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

Abstract

The present invention relates to a 1200MPa grade high-strength tool steel wire rod and a manufacturing method thereof. After rolling and spinning into a wire rod using a V-containing chemical composition, the wire rod is subjected to an online molten salt austempering treatment. The wire rod is cooled from a high-temperature austenite state to a bainite phase region at a cooling rate of 32°C / s or higher to form a quenched structure mainly composed of quenched bainite. The wire rod is isothermally tempered and subjected to a toughening and stress relief treatment to control the precipitation of carbides. Finally, the wire rod is slowly cooled through a roller with an open hood to form a wire rod with a microstructure including tempered bainite and ferrite. The wire rod can avoid the generation of deteriorated structures such as martensite, effectively adjust the structure state, achieve strength-plasticity matching of the wire rod, achieve a tensile strength of 1120-1170MPa, and a cross-sectional reduction rate of 63%-67%. The wire rod is suitable for the manufacture of 1200MPa grade high-strength tool steel and other application fields, and is conducive to being directly processed into tool parts after cold working.

Description

Technical Field

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

[0002] With the continuous advancement of technology, the application areas of tools are constantly expanding, and the durability and quality of tools are improving, which 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 rod as the masterbatch. Therefore, improving the strength grade and quality of tool steel requires improving the strength and plasticity matching of wire rod.

[0003] Conventional tool steel production is generally designed through hardenability elements, combined with Stelmor air-cooled and line-controlled cooling production, but still has the following defects:

[0004] (1) In order to take into account the cold forming, the tool steel wire rod is mainly pearlite based on the limitation of the cooling control capacity of the Stelmor air cooling line. For example, the patent CN119640155A discloses a low-cost, high-plasticity sorbite tool alloy steel wire rod and its manufacturing method, which adopts C-Si-Mn-Cr composition design combined with low-temperature rolling and spinning, air cooling and hood cooling to obtain sorbite wire rod, so as to improve the plasticity of the wire rod and avoid annealing and drawing. However, on the one hand, the addition of Mn and Cr hardenability components can promote the refinement of pearlite lamellae under the slower cooling conditions of the air cooling line to obtain sorbite structure with better plasticity, but it will also aggravate the segregation during the solidification process of the alloy. As the temperature difference between the windward side and the windward side, and from the edge to the core of the wire rod increases during the air cooling process, the wire rod will be more brittle and the brittleness will increase. The uncontrollability increases, and 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 working and heat treatment after cold working in the tool factory, and seriously reduces the quality level of the tool. On the other hand, in order to take into account the hardness of tool steel, the Si and Cr contents in the components are relatively high. At the same time, the minimum cooling control capacity of the air cooling line is limited. Excessive silicon content will make the ferrite grains coarse, resulting in a significant decrease in the toughness of the material and an increased risk of brittle fracture. A high chromium content will increase the difficulty of improving the plasticity of the structure. As the wire rod is in a low temperature state after continuous cooling and incubation of the structural phase transformation, insufficient thermal power leads to limited improvement in the plasticity of the final wire rod. After cold working, tempering and other heat treatments are still required to adjust the product performance, resulting in increased energy consumption and costs for downstream users, affecting the production efficiency of tool steel products.

[0005] (2) In order to improve the problem of high hardness of martensitic structure and easy brittle fracture during packaging, transportation and user processing, some alloy tool steel wire rods use air cooling lines to develop bainite wire rods. For example, patent CN117587211A discloses a high-efficiency and economical production method of alloy tool steel wire rods, which adopts C-Si-Mn-Cr-Ni-Al-Mo-V-Nb composition design, combined with air blowing cooling to the vicinity of bainite precipitation range after spinning, and then enters the hood phase transformation to form wire rods with bainite accounting for 60%~70%. However, on the one hand, the maximum cooling capacity of the air cooling line is limited, and the wire rod passes through the pearlite and upper bainite transformation temperature during the cooling process. The time in the temperature range is too long to effectively inhibit the diffusion of carbon, so that carbon can diffuse faster and precipitate carbides. Austenite is easily transformed in this temperature range to form pearlite and upper bainite, resulting in a loss of strength and plasticity, which requires the addition of higher alloy contents such as V and Nb in the wire rod, and increases the material cost. On the other hand, upper bainite is composed of ferrite laths and cementite distributed between the laths. It has high strength but poor toughness. As the wire rod is continuously cooled and incubated through microstructure phase transformation, it is already in a low-temperature state, and the microstructure stress is difficult to release, which will lead to a decrease in the cold working performance of the wire rod such as cold drawing and cold heading, resulting in processing defects and hand tool quality problems. Summary of the Invention

[0006] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a 1200MPa grade high-strength tool steel wire rod and a manufacturing method thereof, which can avoid the generation of deteriorated structures such as martensite, effectively adjust the organizational state, achieve strength-plasticity matching of the wire rod, and is conducive to direct processing into tool parts after cold forming.

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

[0008] A method for manufacturing a 1200 MPa grade high-strength tool steel wire rod, the manufacturing method comprising:

[0009] The steel billet is rolled and spun into a wire rod at a spinning temperature of ≥900°C, and then subjected to an online molten salt austempering treatment. The wire rod is cooled at a cooling rate of ≥32°C / s from a high-temperature austenite state to a bainite phase region to form a quenched structure mainly composed of quenched bainite, and isothermally tempered and toughened for stress relief to control carbide precipitation. Finally, the steel billet is slowly cooled on a roller with an open hood to form 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 are Fe and unavoidable impurities.

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

[0011] (1) Carbon: As a carbide strengthening element and austenite forming element, C is relatively cheap. As the carbon content increases, it can delay the pearlite transformation and provide the necessary carbon source for the formation of quenched bainite during the online molten salt quenching isothermal treatment. By forming fine dispersed carbides, it gives the wire rod high strength and wear resistance, and improves the ability of 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, increasing the difficulty of isothermal tempering, and affecting the isothermal tempering effect at a lower temperature. Therefore, in order to meet the strength and wear resistance requirements of hand tool steel and control material costs, while reducing the difficulty of controlling the wire rod microstructure during online molten salt isothermal quenching treatment and improving strength-toughness matching, a medium carbon content is used to balance strength and toughness, and the mass percentage of C is controlled to be 0.44%~0.49%.

[0012] (2) Silicon: The Si element can inhibit the grain coarsening during online molten salt quenching isothermal treatment. By hindering the diffusion of carbon into cementite, inhibiting the precipitation of cementite and reducing the nucleation energy of bainitic ferrite, the bainite transformation can still be carried out efficiently at higher temperatures and the tempering temperature is reduced. However, too high a silicon content will lead to excessive solid solution strengthening of silicon, aggravated lattice distortion, reduced toughness of steel, affecting plasticity control, and resulting in decreased plasticity during cold working. Therefore, in order to adapt to the control of the microstructure during online molten salt quenching isothermal treatment and avoid excessive embrittlement, the mass percentage of Si is controlled to be 0.32%~0.40%.

[0013] (3) Manganese: As a strong austenite stabilizing element, Mn can increase the hardenability of wire rod, reduce the tendency of pearlite and ferrite to form, strongly delay the pearlite transformation, expand the bainite transformation temperature range, make it easier to obtain quenched bainite structure in online molten salt austempering treatment, and improve strength and hardness through solid solution strengthening. However, when the Mn content is too high, it will promote the coarsening of austenite grains, inhibit the recrystallization of ferrite and the aggregation of carbides during tempering, require higher energy for stress release, and reduce the toughness and plasticity of steel. Therefore, in order to strengthen the matrix, make the bainite phase transformation and isothermal toughening stress relief of wire rod in a close temperature range, and simplify the molten salt temperature control, the mass percentage of Mn is controlled to 0.35%~0.48%.

[0014] (4) Chromium: As a strong carbide-forming element, Cr 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 the quenched bainite become the main transformation product during online molten salt austempering treatment, thereby improving the wear resistance and corrosion resistance of the tool. However, excessive Cr content will aggravate component segregation, increase the difficulty of controlling organizational uniformity, improve the tempering stability of bainitic ferrite, increase the difficulty of improving plasticity, and reduce cold working properties. Therefore, in order to enhance the wear resistance of steel, the bainite phase transformation and isothermal toughening stress relief of the wire rod are in a close temperature range, simplify the molten salt temperature control, and appropriately increase the Cr content. The mass percentage of Cr is controlled to be 0.55%~0.65%.

[0015] (5) Vanadium: As an alloying element, V element pins the grain boundaries to inhibit the growth of austenite grains, refines the original structure, and promotes the formation of high-density dislocations and twins in bainitic ferrite. It can form nano-scale VC carbides during the isothermal process to compensate for the strength loss of quenched bainite softened by tempering and reduce the crack tendency of hand tools during cold forming. However, the cost of V element is relatively high, and excessive addition is not conducive to controlling the cost of wire rod. Coarse VC particles precipitate along the grain boundaries, which will lead to the deterioration of the impact resistance of the tool. Therefore, based on the role of V element, cost and preparation control, the mass percentage of V is controlled to be 0.035%~0.044%.

[0016] (6) 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%.

[0017] The above-mentioned 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 trace amount of V element is added, which can appropriately reduce material costs. By optimizing the composition ratio, the bainite transformation can still be carried out efficiently at a higher temperature, reducing the difficulty of tempering stress relief, and providing favorable conditions for controlling quenching, toughening and carbide dispersion precipitation in the bainite phase region during online molten salt austempering treatment, simplifying molten salt control, and reducing production energy consumption. The higher spinning temperature, that is, the quenching temperature, makes the carbon content in the austenite uniform and the alloy elements fully dissolved, reducing the carbon concentration gradient and grain boundary segregation of the structure after quenching, making organizational preparations for improving the strength and plasticity of the wire rod. The wire rod after spinning is not air-cooled, but directly undergoes online molten salt austempering treatment:

[0018] 1. Compared with the Stelmor air cooling line, which has unstable temperature control and limited maximum cooling capacity, and leads to the mixture of hard and brittle martensite or low-strength pearlite, it tends to form upper bainite structure in a higher temperature range, which is unfavorable to cold working performance. When the wire rod undergoes online molten salt austempering, on the one hand, the thermal conductivity of molten salt is much higher than that of air, which can significantly increase the cooling rate of the wire rod compared to air cooling, and can cross the pearlite phase region, reduce the kinetic trend of pearlite transformation, and promote the wire rod from the high-temperature austenite state to below the bainite transformation nose temperature. At this time, the stability of austenite is extremely high, and the incubation period of pearlite and martensite transformation is greatly extended, forcing the austenite to transform to quenched bainite first. As the processing time is extended, it is easier to obtain In single quenching bainite structure, bainite transformation tends to form lower bainite rather than coarse upper bainite. Carbides in the lower bainite are evenly distributed in the ferrite needles in the form of nano-scale particles, which combines high strength and dislocation slip ability of fine needle-shaped ferrite, making up for the strength loss caused by reducing carbon or alloy content; on the other hand, during quenching, the wire rod passes through the molten salt, and the molten salt can cover the surface of the wire rod for uniform heat exchange, which will reduce the temperature difference from the edge to the core of the wire rod, avoiding the temperature difference problem between the winded side and the winded side in air cooling. The bainite phase transformation occurs at a higher temperature, avoiding entering the martensite phase region and forming a martensite structure. The phase transformation is sufficient to form a quenched structure dominated by quenched bainite, avoiding residual austenite and forming a low-temperature abnormal structure in the core.

[0019] 2. Compared with the Stelmor air-cooled line, the continuous cooling control and the hood cooling have limited minimum cooling capacity, the resulting microstructure stress is large, and the coarse and fine carbides are mixed, which affects the strengthening and toughening effect. On the one hand, after the bainite phase region is expanded, the bainite transformation can still be carried out efficiently at a higher temperature. As the processing time increases, the wire rod is transformed to the molten salt temperature, which can extend the time the wire rod is in the high-temperature isothermal range. The high-temperature isothermal tempering treatment of the quenched structure can provide more thermal power for toughening, thereby further improving the microstructure state and the 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. As time goes by, the precipitation of vanadium carbides overcomes the diffusion limitation at low temperature, the precipitation amount increases, and it can be fully precipitated and the distribution tends to be dispersed. The fine precipitates can effectively hinder the dislocation movement, improve the strength of the material, and avoid the coarsening or uneven distribution of carbides caused by high temperature, which reduces the toughness of the material. Finally, the wire rod is further toughened by the roller open cover slow cooling to prevent physical shrinkage and stress increase caused by too fast cooling speed, thereby controlling the wire rod organization state and improving the strength-plasticity matching of the wire rod.

[0020] Before the rolling, selecting appropriate heating furnace soaking temperature and furnace time can promote composition homogenization, reduce the impact of segregation, and avoid grain coarsening and overburning risks. In the preferred technical solution, before the rolling, the heating furnace soaking temperature is controlled to be 1130~1180℃, and the furnace time is 120~180min.

[0021] Since the spinning temperature is relatively high, the restrictions on rolling can be reduced. The initial rolling temperature can be appropriately increased to ensure plastic deformation ability, facilitate large deformation rolling, improve deformation uniformity during rolling, reduce cracking risk and wear on the rolling line, and at the same time control appropriate final rolling temperature and final rolling reduction to promote dynamic recrystallization during the final rolling process, refine grains, and avoid excessively low final rolling temperature and increased phase transformation stress during subsequent cooling. In the preferred technical solution, during the rolling, the initial rolling temperature is controlled to be 1030~1080℃, the final rolling temperature is 905~955℃, and the final rolling reduction is 21%~24%.

[0022] During the spinning, the spinning temperature can be further controlled to avoid grain coarsening caused by excessively high spinning temperature and reduce the risk of abnormal Widmanstätten structure. In a preferred technical solution, during the spinning, the spinning temperature is controlled to be 900-945°C.

[0023] The molten salt temperature of the online molten salt austempering treatment is in the bainite phase region. In a preferred technical solution, the online molten salt austempering 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 is less than 410 s. 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, which is conducive to further appropriately reducing production energy consumption and avoiding the total treatment time being too long, which causes carbide coarsening and loss of strength and toughness.

[0024] The front-end molten salt treatment is used to control the wire rod to quickly enter the bainite phase region from the high-temperature austenite state to form a quenched structure mainly composed of quenched bainite. The lower the molten salt temperature and the longer the treatment time, the more favorable it is to inhibit the formation of pearlite, hinder the continuous growth of feather-like ferrite in the upper bainite, promote the nucleation of needle-shaped quenched bainite, and improve the strength of the matrix. However, if the molten salt temperature is too low and the treatment time is too long, the tissue stress will be increased, and the difficulty of isothermal tempering and temperature rise control in the back-end molten salt treatment will be increased, resulting in unnecessary increase in production energy consumption and time. On the contrary, if the molten salt temperature is higher and the treatment time is too long, the stress of the tissue will be increased, and the difficulty of isothermal tempering and temperature rise control in the back-end molten salt treatment will be increased, resulting in unnecessary increase in production energy consumption and time. The shorter the treatment time, the more conducive it is to inhibiting the formation of martensite and reducing the difficulty of toughening and tempering. However, if the molten salt temperature is too high and the treatment time is too short, the diffusion ability of carbon in austenite will be enhanced, affecting the phase transformation and microstructure refinement of quenched bainite, and affecting the strength and toughness of the matrix. Therefore, appropriate molten salt temperature and treatment time can be selected, taking into account production energy consumption, to form a quenched microstructure dominated by quenched bainite, and to prepare the organization for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the front-stage molten salt treatment is 400~490℃, and the treatment time is 25~56s.

[0025] Since the temperature difference between the spinning temperature and the molten salt temperature of the front-stage molten salt treatment is large, selecting a larger molten salt circulation rate can control the molten salt temperature rise and improve the tissue consistency of the wire rod in continuous processing. In the preferred technical solution, the molten salt circulation rate of the front-stage molten salt treatment is 400~500t / h, and the molten salt temperature rise is ≤8°C.

[0026] The latter molten salt treatment is used to control the wire rod to perform high-temperature isothermal tempering on the quenched structure in the high-temperature isothermal range, toughening and stress relief treatment, and control carbide precipitation; the higher the molten salt temperature and the longer the treatment time, the more conducive to reducing the dislocation density of the quenched bainite and reducing the stress of the structure. As time goes on, it is conducive to the full dispersion and 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, production energy consumption increases. Conversely, the lower the molten salt temperature and the shorter the treatment time, the risk of excessive carbide coarsening and production energy consumption can be reduced. However, if the molten salt temperature is too low and the treatment time is too short, insufficient tempering thermal power will lead to uneven carbide distribution and insufficient 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 latter molten salt treatment is 460~485℃ and the treatment time is 250~380s.

[0027] The latter molten salt treatment can appropriately reduce the molten salt circulation volume, reduce production energy consumption, and at the same time accurately control the temperature to promote uniform regulation of the tissue state. In the preferred technical solution, the molten salt circulation volume of the latter molten salt treatment is 250~380t / h.

[0028] In the preferred technical solution, the roller open cover slow cooling controls the wire rod to slowly cool to below 280°C at a cooling rate of 0.4~0.8°C / s, which can prevent the wire rod from cooling too fast during the cooling process, resulting in increased stress, and promote further toughening of the wire rod structure, thereby improving the softening effect of the wire rod.

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

[0030] The above-mentioned wire rod is designed with a medium carbon composition containing a trace amount of V. After online molten salt austempering, a microstructure mainly composed of tempered bainite and a small amount of ferrite is obtained. Compared with the pearlite wire rod formed by the air-cooled line, the above-mentioned wire rod is a layered carbide. The alloy content of the above-mentioned wire rod is lower, which is conducive to reducing material costs. At the same time, the high-density dislocation of the quenched bainite can be used to provide strength characteristics, which has a stronger dislocation barrier effect. After tempering, a highly efficient strengthening structure of tempered bainite + dispersed carbides is formed, which compensates for the strength loss caused by reducing the alloy content without sacrificing plasticity. Compared with the wire rod containing martensite / upper bainite structure formed by the air-cooled wire, which leads to performance fluctuations, the above-mentioned wire rod can avoid the formation of brittle martensite structure. At the same time, coarse cementite is distributed between the feather-like ferrite strips of upper bainite, and the interface bonding strength is poor, which makes it easy to crack along the strips during stretching. The fine needle-like structure of tempered bainite can reduce the source of fatigue cracks. The ferrite matrix of tempered bainite has good plasticity, which makes the wire rod stronger and tougher, and the deformation is uniform during cold working, which is conducive to direct processing into tool parts after cold working and shortening the production cycle.

[0031] The tempered bainite has a stronger ability to hinder dislocation movement than 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 tempered bainite is ≥94%.

[0032] In the preferred technical solution, the diameter of the wire rod is 5.5~9mm, the tensile strength is 1120~1170MPa, the cross-sectional shrinkage rate is 63%~67%, and the mechanical property same-circle difference is ≤35MPa. Small and medium-sized wire rods can be suitable for tools with 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 uniformity of the organization effectively controls the mechanical property same-circle difference, and the cold working formability is better, which is conducive to improving the stability of the production process and reducing the cracking risk of direct cold working forming. At the same time, it can save heat treatment such as tempering after cold working and reduce production costs.

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

[0034] (1) In view of the current situation that the existing tool steel wire rod is difficult to control the deterioration of the structure and the strength and plasticity of the wire rod by combining the design of hardenability elements with the Stelmor air-cooled line controlled cooling production, the manufacturing method of the present invention can control the wire rod after rolling and spinning to quickly enter the bainite phase region from the high-temperature austenite state through the design of V-containing chemical composition and the online molten salt isothermal quenching technology, forming a quenched structure mainly composed of quenched bainite, and then subjecting the quenched structure to high-temperature isothermal tempering in the high-temperature isothermal range, toughening and stress relief treatment, and controlling the precipitation of carbides, and finally promoting further toughening of the wire rod structure through slow cooling with an open hood on the roller, thereby improving the softening effect of the wire rod, avoiding deterioration of martensite and other structures, inhibiting pearlite structure, and improving the strength and plasticity matching of the wire rod by adjusting the structure state. It can also further reduce the molten salt circulation amount of the later molten salt treatment and reduce production energy consumption, and has good industrial adaptability.

[0035] (2) In view of the current situation that the existing tool steel wire rod has high alloy component content, difficult to control deteriorated structure, high tissue stress, and insufficient strength and plasticity, the wire rod of the present invention has relatively low Si / Mn / Cr content and a trace amount of V element is added, which can appropriately reduce the material cost. The wire rod includes a microstructure mainly composed of tempered bainite and a small amount of ferrite, which can compensate for the strength loss caused by reducing the alloy content through the efficient strengthening structure of tempered bainite and dispersed carbides. The wire rod has better strength and toughness, and can achieve a tensile strength of 1120~1170MPa and a cross-sectional shrinkage rate of 63%~67%. It is suitable for the manufacture of 1200MPa grade high-strength tool steel and other application fields, and is conducive to being directly processed into tool parts after cold forming, 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 This is the metallographic structure diagram of Example 2 of the present invention. DETAILED DESCRIPTION

[0039] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention. They are intended to propose the best way to implement the present invention, are intended to explain the present invention, and are sufficient to enable those skilled in the art to practice the present invention, but should not be understood as limiting the scope of the present invention, which is defined solely by the appended claims. The wire rods obtained in the following embodiments and comparative examples are subjected to microstructure and performance testing, including: tensile testing using "GB-T 228.1-2021 Metallic Materials Tensile Testing Part 1: Room Temperature Test Method" to obtain tensile strength and cross-sectional reduction rate; microstructure testing is performed in accordance with the metal microstructure testing method of GB / T13298 standard; mechanical property same-turn difference test method: 2 turns of wire rod are taken 5m away from the end of the coil, and each turn of wire rod is divided into 8 equal sections with the overlap area as the base point. 1 tensile specimen is taken from each section, and the extreme difference in strength of the tensile specimens after the tensile test is the mechanical property same-turn difference. Example 1:

[0040] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.45%, Si: 0.34%, Mn: 0.35%, Cr: 0.55%, V: 0.036%, P: 0.015%, S: 0.014%, with the remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt austempering → roller open cover slow cooling → coiling, specifically:

[0041] 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 component homogenization, and the heating furnace is controlled according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 5.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1130°C, the furnace time is 180min, the initial rolling temperature is 1030°C, the final rolling temperature is 905°C, and the final rolling reduction is 24%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for tissue quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 900°C.

[0042] The online molten salt austempering 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 33°C / s, and quickly enters the bainite phase region from the high-temperature austenite state, inhibits the formation of pearlite, and forms a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to isothermally temper the quenching structure in the bainite phase region, toughening and stress relief treatment, controlling carbide precipitation, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 487°C, the treatment time is 25s, the molten salt circulation volume is 405t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 460°C, the treatment time is 380s, the molten salt circulation volume is 250t / h, and the total treatment time is 405s.

[0043] The roller open cover slow cooling process adopts the method of adjusting the opening of the insulation cover, and the wire rod conveyed by the conveyor roller through the second section of the salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, thereby increasing the stress, promoting further toughening of the wire rod structure, and improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 265°C at a cooling rate of 0.8°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0044] Comparative Example 1:

[0045] A method for manufacturing wire rod, which differs from that of Example 1 in that: the manufacturing method is manufactured according to the process flow of rolling → spinning → Stelmor air cooling line → coiling, specifically: in the rolling process, the heating furnace soaking temperature is controlled to 1080°C, the furnace time is 220min, the initial rolling temperature is 950°C, the final rolling temperature is 845°C, and the spinning temperature is controlled to 830°C. The Stelmor air cooling line adopts the front 1~6# insulation covers to be opened, and the fan is turned on to control the wire rod to be cooled to 695°C at a rate of 3.7°C / s. Thereafter, the insulation cover is closed, the wire rod enters the insulation cover and is cooled to 260°C at a rate of 2.2°C / s, and is collected by the coiling drum to obtain a finished wire rod.

[0046] Comparative Example 2:

[0047] A method for manufacturing a wire rod, which differs from Example 1 in that: the soaking temperature of the heating furnace is controlled to 1080°C, the time in the furnace is 220 minutes, the initial rolling temperature is 950°C, the final rolling temperature is 845°C, the spinning temperature is controlled to be 830°C, the wire rod is treated with molten salt in the front section and cooled at a cooling rate of 29°C / s to obtain a finished wire rod. Example 2:

[0048] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.44%, Si: 0.32%, Mn: 0.46%, Cr: 0.65%, V: 0.039%, P: 0.014%, S: 0.014%, and the remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt austempering → roller open cover slow cooling → coiling, specifically:

[0049] 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 component homogenization, and the heating furnace is controlled according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 8mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1160°C, the time in the furnace is 135min, the initial rolling temperature is 1065°C, the final rolling temperature is 945°C, and the final rolling reduction is 22%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for tissue quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 935°C.

[0050] The online molten salt austempering 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 bainite phase region from the high-temperature austenite state, inhibits the formation of pearlite, and forms a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to isothermally temper the quenching structure in the bainite phase region, toughening and stress relief treatment, controlling carbide precipitation, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 433°C, the treatment time is 46s, the molten salt circulation volume is 475t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 477°C, the treatment time is 290s, the molten salt circulation volume is 365t / h, and the total treatment time is 336s.

[0051] The roller open cover slow cooling process adopts the method of adjusting the opening of the insulation cover, and the wire rod conveyed by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, which leads to an increase in stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 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 a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0052] Comparative Example 3:

[0053] A method for manufacturing 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 31°C / s, the molten salt temperature of the front-stage molten salt treatment is 505°C, the treatment time is 20s, and the total treatment time is 310s to obtain a finished wire rod.

[0054] Comparative Example 4:

[0055] A method for manufacturing 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 40°C / s, the molten salt temperature of the front-stage molten salt treatment is 365°C, the treatment time is 70s, and the total treatment time is 360s to obtain a finished wire rod. Example 3:

[0056] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.45%, Si: 0.4%, Mn: 0.48%, Cr: 0.62%, V: 0.044%, P: 0.012%, S: 0.012%, and the remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt austempering → roller open cover slow cooling → coiling, specifically:

[0057] 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 component homogenization, and the heating furnace is controlled according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 9mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1180°C, the furnace time is 120min, the initial rolling temperature is 1080°C, the final rolling temperature is 955°C, and the final rolling reduction is 21%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for tissue quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 945°C.

[0058] The online molten salt austempering process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 39°C / s, and quickly enters the bainite phase region from the high-temperature austenite state, inhibits the formation of pearlite, and forms a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for the rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to isothermally temper the quenched structure in the bainite phase region, toughening and stress relief treatment, controlling carbide precipitation, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt treatment is 402°C, the treatment time is 56s, the molten salt circulation volume is 500t / h, and the molten salt temperature rise is ≤8°C; the molten salt temperature of the rear-stage molten salt treatment is 485°C, the treatment time is 250s, the molten salt circulation volume is 380t / h, and the total treatment time is 306s.

[0059] The roller open cover slow cooling process adopts the method of adjusting the opening of the insulation cover, and the wire rod transported by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, thereby increasing the stress, promoting further toughening of the wire rod structure, and improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 276°C at a cooling rate of 0.4°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage.

[0060] Comparative Example 5:

[0061] A method for manufacturing a wire rod, which differs from Example 3 in that the molten salt temperature of the latter molten salt treatment is 600°C, the treatment time is 360s, the total treatment time is 456s, and the finished wire rod is obtained.

[0062] Comparative Example 6:

[0063] A method for manufacturing a wire rod, which differs from Example 3 in that the molten salt temperature of the latter molten salt treatment is 560°C, the treatment time is 100s, the total treatment time is 256s, and the finished wire rod is obtained. Example 4:

[0064] A preferred embodiment of the method for manufacturing 1200MPa grade high-strength tool steel wire rod of the present invention comprises the following chemical compositions and mass percentages: C: 0.49%, Si: 0.36%, Mn: 0.42%, Cr: 0.58%, V: 0.035%, P: 0.014%, S: 0.015%, and the remainder being Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt austempering → roller open cover slow cooling → coiling, specifically:

[0065] 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 component homogenization, and the heating furnace is controlled according to a three-stage temperature rise program of a preheating section, a heating section, and a soaking section. The high-temperature steel billet is rolled into a wire rod with a diameter of 6.5mm through a rolling line. Appropriate rolling temperature and reduction are selected to increase the rolling speed, promote dynamic recrystallization during the final rolling process, and refine the grains. Specifically, the soaking temperature of the heating furnace is controlled to be 1140°C, the furnace time is 150min, the initial rolling temperature is 1045°C, the final rolling temperature is 930°C, and the final rolling reduction is 23.5%; the wire-spinning process is used to convert the wire rod exiting the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on a roller and transported along the roller, so that the wire rod is in a high-temperature austenite state and is prepared for structural quenching with a higher quenching temperature; specifically, the wire-spinning temperature is controlled to be 915°C.

[0066] The online molten salt austempering 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 front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 34°C / s, and quickly enters the bainite phase region from the high-temperature austenite state, inhibits the formation of pearlite, and forms a quenching structure mainly composed of quenched bainite. After that, the wire rod is conveyed by a roller through the second salt bath tank for rear-stage molten salt treatment, reducing the molten salt circulation volume, controlling the wire rod to isothermally temper the quenching structure in the bainite phase region, toughening and stress relief treatment, controlling carbide precipitation, and improving the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature of 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 is ≤8°C; the molten salt temperature of 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.

[0067] The roller open cover slow cooling process adopts the method of adjusting the opening of the insulation cover, and the wire rod transported by the conveyor roller through the second salt bath tank is slowly cooled through the insulation cover to prevent the wire rod from cooling too fast during the cooling process, thereby increasing the stress, promoting further toughening of the wire rod structure, and improving the softening effect of the wire rod until it is coiled. Specifically: the wire rod is controlled to slowly cool to 271°C at a cooling rate of 0.6°C / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the finished wire rod is obtained after packaging and storage.

[0068] Comparative Example 7:

[0069] A method for manufacturing a wire rod, which differs from that of Example 4 in that: the manufacturing method is manufactured according to a process flow of rolling → spinning → online molten salt austempering → 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 290°C at a cooling rate of 2.2°C / s to obtain the wire rod.

[0070] The structure and performance of the wire rods obtained in Examples 1 to 4 and Comparative Examples 1 to 7 were tested, and the comparative results are shown in Table 1 below:

[0071] Table 1. Comparison of microstructure and properties of different wire rod compositions and manufacturing methods

[0072]

[0073] From the comparison results of Example 1 and Comparative Example 1, it can be seen that compared with the air-cooled line, the strengthening effect is often weakened due to the uncontrollable cooling rate during the cooling phase transformation process, and abnormal structures such as martensite are generated. The present invention can effectively avoid the risk of C and Cr elements generating brittle structures such as martensite and widmanstattenite by combining the V-containing chemical composition design with the online molten salt isothermal toughening technology, and can maximize the strengthening effect of carbon elements and improve 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 trace amount of V element is added, including a microstructure mainly composed of tempered bainite and a small amount of ferrite, which effectively adjusts the organizational state. The strength loss caused by reducing the alloy content can be compensated by the efficient strengthening structure of tempered bainite and dispersed carbides. The wire rod has better strength and toughness, and can reach a tensile strength of 1120~1170MPa and a cross-sectional shrinkage rate of 63%~67%, which is conducive to being directly processed into tool parts after cold forming.

[0074] From the comparison results of Example 1 and Comparative Example 2, it can be seen that a higher spinning temperature, i.e., quenching temperature, can make the carbon content in austenite uniform, fully dissolve the alloy elements, reduce the carbon concentration gradient and grain boundary segregation of the structure after quenching, and make organizational preparations for improving the strength and plasticity of the wire rod.

[0075] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the molten salt temperature of the front-stage molten salt treatment is in the bainite phase region. The higher the molten salt temperature and the shorter the treatment time, the more conducive to inhibiting the formation of martensite and reducing the difficulty of toughening and 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, which affects the quenching bainite phase transformation and microstructure refinement, and affects the microstructure uniformity and matrix strength and toughness.

[0076] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the molten salt temperature of the front-stage molten salt treatment is in the bainite phase region. The lower the molten salt temperature and the longer the treatment time, the more conducive it is to inhibiting the formation of pearlite, hindering the continuous growth of upper bainite feather-like ferrite, promoting the nucleation of needle-shaped quenched bainite, and improving the matrix strength. However, if the molten salt temperature is too low and the treatment time is too long, the tissue stress will be increased, the difficulty of isothermal tempering and temperature rise control in the back-stage molten salt treatment will be increased, and unnecessary increase in production energy consumption and time will be caused.

[0077] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the higher the molten salt temperature and the longer the treatment time of the later molten salt treatment, the more beneficial it is to reduce the dislocation density of the quenched bainite and reduce the structural stress. As time goes on, it is beneficial to fully disperse and precipitate 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, loss of strength, and a significant decrease in toughness, and at the same time, production energy consumption increases.

[0078] From the comparison results of Example 3 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 risk of excessive coarsening of carbides and the production energy consumption can be reduced. However, if the molten salt temperature is too low and the treatment time is too short, the insufficient tempering thermal power will lead to uneven distribution of carbides and insufficient elimination of residual stress, which will affect the plastic properties of the wire rod.

[0079] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow cooling with the roller cover open can prevent the wire rod from cooling too fast during the cooling process, which leads to increased stress, and promotes further toughening of the wire rod structure, thereby improving the softening effect of the wire rod.

[0080] 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 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 ≥900°C, and then subjected to an online molten salt austempering treatment. The wire rod is cooled from a high-temperature austenite state to a bainite phase region at a cooling rate of ≥32°C / s to form a quenched structure mainly composed of quenched bainite. The steel billet is then isothermally tempered and toughened for stress relief to control carbide precipitation. Finally, the steel billet is slowly cooled on a roller table with an open hood to form a wire rod having a microstructure including tempered bainite and ferrite. The chemical composition and mass percentage of the wire rod include: C: 0.44% to 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 are Fe and unavoidable impurities; the online molten salt austempering 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, the molten salt circulation amount of the front-stage molten salt treatment is greater than the molten salt circulation amount of the rear-stage molten salt treatment, and the treatment time of the rear-stage molten salt treatment is 250~380s.

2. The method for manufacturing 1200MPa 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 soaking time in the furnace is 120-180 min.

3. The method for manufacturing 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 controlled to be 905-955° C., and the final rolling reduction is controlled to be 21%-24%; during the wire drawing, the wire drawing temperature is controlled to be 900-945° C.

4. The method for manufacturing 1200 MPa grade high strength tool steel wire rod according to claim 1, characterized in that: The total processing time of the online molten salt austempering treatment is less than 410 s.

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

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

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

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

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

10. The 1200 MPa grade high strength tool steel wire rod according to claim 8, characterized in that: The diameter of the wire rod is 5.5-9 mm, the tensile strength is 1120-1170 MPa, the cross-sectional shrinkage is 63%-67%, and the mechanical property difference within the same circle is ≤35 MPa.

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