelmor air cooling line. For example, patent CN119640155A discloses a low-cost, high-plasticity bainite tool alloy steel wire rod and its manufacturing method, which adopts a C-Si-Mn-Cr component system combined with low-temperature rolling and spinning, Stelmor air cooling and heat preservation cooling to make a microstructure wire rod with a high bainite content, achieving a tensile strength of 1045~1074MPa and a surface reduction rate of 50%~55%. However, on the one hand, in order to retain part of the rolling distortion energy to enhance the instability of the high-temperature structure, so as to control the bainite phase transformation and reduce the risk of abnormal bainite or martensite structure, it is necessary to adopt low-temperature rolling and spinning, which will lead to The load requirements for the rolling line are high, which aggravates the wear of the rolling line and affects the rolling efficiency. On the other hand, in order to increase the sorbitization rate in the structure, the air cooling intensity needs to be increased after wire drawing. However, due to the limitation of the maximum cooling capacity of the air cooling line, the core is prone to form coarse pearlite due to too slow cooling rate. There are more ferrite soft phases in the mixed structure of pearlite and sorbite, and the strength of the wire rod is insufficient. Additional heat treatment is required after cold working to improve the performance of tool steel. Continuing to add alloy elements will also lead to increased material costs. The increase in air cooling intensity will also increase the temperature difference between the winded and winded sides of the wire rod, and the overlap and non-overlap parts. Affected by the segregation of hardenable components such as Mn and Cr, the difficulty of microstructure control increases. Overcooling in some positions is prone to form uncontrollable bainite or martensite brittle abnormal structures, resulting in fluctuations in the mechanical properties of the wire rod and increased brittleness, affecting the cold working performance.

[0004] (2) In order to improve the hardness of tool steel wire rod, the wire rod will adopt the synergistic strengthening of high carbon and multi-element alloy, and be made into bainite or martensite structure wire rod with Stelmor air cooling line. For example, patent CN117051308B discloses a production method of tool steel wire rod, which adopts C-Si-Mn-Cr-Mo-V-Ni-Nb composition design combined with low temperature spinning, first quickly air cooling to the slow cooling area and then keeping warm and slow cooling to obtain tool steel wire rod with metallographic structure of bainite + martensite. However, on the one hand, due to the maximum cooling capacity limitation of Stelmor air cooling line, the wire rod will pass through the upper pearlite and upper bainite areas more slowly, forming soft phase or brittle feather-like upper bainite. At the same time, high carbon and alloying elements such as Cr and Mo are easy to produce central segregation during continuous casting, resulting in the enrichment of carbon and alloying elements in the core of the wire rod. With the increase of air cooling intensity, phase transformation and organizational uniformity are difficult to control. Local martensite is easily formed first during cooling, which delays the incubation period of bainite transformation. At the same time, if the temperature distribution in the slow cooling area is uneven, the phase transformation will be incomplete, and the retained austenite will be transformed into coarse martensite in the subsequent cooling, exacerbating performance fluctuations. On the other hand, the phase transformation range temperature of the wire rod is relatively low. During the continuous slow cooling process, it is in a low temperature state after the phase transformation. The bainite lamellar structure is coarse, the carbide is unevenly distributed, and local stress is concentrated. Martensite is formed by rapid cooling of austenite. High silicon solid solution in martensite will further increase the lattice distortion, resulting in high-density internal stress inside the matrix, which increases the brittleness of the wire rod and significantly reduces the plastic deformation ability. It not only affects the cold working performance, but also makes the wire rod easy to break during transportation and coiling, affecting the yield rate. Summary of the Invention

[0005] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 1500MPa grade high-strength tool steel wire rod and a manufacturing method thereof, which can appropriately control material costs, strongly regulate the phase transformation of the wire rod, and improve the strength-plasticity matching of the wire rod so that it can be directly processed into parts after cold forming, thereby reducing the energy consumption and cost of tool steel production and improving efficiency.

[0006] The technical solution adopted by the present invention to solve its technical problem is: A method for manufacturing a 1500MPa grade high-strength tool steel wire rod, the manufacturing method comprising: The steel billet is rolled and spun into wire rod at a spinning temperature of ≥915℃, and then is subjected to online molten salt supercooling quenching isothermal treatment. First, the wire rod is controlled to enter the martensite phase region from the high-temperature austenite state at a cooling rate of ≥37℃ / s in the front section of the molten salt, promoting the transformation of part of the austenite to the martensite structure. Then, the temperature is raised to the sorbite phase region in the rear section of the molten salt, promoting the transformation of the austenite residue to the sorbite structure. The structure is subjected to isothermal tempering toughening and stress relief treatment, and finally slowly cooled by rollers to form a microstructure. The fabric includes a wire rod of tempered bainite and tempered martensite, and the chemical composition and mass percentage of the wire rod include: C: 0.61%-0.66%, Si: 0.35%-0.45%, Mn: 0.90%-1.0%, Cr: 0.70%-0.80%, Nb: 0.028%-0.038%, V: 0.055%-0.075%, P≤0.015%, S≤0.015%, and the rest is Fe and unavoidable impurities.

[0007] The chemical composition and mass percentage of the above-mentioned wire rod are designed based on the following: (1) Carbon: C is a carbide strengthening element and austenite forming element in steel. It is relatively cheap and can improve the tensile strength and wear resistance of wire rods by solid solution strengthening and dispersing carbides. As the carbon content increases, the stability of austenite can be improved, making it difficult for austenite in the wire rod to decompose into pearlite in advance at high cooling rate, and more likely to trigger martensite phase transformation. The carbon content in the austenite residue is relatively high. When the sorbite zone is isothermal in the subsequent process, it can promote its transformation into lamellar sorbite through carbon diffusion, ensuring strength and hardness while taking into account a certain toughness. However, if the carbon content is too high, the martensite quenching stress increases, causing the difficulty of isothermal tempering to increase, resulting in reduced plasticity and toughness. At the same time, it will increase the decarburization tendency, easily form proeutectoid network carbides, reduce grain boundary connectivity, and worsen fatigue performance. Therefore, in order to meet the strength and wear resistance requirements of tool steel, control material costs, and facilitate microstructure control and toughening stress relief treatment to improve strength and toughness matching, the mass percentage of C is controlled to be 0.61%~0.66%.

[0008] (2) Silicon: The Si element can inhibit the coarsening of grains during online molten salt treatment, prolong the stability time of austenite in the medium temperature zone, and make it easier to obtain martensite with a high cooling rate. In the subsequent isothermal treatment of the sorbite zone, silicon can refine the spacing between the sorbite layers and hinder the aggregation and growth of carbides during tempering, so that the tempered martensite maintains a high hardness and strength, ensuring the toughening effect of the wire rod and adapting to the wear resistance requirements of tool steel. However, too high a silicon content will aggravate the decarburization phenomenon, reduce the surface quality of the wire rod, increase the number of inclusions, reduce the plasticity and impact toughness of the wire rod, and affect the cold working performance. Therefore, in order to adapt to the regulation of the organization by online molten salt, take into account the cold working performance of the wire rod, and improve the strength and toughness of the tempered martensite, a high silicon content is not used, and the mass percentage of Si is controlled to be 0.35%~0.45%.

[0009] (3) Manganese: Mn is a strong austenite stabilizing element and a good hardenability element. It can expand the austenite zone, reduce the critical cooling rate of steel and the starting temperature of martensite transformation, reduce the amount of martensite transformation during quenching of the wire rod, avoid excessive internal stress caused by a large amount of transformation at one time, and make the sorbite phase transformation temperature range lower, which is conducive to improving the sorbite nucleation rate, promoting the transformation of austenite residue to sorbite structure, and improving the strength of the wire rod. Manganese can also synergize with elements such as chromium to form interstitial solid solution and enhance the cluster strengthening effect. However, when the Mn content is too high, it will promote the growth of austenite grains. When heated at high temperature, it is easy to cause grain coarsening, promote central segregation, and cause uneven phase transformation structure or increase temper brittleness, affecting the impact toughness of the wire rod and deteriorating the fatigue resistance of the tool. Therefore, in order to facilitate the regulation of microstructure phase transformation by molten salt treatment, improve the matrix strength, and take into account plasticity and microstructure uniformity, the Mn content is appropriately increased, and the mass percentage of Mn is controlled to be 0.90%~1.0%.

[0010] (4) Chromium: Cr can significantly improve the hardenability of steel, inhibit the precipitation of ferrite, promote the transformation of austenite to martensite, and keep the steel at a high strength and hardness after tempering. Chromium also increases the solubility of carbon in the austenite residue. When isothermal in the sorbite zone, it can refine the sorbite lamellae. Chromium combines with carbon to form alloy carbides, refines the carbide particles, and strengthens the second phase strengthening effect, improves the steel's resistance to tempering softening, increases the wear resistance and corrosion resistance of tool steel wire rods, and improves the high-temperature service stability of tools. However, excessive Cr content will aggravate composition segregation. Local Cr-rich areas will increase the difficulty of controlling the uniformity of the structure and the difficulty of improving the plasticity during tempering, and reduce toughness. Therefore, in order to take into account the wear resistance requirements of tool steel and facilitate the control of mixed structure and stress relief of the structure, the mass percentage of Cr is controlled to be 0.70%~0.80%.

[0011] (5) Niobium: Niobium will precipitate in the high-temperature austenite stage and refine the original austenite grains by pinning the austenite grain boundaries. The fine austenite grains can make the subsequent martensite transformation more uniform, reduce the coarse martensite blocks, and the layers are finer during the sorbite transformation, avoiding the uneven structure caused by coarse grains. The precipitation of NbC during isothermal tempering can produce a dispersion strengthening effect, which can improve the toughness while ensuring the strength. However, the cost of Nb element is relatively high, and excessive addition is not conducive to controlling the cost of wire rod. Therefore, based on the role, cost and manufacturing control of Nb element, the mass percentage of Nb is controlled to be 0.028%~0.038%.

[0012] (6) Vanadium: The V element forms high-melting-point carbides with carbon to pin the grain boundaries, which can prevent grain growth and refine the martensite laths. At the same time, fine dispersed carbides are precipitated during the tempering process to produce secondary hardening. The dispersed VC particles and niobium synergistically refine the grains, thereby improving the strength and toughness of the steel. This is beneficial for inhibiting the initiation of fatigue cracks and improving the fatigue limit of tool steel. 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 aggregation and coarsening of carbides will lead to a decrease in the toughness of the matrix. Therefore, based on the role of the V element, cost and manufacturing control, the mass percentage of V is controlled to be 0.055%~0.075%.

[0013] (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%.

[0014] The above-mentioned wire rod adopts the C-Si-Mn-Cr-Nb-V composition design, with relatively low C and Si contents, an appropriate increase in Mn, and trace additions of Nb and V. This can appropriately control material costs and regulate the hardenability of the wire rod at the same time, thereby providing favorable conditions for promoting the transformation of part of austenite to martensite, fully transforming the austenite residue in the sorbite phase, refining the structure and dispersing stress through microalloying elements, appropriately controlling the difficulty of tempering softening, and avoiding the risk of martensite embrittlement and cracking. On this basis, the wire rod adopts high-temperature spinning, i.e., a higher quenching temperature, so that more alloy elements are dissolved in the austenite, thereby improving its ability to resist pearlite or bainite transformation, increasing the stability of the supercooled austenite, and preparing for subsequent microstructure quenching. After spinning, the wire rod is not air-cooled but undergoes online molten salt supercooling quenching isothermal treatment: 1. Compared with the difficulty in suppressing coarse pearlite and bainite due to the limitation of the maximum cooling capacity of the Stelmor air-cooled line, the abnormal martensite structure is difficult to control. On the one hand, the wire rod passes through the molten salt, and the high heat exchange capacity of the molten salt can be used to promote the rapid cooling of the wire rod, so that the wire rod quickly passes through the pearlite and bainite phase transformation temperature range from the high-temperature austenite state to the martensite phase region, forming a large degree of supercooling, inhibiting the incubation of pearlite and bainite, promoting martensite nucleation, improving the matrix strength, and compensating for the strength loss caused by reducing the carbon or alloy content. At the same time, it matches the hardenability and composition of the steel to avoid excessive martensiticization affecting the subsequent sorbite transformation, or coarse martensite lamellae, and a large amount of one-time The transformation leads to stress peaks and causes quenching cracks; on the other hand, when the wire rod passes through the molten salt, the molten salt can cover the surface of the wire rod for uniform heat exchange, and there is no temperature difference problem between the winded side and the winded side, or between the overlap and the non-overlap. The temperature difference from the surface to the core of the wire rod can be reduced, and a small amount of austenite can be promoted to uniformly transform into martensite structure, thereby improving the uniformity of the structure, making the martensite phase change more controllable, and strongly regulating the structure. At the same time, the precipitation of proeutectoid carbides is suppressed. The martensite phase transformation is accompanied by lattice distortion, which will produce high-density dislocations and internal stress at the interface, providing a large number of nucleation sites for the subsequent precipitation of Nb and V. The internal stress can reduce the activation energy of precipitation, making it easier for subsequent fine carbides to nucleate.

[0015] 2. Compared with the limitation of the minimum cooling capacity and continuous cooling control of the Stelmor air-cooled line, the coarse tissue lamellae, internal stress, brittleness or tissue uniformity are difficult to control. On the one hand, the molten salt in the latter stage can control the wire rod to heat up to the sorbite phase region. Compared with continuous cooling, on the basis of controlling the martensite phase transformation, it can prolong the time that the wire rod is in the sorbite phase region, promote the full transformation of the austenite residue to the sorbite with finer interlamellar spacing, avoid the austenite residue from continuing to form bainite or martensite structure in the subsequent cooling process, make the structure more uniform, reduce the performance fluctuation caused by anisotropy, and at the same time be in the temperature range of Nb and V carbide dispersion precipitation, which can promote the precipitation and dispersion distribution of nano-scale carbides of Nb and V, further improve Increase the strength of the martensite and troostite matrix; on the other hand, the temperature of the troostite phase region is higher than that of the martensite phase region, which can extend the time that the wire rod is in the high temperature range, and isothermal treatment instead of continuous cooling can enhance the atomic diffusion ability, and dislocations can be relaxed through slip and climb motion, so that the micro stress caused by lattice distortion can be released through stress redistribution, and stress relief and toughening treatment can be performed to improve the brittleness of martensite, which is conventionally regarded as an abnormal organization, and enhance the matching of strength and plasticity. The temperature of the wire rod out of the molten salt is high, and the roller slow cooling is used to control the slow cooling of the wire rod, which can prevent the wire rod from increasing stress due to too fast cooling rate during the cooling process, continue the softening effect of the molten salt in the later stage, promote further toughening of the wire rod organization, and realize organization state regulation.

[0016] Before the rolling, the soaking temperature and the time in the furnace are controlled to promote uniform diffusion of alloy elements, reduce the influence of segregation, provide favorable conditions for strengthening and refining grains through precipitation during subsequent rolling, and avoid overburning caused by excessive soaking temperature or excessive time in the furnace. In the preferred technical solution, before the rolling, the soaking temperature of the heating furnace is controlled to be 1180~1220℃, and the time in the furnace is 140~200min.

[0017] Due to the high spinning temperature, the restrictions on rolling can be reduced. A higher initial rolling temperature is selected to improve the plasticity of the steel billet, reduce the wear on the rolling line, increase the rolling speed, and inhibit the premature and large-scale precipitation of microalloy carbides, which hinders the recrystallization process and causes grain coarsening and unevenness. The selection of appropriate final rolling temperature and final rolling reduction can promote dynamic recrystallization and achieve strain-induced precipitation and grain refinement. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1055~1080℃, the final rolling temperature is 920~950℃, and the final rolling reduction is 24%~30%.

[0018] During the spinning, the spinning temperature can be further controlled to avoid excessively high spinning temperature and increased risk of grain coarsening. In a preferred technical solution, during the spinning, the spinning temperature is controlled to be 915-940°C.

[0019] The molten salt temperature of the front-stage molten salt treatment is in the martensite phase region. The lower the molten salt temperature and the longer the treatment time, the higher the degree of supercooling and the driving force for the transformation of austenite to martensite can be, and the martensite transformation speed, matrix strength and tissue stress can be improved. However, if the molten salt temperature is too low and the treatment time is too long, the stability of austenite will drop sharply, and excessive transformation of austenite to martensite will affect the residual austenite and subsequent sorbite transformation, increasing the difficulty of stress relief and energy consumption cost. On the contrary, if the molten salt temperature is higher and the treatment time is shorter, the thermal motion of austenite atoms can be enhanced, and its stability can be improved to a certain extent, and the martensite transformation speed can be controlled, and excessive martensite can be suppressed. However, the molten salt temperature is too high and the treatment time is too short, the driving force for austenite transformation is too low, the unstable austenite is not fully transformed, and the target amount of a small amount of martensite cannot be achieved. Even bainite structure is formed, which will affect the strength of the matrix. Therefore, the front-stage molten salt treatment can adopt appropriate molten salt temperature and treatment time to control the wire rod to quickly cool down to the martensite phase region, promote the transformation of a small amount of austenite to martensite, and make organizational preparations for subsequent sorbite phase transformation control and appropriate toughening treatment. In the preferred technical solution, the molten salt temperature of the front-stage molten salt treatment is 310~360℃, and the treatment time is 28~38s.

[0020] The spinning temperature has a large temperature difference with the previous molten salt treatment. Selecting a larger molten salt circulation rate can quickly take away the heat of the wire rod, control the temperature rise of the molten salt, and improve the temperature uniformity of the wire rod surface. In the preferred technical solution, the molten salt circulation rate of the previous molten salt treatment is 550~850t / h, and the molten salt temperature rise is ≤8°C.

[0021] The latter molten salt treatment is in the sorbite phase region. The higher the molten salt temperature and the longer the treatment time of the latter molten salt, the more it can promote the full transformation of austenite residues to sorbite structure, improve the uniformity of the structure, enhance the activity of atoms inside the wire rod, release residual stress through dislocation slip, grain boundary migration, etc., and significantly improve the impact toughness of the material. However, if the molten salt temperature is too high and the treatment time is too long, the risk of coarsening of the sorbite structure will increase, or excessive softening will reduce the strength, and carbide precipitation and coarsening will cause loss of strong and plastic properties, increasing production energy consumption. On the contrary, the lower the molten salt temperature, the more it can promote the refinement of the sorbite interlamellar spacing and the dispersion and precipitation of microalloy carbides. As the treatment time is shortened, the production energy consumption and the risk of carbide coarsening can be reduced. However, if the molten salt temperature is too low and the treatment time is too short, the untransformed austenite residue will cause the material hardness to fluctuate and the toughness to decrease, or the martensite and sorbite stresses cannot be released in time after the sorbite is inoculated, affecting the full precipitation of microalloy carbides, which will lead to strength loss and obvious structural brittleness. Therefore, the latter molten salt treatment can control the wire rod in the high-temperature isothermal range, promote the full transformation of the austenite residue into sorbite, and then perform isothermal tempering and toughening stress relief treatment on the sorbite and martensite quenching structure to avoid the precipitation and coarsening of carbides and improve the strength-plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the latter molten salt treatment is 530~560℃, and the treatment time is 290~590s.

[0022] Controlling the molten salt circulation rate of the latter molten salt treatment can accurately control the temperature and improve the consistency of the transformation of retained austenite to sorbite, stress relief toughening and Nb and V precipitation. In the preferred technical solution, the molten salt circulation rate of the latter molten salt treatment is 400~620t / h.

[0023] The slow cooling on the roller can further control the cooling rate of the wire rod, avoid the increase of stress caused by excessive cooling of the wire rod, promote further toughening of the wire rod structure, improve the softening effect, and avoid the slow cooling rate of the wire rod affecting the offline speed. In the preferred technical solution, the slow cooling on the roller controls the wire rod to slowly cool to below 260°C at a cooling rate of 0.2~0.5°C / s.

[0024] A 1500MPa grade high-strength tool steel wire rod is manufactured by any one of the above-mentioned methods for manufacturing a 1500MPa grade high-strength tool steel wire rod.

[0025] The above-mentioned wire rod is designed through the Nb-V chemical composition, with low carbon and silicon content and trace addition of niobium and vanadium, which can appropriately reduce material costs. At the same time, it can form a microstructure including a mixed structure of a small amount of tempered martensite and a large amount of tempered bainite. Compared with air-cooled bainite tool steel wire rod, it can avoid the formation of coarsened pearlite structure. The interlamellar spacing of bainite is finer and the strength and toughness are better. Through toughening and stress relief treatment, it is transformed into tempered bainite structure, which can further improve the plasticity characteristics. At the same time, martensite has higher strength than bainite. After stress relief and toughening treatment, the brittleness of the structure can be improved, so that the tempered martensite retains the strength characteristics of martensite and has better toughness, which can effectively improve the strengthening effect of carbon and enhance the matrix strength. , to make up for the strength loss caused by reducing the content of alloys such as silicon and vanadium, and take into account the hardness and wear resistance requirements of tool steel; compared with air-cooled bainite and martensite tool steel wire rods, it can avoid the formation of feather-like brittle upper bainite structure, and at the same time, the martensite phase change becomes controllable and transforms into tempered martensite that is both strong and tough, which can avoid the risk of brittle fracture during transportation and coiling, and improve the yield rate. Combined with the dispersed distribution of tempered bainite and microalloy carbides, it can improve impact toughness and organizational uniformity, reduce mechanical property fluctuations, and uniform organization can reduce stress concentration points, taking into account cold working performance, and improving the strength and plasticity matching of wire rods, so as to avoid additional heat treatment after cold working, which is suitable for tool steel production and application needs.

[0026] The greater the proportion of the tempered bainite in the microstructure and the finer the interlamellar spacing, the better the toughness. In a preferred technical solution, the volume percentage of the tempered bainite is 75% to 85%, and the interlamellar spacing is 75 to 115 nm.

[0027] The greater the proportion of tempered martensite shown in the microstructure, the better the matrix strength and wear resistance. In a preferred technical solution, the volume percentage of the tempered martensite is 15% to 25%.

[0028] In the preferred technical solution, the diameter of the wire rod is 5~10mm, the tensile strength is 1470~1520MPa, the cross-sectional shrinkage rate is 52%~57%, and the mechanical property same-circle difference is ≤38MPa. The wire rod diameter belongs to small and medium-sized wire rods, and can be manufactured into high-strength cutting tools, impact tools, hand tools and other tools through cold processing such as drawing and cold heading. The material utilization rate is good. The wire rod has high tensile strength and can resist static load fracture in service. It has a wider adaptability to the service environment. The wire rod has high plasticity and toughness, can relieve stress concentration, and can be directly cold processed and reduce the risk of cracking during cold processing. At the same time, combined with the high strength characteristics, it can avoid additional heat treatment after cold processing, so as to reduce the tool steel production process and reduce the energy consumption and cost of tool steel production.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) In view of the current situation that the abnormal structure of tool steel wire rod is difficult to control and the strength and plasticity are difficult to be balanced due to the limitation of Stelmor air cooling line, the manufacturing method of the present invention combines the Nb-V chemical composition design with the online molten salt supercooling quenching isothermal technology, firstly controls the wire rod to quickly pass through the pearlite and bainite phase region from the high temperature austenite state in the front section of molten salt and enter the martensite phase region, promotes the transformation of part of austenite to martensite structure, inhibits the pearlite and bainite phase transformation, and makes the martensite phase transformation uniform and controllable, then heats up to the sorbite phase region in the rear section of molten salt, promotes the transformation of austenite residue to sorbite structure, performs isothermal tempering and toughening stress relief treatment on the structure, releases the structure stress, and finally passes through the roller to slowly cool to avoid stress increase, promotes further toughening of the wire rod structure, strongly regulates the wire rod phase transformation, improves the strength and plasticity matching of the wire rod, avoids the risk of brittle fracture during transportation and winding, improves the yield rate, and has good industrial adaptability.

[0030] (2) In view of the problem that the existing tool steel wire rods are not strong enough in terms of plasticity and need to be combined with heat treatment after cold working to release the material properties, which leads to increased manufacturing costs and energy consumption and decreased production efficiency, the wire rods of the present invention have low carbon and silicon content and are added with trace amounts of niobium and vanadium, which can appropriately reduce the material cost. At the same time, the microstructure includes tempered martensite and tempered martensite, which can avoid the strength loss caused by the formation of coarsened pearlite structure. Combined with the tempered martensite that is both strong and tough, it compensates for the strength loss caused by reducing the content of alloys such as silicon and vanadium. The microalloy carbides are dispersed, taking into account the hardness and wear resistance requirements of tool steel and the cold working performance, and improving the strength and plasticity matching of the wire rods. The tensile strength can reach 1470~1520MPa, the cross-sectional shrinkage rate is 52%~57%, and the mechanical property difference is ≤38MPa. It is used in the application fields of manufacturing high-strength tool steel, and can be directly processed into parts after cold working, eliminating the need for heat treatment after cold working, so as to achieve production energy consumption, cost reduction and efficiency improvement, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] 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: Figure 1 is a metallographic structure diagram of Example 1 of the present invention; Figure 2 is a metallographic structure diagram of Example 2 of the present invention; Figure 3 This is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0032] 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 to propose the best way to implement the present invention, are intended to be used 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 limited only by the appended claims; the wire rods obtained in the following embodiments and comparative examples are subjected to organizational and performance testing, including: tensile testing using "GB-T228.1-2021 Metallic Materials Tensile Testing Part 1: Room Temperature Test Method" to obtain tensile strength and cross-sectional shrinkage; organizational testing is performed in accordance with the metal microstructure detection method of GB / T13298 standard; mechanical property same-circle difference test method: take 2 circles of wire rod 5m away from the end of the coil, and divide each circle of wire rod into 8 sections with the overlap area position as the base point, and take 1 tensile specimen on each section. The extreme difference in strength of the tensile specimens after tensile testing is the mechanical property same-circle difference. Example 1:

[0033] A preferred embodiment of the method for manufacturing 1500MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.66%, Si: 0.38%, Mn: 0.90%, Cr: 0.77%, Nb: 0.032%, V: 0.074%, P: 0.015%, 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 supercooling quenching isothermal treatment → roller slow cooling → coiling, specifically: 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 7mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1195°C, the furnace time is 180min, the initial rolling temperature is 1065°C, the final rolling temperature is 935°C, and the final rolling reduction is 28.5%; 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 with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 925°C.

[0034] The online molten salt supercooling quenching 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 molten salt treatment, so that the wire rod is cooled at a cooling rate of 40°C / s, passing through the pearlite and bainite phase regions from the high-temperature austenite state and entering the martensite phase region, promoting the transformation of a small part of austenite to martensite structure, retaining most of the austenite residues, and then conveyed by a roller through the second salt bath tank for the rear molten salt treatment, controlling the wire rod to heat up to the sorbite phase region, and then at the high temperature. The isothermal zone promotes the transformation of austenite residues to sorbite structure, promotes the dispersion and precipitation of Nb and V carbides, and performs isothermal tempering and toughening stress relief treatment on the quenched martensite and sorbite structures to avoid carbide coarsening and improve the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 342°C, the treatment time is 32s, the molten salt circulation rate is 635t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 538°C, the treatment time is 430s, and the molten salt circulation rate is 435t / h.

[0035] The roller slow cooling process adopts the method of closing the insulation cover, inputting the hot air in the online molten salt supercooling quenching isothermal treatment into 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 promoting 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 255°C at a cooling rate of 0.42°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.

[0036] Comparative Example 1: 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 1080°C, the furnace time is 245 minutes, the initial rolling temperature is 980°C, the final rolling temperature is 830°C, and the spinning temperature is controlled to 800°C. The Stelmor air cooling line adopts a closed insulation cover, controls the wire rod to enter the insulation cover and cools it to 295°C at a rate of 2.4°C / s, and is collected by a coiling drum to obtain a finished wire rod.

[0037] Comparative Example 2: A method for manufacturing a wire rod, which differs from Example 1 in that: the soaking temperature of the heating furnace is controlled to 1130°C, the time in the furnace is 220 minutes, the initial rolling temperature is 1020°C, the final rolling temperature is 890°C, the spinning temperature is controlled to be 870°C, the wire rod is treated with molten salt in the front section and cooled at a cooling rate of 35°C / s to obtain a finished wire rod. Example 2:

[0038] A preferred embodiment of the method for manufacturing 1500MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.62%, Si: 0.45%, Mn: 0.93%, Cr: 0.70%, Nb: 0.028%, V: 0.075%, 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 supercooling quenching isothermal treatment → roller slow cooling → coiling, specifically: 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 5mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1180°C, the furnace time is 200min, the initial rolling temperature is 1055°C, the final rolling temperature is 920°C, and the final rolling reduction is 30%; 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 with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 915°C.

[0039] The online molten salt supercooling quenching 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 molten salt treatment, so that the wire rod is cooled at a cooling rate of 38°C / s, passing through the pearlite and bainite phase regions from the high-temperature austenite state and entering the martensite phase region, promoting the transformation of a small part of austenite to martensite structure, retaining most of the austenite residues, and then conveyed by a roller through the second salt bath tank for the rear molten salt treatment, controlling the wire rod to heat up to the sorbite phase region, and then at the high temperature. The isothermal zone promotes the transformation of austenite residues to sorbite structure, promotes the dispersion and precipitation of Nb and V carbides, and performs isothermal tempering and toughening stress relief treatment on the quenched martensite and sorbite structures to avoid carbide coarsening and improve the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 359°C, the treatment time is 28s, the molten salt circulation rate is 550t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 532°C, the treatment time is 590s, and the molten salt circulation rate is 400t / h.

[0040] The roller slow cooling process adopts the method of closing the insulation cover, inputting the hot air in the online molten salt supercooling quenching isothermal treatment into 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 promoting 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 251°C at a cooling rate of 0.5°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.

[0041] Comparative Example 3: 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 36°C / s, the molten salt temperature of the front-stage molten salt treatment is 375°C, and the treatment time is 15s to obtain a finished wire rod.

[0042] Comparative Example 4: 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 43°C / s, the molten salt temperature of the front-stage molten salt treatment is 300°C, and the treatment time is 40s to obtain a finished wire rod. Example 3:

[0043] A preferred embodiment of the method for manufacturing 1500MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.62%, Si: 0.44%, Mn: 0.97%, Cr: 0.76%, Nb: 0.038%, V: 0.055%, 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 supercooling quenching isothermal treatment → roller slow cooling → coiling, specifically: 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, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1220°C, the time in the furnace is 140min, the initial rolling temperature is 1080°C, the final rolling temperature is 950°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 tissue quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 940°C.

[0044] The online molten salt supercooling quenching 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 molten salt treatment, so that the wire rod is cooled at a cooling rate of 45°C / s, passing through the pearlite and bainite phase regions from the high-temperature austenite state and entering the martensite phase region, promoting the transformation of a small part of austenite to martensite structure, retaining most of the austenite residues, and then the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, controlling the wire rod to heat up to the sorbite phase region, and then at the high temperature. The isothermal zone promotes the transformation of austenite residues to sorbite structure, promotes the dispersion and precipitation of Nb and V carbides, and performs isothermal tempering and toughening stress relief treatment on the quenched martensite and sorbite structures to avoid carbide coarsening and improve the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 310°C, the treatment time is 38s, the molten salt circulation rate is 850t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 560°C, the treatment time is 290s, and the molten salt circulation rate is 620t / h.

[0045] The roller slow cooling process adopts the method of closing the insulation cover, inputting the hot air in the online molten salt supercooling quenching isothermal treatment into 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 promoting 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 0.25°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 3 shown.

[0046] Comparative Example 5: A method for manufacturing a wire rod, which differs from the manufacturing method of Example 3 in that the molten salt temperature of the latter molten salt treatment is 595° C., the treatment time is 600 s, and the finished wire rod is obtained.

[0047] Comparative Example 6: A method for manufacturing a wire rod, which differs from the manufacturing method of Example 3 in that the molten salt temperature of the latter molten salt treatment is 500° C., the treatment time is 250 s, and the finished wire rod is obtained. Example 4:

[0048] A preferred embodiment of the method for manufacturing 1500MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.61%, Si: 0.35%, Mn: 0.10%, Cr: 0.80%, Nb: 0.033%, V: 0.064%, P: 0.015%, S: 0.013%, and the remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt supercooling quenching isothermal treatment → roller slow cooling → coiling, specifically: 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, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1205°C, the time in the furnace is 165min, the initial rolling temperature is 1070°C, the final rolling temperature is 945°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 tissue quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 930°C.

[0049] The online molten salt supercooling quenching 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 molten salt treatment, so that the wire rod is cooled at a cooling rate of 43°C / s, passing through the pearlite and bainite phase regions from the high-temperature austenite state and entering the martensite phase region, promoting the transformation of a small part of austenite to martensite structure, retaining most of the austenite residues, and then conveyed by a roller through the second salt bath tank for the rear molten salt treatment, controlling the wire rod to heat up to the sorbite phase region, and then at the high temperature. The isothermal zone promotes the transformation of austenite residues to sorbite structure, promotes the dispersion and precipitation of Nb and V carbides, and performs isothermal tempering and toughening stress relief treatment on the quenched martensite and sorbite structures to avoid carbide coarsening and improve the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 328°C, the treatment time is 35s, the molten salt circulation rate is 720t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 546°C, the treatment time is 375s, and the molten salt circulation rate is 550t / h.

[0050] The roller slow cooling process adopts the method of closing the insulation cover, inputting the hot air in the online molten salt supercooling quenching isothermal treatment into the insulation cover, and conveying the wire rod through the second salt bath tank by the conveyor roller to be slowly cooled through the insulation cover, so as to prevent the wire rod from cooling too fast and causing stress increase during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 256°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 the finished wire rod is obtained after packaging and storage.

[0051] Comparative Example 7: 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 supercooling quenching isothermal treatment → 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 270°C at a cooling rate of 1.7°C / s to obtain a wire rod. The wire rod has a tensile strength of 1537 MPa, a cross-sectional shrinkage rate of 44%, and a mechanical property difference of 53 MPa.

[0052] The structure and performance of the wire rods obtained in Examples 1 to 4 and Comparative Examples 1 to 6 were tested, and the comparative results are shown in Table 1 below: Table 1. Comparison of microstructure and properties of different wire rod compositions and manufacturing methods

[0053] In the above table, the interlamellar spacing of comparative example 1 is the interlamellar spacing of pearlite, and the interlamellar spacing of other examples is the interlamellar spacing of tempered bainite. From the comparison results of Example 1 and Comparative Example 1, it can be seen that C and Cr, as carbide strengthening elements and austenite forming elements, often weaken the strengthening effect due to the uncontrollable cooling rate during the air-cooled line cooling phase transformation process, and increasing the air-cooled strength will further lead to the generation of abnormal tissues such as martensite, increasing the fluctuation of mechanical properties. The present invention combines the Nb-V chemical composition design with the online molten salt supercooling quenching isothermal technology to first control the wire rod to quickly enter the high-temperature austenite state in the front section of the molten salt. The martensite phase region can inhibit the pearlite and bainite phase transformation, making the martensite phase transformation more uniform and controllable. Then, the temperature is raised to the sorbite phase region through the molten salt in the latter stage, promoting the transformation of the austenite residue to the sorbite structure, and the structure is subjected to isothermal tempering toughening and stress relief treatment, which strongly regulates the phase transformation of the wire rod and improves the strength-plasticity matching of the wire rod. It can be seen from the results of Examples 1 to 4 that the present invention can achieve a tensile strength of 1470 to 1520 MPa and a cross-sectional shrinkage rate of 52% to 57%. It is used in the application fields of manufacturing high-strength tool steel, so as to avoid heat treatment after cold working and can be directly processed into parts after cold working. From the comparison results of Example 1 and Comparative Example 2, it can be seen that the use of high-temperature spinning, i.e., a higher quenching temperature, for wire rods can increase the stability of supercooled austenite, prepare for subsequent tissue quenching, and at the same time reduce the restrictions on rolling, select a higher rolling temperature, reduce the wear on the rolling line, and increase the rolling speed.

[0054] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the higher the molten salt temperature and the shorter the treatment time of the front-stage molten salt treatment, the martensite transformation rate can be controlled and the formation of excessive martensite can be suppressed. However, if the molten salt temperature is too high and the treatment time is too short, the target amount of a small amount of martensite cannot be achieved, which will affect the matrix strength.

[0055] From the comparison results of 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-stage molten salt treatment, the higher the martensite transformation rate, matrix strength and tissue stress. However, if the molten salt temperature is too low and the treatment time is too long, the austenite will transform too much into martensite, which will affect the austenite residual amount and subsequent sorbite transformation, and increase the difficulty of stress relief and energy consumption cost.

[0056] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the higher the molten salt temperature and the longer the treatment time of the later molten salt treatment, the more it can promote the full transformation of the austenite residue to the sorbite structure, which is beneficial to the release of residual stress and the significant improvement of the impact toughness of the material. However, if the molten salt temperature is too high and the treatment time is too long, it is easy to over-soften and reduce the strength, and lose the strong and plastic properties due to the coarsening of carbides, while increasing production energy consumption.

[0057] From the comparison results of Example 3 and Comparative Example 6, it can be seen that the lower the molten salt temperature of the later molten salt treatment, the more refined the troostite interlamellar spacing and the dispersed precipitation of microalloy carbides can be promoted. As the treatment time is shortened, the production energy consumption and the risk of carbide coarsening can be reduced. However, if the molten salt temperature is too low and the treatment time is too short, the untransformed austenite residue will cause the material hardness to fluctuate and the toughness to decrease, affecting the stress release of the tissue and the sufficient precipitation of microalloy carbides, which will result in strength loss and obvious tissue brittleness.

[0058] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow cooling of the roller can avoid the increase of stress caused by excessive cooling of the wire rod, promote further toughening of the wire rod structure, and improve the softening effect. The hot air in the isothermal treatment of the molten salt supercooling quenching isothermal treatment is input into the insulation cover, which can recycle the heat energy and appropriately reduce the production energy consumption.

[0059] 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 1500MPa grade high strength tool steel wire rod, characterized in that: The manufacturing method includes: The steel billet is rolled and spun into wire rod at a spinning temperature of ≥915℃, and then is subjected to online molten salt supercooling quenching isothermal treatment. First, the wire rod is controlled to enter the martensite phase region from the high-temperature austenite state at a cooling rate of ≥37℃ / s in the front section of the molten salt, promoting the transformation of part of the austenite to the martensite structure. Then, the temperature is raised to the sorbite phase region in the rear section of the molten salt, promoting the transformation of the austenite residue to the sorbite structure. The structure is subjected to isothermal tempering toughening and stress relief treatment, and finally slowly cooled by rollers to form a microstructure. The fabric includes a wire rod of tempered bainite and tempered martensite, and the chemical composition and mass percentage of the wire rod include: C: 0.61%-0.66%, Si: 0.35%-0.45%, Mn: 0.90%-1.0%, Cr: 0.70%-0.80%, Nb: 0.028%-0.038%, V: 0.055%-0.075%, P≤0.015%, S≤0.015%, and the rest is Fe and unavoidable impurities.

2. The method for manufacturing 1500MPa 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 1180-1220° C., and the soaking time in the furnace is 140-200 minutes.

3. The method for manufacturing 1500MPa 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 1055-1080° C., the final rolling temperature is controlled to be 920-950° C., and the final rolling reduction is controlled to be 24%-30%.

4. The method for manufacturing 1500MPa 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 915-940°C.

5. The method for manufacturing 1500MPa grade high strength tool steel wire rod according to claim 1, characterized in that: The molten salt temperature of the front-stage molten salt treatment is 310-360° C., and the treatment time is 28-38 seconds; the molten salt temperature of the back-stage molten salt treatment is 530-560° C., and the treatment time is 290-590 seconds.

6. The method for manufacturing 1500MPa grade high strength tool steel wire rod according to claim 5, characterized in that: The molten salt circulation rate of the front-stage molten salt treatment is 550~850t / h, and the molten salt temperature rise is ≤8°C; the molten salt circulation rate of the back-stage molten salt treatment is 400~620t / h.

7. The method for manufacturing 1500MPa grade high strength tool steel wire rod according to claim 5, characterized in that: The roller slow cooling controls the wire rod to be slowly cooled to below 260° C. at a cooling rate of 0.2-0.5° C. / s.

8. A 1500MPa grade high strength tool steel wire rod, characterized in that: The wire rod is manufactured by the manufacturing method of 1500MPa grade high-strength tool steel wire rod according to any one of claims 1 to 7.

9. The 1500 MPa grade high strength tool steel wire rod according to claim 8, characterized in that: The volume percentage of the tempered bainite is 75% to 85%, and the interlamellar spacing is 75 to 115 nm; the volume percentage of the tempered martensite is 15% to 25%.

10. The 1500 MPa grade high strength tool steel wire rod according to claim 8, characterized in that: The diameter of the wire rod is 5-10 mm, the tensile strength is 1470-1520 MPa, the cross-sectional shrinkage rate is 52%-57%, and the mechanical property difference within the same circle is ≤38 MPa.

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