1200MPa-grade high-strength tool steel wire rod and manufacturing method thereof

Abstract

The invention relates to a 1200MPa-grade high-strength tool steel wire rod and a manufacturing method thereof, after V-containing chemical components are designed, rolled and spun into a wire rod, the wire rod is subjected to online molten salt isothermal quenching treatment, and the wire rod is controlled to enter a bainite phase region from a high-temperature austenite state at a cooling speed greater than or equal to 32 DEG C / s; the steel wire rod with a microscopic structure comprising tempered bainite and ferrite is prepared through isothermal tempering, toughening and destressing treatment, carbide precipitation control and roller way cover opening and slow cooling, so that generation of deteriorated structures such as martensite can be avoided, the structure state can be effectively adjusted, strong plasticity matching of the steel wire rod is achieved, and the service life of the steel wire rod is prolonged. And the tensile strength is 1120-110MPa, the percentage reduction of area is 63-67%, and the steel is suitable for the application fields of manufacturing 1200MPa-grade high-strength tool steel and the like, and is favorable for being directly processed into tool parts after being subjected to cold processing molding.

Description

Technical Field

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

[0002] With the continuous improvement of the scientific and technological level, the application fields of tools are constantly expanding. Therefore, improving the durability and quality level of tools can rapidly promote the industrialization process. Alloy tool steel has better hardness and wear resistance than carbon tool steel and is suitable for complex working conditions such as taps, dies, and cutting tools. Tool factories generally use hot-rolled wire rods as the base material. Therefore, to improve the strength grade and quality level of tool steel, it is necessary to improve the strength-ductility performance matching of the wire rod.

[0003] Conventional tool steel production generally designs through hardenability elements and combines controlled cooling production on a Stelmor air-cooling line, and there are still the following defects: (1) To balance cold forming, due to the limitation of the controlled cooling capacity of the Stelmor air-cooling line for tool steel wire rods, the obtained structure is mainly pearlite. For example, a low-cost high-plasticity sorbite tool alloy steel wire rod and a manufacturing method thereof disclosed in Patent CN119640155A adopt a C-Si-Mn-Cr composition design, combined with low-temperature rolling and spinning, air-cooling and cover-cooling to obtain a sorbite structure wire rod to improve the plasticity of the wire rod and enable annealing-free drawing processing. However, on the one hand, the addition of hardenability components such as Mn and Cr can promote the refinement of pearlite lamellae in the wire rod under the slower cooling conditions of the air-cooling line and obtain a sorbite structure with better strength and plasticity. However, it will also exacerbate the segregation during the alloy solidification process. As the temperature difference and uncontrollability between the windward surface and the leeward surface, and from the edge to the core of the wire rod increase during the air-cooling process, it is difficult to avoid the generation of deteriorated structures such as martensite due to the increase in hardenability, which increases the difficulty of cold processing and post-cold processing heat treatment in tool factories and seriously reduces the quality level of tools. On the other hand, to balance the hardness of tool steel, the Si and Cr contents in the components are relatively high, and at the same time, the minimum controlled cooling capacity of the air-cooling line is limited. Excessive silicon content will coarsen the ferrite grains, resulting in a significant reduction in the toughness of the material and increasing the risk of brittle fracture. A higher chromium content will increase the difficulty of improving the plasticity of the structure. As the wire rod has been in a low-temperature state after continuous cooling and through the structure phase transformation incubation, the thermal driving force is insufficient, resulting in limited improvement in the final plasticity of the wire rod. After cold processing, heat treatment such as tempering is still required to adjust the product performance, resulting in increased production energy consumption and costs for downstream users and affecting the production efficiency of tool steel products.

[0004] (2) In order to improve the problem that the martensite structure has high hardness and is prone to brittle fracture during packing, transportation and user processing, some alloy tool steel wire rods adopt an air-cooling line to develop bainite wire rods. For example, a production method of an efficient and economical alloy tool steel wire rod disclosed in Patent CN117587211A adopts a C-Si-Mn-Cr-Ni-Al-Mo-V-Nb composition design, combined with blowing and cooling after spinning to near the bainite precipitation range, and then entering a cover for phase transformation to produce a wire rod with 60% - 70% bainite. However, on the one hand, the maximum cooling capacity of the air-cooling line is limited, and the wire rod spends a long time in the pearlite and upper bainite transformation temperature ranges during cooling, unable to effectively inhibit the diffusion of carbon, allowing carbon to diffuse and precipitate carbides relatively quickly, and austenite is prone to transform in this temperature range to form pearlite and upper bainite, resulting in a loss of strength and plasticity, and leading to the need to add higher alloy contents such as V and Nb in the wire rod, increasing the material cost; on the other hand, upper bainite is composed of ferrite laths and cementite distributed between the laths, with relatively high strength but poor toughness. As the wire rod is in a low-temperature state after continuous cooling and through the incubation of tissue phase transformation, it is difficult to release the tissue stress, which will lead to a decline in the cold working properties such as cold drawing and cold heading of the wire rod, bringing processing defects and hand tool quality problems. Summary of the Invention

[0005] The present invention aims to solve at least one of the above technical problems to some extent. The present invention provides a 1200 MPa grade high-strength tool steel wire rod and its manufacturing method, which can avoid the generation of deteriorated structures such as martensite, effectively adjust the tissue state, achieve the strength-plasticity matching of the wire rod, and is beneficial to directly processing into tool parts after cold forming.

[0006] The technical solution adopted by the present invention to solve its technical problems is as follows: A manufacturing method of a 1200 MPa grade high-strength tool steel wire rod, the manufacturing method comprising: After the steel billet is rolled and spun into a wire rod at a spinning temperature of ≥900 °C, it is subjected to online molten salt isothermal quenching treatment, controlling the wire rod to enter the bainite phase region from the high-temperature austenite state at a cooling rate of ≥32 °C / s, forming a quenched structure mainly composed of quenched bainite and isothermal tempering and toughening stress relief treatment to control the precipitation of carbides, and finally slow cooling through a roller table with the cover opened to produce a wire rod with a microstructure including tempered bainite and ferrite. The chemical composition and mass percentage of the wire rod include: C: 0.44% - 0.49%, Si: 0.32% - 0.40%, Mn: 0.35% - 0.48%, Cr: 0.55% - 0.65%, V: 0.035% - 0.044%, P ≤ 0.015%, S ≤ 0.015%, and the rest is Fe and inevitable impurities.

[0007] The design basis for the chemical composition and mass percentage of the above wire rod includes: (1) Carbon: As a carbide strengthening element and austenite forming element, the element C has a relatively lower price. With the increase of carbon content, it can delay the pearlite transformation, provide the necessary carbon source for the formation of quenched bainite during the online molten salt quenching and isothermal treatment process, endow the wire rod with high strength and wear resistance by forming fine and dispersed carbides, and improve the ability of the tool steel to withstand alternating loads. However, if the carbon content is too high, it is easy to form coarse cementite or martensite, resulting in a decrease in toughness, an increase in the difficulty of isothermal tempering, and an impact on the isothermal tempering effect at lower temperatures. Therefore, in order to meet the strength and wear resistance requirements of hand tool steel, control the material cost, reduce the difficulty of controlling the microstructure state of the wire rod during the online molten salt isothermal quenching treatment, and improve the strength-toughness matching, a medium carbon content is adopted to balance the strength and toughness, and the mass percentage of C is controlled to be 0.44% - 0.49%.

[0008] (2) Silicon: Element Si can inhibit the grain coarsening during the online molten salt quenching and isothermal process. By hindering the diffusion of carbon into cementite, inhibiting the precipitation of cementite, and reducing the nucleation energy of bainite ferrite, the bainite transformation can still proceed efficiently at a higher temperature, and the tempering temperature decreases. However, if the silicon content is too high, the solution strengthening effect of silicon will be too strong, the lattice distortion will intensify, the toughness of the steel will be reduced, the plasticity regulation will be affected, and the plasticity will decrease during cold working. Therefore, in order to adapt to the regulation of the microstructure by the online molten salt quenching and isothermal process and avoid excessive embrittlement, the mass percentage of Si is controlled to be 0.32% - 0.40%.

[0009] (3) Manganese: As a strong austenite stabilizing element, Mn can increase the hardenability of the wire rod, reduce the tendency of pearlite and ferrite formation, strongly delay the pearlite transformation, expand the bainite transformation temperature range, make it easier to obtain quenched bainite structure during the online molten salt isothermal quenching treatment, and improve the strength and hardness through the solution strengthening effect. However, when the content of Mn is too high, it will promote the coarsening of austenite grains, inhibit the recrystallization of ferrite and the aggregation of carbides during the tempering process, make the stress release require higher energy, and reduce the toughness and plasticity of the steel. Therefore, in order to strengthen the matrix, make the bainite phase transformation and isothermal toughening and stress relief of the wire rod in a close temperature range, and simplify the molten salt temperature control, the mass percentage of Mn is controlled to be 0.35% - 0.48%.

[0010] (4) Chromium: As a strong carbide-forming element, the Cr element can form high-hardness alloy carbides, improve the hardenability of steel, reduce the diffusion coefficient of carbon in austenite, delay the precipitation of ferrite and pearlite, and make quenched bainite the main transformation product during online molten salt isothermal quenching treatment, enhancing the wear resistance and corrosion resistance of tools. However, excessive Cr content will exacerbate composition segregation, increase the difficulty of controlling tissue uniformity, improve the tempering stability of bainite ferrite, increase the difficulty of enhancing plasticity, and reduce cold workability. Therefore, to enhance the wear resistance of steel, make the bainite phase transformation and isothermal toughening stress relief of the wire rod occur in a close temperature range, and simplify the molten salt temperature control, the Cr content is appropriately increased, and the mass percentage of Cr is controlled at 0.55% - 0.65%.

[0011] (5) Vanadium: As an alloying element, the V element pins the grain boundaries to inhibit the growth of austenite grains, refines the original structure, promotes the formation of high-density dislocations and twins in bainite ferrite, and can form nano-scale VC carbides during the isothermal process, compensating for the strength loss caused by tempering softening of quenched bainite and reducing the crack tendency during cold forming of hand tools. However, the cost of the V element is relatively high, and excessive addition is not conducive to controlling the cost of the wire rod. The precipitation of coarse VC particles along the grain boundaries will deteriorate the impact resistance of the tool. Therefore, based on the role, cost, and preparation control of the V element, the mass percentage of V is controlled at 0.035% - 0.044%.

[0012] (6) Phosphorus and sulfur: The P element and the S element belong to impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% are controlled.

[0013] The above wire rod adopts a medium-carbon composition design of C-Si-Mn-Cr-V, taking into account the wear resistance requirements of hand tools. The Si / Mn / Cr content is relatively low, and a small amount of V element is added, which can appropriately reduce the material cost. By optimizing the composition ratio, the bainite transformation can still proceed efficiently at a relatively high temperature, reducing the difficulty of tempering stress relief, providing favorable conditions for controlling quenching, toughening, and carbide dispersion precipitation in the bainite phase region during online molten salt isothermal quenching treatment, simplifying the molten salt control, and reducing production energy consumption. With a relatively high wire spooling temperature, i.e., the quenching inlet temperature, the carbon content in austenite is homogenized, and alloying elements are fully dissolved, reducing the carbon concentration gradient and grain boundary segregation in the structure after quenching, and making organizational preparations for improving the strength and plasticity of the wire rod. The wire rod after wire spooling is directly subjected to online molten salt isothermal quenching treatment without air cooling: I. Compared with the unstable temperature control and limited maximum cooling capacity of the Stelmor air-cooled line, which result in the mixture of brittle martensite or low-strength pearlite, and tend to form upper bainite tissue that is unfavorable for cold working performance in a relatively high temperature range. When the wire rod undergoes online molten salt isothermal quenching, on the one hand, the thermal conductivity of the molten salt is much higher than that of air, which can significantly increase the cooling rate of the wire rod compared with air cooling. It can bypass the pearlite phase region, reduce the kinetic trend of pearlite transformation, and promote the wire rod to enter the temperature below the nose temperature of bainite transformation from the high-temperature austenite state. At this time, the stability of austenite is extremely high, and the incubation periods of pearlite and martensite transformations are greatly extended, forcing austenite to preferentially transform into quenched bainite. With the extension of the treatment time, it is easier to obtain a single quenched bainite structure during quenching. The bainite transformation tends to form lower bainite rather than coarse upper bainite. The carbides in lower bainite are uniformly distributed in the ferrite needles in the form of nanoscale particles, combining high strength and the dislocation slip ability of fine acicular ferrite to make up for the strength loss caused by reducing carbon or alloy content. On the other hand, when the wire rod passes through the molten salt during quenching, the molten salt can cover the surface of the wire rod for uniform heat transfer, reducing the temperature difference from the edge to the core of the wire rod and avoiding the temperature difference problem between the windward side and the leeward side in air cooling. The bainite phase transformation occurs at a relatively high temperature, avoiding entering the martensite phase region to form martensite tissue, fully transforming to form a quenched structure mainly composed of quenched bainite, avoiding austenite residue, and preventing the formation of abnormal low-temperature tissue in the core.

[0014] II. Compared with the limited minimum cooling capacity of continuous cooling control and hood cooling of the Stelmor air-cooled line, the resulting tissue stress is relatively large, and the uneven thickness of carbides affects the strengthening and toughening effect. On the one hand, after expanding the bainite phase region, the bainite transformation can still proceed efficiently at a relatively high temperature. With the extension of the treatment time, the wire rod transforms to the molten salt temperature, which can extend the time of the wire rod in the high-temperature isothermal range and perform high-temperature isothermal tempering treatment on the quenched structure, providing more thermal power for toughening and further improving the tissue state and plasticity of the wire rod. On the other hand, the temperature of the bainite phase region is lower than that of the pearlite phase region. With the extension of time, the precipitation of vanadium carbides overcomes the diffusion limitation at low temperature, and the precipitation amount increases. It can be fully precipitated and the distribution tends to be dispersed. The fine precipitates can effectively hinder the movement of dislocations, improve the material strength, and avoid the coarsening of carbide precipitation or uneven distribution at high temperature, which reduces the toughness of the material. Finally, through the slow cooling of the roller table with the hood opened, the wire rod is further toughened, and physical shrinkage and stress increase caused by too fast cooling speed are prevented, thereby controlling the tissue state of the wire rod and improving the strength-plasticity matching of the wire rod.

[0015] Before rolling, by selecting an appropriate soaking temperature and soaking time in the heating furnace, it is possible to promote compositional homogenization, reduce the influence of segregation, and avoid the risks of grain coarsening and overburning. In the preferred technical solution, before rolling, the soaking temperature of the heating furnace is controlled at 1130 - 1180 °C, and the soaking time is 120 - 180 min.

[0016] Due to the relatively high spinning temperature, the restrictions on rolling can be reduced. Appropriately increasing the initial rolling temperature can ensure the plastic deformation ability, facilitate large deformation rolling, improve the deformation uniformity during rolling, reduce the risk of cracking and the wear impact on the rolling line. At the same time, controlling the appropriate finishing rolling temperature and finishing rolling reduction can promote dynamic recrystallization during the finishing rolling process, refine the grains, and avoid excessive phase transformation stress during subsequent cooling due to too low finishing rolling temperature. In the preferred technical solution, during rolling, the initial rolling temperature is controlled at 1030 - 1080 °C, the finishing rolling temperature is controlled at 905 - 955 °C, and the finishing rolling reduction is 21% - 24%.

[0017] During spinning, the spinning temperature can be further controlled to avoid grain coarsening caused by too high spinning temperature and reduce the risk of abnormal Widmanstätten structure. In the preferred technical solution, during spinning, the spinning temperature is controlled at 900 - 945 °C.

[0018] The molten salt temperature of the online molten salt isothermal quenching treatment is in the bainite phase region. In the preferred technical solution, the online molten salt isothermal quenching treatment is divided into the front - stage molten salt treatment and the back - stage molten salt treatment. The molten salt temperature is 400 - 490 °C, and the total treatment time < 410 s. The molten salt circulation volume of the front - stage molten salt treatment is greater than that of the back - stage molten salt treatment, which is beneficial to further appropriately reduce production energy consumption and avoid carbide coarsening and loss of strength and toughness properties caused by too long total treatment time.

[0019] The front - stage molten salt treatment is used to control the wire rod to quickly enter the bainite phase region from the high - temperature austenite state, forming a quenched structure mainly composed of quenched bainite. The lower the molten salt temperature and the longer the treatment time, the more beneficial it is to inhibit the formation of pearlite, hinder the continuous growth of the feather - shaped ferrite of upper bainite, promote the nucleation of needle - shaped quenched bainite, and improve the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, the tissue stress will increase, and the isothermal tempering and temperature rise control difficulty of the back - stage molten salt treatment will increase, resulting in unnecessary increases in production energy consumption and time. On the contrary, the higher the molten salt temperature and the shorter the treatment time, the more beneficial it is to inhibit the formation of martensite and reduce the tempering hardening difficulty. However, if the molten salt temperature is too high and the treatment time is too short, the diffusion ability of carbon in austenite will increase, affecting the bainite phase transformation and tissue refinement of quenching, and affecting the strength and toughness properties of the matrix. Therefore, appropriate molten salt temperature and treatment time can be selected to balance production energy consumption, form a quenched structure mainly composed of quenched bainite, and prepare the tissue for the back - stage molten salt treatment. In the preferred technical solution, the molten salt temperature of the front - stage molten salt treatment is 400 - 490 °C, and the treatment time is 25 - 56 s.

[0020] Due to the large temperature difference between the spinning temperature and the molten salt temperature of the front - stage molten salt treatment, a larger molten salt circulation volume can be selected to control the molten salt temperature rise and improve the tissue consistency of the wire rod during continuous treatment. In the preferred technical solution, the molten salt circulation volume of the front - stage molten salt treatment is 400 - 500 t / h, and the molten salt temperature rise ≤ 8 °C.

[0021] The subsequent molten salt treatment is used to control the high-temperature isothermal tempering of the quenched structure of the wire rod in the high-temperature isothermal range, toughen and remove stress, and control the precipitation of carbides. The higher the molten salt temperature and the longer the treatment time, the more beneficial it is to reduce the dislocation density of quenched bainite and the tissue stress. As the time prolongs, it is beneficial to the full and uniform precipitation of vanadium-containing carbides, providing an appropriate toughening effect. However, when the molten salt temperature is too high and the treatment time is too long, there is a risk of carbide coarsening and a significant decrease in toughness, and at the same time, the production energy consumption increases. On the contrary, when the molten salt temperature is too low and the treatment time is too short, it can reduce the risk of excessive carbide coarsening and the production energy consumption. However, when the molten salt temperature is too low and the treatment time is too short, insufficient tempering driving force will lead to uneven carbide distribution and incomplete elimination of residual stress, affecting the plastic properties of the wire rod. Therefore, the molten salt temperature and treatment time can be further controlled to improve the strength-plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature of the subsequent molten salt treatment is 460-485°C, and the treatment time is 250-380 s.

[0022] The subsequent molten salt treatment can appropriately reduce the molten salt circulation volume, reduce the production energy consumption, and at the same time accurately control the temperature, promoting the uniform regulation of the tissue state. In the preferred technical solution, the molten salt circulation volume of the subsequent molten salt treatment is 250-380 t / h.

[0023] In the preferred technical solution, the slow cooling with the roller table cover open controls the wire rod to cool slowly at a cooling rate of 0.4-0.8°C / s to below 280°C, which can prevent the stress increase caused by too fast cooling rate during the cooling process of the wire rod and promote the further toughening of the wire rod tissue, improving the softening effect of the wire rod.

[0024] A 1200 MPa grade high-strength tool steel wire rod, which is manufactured by the manufacturing method of the 1200 MPa grade high-strength tool steel wire rod described in any one of the above.

[0025] The above-mentioned wire rod adopts a medium-carbon composition design containing trace amounts of V. After being combined with online molten salt isothermal quenching, a microstructure mainly composed of tempered bainite and containing a small amount of ferrite is obtained. Compared with the pearlitic wire rod formed by the air-cooled line, which is lamellar carbide, the above-mentioned wire rod has a lower alloy content, which is beneficial to reducing the material cost. At the same time, the high-density dislocations of quenched bainite can provide strength characteristics, and the dislocation obstruction effect is stronger. After tempering, an efficient strengthening structure of tempered bainite + dispersed carbide is formed, compensating for the strength loss caused by the reduction of alloy content without sacrificing plasticity. Compared with the wire rod containing martensite / upper bainite structure formed by the air-cooled line, which causes performance fluctuations, the above-mentioned wire rod can avoid the formation of brittle martensite structure. At the same time, the coarse cementite is distributed between the feathery ferrite strips of upper bainite, and the interfacial bonding force is poor, and it is easy to crack along the strips during tension. The fine needle-like structure of tempered bainite can reduce the fatigue crack source, and the ferrite matrix of tempered bainite has good plasticity, making the wire rod have better strength and toughness, uniform deformation during cold working, and being beneficial to directly machining into tool parts after cold processing forming, shortening the production cycle.

[0026] The above-mentioned tempered bainite has a stronger ability to obstruct dislocation movement compared with coarse pearlite. A high volume percentage of tempered bainite can improve the good matching of strength and toughness. In the preferred technical solution, the volume percentage of the above-mentioned tempered bainite ≥ 94%.

[0027] In the preferred technical solution, the diameter of the above-mentioned wire rod is 5.5 - 9 mm, the tensile strength is 1120 - 1170 MPa, the reduction of area is 63% - 67%, the difference in mechanical properties within the same coil ≤ 35 MPa. The medium and small-sized wire rods can be applied to tools under complex working conditions such as taps, dies, and cutting tools. The wire rod has good tensile strength and significantly improved toughness. At the same time, the high tissue uniformity effectively controls the difference in mechanical properties within the same coil, and the cold working formability is better, which is beneficial to improving the stability of the production process, reducing the cracking risk of direct cold working forming, and at the same time, it can save heat treatments such as quenching and tempering after cold working, reducing the production cost.

[0028] Compared with the prior art, the beneficial effects of the present invention are at least as follows: (1)In view of the current situation that for the existing tool steel wire rods, it is difficult to control the deteriorated microstructure and the insufficient strength and plasticity of the wire rods through the design of hardenability elements combined with the controlled cooling in the Stelmor air-cooling line, the manufacturing method of the present invention combines the chemical composition design containing V with the on-line molten salt isothermal quenching technology, which can control the wire rods after rolling and coiling to quickly enter the bainite phase region from the high-temperature austenite state, forming a quenched microstructure mainly composed of quenched bainite. Then, the quenched microstructure is subjected to high-temperature isothermal tempering in the high-temperature isothermal range for toughening and stress relief treatment to control the precipitation of carbides. Finally, the wire rods are slowly cooled with the cover opened through the roller table to further toughen the microstructure of the wire rods, improve the softening effect of the wire rods, avoid deteriorated microstructures such as martensite, inhibit the pearlite microstructure, improve the strength-plasticity matching of the wire rods by adjusting the microstructure state, and further reduce the molten salt circulation amount of the subsequent molten salt treatment and the production energy consumption, having good industrial adaptability.

[0029] (2)In view of the current situation that for the existing tool steel wire rods, the alloying element content is high, the deteriorated microstructure is difficult to control, the tissue stress is high, and the strength and plasticity are insufficient, the Si / Mn / Cr content of the wire rods of the present invention is relatively low, and a small amount of V element is added, which can appropriately reduce the material cost. The microstructure mainly consists of tempered bainite and contains a small amount of ferrite. The high-efficiency strengthening structure of tempered bainite and dispersed carbides can make up for the strength loss caused by the reduction of the alloy content. The wire rods have better strength and toughness, and can reach a tensile strength of 1120 - 1170 MPa and a reduction of area of 63% - 67%. It is suitable for application fields such as manufacturing high-strength tool steels of 1200 MPa level, which is beneficial to directly processing into tool parts after cold processing forming, and has good market application prospects. Brief Description of the Drawings

[0030] The above and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, where: Figure 1 is the metallographic structure diagram of Embodiment 1 of the present invention; Figure 2 is the metallographic structure diagram of Embodiment 2 of the present invention. Detailed Embodiments

[0031] The embodiments described below with reference to the accompanying drawings are exemplary and are for illustrative purposes only, without limiting the description of the features and characteristics of the present invention. They are intended to present the best mode of implementing the present invention, for the purpose of explaining the present invention and enabling those skilled in the art to implement the present invention. It should not be construed as imposing any limitation on the scope of the present invention, which is defined only by the appended claims. The detection of the structure and properties of the wire rods obtained from the following examples and comparative examples includes: the tensile test is carried out in accordance with "GB-T 228.1-2021 Metallic materials-Tensile testing-Part 1: Method of test at room temperature" to obtain the tensile strength and reduction of area; the structure detection is carried out in accordance with the metallic microstructure detection method of the GB / T13298 standard; the method for testing the within-coil difference of mechanical properties: 2 coils of wire rods are taken at a distance of 5 m from the end of the coil. Taking the lap area position as the base point, each coil of wire rod is evenly divided into 8 segments on average, and 1 tensile specimen is taken on each segment. The strength range of the tensile specimens after the tensile test is the within-coil difference of mechanical properties. Example 1:

[0032] A preferred embodiment of the manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to the present invention. The chemical composition and mass percentage of the wire rod include C: 0.45%, Si: 0.34%, Mn: 0.35%, Cr: 0.55%, V: 0.036%, P: 0.015%, S: 0.014%, and the rest are Fe and unavoidable impurities. Its manufacturing method is carried out according to the technological process of rolling → wire laying → in-line molten salt isothermal quenching → slow cooling with the rollway cover open → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet with rollable plasticity through a heating furnace to promote the homogenization of components. The heating furnace is controlled according to the three-stage temperature rising program of the preheating section, heating section, and soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 5.5 mm through the rolling line. Appropriate rolling temperature and reduction ratio are selected to increase the rolling speed and promote the dynamic recrystallization in the finishing rolling process to refine the grains. Specifically: the soaking temperature of the heating furnace is controlled at 1130 °C, the residence time in the furnace is 180 min, the initial rolling temperature is 1030 °C, the finishing rolling temperature is 905 °C, and the finishing rolling reduction ratio is 24%; the wire laying process is used to make the wire rod exiting the rolling line into a coil through a wire laying machine. The coil is scattered on the rollway and transported along the rollway, so that the coil is in a high-temperature austenite state, preparing for quenching the structure with a relatively high quenching temperature. Specifically: the wire laying temperature is controlled at 900 °C.

[0033] The online molten salt isothermal quenching process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is transported through the first-stage salt bath tank by a roller table for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 33 °C / s, quickly enters the bainite phase region from the high-temperature austenite state, inhibits the formation of pearlite, and forms a quenched structure mainly composed of quenched bainite. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the isothermal tempering of the quenched structure in the bainite phase region of the wire rod, toughening and stress-relieving treatment, controlling the precipitation of carbides, and improving the strength-ductility matching of the wire rod. Specifically: the molten salt temperature for the front-stage molten salt treatment is 487 °C, the treatment time is 25 s, the molten salt circulation volume is 405 t / h, and the molten salt temperature rise ≤ 8 °C; the molten salt temperature for the rear-stage molten salt treatment is 460 °C, the treatment time is 380 s, the molten salt circulation volume is 250 t / h, and the total treatment time is 405 s.

[0034] In the slow cooling process with the roller table cover opened, the opening degree of the heat preservation cover is adjusted, and the wire rod transported by the conveying roller table after passing through the second-stage salt bath tank is slowly cooled through the heat preservation cover, preventing the wire rod from increasing stress due to too fast cooling rate during the cooling process, and promoting further toughening of the wire rod structure, improving the softening effect of the wire rod until coiling. Specifically: controlling the wire rod to slowly cool at a cooling rate of 0.8 °C / s to 265 °C; the coiling process is used to coil the wire rod into a coil by a coiling drum, and after packaging and storing in the warehouse, the finished wire rod is obtained, and its metallographic structure diagram is as Figure 1 shown.

[0035] Comparative Example 1: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 1 is that: its manufacturing method is carried out according to the technological process of rolling → wire laying → Stelmor air cooling line → coiling. Specifically: in the rolling process, the soaking temperature of the heating furnace is controlled at 1080 °C, the time in the furnace is 220 min, the initial rolling temperature is 950 °C, the final rolling temperature is 845 °C, the wire laying temperature is controlled at 830 °C. In the Stelmor air cooling line, the front 1-6# heat preservation covers are opened, and the fan is turned on to control the wire rod to cool at a speed of 3.7 °C / s to 695 °C. Then the heat preservation cover is closed, and the wire rod enters the heat preservation cover and cools at a speed of 2.2 °C / s to 260 °C, and is collected by the coiling drum to obtain the finished wire rod.

[0036] Comparative Example 2: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 1 is that: the soaking temperature of the heating furnace is controlled at 1080 °C, the time in the furnace is 220 min, the initial rolling temperature is 950 °C, the final rolling temperature is 845 °C, the wire laying temperature is controlled at 830 °C, and the wire rod undergoes the front-stage molten salt treatment and cools down at a cooling rate of 29 °C / s to obtain the finished wire rod. Example 2:

[0037] A preferred embodiment of the manufacturing method of the 1200 MPa grade high-strength tool steel wire rod. The chemical composition and mass percentage of the wire rod include C: 0.44%, Si: 0.32%, Mn: 0.46%, Cr: 0.65%, V: 0.039%, P: 0.014%, S: 0.014%, and the rest are Fe and unavoidable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt isothermal quenching → slow cooling with the cover open on the roller table → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet that reaches the plastic state for rolling, promoting the homogenization of components. The heating furnace is controlled according to the three-stage heating program of the preheating section, heating section, and soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 8 mm through the rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed and promote dynamic recrystallization during the finishing rolling process, refining the grains. Specifically: control the soaking temperature of the heating furnace to be 1160 °C, the residence time in the furnace to be 135 min, the initial rolling temperature to be 1065 °C, the finishing rolling temperature to be 945 °C, and the finishing rolling reduction to be 22%; The wire laying process is used to make the wire rod coming out of the rolling line into a wire rod through the wire laying machine. The wire rod is scattered on the roller table and transported along the roller table, making the wire rod in the high-temperature austenite state to prepare for quenching the structure with a relatively high quenching temperature. Specifically: control the wire laying temperature to be 935 °C.

[0038] The on-line molten salt isothermal quenching process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is transported through the roller table and passes through the first-stage salt bath tank for the front-stage molten salt treatment, making the wire rod cool down at a cooling rate of 38 °C / s, quickly entering the bainite phase region from the high-temperature austenite state, inhibiting the formation of pearlite, and forming a quenched structure mainly composed of quenched bainite. Then the wire rod is transported through the roller table and passes through the second-stage salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the isothermal tempering of the quenched structure in the bainite phase region, toughening and stress relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature for the front-stage molten salt treatment is 433 °C, the treatment time is 46 s, the molten salt circulation volume is 475 t / h, and the molten salt temperature rise ≤ 8 °C; the molten salt temperature for the rear-stage molten salt treatment is 477 °C, the treatment time is 290 s, the molten salt circulation volume is 365 t / h, and the total treatment time is 336 s.

[0039] The slow cooling process with the cover open on the roller table uses the adjustment of the opening degree of the heat preservation cover. The wire rod passing through the second-stage salt bath tank is slowly cooled by the conveying roller table through the heat preservation cover, preventing the wire rod from cooling too fast during the cooling process, resulting in an increase in stress, and promoting the further toughening of the wire rod structure, improving the softening effect of the wire rod until coiling. Specifically: control the wire rod to slowly cool to 273 °C at a cooling rate of 0.5 °C / s; The coiling process is used to coil the wire rod into a coil through the coiling drum. After packaging and storing in the warehouse, the finished wire rod is obtained. Its metallographic structure diagram is asFigure 2 as shown

[0040] Comparative Example 3: A manufacturing method of wire rod, the difference from that of Example 2 lies in that: the wire rod is subjected to molten salt treatment in the front stage and cooled at a cooling rate of 31 °C / s. The molten salt temperature of the front-stage molten salt treatment is 505 °C, the treatment time is 20 s, and the total treatment time is 310 s to obtain the finished wire rod product.

[0041] Comparative Example 4: A manufacturing method of wire rod, the difference from that of Example 2 lies in that: the wire rod is subjected to molten salt treatment in the front stage and cooled at a cooling rate of 40 °C / s. The molten salt temperature of the front-stage molten salt treatment is 365 °C, the treatment time is 70 s, and the total treatment time is 360 s to obtain the finished wire rod product. Example 3:

[0042] A preferred implementation of the manufacturing method of the 1200 MPa grade high-strength tool steel wire rod of the present invention. The chemical composition and mass percentage of the wire rod include C: 0.45%, Si: 0.4%, Mn: 0.48%, Cr: 0.62%, V: 0.044%, P: 0.012%, S: 0.012%, and the rest are Fe and unavoidable impurities; its manufacturing method is manufactured according to the process flow of rolling → wire laying → online molten salt isothermal quenching → slow cooling with the rollway cover open → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 180 mm × 180 mm into a high-temperature steel billet that reaches the plastic state for rolling, promoting the homogenization of components. The heating furnace is controlled according to the three-stage heating program of the preheating section, heating section, and soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 9 mm through the rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed and promote dynamic recrystallization during the finishing rolling process, refining the grains. Specifically: control the soaking temperature of the heating furnace to be 1180 °C, the residence time in the furnace to be 120 min, the initial rolling temperature to be 1080 °C, the finishing rolling temperature to be 955 °C, and the finishing rolling reduction to be 21%; the wire laying process is used to make the wire rod coming out of the rolling line into a wire rod through the wire laying machine. The wire rod is scattered on the rollway and transported along the rollway, making the wire rod in the high-temperature austenite state to prepare for quenching with a relatively high quenching temperature. Specifically: control the wire laying temperature to be 945 °C.

[0043] The online molten salt isothermal quenching process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is transported through the first-stage salt bath tank by a roller path for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 39 °C / s, quickly enters the bainite phase region from the high-temperature austenite state, inhibits the formation of pearlite, and forms a quenched structure mainly composed of quenched bainite. Then the wire rod is transported through the second-stage salt bath tank by a roller path for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the isothermal tempering of the quenched structure in the bainite phase region of the wire rod, toughening and stress relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature for the front-stage molten salt treatment is 402 °C, the treatment time is 56 s, the molten salt circulation volume is 500 t / h, and the molten salt temperature rise ≤ 8 °C; the molten salt temperature for the rear-stage molten salt treatment is 485 °C, the treatment time is 250 s, the molten salt circulation volume is 380 t / h, and the total treatment time is 306 s.

[0044] The roller path open-hood slow cooling process uses adjusting the opening degree of the heat preservation hood. The wire rod transported by the conveying roller path after passing through the second-stage salt bath tank is slowly cooled through the heat preservation hood, preventing the wire rod from having too fast a cooling rate during the cooling process, which may lead to an increase in stress, and promoting the further toughening of the wire rod structure, improving the softening effect of the wire rod, until coiling. Specifically: controlling the wire rod to slowly cool at a cooling rate of 0.4 °C / s to 276 °C; 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 warehousing.

[0045] Comparative Example 5: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 3 is that: the molten salt temperature for the rear-stage molten salt treatment is 600 °C, the treatment time is 360 s, the total treatment time is 456 s, and the finished wire rod is obtained.

[0046] Comparative Example 6: A manufacturing method of a wire rod, the difference between its manufacturing method and that of Example 3 is that: the molten salt temperature for the rear-stage molten salt treatment is 560 °C, the treatment time is 100 s, the total treatment time is 256 s, and the finished wire rod is obtained. Example 4:

[0047] A preferred implementation of the manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to the present invention. The chemical composition and mass percentage of the wire rod include C: 0.49%, Si: 0.36%, Mn: 0.42%, Cr: 0.58%, V: 0.035%, P: 0.014%, S: 0.015%, and the rest are Fe and inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → online molten salt isothermal quenching → roller path open-hood slow cooling → coiling. Specifically: The rolling process is used to heat a billet with a specification of 180mm×180mm into a hot billet with rollable plasticity through a heating furnace, promoting the homogenization of components. The heating furnace is controlled according to a three-stage heating program of a preheating section, a heating section, and a soaking section. The hot billet is rolled into wire rods with a diameter specification of 6.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the finishing rolling process, and refine the grains. Specifically: control the soaking temperature of the heating furnace at 1140°C, the residence time in the furnace at 150min, the initial rolling temperature at 1045°C, the finishing rolling temperature at 930°C, and the finishing rolling reduction at 23.5%; The wire laying process is used to make the wire rods exiting the rolling line into coiled rods through a wire laying machine. The coiled rods are scattered on the roller table and transported along the roller table, keeping the coiled rods in a high-temperature austenite state to prepare for quenching the structure at a relatively high quenching temperature. Specifically: control the wire laying temperature at 915°C.

[0048] The online molten salt isothermal quenching process uses a two-stage salt bath tank with molten salt inside. The coiled rods after wire laying are transported through the first-stage salt bath tank by the roller table for the front-stage molten salt treatment, cooling the coiled rods at a cooling rate of 34°C / s, quickly entering the bainite phase region from the high-temperature austenite state, inhibiting the formation of pearlite, and forming a quenched structure mainly composed of quenched bainite. Then the coiled rods are transported through the second-stage salt bath tank by the roller table for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the coiled rods to perform isothermal tempering on the quenched structure in the bainite phase region, toughening and stress relieving treatment, controlling the precipitation of carbides, and improving the strength-plasticity matching of the coiled rods. Specifically: the molten salt temperature for the front-stage molten salt treatment is 462°C, the treatment time is 34s, the molten salt circulation volume is 420t / h, and the molten salt temperature rise ≤ 8°C; the molten salt temperature for the rear-stage molten salt treatment is 470°C, the treatment time is 355s, the molten salt circulation volume is 295t / h, and the total treatment time is 389s.

[0049] The slow cooling process with the roller table cover open uses the adjustment of the opening degree of the heat preservation cover. The coiled rods transported by the conveying roller table through the second-stage salt bath tank are slowly cooled through the heat preservation cover, preventing the coiled rods from having too fast a cooling rate during the cooling process, which may lead to an increase in stress, and promoting the further toughening of the coiled rod structure, improving the softening effect of the coiled rods until coiling. Specifically: control the coiled rods to be slowly cooled at a cooling rate of 0.6°C / s to 271°C; The coiling process is used to coil the coiled rods into coils through a coiling drum, and the finished coiled rod products are obtained after packaging and warehousing.

[0050] Comparative Example 7: A manufacturing method of coiled rods, the difference between its manufacturing method and that of Example 4 lies in: its manufacturing method is carried out according to the technological process of rolling → wire laying → online molten salt isothermal quenching → air cooling → coiling. Specifically: the air cooling process uses the coiled rods transported by the conveying roller table through the second-stage salt bath tank to be naturally cooled in the air, and the coiled rods are cooled to 290°C at a cooling rate of 2.2°C / s to obtain coiled rods.

[0051] The wire rods obtained from the above Examples 1 to 4 and Comparative Examples 1 to 7 were subjected to microstructure and property tests, and the comparison results obtained are shown in Table 1 below: Table 1. Comparison results of microstructure and properties of different wire rod compositions and manufacturing methods

[0052] From the comparison results of Example 1 and Comparative Example 1, it can be seen that compared with the air-cooled line, which often weakens the strengthening effect due to the uncontrollable cooling rate during the cooling phase transformation and leads to the appearance of abnormal microstructures such as martensite, the present invention combines the design of V-containing chemical composition with the online molten salt isothermal toughening technology, which can effectively avoid the risk of brittle microstructures such as martensite and Widmanstatten structure caused by C and Cr elements, and can maximize the strengthening effect of carbon elements, improving the overall strength and plasticity of the wire rod; from the results of Examples 1 to 4, it can be seen that the Si / Mn / Cr content of the wire rod of the present invention is relatively low, and a small amount of V element is added. The microstructure mainly consists of tempered bainite and contains a small amount of ferrite, effectively adjusting the microstructure state. The high-strength structure of tempered bainite and dispersed carbides can make up for the strength loss caused by the reduction of alloy content, and the wire rod has better strength and toughness properties, with a tensile strength of 1120 - 1170 MPa and a reduction of area of 63% - 67%, which is beneficial to directly processing into tool parts after cold processing and forming.

[0053] From the comparison results of Example 1 and Comparative Example 2, it can be seen that at a higher spinning temperature, i.e., the quenching temperature, the carbon content in austenite is homogenized and alloying elements are fully dissolved, reducing the carbon concentration gradient and grain boundary segregation in the microstructure after quenching, and making organizational preparations for improving the strength and plasticity of the wire rod.

[0054] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the molten salt temperature of the front-section molten salt treatment is in the bainite phase region. The higher the molten salt temperature and the shorter the treatment time, the more beneficial it is to inhibit the formation of martensite and reduce the difficulty of toughening tempering. However, if the molten salt temperature is too high and the treatment time is too short, the diffusion ability of carbon in austenite is enhanced, affecting the quenching bainite phase transformation and microstructure refinement, and affecting the microstructure uniformity and matrix strength and toughness properties.

[0055] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the molten salt temperature of the front-section molten salt treatment is in the bainite phase region. The lower the molten salt temperature and the longer the treatment time, the more beneficial it is to inhibit the formation of pearlite, hinder the continuous growth of the feathery ferrite of upper bainite, promote the nucleation of acicular quenched bainite, and improve the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, it will increase the tissue stress, increase the difficulty of isothermal tempering and temperature rise control in the subsequent molten salt treatment, causing unnecessary increases in production energy consumption and time.

[0056] It can be seen from the comparison results between Example 3 and Comparative Example 5 that the higher the molten salt temperature and the longer the treatment time in the post-stage molten salt treatment, the more beneficial it is to reduce the dislocation density of quenched bainite and the tissue stress. As the time prolongs, it is beneficial for the vanadium-containing carbides to precipitate fully and dispersedly, providing an appropriate toughening effect. However, when the molten salt temperature is too high and the treatment time is too long, there is a risk of carbide coarsening, loss of strength, and significant decrease in toughness, and at the same time, the production energy consumption increases.

[0057] It can be seen from the comparison results between Example 3 and Comparative Example 6 that the lower the molten salt temperature and the shorter the treatment time in the post-stage molten salt treatment, the risk of excessive carbide coarsening can be reduced and the production energy consumption can be decreased. However, when the molten salt temperature is too low and the treatment time is too short, insufficient tempering driving force will lead to uneven carbide distribution and incomplete elimination of residual stress, affecting the plastic properties of the wire rod.

[0058] It can be seen from the comparison results between Example 4 and Comparative Example 7 that slow cooling with the roller path cover opened can prevent the stress from increasing due to too fast cooling rate during the cooling process of the wire rod, and promote further toughening of the wire rod structure, improving the softening effect of the wire rod.

[0059] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or modifications made without departing from the technical spirit of the present invention should be included within the protection scope of the present invention.

Claims

1. A manufacturing method of a 1200MPa grade high-strength tool steel wire rod, characterized in that, The manufacturing method thereof includes: After subjecting a steel billet to rolling and then wire-spinning it into wire rods at a wire-spinning temperature of ≥900 °C, an on-line molten salt isothermal quenching treatment is carried out. The wire rods are controlled to enter the bainite phase region from the high-temperature austenite state at a cooling rate of ≥32 °C / s, forming a quenched structure mainly composed of quenched bainite, followed by isothermal tempering and toughening stress relief treatment to control the precipitation of carbides. Finally, slow cooling is carried out with the cover opened on a roller table to produce wire rods with a microstructure including tempered bainite and ferrite. The chemical composition and mass percentage of the wire rods include: C: 0.44% - 0.49%, Si: 0.32% - 0.40%, Mn: 0.35% - 0.48%, Cr: 0.55% - 0.65%, V: 0.035% - 0.044%, P ≤ 0.015%, S ≤ 0.015%, and the balance is Fe and unavoidable impurities.

2. The manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to claim 1, characterized in that, Before the rolling, the soaking temperature of the heating furnace is controlled to be 1130 - 1180 °C, and the time in the furnace is 120 - 180 min.

3. The manufacturing method of the 1200MPa 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 1030 - 1080 °C, the final rolling temperature is 905 - 955 °C, and the reduction in the final rolling is 21% - 24%; during the wire-spinning, the wire-spinning temperature is controlled to be 900 - 945 °C.

4. The manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to claim 1, characterized in that, The on-line molten salt isothermal quenching treatment is divided into a front-stage molten salt treatment and a rear-stage molten salt treatment. The molten salt temperature is 400 - 490 °C, and the total treatment time < 410 s. The molten salt circulation volume in the front-stage molten salt treatment is greater than that in the rear-stage molten salt treatment.

5. The manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to claim 4, characterized in that, The molten salt temperature in the front-stage molten salt treatment is 400 - 490 °C, the treatment time is 25 - 56 s, the molten salt circulation volume is 400 - 500 t / h, and the temperature rise of the molten salt ≤ 8 °C.

6. The manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to claim 4, characterized in that, The molten salt temperature in the rear-stage molten salt treatment is 460 - 485 °C, the treatment time is 250 - 380 s, and the molten salt circulation volume is 250 - 380 t / h.

7. The manufacturing method of the 1200 MPa grade high-strength tool steel wire rod according to claim 4, characterized in that, The slow cooling with the cover opened on the roller table controls the wire rods to be slowly cooled to below 280 °C at a cooling rate of 0.4 - 0.8 °C / s.

8. A wire rod of 1200 MPa grade high-strength tool steel, characterized in that, The wire rods are obtained by the manufacturing method of the 1200 MPa grade high-strength tool steel wire rods described in any one of claims 1 - 7.

9. The wire rod of 1200 MPa grade high-strength tool steel according to claim 8, characterized in that, The volume percentage of the tempered bainite ≥ 94%.

10. The wire rod of 1200 MPa grade high-strength tool steel according to claim 8, characterized in that, The diameter of the wire rods is 5.5 - 9 mm, the tensile strength is 1120 - 1170 MPa, the reduction of area is 63% - 67%, and the difference in mechanical properties within the same coil ≤ 35 MPa.

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