A 1200MPa grade high-strength carbon tool steel wire rod and its manufacturing method

Abstract

The invention relates to a 1200MPa-grade high-strength carbon tool steel wire rod and a manufacturing method thereof. The chemical composition and mass percentage of the wire rod include: C: 0.63%-0.67%, Si: 0.30%-0.40%, Mn: 0.57%-0.67%, P≤0.015%, S≤0.015%, and the remainder is Fe and inevitable impurities. After being rolled and spun into wire rods, the wire rods are subjected to an online molten salt low-temperature quenching isothermal treatment to control the wire rods from a high-temperature austenite state to enter a sorbite phase region to form a structure dominated by sorbite with fine lamellar spacing, and are isothermally tempered. Finally, the wire rods are slowly cooled on a roller to form wire rods with a microstructure comprising tempered sorbite, ferrite, and fused sorbite. The wire rods can reduce material costs, improve the overall strength and plasticity of the wire rods, achieve a tensile strength of 1130-1180MPa, and a cross-sectional reduction rate of 38%-43%, so that they can be directly processed into tool parts after cold forming.

Description

Technical Field

[0001] The present invention belongs to the technical field related to hot-rolled wire rods, and specifically relates to a 1200MPa grade high-strength carbon tool steel wire rod and a manufacturing method thereof. Background Art

[0002] The rapid development of high-end equipment manufacturing, precision machining, and aerospace has placed higher demands on the strength, wear resistance, and fatigue performance of carbon tool steel. Traditional carbon tool steels like T8 and T10 typically have tensile strengths below 1000 MPa, making them difficult to meet the demands of extreme working conditions. The development of high-strength carbon tool steel wire rod has become an urgent need in the industry.

[0003] Carbon tool steels such as T8 and T10 are hypereutectoid steels with a high carbon content and are produced using a Stelmor air-cooled line. To improve the strength and plasticity of the wire rod, the carbon content is increased or alloying elements such as Cr and V are added to the steel. For example, patent CN110791717B discloses a high-quality hypoeutectoid alloy tool steel wire and its production method, which uses a C-Si-Mn-Cr-Mo-V-Ni composition design, combined with high-temperature coiling and long-term heat preservation to produce a wire composed of ferrite, granular pearlite, and carbides. However, increasing the Cr and V content can easily lead to the formation of brittle structures such as martensite and increase alloy costs. Omitting alloying elements and reducing alloy content to reduce material costs and the risk of brittle structures can lead to the following technical problems:

[0004] 1. After eliminating strong carbide-forming elements such as V and Ni, the austenite grains are more likely to grow during heating and rolling. The ferrite precipitated during the subsequent holding process will also appear coarse and flaky due to the coarse austenite grains, resulting in coarsening of the structure and a simultaneous decrease in strength and toughness. Refining the grains through low-temperature and high-reduction rolling will increase the deformation resistance of the rolled piece, aggravate the wear of the rolling line, and affect the rolling efficiency.

[0005] 2. After eliminating hardenability elements such as Cr and Mo, the critical temperature of pearlite transformation increases and the transformation rate slows down. Under the same cooling conditions, the time for pearlite to start transformation is prolonged. Limited by the maximum cooling capacity of the Stelmor air-cooling line, ferrite is prone to preferential precipitation and coarsening along the austenite grain boundaries during the phase transformation, and it is easier to generate pearlite structure with a larger interlamellar spacing, resulting in insufficient carbon supply for sorbite transformation in subsequent cooling, and a decrease in the proportion of sorbite in the structure, which will bring about a large strength loss and affect the wear resistance and toughness of the wire rod. After cold forming, the blank needs to be strengthened by heat treatment to meet the use requirements. Additional heat treatment leads to increased energy consumption and cost of tool steel product production and reduced efficiency, and increasing the C content is likely to lead to the precipitation of network carbides, which deteriorates the cold working performance.

[0006] 3. Due to the decrease in hardenability of wire rod and the acceleration of carbon diffusion rate, in order to refine the organization and improve the air cooling strength, due to the unstable control of wind temperature and air volume of the air cooling line, the cooling rate difference between different positions of the wire rod, such as the winded surface and the winded surface, the overlap and non-overlap, the surface and the core, will be amplified. The surface or local area will preferentially form fine pearlite due to the higher cooling rate, while the winded surface or the core will have a slower cooling rate, and ferrite will easily precipitate along the grain boundary to form coarsened pearlite, making it difficult to form a uniform organization, resulting in large fluctuations in the mechanical properties of the material, forming a gradient distribution of surface tensile stress and core compressive stress, and high organizational stress, which becomes a hidden danger of cracking in subsequent processing and affects the fatigue performance of tool steel. Long-term heat preservation treatment is limited by the length and heat preservation capacity of the air cooling line, and has limited control over organizational plasticity, and significantly affects the line speed and production efficiency. Summary of the Invention

[0007] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 1200MPa grade high-strength carbon tool steel wire rod and a manufacturing method thereof, which can reduce material costs, improve the overall strength and plasticity of the wire rod and production efficiency, so that it can be directly processed into tool parts after cold forming to meet working conditions.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] A method for manufacturing a 1200MPa grade high-strength carbon tool steel wire rod, the manufacturing method comprising:

[0010] The steel billet is rolled and spun into a wire rod at a spinning temperature of ≥860°C, and then subjected to an online molten salt low-temperature quenching isothermal treatment. The wire rod is controlled to enter the sorbite phase region from the high-temperature austenite state at a cooling rate of ≥38°C / s to form a structure mainly composed of sorbite. The wire rod is controlled to be isothermal tempered in the isothermal range to promote the melting of the sorbite lamellar layer and toughening and stress relief treatment. Finally, it is slowly cooled by a roller to form a wire rod with a microstructure including tempered sorbite, ferrite and fused sorbite. The chemical composition and mass percentage of the wire rod include: C: 0.63%~0.67%, Si: 0.30%~0.40%, Mn: 0.57%~0.67%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities.

[0011] The chemical composition and mass percentage of the above-mentioned wire rod are designed based on the following:

[0012] (1) Carbon: As an effective carbide strengthening element and an element that expands the austenite zone, C is relatively cheap. It can improve the hardness of the wire rod through solid solution strengthening and meet the wear resistance requirements of tool steel. As the carbon content increases, it can reduce the self-diffusion activation energy of iron atoms, causing the continuous cooling transformation curve to move to the lower right, delaying the nucleation and growth of pearlite, and allowing the sorbite phase transformation to occur at a lower temperature. Under low-temperature quenching, the spacing between sorbite lamellae is thinned and the density of lamellae interfaces is increased, thereby significantly hindering the dislocation movement by the lamellae interfaces, providing basic strength. However, too high a carbon content will increase the decarburization sensitivity and segregation. The cementite lamellae are coarsened and unevenly distributed, easily forming network carbides, resulting in a decrease in microstructure uniformity and toughness. At the same time, it increases the difficulty of lamellae melting in subsequent isothermal treatment, affects the processing time, and reduces the plasticity of the steel. Therefore, in order to meet the strength and durability requirements of tool steel, avoid the cost increase caused by complex elements, and avoid excessive toughening difficulty, the mass percentage of C is controlled to be 0.63%~0.67%.

[0013] (2) Silicon: Si is a deoxidizing element that can improve the purity of steel, supplement strength through solid solution strengthening, inhibit grain coarsening and cementite nucleation and growth during molten salt treatment, slow down the pearlite transformation rate, and help make the phase transformation product after quenching mainly fine troostite, avoid the formation of coarse pearlite or bainite, and slow down the precipitation and coarsening of carbides during tempering, improve the tempering resistance, slow down the hardness decline, and keep the hardness and wear resistance stable after cold working. However, too high silicon content will increase the cost and difficulty of steelmaking, increase the austenitizing temperature, expand the bainite transformation zone, increase the risk of microstructure control, and the high residual stress of the hard and brittle phase will reduce the plasticity and impact toughness of the wire rod, become a source of crack initiation, affect the cold working performance and reduce the fatigue life of the tool steel. Therefore, in order to improve the matrix strength, adapt to the microstructure control of online molten salt, and take into account the toughness, processing and fatigue performance of the wire rod, the Si content is appropriately reduced, and the mass percentage of Si is controlled to be 0.30%~0.40%.

[0014] (3) Manganese: Mn is a hardenability element, which helps to reduce the hardness difference between the core and the surface, expand the austenite area, delay the transformation of austenite to pearlite, and significantly reduce the critical temperature of the sorbite transformation. The lower transformation temperature leads to a lower carbon diffusion rate, which reduces the spacing between sorbite lamellae and improves the strength. By improving the stability of austenite, the nucleation and growth of ferrite at the grain boundary can be delayed, and the strength reduction caused by excessive ferrite can be avoided. The tempering resistance can be improved by inhibiting the coarsening of carbides. However, when the Mn content is too high, it tends to promote the growth of steel grains. High-temperature heating can easily lead to grain coarsening, increase the risk of segregation during the solidification process of the steel billet, lead to increased phase transformation stress, and increase the risk of forming abnormal martensite structure in local Mn-rich areas during quenching, affecting the impact toughness and fatigue properties of the wire rod. Therefore, in order to take into account the strength and fatigue properties of tool steel wire rods and reduce the difficulty of controlling plasticity and structural uniformity, the mass percentage of Mn is controlled to be 0.57%~0.67%.

[0015] (4) 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%.

[0016] The above-mentioned wire rod is designed with carbon composition of C, Si and Mn. The carbon content is lower than that of carbon tool steel such as T8 and T10, and it does not contain alloying elements such as Cr and V compared to alloy tool steel, which can reduce material cost. At the same time, the hardenability of the wire rod is properly regulated by optimizing the ratio of C, Si and Mn, which provides favorable conditions for reducing the phase transformation temperature, delaying the pearlite transformation, promoting the formation of fine sorbite structure, inhibiting excessive softening during isothermal tempering, providing appropriate wear resistance, and improving fatigue life. On this basis, the selection of a higher spinning temperature can promote the wire rod to be in a uniform high-temperature austenite state, providing a mother phase with uniform composition for the subsequent sorbite phase transformation, so as to promote the refinement of the sorbite lamellae by combining a higher quenching temperature with low-temperature quenching. The wire rod after spinning is directly subjected to online molten salt low-temperature isothermal treatment without air cooling:

[0017] 1. Compared with the limitations of the maximum cooling capacity and temperature control instability of the Stelmor air-cooled line, it is difficult to suppress the pearlite formation, uniformity of organization and performance, and online time of carbon tool steel wire rods. On the one hand, when the wire rod passes through the molten salt, the higher thermal conductivity of the molten salt than that of air can be used to promote the rapid cooling of the wire rod, and it can quickly cool down to the sorbite zone across the pearlite phase transformation zone for low-temperature quenching. The high cooling rate is used to shorten the phase transformation time, avoid the precipitation of network carbides of carbon at the austenite grain boundary to reduce the strength and toughness, inhibit the coarsening of pearlite lamellae, provide more carbon sources for the sorbite phase transformation, make sorbite a better phase transformation product, obtain a high sorbitization rate and fine lamellar organization, improve the sorbite strengthening effect and the continuity of the matrix, and make up for the reduction Low alloy content and elimination of strength loss caused by alloying elements provide basic strength and hardness to meet the wear resistance requirements of tool steel. On the other hand, when the wire rod passes through the molten salt, the molten salt covers the surface of the wire rod for uniform heat exchange. Compared with the air-cooled line, there is no temperature difference problem between the winded surface and the winded surface, and between the overlap and non-overlap. It can avoid local overcooling to form brittle abnormal structures such as martensite, and effectively reduce the temperature gradient from the surface to the core of the wire rod, so that the core of the wire rod can also enter the bainite phase transformation zone with the surface, inhibiting the formation of pearlite and ferrite structures in the core, so that bainite is uniformly generated in the entire cross section, improving the uniformity of the wire rod structure and mechanical properties, and the uniform structure distribution reduces stress concentration points, so that the fatigue life of the tool steel under alternating loads is improved.

[0018] Second, compared with the minimum cooling capacity and continuous cooling limit of the Stelmor air-cooled line, which leads to large residual stress, difficulty in improving plasticity and toughness, and affects production efficiency, on the one hand, the uniform cooling of the wire rod reduces the temperature difference between different parts of the wire rod, reduces thermal stress and tissue stress, and the molten salt can control the wire rod after full phase change to be consistent with the molten salt temperature, and perform isothermal treatment in the sorbite phase temperature range for a longer time instead of continuously cooling to too low a temperature. High temperature is used to provide thermal power and enhance atomic diffusion capacity, so that the sorbite layer is melted during the isothermal process, and the lamellar structure that is not completely broken still maintains the interface strengthening effect to meet the tool's demand for wear resistance. The broken melted sorbite reduces the brittleness of the lamellar structure, which quickly further reduces the residual stress. The steel wire rod is released in one step to avoid the residual stress becoming a hidden danger of cracking in subsequent processing, and the plasticity and cold processing properties of the steel wire rod, as well as the fatigue performance of the tool steel, are improved. The phase change temperature is reduced under the combined composition design, and the isothermal temperature is not too high and the steel wire rod is over-softened, thereby improving the matching of plasticity and toughness with strength. On the other hand, the steel wire rod is slowly cooled on a roller after leaving the molten salt, which can avoid the stress increase caused by too fast cooling rate, and can also use the slow cooling in a high temperature state to promote further toughening of the organization and improve the softening effect. At the same time, since the stress of the steel wire rod has been quickly released after the online molten salt low-temperature quenching isothermal treatment, the slow roller cooling does not need to adopt too long a period of heat preservation treatment, thereby shortening the total processing time, reducing the online time of steel wire rod production, and thus improving production efficiency.

[0019] Before the rolling, a higher soaking temperature of the heating furnace and an appropriate time in the furnace are selected to promote the homogenization of the steel billet composition, reduce the influence of segregation, and improve the rolling plasticity of the steel billet, so as to reduce the rolling force and reduce the risk of rolling cracks, while avoiding grain coarsening caused by too long a time in the furnace. In the preferred technical solution, before the rolling, the soaking temperature of the heating furnace is controlled to be 1125~1170℃, and the time in the furnace is 160~200min.

[0020] Since the spinning temperature is relatively high and the nucleation of sorbite can be promoted by molten salt, the restriction on the rolling temperature can be reduced. During the rolling, a higher initial rolling temperature can be selected to reduce the deformation resistance and the wear of the rolling line, and to improve the rolling speed and efficiency. With the appropriate final rolling temperature and final rolling reduction, the dynamic recrystallization and grain refinement of the final rolling process are promoted, the dislocation energy storage is strengthened, and the premature precipitation of ferrite or pearlite during the rolling process due to too low temperature is avoided. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1040~1080℃, the final rolling temperature is 870~930℃, and the final rolling reduction is 24.5%~28%.

[0021] During the spinning process, in order to further avoid the risk of austenite grain coarsening and increased coil deformation of the wire rod, the spinning temperature can be further controlled. In a preferred technical solution, the spinning temperature is controlled to be 860-890°C during the spinning process.

[0022] In the preferred technical solution, the online molten salt low-temperature quenching isothermal treatment is divided into a front-stage molten salt treatment and a rear-stage molten salt treatment. The molten salt circulation volume of the front-stage molten salt treatment is greater than the molten salt circulation volume of the rear-stage molten salt treatment. The phase change temperature can be reduced under the composition design. The front-stage molten salt treatment uses a larger molten salt circulation volume, which can increase the cooling rate of the wire rod, shorten the residence time of the supercooled austenite, avoid the precipitation of proeutectoid ferrite or coarsening of pearlite, and at the same time, the rear-stage molten salt treatment uses a smaller molten salt circulation volume, which can control the wire rod to undergo isothermal toughening treatment and reduce production energy consumption.

[0023] The molten salt temperature of the front-stage molten salt treatment is in the sorbite phase region. The lower the molten salt temperature is, the more it can inhibit the precipitation of network carbides, proeutectoid ferrite and coarsened pearlite, promote the rapid nucleation of sorbite with fine lamellar spacing, promote the formation of fine lamellar structure by low-temperature quenching, and promote sufficient phase transformation of the organization by extending the treatment time of the peak precipitation temperature, providing a basis for strength and hardness. However, if the molten salt temperature is too low, it will aggravate the temperature difference of the wire rod cross section, increase the organization stress and the risk of precipitation of low-temperature brittle organization. As the treatment time is too long, the production energy consumption will be unnecessarily increased. On the contrary, the higher the molten salt temperature is, the more it can improve the uniformity of the temperature from the surface to the core, reduce thermal stress and organization stress, improve organization uniformity, and as the treatment time is shortened , which can reduce production energy consumption, but the molten salt temperature is too high and the treatment time is too short, the phase transformation driving force is reduced, which will promote the nucleation of proeutectoid ferrite at the austenite grain boundary, reduce the strength and wear resistance, or because the phase transformation is not fully carried out, the remaining austenite is left too much to the later molten salt treatment process with a smaller molten salt circulation volume, which increases the fluctuation of mechanical properties. Therefore, the molten salt temperature and treatment time of the front molten salt treatment can be controlled to control the wire rod to quickly enter the sorbite phase region from the high-temperature austenite state, forming a structure dominated by sorbite with fine lamellar spacing, so as to make organizational preparations for the later molten salt treatment. In the preferred technical solution, the molten salt temperature of the front molten salt treatment is 445~485℃, and the treatment time is 80~120s.

[0024] Since the spinning temperature is significantly different from the molten salt temperature of the front-stage molten salt treatment, the front-stage molten salt treatment can further control the molten salt circulation rate, reduce the molten salt temperature rise, quickly remove heat, improve the temperature uniformity of the wire rod cross section, avoid local tissue abnormalities, and reduce the superposition stress of thermal stress and tissue stress. In the preferred technical solution, the molten salt circulation rate of the front-stage molten salt treatment is 520~700t / h, and the molten salt temperature rise is ≤8°C.

[0025] The higher the molten salt temperature and the longer the treatment time of the latter molten salt treatment, the further the temperature gradient of the wire rod cross section can be reduced, the stress concentration source can be reduced, more thermal power can be provided, the atomic diffusion can be accelerated, the melting of the sorbite lamellae and the stress release can be promoted, and the melted carbides can be further evenly dispersed by extending the time, thereby improving the plasticity and toughness of the wire rod, making it less likely to crack due to stress concentration during subsequent cold processing. However, if the molten salt temperature is too high and the treatment time is too long, the matrix will soften too quickly due to high temperature recovery, the carbides will continue to coarsen, and the lattice distortion will be reduced, which will result in a large loss of strength and will not be able to meet the requirements of tool steel for wear resistance and deformation resistance, and the load-bearing capacity of the tool steel will decrease. On the contrary, the lower the molten salt temperature and the shorter the treatment time, the more conducive it is to reducing the stress relief toughening speed and production energy consumption, retaining the sorbite The strengthening effect of the body layer is achieved, but the molten salt temperature is too low and the treatment time is too short. The insufficient thermal power leads to slow atomic diffusion, which is not conducive to stress release, and will lead to insufficient plastic and toughness of the wire rod and excessively high residual stress, affecting the cold processing and fatigue performance of tool steel. Therefore, the molten salt temperature and treatment time of the latter molten salt treatment can be controlled so that the structure after low-temperature quenching can be properly stress-relieved and toughened, and the melting is not coarsened, the dispersion is not agglomerated, and the loss of strength due to excessive softening is avoided, thereby improving the overall strength-plasticity matching of the wire rod. The molten salt temperature difference between the front molten salt treatment and the rear molten salt treatment is smaller, which is more convenient to control the temperature rise of the molten salt, which is beneficial to reducing the molten salt circulation volume and thus production energy consumption. In the preferred technical solution, the molten salt temperature of the latter molten salt treatment is 430~455℃, and the treatment time is 150~300s.

[0026] Since the wire rod has undergone a period of front-stage molten salt treatment, the temperature difference between the wire rod temperature and the molten salt temperature of the rear-stage molten salt treatment is small. The rear-stage molten salt treatment can appropriately reduce the molten salt circulation volume, control the molten salt temperature rise, promote uniform stress release and reduce production energy consumption. In the preferred technical solution, the molten salt circulation volume of the rear-stage molten salt treatment is 240~440t / h, and the molten salt temperature rise is ≤3°C.

[0027] The roller slow cooling can control the wire rod to cool slowly, prevent the wire rod from increasing stress due to excessively fast cooling during the cooling process, promote further toughening of the wire rod structure, and improve the softening effect of the wire rod. In the preferred technical solution, the roller slow cooling controls the wire rod to cool slowly to below 260°C at a cooling rate of 0.25~0.55°C / s.

[0028] In the preferred technical solution, the roller slow cooling passes the hot air from the online molten salt low-temperature quenching isothermal treatment into the insulation cover, and the wire rod is transported by the conveying roller through the insulation cover for slow cooling, which can further recycle and utilize heat energy and reduce production energy consumption.

[0029] A 1200 MPa grade high-strength carbon tool steel wire rod is manufactured by any one of the above-mentioned methods for manufacturing a 1200 MPa grade high-strength carbon tool steel wire rod.

[0030] The above-mentioned wire rod adopts the carbon chemical composition design combined with the online molten salt low-temperature quenching and isothermal toughening technology to form a microstructure consisting mainly of tempered troostite and fused troostite, with a small amount of ferrite mixed structure. Compared with the existing air-cooled high-carbon carbon steel wire rod, the carbon content is lower, which not only avoids the adverse problems of network carbides on cold working performance, but also improves the organizational continuity and the strengthening effect of carbon elements through interface strengthening, making the strength and wear resistance of carbon tool steel better. By promoting the melting of troostite lamellar layers and toughening and stress relief treatment, the brittle fracture of high carbon steel is avoided, so that the stress can be evenly dispersed during cold working, avoiding cracking due to local stress exceeding the plastic limit; compared with the existing air-cooled alloy hypoeutectoid tool steel wire rod, it does not contain elements such as Cr, has a simpler composition, can reduce the difficulty of smelting, and has relatively low Si and Mn contents. It is relatively low and does not contain precious metal elements such as Mo and V, which can effectively reduce material costs. It not only inhibits the coarsening of pearlite and the precipitation of proeutectoid ferrite, increases the sorbitization rate, reduces the interlamellar spacing and the proportion of ferrite, and improves the matrix strength, making up for the strength loss and adverse effects on organizational uniformity caused by reducing the alloy content and eliminating alloying elements, but can retain the strength of sorbite and avoid carbide aggregation. It is more wear-resistant than low-carbon steel, and through the melting of sorbite lamellae, it is transformed into intermediate tempered sorbite and fused sorbite that transition to spheroidized organization, reducing brittle interfaces, so that the wire rod has both good plasticity and toughness and organizational uniformity, thereby reducing the risk of cracking during cold working, so that it can be directly processed into tool parts after cold forming, eliminating additional heat treatment after cold working, and meeting the requirements of carbon tool steel for strength, wear resistance and fatigue performance.

[0031] The larger the volume percentage of tempered bainite in the microstructure, the smaller the interlamellar distance, the larger the interface area, the stronger the hindering effect on dislocation movement, the higher the wire rod strength, the higher the proportion of fused bainite and ferrite, and the higher the wire rod plasticity. In the preferred technical solution, the volume percentage of tempered bainite is 73%~88%, the interlamellar distance is 80~115nm, the volume percentage of fused bainite is 10%~20%; the volume percentage of ferrite is 2%~7%.

[0032] In the preferred technical solution, the diameter of the wire rod is 5.5~9mm, the tensile strength is 1130~1180MPa, the cross-sectional shrinkage rate is 38%~43%, the mechanical property same circle difference is ≤30MPa, the wire rod is in small and medium sizes, and can be directly processed into small and medium-sized tool parts by cold forming, reducing the material loss rate. The wire rod has high tensile strength and can meet the load-bearing requirements when the tool is in use. After cold processing, no additional heat treatment is required to regulate product performance, further reducing subsequent processing costs and energy consumption. The wire rod has a good cross-sectional shrinkage rate and a small mechanical property same circle difference, so that it can withstand a large deformation during cold processing and the stress can be evenly dispersed. It will not cause excessive deformation or cracking in some areas due to local strength or plasticity differences, thereby improving the stability and yield of cold processing. During use, the tool can resist wear and withstand certain impacts and alternating stresses, thereby improving fatigue performance and service life.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) In view of the current situation that adding or increasing C, Cr, V and other elements that improve hardenability in steel will increase the difficulty of manufacturing hot-rolled wire rods of tool steel, the present invention combines the design of carbon chemical composition with the online molten salt low-temperature quenching and isothermal toughening technology to reduce the difficulty of smelting and the restriction on rolling temperature, improve rolling efficiency, and reduce the wear effect on the rolling line. By controlling the wire rod to quickly enter the sorbite phase region from the high-temperature austenite state, the precipitation of network carbides, coarsened pearlite and proeutectoid ferrite can be inhibited. Low-temperature quenching forms a structure mainly composed of sorbite with fine lamellar spacing, and then isothermal tempering and toughening are performed to relieve stress, promote the melting of sorbite lamellar layers, avoid excessive softening, and finally slow roller cooling prevents the wire rod from increasing stress during the cooling process, promotes further toughening of the wire rod structure, and improves the strength-plasticity matching of the wire rod. There is no need to use long-term heat preservation treatment, and production efficiency can be taken into account. It has good industrial adaptability.

[0035] (2) In view of the current situation that increasing the C content will easily deteriorate the cold working performance, increasing the Cr and V content will easily lead to the formation of brittle structures such as martensite and increase the alloy cost, and the blank needs to be heat-treated and strengthened after cold forming to meet the use requirements, the present invention is designed through carbon chemical composition, with a relatively low carbon or alloy element content, and does not contain alloy elements such as Cr, Mo, and V, which can effectively reduce material costs. The microstructure includes tempered bainite, ferrite and melted bainite, and can increase the bainite rate by regulating the phase transformation of the structure, reduce the interlamellar spacing and the proportion of ferrite, and achieve stress release by regulating the tempering state of the structure. Compared with the existing air-cooled high-carbon carbon steel wire rod, the overall strength and plasticity of the wire rod are improved, and the tensile strength can reach 1130~1180MPa and the cross-sectional shrinkage rate can reach 38%~43%. It is used in the application fields of manufacturing carbon tool steel, etc. After cold forming, it can be directly processed into tool parts, eliminating the additional heat treatment after cold working, so as to reduce the subsequent processing cost and energy consumption, meet the use requirements of working conditions, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] 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:

[0037] Figure 1 is a metallographic structure diagram of Example 1 of the present invention;

[0038] Figure 2 is a metallographic structure diagram of Example 2 of the present invention;

[0039] Figure 3 This is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0040] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention. They are 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:

[0041] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.66%, Si: 0.33%, Mn: 0.57%, P: 0.014%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows a process flow of rolling → spinning → online molten salt low-temperature quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0042] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote sufficient diffusion of components, and reduce the influence of segregation. The heating furnace is controlled according to a three-stage temperature increase 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 5.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote dynamic recrystallization during the final rolling process to refine grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1125°C, the furnace time is 200min, the initial rolling temperature is 1040°C, the final rolling temperature is 870°C, and the final rolling reduction is 28%. The wire-laying process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-laying mechanism. 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 the microstructure to promote the refinement of the sorbite layer in conjunction with low-temperature quenching. Specifically, the wire-laying temperature is controlled to be 860°C.

[0043] The online molten salt low-temperature 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, quickly entering the sorbite phase region from the high-temperature austenite state, inhibiting coarse pearlite and proeutectoid ferrite, obtaining a high sorbitization rate, and forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, reducing the temperature. Low molten salt circulation volume controls the isothermal tempering of the wire rod in the isothermal range, promotes the melting of the sorbite lamellar layer, toughening and stress relief treatment, avoids excessive softening, and improves the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front molten salt treatment is 445℃, the treatment time is 120s, the molten salt circulation volume is 525t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the rear molten salt treatment is 455℃, the treatment time is 150s, the molten salt circulation volume is 240t / h, and the molten salt temperature rise is ≤3℃.

[0044] The roller slow cooling process adopts the method of closing the insulation cover, passing the hot air of the online molten salt low-temperature 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 258°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 1 shown.

[0045] Comparative Example 1:

[0046] A method for manufacturing a wire rod, which differs from 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 be 1060°C, the furnace time is 245min, the initial rolling temperature is 980°C, the final rolling temperature is 835°C, the spinning temperature is controlled to be 805°C, the Stelmor air cooling line uses 75% of the air volume of 1~4# fans to control the wire rod to cool to 675°C at a speed of 7.5°C / s, 5~14# fans are opened at 20%, the wire rod is controlled to cool to 265°C at a speed of 2.1°C / s, and the total air volume of the fan is 200,000 m 3 / h, and is collected by a collecting drum to obtain a finished wire rod. The volume percentage of sorbite in the wire rod microstructure is 54%, the sorbite lamellar spacing is 141nm, the volume percentage of pearlite is 24%, the pearlite lamellar spacing is 211nm, the volume percentage of ferrite is 22%, the tensile strength is 845MPa, the cross-sectional shrinkage rate is 30%, and the mechanical property difference in the same circle is 84MPa.

[0047] Comparative Example 2:

[0048] A method for manufacturing a wire rod, which differs from Example 1 in that: the soaking temperature of the heating furnace is controlled to 1075°C, the time in the furnace is 235 minutes, the initial rolling temperature is 990°C, the final rolling temperature is 840°C, the spinning temperature is controlled to be 815°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:

[0049] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.63%, Si: 0.40%, Mn: 0.66%, P: 0.015%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt low-temperature quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0050] 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 sufficient diffusion of components, and reduce the influence of segregation. The heating furnace is controlled according to a three-stage temperature increase 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 improve rolling efficiency and promote dynamic recrystallization during the final rolling process to refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1155°C, the furnace time is 170min, the initial rolling temperature is 1070°C, the final rolling temperature is 915°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. 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 the microstructure to promote the refinement of the sorbite layer in conjunction with low-temperature quenching. Specifically, the wire-spinning temperature is controlled to be 880°C.

[0051] The online molten salt low-temperature 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-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 38°C / s, and quickly enters the sorbite phase region from the high-temperature austenite state, suppressing coarse pearlite and proeutectoid ferrite, obtaining a high sorbitization rate, and forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing Low molten salt circulation volume controls the isothermal tempering of the wire rod in the isothermal range, promotes the melting of the sorbite lamellar layer, toughening and stress relief treatment, avoids excessive softening, and improves the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front molten salt treatment is 471°C, the treatment time is 91s, the molten salt circulation volume is 650t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear molten salt treatment is 440°C, the treatment time is 245s, the molten salt circulation volume is 365t / h, and the molten salt temperature rise is ≤3°C.

[0052] The roller slow cooling process adopts the method of closing the insulation cover, passing the hot air of the online molten salt low-temperature 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 256°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.

[0053] Comparative Example 3:

[0054] A method for manufacturing a wire rod, which differs from Example 2 in that: the wire rod undergoes a front-stage molten salt treatment and is cooled at a cooling rate of 35°C / s, the molten salt temperature of the front-stage molten salt treatment is 500°C, and the treatment time is 70s to obtain a finished wire rod.

[0055] Comparative Example 4:

[0056] A method for manufacturing wire rod, which differs from Example 2 in that: the wire rod is subjected to a front-stage molten salt treatment and cooled at a cooling rate of 42°C / s, the molten salt temperature of the front-stage molten salt treatment is 425°C, and the treatment time is 150s to obtain a finished wire rod. Example 3:

[0057] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.67%, Si: 0.30%, Mn: 0.63%, P: 0.014%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt low-temperature quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0058] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote sufficient diffusion of components, and reduce the influence of segregation. The heating furnace is controlled according to a three-stage temperature increase 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 6.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to improve rolling efficiency and promote dynamic recrystallization during the final rolling process to refine grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1135°C, the furnace time is 175min, the initial rolling temperature is 1065°C, the final rolling temperature is 880°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. 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 the microstructure to promote the refinement of the sorbite layer in conjunction with low-temperature quenching. Specifically, the wire-spinning temperature is controlled to be 875°C.

[0059] The online molten salt low-temperature 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, quickly entering the sorbite phase region from the high-temperature austenite state, inhibiting coarse pearlite and proeutectoid ferrite, obtaining a high sorbitization rate, and forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear molten salt treatment, reducing the temperature. Low molten salt circulation volume controls the isothermal tempering of the wire rod in the isothermal range, promotes the melting of the sorbite lamellar layer, toughening and stress relief treatment, avoids excessive softening, and improves the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front molten salt treatment is 458℃, the treatment time is 105s, the molten salt circulation volume is 568t / h, and the molten salt temperature rise is ≤8℃; the molten salt temperature of the rear molten salt treatment is 445℃, the treatment time is 185s, the molten salt circulation volume is 295t / h, and the molten salt temperature rise is ≤3℃.

[0060] The roller slow cooling process adopts the method of closing the insulation cover, passing the hot air of the online molten salt low-temperature 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 257°C at a cooling rate of 0.43°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.

[0061] Comparative Example 5:

[0062] 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 480° C., the treatment time is 315 s, and the finished wire rod is obtained.

[0063] Comparative Example 6:

[0064] 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 400° C., the treatment time is 140 s, and the finished wire rod is obtained. Example 4:

[0065] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength carbon tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.67%, Si: 0.38%, Mn: 0.67%, P: 0.013%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt low-temperature quenching isothermal treatment → roller slow cooling → coiling, specifically:

[0066] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity, promote sufficient diffusion of components, and reduce the influence of segregation. The heating furnace is controlled according to a three-stage temperature increase 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 improve rolling efficiency and promote dynamic recrystallization during the final rolling process to refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1170°C, the furnace time is 160min, the initial rolling temperature is 1080°C, the final rolling temperature is 930°C, and the final rolling reduction is 24.5%. The wire-laying process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-laying mechanism. 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 the microstructure to promote the refinement of the sorbite layer in conjunction with low-temperature quenching. Specifically, the wire-laying temperature is controlled to be 890°C.

[0067] The online molten salt low-temperature 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-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 38°C / s, and quickly enters the sorbite phase region from the high-temperature austenite state, suppressing coarse pearlite and proeutectoid ferrite, obtaining a high sorbitization rate, and forming a structure dominated by sorbite with fine lamellar spacing. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing Low molten salt circulation volume controls the isothermal tempering of the wire rod in the isothermal range, promotes the melting of the sorbite lamellar layer, toughening and stress relief treatment, avoids excessive softening, and improves the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front molten salt treatment is 485°C, the treatment time is 80s, the molten salt circulation volume is 700t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear molten salt treatment is 430°C, the treatment time is 300s, the molten salt circulation volume is 440t / h, and the molten salt temperature rise is ≤3°C.

[0068] The roller slow cooling process adopts the method of closing the insulation cover, passing the hot air of the online molten salt low-temperature 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, thereby causing stress increase, 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 254°C at a cooling rate of 0.55°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage.

[0069] Comparative Example 7:

[0070] A method for manufacturing a wire rod, which differs from Example 4 in that: the manufacturing method follows a process flow of rolling → spinning → online molten salt low-temperature 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, and the wire rod is cooled to 250°C at a cooling rate of 1.4°C / s to obtain the wire rod.

[0071] The structure and performance of the wire rods obtained in Examples 1 to 4 and Comparative Examples 2 to 7 were tested, and the comparative results are shown in Table 1 below:

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

[0073]

[0074] From the comparison results of Example 1 and Comparative Example 1, it can be seen that compared with omitting alloy elements and reducing alloy content, the air-cooled line process will make the ferrite in the wire rod preferentially precipitate and coarsen along the austenite grain boundary during the phase transformation, and it is easier to generate pearlite structure with a larger interlamellar spacing. The proportion of sorbite in the structure decreases, and it is difficult to generate a uniform structure, which will bring about a large strength loss, fluctuation of material mechanical properties and residual stress. The present invention combines the carbon chemical composition design with the online molten salt low-temperature quenching and isothermal toughening technology. By controlling the wire rod to quickly enter the sorbite phase region from the high-temperature austenite state, it can inhibit the precipitation of network carbides, coarsened pearlite and proeutectoid ferrite, effectively avoid the risk of C element generating brittle structures such as martensite and widmanstattenite, and low-temperature quenching forms a thin flake structure. The interlaminar spacing is mainly composed of sorbite, which improves the strengthening effect of carbon elements, and then isothermal tempering and toughening to relieve stress, promotes the melting of sorbite sheets, avoids excessive softening, and improves the strength-plasticity matching of the wire rod. It can be seen from the results of Examples 1 to 4 that the tensile strength can reach 1130~1180MPa and the cross-sectional shrinkage rate is 38%~43%, so that it can be directly processed into tool parts after cold forming, reducing subsequent processing costs and energy consumption. The wire rod has been quickly released from stress after the molten salt treatment in the later stage. The temperature of the hood in the slow cooling stage of the roller is relatively lower than that of the existing air cooling line insulation treatment. There is no need to use high-temperature coiling and long-term insulation treatment, which can avoid the large ring shape differences and wear problems caused by high-temperature coiling, and at the same time promote the rapid offline of the wire rod, thereby taking into account production efficiency.

[0075] From the comparison results of Example 1 with Comparative Examples 1 and 2, it can be seen that compared with the air-cooled wire process that requires low-temperature rolling and spinning to refine the grains after omitting alloy elements and reducing the alloy content, the present invention can use a higher spinning temperature to promote the wire rod to be in a uniform high-temperature austenite state, providing a mother phase with uniform composition for the subsequent sorbite phase transformation, so as to promote the refinement of the sorbite layer with a higher quenching temperature in combination with low-temperature quenching, while reducing the restriction on the rolling temperature. A higher initial rolling temperature can be used to reduce deformation resistance and wear on the rolling line, and improve rolling speed and efficiency.

[0076] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the higher the molten salt temperature in the front-stage molten salt treatment, the higher the uniformity of the temperature from the surface to the core can be, the thermal stress and tissue stress can be reduced, and the tissue uniformity can be improved. As the treatment time is shortened, the production energy consumption can be reduced. However, if the molten salt temperature is too high and the treatment time is too short, the driving force of the phase transformation will be reduced, which will promote the nucleation of proeutectoid ferrite at the austenite grain boundary, reduce the strength and wear resistance, or because the phase transformation is not fully carried out, too much residual austenite will be left in the back-stage molten salt treatment process with a smaller molten salt circulation amount, thereby increasing the fluctuation of mechanical properties.

[0077] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the lower the molten salt temperature of the front-stage molten salt treatment, the precipitation of network carbides, proeutectoid ferrite and coarsened pearlite can be suppressed, the rapid nucleation of troostite with fine lamellar spacing can be promoted, the formation of fine lamellar structure can be promoted through low-temperature quenching, and the sufficient phase transformation of the organization can be promoted by extending the treatment time of the peak precipitation temperature, thereby improving the strength and hardness foundation. However, if the molten salt temperature is too low, the temperature difference of the wire rod cross section will be aggravated, the organizational stress will be increased, and as the treatment time is too long, the production energy consumption will increase.

[0078] From the comparison results of Example 2 and Comparative Example 5, it can be seen that the higher the molten salt temperature and the longer the treatment time of the latter molten salt treatment, the temperature gradient of the wire rod cross section can be further reduced, the stress concentration source can be reduced, the melting of the sorbite lamellae can be promoted, and the melted carbides can be further evenly dispersed by prolonging the time, thereby improving the plasticity and toughness of the wire rod. However, if the molten salt temperature is too high and the treatment time is too long, the matrix will soften too quickly due to high temperature recovery, and the carbides will continue to coarsen, which will result in a large loss of strength.

[0079] From the comparison results of Example 2 and Comparative Example 6, it can be seen that the lower the molten salt temperature and the shorter the treatment time of the later molten salt treatment, the more conducive it is to reducing the stress relief toughening rate and production energy consumption, and retaining the strengthening effect of the troostite layer. However, if the molten salt temperature is too low and the treatment time is too short, the insufficient thermal power will lead to slow atomic diffusion, which is not conducive to stress release, and will result in insufficient plastic and toughness properties of the wire rod and excessively high residual stress.

[0080] From the comparison results of Example 2 and Comparative Example 7, it can be seen that the wire rod has undergone sufficient phase change due to the subsequent molten salt treatment, which can avoid the re-formation of low-temperature brittle structure during the subsequent cooling process. However, in order to reduce the stress increase caused by physical shrinkage, slow roller cooling treatment can be adopted to prevent the wire rod from increasing stress due to excessively fast cooling rate during the cooling process, and to promote further toughening of the wire rod structure and improve the softening effect of the wire rod.

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

Claims

1. A method for manufacturing 1200MPa grade high strength carbon 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 ≥860°C, and then subjected to an online molten salt low-temperature quenching isothermal treatment. The wire rod is controlled to enter the sorbite phase region from the high-temperature austenite state at a cooling rate of ≥38°C / s to form a structure mainly composed of sorbite. The wire rod is controlled to be isothermal tempered in the isothermal range to promote the melting of the sorbite lamellae and toughening and stress relief treatment. Finally, it is slowly cooled by a roller to form a wire rod with a microstructure including tempered sorbite, ferrite and fused sorbite. The chemical composition and mass percentage of the wire rod include: C: 0 .63%~0.67%, Si: 0.30%~0.40%, Mn: 0.57%~0.67%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; the online molten salt low-temperature quenching isothermal treatment is divided into a front-stage molten salt treatment and a rear-stage molten salt treatment, the molten salt temperature of the front-stage molten salt treatment is 445~485℃, and the treatment time is 80~120s; the molten salt temperature of the rear-stage molten salt treatment is 430~455℃, and the treatment time is 150~300s.

2. The method for manufacturing 1200MPa grade high strength carbon 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 1125-1170° C., and the soaking time in the furnace is 160-200 minutes.

3. The method for manufacturing 1200MPa grade high strength carbon tool steel wire rod according to claim 1, characterized in that: During the rolling, the initial rolling temperature is controlled to be 1040-1080° C., the final rolling temperature is controlled to be 870-930° C., and the final rolling reduction is controlled to be 24.5%-28%; during the wire drawing, the wire drawing temperature is controlled to be 860-890° C.

4. The method for manufacturing 1200MPa grade high strength carbon tool steel wire rod according to claim 1, characterized in that: The molten salt circulation amount of the front-stage molten salt treatment is greater than the molten salt circulation amount of the back-stage molten salt treatment.

5. The method for manufacturing 1200MPa grade high strength carbon tool steel wire rod according to claim 4, characterized in that: The molten salt circulation rate of the front-stage molten salt treatment is 520~700t / h, and the molten salt temperature rise is ≤8°C; the molten salt circulation rate of the back-stage molten salt treatment is 240~440t / h, and the molten salt temperature rise is ≤3°C.

6. The method for manufacturing 1200 MPa grade high strength carbon tool steel wire rod according to claim 1, characterized in that: The roller slow cooling controls the wire rod to be slowly cooled to below 260° C. at a cooling rate of 0.25-0.55° C. / s.

7. A 1200MPa grade high strength carbon tool steel wire rod, characterized in that: The wire rod is manufactured by the manufacturing method of 1200MPa grade high-strength carbon tool steel wire rod according to any one of claims 1 to 6.

8. The 1200 MPa grade high strength carbon tool steel wire rod according to claim 7, characterized in that: The volume percentage of the tempered bainite is 73% to 88%, the interlamellar spacing is 80 to 115 nm, the volume percentage of the fused bainite is 10% to 20%, and the volume percentage of the ferrite is 2% to 7%.

9. The 1200 MPa grade high strength carbon tool steel wire rod according to claim 7, characterized in that: The diameter of the wire rod is 5.5-9 mm, the tensile strength is 1130-1180 MPa, the cross-sectional shrinkage rate is 38%-43%, and the mechanical property difference within the same circle is ≤30 MPa.

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