A 1450mpa grade high strength tool steel wire rod and a manufacturing method thereof

Abstract

The application relates to a 1450MPa-grade high-strength tool steel wire rod and a manufacturing method thereof. After rolling and wire drawing into a wire rod by adopting a Nb-V chemical composition design, the wire rod is subjected to on-line molten salt phase transformation control quenching isothermal treatment. The wire rod is cooled at a cooling speed of greater than or equal to 36 DEG C / s from a high-temperature austenite state in a front-stage molten salt, partial austenite is promoted to transform into a martensite structure, the wire rod is heated to a sorbite phase zone in a rear-stage molten salt, residual austenite is promoted to transform into a sorbite structure, isothermal tempering and stress relief treatment is performed on the structure, and finally, the wire rod is slowly cooled on a roller bed, the wire rod is prepared, the microstructure of the wire rod comprises isothermal sorbite and tempered martensite, the strength and plasticity matching of the wire rod is improved, the tensile strength is 1410-1460MPa, the reduction of area is 37%-42%, after cold working, additional heat treatment is not needed, the tool steel part is directly processed, the production energy consumption and cost of the high-strength tool steel are reduced, and the production efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the field related to hot-rolled wire rods, and in particular relates to a 1450MPa grade high-strength tool steel wire rod and a manufacturing method thereof. Background Art

[0002] With the rapid development of the automotive industry, new energy and other fields, the market demand for tool steel is on the rise. In order to adapt to the expansion of application environment and application fields, tool steel needs to continuously innovate in hot-rolled wire rod raw materials and production and manufacturing processes. Existing tool steel hot-rolled wire rods are usually produced by Stelmore air-cooling line controlled cooling. Due to the cooling disadvantages of Stelmore air-cooling line equipment, tool steel needs to be continuously adjusted in alloy composition design to meet cold forming requirements. After cold forming, the wire rod is heat treated to release the material potential to improve tool performance. However, additional heat treatment also leads to increased production costs and energy consumption of tool steel and limited production efficiency. Therefore, it is necessary to develop a heat-treatment-free high-strength tool steel wire rod and its manufacturing method to meet the production and market use needs of the steel industry.

[0003] The main reasons why existing tool steel wire rods cannot meet the performance and heat treatment requirements are:

[0004] (1) In order to make the wire rod have good toughness, reduce the risk of brittle fracture, and take into account the hardness and strength of tool steel, alloy tool steel wire rod generally adopts a medium-high carbon and high silicon composition system, combined with the Stelmor air-cooled line to form a pearlite and ferrite structure wire rod. For example: Patent CN110791717B discloses a high-quality hypoeutectoid alloy tool steel wire rod and its production method, which adopts a hypoeutectoid C-Si-Mn-Cr-Mo-V-Ni composition system. After spinning, it is rolled up at high temperature and slowly cooled and kept warm to form a microstructure composed of ferrite + granular pearlite + carbide. However, on the one hand, due to the slow cooling limit of the cooling line, the wire rod slowly passes through the high-temperature pearlite phase region to form a pearlite and ferrite soft phase structure. Excessive ferrite will reduce the strength and hardness. If the ferrite distribution is uneven, it is easy to form weak areas of mechanical properties, resulting in insufficient wear resistance and strength of tool steel, and an imbalance in the matching of strength and toughness. For the production of high-strength tool steel, heat treatment is required to adjust the performance after cold working. Continuing to add alloy content will lead to increased material costs and increased risk of abnormal tissue precipitation. On the other hand, wire rods are soft at high temperatures, and high-temperature coiling can easily cause deformation defects such as loose coiling or collapse, or surface defects due to friction with coiling equipment. At the same time, under long-term processing, carbide particles are too large, irregular in shape or locally aggregated, which can easily become the source of crack initiation, resulting in significant material anisotropy, affecting cold working and tool service performance. Long-term slow cooling and heat preservation will also lead to a long online time of wire rods and low production efficiency.

[0005] (2) In order to make the wire rod have higher hardness or strength, the alloy tool steel wire rod will use medium-high carbon, high silicon and high manganese components, combined with the Stelmor air cooling line to make bainite or martensite structure wire rod. For example, patent CN117051308B discloses a production method of tool steel wire rod, which adopts C-Si-Mn-Cr-Mo-V-Ni-Nb component system, combined with strong air cooling and slow cooling after low-temperature spinning, to make tool steel wire rod with metallographic structure of bainite + martensite. However, on the one hand, due to the maximum cooling capacity limit of the Stelmor air cooling line, the wire rod slowly passes through the upper bainite phase region, which easily causes coarse feather-like upper bainite to appear in the metallographic structure. At the same time, as the air cooling intensity increases, it is affected by the segregation of hardenability components such as Mn. The temperature difference between the windward side and the windward side, the overlap and non-overlap, and the edge to the core of the wire rod will be further increased. The stress field generated by the bainite phase transformation hinders the martensite shear process, which can easily lead to incomplete and uncontrollable growth of the martensite lamellae, uneven carbide precipitation, large fluctuations in organizational uniformity and mechanical properties, and the inhomogeneous organizational zone will become a fracture-sensitive area, affecting the cold working performance; on the other hand, as the wire rod continues to cool down on the Stelmor air cooling line, the wire rod is in a low-temperature state after the phase transformation. The phase transformation stress caused by the upper bainite ferrite lath and the martensite lath is high, and it is easy to form a microcrack source at the interface, resulting in high brittleness and insufficient toughness of the wire rod, which is easy to break during cold working or even coiling and transportation, affecting the yield of wire rod and tool steel. 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 1450MPa grade high-strength tool steel wire rod and a manufacturing method thereof, which can strongly regulate the phase transformation of the wire rod, improve the strength-plasticity matching of the wire rod, promote the rapid production of the wire rod, avoid brittle fracture of the coil, facilitate cold forming without additional heat treatment, and directly process it into tool steel parts, so as to reduce the energy consumption and cost of high-strength tool steel production and improve production efficiency.

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

[0008] A method for manufacturing a 1450MPa grade high-strength tool steel wire rod, the manufacturing method comprising:

[0009] After the billet is rolled and spun into wire rod at a spinning temperature of ≥885℃, it is subjected to online molten salt phase transformation controlled quenching isothermal treatment. First, the wire rod is controlled to cool from the high temperature austenite state at a cooling rate of ≥36℃ / s in the front section of the molten salt to promote 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 to promote 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 through a roller to form a microstructure package. The invention relates to a wire rod comprising isothermal sorbite and tempered martensite, wherein the chemical composition and mass percentage of the wire rod include: C: 0.60%-0.65%, Si: 0.35%-0.45%, Mn: 0.73%-0.80%, Cr: 0.65%-0.74%, Nb: 0.015%-0.025%, V: 0.065%-0.080%, P≤0.015%, S≤0.015%, and the remainder is 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: C is the main hardening element in steel and is relatively cheap. It improves the strength and hardness of steel by solid solution strengthening and forming carbides. As the carbon content increases, it can increase the stability of austenite, reduce the martensite transformation temperature, promote the formation of martensite, aggravate the martensite lattice distortion, solid solution strengthen the martensite matrix, and increase the resistance to dislocation movement. It can cooperate with molten salt treatment to promote the refinement of pearlite lamellae to form sorbite structure, and form alloy carbides with alloy elements such as Cr and V to improve wear resistance and tempering stability, giving tool steel wire rod high strength. However, excessive carbon content will lead to an increase in undissolved carbides, which will be distributed in the form of coarse particles and split the matrix, increase the martensite quenching stress, increase the difficulty of isothermal tempering, and lead to a decrease in toughness. Therefore, in order to meet the strength and wear resistance requirements of tool steel, control material costs, and facilitate organizational control and toughening stress relief treatment to improve strength and toughness matching, the mass percentage of C is controlled to be 0.60%~0.65%.

[0012] (2) Silicon: The Si element can inhibit the coarsening of grains during online molten salt treatment, and at the same time reduce the carbon diffusion coefficient, reduce the interlamellar spacing of the troostite, increase the strength, delay the decomposition of martensite, and keep the strength of the steel at high temperature, which is suitable for the wear resistance requirements of tool steel. However, too high a silicon content will lead to an increase in inclusions, reduce the plasticity and impact toughness of the wire rod, increase the deformation resistance of the tool steel wire rod during cold working, and make it easy to break during cold working. Therefore, in order to adapt to the regulation of the organization of the 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%.

[0013] (3) Manganese: Mn can expand the austenite zone, reduce the pearlite transformation rate, make it easier for steel to form martensite during the front-stage molten salt cooling, improve hardenability, and make the sorbite phase transformation temperature range lower, which is beneficial to improving the sorbite nucleation rate, promoting the transformation of austenite residue to sorbite structure, and improving the strength of wire rod. However, when the Mn content is too high, it will promote the growth of austenite grains, easily lead to grain coarsening when heated at high temperature, and easily lead to uneven austenite composition due to dendrite segregation. The phase transformation structure is uneven or the toughness is significantly reduced, affecting the impact toughness of the wire rod and deteriorating the fatigue resistance of the tool. Therefore, in order to make the C and Mn partitioning promote martensite phase transformation during online molten salt treatment, improve the matrix strength, and take into account both plasticity and structural uniformity, the mass percentage of Mn is controlled to be 0.73%~0.80%.

[0014] (4) Chromium: Cr forms carbides with carbon, which have high hardness and good wear resistance. When evenly distributed, it can refine the grains, improve the wear resistance of tool steel wire rods, enhance the high-temperature service stability of tools, and significantly improve the hardenability of steel, which is beneficial to inhibit ferrite precipitation. Combined with a large degree of undercooling, it promotes the transformation of austenite to martensite and inhibits the formation of bainite, so that the steel maintains high strength and hardness after tempering. However, too high Cr content will aggravate component segregation. Local Cr-rich areas will increase the uniformity of the structure and the difficulty of controlling coarsening carbides, while increasing the difficulty of improving plasticity and reducing toughness. Therefore, in order to take into account the wear resistance requirements of tool steel and facilitate the control of mixed structure and structure stress relief, the mass percentage of Cr is controlled to be 0.65%~0.74%.

[0015] (5) Niobium: Niobium forms extremely fine carbide particles with carbon, which can hinder the growth of austenite grains during heating and rolling. After cooling, a fine-grained structure is obtained, which maintains the deformation strengthening effect, can improve the uniformity of the mechanical properties of the wire rod and increase its strength and toughness. During the isothermal tempering process, NbC precipitates, which can produce a dispersion strengthening effect and improve the wear resistance of steel. 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 0.015%~0.025%.

[0016] (6) Vanadium: The V element forms high-melting-point carbides with carbon to pin the grain boundaries, which can prevent grain growth and promote the uniform formation of lamellar troostite. At the same time, fine dispersed carbides are precipitated during the tempering process to produce secondary hardening. The refined grains and dispersed VC particles improve the strength and toughness of the steel, reduce the brittle transition temperature, and are beneficial to inhibit the initiation of fatigue cracks and improve 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 wire rods. 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 0.065%~0.080%.

[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 adopts the C-Si-Mn-Cr-Nb-V composition design, with relatively low carbon and silicon content, trace addition of Nb and V, and combined with Mn and Cr to control the hardenability of the wire rod, taking into account the wear resistance of the tool steel, and reducing the martensite transformation temperature, so as to promote the formation of martensite, increase lattice distortion, control the residual austenite, delay the transformation of pearlite, promote the formation of fine sorbite, and improve the strength and toughness of martensite in the previous molten salt treatment, thereby providing favorable conditions for balancing the strength, toughness and wear resistance of the wire rod. On this basis, a higher spinning temperature, i.e., quenching temperature, is adopted to make carbon and alloy elements evenly distributed in the austenite, avoiding the preferential precipitation of proeutectoid ferrite at the grain boundary of the austenite before cooling, or the formation of coarse pearlite, which reduces the strength and hardness of the steel, and makes organizational preparations for the subsequent direct transformation of austenite to martensite. The wire rod after spinning is not air-cooled or high-temperature coiled, but directly undergoes online molten salt phase transformation controlled quenching isothermal:

[0019] 1. Compared with the limitation of the cooling capacity of the Stelmor air-cooling line, which makes it impossible to suppress the formation of ferrite, coarse pearlite and bainite, the wire rod can be quickly cooled down when passing through the front molten salt. On the one hand, the high heat exchange capacity of the molten salt is used to significantly improve the cooling rate of the wire rod, so that the wire rod can quickly pass through the pearlite phase region from the high-temperature austenite state without falling into the bainite, which can inhibit the formation of ferrite, coarse pearlite and bainite. Combined with a large degree of undercooling, a small amount of austenite is promoted to quickly transform into martensite after short quenching, thereby improving the matrix strength to make up for the strength loss caused by reducing the alloy content; 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 non-overlap, which can promote uniform phase transformation of the wire rod, make the martensitic phase transformation more controllable and uniform, and do not need to use high-temperature coiling, which can avoid coiling deformation and surface defects caused by high-temperature coiling.

[0020] 2. Compared with the limitation of continuous cooling and slow cooling capacity of Stelmor air cooling line, which makes it impossible to control martensite and sorbite phase transformation and stress release, the wire rod can be heated to the sorbite phase region after passing through the molten salt in the latter stage. On the one hand, the temperature of the sorbite phase region is lower than that of the pearlite high temperature phase region, which can promote the refinement of pearlite lamellae to form a sorbite structure with better toughness, and avoid the loss of wire rod toughness due to coarse pearlite. As the processing time increases, the temperature of the wire rod and the molten salt tends to be consistent, which can extend the isothermal phase transformation time, promote the full transformation of austenite residue to sorbite structure, avoid the formation of low-temperature brittle structure of austenite residue in the subsequent cooling process, and improve the uniformity of the structure. On the other hand, the molten salt temperature of the latter stage is higher than that of the former stage, which can control the wire rod to perform isothermal tempering of martensite and sorbite structure in the high temperature isothermal range, toughening and stress relief treatment, and effectively improve Martensite brittleness can not only avoid the risk of cold working cracking or brittle fracture caused by martensite brittleness, but also improve the plasticity and toughness of wire rod. At the same time, it also prolongs the time in the temperature range of dispersion precipitation of niobium and vanadium carbides, which can promote the full precipitation of carbides in the tempered martensite and isothermal troostite matrix, play a strengthening and toughening role, so that the stress release process does not lose strength characteristics excessively. Compared with air cooling control, it can avoid the loss of strength and toughness and fluctuation of mechanical properties caused by uneven coarseness of carbides. Slow cooling and heat preservation control under high temperature coiling can promote the rapid processing of wire rod off the line. The wire rod after leaving the molten salt is kept at a high temperature. The wire rod is slowly cooled by roller cooling, which can prevent the stress increase of the wire rod due to excessive cooling rate during the cooling process. At the same time, it continues the treatment effect of the molten salt in the later stage, promotes further toughening of the wire rod structure, and realizes the matching of structure regulation and strength and plasticity of the wire rod.

[0021] Before the rolling, appropriate heating furnace soaking temperature and furnace time are selected to enable sufficient diffusion of alloy elements in the austenite, promote the homogenization of the billet composition and the consistency of the temperature inside and outside the billet, reduce the segregation effect of the casting process, avoid uneven performance due to local composition differences during subsequent processing, make the austenite grains refined and uniform, and provide a basis for subsequent rolling. In the preferred technical solution, before the rolling, the heating furnace soaking temperature is controlled to be 1160~1190℃, and the furnace time is 120~180min.

[0022] Since the spinning temperature is relatively high, low-temperature rolling is not necessary. A higher initial rolling temperature can be selected to reduce the deformation resistance of the steel billet, increase the rolling speed and efficiency, reduce the load demand on the rolling line, ensure that the entire rolling process is carried out in the austenite region, and trigger dynamic recrystallization through initial deformation, inducing Nb to precipitate, pin the grain boundaries, and refine the grains at different rolling stages. The final rolling temperature and final rolling reduction are controlled to promote sufficient dynamic recrystallization of austenite, improve material density, and eliminate internal defects. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1010~1050℃, the final rolling temperature is controlled to be 900~930℃, and the final rolling reduction is controlled to be 25%~31%.

[0023] During the spinning process, the spinning temperature can be further controlled to promote the homogenization of austenite and avoid excessively high spinning temperature, which causes the austenite grains to stay in the high temperature zone for too long and thus coarsen the austenite grains. In a preferred technical solution, during the spinning process, the spinning temperature is controlled to be 885-910°C.

[0024] The molten salt temperature of the front-stage molten salt treatment is below the sorbite phase region. The lower the molten salt temperature and the longer the treatment time, the greater the supercooling degree. The carbon atoms in the austenite will diffuse appropriately, which will reduce the local stability of the austenite and enhance the driving force of the martensite transformation. This can promote a small amount of austenite to rapidly transform into martensite and improve the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, the martensite transformation is more and the austenite residual amount is reduced, which will affect the subsequent sorbite phase transformation and increase the difficulty of toughening and stress relief. If the molten salt temperature is too low and enters the bainite phase region, the driving force of the bainite phase transformation will be enhanced, and the diffusion-type phase transformation will preferentially consume austenite, which will affect the martensite phase transformation. On the contrary, the molten salt The higher the temperature and the shorter the treatment time, the lower the martensitic phase transformation stress and production energy consumption can be, and the difficulty of subsequent softening can be reduced. However, if the molten salt temperature is too high and the treatment time is too short, the carbon diffusion is insufficient, the overall stability of the austenite is improved, and the amount of martensite transformation during cooling will be less, which will affect the matrix strength. Therefore, the molten salt temperature and treatment time of the front-stage molten salt treatment can be controlled to control the rapid phase transformation of the wire rod from the high-temperature austenite state. Through short-time treatment, a small amount of austenite is promoted to transform into martensite, and organizational preparation is made for the subsequent rear-stage molten salt treatment. In the preferred technical solution, the molten salt temperature of the front-stage molten salt treatment is 420~470℃, and the treatment time is 20~35s.

[0025] The spinning temperature is quite different from the molten salt temperature of the previous molten salt treatment. A larger molten salt circulation volume can allow the molten salt to quickly take away the heat of the wire rod, reduce the temperature rise of the molten salt, and reduce the difference between different positions in the same circle of the wire rod. In the preferred technical solution, the molten salt circulation volume of the previous molten salt treatment is 520~620t / h, and the molten salt temperature rise is ≤8°C.

[0026] The latter stage molten salt treatment is in the sorbite phase region. The lower the molten salt temperature, the more suitable thermodynamic conditions are provided for the transformation of austenite residues to sorbite structure, which is conducive to the formation of fine sorbite structure and avoids coarse pearlite. At the same time, it is conducive to the combination of alloying elements such as Nb and V with carbon to form carbides and precipitate in the form of fine dispersion, thereby improving the strength and toughness of the matrix. As the treatment time is shortened, it is conducive to reducing production energy consumption and avoiding the precipitation and coarsening of carbides. However, if the molten salt temperature is too low, the treatment time is too short, the supercooling is insufficient, and the transformation speed is slow, it will affect the full transformation of the sorbite structure, and the vanadium-containing carbides will not have time to fully precipitate and disperse. The precipitation strengthening effect is not good, and the strength and toughness will be lost. At the same time, it is difficult to provide more thermal power for tempering, and the stress release of the structure is insufficient, which will bring about a large loss of plasticity. On the contrary, the higher the molten salt temperature and the longer the treatment time, the more effective it is to reduce the internal stress in the martensite, reduce brittleness, and improve Toughness, promote the transition of troostite lamellae to carbide spheroidization for toughening, improve the plasticity of the matrix, sufficient treatment time is conducive to the uniform dispersion of Nb and V carbides in the matrix, and the uniformly dispersed carbides can more effectively hinder dislocation movement. However, if the molten salt temperature is too high and the treatment time is too long, the carbides will aggregate and grow, and the dispersion strengthening effect will be weakened, which cannot effectively improve the strength and hardness of the material. Excessive softening of the wire rod is not conducive to strength, and will lose strong and plastic properties, while increasing production energy consumption. Therefore, the molten salt temperature and treatment time of the later molten salt treatment can be further controlled, and the wire rod can be controlled to isothermal temper the organization in the high temperature isothermal range to promote the transformation of austenite residue into troostite, and then toughen and stress relieve the martensite to avoid carbide coarsening and improve the strength and plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the later molten salt treatment is 565~590℃, and the treatment time is 350~500s.

[0027] The temperature difference between the wire rod after the front-stage molten salt treatment and the rear-stage molten salt is small. Appropriately reducing the molten salt circulation volume can control the molten salt temperature rise, which not only meets the heat exchange requirements but also avoids energy waste. In the preferred technical solution, the molten salt circulation volume of the rear-stage molten salt treatment is 220~320t / h.

[0028] Since the wire rod undergoes online molten salt phase transformation controlled quenching isothermal treatment, the structure is fully transformed, which can avoid the formation of abnormal structure during the subsequent cooling process due to austenite retention. However, considering that excessive cooling may cause physical shrinkage stress in the wire rod, roller slow cooling treatment is selected to avoid excessive cooling and increase stress, making the structure more uniform. At the same time, the temperature of the wire rod is relatively high after leaving the molten salt. Properly controlling the cooling rate of the wire rod can promote further toughening of the structure and improve the softening effect of the wire rod. In the preferred technical solution, the roller slow cooling controls the wire rod to slowly cool to below 280°C at a cooling rate of 0.4~0.7°C / s.

[0029] A 1450MPa grade high-strength tool steel wire rod is manufactured by any one of the above-mentioned methods for manufacturing a 1450MPa grade high-strength tool steel wire rod.

[0030] The above-mentioned wire rod adopts Nb-V chemical composition design combined with online molten salt phase transformation control quenching isothermal technology. The carbon and silicon contents are relatively low, and niobium and vanadium are added in trace amounts, which can appropriately control the material cost. At the same time, the microstructure includes a mixed structure of a small amount of tempered martensite and a large amount of isothermal sorbite. Compared with the air-cooled wire tool steel pearlite + ferrite wire rod, the interlamellar spacing of sorbite is finer than that of pearlite, and the fine grain strengthening and stacking fault strengthening effects are more significant. By combining a small amount of tempered martensite, the matrix strength can be further improved to make up for the strength loss caused by reducing the alloy content and effectively resist wear. At the same time, the internal stress is fully eliminated through isothermal tempering and transformed into isothermal sorbite. The uniform lamellar structure reduces the stress concentration point, and the toughness is better than that of coarse pearlite, avoiding the brittle fracture of the coarse lamellar layer of air-cooled pearlite due to the obstruction of slip, and at the same time avoiding It avoids local weak strength areas and uneven deformation caused by blocky distribution of ferrite, avoids carbide coarsening, and makes it less likely to produce microcracks during cold working; compared with air-cooled wire tool steel, the wire rod containing martensite structure can inhibit the transformation of bainite structure, and the strength of martensite is higher than that of bainite. A small amount of martensite phase change becomes uniform and controllable. After high-temperature tempering, the brittleness is reduced and it is transformed into tempered martensite. The strength is further improved through dispersion strengthening and its distribution is uniform. Nb and V carbides can be dispersed and precipitated during the isothermal process of molten salt, and the strength, surface hardness and wear resistance are further improved through precipitation strengthening, which reduces the initiation and expansion of fatigue cracks. Combined with the isothermal bainite structure, the overall impact toughness and elongation of the wire rod are higher, and it is not easy to break brittlely during coiling, transportation, cold working and use. It is especially suitable for tool steels that withstand impact loads.

[0031] In the microstructure, the higher the volume percentage of the isothermal troostite and the finer the interlamellar spacing, the better the plasticity and toughness. In the preferred technical solution, the volume percentage of the isothermal troostite is 65%~75%, and the interlamellar spacing is 80~115nm.

[0032] In the microstructure, the higher the proportion of the tempered martensite is, the higher the matrix strength is. In a preferred technical solution, the volume percentage of the tempered martensite is 25% to 35%.

[0033] In the preferred technical solution, the diameter of the wire rod is 6~11mm, the tensile strength is 1410~1460MPa, the cross-sectional shrinkage rate is 37%~42%, the mechanical property same-circle difference is ≤37MPa, and the wire rod specifications can be processed through drawing, cold heading and other cold processing to manufacture high-strength tool steels such as cutting tools, drills, and wrenches. It has good processing adaptability and material utilization. The wire rod has good plasticity and toughness, and smaller fluctuations in mechanical properties, which can avoid brittle fracture during cold processing, improve the processing dimensional accuracy of tool steel and service toughness under impact loads. The wire rod has high strength, which can enable tool steel to withstand complex stresses such as cutting and impact, and avoid deformation during service. Furthermore, the high strength and high plasticity can avoid additional heat treatment after cold processing and can be directly processed into parts.

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

[0035] (1) In view of the current situation that hot-rolled wire rods of tool steel are usually produced by Stelmor air-cooled line controlled cooling, which leads to high alloy content, difficult to control abnormal structure, and insufficient strength or plasticity, the present invention combines the Nb-V chemical composition design with the online molten salt phase transformation control quenching isothermal technology to first control the wire rod to cool down from the high-temperature austenite state in the front section of the molten salt, promote the transformation of part of the austenite to the martensite structure, inhibit ferrite and coarse pearlite, avoid entering the bainite phase region to form a brittle phase, and then heat up to the bainite phase region in the back section of the molten salt to promote the austenite The residue is transformed into sorbite structure, and the structure is subjected to isothermal tempering toughening and stress relief treatment. Finally, it is slowly cooled through rollers to prevent the wire rod from cooling too fast during the cooling process, which will cause stress increase, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. It can avoid coiling deformation and surface defects caused by high-temperature coiling, and avoid carbide coarsening. By strongly controlling the phase transformation and organizational state of the wire rod, the strength and plasticity matching of the wire rod is improved, and the risk of brittle fracture caused by martensite during coiling and transportation is avoided, thereby improving production efficiency and yield rate, and having good industrial adaptability.

[0036] (2) In view of the fact that the existing tool steel wire rod needs to be heat treated after cold working to release the material potential in order to improve the tool performance, there is a lack of development of heat treatment-free tool steel. The carbon and silicon content is relatively low, and a trace amount of niobium and vanadium is added to appropriately control the material cost. At the same time, the microstructure includes a mixed structure of tempered martensite and isothermal troostite, which can avoid the formation of pearlite and ferrite soft phase structure. Combined with the dispersion precipitation strengthening of Nb and V carbides, it can make up for the strength loss caused by reducing the alloy content and effectively resist wear. The brittleness of martensite is reduced after high-temperature tempering, and the strength-plasticity matching of the wire rod is improved with the isothermal troostite structure, which can achieve a tensile strength of 1410~1460MPa and a cross-sectional shrinkage rate of 37%~42%. It is used in the manufacture of high-strength tool steel and other application fields, so as to avoid additional heat treatment after cold working. After cold working, it can be directly processed into parts, reducing production energy consumption and cost and improving 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 is a metallographic structure diagram of Example 2 of the present invention;

[0040] Figure 3 This is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0041] 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:

[0042] A preferred embodiment of the method for manufacturing 1450MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.65%, Si: 0.39%, Mn: 0.77%, Cr: 0.66%, Nb: 0.019%, V: 0.080%, 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 phase transformation controlled quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0043] 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, promote the homogenization of alloy composition, and the heating furnace is controlled 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 1180°C, the time in the furnace is 145min, the initial rolling temperature is 1045°C, the final rolling temperature is 925°C, and the final rolling reduction is 27%; 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 905°C.

[0044] The online molten salt phase change control 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 39°C / s, from the high-temperature austenite state to below the sorbite phase region, and a large degree of supercooling is used to promote the transformation of a small amount of austenite to martensite structure, forming a quenching structure mainly composed of austenite residues. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, and the wire rod is controlled to heat up to the sorbite phase region. The warm 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 435°C, the treatment time is 32s, the molten salt circulation rate is 590t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 584°C, the treatment time is 375s, and the molten salt circulation rate is 290t / h.

[0045] The roller slow cooling process adopts the method of controlling the opening of 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 be slowly cooled to 273°C at a cooling rate of 0.6°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.

[0046] Comparative Example 1:

[0047] 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 1120°C, the furnace time is 195 minutes, the initial rolling temperature is 995°C, the final rolling temperature is 860°C, and the spinning temperature is controlled to 830°C. The Stelmor air cooling line uses the front fans No. 1 to No. 6 to turn on, and controls the wire rod to cool to 689°C at a rate of 3.4°C / s. Then, the insulation cover is closed, the wire rod enters the insulation cover and is cooled to 270°C at a rate of 1.8°C / s, and is collected by the coiling drum to obtain a finished wire rod.

[0048] Comparative Example 2:

[0049] A method for manufacturing a wire rod, which differs from Example 1 in that: the soaking temperature of the heating furnace is controlled to 1140°C, the time in the furnace is 185 minutes, the initial rolling temperature is 1000°C, the final rolling temperature is 880°C, the spinning temperature is controlled to be 845°C, the wire rod is treated with molten salt in the front section and cooled at a cooling rate of 34°C / s to obtain a finished wire rod. Example 2:

[0050] A preferred embodiment of the method for manufacturing 1450MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.62%, Si: 0.35%, Mn: 0.75%, Cr: 0.65%, Nb: 0.019%, V: 0.069%, 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 phase transformation controlled quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0051] 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, promote the homogenization of alloy composition, and the heating furnace is controlled 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, 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 180min, the initial rolling temperature is 1010°C, the final rolling temperature is 900°C, and the final rolling reduction is 31%; 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 885°C.

[0052] The online molten salt phase change control 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 36°C / s, from the high-temperature austenite state to below the sorbite phase region, and a large degree of supercooling is used to promote the transformation of a small part of austenite to martensite structure, forming a quenching structure mainly composed of austenite residues. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, and the wire rod is controlled to heat up to the sorbite phase region. The warm 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 469°C, the treatment time is 20s, the molten salt circulation rate is 520t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 566°C, the treatment time is 500s, and the molten salt circulation rate is 220t / h.

[0053] The roller slow cooling process adopts the method of controlling the opening of 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 277°C at a cooling rate of 0.4°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.

[0054] Comparative Example 3:

[0055] 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 32°C / s, the molten salt temperature of the front-stage molten salt treatment is 485°C, and the treatment time is 14s to obtain a finished wire rod.

[0056] Comparative Example 4:

[0057] 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 39°C / s, the molten salt temperature of the front-stage molten salt treatment is 415°C, and the treatment time is 35s to obtain a finished wire rod. Example 3:

[0058] A preferred embodiment of the method for manufacturing 1450MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.60%, Si: 0.45%, Mn: 0.80%, Cr: 0.73%, Nb: 0.015%, V: 0.072%, 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 phase transformation controlled quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0059] 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, promote the homogenization of alloy composition, and the heating furnace is controlled 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 11mm 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 1190°C, the time in the furnace is 120min, the initial rolling temperature is 1050°C, the final rolling temperature is 930°C, and the final rolling reduction is 25%; 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 910°C.

[0060] The online molten salt phase change control 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, from the high-temperature austenite state to below the sorbite phase region, and a large degree of supercooling is used to promote the transformation of a small part of austenite to martensite structure, forming a quenching structure mainly composed of austenite residues. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, and the wire rod is controlled to heat up to the sorbite phase region. The warm isothermal zone promotes the transformation of austenite residue 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 molten salt treatment is 420℃, the treatment time is 35s, the molten salt circulation rate is 620t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the rear molten salt treatment is 590℃, the treatment time is 350s, and the molten salt circulation rate is 320t / h.

[0061] The roller slow cooling process adopts the method of controlling the opening of 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, thereby increasing the stress, promoting further toughening of the wire rod structure, and improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 271°C at a cooling rate of 0.7°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.

[0062] Comparative Example 5:

[0063] 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 510 s, and the finished wire rod is obtained.

[0064] Comparative Example 6:

[0065] 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 520° C., the treatment time is 150 s, and the finished wire rod is obtained. Example 4:

[0066] A preferred embodiment of the method for manufacturing the 1450MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.62%, Si: 0.35%, Mn: 0.73%, Cr: 0.74%, Nb: 0.025%, V: 0.065%, 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 phase transformation controlled quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0067] 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, promote the homogenization of alloy composition, and the heating furnace is controlled 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, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1175°C, the time in the furnace is 165min, the initial rolling temperature is 1025°C, the final rolling temperature is 910°C, and the final rolling reduction is 29%; 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 895°C.

[0068] The online molten salt phase change control 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, from the high-temperature austenite state to below the sorbite phase region, and a large degree of supercooling is used to promote the transformation of a small part of austenite to martensite structure, forming a quenching structure mainly composed of austenite residues. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, and the wire rod is controlled to heat up to the sorbite phase region. The warm 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 453°C, the treatment time is 28s, the molten salt circulation rate is 545t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 572°C, the treatment time is 435s, and the molten salt circulation rate is 265t / h.

[0069] The roller slow cooling process adopts the method of controlling 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, thereby increasing the stress, promoting further toughening of the wire rod structure, and improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 274°C at a cooling rate of 0.55°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.

[0070] Comparative Example 7:

[0071] A method for manufacturing a wire rod, which differs from that of Example 4 in that: the manufacturing method is manufactured according to a process flow of rolling → spinning → online molten salt phase change controlled 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 276°C at a cooling rate of 2.1°C / s to obtain a wire rod. The wire rod has a tensile strength of 1441 MPa, a cross-sectional shrinkage rate of 33%, and a mechanical property difference of 48 MPa.

[0072] 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:

[0073] Table 1. Comparison of microstructure and properties of different wire rod compositions and manufacturing methods

[0074]

[0075] In the above table, the lamellar spacing of comparative example 1 is the pearlite lamellar spacing, and the lamellar spacing of other examples is the isothermal troostite lamellar spacing; from the comparison results of Example 1 and Comparative Example 1, it can be seen that due to the slow cooling limitation of the Stelmor air-cooled line equipment, the wire rod slowly passes through the high-temperature pearlite phase region to form pearlite and ferrite soft phase structures, resulting in insufficient wear resistance and strength of the wire rod, the presence of tissue stress, and poor plasticity and tissue uniformity. The present invention adopts Nb-V chemical composition design, with relatively low carbon and silicon contents, and trace additions of niobium and vanadium, which can appropriately control material costs. A small amount of martensite phase change becomes uniform and controllable, and the microstructure includes a mixed structure of a small amount of tempered martensite and a large amount of isothermal troostite, which can avoid the formation of pearlite and ferrite soft phase structures, and cooperate with the dispersion precipitation strengthening of Nb and V carbides to reduce the fluctuation of mechanical properties and improve the strength-plasticity matching of the wire rod. It can be seen from Examples 1 to 4 that the wire rod can achieve a tensile strength of 1410 to 1460 MPa and a cross-sectional reduction rate of 37% to 42%. It is used in application fields such as the manufacture of high-strength tool steel, so that additional heat treatment can be eliminated after cold forming and it can be directly processed into parts.

[0076] From the comparison results of Example 1 and Comparative Example 2, it can be seen that the use of a higher spinning temperature, i.e., the quenching temperature, can prepare the structure for the subsequent direct transformation of austenite to martensite. At the same time, there is no need to use low-temperature rolling. A higher initial rolling temperature can be selected to reduce the deformation resistance of the steel billet and improve the rolling speed and efficiency.

[0077] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the molten salt temperature of the front-stage molten salt treatment is below the troostite phase region. The higher the molten salt temperature and the shorter the treatment time, the martensitic phase transformation stress and production energy consumption can be reduced, and the subsequent softening difficulty can be reduced. However, if the molten salt temperature is too high and the treatment time is too short, the carbon diffusion is insufficient, and the amount of martensitic transformation during cooling will be less, which will affect the matrix strength.

[0078] 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 treatment and the longer the treatment time, the greater the supercooling, which can promote the rapid transformation of a small amount of austenite to martensite and improve the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, the martensite transformation is more and the austenite residual amount is reduced, which will affect the subsequent troostite phase transformation and increase the difficulty of toughening and stress relief.

[0079] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the latter molten salt treatment is in the sorbite phase region. The higher the molten salt temperature and the longer the treatment time, the more effective it is to reduce the internal stress in the martensite, reduce brittleness, and improve toughness, and promote the transition of the sorbite lamellae to carbide spheroidization for toughening. Sufficient treatment time helps to evenly disperse Nb and V carbides in the matrix and improve the plasticity of the matrix. However, if the molten salt temperature is too high and the treatment time is too long, the carbides will aggregate and grow, the dispersion strengthening effect will be weakened, and excessive softening of the wire rod will be detrimental to the strength, which will result in loss of strong and plastic properties, and at the same time increase production energy consumption.

[0080] 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 conducive it is to the formation of a fine sorbite structure and the avoidance of coarse pearlite. At the same time, it is conducive to the combination of alloying elements such as Nb and V with carbon to form carbides and precipitate in the form of fine dispersions. However, if the molten salt temperature is too low and the treatment time is too short, the sorbite transformation rate is slow, the vanadium-containing carbides do not have time to fully precipitate and disperse, the stress release of the structure is insufficient, and a large plastic loss will be caused.

[0081] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow cooling by rollers can avoid stress increase caused by excessive cooling. Proper control of the cooling speed of the wire rod can promote further toughening of the structure and improve the softening effect of the wire rod.

[0082] 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 1450MPa 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 ≥885°C, and then subjected to an online molten salt phase transformation controlled quenching isothermal treatment. The wire rod is first controlled to cool from a high-temperature austenite state at a cooling rate of ≥36°C / s in the front section of the molten salt to promote the transformation of part of the austenite to the martensite structure. The wire rod is then heated to the sorbite phase region in the rear section of the molten salt to promote 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 a roller to produce a wire rod with a microstructure including isothermal sorbite and tempered martensite. The chemical composition and mass percentage of the wire rod include: C: 0.60%~0.65%, Si: 0.35 %~0.45%, Mn: 0.73%~0.80%, Cr: 0.65%~0.74%, Nb: 0.015%~0.025%, V: 0.065%~0.080%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; the molten salt temperature of the front-stage molten salt treatment is 420~470℃, and the treatment time is 20~35s, and the molten salt temperature of the back-stage molten salt treatment is 565~590℃, and the treatment time is 350~500s; the roller slow cooling controls the wire rod to slowly cool to below 280℃ at a cooling rate of 0.4~0.7℃ / s.

2. The method for manufacturing 1450MPa 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 1160-1190° C., and the soaking time in the furnace is 120-180 minutes.

3. The method for manufacturing 1450 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 1010-1050° C., the final rolling temperature is controlled to be 900-930° C., and the final rolling reduction is controlled to be 25%-31%.

4. The method for manufacturing 1450 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 885-910°C.

5. The method for manufacturing 1450 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 treatment is 520~620t / h, and the molten salt temperature rise is ≤8°C; the molten salt circulation rate of the back-stage molten salt treatment is 220~320t / h.

6. A 1450MPa grade high strength tool steel wire rod, characterized in that: The wire rod is manufactured by the manufacturing method of 1450MPa grade high-strength tool steel wire rod according to any one of claims 1 to 5.

7. The 1450 MPa grade high strength tool steel wire rod according to claim 6, characterized in that: The volume percentage of the isothermal troostite is 65% to 75%, and the interlamellar spacing is 80 to 115 nm; the volume percentage of the tempered martensite is 25% to 35%.

8. The 1450 MPa grade high strength tool steel wire rod according to claim 6, characterized in that: The diameter of the wire rod is 6-11 mm, the tensile strength is 1410-1460 MPa, the cross-sectional shrinkage rate is 37%-42%, and the mechanical property difference within the same circle is ≤37 MPa.

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