High-plasticity wire rod for hand tools and manufacturing method thereof

Abstract

The present invention relates to a high-plasticity wire rod for hand tools and a manufacturing method thereof. The method comprises subjecting a steel billet to rolling, spinning, online molten salt isothermal toughening and roller slow cooling treatment to produce a wire rod having a microstructure including tempered bainite and ferrite. The online molten salt isothermal toughening treatment controls the phase transformation of the wire rod in the mixed phase region of ferrite and bainite and the isothermal toughening treatment. The wire rod is designed with a Cr-V chemical composition and has relatively low contents of alloying elements such as Mn and V. Combined with the online molten salt isothermal toughening technology, the material cost and the fluctuation of the mechanical properties of the wire rod can be reduced, and the plasticity and cold working performance of the wire rod can be effectively improved, achieving a tensile strength of 910-960 MPa and a cross-sectional reduction rate of 58%-65%. This is beneficial for eliminating additional heat treatment before cold working of hand tools, reducing the risk of cracking of the wire rod during direct cold working, and reducing the processing cost of hand tools.

Description

Technical Field

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

[0002] Hand tools include screwdrivers, pliers, hammers, wrenches and other daily installation and maintenance tools. As essential tools for life and production, the market's requirements for their durability and service life are increasing year by year. Hand tools often require hot-rolled wire rods as the base material and are manufactured in an online process through cold processing methods such as cold drawing and cold heading. Therefore, the selection of wire rods for hand tools is crucial to the quality and market benefits of hand tools. Traditional wire rods for hand tools are generally produced using the Stelmor air-cooled line after spinning, and usually contain high levels of alloy elements such as carbon, silicon, manganese, chromium, vanadium, and tungsten to ensure the performance of the material after cold working. However, with the addition of alloy elements, the probability of forming brittle structures such as bainite and martensite in the rolling and cooling process of the wire rods for hand tools continues to increase, reducing its cold working performance, resulting in cracking and breakage of the wire rods for hand tools during the cold working process, and even requiring additional heat treatment such as annealing before cold working to improve its cold working performance, resulting in 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 needs.

[0003] The following technical difficulties exist in manufacturing high-plasticity wire rod for hand tools with lower material cost and better cold working performance:

[0004] In order to ensure that the steel wire rod can obtain high hardness and high wear resistance, and meet the requirements of hand tools to resist wear and deformation during use, the wire rod for hand tools needs to be added with higher contents of alloy elements such as carbon, manganese, and chromium. However, due to the partial concentration of alloy elements in the core, the influence of the hardenability of steel, and the wire rod after spinning in the air cooling line, the minimum cooling control capacity of the Stelmor cooling line is limited, and the instability of the cooling control by controlling the heat exchange between the wire rod and the air through the fan air volume is large, resulting in greater difficulty in cooling. Some positions in the wire rod are prone to form low-temperature brittle structures such as bainite and martensite due to excessive cooling, segregation and hardenability, resulting in insufficient plasticity of the wire rod and the same circle. The mechanical properties fluctuate greatly, and the risk of cracking during subsequent cold working is high. Therefore, in order to improve the cold working performance, the existing technology generally hopes to eliminate the influence of low-temperature phase transformation structures such as bainite and martensite, and reduce the initial strength of the hot-rolled wire rod and improve the plasticity, so that users can improve and achieve the final comprehensive mechanical properties of hand tools through heat treatment such as quenching and tempering after cold working. For example, the chrome-vanadium hot-rolled steel wire rod and its preparation method, as well as the preparation method of steel wire and hand tools disclosed in patent CN111876679B, the chrome-vanadium wire rod obtains pearlite by controlling the total content of Mn and Cr, the alloy core segregation index and combining air cooling and slow cooling treatment. + ferrite + sorbite structure wire rod, reaching a tensile strength of 800~950MPa, cross-sectional shrinkage ≥50%, but on the one hand, in order to improve the strength and toughness of the material, the vanadium content in the material is relatively high, but vanadium as a precious alloy will lead to an increase in material cost, and in the process of air-cooled wire-controlled cooling, the wire rod is continuously and slowly cooled, which affects the precipitation driving force of vanadium-containing carbides, and there is a risk of insufficient precipitation or rapid coarsening of carbides due to unstable air-cooling control, which will affect the plasticity of the wire rod and increase the fluctuation of the mechanical properties of the wire rod; on the other hand, the segregation control of the alloying elements in the steel billet is limited. As the specifications of the wire rod increase, the alloy content of the wire rod and the air-cooled cooling process increase. During the cooling process, the temperature gradient on the same cross section will further increase, and the minimum cooling control capacity of the air cooling line is limited, which will increase the difficulty of controlling the structure and performance of the wire rod. At the same time, the wire rod will pass through the ferrite, sorbite and low-temperature structure phase regions during the continuous and slow cooling process. The wire rod is in a low-temperature state after the phase transformation, and there will be residual structural stress in the structure, which will limit the improvement of the plasticity of the wire rod. The untransformed austenite residue will continue to form martensite low-temperature brittle structure in the subsequent cooling, affecting the structure and performance of the wire rod in the same circle, and will cause local stress concentration and uneven deformation during cold working, thereby increasing the risk of cold working cracking and processing costs for downstream users. Summary of the Invention

[0005] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a high-plasticity wire rod for hand tools and a manufacturing method thereof, which can reduce material costs and fluctuations in the mechanical properties of the wire rod, effectively improve the plasticity and cold working performance of the wire rod, and is conducive to eliminating additional heat treatment before cold working of hand tools, reducing the risk of cracking of the wire rod during direct cold working, and reducing the processing cost of hand tools.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A first aspect of the present invention is to provide a method for manufacturing a high-plasticity wire rod for hand tools, the manufacturing method comprising:

[0008] The steel billet is subjected to rolling, wire drawing, online molten salt isothermal toughening and roller slow cooling treatment to prepare a wire rod whose microstructure includes 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 unavoidable impurities.

[0009] The chemical composition and mass percentage of the above-mentioned wire rod are designed based on the following:

[0010] (1) Carbon: As an effective carbide strengthening element and austenite forming element, C element has a relatively low price. With the increase of carbon content, it can produce solid solution strengthening effect, reduce the austenite transformation temperature, increase the driving force of bainite transformation and bainite transformation temperature, and promote the formation of a mixed structure dominated by bainite in the mixed phase area of ​​ferrite and bainite after the wire rod is subjected to online molten salt isothermal toughening control after spinning, so as to meet the requirements of hand tools for hardness, wear resistance and shear resistance. However, excessive carbon content will increase the tendency of carbon segregation during the solidification process of the steel billet, inhibit ferrite precipitation, cause pearlite transformation, and affect the plasticity and 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 material costs, and facilitate microstructure, carbide precipitation control and improve material plasticity, a medium carbon content is used, and the mass percentage of C is controlled to be 0.43%~0.48%.

[0011] (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 formed first, solid solution strengthens ferrite and bainite matrix, and can also improve the elastic limit. 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%.

[0012] (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, thereby enabling online molten salt isothermal toughening to 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, promoting the mixing of the two to increase the matrix strength, which 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%.

[0013] (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 structure 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%.

[0014] (5) Vanadium: As an alloying element, V can pin the austenite grain boundaries during the controlled rolling stage, 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 fatigue strength and toughening the matrix. It is helpful 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. Excessive 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%.

[0015] (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%.

[0016] The above-mentioned wire rod is designed with a Cr-V chemical composition. The V content is low, which can reduce material costs. Combined with the optimized ratio of C, Si, and Mn, it takes into account the strength, high toughness and wear resistance required by hand tool steel. The phase transformation thermodynamics and kinetics are regulated to make the ferrite and bainite phase transformation phase regions have the characteristics of intersection. The preferential nucleation of ferrite and bainite will consume carbon and other alloying elements in the surrounding austenite, change the local composition and structure, and make the nucleation conditions of pearlite unfavorable. Ferrite is then used to suppress the pearlite phase transformation, providing favorable conditions for online molten salt isothermal toughening treatment to control microstructure phase transformation, carbide precipitation and toughening. On this basis, the wire rod after spinning is not air-cooled but directly treated online with molten salt:

[0017] 1. Compared with the Stelmor air-cooled line, which has limited cooling capacity, great difficulty in controlling cooling, and continuous cooling treatment, resulting 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 exchange capacity than the air-cooled line, and the wire rod can quickly enter the mixed phase area of ​​ferrite and bainite, inhibiting grain coarsening, increasing the precipitation driving force of bainite and vanadium-containing carbides, promoting the transformation of high-temperature austenite in the organization to ferrite and bainite, inhibiting ferrite coarsening and pearlite formation, and quickly forming a mixed organization dominated by bainite and a small amount of ferrite to ensure matrix strength. On the other hand, the high heat exchange capacity of molten salt can be utilized The ability and molten salt cover the surface of the wire rod for uniform heat exchange, reducing the temperature gradient on the same cross section of the wire rod with larger specifications, so that the wire rod can be maintained at the molten salt temperature for isothermal treatment, that is, extending the treatment time of the wire rod in the mixed phase area of ​​ferrite and bainite, which can promote the full transformation of austenite to ferrite and bainite, make the precipitation of bainite controllable, and avoid the core austenite residue from continuing to form martensite low-temperature brittle structure in the subsequent cooling process, effectively control abnormal structure, and also extend the treatment time of the wire rod in the vanadium carbide precipitation temperature range, promote the full dispersion and precipitation of vanadium carbides, avoid the coarsening of carbide precipitation and affect the uniformity of the structure, and thus reduce the fluctuation of the mechanical properties of the wire rod.

[0018] 2. Compared with the limited minimum cooling capacity of the Stelmor air cooling line and the insufficient plasticity of the wire rod caused by continuous cooling treatment, since the temperature of the wire rod in the mixed phase region of ferrite and bainite is higher, isothermal treatment rather than continuous cooling prolongs the treatment time of the wire rod in the higher temperature range, which can promote the isothermal toughening of ferrite and bainite, eliminate internal stress, make the carbide particles in the bainite structure smaller and more dispersed, and transform it into tempered bainite that is both strong and tough, reducing the splitting effect on the matrix, which is beneficial to the movement and slip of dislocations, quickly improving the plasticity of the material, and adjusting the strength and plasticity matching of the wire rod. In the subsequent roller slow cooling treatment, the wire rod is still at a higher temperature after passing through the mixed phase region of ferrite and bainite. Under slow cooling, the toughening effect of the late stage of online molten salt isothermal toughening can be continued, further promoting the tempering and softening of the matrix structure, so that the bainite structure, which is conventionally regarded as an abnormal structure, is effectively utilized. At the same time, the ferrite maintains a certain plasticity, and finally a mixed structure with matching strength and toughness is obtained, realizing high plasticity of the wire rod.

[0019] In order to further improve the surface quality of rolling and increase rolling efficiency, an appropriate initial rolling descaling water pressure is selected to be sprayed onto the surface of the steel billet to break and remove the oxide scale. In the preferred technical solution, before rolling, the initial rolling descaling water pressure is controlled to be ≥16MPa after the steel billet leaves the heating furnace.

[0020] Since online molten salt isothermal toughening can control the strengthening and toughening and plasticity improvement effects, it is not necessary to use an excessively low rolling temperature in the rolling stage. Selecting an appropriate final rolling inlet temperature during rolling is conducive to achieving uniform deformation during the final rolling process, avoiding deformation difficulties caused by too low a temperature, generating excessive internal stress or even cracks, and at the same time, with an appropriate final rolling deformation, the austenite grains are induced to refine through deformation. The austenite grain size is relatively small and uniform, which is conducive to the refinement of the microstructure. In the preferred technical solution, during the rolling, the final rolling inlet is controlled to be 900~935℃, and the final rolling deformation is 20%~25%.

[0021] In order to better control the strength and plasticity of ferrite and matrix, a smaller molten salt circulation volume can be selected in the early stage of the online molten salt isothermal toughening treatment than in the later stage to appropriately reduce the cooling rate of the wire rod, so that the ferrite can be precipitated better, avoid the cooling rate being too fast to affect or inhibit the precipitation of ferrite, and avoid the cooling rate being too slow to cause grain and precipitation coarsening or affect the rapid phase transformation of bainite. In the later stage of the online molten salt isothermal toughening treatment, the molten salt circulation volume can be increased to improve the temperature accuracy and promote uniform toughening of the structure. In the preferred technical solution, the online molten salt isothermal toughening controls the wire rod to first pass through the front section of molten salt, cool down at a cooling rate of ≥10℃ / s and enter the mixed phase zone of ferrite and bainite from the austenite state to form a mixed structure dominated by bainite, and then pass through the rear section of molten salt to increase the molten salt circulation volume and promote the isothermal toughening of ferrite and bainite.

[0022] The temperature of the front-stage molten salt is in the mixed phase region of ferrite and bainite. The lower the temperature of the front-stage molten salt and the longer the treatment time, the slower the formation rate of ferrite, resulting in smaller ferrite grain size, greater driving force for bainite transformation and vanadium-containing carbides, increased nucleation rate of bainite, increased matrix strength and decreased plasticity. 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 improving plasticity, and is not conducive to the control of wire rod plasticity and organizational uniformity. On the contrary, if the temperature of the front-stage molten salt is higher and the treatment time is shorter, the cooling rate of the wire rod is reduced, the ferrite phase transformation increases, and the matrix plasticity increases. 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 improving plasticity, and is not conducive to the control of wire rod plasticity and organizational uniformity. If the 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 is reduced, and the nucleation rate decreases, which is not conducive to the formation of a mixed structure dominated by bainite, affecting the strength of the matrix. Therefore, controlling the molten salt temperature and treatment time of the front-stage molten salt can promote the wire rod to quickly enter the ferrite and bainite mixed phase region in the high-temperature austenite state, appropriately reducing the cooling rate of the wire rod to promote the formation of ferrite, taking into account the driving force for bainite transformation, forming a mixed structure dominated by bainite, and making organizational preparations for the subsequent molten salt treatment. In the preferred technical solution, the molten salt temperature of the front-stage molten salt is 540~570℃, and the treatment time is 30~120s.

[0023] The molten salt circulation rate of the front-stage molten salt is positively correlated with the wire rod cooling rate. Selecting an appropriate molten salt circulation rate can further coordinate the molten salt temperature and processing time to control the phase change of the wire rod. In the preferred technical solution, the molten salt circulation rate of the front-stage molten salt is 50~150t / h.

[0024] The molten salt temperature of the latter molten salt is in the mixed phase region of ferrite and bainite. The higher the molten salt temperature of the latter molten salt and the longer the treatment time, the more thermal power is provided to the isothermal toughening, so that the plasticity of the matrix is ​​improved faster. However, if the molten salt temperature is too high and the treatment time is too long, it is not conducive to controlling the dispersion and precipitation of carbides, and there is a risk of carbide coarsening, which affects the strength and plasticity of the matrix. On the contrary, the lower the molten salt temperature of the latter molten salt and the shorter the treatment time, the softening and plasticity improvement effect decreases, which is conducive to controlling the fine precipitation of carbides, but If the molten salt temperature is too low and the treatment time is too short, the isothermal toughening is insufficient, the phase structure of ferrite and bainite still retains some characteristics before tempering, and the dislocation density is still high, which will limit the improvement of the material's toughness and plasticity. Therefore, appropriate molten salt temperature and treatment time are selected to promote 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 of the latter stage is 540~570℃, and the treatment time is 20~120s.

[0025] The molten salt circulation volume of the rear-stage molten salt is higher than that of the front-stage molten salt. Selecting an appropriate molten salt circulation volume can control the temperature rise of the molten salt, control the temperature accuracy, promote uniform isothermal toughening transformation, avoid excessive molten salt circulation volume and unnecessary increase in production energy consumption. In the preferred technical solution, the molten salt circulation volume of the rear-stage molten salt is 350~450t / h, and the molten salt temperature rise is ≤3°C.

[0026] The roller slow cooling controls the wire rod to continuously soften at an appropriate cooling rate, which can further promote the tempering and softening of the matrix structure and avoid the slow cooling rate affecting the production efficiency. In the preferred technical solution, the roller slow cooling controls the wire rod to slowly cool at a cooling rate of 0.15~0.55℃ / s.

[0027] In the preferred technical solution, the roller slow cooling adopts the insulation cover to control cooling, which can use the residual heat of the wire rod itself to form a heat field in the cover to slowly cool down and reduce production energy consumption. Preferably, the wire rod that has undergone online molten salt isothermal toughening treatment is transported into the insulation cover by the roller until it is coiled. The coiling temperature is preferably below 300°C to form appropriate tempering softening.

[0028] The roller slow cooling can use an insulation cover in combination with the roller speed to further control the cooling speed of the wire rod. In the preferred technical solution, the roller slow cooling controls the roller speed of the wire rod after entering the insulation cover to be 0.3~0.6m / s, and the roller speed of the wire rod before entering the insulation cover is preferably 0.8~1m / s, which can take into account production efficiency.

[0029] A second aspect of the present invention is to provide a high-plasticity wire rod for hand tools, wherein the wire rod is manufactured by any one of the above-mentioned methods for manufacturing a high-plasticity wire rod for hand tools.

[0030] The above-mentioned wire rod adopts the chemical composition of C-Si-Mn-Cr-V, with a relatively low V content and low material cost. The microstructure includes a mixed structure mainly composed of tempered bainite and a small amount of ferrite. Compared with the traditional ferrite + pearlite / sorbite structure, the bainite has a denser structure than pearlite / ferrite and a more obvious solid solution strengthening effect. After isothermal toughening, the carbides in the bainite will gradually become dispersed and uniform, and transformed into tempered bainite that is both strong and tough. The plasticity and cold working properties are significantly improved, which is beneficial to eliminating additional heat treatment before cold working of hand tools, while reducing the risk of cracking in direct cold working of wire rods, thereby reducing the material and processing costs of hand tools. At the same time, the microstructure of tempered bainite is conducive to dispersing stress concentration. The fine grains and evenly distributed carbides enable the material to better resist crack initiation and expansion under cyclic loads. The fatigue performance is good and can meet the needs of some hand tool steels with high requirements for wear resistance and deformation resistance.

[0031] 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 act as a soft phase. The ferrite phase can undergo plastic deformation through dislocation slip and other means, and cooperate with the tempered bainite to adjust the strength of the wire rod to a certain extent, so that it is not too high or the toughness is too poor, so that the organization has good toughness and cold working performance, and can withstand large deformation 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%.

[0032] Due to the uniform phase transformation and isothermal toughening of the structure in the wire rod, and the absence of hard and brittle structures such as martensite, the structure has better uniformity, which can reduce the fluctuation of the mechanical properties of the wire rod and further reduce the risk of cracking during cold working of the wire rod. In the preferred technical solution, the mechanical property difference of the wire rod is ≤30MPa.

[0033] The wire rod has a moderate tensile strength, which can ensure that the hand tools processed with the wire rod are at the corresponding strength level, helping to improve the performance of the hand tools and avoid the increase in processing difficulty due to excessive tensile strength. The wire rod has high plasticity, which is manifested in a significantly improved cross-sectional shrinkage rate, which can improve the cold processing performance of the wire rod, so that the hand tools do not need to adopt annealing and other heat treatment processes before cold processing, and can be drawn, cold headed, cold rolled and other processes. It can withstand large plastic deformation without obvious cracks or fractures. The higher the plasticity of the wire rod, the more conducive it is to the manufacture of hand tools with complex shapes. In the preferred technical solution, the diameter of the wire rod is 8.0~15.0mm, the tensile strength is 910~960MPa, and the cross-sectional shrinkage rate is 58%~65%.

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

[0035] (1) In view of the current situation that the risk of forming brittle structures such as bainite and martensite in traditional air-cooled wire rods for hand tools is high and its cold working performance is reduced, the manufacturing method of the present invention combines the online molten salt isothermal toughening technology through the design of Cr-V chemical composition, and utilizes the characteristics of the intersection of ferrite and bainite phase regions and the suppression of pearlite phase transformation to promote the wire rod to quickly enter the ferrite and bainite mixed phase region in the high-temperature austenite state, forming a mixed structure dominated by bainite, promoting the isothermal toughening of ferrite and bainite, so as to improve the plasticity of the wire rod, avoid the enrichment of V elements and the coarsening of carbides, refine the grains of the material, adjust the strength and plasticity matching of the wire rod, avoid abnormal martensite structure, and finally continue to soften through roller slow cooling, further promoting the tempering softening of the matrix structure, thereby reducing material costs and fluctuations in the mechanical properties of the wire rod, effectively improving the plasticity and cold working performance of the wire rod, and having good market application prospects.

[0036] (2) In view of the fact that the pearlite wire rod for traditional hand tools contains relatively high carbon, silicon, manganese, chromium, vanadium, tungsten and other alloying elements, the material cost is high, the plasticity and cold working performance are insufficient, and the processing cost of hand tools is sharply increased, the content of alloying elements such as Mn and V in the present invention is relatively low, which can take into account the wear resistance and corrosion resistance of hand tools and reduce material costs. The microstructure of the wire rod includes a mixed structure composed of tempered bainite and ferrite, which can improve the strength of the wire rod and match it with high plasticity, achieving a tensile strength of 910~960MPa and a cross-sectional shrinkage rate of 58%~65%. It is used in application fields such as manufacturing hand tools, which is beneficial to eliminating additional heat treatment before cold working of hand tools, reducing the risk of cracking of wire rods during direct cold working, and reducing the processing cost of hand tools, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0038] Figure 1 is a metallographic structure diagram of Example 1 of the present invention;

[0039] Figure 2 is a metallographic structure diagram of Example 2 of the present invention;

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

[0041] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention. They are 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:

[0042] A preferred embodiment of the method for manufacturing high-plasticity wire rod for hand tools of the present invention comprises the following chemical compositions and mass percentages: C: 0.44%, Si: 0.3%, Mn: 0.68%, Cr: 0.55%, V: 0.051%, P: 0.015%, S: 0.013%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows a process flow of rolling → spinning → online molten salt isothermal toughening → roller slow cooling → coiling, specifically:

[0043] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity. After the steel billet comes out of the heating furnace, initial rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface oxide scale. The high-temperature steel billet is rolled into a wire with a diameter of 15mm through a rolling line. Appropriate final rolling inlet temperature and final rolling deformation are selected to promote microstructure grain refinement and strengthen the matrix. Specifically: the initial rolling descaling water pressure is controlled to 18.5MPa, the final rolling inlet is 935℃, and the final rolling deformation is 20%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism. The wire rod is spread on the roller and transported along the roller so that the wire rod is in a high-temperature austenite state.

[0044] The online molten salt isothermal toughening process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by rollers through the first salt bath tank for front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 12°C / s, and quickly enters the ferrite and bainite mixed phase region from the high-temperature austenite state. A smaller molten salt circulation volume is selected to control the ferrite and bainite phase transformation and the dispersion and precipitation of vanadium-containing carbides to form a mixed structure dominated by bainite. The wire rod is then conveyed by rollers through the second salt bath tank for Carry out the molten salt treatment in the back stage, increase the molten salt circulation volume to control the molten salt temperature rise, promote the isothermal toughening of ferrite and bainite, promote the dispersion and precipitation of vanadium-containing carbides, inhibit the growth of carbide strengthening phase, and improve the strength-plasticity matching of wire rod. Specifically: the molten salt temperature of the front stage is 553℃, the treatment time is 105s, and the molten salt circulation volume is 150t / h; the molten salt temperature of the back stage is 565℃, the treatment time is 45s, the molten salt circulation volume is 450t / h, and the molten salt temperature rise is ≤3℃.

[0045] The roller slow cooling process adopts the method of closing the insulation cover, and the conveyor roller conveys the wire rod that has passed the second salt bath tank into the insulation cover, so that the wire rod can be slowly cooled and continuously softened at high temperature, further promoting the tempering and softening of the matrix structure until it is coiled. Specifically: the roller speed before the wire rod enters the insulation cover is 0.95m / s, and the roller speed is reduced to 0.45m / s after entering the insulation cover. The wire rod is controlled to slowly cool to 282℃ at a cooling rate of 0.27℃ / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0046] Comparative Example 1:

[0047] A method for manufacturing wire rods, which differs from that of Example 1 in that: the method is manufactured according to the process flow of rolling → spinning → Stelmore air cooling line → coiling, specifically: in the rolling process, the final rolling entrance is 865°C, the Stelmore air cooling line uses a closed fan, the first two insulation covers are opened, and the remaining insulation covers are closed, the wire rods after spinning are transported along the Stelmore air cooling line by rollers, the cooling rate of the wire rods in the insulation cover is controlled to 2.8°C / s, the temperature is reduced to 280°C and collected by a coiling drum to obtain a finished wire rod.

[0048] Comparative Example 2:

[0049] A method for manufacturing wire rod, which differs from Example 1 in that: the final rolling entrance in the rolling process is 990°C, the wire rod is treated with a front-stage molten salt and cooled at a cooling rate of 9°C / s, the molten salt circulation rate of the front-stage molten salt is 45t / h, and a finished wire rod is obtained. Example 2:

[0050] A preferred embodiment of the method for manufacturing high-plasticity wire rod for hand tools of the present invention comprises the following chemical compositions and mass percentages: C: 0.46%, Si: 0.3%, Mn: 0.5%, Cr: 0.49%, V: 0.045%, P: 0.013%, S: 0.015%, with the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt isothermal toughening → roller slow cooling → coiling, specifically:

[0051] The rolling process is used to heat a steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet with rollable plasticity. After the steel billet comes out of the heating furnace, initial rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface oxide scale. The high-temperature steel billet is rolled into a wire with a diameter of 8mm through a rolling line. Appropriate final rolling inlet temperature and final rolling deformation are selected to promote microstructure grain refinement and strengthen the matrix. Specifically: the initial rolling descaling water pressure is controlled to 16MPa, the final rolling inlet is 900℃, and the final rolling deformation is 25%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism. The wire rod is spread on the roller and transported along the roller so that the wire rod is in a high-temperature austenite state.

[0052] The online molten salt isothermal toughening process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by rollers through the first salt bath tank for front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 10°C / s, and quickly enters the ferrite and bainite mixed phase region from the high-temperature austenite state. A smaller molten salt circulation volume is selected to control the ferrite and bainite phase transformation and the dispersion and precipitation of vanadium-containing carbides to form a mixed structure dominated by bainite. The wire rod is then conveyed by rollers through the second salt bath tank. Carry out the rear-stage molten salt treatment, increase the molten salt circulation volume to control the molten salt temperature rise, promote the isothermal toughening of ferrite and bainite, promote the dispersion and precipitation of vanadium-containing carbides, inhibit the growth of carbide strengthening phase, and improve the strength-plasticity matching of wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 562℃, the treatment time is 68s, and the molten salt circulation volume is 50t / h; the molten salt temperature of the rear-stage molten salt is 552℃, the treatment time is 85s, the molten salt circulation volume is 350t / h, and the molten salt temperature rise is ≤3℃.

[0053] The roller slow cooling process adopts the method of closing the insulation cover, and the conveyor roller conveys the wire rod that has passed the second salt bath tank into the insulation cover, so that the wire rod can be slowly cooled and continuously softened at high temperature, further promoting the tempering and softening of the matrix structure until it is coiled. Specifically: the roller speed before the wire rod enters the insulation cover is 0.9m / s, and the roller speed is reduced to 0.5m / s after entering the insulation cover. The wire rod is controlled to slowly cool to 280℃ at a cooling rate of 0.35℃ / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0054] Comparative Example 3:

[0055] A method for manufacturing wire rod, which differs from Example 2 in that: the wire rod is treated with a front-stage molten salt and cooled at a cooling rate of 8°C / s, the molten salt temperature of the front-stage molten salt is 585°C, and the treatment time is 25s to obtain a finished wire rod.

[0056] Comparative Example 4:

[0057] A method for manufacturing wire rod, which differs from Example 2 in that: the wire rod is treated with a front-stage molten salt and cooled 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 130s, and the molten salt circulation volume is 200t / h; and a finished wire rod is obtained. Example 3:

[0058] A preferred embodiment of the method for manufacturing high-plasticity wire rod for hand tools of the present invention comprises the following chemical compositions and mass percentages: C: 0.43%, Si: 0.2%, Mn: 0.7%, Cr: 0.45%, V: 0.055%, P: 0.013%, S: 0.013%, and the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt isothermal toughening → roller slow cooling → coiling, specifically:

[0059] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity. After the steel billet comes out of the heating furnace, initial rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface oxide scale. The high-temperature steel billet is rolled into a wire with a diameter of 11mm through a rolling line. Appropriate final rolling inlet temperature and final rolling deformation are selected to promote microstructure grain refinement and strengthen the matrix. Specifically: the initial rolling descaling water pressure is controlled to 17MPa, the final rolling inlet is 915℃, and the final rolling deformation is 23%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism. The wire rod is spread on the roller and transported along the roller so that the wire rod is in a high-temperature austenite state.

[0060] The online molten salt isothermal toughening process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by rollers through the first salt bath tank for front-stage molten salt treatment, so that the wire rod is cooled at a cooling rate of 11°C / s, and quickly enters the ferrite and bainite mixed phase region from the high-temperature austenite state. A smaller molten salt circulation volume is selected to control the ferrite and bainite phase transformation and the dispersion and precipitation of vanadium-containing carbides to form a mixed structure dominated by bainite. The wire rod is then conveyed by rollers through the second salt bath tank. Carry out the rear-stage molten salt treatment, increase the molten salt circulation volume to control the molten salt temperature rise, promote the isothermal toughening of ferrite and bainite, promote the dispersion and precipitation of vanadium-containing carbides, inhibit the growth of carbide strengthening phase, and improve the strength-plasticity matching of wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 570℃, the treatment time is 30s, and the molten salt circulation volume is 85t / h; the molten salt temperature of the rear-stage molten salt is 540℃, the treatment time is 120s, the molten salt circulation volume is 375t / h, and the molten salt temperature rise is ≤3℃.

[0061] The roller slow cooling process adopts the method of closing the insulation cover, and the conveyor roller conveys the wire rod that has passed the second salt bath tank into the insulation cover, so that the wire rod can be slowly cooled and continuously softened at high temperature, further promoting the tempering and softening of the matrix structure until it is coiled. Specifically: the roller speed before the wire rod enters the insulation cover is 0.8m / s, and the roller speed is reduced to 0.6m / s after entering the insulation cover. The wire rod is controlled to slowly cool to 275℃ at a cooling rate of 0.55℃ / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the finished wire rod is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 3 shown.

[0062] Comparative Example 5:

[0063] A method for manufacturing a wire rod, which differs from Example 3 in that the molten salt temperature of the rear-stage molten salt is 585° C., the processing time is 130 seconds, and the finished wire rod is obtained.

[0064] Comparative Example 6:

[0065] A method for manufacturing a wire rod, which differs from Example 3 in that the molten salt temperature of the rear-stage molten salt is 525° C., the processing time is 20 seconds, and the finished wire rod is obtained. Example 4:

[0066] A preferred embodiment of the method for manufacturing high-plasticity wire rod for hand tools of the present invention comprises the following chemical compositions and mass percentages: C: 0.48%, Si: 0.26%, Mn: 0.59%, Cr: 0.6%, V: 0.049%, P: 0.014%, S: 0.014%, and the remainder being Fe and unavoidable impurities; the manufacturing method follows the process flow of rolling → spinning → online molten salt isothermal toughening → roller slow cooling → coiling, specifically:

[0067] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity. After the steel billet comes out of the heating furnace, initial rolling descaling water is sprayed onto the surface of the high-temperature steel billet to remove the surface oxide scale. The high-temperature steel billet is rolled into a wire with a diameter of 13mm through a rolling line. Appropriate final rolling inlet temperature and final rolling deformation are selected to promote microstructure grain refinement and strengthen the matrix. Specifically: the initial rolling descaling water pressure is controlled to 18MPa, the final rolling inlet is 925℃, and the final rolling deformation is 21%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism. The wire rod is spread on the roller and transported along the roller so that the wire rod is in a high-temperature austenite state.

[0068] The online molten salt isothermal toughening process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first salt bath tank for the front molten salt treatment, so that the wire rod is cooled at a cooling rate of 13°C / s, and quickly enters the ferrite and bainite mixed phase region from the high temperature austenite state. A smaller molten salt circulation volume is selected to control the ferrite and bainite phase transformation and the dispersion and precipitation of vanadium-containing carbides to form a mixed structure dominated by bainite. The wire rod is then conveyed by a roller through the second salt bath tank for Carry out the rear-stage molten salt treatment, increase the molten salt circulation volume to control the molten salt temperature rise, promote the isothermal toughening of ferrite and bainite, promote the dispersion and precipitation of vanadium-containing carbides, inhibit the growth of carbide strengthening phase, and improve the strength-plasticity matching of wire rod. Specifically: the molten salt temperature of the front-stage molten salt is 540℃, the treatment time is 120s, and the molten salt circulation volume is 115t / h; the molten salt temperature of the rear-stage molten salt is 570℃, the treatment time is 20s, the molten salt circulation volume is 415t / h, and the molten salt temperature rise is ≤3℃.

[0069] The roller slow cooling process adopts the method of closing the insulation cover, and the wire rod that has passed the second salt bath tank is transported by the conveyor roller into the insulation cover, so that the wire rod can be slowly cooled and continuously softened at high temperature, further promoting the tempering and softening of the matrix structure until it is coiled. Specifically: the roller speed of the wire rod before entering the insulation cover is 1m / s, and the roller speed is reduced to 0.32m / s after entering the insulation cover. The wire rod is controlled to slowly cool to 285℃ at a cooling rate of 0.18℃ / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the finished wire rod is obtained after packaging and storage.

[0070] Comparative Example 7:

[0071] A method for manufacturing a wire rod, which differs from Example 4 in that: the manufacturing method follows a process flow of rolling → spinning → online molten salt isothermal toughening → air cooling → coiling. Specifically: the air cooling process is to open the insulation cover and transport the wire rod through the second salt bath tank by a conveyor roller. The wire rod is slowly cooled to 280°C at a cooling rate of 1.2°C / s to obtain the wire rod.

[0072] 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:

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

[0074]

[0075] From the comparison results of Examples 1 to 4 and Comparative Example 1, it can be seen that even if low-temperature rolling and Stelmor air-cooling line slow cooling post-treatment are adopted, brittle structures such as bainite and martensite will still appear, the mechanical properties fluctuate greatly, the plasticity of the structure is insufficient, and its cold working performance is reduced. The present invention can appropriately reduce the content of alloy elements to reduce material costs through the Cr-V chemical composition design combined with the online molten salt isothermal toughening technology. The wire rod is controlled by online molten salt isothermal toughening in the ferrite and bainite mixed phase region and isothermal toughening treatment to produce a wire rod with a microstructure type including a mixed structure composed of tempered bainite and ferrite. The product tensile strength can reach 910~960MPa and the cross-sectional shrinkage rate is 58%~65%. The mechanical properties fluctuate less and the cold working performance is better. It is used in application fields such as manufacturing hand tools without adding additional heat treatment, effectively reducing the risk of cracking and fracture during direct cold working of the wire rod for hand tools, and can achieve the corresponding hand tool strength grade.

[0076] From the comparison results of Example 1 and Comparative Example 2, it can be seen that an appropriate final rolling inlet temperature combined with the final rolling deformation is conducive to grain refinement of the structure, controlling the wire rod to quickly enter the ferrite and bainite mixed phase region in the high-temperature austenite state, which can increase the driving force of bainite transformation and vanadium-containing carbides, and improve the strength and plasticity of the wire rod.

[0077] From the comparison results of Example 2 and Comparative Example 3, it can be seen that the higher the temperature of the molten salt in the front section 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 is reduced and the nucleation rate is reduced, which is not conducive to the formation of a mixed structure dominated by bainite, affecting the matrix strength. At the same time, as the total treatment time of online molten salt isothermal toughening is shortened, a certain amount of plasticity will be lost.

[0078] From the comparison results of Example 2 and Comparative Example 4, it can be seen that the lower the molten salt temperature of the front-stage molten salt and the longer the treatment time, the slower the formation rate of ferrite, the smaller the ferrite grain size, the greater the driving force of bainite transformation and vanadium-containing carbides, the nucleation rate of bainite increases, the matrix strength increases, and the rear-stage molten salt treatment makes organizational preparations. However, the molten salt temperature is too low and the treatment time is too long, which is not conducive to the transformation and formation of ferrite, affects the temperature control accuracy, increases the difficulty of improving plasticity, and is not conducive to the control of wire rod plasticity and organizational uniformity.

[0079] From the comparison results of Example 3 and Comparative Example 5, it can be seen that the higher the molten salt temperature of the latter stage and the longer the treatment time are, the more conducive it is to provide more thermal power for isothermal toughening and to improve the plasticity of the matrix faster. However, if the molten salt temperature is too high and the treatment time is too long, it is not conducive to controlling the dispersion and precipitation of carbides, and there is a risk of carbide coarsening, which affects the strength and plasticity of the matrix.

[0080] From the comparison results of Example 3 and Comparative Example 6, it can be seen that the lower the molten salt temperature of the latter molten salt and the shorter the treatment time, the lower the softening and plasticity improvement effect, 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, the phase structure of ferrite and bainite still retains some characteristics before tempering, and the dislocation density is still high, which will limit the improvement of the toughness and plasticity of the material.

[0081] From the comparison results of Example 4 and Comparative Example 7, it can be seen that during the roller slow cooling treatment, the wire rod can continue the toughening effect of the late stage of online molten salt isothermal toughening under slow cooling, and further promote the tempering and softening of the matrix structure.

[0082] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing a high-plasticity wire rod for hand tools, characterized in that: The manufacturing method includes: The steel billet is subjected to rolling, wire drawing, online molten salt isothermal toughening and roller slow cooling treatment to be made into a wire rod with a microstructure including tempered bainite and ferrite. The online molten salt isothermal toughening control wire rod first passes through the front section molten salt, is cooled at a cooling rate of ≥10℃ / s and enters the ferrite and bainite mixed phase region from the austenite state to form a mixed structure mainly composed of bainite, and then passes through the back section molten salt to increase the molten salt circulation amount and promote the isothermal toughening of ferrite and bainite; the molten salt temperature of the front section molten salt is 540~570℃, and the treatment time is 30~1 20s, the molten salt circulation rate is 50~150t / h; the molten salt temperature of the rear-stage molten salt is 540~570℃, and the processing time is 20~120s; 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 unavoidable impurities.

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

3. The method for manufacturing a high-plasticity wire rod for hand tools according to claim 1, wherein: During the rolling, the final rolling entrance temperature is controlled to be 900-935° C., and the final rolling deformation is 20%-25%.

4. The method for manufacturing a high-plasticity wire rod for hand tools according to claim 1, wherein: The molten salt circulation rate of the rear-stage molten salt is 350-450t / h, and the molten salt temperature rise is ≤3°C.

5. The method for manufacturing a high-plasticity wire rod for hand tools according to claim 1, wherein: The roller slow cooling controls the wire rod to slowly cool at a cooling rate of 0.15-0.55°C / s.

6. A high plasticity wire rod for hand tools, characterized in that: The wire rod is manufactured by the method for manufacturing high-plasticity wire rod for hand tools according to any one of claims 1 to 5.

7. The high plasticity wire rod for hand tools according to claim 6, characterized in that: The volume percentage of the tempered bainite is 70% to 80%.

8. The high plasticity wire rod for hand tools according to claim 6, 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 is 58%-65%, and the mechanical property difference within the same circle is ≤30 MPa.

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