High-plasticity wire rod for hand tool and manufacturing method of high-plasticity wire rod

Abstract

The invention relates to a high-plasticity wire rod for a hand tool and a manufacturing method of the high-plasticity wire rod, and the manufacturing method comprises the following steps: carrying out rolling, spinning, on-line molten salt isothermal toughening and roller way slow cooling treatment on a steel billet to prepare the wire rod with a microscopic structure comprising tempered bainite and ferrite, phase change and isothermal toughening treatment of the wire rod in a ferrite and bainite mixed phase region are controlled through online molten salt isothermal toughening, the wire rod adopts Cr-V chemical component design, the content of alloy elements such as Mn and V is relatively low, the online molten salt isothermal toughening technology is combined, the material cost and the mechanical property fluctuation of the wire rod can be reduced, the plasticity and cold machining performance of the wire rod are effectively improved, and the service life of the wire rod is prolonged. The tensile strength of the wire rod is 910-960 MPa, the percentage reduction of area is 58%-65%, extra heat treatment before cold machining of a hand tool is omitted, the cracking risk of direct cold machining of the wire rod is reduced, and the machining cost of the hand tool is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of hot-rolled wire rods, and particularly relates to a high-plasticity wire rod for hand tools and a manufacturing method thereof. Background Art

[0002] Hand tools include daily installation and repair tools such as screwdrivers, pliers, hammers, wrenches, etc. As essential tools for life and production, the market's requirements for their durability and service life have been increasing year by year. Moreover, hand tools often use hot-rolled wire rods as the base material and are continuously manufactured through cold working methods such as cold drawing and cold heading. Therefore, the material selection of wire rods for hand tools is crucial for the quality and market benefits of hand tools. Traditional wire rods for hand tools are generally produced using the Stelmor air-cooling line after wire laying and usually contain relatively high alloying elements such as carbon, silicon, manganese, chromium, vanadium, tungsten, etc. to ensure the use performance of the material after cold working manufacturing. However, with the addition of alloying elements, the probability of forming brittle structures such as bainite and martensite in the wire rods for hand tools during rolling and cooling continuously increases, reducing their cold working performance and resulting in cracking and fracture of the wire rods for hand tools during cold working. Even before cold working, additional heat treatments such as annealing need to be carried out to improve their cold working performance, leading to a sharp increase in processing costs. Therefore, it is necessary to develop a high-plasticity wire rod for hand tools and a manufacturing method thereof to meet the development of the hand tool industry and market usage requirements.

[0003] There are also the following technical difficulties in manufacturing high-plasticity wire rods for hand tools with lower manufacturing material costs and better cold working performance: In order to ensure that the steel wire rod can obtain high hardness and high wear resistance and meet the requirements of hand tools for resisting wear and deformation during use, the wire rod for hand tools needs to add relatively high contents of alloying elements such as carbon, manganese, and chromium. However, affected by the segregation and aggregation of alloying elements in the core, the hardenability of the steel, and the limited minimum controlled cooling capacity of the Stelmor cooling line for the wire rod after spinning in the air cooling line, and the large instability of controlling the heat exchange between the wire rod and air by the air volume of the fan, the controlled cooling is difficult. In some positions of the wire rod, due to rapid cooling, segregation, and hardenability effects, low-temperature brittle structures such as bainite and martensite are easily formed, resulting in insufficient plasticity of the wire rod and large fluctuations in the mechanical properties of the same coil. Furthermore, the risk of cracking in subsequent cold processing is relatively high. Therefore, in order to improve the cold processing performance, in the prior art, it is generally desired to eliminate the influence of low-temperature phase transformation structures such as bainite and martensite, reduce the initial strength of the hot-rolled wire rod, improve the plasticity, and enable users to improve and achieve the final comprehensive mechanical properties of hand tools through heat treatments such as quenching and tempering after cold processing. For example, the chromium-vanadium series hot-rolled steel wire rod and its preparation method, as well as the preparation methods of steel wires and hand tools, disclosed in Patent CN111876679B. The chromium-vanadium series wire rod obtains a wire rod with a pearlite + ferrite + sorbite structure by controlling the total content of Mn and Cr and the alloy core segregation index combined with air-cooled slow cooling, achieving a tensile strength of 800 - 950 MPa and a reduction of area ≥ 50%. However, on the one hand, in order to improve the strength and toughness of the material, the content of vanadium in the material is relatively high. However, as a precious alloy, vanadium will increase the material cost, and during the controlled cooling process in the air cooling line, the wire rod cools continuously and slowly, affecting the precipitation driving force of vanadium carbides, with the risks of incomplete precipitation or rapid coarsening of carbides due to unstable air cooling control, which will affect the plasticity of the wire rod and increase the fluctuations in the mechanical properties of the wire rod. On the other hand, the control of the segregation of alloying elements in the steel billet is limited. As the wire rod specification increases, the alloy content of the wire rod and the temperature gradient on the same cross-section during air cooling will further increase, and the limited minimum controlled cooling capacity of the air cooling line will increase the difficulty of controlling the structure and properties of the wire rod. At the same time, during the continuous slow cooling process of the wire rod, it will pass through the ferrite, sorbite, and low-temperature tissue phase regions. After the wire rod undergoes phase transformation, it is already in a low-temperature state, and residual tissue stress will remain in the tissue, restricting the improvement of the plasticity of the wire rod. The remaining austenite that has not undergone phase transformation will still continue to form martensite low-temperature brittle tissue during subsequent cooling, affecting the structure and properties of the same coil of the wire rod, and causing phenomena such as local stress concentration and uneven deformation during cold processing, thereby increasing the cold processing cracking risk and processing cost of downstream users. Summary of the Invention

[0004] The present invention aims to solve at least one of the above technical problems to some extent. The present invention provides a high-plasticity wire rod for hand tools and a manufacturing method thereof, which can reduce the material cost and the fluctuation of the mechanical properties of the wire rod, effectively improve the plasticity and cold working performance of the wire rod, and is beneficial to eliminating the additional heat treatment before the cold working of hand tools, reducing the cracking risk of direct cold working of the wire rod, and reducing the processing cost of hand tools.

[0005] The technical solution adopted by the present invention to solve its technical problems is as follows: The first aspect of the present invention lies in providing a manufacturing method of a high-plasticity wire rod for hand tools, and the manufacturing method includes: The steel billet is subjected to rolling, wire laying, online molten salt isothermal toughening and roller table slow cooling treatments to make a wire rod with a microstructure including tempered bainite and ferrite. The online molten salt isothermal toughening controls the phase transformation and isothermal toughening treatment of the wire rod in the mixed phase region of ferrite and bainite. The chemical composition and mass percentage of the wire rod include: C: 0.43% - 0.48%, Si: 0.20% - 0.30%, Mn: 0.50% - 0.70%, Cr: 0.45% - 0.60%, V: 0.045% - 0.055%, P ≤ 0.015%, S ≤ 0.015%, and the rest are Fe and inevitable impurities.

[0006] The design basis for the chemical composition and mass percentage of the above wire rod includes: (1) Carbon: As an effective carbide strengthening element and austenite forming element, the C element has a relatively lower price. With the increase of the carbon content, solid solution strengthening can be generated, the austenite transformation temperature can be reduced, the bainite transformation driving force and bainite transformation temperature can be increased, and it can promote the formation of a mixed structure mainly composed of bainite in the mixed phase region of ferrite and bainite when the wire rod after wire laying is controlled by online molten salt isothermal toughening, meeting the requirements of hand tools for hardness, wear resistance and shear resistance. However, if the carbon content is too high, it will increase the carbon segregation tendency during the solidification of the steel billet, inhibit the precipitation of ferrite, cause pearlite transformation, and affect the plastic toughness of the material and the control of carbide precipitation. Therefore, in order to meet the high hardness and wear resistance requirements of hand tool steel, control the material cost, and facilitate the control of the microstructure, carbide precipitation and the improvement of the material plasticity, a medium carbon content is adopted, and the mass percentage of C is controlled at 0.43% - 0.48%.

[0007] (2) Silicon: Si is the main deoxidizing element in steel. It can also inhibit grain coarsening during online molten salt isothermal toughening. By reducing carbon activity, the cooling curve is shifted to the bainite region to inhibit pearlite transformation. During the online molten salt isothermal toughening control process after spinning, ferrite + bainite structure is preferentially formed, solid solution strengthening ferrite and bainite matrix, and the elastic limit can also be improved. It is suitable for tools that need to resist deformation, such as wrenches and pliers. However, too high silicon content will reduce the toughness of steel and lead to a decrease in plasticity during cold heading. Therefore, the mass percentage of Si is controlled to be 0.20%~0.30%.

[0008] (3) Manganese: As an austenite-forming element, Mn can increase the hardenability of wire rod, reduce the ferrite formation temperature and bainite transformation temperature, inhibit the early coarse precipitation of ferrite, expand the bainite transformation range, and delay the precipitation of pearlite. In addition, the online molten salt isothermal toughening can control the phase transformation of wire rod in the mixed phase region of ferrite and bainite. Ferrite can overlap and precipitate with the bainite phase region at a lower temperature, promote the mixing of the two and improve the matrix strength. It is suitable for tools subjected to impact loads. However, when the Mn content is too high, it will aggravate the segregation of alloy elements, increase the difficulty of isothermal toughening, and reduce the toughness and plasticity of steel. Therefore, in order to take into account the impact fracture resistance of hand tools and facilitate the phase transformation and tempering toughening control of wire rod, the mass percentage of Mn is controlled to be 0.50%~0.70%.

[0009] (4) Chromium: As a carbide-forming element, Cr can form fine carbides, pin grain boundaries and inhibit the growth of austenite grains. At the same time, it improves the hardenability of steel, reduces the diffusion coefficient of carbon in austenite, shifts the pearlite transformation kinetic curve to the right, delays the pearlite transformation, and creates favorable conditions for the preferential formation of ferrite and bainite during the controlled cooling process, which is beneficial to improving the wear resistance and corrosion resistance of steel. However, excessive Cr content will aggravate component segregation, increase the difficulty of controlling the uniformity of the organization and the improvement of plasticity, and reduce the cold working performance of the wire rod. Therefore, in order to take into account the wear resistance of hand tools and the service life in a humid environment, and to facilitate the microstructure and plasticity control of the wire rod, the mass percentage of Cr is controlled to be 0.45%~0.60%.

[0010] (5) Vanadium: As an alloying element, V can pin the austenite grain boundaries during the controlled rolling stage, thereby refining the original austenite grains. It can be dispersed and precipitated during the online molten salt isothermal toughening process, effectively refining the material grains and providing a strong precipitation strengthening effect, thereby refining the structure and significantly improving the fatigue strength and toughening the matrix. It is beneficial to reduce cracks during cold forming of hand tools. However, the cost of V is relatively high, and excessive addition is not conducive to controlling the cost of wire rods. Too much V will also cause element enrichment and rapid coarsening of carbides. Coarsened carbides affect strength and toughness. Therefore, based on the role of V, cost and preparation control, the mass percentage of V is controlled at 0.045%~0.055%.

[0011] (6) Phosphorus and sulfur: Elements P and S are impurity elements, and the lower the better. Therefore, P ≤ 0.015% and S ≤ 0.015% are controlled.

[0012] The above wire rod adopts the Cr-V chemical composition design. The content of V is relatively low, which can reduce the material cost. Combining with the optimized ratio of C, Si, and Mn, taking into account the requirements of strength, high toughness, and wear resistance for hand tool steel, regulating the thermodynamics and kinetics of phase transformation, making the ferrite and bainite phase transformation regions have the characteristic of intersection. The preferential nucleation of ferrite and bainite will consume carbon and other alloying elements in the surrounding austenite, changing the local composition and structure, making the nucleation conditions of pearlite unfavorable. Furthermore, the ferrite is used to suppress the pearlite phase transformation, providing favorable conditions for controlling tissue phase transformation, carbide precipitation, and toughening during online molten salt isothermal toughening treatment. On this basis, the wire rod after spinning is directly treated online with molten salt without air cooling: I. Compared with the limited cooling capacity, difficult controlled cooling, and continuous cooling treatment of the Stelmor air cooling line, which result in uncontrollable bainite phase transformation and carbide precipitation and large fluctuations in mechanical properties, the wire rod in the high-temperature austenite state after spinning is treated with molten salt. On the one hand, the molten salt has a stronger heat transfer capacity than the air cooling line. The wire rod can quickly enter the ferrite and bainite mixed phase region, inhibiting grain coarsening, increasing the precipitation driving force of bainite and vanadium-containing carbides, promoting the transformation of high-temperature austenite in the tissue into ferrite and bainite, inhibiting ferrite coarsening and pearlite formation, and quickly forming a mixed tissue mainly composed of bainite and a small amount of ferrite to ensure the matrix strength. On the other hand, the high heat transfer capacity of the molten salt and the uniform heat transfer by the molten salt covering the surface of the wire rod can reduce the temperature gradient across the cross-section of the larger-sized wire rod, enabling the wire rod to be maintained at the molten salt temperature for isothermal treatment, that is, extending the treatment time of the wire rod in the ferrite and bainite mixed phase region, promoting the full transformation of austenite into ferrite and bainite, making the precipitation of bainite controllable, and avoiding the formation of martensite low-temperature brittle tissue by the residual austenite in the core during subsequent cooling, effectively controlling abnormal tissues. It also extends the treatment time of the wire rod in the temperature range of vanadium-containing carbide precipitation, promoting the full dispersion precipitation of vanadium-containing carbides, avoiding the coarsening of carbide precipitation and affecting the tissue uniformity, and thus reducing the fluctuations in the mechanical properties of the wire rod.

[0013] Second, compared with the limited minimum cooling capacity of the Stelmor air-cooled line and the insufficient plasticity of the wire rod caused by continuous cooling treatment, since the temperature of the wire rod in the ferrite and bainite mixed phase region is relatively high, by isothermal treatment instead of continuous cooling, that is, the treatment time of the wire rod in the higher temperature range is extended, isothermal toughening of ferrite and bainite can be promoted, internal stress can be eliminated, the carbide particles in the bainite structure become finer and more dispersed, and are transformed into tempered bainite that is both strong and tough, reducing the splitting effect on the matrix, facilitating the movement and slip of dislocations, rapidly improving the plasticity of the material, adjusting the strength-plasticity matching of the wire rod. In the subsequent slow cooling treatment on the roller table, taking advantage of the fact that the wire rod is still at a relatively high temperature after passing through the ferrite and bainite mixed phase region, slow cooling can continue the toughening effect in the later stage of in-line molten salt isothermal toughening, further promoting the temper softening of the matrix structure, so that the bainite structure, which is conventionally regarded as an abnormal structure, can be effectively utilized. At the same time, the ferrite maintains a certain plasticity, and finally a mixed structure with good strength-toughness matching is obtained, realizing the high plasticity of the wire rod.

[0014] In order to further improve the surface quality of rolling and increase the rolling efficiency, an appropriate initial rolling descaling water pressure is selected and sprayed onto the surface of the steel billet to break and remove the scale. In the preferred technical solution, before rolling, the initial rolling descaling water pressure after the steel billet exits the heating furnace is controlled to be ≥16 MPa.

[0015] Since in-line molten salt isothermal toughening can control the strengthening-toughening and plasticity improvement effects, it is not necessary to adopt too low a rolling temperature in the rolling stage. When rolling, an appropriate final rolling entry temperature is selected, which is beneficial to achieving uniform deformation during final rolling, avoiding difficult deformation caused by too low a temperature, generating excessive internal stress or even cracks. At the same time, with an appropriate final rolling deformation amount, through deformation-induced austenite grain refinement, the austenite grain size is relatively small and uniform, which helps to refine the tissue grains. In the preferred technical solution, when rolling, the final rolling entry temperature is controlled to be 900 - 935 °C, and the final rolling deformation amount is 20% - 25%.

[0016] In order to better control the strength and plasticity of ferrite and the matrix, a smaller molten salt circulation volume can be selected in the early stage of the in-line molten salt isothermal toughening treatment than in the later stage, so as to appropriately reduce the cooling rate of the wire rod, enable ferrite to precipitate better, avoid the influence or inhibition of ferrite precipitation caused by too fast a cooling rate, and avoid grain coarsening or affecting the rapid phase transformation of bainite caused by too slow a cooling rate. In the later stage of the in-line molten salt isothermal toughening treatment, the molten salt circulation volume can be increased to improve the temperature accuracy and promote uniform toughening of the tissue. In the preferred technical solution, the in-line molten salt isothermal toughening controls the wire rod to first pass through the front-section molten salt, cool down at a cooling rate of ≥10 °C / s and enter the ferrite and bainite mixed phase region from the austenite state, forming a mixed structure mainly composed of bainite, and then pass through the rear-section molten salt to increase the molten salt circulation volume and promote the isothermal toughening of ferrite and bainite.

[0017] The molten salt temperature in the front stage is in the ferrite and bainite mixed phase region. The lower the molten salt temperature and the longer the treatment time in the front stage, the slower the formation rate of ferrite and the smaller the ferrite grain size will be, the driving force for bainite transformation and vanadium-containing carbide increases, the nucleation rate of bainite increases, the matrix strength rises and the plasticity decreases. However, if the molten salt temperature is too low and the treatment time is too long, it is not conducive to the formation of ferrite transformation, affects the temperature control accuracy, increases the difficulty of plasticity improvement, and is not conducive to the control of the plasticity and tissue uniformity of the wire rod. On the contrary, the higher the molten salt temperature and the shorter the treatment time in the front stage, the lower the cooling rate of the wire rod, the more ferrite phase transformation, and the plasticity of the matrix rises. However, if the molten salt temperature is too high and the treatment time is too short, the ferrite grain size increases and the toughness is lost, the driving force for bainite transformation decreases and the nucleation rate decreases, which is not conducive to the formation of a mixed structure mainly composed of bainite and affects the matrix strength. Therefore, controlling the molten salt temperature and treatment time in the front stage can promote the wire rod to quickly enter the ferrite and bainite mixed phase region in the high-temperature austenite state, appropriately reduce the cooling rate of the wire rod to promote the formation of ferrite, take into account the driving force for bainite transformation, and form a mixed structure mainly composed of bainite, making organizational preparations for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature in the front stage is 540-570 °C, and the treatment time is 30-120 s.

[0018] The molten salt circulation volume of the front-stage molten salt is positively correlated with the wire rod cooling rate. Selecting an appropriate molten salt circulation volume can further cooperate with the molten salt temperature and treatment time to control the phase transformation of the wire rod. In the preferred technical solution, the molten salt circulation volume of the front-stage molten salt is 50-150 t / h.

[0019] The molten salt temperature in the latter stage is in the ferrite and bainite mixed phase region. The higher the molten salt temperature and the longer the treatment time in the latter stage, it is beneficial to provide more thermal power for isothermal toughening and enable the matrix plasticity to be rapidly improved. However, if the molten salt temperature is too high and the treatment time is too long, it is not conducive to controlling the dispersion precipitation of carbides and there is a risk of carbide coarsening, affecting the strength and plasticity of the matrix. On the contrary, if the molten salt temperature in the latter stage is too low and the treatment time is too short, the softening and plasticity improvement effects decline, which is conducive to controlling the fine precipitation of carbides. However, if the molten salt temperature is too low and the treatment time is too short, the isothermal toughening is insufficient, and the phase structures of ferrite and bainite still retain some characteristics before tempering, and the dislocation density is still relatively high, which will limit the improvement of the toughness and plasticity of the material. Therefore, selecting an appropriate molten salt temperature and treatment time to promote the isothermal toughening of ferrite and bainite to improve the plasticity of the wire rod and adjust the strength-plasticity matching of the wire rod. In the preferred technical solution, the molten salt temperature in the latter stage is 540-570 °C, and the treatment time is 20-120 s.

[0020] The molten salt circulation rate of the latter-stage molten salt is higher than that of the former-stage molten salt. Selecting an appropriate molten salt circulation rate can control the temperature rise of the molten salt, control the temperature accuracy, promote the uniform transformation of isothermal toughening, and avoid excessive molten salt circulation rate and unnecessary increase in production energy consumption. In the preferred technical solution, the molten salt circulation rate of the latter-stage molten salt is 350 - 450 t / h, and the temperature rise of the molten salt ≤ 3°C.

[0021] The roller table slow cooling controls the appropriate cooling rate of the wire rod for continuous softening, which can further promote the tempering softening of the matrix structure and avoid the influence of too slow cooling rate on production efficiency. In the preferred technical solution, the roller table slow cooling controls the wire rod to cool slowly at a cooling rate of 0.15 - 0.55°C / s.

[0022] In the preferred technical solution, the roller table slow cooling adopts heat preservation cover controlled cooling, which can utilize the residual heat of the wire rod itself to form a slow cooling heat field inside the cover and reduce production energy consumption. Preferably, the wire rod after online molten salt isothermal toughening treatment is conveyed into the heat preservation cover by the roller table until coiling. Preferably, the coiling temperature is below 300°C to form appropriate tempering softening.

[0023] The roller table slow cooling can adopt a heat preservation cover and cooperate with the roller table speed to further control the cooling rate of the wire rod. In the preferred technical solution, the roller table speed of the wire rod after entering the heat preservation cover is 0.3 - 0.6 m / s, and preferably the roller table speed of the wire rod before entering the heat preservation cover is 0.8 - 1 m / s, which can take into account production efficiency.

[0024] The second aspect of the present invention is to provide a high-plasticity wire rod for hand tools, which is manufactured by the manufacturing method of the high-plasticity wire rod for hand tools described in any one of the above.

[0025] The above wire rod adopts a C-Si-Mn-Cr-V chemical composition, with a relatively low V content and low material cost. The microstructure includes a mixed structure mainly composed of tempered bainite and containing a small amount of ferrite. Compared with the traditional ferrite + pearlite / sorbite structure, the bainite structure is denser than the pearlite / ferrite structure, and the solid solution strengthening effect is obvious. After isothermal toughening, the carbides in the bainite will gradually become dispersed and uniform, turning into tempered bainite that is both strong and tough. The plasticity and cold working performance are significantly improved, which is beneficial to eliminating the additional heat treatment before cold working of hand tools, reducing the cracking risk of direct cold working of the wire rod, and then reducing the material and processing costs of hand tools. At the same time, the microstructure of tempered bainite is beneficial to dispersing stress concentration. The fine grains and uniformly distributed carbides enable the material to better resist the initiation and propagation of cracks under cyclic loading, with good fatigue performance, and can meet the requirements of some hand tool steels with high requirements for wear resistance and anti-deformation ability.

[0026] The higher the proportion of the tempered bainite, the higher the strength of the wire rod. An appropriate amount of tempered bainite is beneficial to improving the toughness and fatigue resistance of the wire rod. An appropriate amount of ferrite can play the role of a soft phase. The ferrite phase can undergo plastic deformation through methods such as dislocation slip, and cooperate with the tempered bainite phase to adjust the strength of the wire rod to a certain extent, so that it is not too high or the toughness is not too poor, making the structure have good toughness and cold working performance, and can withstand large deformations during cold working without defects such as cracking. In the preferred technical solution, the volume percentage of the tempered bainite is 70% - 80%, and the volume percentage of the ferrite is 20% - 30%.

[0027] Due to the uniform phase transformation and isothermal toughening of the structure in the wire rod, and there is no hard and brittle structure such as martensite, the structure uniformity is better, which can reduce the mechanical property fluctuation of the wire rod and further reduce the risk of cold working cracking of the wire rod. In the preferred technical solution, the mechanical property difference within the same coil of the wire rod ≤ 30 MPa.

[0028] The tensile strength of the wire rod is moderate, which can make the hand tools processed from the wire rod be at the corresponding strength level, help improve the use performance of the hand tools, and avoid the increase in processing difficulty due to too high tensile strength. The wire rod has the characteristic of high plasticity, which is manifested in the obvious increase in the reduction of area, which can improve the cold working performance of the wire rod, so that the hand tools can be subjected to processing such as drawing, cold heading, and cold rolling without heat treatment processes such as annealing before cold working, and can withstand large plastic deformations without obvious cracks or fractures. The higher the plasticity of the wire rod, the more beneficial it is to manufacture hand tools with complex shapes. In the preferred technical solution, the diameter of the wire rod is 8.0 - 15.0 mm, the tensile strength is 910 - 960 MPa, and the reduction of area is 58% - 65%.

[0029] Compared with the prior art, the beneficial effects of the present invention are at least as follows: (1) Aiming at the current situation that there is a high risk of forming brittle structures such as bainite and martensite in the air-cooled wire rod for traditional hand tools, which reduces its cold working performance, the manufacturing method of the present invention combines Cr-V chemical composition design with on-line molten salt isothermal toughening technology, utilizes the characteristics of the intersection of the ferrite and bainite phase regions and the suppression of pearlite phase transformation, promotes the wire rod to quickly enter the ferrite and bainite mixed phase region in the high-temperature austenite state, forms a mixed structure mainly composed of bainite, promotes the isothermal toughening of ferrite and bainite, improves the plasticity of the wire rod, avoids the enrichment of V element and the coarsening of carbides, refines the grains of the material, adjusts the strength-plasticity matching of the wire rod, avoids abnormal martensite structure, and finally undergoes slow cooling through the roller table to continuously soften, further promoting the tempering softening of the matrix structure, thereby being able to reduce the material cost and the mechanical property fluctuation of the wire rod, effectively improve the plasticity and cold working performance of the wire rod, and has good market application prospects.

[0030] (2) In view of the fact that the pearlite wire rod for traditional hand tools contains relatively high alloy elements such as carbon, silicon, manganese, chromium, vanadium, and tungsten, resulting in relatively high material costs, insufficient plasticity and cold working performance, and a sharp increase in the processing cost of hand tools, the content of alloy elements such as Mn and V in the present invention is relatively low, which can balance the wear resistance and corrosion resistance of hand tools, reduce material costs. The microstructure of the wire rod includes a mixed structure composed of tempered bainite and ferrite, which can improve the strength and high plasticity matching of the wire rod, achieving a tensile strength of 910-960 MPa and a reduction of area of 58%-65% for the wire rod. It is used in application fields such as manufacturing hand tools, which is beneficial for eliminating the additional heat treatment before cold working of hand tools, reducing the cracking risk of direct cold working of the wire rod, and reducing the processing cost of hand tools, having good market application prospects. Brief Description of the Drawings

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the following description of the embodiments in conjunction with the accompanying 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; Figure 3 is the metallographic structure diagram of Embodiment 3 of the present invention. Detailed Embodiments

[0032] The embodiments described below with reference to the accompanying drawings are exemplary, only for illustrative purposes and do not limit the description of the features and characteristics of the present invention. To propose the best mode for implementing the present invention, it is intended to explain the present invention and is sufficient to enable those skilled in the art to implement the present invention, and should not be construed as having any limitation on the scope of the present invention. The scope of the present invention is only defined by the appended claims; the organization and performance detection of the wire rods obtained in the following embodiments and comparative examples include: 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 microstructure detection is carried out in accordance with the metallic microstructure detection method of the GB / T13298 standard; the method for testing the difference in mechanical properties within the same coil: 2 coils of wire rods are taken at a distance of 5 m from the end of the coil. Taking the lap joint 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 difference in mechanical properties within the same coil. Embodiment 1:

[0033] A preferred embodiment of the manufacturing method of the high-plasticity wire rod for hand tools according to the present invention. The chemical composition and mass percentage of the wire rod include C: 0.44%, Si: 0.3%, Mn: 0.68%, Cr: 0.55%, V: 0.051%, P: 0.015%, S: 0.013%, and the rest are Fe and inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt isothermal toughening → slow cooling on the roller table → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the rollable plasticity through a heating furnace. After the steel billet exits the heating furnace, primary rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface scale. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 15mm through the rolling line. Appropriate final rolling inlet temperature and final rolling deformation amount are selected to promote the refinement of the tissue grains and strengthen and toughen the matrix. Specifically: control the pressure of the primary rolling descaling water to be 18.5MPa, the final rolling inlet to be 935°C, and the final rolling deformation amount to be 20%; the wire laying process is used to make the wire rod exiting the rolling line into a wire rod through a wire laying machine. The wire rod is scattered on the roller table and conveyed along the roller table, so that the wire rod is in a high-temperature austenite state.

[0034] The on-line molten salt isothermal toughening process uses a two-section salt bath tank with molten salt inside. The wire rod after wire laying is conveyed through the roller table and passes through the first section of the salt bath tank for the front-section molten salt treatment, so that the wire rod cools down at a cooling rate of 12°C / s, quickly enters the mixed phase region of ferrite and bainite from the high-temperature austenite state. A smaller molten salt circulation amount is selected to control the phase transformation of ferrite and bainite and the precipitation of vanadium-containing carbides in a dispersed manner, forming a mixed structure mainly composed of bainite. Then the wire rod is conveyed through the roller table and passes through the second section of the salt bath tank for the rear-section molten salt treatment. The molten salt circulation amount is increased to control the temperature rise of the molten salt, promote the isothermal toughening of ferrite and bainite, promote the dispersed precipitation of vanadium-containing carbides, inhibit the growth of carbide strengthening phases, and improve the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-section molten salt is 553°C, the treatment time is 105s, and the molten salt circulation amount is 150t / h; the molten salt temperature of the rear-section molten salt is 565°C, the treatment time is 45s, the molten salt circulation amount is 450t / h, and the temperature rise of the molten salt ≤ 3°C.

[0035] The slow cooling process on the roller table uses a closed heat preservation cover. The wire rod passing through the second section of the salt bath tank is conveyed into the heat preservation cover by the conveying roller table, so that the wire rod can be slowly cooled at a high temperature for continuous softening, further promoting the tempering softening of the matrix structure until coiling. Specifically: the roller table speed before the wire rod enters the heat preservation cover is 0.95m / s, and the roller table speed decreases to 0.45m / s after entering the heat preservation cover. The wire rod is controlled to slowly cool to 282°C at a cooling rate of 0.27°C / s; the coiling process is used to coil the wire rod into a coil through a coiling reel. After packaging and warehousing, the finished wire rod is obtained, and its metallographic structure diagram is as Figure 1 shown.

[0036] Comparative Example 1: A manufacturing method of wire rod, the difference between its manufacturing method and that of Example 1 lies in: 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 final rolling inlet temperature is 865 °C, the Stelmor air cooling line is with the fan turned off, the first two heat preservation covers are opened, and the rest of the heat preservation covers are closed. The wire rod after wire laying is conveyed along the Stelmor air cooling line by roller table. The cooling rate of the wire rod in the heat preservation cover is controlled at 2.8 °C / s, cooled down to 280 °C and collected by the coiling drum to obtain the finished wire rod product.

[0037] Comparative Example 2: A manufacturing method of wire rod, the difference between its manufacturing method and that of Example 1 lies in: in the rolling process, the final rolling inlet temperature is 990 °C, the wire rod undergoes front-section molten salt treatment and cools down at a cooling rate of 9 °C / s, and the molten salt circulation volume of the front-section molten salt is 45 t / h to obtain the finished wire rod product. Example 2:

[0038] A preferred implementation of the manufacturing method of the high-plasticity wire rod for hand tools of the present invention. The chemical composition and mass percentage of the wire rod include C: 0.46%, Si: 0.3%, Mn: 0.5%, Cr: 0.49%, V: 0.045%, P: 0.013%, S: 0.015%, and the rest are Fe and inevitable impurities; its manufacturing method is carried out according to the technological process of rolling → wire laying → in-line molten salt isothermal toughening → roller table slow cooling → 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 rollable plasticity through a heating furnace. After the steel billet exits the heating furnace, primary rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface scale, and the high-temperature steel billet is rolled into a wire rod with a diameter specification of 8 mm through a rolling line. Appropriate final rolling inlet temperature and final rolling reduction are selected to promote the refinement of the tissue grains and strengthen and toughen the matrix. Specifically: control the primary rolling descaling water pressure at 16 MPa, the final rolling inlet at 900 °C, and the final rolling reduction at 25%; the wire laying process is used to make the wire rod exiting the rolling line into a wire coil through a wire laying machine, and the wire coil is scattered on the roller table and conveyed along the roller table, keeping the wire coil in a high-temperature austenite state.

[0039] The online molten salt isothermal toughening process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire drawing 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 10 °C / s, quickly enters the mixed phase region of ferrite and bainite from the high-temperature austenite state, selects a smaller molten salt circulation volume, controls the phase transformation of ferrite and bainite and the dispersion precipitation of vanadium-containing carbides, forms a mixed structure mainly composed of bainite. Then the wire rod is transported through the second-stage salt bath tank by a roller table for the back-stage molten salt treatment, increases the molten salt circulation volume to control the temperature rise of the molten salt, promotes the isothermal toughening of ferrite and bainite, promotes the dispersion precipitation of vanadium-containing carbides, inhibits the growth of carbide strengthening phases, and improves the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 562 °C, the treatment time is 68 s, and the molten salt circulation volume is 50 t / h; the molten salt temperature of the back-stage molten salt is 552 °C, the treatment time is 85 s, the molten salt circulation volume is 350 t / h, and the temperature rise of the molten salt ≤ 3 °C.

[0040] The roller table slow cooling process adopts to close the heat preservation cover. The wire rod transported by the conveying roller table after passing through the second-stage salt bath tank enters the heat preservation cover, so that the wire rod can be slowly cooled at high temperature for continuous softening, further promoting the tempering softening of the matrix structure until coiling. Specifically: the roller table speed before the wire rod enters the heat preservation cover is 0.9 m / s, and the roller table speed decreases to 0.5 m / s after entering the heat preservation cover, controlling the wire rod to slowly cool to 280 °C at a cooling rate of 0.35 °C / s; the coiling process is used to coil the wire rod into a coil by a coiler, and after packaging and storing in the warehouse, the finished wire rod is obtained, and its metallographic structure diagram is as Figure 2 shown.

[0041] Comparative Example 3: A manufacturing method of a wire rod, the difference in its manufacturing method from that of Example 2 is that: the wire rod undergoes front-stage molten salt treatment and cools down at a cooling rate of 8 °C / s, the molten salt temperature of the front-stage molten salt is 585 °C, the treatment time is 25 s, and the finished wire rod is obtained.

[0042] Comparative Example 4: A manufacturing method of a wire rod, the difference in its manufacturing method from that of Example 2 is that: the wire rod undergoes front-stage molten salt treatment and cools down at a cooling rate of 17 °C / s, the molten salt temperature of the front-stage molten salt is 525 °C, the treatment time is 130 s, and the molten salt circulation volume is 200 t / h; the finished wire rod is obtained. Example 3:

[0043] A preferred embodiment of the manufacturing method of the high-plasticity wire rod for hand tools of the present invention. The chemical composition and mass percentage of the wire rod include C: 0.43%, Si: 0.2%, Mn: 0.7%, Cr: 0.45%, V: 0.055%, P: 0.013%, S: 0.013%, and the rest are Fe and inevitable impurities. Its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt isothermal toughening → slow cooling on the roller path → coiling. Specifically: The rolling process is used to heat a steel billet with a specification of 220mm×220mm into a high-temperature steel billet that reaches the plastic state for rolling through a heating furnace. After the steel billet exits the heating furnace, primary rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface scale. The high-temperature steel billet is rolled into a wire rod with a diameter specification of 11mm through the rolling line. Appropriate finishing rolling inlet temperature and finishing rolling deformation amount are selected to promote the refinement of the tissue grains and strengthen and toughen the matrix. Specifically: control the pressure of the primary rolling descaling water to be 17MPa, the finishing rolling inlet to be 915°C, and the finishing rolling deformation amount to be 23%; The wire laying process is used to make the wire rod exiting the rolling line into a wire rod through a wire laying machine. The wire rod is scattered on the roller path and transported along the roller path, making the wire rod in a high-temperature austenite state.

[0044] The on-line molten salt isothermal toughening 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 the roller path for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 11°C / s, quickly enters the mixed phase region of ferrite and bainite from the high-temperature austenite state, selects a smaller molten salt circulation volume, controls the phase transformation of ferrite and bainite and the dispersion precipitation of vanadium-containing carbides, forms a mixed structure mainly composed of bainite. Then the wire rod is transported through the second-stage salt bath tank by the roller path for the back-stage molten salt treatment, increases the molten salt circulation volume to control the temperature rise of the molten salt, promotes the isothermal toughening of ferrite and bainite, promotes the dispersion precipitation of vanadium-containing carbides, inhibits the growth of carbide strengthening phases, and improves the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 570°C, the treatment time is 30s, and the molten salt circulation volume is 85t / h; the molten salt temperature of the back-stage molten salt is 540°C, the treatment time is 120s, the molten salt circulation volume is 375t / h, and the temperature rise of the molten salt ≤ 3°C.

[0045] The slow cooling process on the roller path adopts to close the heat preservation cover. The wire rod passing through the second-stage salt bath tank is transported into the heat preservation cover by the conveying roller path, so that the wire rod can be slowly cooled at high temperature for continuous softening, further promoting the tempering softening of the matrix structure until coiling. Specifically: the roller path speed before the wire rod enters the heat preservation cover is 0.8m / s, the roller path speed after entering the heat preservation cover is reduced to 0.6m / s, and the wire rod is controlled to slowly cool to 275°C at a cooling rate of 0.55°C / s; The coiling process is used to coil the wire rod into a coil through a coiling drum. After packaging and warehousing, the finished wire rod is obtained, and its metallographic structure diagram is as Figure 3 shown.

[0046] Comparative Example 5: A manufacturing method of wire rod, the difference between the manufacturing method and that of Example 3 lies in that: the molten salt temperature of the latter-stage molten salt is 585 °C, the treatment time is 130 s, and the wire rod finished product is obtained.

[0047] Comparative Example 6: A manufacturing method of wire rod, the difference between the manufacturing method and that of Example 3 lies in that: the molten salt temperature of the latter-stage molten salt is 525 °C, the treatment time is 20 s, and the wire rod finished product is obtained. Example 4:

[0048] A preferred implementation manner of the manufacturing method of the high-plasticity wire rod for hand tools according to the present invention, the chemical composition and mass percentage of the wire rod include C: 0.48%, Si: 0.26%, Mn: 0.59%, Cr: 0.6%, V: 0.049%, P: 0.014%, S: 0.014%, and the rest are Fe and inevitable impurities; its manufacturing method is manufactured according to the technological process of rolling → wire laying → on-line molten salt isothermal toughening → roller table slow cooling → coiling, specifically: The rolling process is used to heat a steel billet with a specification of 220 mm × 220 mm into a high-temperature steel billet that reaches the rollable plasticity through a heating furnace. After the steel billet exits the heating furnace, primary rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface scale, and the high-temperature steel billet is rolled into a wire rod with a diameter specification of 13 mm through a rolling line. Appropriate finishing rolling inlet temperature and finishing rolling reduction are selected to promote the refinement of the tissue grains and strengthen and toughen the matrix. Specifically: control the primary rolling descaling water pressure to be 18 MPa, the finishing rolling inlet to be 925 °C, and the finishing rolling reduction to be 21%; the wire laying process is used to make the wire rod exiting the rolling line into a wire rod through a wire laying machine, and the wire rod is scattered on the roller table and conveyed along the roller table, so that the wire rod is in a high-temperature austenite state.

[0049] The on-line molten salt isothermal toughening process uses a two-stage salt bath tank with molten salt inside. The wire rod after wire laying is conveyed through the roller table and passes through the first-stage salt bath tank for the front-stage molten salt treatment, so that the wire rod cools down at a cooling rate of 13 °C / s, quickly enters the mixed phase region of ferrite and bainite from the high-temperature austenite state, selects a smaller molten salt circulation amount, controls the ferrite, bainite phase transformation and the dispersion precipitation of vanadium-containing carbides, forms a mixed structure mainly composed of bainite. Then the wire rod is conveyed through the roller table and passes through the second-stage salt bath tank for the latter-stage molten salt treatment, increases the molten salt circulation amount to control the molten salt temperature rise, promotes the isothermal toughening of ferrite and bainite, promotes the dispersion precipitation of vanadium-containing carbides, inhibits the growth of carbide strengthening phases, and improves the strength-plasticity matching of the wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 540 °C, the treatment time is 120 s, and the molten salt circulation amount is 115 t / h; the molten salt temperature of the latter-stage molten salt is 570 °C, the treatment time is 20 s, the molten salt circulation amount is 415 t / h, and the molten salt temperature rise ≤ 3 °C.

[0050] In the slow cooling process of the roller path, the heat preservation cover is closed, and the wire rod passing through the second salt bath tank is conveyed by the conveying roller path into the heat preservation cover, enabling the wire rod to be slowly cooled at high temperature for continuous softening, further promoting the tempering softening of the matrix structure until coiling. Specifically: the speed of the roller path before the wire rod enters the heat preservation cover is 1 m / s, and the speed of the roller path after entering the heat preservation cover is reduced to 0.32 m / s, controlling the wire rod to be slowly cooled at a cooling rate of 0.18 °C / s to 285 °C; the coiling process is used to coil the wire rod into a coil by a coiling drum, and after packaging and warehousing, the finished wire rod is obtained.

[0051] Comparative Example 7: A manufacturing method of a wire rod, the difference in its manufacturing method from 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 toughening → air cooling → coiling. Specifically: in the air cooling process, the heat preservation cover is opened, and the wire rod passing through the second salt bath tank is conveyed by the conveying roller path, and the wire rod is slowly cooled at a cooling rate of 1.2 °C / s to 280 °C to obtain the wire rod.

[0052] The wire rods obtained from the above Examples 1-4 and Comparative Examples 1-7 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

[0053] From the comparison results of Examples 1-4 and Comparative Example 1, it can be seen that compared with the situation where even after low-temperature rolling and slow cooling post-treatment with a Stelmor air-cooling line, brittle structures such as bainite and martensite still appear, with large fluctuations in mechanical properties, insufficient tissue plasticity, and reduced cold working performance. Through the Cr-V chemical composition design combined with the online molten salt isothermal toughening technology in the present invention, the alloy element content can be appropriately reduced to reduce the material cost. By controlling the phase transformation of the wire rod in the ferrite and bainite mixed phase region and isothermal toughening treatment through online molten salt isothermal toughening, a wire rod with a mixed structure composed of tempered bainite and ferrite in the microstructure type can be manufactured, with a product tensile strength of 910-960 MPa, a reduction of area of 58%-65%, smaller fluctuations in mechanical properties, better cold working performance. When used in the manufacturing of hand tools and other application fields, additional heat treatment is not required, effectively reducing the cracking and fracture risks during the direct cold working process of the wire rod for hand tools, and being able to meet the corresponding strength grade of hand tools.

[0054] From the comparison results of Example 1 and Comparative Example 2, it can be seen that an appropriate finish rolling inlet temperature combined with the finish rolling deformation amount helps to refine the tissue grains, control the wire rod to quickly enter the ferrite and bainite mixed phase region in the high-temperature austenite state, and can increase the driving force for bainite transformation and vanadium carbide, enhancing the strength and plasticity of the wire rod.

[0055] It can be seen from the comparison results of Example 2 and Comparative Example 3 that the higher the temperature of the molten salt in the front stage and the shorter the treatment time, the lower the cooling rate of the wire rod, the more ferrite phase transformation, which is beneficial to improving the plasticity of the wire rod. However, if the molten salt temperature is too high and the treatment time is too short, the driving force for bainite transformation decreases and the nucleation rate decreases, which is not conducive to the formation of a mixed structure mainly composed of bainite, affecting the matrix strength. At the same time, with the shortening of the total treatment time of on-line molten salt isothermal toughening, a certain amount of plasticity will be lost.

[0056] It can be seen from the comparison results of Example 2 and Comparative Example 4 that the lower the temperature of the molten salt in the front stage and the longer the treatment time, the slower the formation rate of ferrite, resulting in smaller ferrite grain size, the increased driving force for bainite transformation and the precipitation of vanadium carbides, the increased nucleation rate of bainite, and the increase of matrix strength. The molten salt treatment in the latter stage prepares the organization. However, if the molten salt temperature is too low and the treatment time is too long, it is not conducive to the formation of ferrite transformation, affects the temperature control accuracy, increases the difficulty of plasticity improvement, and is not conducive to the control of the plasticity and tissue uniformity of the wire rod.

[0057] It can be seen from the comparison results of Example 3 and Comparative Example 5 that the higher the temperature of the molten salt in the latter stage and the longer the treatment time, it is beneficial to provide more thermal power for isothermal toughening, enabling the matrix plasticity to be improved rapidly. However, if the molten salt temperature is too high and the treatment time is too long, it is not conducive to controlling the dispersion precipitation of carbides and there is a risk of carbide coarsening, affecting the strength and plasticity of the matrix.

[0058] It can be seen from the comparison results of Example 3 and Comparative Example 6 that the lower the temperature of the molten salt in the latter stage and the shorter the treatment time, the softening and plasticity improvement effects decrease, which is conducive to controlling the fine precipitation of carbides. However, if the molten salt temperature is too low and the treatment time is too short, the isothermal toughening is insufficient, and the phase structures of ferrite and bainite still retain some characteristics before tempering, and the dislocation density is still relatively high, which will limit the improvement of the toughness and plasticity of the material.

[0059] It can be seen from the comparison results of Example 4 and Comparative Example 7 that in the roller table slow cooling treatment, the wire rod can continue the toughening effect in the later stage of on-line molten salt isothermal toughening under slow cooling, further promoting the tempering softening of the matrix structure.

[0060] 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 changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A manufacturing method of a high-plasticity wire rod for hand tools, characterized in that, The manufacturing method includes: The steel billet is subjected to rolling, wire laying, on-line molten salt isothermal toughening and slow cooling on the roller table to produce a wire rod with a microstructure including tempered bainite and ferrite. The on-line molten salt isothermal toughening controls the phase transformation and isothermal toughening treatment of the wire rod in the mixed phase region of ferrite and bainite. The chemical composition and mass percentage of the wire rod include: C: 0.43% - 0.48%, Si: 0.20% - 0.30%, Mn: 0.50% - 0.70%, Cr: 0.45% - 0.60%, V: 0.045% - 0.055%, P≤0.015%, S≤0.015%, and the rest is Fe and unavoidable impurities.

2. The manufacturing method of the high-plasticity wire rod for hand tools according to claim 1, characterized in that Before the rolling, control the initial rolling descaling water pressure of the steel billet after leaving the heating furnace to be ≥16 MPa.

3. The manufacturing method of the highly plastic wire rod for hand tools according to claim 1, characterized in that, During the rolling, control the final rolling inlet temperature to be 900 - 935 °C, and the final rolling deformation amount to be 20% - 25%.

4. The manufacturing method of the high-plasticity wire rod for hand tools according to claim 1, characterized in that, The on-line molten salt isothermal toughening controls the wire rod to first pass through the front-section molten salt, cool down at a cooling rate of ≥10 °C / s and enter the mixed phase region of ferrite and bainite from the austenite state to form a mixed structure mainly composed of bainite, and then pass through the rear-section molten salt to increase the molten salt circulation amount and promote the isothermal toughening of ferrite and bainite.

5. The manufacturing method of the highly plastic wire rod for hand tools according to claim 4, characterized in that, The molten salt temperature of the front-section molten salt is 540 - 570 °C, and the treatment time is 30 - 120 s; the molten salt temperature of the rear-section molten salt is 540 - 570 °C, and the treatment time is 20 - 120 s.

6. The manufacturing method of the high-plasticity wire rod for hand tools according to claim 5, characterized in that, The molten salt circulation amount of the front-section molten salt is 50 - 150 t / h; the molten salt circulation amount of the rear-section molten salt is 350 - 450 t / h, and the molten salt temperature rise ≤3 °C.

7. The manufacturing method of the high-plasticity wire rod for hand tools according to claim 5, characterized in that, The slow cooling on the roller table controls the wire rod to cool slowly at a cooling rate of 0.15 - 0.55 °C / s.

8. A high-plasticity wire rod for hand tools, characterized in that, The wire rod is obtained by the manufacturing method of the high-plasticity wire rod for hand tools described in any one of claims 1 - 7.

9. The high-plasticity wire rod for hand tools according to claim 8, characterized in that, The volume percentage of the tempered bainite is 70% - 80%.

10. The high-plasticity wire rod for hand tools according to claim 8, characterized in that, The diameter of the wire rod is 8.0 - 15.0 mm, the tensile strength is 910 - 960 MPa, the cross-sectional shrinkage rate is 58% - 65%, and the difference in mechanical properties within the same coil ≤30 MPa.

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