A Grade 8.8 Boron-Containing Cold Heading Steel Wire Rod and Its Production Method

Abstract

This invention relates to an 8.8 grade boron-containing cold heading steel wire rod and its production method. The process involves rolling, wire drawing, hot coiling, and slow cooling in a holding tunnel furnace. The wire drawing temperature is controlled at 900-950℃. After wire drawing, the wire rod is conveyed to the coiling process under holding conditions. The coiling temperature is 800-850℃. During slow cooling in the holding tunnel furnace, the furnace entry temperature is controlled at 720-770℃. The cooling rate of the coil in the 700-600℃ temperature range is ≤0.1℃ / s, and the exit temperature is ≤500℃. Utilizing the residual heat of the coil in the holding tunnel furnace for slow cooling results in a higher ferrite content and online softening of the microstructure. This effectively reduces the tensile strength and hardness of the boron-containing cold heading steel wire rod while improving its plasticity. The wire rod can achieve a tensile strength ≤450MPa, a reduction of area ≥58%, and a hardness ≤75HRB. It can be used for the non-annealing production of 8.8 grade nuts and other large deformation fasteners.

Description

Technical Field

[0001] This invention belongs to the field of cold heading steel wire rod technology, specifically relating to an 8.8 grade boron-containing cold heading steel wire rod and its production method. Background Technology

[0002] Cold heading steel wire rod is of great significance as a base material for fastener production, especially for the efficient and stable production, quality assurance, and application expansion of products with large cold heading deformation. For example, nuts and bolts are used to fasten structural components and are widely used in automobiles, construction machinery, and other fields. However, the deformation of nuts during the cold heading process is greater than that of ordinary bolts, requiring higher plasticity of the material. If the material used to produce bolts is used directly to produce nuts, it is easy to cause large mold wear and insufficient plasticity, resulting in a high cracking rate. Therefore, it is necessary to develop an 8.8 grade cold heading steel wire rod and its production method to meet the needs of downstream users to produce nuts and other large deformation fasteners without cracking during the deformation process.

[0003] Currently, cold heading steel wire rods are generally produced using controlled rolling and controlled cooling on rolling lines and Stellmore cooling lines. However, the following technical bottlenecks still exist in manufacturing cold heading steel wire rods with low strength, high plasticity, and performance requirements for fasteners with large deformation:

[0004] To ensure the performance of fasteners while balancing the hardenability of steel and reducing hot-rolled strength, existing cold-heading steel wire rods often employ boron-containing components combined with slow cooling. For example, patent CN113025917A discloses a low-strength, high-plasticity, anneal-free cold-heading steel wire rod and its manufacturing method. This method uses a medium-low carbon composition of C-Mn-Cr-Ti-Al-B, combined with low-temperature rolling and a Stellmore slow cooling process to obtain a ferrite and pre-spheroidized pearlite microstructure, achieving a tensile strength of 460~540MPa. However, to increase the ferrite phase ratio and promote the precipitation of carbides in spherical or short rod-like morphologies, thereby reducing wire rod strength and increasing the reduction of area, a Stellmore cooling process is used after low-temperature wire drawing. While the wire rod is kept warm and cooled, the solid solution strengthening and hardening effects of elements such as Mn, Cr, B, and Si delay the pearlite transformation. Due to the minimum cooling capacity and cooling line length limitations of the insulation cooling, the outer layer of the wire rod dissipates heat faster when in contact with the air, while the inner layer dissipates heat slower. This temperature difference easily leads to the superposition of thermal stress, generating mechanical stress and phase transformation stress. In some areas, fine pearlite with high hardness is formed, while in other areas, coarse ferrite with uneven plasticity is formed. This increases the surface hardness of the wire rod, increases the wear and cracking risk of the molds in subsequent cold heading processes, and provides limited improvement by reducing the rolling and wire drawing temperatures and the roller conveyor speed, and extending the time the wire rod spends in the insulation cover. In addition, it increases the risk of cracking during the rolling stage and affects continuous production and offline efficiency.

[0005] On the other hand, in order to obtain higher deformation energy storage and high dislocation density, low-temperature large deformation rolling is adopted. However, at low temperatures, the resistance to dislocation movement increases, Mn and Cr increase the deformation resistance of the billet, and B has weak diffusion ability and is prone to uneven grain boundary segregation to form embrittled phases. Especially for rolling large square billets with large cross sections, this will lead to increased equipment load, reduced rolling efficiency, local overcooling causing increased hardness, and surface defects. In order to improve production efficiency, the rolling and wire drawing temperatures are increased. However, the temperature of the shroud increases and the cooling is accelerated under the subsequent slow cooling effect. The proportion of ferrite phase will be affected, and the coarsening of pearlite lamellae will lead to the obstruction of pre-spheroidization, resulting in increased wire rod strength and hardness and decreased plasticity, which is difficult to meet the production requirements of large deformation fasteners. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the above-mentioned technical problems. The present invention provides an 8.8 grade boron-containing cold heading steel wire rod and its production method, which can effectively reduce the strength and hardness of cold heading steel wire rod, improve plasticity, take into account production efficiency, and meet the needs of downstream users to produce large deformation fasteners such as nuts.

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

[0008] The first aspect of this invention is to provide a method for producing 8.8 grade boron-containing cold heading steel wire rod, wherein the chemical element composition of the cold heading steel wire rod, by weight percentage, comprises: C: 0.18%~0.20%, Si: ≤0.05%, Mn: 0.60%~0.80%, P: ≤0.015%, S: ≤0.010%, Cr: ≤0.10%, Ni: ≤0.10%, Cu: ≤0.10%, Mo: ≤0.10%, Al: ≥0.02%, B: 0.0015%~0.0030%, Ti: 0.02%~0.05%, N: ≤0.0060%, O: ≤0.0015%, with the remainder being Fe and unavoidable impurities; the production method follows a process of rolling, wire drawing, hot coiling, and slow cooling in a holding tunnel furnace.

[0009] During the spinning process, the spinning temperature is controlled at 900~950℃. After spinning, the wire is conveyed into the winding process under heat preservation.

[0010] During the hot winding process, the winding collection temperature is controlled at 800~850℃, and the wire rod winding is a coil.

[0011] During slow cooling in the insulated tunnel furnace, the furnace inlet temperature is controlled at 720~770℃, the residence time of the coil in the insulated tunnel furnace is 60~90min, the cooling rate in the temperature range of 700~600℃ is ≤0.1℃ / s, and the furnace outlet temperature is ≤500℃.

[0012] The aforementioned boron-containing cold heading steel wire rods increase the ferrite transformation initiation temperature by appropriately reducing the carbon content, making it easier for ferrite to nucleate and precipitate from austenite at higher temperatures, reducing the pearlite proportion, and improving cold heading plasticity. Simultaneously, it meets the 8.8 grade performance requirements, avoiding insufficient strength after cold heading due to excessively low carbon content. By reducing the silicon content, solid solution strengthening is weakened and ferrite hardness is reduced, thus lowering the deformation resistance of pearlite during slow cooling in the holding tunnel furnace, reducing intragranular deformation resistance during cold heading, and improving cold heading plasticity. Appropriate adjustments are also made to... The Mn content further reduces the risk of component segregation and hard and brittle phases, balancing hardenability and plasticity. By reducing the Cr content, the rolling deformation resistance is reduced, which is conducive to the rolling of large billets with high compression ratios and to the protection of rolling mill equipment. At the same time, the low Si and Cr content reduces the obstruction to cementite diffusion, which is conducive to increasing the pearlite transformation rate, shortening the tunnel furnace residence time, increasing production capacity, and reducing carbide precipitation, avoiding over-strengthening. This provides favorable conditions for hot coiling under high temperature rolling and slow cooling and stable production in the tunnel furnace.

[0013] Based on this, compared with the existing low-temperature rolling and Stellmore slow cooling treatment, the above production method uses high-temperature rolling before hot coiling. On the one hand, it can promote the uniformity of austenite composition, reduce the local enrichment of B and Mn, and avoid excessive segregation of B at local grain boundaries to form embrittled phases, thus creating favorable conditions for subsequent microstructure transformation and avoiding the formation of abnormal structures such as hard and brittle bainite in the segregation zone during subsequent slow cooling. On the other hand, it can match the temperature requirements of hot coiling and slow cooling in the tunnel furnace, ensuring the efficient utilization of residual heat. Combined with conveying the wire rod into the coiling process under heat preservation after rolling, it can reduce the heat loss of the wire rod during transportation, allowing it to cool at a slower rate and avoiding the formation of undesirable structures or affecting the coiling temperature due to excessively rapid cooling.

[0014] Based on this, compared to the existing Stellmore slow-cooling and coiling method, the above production method uses hot coiling, which keeps the wire rod in an austenitic state, preventing premature phase transformation. Bending stress during coiling can be released through high-temperature creep, preventing surface micro-cracks. Simultaneously, the coiled wire rod can be fed into the insulated tunnel furnace as a whole. On one hand, the tight stacking within the coil allows the outer layer of wire rod to form an insulating layer, preventing the residual heat of the inner layer from dissipating easily. The overall heat distribution of the coil is more uniform, and the residual heat allows for rapid entry into a more stable insulated environment, reducing production energy consumption while avoiding localized rapid heat dissipation in individual wire rods that could lead to increased surface hardness and loss of the soft ferrite phase. It also effectively reduces the temperature difference between the wire rod surface and core, significantly reducing overall thermal stress and improving the overall plasticity of the wire rod. On the other hand, temperature compensation within the insulated tunnel furnace further enhances the thermal stability. The prolonged residence of wire rod in the insulated tunnel furnace at 700~600℃ reduces the undercooling of pearlite transformation, allowing for more precise microstructure control. The extremely slow cooling rate provides ample diffusion time for austenite, enabling ferrite to uniformly precipitate at austenite grain boundaries and within grains, resulting in higher ferrite content. This, in turn, reduces the overall tensile strength and hardness of the wire rod. Simultaneously, the higher temperature enhances atomic diffusion, allowing carbon atoms to migrate uniformly to the pearlite lamellae, further releasing phase transformation stress and achieving online softening of the microstructure. This reduces the precipitation of carbonitrides, avoids over-strengthening, and ultimately forms a uniform microstructure of soft ferrite and ductile pearlite, without the formation of brittle martensite or bainite. The uniform distribution of high ferrite content provides slip channels for plastic deformation. Combined with the synergistic effect of composition design, this benefits downstream users in cold heading deformation and reduces the risk of cracking.

[0015] Because the wire drawing temperature is relatively high, the limitation on the rolling temperature can be reduced. The optimization of C, Si, Mn, and Cr further reduces the resistance to high-temperature deformation. During rolling, selecting an appropriate heating temperature can promote the full dissolution and uniform distribution of B, leverage the grain refinement effect of TiN, improve rolling plasticity, and reduce the risk of decarburization. The higher initial rolling temperature reduces rolling deformation resistance, lowers the rolling load on the mill, avoids excessive roll wear, and reduces the risk of rolling cracks. Controlling the sizing and reduction temperatures inhibits premature austenite grain growth and improves dimensional accuracy, facilitating the connection with subsequent wire drawing temperatures. In the preferred technical solution, during rolling, the heating temperature is controlled at 1100~1150℃, the initial rolling temperature at 1000~1060℃, and the sizing and reduction temperatures at 880~930℃.

[0016] During the rolling process, a 240mm square billet can be used as the basis for a high compression ratio, reducing intermediate billet opening stages. Combined with high-temperature, high-compression-ratio rolling, this improves rolling production efficiency. Multiple rolling passes rationally distribute deformation, avoiding localized stress concentration. The high compression ratio allows deformation to penetrate from the surface to the core, thoroughly breaking down the original columnar crystals and loose coarse-grained defects in the billet. This prevents uneven plasticity during cold heading caused by these coarse-grained areas. Combined with TiN pinning austenite grain boundaries, the grains are refined. After austenite grain refinement, the number of nucleation points for ferrite and pearlite increases, which is beneficial for improving the plasticity of the wire rod. In the preferred technical solution, a billet with a cross-section ≥240mm×240mm is used during rolling, undergoing 22-26 rolling passes, with a total compression ratio of 226-510. Preferably, an 850mm ultra-heavy-duty rolling mill is used in the roughing stage to increase the roughing compression ratio.

[0017] To further avoid surface defects in the billet caused by residual oxide scale during the heating process, high-pressure water descaling and peeling off the iron oxide scale on the billet surface can be used before rolling after heating. In the preferred technical solution, the water descaling pressure is ≥22MPa during rolling.

[0018] To further ensure good surface quality of the billet and prevent billet defects from being inherited to the wire rod surface, in the preferred technical solution, shot blasting and magnetic particle testing are used to finish the billet before rolling.

[0019] After spinning, the wire rod temperature can be controlled and the production cycle shortened by further controlling the roller speed, time, and wire rod cooling rate. This also helps to adapt to low Si content, reduce surface oxidation defects, and avoid grain coarsening, which can affect production efficiency and winding temperature due to excessively long conveying time, low roller speed, and low cooling rate. In the preferred technical solution, after spinning, the wire rod is conveyed by rollers through an insulation cover. The roller speed is ≥0.6m / s, and the wire rod enters the winding process within 1 minute. The wire rod cooling rate is >1.5℃ / s.

[0020] To further reduce heat loss and the risk of poor structure during the transportation of wire rod, in the preferred technical solution, the coiled wire rod is put into the heat preservation tunnel oven within 1 minute after being assembled.

[0021] In order to further control the cooling rate of wire rod in the insulated tunnel furnace, in a preferred technical solution, the temperature of the atmosphere inside the furnace is controlled at 400~600℃ during slow cooling in the insulated tunnel furnace, and the furnace exit temperature is further preferred to be >400℃.

[0022] To further improve the uniformity of wire rod structure and facilitate deformation, the preferred technical solution includes a continuous casting process before rolling. This continuous casting process employs full-process protective casting and dual electric stirring: controlling the superheat to 17-27°C; the crystallizer electric stirring current is 300-400A with a frequency of 2-5Hz; the end electric stirring current is 200-300A with a frequency of 8-10Hz. Full-process protective casting prevents secondary oxidation of the molten steel by air, thus avoiding its cleanliness. The combination of superheat and dual electric stirring promotes the growth of equiaxed crystals. The crystallizer electric stirring parameters ensure more uniform solidification of the molten steel within the crystallizer, reducing columnar crystal regions and increasing the equiaxed crystal ratio. The end electric stirring parameters mitigate defects such as billet segregation, central porosity, and shrinkage cavities, resulting in a more uniform and dense internal structure of the billet.

[0023] To further improve the purity of molten steel before continuous casting, the preferred technical solution includes an LF refining process before the continuous casting process. The LF refining process controls the white slag holding time to be ≥18 min, and uses the white slag for deep desulfurization and inclusion adsorption. Preferably, lime and fluorite are added to adjust the slag fluidity, with lime dosage ≤100 kg / furnace and fluorite dosage ≤100 kg / furnace. After the white slag, titanium-iron alloy is added to avoid titanium oxidation loss, and calcium wire is fed in for calcium treatment. Calcium treatment makes the inclusions easier to float and remove. After calcium treatment, boron-iron wire is added to improve the boron yield. At the same time, soft blowing of argon gas is started for 18~25 min to promote inclusion flotation and composition homogenization.

[0024] To further achieve efficient dephosphorization, precise alloying, and stable steel quality, the preferred technical solution includes a converter smelting process before the LF refining process. This converter smelting process uses a 150-200 ton converter with a scrap steel ratio controlled at 15%-25%, reducing the cost per ton of steel. The converter tapping process employs a double-plate sliding plate to effectively control the amount of slag and improve the purity of the molten steel. During tapping, deoxidizer, carburizer, alloy, and top slag are added sequentially. The deoxidizer is added first for deep deoxidation, while the carburizer and alloy are added in sequence to improve alloy absorption rate, promote carbon content homogenization, and avoid excessive carbon addition leading to carbon exceeding the standard. The top slag uses quicklime and calcium aluminate. Calcium aluminate lowers the slag melting point, promotes CaO dissolution, and improves desulfurization efficiency, creating conditions for rapid white slag formation during LF refining. The final carbon content at the converter tapping point is 0.08%-0.15%. Feeding aluminum wire to the argon blowing station is prohibited to reduce exposed steel gas absorption and avoid prolonged production cycles caused by feeding aluminum wire to the argon blowing station.

[0025] A second aspect of the present invention is to provide an 8.8 grade boron-containing cold heading steel wire rod, wherein the cold heading steel wire rod is produced by the production method of any one of the above-described 8.8 grade boron-containing cold heading steel wire rods, and the microstructure includes ferrite and pearlite, wherein the proportion of ferrite is ≥70%, the ferrite can provide high plasticity through dislocation slip and grain boundary coordination, adapting to large cold heading deformation and avoiding excessive work hardening during cold heading, the pearlite has moderate strength, can provide basic strength, avoid excessively soft deformation during cold heading, so as to maintain the dimensional accuracy of fasteners after cold heading, and has no hard and brittle phases, so as to avoid stress concentration in local hard and brittle areas leading to cracking.

[0026] In the preferred technical solution, the diameter of the cold heading steel wire rod is 12~18mm, the tensile strength is ≤450MPa, the reduction of area is ≥58%, and the hardness is ≤75HRB. The lower tensile strength is to reduce the load on the cold heading machine, avoid equipment overload, and reduce production costs. The lower hardness is to reduce surface friction and scratches during cold heading, reduce the risk of mold seizure and wear. The increased reduction of area is beneficial for downstream users to use without annealing, cold heading with large deformation processing, reduce the risk of fracture, and improve forming quality.

[0027] In the preferred technical solution, the total decarburization layer depth of the cold heading steel wire rod is ≤1%D, where D represents the wire rod diameter. A low decarburization layer depth can improve the coordination of cold heading deformation. The grain size is grade 8 to 10. Fine grains can improve the cold heading plasticity and service resistance. 1 / 3 of the cold upsetting is qualified, and the surface defect depth is ≤0.05mm, which can prevent defects from becoming crack sources during cold heading or causing fatigue fracture during service.

[0028] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0029] This invention optimizes the chemical composition of boron-containing cold heading steel wire rod, combines high-temperature wire drawing and hot coiling for rapid coil collection, and utilizes the residual heat of the coil in a heat-insulating tunnel furnace for slow cooling. This results in a higher ferrite content and online softening of the microstructure, effectively reducing the tensile strength and hardness of the boron-containing cold heading steel wire rod while improving its plasticity. The wire rod achieves a tensile strength ≤450MPa, a reduction of area ≥58%, and a hardness ≤75HRB. Furthermore, it allows for direct rolling with large billets and high compression ratios, balancing plasticity and production efficiency. The finished cold heading steel wire rod can be used as a base material to produce large deformation fasteners such as 8.8 grade nuts, better meeting the downstream users' need for crack-free production during deformation without annealing. It exhibits excellent industrial adaptability and application prospects. Attached Figure Description

[0030] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0031] Figure 1This is a metallographic diagram of Embodiment 1 of the present invention.

[0032] Figure 2 This is a metallographic diagram of Comparative Example 1 of the present invention. Detailed Implementation

[0033] The present invention will be further described in detail below through specific preferred embodiments, but the present invention is not limited to the following embodiments. It should be noted that, unless otherwise specified, all raw and auxiliary materials involved in the present invention are purchased through commercial channels.

[0034] Example 1:

[0035] A preferred embodiment of the production method of the 8.8 grade boron-containing cold heading steel wire rod of the present invention, wherein the chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.18%, Si: 0.05%, Mn: 0.60%, P: 0.015%, S: 0.010%, Cr: 0.10%, Ni: 0.10%, Cu: 0.10%, Mo: 0.10%, Al: 0.02%, B: 0.0015%, Ti: 0.05%, N: 0.0060%, O: 0.0015%, with the remainder being Fe and unavoidable impurities; the production method follows the process flow of converter smelting → LF refining → large billet continuous casting → billet finishing → heating → rolling → wire drawing → hot coiling → slow cooling in a tunnel furnace with heat preservation → inspection → packaging → weighing → marking → warehousing. Specifically:

[0036] The converter smelting process is used to smelt steelmaking raw materials into molten steel. A 150-ton converter is used, and the scrap steel ratio is controlled at 25%. The converter tapping adopts a double-plate sliding plate to effectively control the amount of slag and improve the purity of the molten steel. During tapping, deoxidizer → carbon raiser → alloy → top slag are added in sequence. The top slag is made of quicklime and calcium aluminate. The carbon content at the end of the converter is 0.08%. Aluminum wire is prohibited from being fed to the argon blowing station.

[0037] The LF refining process is used to perform LF refining treatment on molten steel that has passed through the argon blowing station. The fluidity of the slag is adjusted by adding lime and fluorite, and the white slag is maintained for ≥18 minutes. After the white slag, titanium-iron alloy is added, and calcium wire is fed in for calcium treatment. After calcium treatment, boron-iron wire is added, and soft blowing of argon gas is started at the same time for 25 minutes.

[0038] The continuous casting process is used to feed molten steel that has undergone LF refining into a continuous casting machine and continuously cast it into a steel billet. The steel billet is a large square billet with a cross-sectional length × width of 240mm × 240mm. The continuous casting process adopts full-process protective casting, with a superheat of 27℃ and a casting speed of 1.15m / s. Double electric stirring is used to improve the homogeneity of the billet. The electric stirring current in the crystallizer is 300A and the frequency is 2Hz; the electric stirring current at the end is 200A and the frequency is 8Hz.

[0039] The billet finishing process is used to perform shot blasting and magnetic particle testing on the billet after continuous casting to ensure good surface quality of the billet and prevent billet defects from being inherited to the surface of the wire rod.

[0040] The rolling process involves placing the finished steel billet in a heating furnace, controlling the heating temperature at 1150℃, and heating it to achieve rollable plasticity. After heating, the billet undergoes high-pressure water descaling to remove surface oxide scale, with the water descaling pressure controlled at ≥22MPa. The descaled billet then enters a continuous rolling mill, with the initial rolling temperature controlled at 1060℃. The rolling coefficient and deformation are reasonably allocated, and 26 passes are used to obtain a wire rod with a diameter of 12mm and a total compression ratio of 510. In the roughing stage, an 850mm ultra-heavy-duty rolling mill is used to increase the roughing compression ratio, and the entry sizing temperature is controlled at 930℃.

[0041] The spinning process is used to feed the rolled wire into the spinning machine and spin it into coils at high temperature. The spinning temperature is controlled at 950°C. After spinning, the coils are fully covered by the Stellmore cooling line insulation cover. The coils are conveyed through the insulation cover by roller conveyors at a speed of 0.6 m / s, so that the coils pass through 5 sections of the Stellmore cooling line within 1 minute. The coil cooling rate is >1.5°C / s. The coils are then conveyed into the coiling process under insulation.

[0042] The hot coiling process is used to coil the wire rod into coils, and the coiling temperature is controlled at 850°C. After coiling, the coils are quickly fed into the heat preservation tunnel furnace via roller conveyor within 1 minute.

[0043] The slow cooling process in the insulated tunnel furnace is used to feed the coil into the furnace, promote the phase transformation of ferrite and pearlite, control the furnace inlet temperature at 770℃, the furnace atmosphere temperature at 400~600℃, the residence time of the coil in the furnace at 90 minutes, and the cooling rate ≤0.1℃ / s in the 700~600℃ temperature range to promote uniform precipitation of ferrite. The outlet temperature is 450℃, utilizing the residual heat of the coil in the furnace for slow cooling, achieving online softening of the microstructure. Finally, after inspection, packaging, weighing, marking, and warehousing, the finished cold-heading steel wire rod is obtained, and its metallographic structure is shown in the figure. Figure 1 As shown.

[0044] Example 2:

[0045] Another preferred embodiment of the production method of the 8.8 grade boron-containing cold heading steel wire rod of the present invention, wherein the chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.20%, Si: 0.03%, Mn: 0.80%, P: 0.012%, S: 0.003%, Cr: 0.03%, Ni: 0.03%, Cu: 0.02%, Mo: 0.02%, Al: 0.04%, B: 0.0030%, Ti: 0.05%, N: 0.0040%, O: 0.0010%, with the remainder being Fe and unavoidable impurities; the production method follows the process flow of converter smelting → LF refining → large billet continuous casting → billet finishing → heating → rolling → wire drawing → hot coiling → slow cooling in a tunnel furnace with heat preservation → inspection → packaging → weighing → marking → warehousing, specifically:

[0046] The converter smelting process is used to smelt steelmaking raw materials into molten steel. A 200-ton converter is used, and the scrap steel ratio is controlled at 15%. The converter tapping adopts a double-plate sliding plate to effectively control the amount of slag and improve the purity of the molten steel. During tapping, deoxidizer → carbon raiser → alloy → top slag are added in sequence. The top slag is made of quicklime and calcium aluminate. The carbon content at the end of the converter is 0.15%. Aluminum wire is prohibited from being fed into the argon blowing station.

[0047] The LF refining process is used to perform LF refining treatment on molten steel that has passed through the argon blowing station. The fluidity of the slag is adjusted by adding lime and fluorite, and the white slag is maintained for ≥18 minutes. After the white slag, titanium-iron alloy is added, and calcium wire is fed in for calcium treatment. After calcium treatment, boron-iron wire is added, and soft blowing of argon gas is started at the same time for 18 minutes.

[0048] The large billet continuous casting process is used to feed molten steel that has undergone LF refining into a continuous casting machine and continuously cast it into a billet. The billet is a large square billet with a cross-sectional length × width of 240mm × 240mm. The continuous casting process adopts full-process protective casting, with a superheat of 17℃ and a casting speed of 1.15m / s. Double electric stirring is used to improve the homogeneity of the billet. The electric stirring current in the crystallizer is 400A and the frequency is 5Hz; the electric stirring current at the end is 300A and the frequency is 10Hz.

[0049] The billet finishing process is used to perform shot blasting and magnetic particle testing on the billet after continuous casting to ensure good surface quality of the billet and prevent billet defects from being inherited to the surface of the wire rod.

[0050] The rolling process involves placing the finished steel billet in a heating furnace, controlling the heating temperature at 1100℃, and heating it to achieve rollable plasticity. After heating, the billet undergoes high-pressure water descaling to remove surface oxide scale, with the water descaling pressure controlled at ≥22MPa. The descaled billet then enters a continuous rolling mill, where the initial rolling temperature is controlled at 1000℃. The rolling coefficient and deformation are rationally allocated, and 22 passes are used to obtain a wire rod with a diameter of 18mm and a total compression ratio of 226. In the roughing stage, an 850mm ultra-heavy-duty rolling mill is used to increase the roughing compression ratio, and the sizing temperature is controlled at 880℃.

[0051] The spinning process is used to feed the rolled wire into the spinning machine and spin it into coils at high temperature. The spinning temperature is controlled at 900°C. After spinning, the coils are fully covered by the Steilmo cooling line insulation cover. The coils are conveyed through the insulation cover by roller conveyors at a speed of 0.6 m / s, so that the coils can quickly pass through 5 sections of the Steilmo cooling line within 1 minute. The coil cooling rate is >1.5°C / s. The coils are then conveyed into the coiling process under insulation.

[0052] The hot coiling process is used to coil the wire rod into coils, and the coiling temperature is controlled at 800°C. After coiling, the coils are quickly fed into the heat preservation tunnel furnace via roller conveyor within 1 minute.

[0053] The slow cooling process in the insulated tunnel furnace is used to feed the coil into the furnace, promote the phase transformation of ferrite and pearlite, control the furnace inlet temperature to 720℃, the furnace atmosphere temperature to 400~600℃, the residence time of the coil in the furnace to 60min, and the cooling rate to ≤0.1℃ / s in the 700~600℃ temperature range to promote uniform precipitation of ferrite. The outlet temperature is 480℃, and the residual heat of the coil is used to achieve slow cooling in the furnace, realizing online softening of the microstructure. Finally, after inspection, packaging, weighing, marking, and warehousing, the finished cold heading steel wire rod is obtained.

[0054] Comparative Example 1:

[0055] A method for producing boron-containing cold heading steel wire rod differs from Example 1 in that the chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.20%, Si: 0.06%, Mn: 0.82%, P: 0.015%, S: 0.010%, Cr: 0.16%, Ni: 0.10%, Cu: 0.10%, Mo: 0.10%, Al: 0.04%, B: 0.0020%, Ti: 0.04%, N: 0.0060%, O: 0.0015%, with the remainder being Fe and unavoidable impurities. The product is manufactured according to the following process flow: converter smelting → LF refining → small billet continuous casting → billet finishing → heating → rolling → wire drawing → Steyrmore cooling line cooling. Specifically: the billet in the small billet continuous casting process is a small square billet with a cross-sectional length × width of 160mm × 160mm; the sizing temperature in the rolling process is 900℃, the total compression ratio is 226; the wire drawing temperature in the wire drawing process is 900℃; the roller speed of the Steyrmore cooling line is 0.14m / s; the fans are all off; the insulation cover is fully covered; and the wire cooling rate is 0.2~0.3℃ / s. Its metallographic structure is shown in the figure below. Figure 2 As shown.

[0056] Comparative Example 2:

[0057] A method for producing boron-containing cold heading steel wire rod differs from Example 2 in that the production method follows a process flow of converter smelting → LF refining → small billet continuous casting → billet finishing → heating → rolling → wire drawing → Steyrmore cooling line cooling. Specifically, the billet in the small billet continuous casting process is a small billet with a cross-sectional length × width of 160mm × 160mm; the total compression ratio is 100; the fans in the Steyrmore cooling line are all turned off, the insulation cover is fully covered, and the wire rod cooling rate is 0.2~0.3℃ / s.

[0058] The tensile strength and reduction of area of ​​the cold-heading steel wire rods obtained in the examples and comparative examples were tested according to GB / T228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature"; the hardness was tested according to GB / T230.1 "Metallic materials - Rockwell hardness testing - Part 1: Test method"; the grain size was tested according to GB / T6394 "Metallic materials - Determination of average grain size"; the microstructure was tested according to GB / T13298 "Metallic materials - Microstructure testing method"; and a 1 / 3 cold upsetting experiment was conducted according to YB / T 5293 "Metallic materials - Upsetting test method". The test results are shown in Table 1 below:

[0059] Table 1. Test results of technical properties of different cold heading steel wire rods

[0060]

[0061] The comparison results between Example 1 and Comparative Example 1 show that, compared to wire rods with higher C, Si, Mn, and Cr content, which experience rapid cooling via the Stellmore cooling line and poor slow cooling effect, leading to increased hardness and decreased plasticity, this invention, while ensuring the basic performance characteristics of fasteners with large deformations such as nuts (e.g., hardenability), achieves higher ferrite content through optimized chemical composition design of boron-containing cold heading steel wire rods. This is achieved by combining hot coiling after high-temperature wire drawing with slow cooling in a heat-insulating tunnel furnace, resulting in a lower cooling rate compared to the traditional Stellmore cooling line. The ferrite content reaches ≥70%, achieving online softening of the microstructure. After cooling, the tensile strength and hardness are lower, and the reduction of area is higher. As can be seen from the results of Examples 1 and 2, the tensile strength of cold heading steel wire rod can reach ≤450MPa, the reduction of area can reach ≥58%, and the hardness can reach ≤75HRB. Combining the comparison results of Example 2 and Comparative Example 2, it can be seen that large billet and large compression ratio can be used for direct rolling, taking into account both plasticity indicators and production efficiency. This is beneficial for reducing the risk of cracking and mold wear when used downstream without annealing.

[0062] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing a 8.8 grade boron containing cold heading steel wire rod, characterized in that, The chemical element composition of the cold heading steel wire rod includes, in terms of percentage by weight, C: 0.18%-0.20%, Si: ≤0.05%, Mn: 0.60%-0.80%, P: ≤0.015%, S: ≤0.010%, Cr: ≤0.10%, Ni: ≤0.10%, Cu: ≤0.10%, Mo: ≤0.10%, Al: ≥0.02%, B: 0.0015%-0.0030%, Ti: 0.02%-0.05%, N: ≤0.0060%, O: ≤0.0015%, and the rest is Fe and inevitable impurities; and the production method is produced according to the process of rolling, wire drawing, hot collecting and slow cooling in a heat preservation tunnel furnace. In the wire drawing, the wire drawing temperature is controlled to be 900-950℃, and the wire rod is conveyed by a roller after the wire drawing, passes through a heat preservation cover, the roller speed is ≥0.6m / s, the cooling speed of the wire rod is >1.5℃ / s, the wire rod is conveyed into the collecting process within 1min under the heat preservation, and the coil is put into the heat preservation tunnel furnace within 1min after the collecting. In the hot collecting, the collecting temperature is controlled to be 800-850℃, and the wire rod is collected as a coil. In the slow cooling in the heat preservation tunnel furnace, the furnace temperature is controlled to be 720-770℃, the atmosphere temperature in the furnace is 400-600℃, the residence time of the coil in the heat preservation tunnel furnace is 60-90min, the cooling rate in the temperature range of 700-600℃ is ≤0.1℃ / s, and the discharge temperature is >400℃ and ≤500℃.

2. A process for producing a 8.8 grade boron-containing cold- heading steel rod as claimed in claim 1, characterized in that, In the rolling, the billet with a section ≥240mm*240mm is adopted, the heating temperature is controlled to be 1100-1150℃, the starting rolling temperature is 1000-1060℃, the water descaling pressure is ≥22MPa, the total compression ratio is 226-510 after 22-26 rolling passes, and the finishing reducing temperature is 880-930℃.

3. The process for production of 8.8 grade boron containing cold heading steel wire rod as claimed in claim 2 wherein, Before the rolling, the steel billet is finished by adopting shot blasting and magnetic powder detection.

4. A process for producing a 8.8 grade boron-containing cold- heading steel rod as claimed in claim 1, characterized in that, The continuous casting process before the rolling is included, the whole process protection casting and double electric stirring are adopted in the continuous casting process, the superheat is controlled to be 17-27℃, the electric stirring current of the crystallizer is 300-400A, and the frequency is 2-5Hz; the electric stirring current of the tail end is 200-300A, and the frequency is 8-10Hz.

5. The process for producing a 8.8 grade boron-containing cold- heading steel rod as claimed in claim 4, characterized in that, The LF refining process before the continuous casting process is included, the white slag is kept for ≥18min in the LF refining process, the ferro-titanium alloy is added after the white slag, the calcium wire is fed for calcium treatment, the ferro-boron wire is added after the calcium treatment, and the soft argon blowing is started at the same time, and the soft blowing time is 18-25min.

6. The process for producing a 8.8 grade boron-containing cold- heading steel rod as claimed in claim 5, characterized in that, The converter smelting process before the LF refining process is included, the converter with a capacity of 150-200 tons is adopted in the converter smelting process, the scrap steel ratio is controlled to be 15%-25%, the deoxidizer, the carbon additive, the alloy and the top slag are sequentially added when the molten steel is discharged, the top slag adopts the quicklime and the calcium aluminate, the carbon content at the end of the converter is 0.08%-0.15%, and the aluminum wire is prohibited to be fed at the argon blowing station.

7. A cold heading steel rod of grade 8.8 containing boron, characterized in that, The cold heading steel wire rod is obtained by the production method of the 8.8-grade boron-containing cold heading steel wire rod according to any one of claims 1-6.

8. The 8.8 grade boron-containing cold- heading steel wire rod according to claim 7, characterized in that The diameter of the cold heading steel wire rod is 12-18 mm, the microstructure includes ferrite and pearlite, the tensile strength is less than or equal to 450 MPa, the reduction of area is greater than or equal to 58%, and the hardness is less than or equal to 75 HRB.

Images