CrMo cold heading steel wire rod for 10.9-grade fastener and production method of CrMo cold heading steel wire rod

Abstract

The invention relates to a CrMo cold heading steel wire rod for a 10.9-grade fastener and a production method thereof, the production method comprises the following steps: producing according to the flow of rolling, spinning, heat collecting and coiling and heat preservation tunnel furnace slow cooling, rolling by adopting a steel billet with the section of 240 mm * 240 mm, controlling the total compression ratio to be 183-510, controlling the spinning temperature to be 910-960 DEG C, conveying the steel wire rod under heat preservation after spinning to enter a coiling process, and collecting the coiling temperature to be 810-860 DEG C, the charging temperature is 750-790 DEG C, the retention time of a coil in a heat preservation tunnel furnace is 60-90 min, the cooling rate in the temperature interval of 750-600 DEG C is smaller than or equal to 0.1 DEG C / s, the discharging temperature is smaller than or equal to 500 DEG C, the tensile strength of a medium-large-specification wire rod is smaller than or equal to 780 MPa, the hardness is smaller than or equal to 92 HRB, the percentage reduction of area is larger than or equal to 40%, the one-ball two-drawing process of a user is met, rough drawing production with the large reducing rate of 20-30% is achieved, and the annealing time is shortened.

Description

Technical Field

[0001] This invention belongs to the field of cold heading steel wire rod technology, specifically relating to a CrMo cold heading steel wire rod for 10.9 grade fasteners and its production method. Background Technology

[0002] Most cold heading steel materials for 10.9 grade fasteners are produced using CrMo alloy cold heading steel such as SCM435. These materials offer advantages such as better adaptability to quenching and tempering after cold heading and excellent fatigue performance. However, due to the high content of alloying elements in the steel, Cr and Mo significantly improve the hardenability of the wire rod. After wire drawing, martensitic or bainitic phase transformations easily occur on the Steyrmo line, resulting in increased wire rod strength and hardness. This makes it unsuitable for downstream users' direct drawing production. Furthermore, due to high deformation resistance and rapid die wear, the direct drawing reduction rate is forced to decrease. The decomposition of hard and brittle phases requires higher temperatures and longer times, leading to longer annealing cycles, increased energy consumption, and higher costs. Therefore, it is necessary to develop an online softening CrMo cold heading steel wire rod for 10.9 grade fasteners and its production method. This would allow users to omit the pre-drawing softening and annealing process while achieving rough drawing production with a large reduction rate, while simultaneously reducing subsequent annealing time, energy consumption, and costs.

[0003] To reduce the strength of CrMo alloy cold heading steel wire rod, traditional methods often employ low-temperature rolling and reduced speeds on the Steyrmo line rollers. For example, patent CN119464931A discloses a method for producing high-strength alloy cold heading steel wire rod of grade 10.9 and above without annealing. After low-temperature rolling and wire drawing, the wire is cooled in an insulating hood to obtain a ferrite + pearlite microstructure. However, this method still has the following drawbacks:

[0004] Due to the characteristics of the Steilmo line's uncoiled cooling, the wire rod, after being spun out, falls spirally onto the roller conveyor and is cooled naturally by air. The inherent shape of its uncoiled stacking results in overlapping points on the surface of the wire rod, i.e., lap points. Heat dissipation at these lap points relies on the wire rod's own heat conduction and the roller conveyor's heat conduction, resulting in slow heat dissipation. The non-lap exposed areas have high heat dissipation efficiency. Even with insulation, the difference between lap and non-lap points cannot be avoided, and the cooling rates of the two differ significantly. Influenced by the steel composition, a cooling rate slightly exceeding the critical value triggers the bainitic phase transformation, while an excessively high cooling rate generates martensite. The generation of martensite or bainite cannot be completely avoided, resulting in significant differences in the uniformity of wire rod strength and hardness. The insulation capacity and length limitations of the Steilmo line also lead to insufficient plasticity in the final wire rod. Excessively reducing the speed of the Steilmo line roller conveyor will affect production efficiency.

[0005] In addition, low-temperature rolling and low-temperature wire drawing result in greater deformation resistance of CrMo steel coils, which can cause significant damage to equipment and affect production efficiency. The risk of production interruption due to poor wire drawing is also increased. This is especially true for medium and large-sized wire rods, which have large cross-sectional dimensions, high heat capacity, and significant heat generation during deformation. Unlike small-sized wire rods, the final rolling temperature cannot be controlled by low-temperature rolling, further exceeding equipment load limits and making low-temperature rolling difficult to achieve. Summary of the Invention

[0006] The present invention aims to solve at least one of the above-mentioned technical problems. The present invention provides a CrMo cold heading steel wire rod for 10.9 grade fasteners and its production method, which can realize online softening of medium and large-sized wire rods, take into account production efficiency, and while meeting the user's one-ball two-drawing process, can achieve rough drawing production with a large reduction rate of 20%~30%, while reducing the user's subsequent annealing time, reducing energy consumption and cost.

[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 CrMo cold heading steel wire rod for 10.9 grade fasteners, wherein the chemical element composition of the cold heading steel wire rod, by weight percentage, comprises: C: 0.33%~0.35%, Si: 0.15%~0.30%, Mn: 0.60%~0.70%, P: ≤0.015%, S: ≤0.010%, Cr: 0.90%~1.10%, Ni: ≤0.20%, Cu: ≤0.20%, Mo: 0.15%~0.25%, Al: 0.01%~0.03%, 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 heat-insulating tunnel furnace.

[0009] During the rolling process, a steel billet with a cross-section of 240mm×240mm is used, and the total compression ratio is 183~510;

[0010] During the spinning process, the spinning temperature is controlled at 910~960℃. After spinning, the wire is conveyed into the winding process under heat preservation.

[0011] During the hot winding process, the winding collection temperature is controlled at 810~860℃, and the wire rod winding is a coil.

[0012] During slow cooling in the insulated tunnel furnace, the furnace inlet temperature is controlled at 750~790℃, the residence time of the coil in the insulated tunnel furnace is 60~90min, the cooling rate in the temperature range of 750~600℃ is ≤0.1℃ / s, and the furnace outlet temperature is ≤500℃.

[0013] The aforementioned cold heading steel wire rod adopts a CrMo composition system, narrowing the range of carbon and chromium content to avoid excessive carbide segregation. Appropriate Al elements are added to prevent coarse, hard, and brittle phase inclusions and suppress grain coarsening during rolling. Furthermore, the higher wire drawing temperature reduces the limitations on rolling temperature. Using large-section billets and a high compression ratio during rolling effectively breaks up dendritic segregation and porosity in the as-cast structure, reducing compositional segregation and thus mitigating the risk of abnormal microstructure caused by alloy segregation. The high compression ratio further promotes multiple dynamic recrystallizations of austenite during rolling to refine the grains, laying the foundation for a fine-grained structure after phase transformation and improving matrix plasticity. Simultaneously, the higher wire drawing temperature ensures the wire rod is entirely austenitic during wire drawing, without ferrite pre-precipitation, avoiding microstructure inhomogeneity or poor wire drawing caused by excessively low wire drawing temperatures and premature phase transformation.

[0014] After high-temperature spinning, the wire rod is rapidly conveyed under heat preservation rather than air cooling. This ensures a stable temperature before entering the coiling stage and prevents rapid surface cooling. Compared to the continuous cooling on the Stellmore line after low-temperature spinning, which introduces the risk of brittle structures and increases strength and hardness, hot coiling at high temperature eliminates the heat dissipation difference between overlapping and non-overlapping points, preventing the formation of brittle structures due to premature entry into the bainitic transformation zone caused by localized low temperatures in traditional loose coils. Furthermore, the compact coils offer better overall heat preservation, reducing the initial temperature difference between the surface and core. Residual heat allows for rapid entry into a more stable heat preservation environment, reducing production energy consumption and preparing for slow cooling in the tunnel furnace. Upon entering the tunnel furnace, the temperature is more uniform and higher than that of the Stellmore line under heat preservation and cooling. The sensitive zone of pearlite phase transformation has a lower cooling rate, lower than the critical cooling rate of bainite in CrMo steel, providing sufficient time for carbon diffusion. This promotes the uniform transformation of austenite into ferrite and pearlite, preventing residual austenite from continuing to form low-temperature brittle structures during subsequent cooling. It further eliminates the risk of abnormal structures such as bainite and martensite. On the other hand, Cr and Mo elements reduce the carbon diffusion rate, and the extended residence time of the coil in the holding tunnel furnace promotes the full precipitation of cementite, which is uniformly distributed in the ferrite matrix. The migration of carbon atoms in the ferrite promotes grain homogenization, eliminates internal stress, and achieves online softening of the structure. This reduces the tensile strength and hardness of the wire rod and improves its uniformity. At the same time, it avoids the impact of excessive residence time and excessively low furnace exit temperature on production efficiency.

[0015] During the rolling process, a higher heating temperature can soften the steel billet, reduce deformation resistance, and decrease dendritic segregation of Cr and Mo, facilitating high compression ratio and medium-to-large-sized rolling. Simultaneously, it avoids excessively high temperatures that could lead to austenite grain coarsening. A higher initial rolling temperature can be used in conjunction with high compression ratio rolling of large-section steel billets, breaking up coarse-grained structures and preventing exceeding the mill load. Appropriate entry / reduction sizing temperatures can ensure dimensional accuracy and ensure proper connection to the wire drawing temperature, preventing premature ferrite precipitation due to excessively low entry / reduction sizing temperatures. In the preferred technical solution, during rolling, the heating temperature is controlled at 1120~1170℃, the initial rolling temperature at 1020~1080℃, and the entry / reduction sizing temperature at 890~950℃.

[0016] 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.

[0017] During the rolling process, appropriate rolling passes can be selected to gradually refine the grains and avoid surface or internal defects caused by large deformation in a single pass. In the preferred technical solution, the rolling process involves 22 to 26 passes. Preferably, an 850mm ultra-heavy-duty rolling mill is used in the roughing stage to increase the roughing compression ratio. The single-pass compression ratio in the roughing is 1.25 to 1.30, which provides sufficient rolling force while shortening the load of subsequent passes.

[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] To further control the rate of temperature drop of the wire rod, the connection between the wire drawing temperature and the coiling and collecting temperature, an insulation cover can be used to suppress rapid heat dissipation from the wire rod surface and reduce the temperature difference between the surface and the inside. At the same time, rapid conveying can suppress the coarsening of austenite grains and improve production continuity. In the preferred technical solution, after wire drawing, the wire rod is conveyed by a roller conveyor through the insulation cover. The roller conveyor speed is ≥0.6m / s, and the wire rod enters the coiling process within 1 minute.

[0020] To further control the rate of temperature drop in the coil, ensure proper connection between the coil collection temperature and the furnace entry temperature, facilitate rapid conveying to prevent premature phase transformation and microstructure stratification, and avoid excessively low surface cooling leading to the precipitation of abnormal microstructure hard spots, the furnace atmosphere temperature and exit temperature can be further controlled. This further reduces the temperature difference between the surface and interior of the coil, controls the cooling rate in the high-temperature range, ensures complete phase transformation and microstructure softening, and balances production efficiency. In the preferred technical solution, the coil enters the heat-insulating tunnel furnace within 1 minute after collection. During slow cooling in the heat-insulating tunnel furnace, the furnace atmosphere temperature is controlled at 400~600℃, and the exit temperature is >400℃.

[0021] In the preferred technical solution, a continuous casting process before rolling is included. This continuous casting process adopts full-process protective casting and double electric stirring: the superheat is controlled at 20~30℃, and the cleanliness of the molten steel is ensured through full-process protective casting, avoiding excessive superheat that leads to the development of columnar crystals and central porosity and shrinkage defects; the crystallizer electric stirring current is 300~400A and the frequency is 2~5Hz. The crystallizer electric stirring acts on the initial solidification stage, which can refine the surface grains, homogenize the composition and temperature, and promote equiaxed crystal nucleation by using low-frequency stirring; the end electric stirring current is 200~250A and the frequency is 6~8Hz. The end electric stirring acts on the end of solidification, and the low-current high-frequency stirring eliminates the porosity in the core and further reduces compositional segregation.

[0022] To further eliminate central porosity and shrinkage cavities, light reduction and appropriate total reduction can be used to reduce the central segregation of Cr, Mo, and C, providing a basis for high compression ratio rolling of large-section steel billets. In the preferred technical solution, the total reduction of light reduction in the continuous casting process is 15~20mm.

[0023] In the preferred technical solution, an LF refining process is included before the continuous casting process. The LF refining process controls the white slag holding time to be ≥20 min. Preferably, quicklime with a dosage of ≤200 kg and fluorite with a dosage of ≤150 kg are used to adjust the slag fluidity, so as to deeply desulfurize and deoxidize, avoid the oxidation of Mo alloy and the formation of hard and brittle inclusions, and improve the purity of molten steel. After the white slag, ferromolybdenum alloy is added to precisely control the Mo content and avoid oxidation loss. Calcium wire is fed in for calcium treatment, with a calcium wire feeding amount of ≤100 m, to reduce inclusions, avoid nodule formation at the continuous casting nozzle, and avoid excessive calcium feeding of CaS inclusions. At the same time, soft blowing of argon gas is started for 15~25 min to homogenize the molten steel, avoid Mo segregation, ensure consistent hardenability, and promote the final flotation of inclusions.

[0024] In the preferred technical solution, a converter smelting process is included before the LF refining process. This converter smelting process uses a 150-200 ton converter, with the scrap steel ratio controlled at 15%-25%. The converter tapping process employs a double-plate sliding plate to effectively control the slag amount and improve the purity of the molten steel. During tapping, deoxidizer, recarburizer, alloy, and top slag are added sequentially. The deoxidizer is selected from aluminum blocks, which can adsorb inclusions in the steel. A low-nitrogen recarburizer is selected to avoid excessive nitrogen content. The alloy is selected from silicon-manganese alloy. Gold, high-carbon ferromanganese, and high-carbon ferrochrome can balance deoxidation and alloying effects and optimize yield. The top slag uses quicklime and calcium aluminate, which can force dephosphorization. The carbon content at the converter endpoint is 0.08%~0.17%. The converter endpoint carbon control can take into account both dephosphorization and refining efficiency. The converter endpoint P≤0.011%. The converter tapping temperature ≥1600℃. The high tapping temperature design can compensate for the temperature drop during the refining process and ensure smooth continuous casting. Aluminum wire feeding is prohibited at the argon blowing station to avoid the deterioration of inclusion morphology.

[0025] The second aspect of the present invention is to provide a 10.9 grade fastener CrMo cold heading steel wire rod, wherein the cold heading steel wire rod is produced by any of the above-described production methods for 10.9 grade fastener CrMo cold heading steel wire rods, the microstructure being uniform ferrite and pearlite, the volume percentage of pearlite being ≤65%, the total decarburized layer depth being ≤1%D diameter, eliminating abnormal structures such as bainite and martensite, and further reducing the tensile strength and hardness of the wire rod through online softening, while improving its uniformity, achieving 1 / 3 cold upsetting qualification, and surface defect depth being ≤0.05mm, which can meet the user's requirement of a 20%~30% reduction in surface area during rough drawing without annealing, while also allowing for further shortening of the user's annealing time, reducing the user's energy consumption and manufacturing costs.

[0026] In the preferred technical solution, the diameter of the cold heading steel wire rod is 12~20mm, the tensile strength is ≤780MPa, the difference in tensile strength between the same ring is ≤100MPa, the hardness is ≤92HRB, and the reduction of area is ≥40%. The cold heading steel wire rod has low tensile strength and hardness, and high reduction of area. It can be used for the direct drawing production of 10.9 grade fasteners to avoid the risk of cracking. At the same time, it can improve the direct drawing reduction rate, reduce the cold heading deformation resistance and the subsequent annealing cycle.

[0027] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0028] This invention optimizes the composition of CrMo cold heading steel wire rod, combining large-section billets with high compression ratio rolling to improve matrix plasticity and uniformity. Combined with rapid hot-rolling collection of the coil, the residual heat of the coil can be utilized in a heat-insulating tunnel furnace for slow cooling, promoting the uniform transformation of ferrite and pearlite and eliminating the risk of abnormal structures such as bainite and martensite. Furthermore, it overcomes the limitations of low-temperature rolling, enabling online softening of medium and large-diameter wire rods, further reducing tensile strength and hardness while improving uniformity. The resulting wire rods achieve tensile strength ≤780MPa, tensile strength difference within the same ring ≤100MPa, hardness ≤92HRB, reduction of area ≥40%, and 1 / 3 cold forging qualification. While meeting the user's one-ball two-drawing process, it can achieve rough drawing production with a large reduction rate of 20%~30%, while reducing the user's subsequent annealing time from 32h to 24h, thus reducing energy consumption and costs. It has good industrial adaptability and application prospects. Attached Figure Description

[0029] 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:

[0030] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention.

[0031] Figure 2This is a metallographic diagram of Embodiment 2 of the present invention.

[0032] Figure 3 This is a metallographic diagram of Comparative Example 1 of the present invention.

[0033] Figure 4 This is a metallographic diagram of Comparative Example 2 of the present invention. Detailed Implementation

[0034] The present invention will be further described in detail below through specific preferred embodiments. However, the present invention is not limited to the following embodiments. It should be noted that, unless otherwise specified, the raw materials and alloy materials involved in the present invention, such as low-nitrogen carbon raisers, silicon-manganese alloys, high-carbon ferromanganese, and high-carbon ferrochrome, are all purchased through commercial channels.

[0035] Example 1:

[0036] A preferred embodiment of the production method of CrMo cold heading steel wire rod for 10.9 grade fasteners according to the present invention, wherein the chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.33%, Si: 0.30%, Mn: 0.60%, P: 0.015%, S: 0.010%, Cr: 0.90%, Ni: 0.10%, Cu: 0.20%, Mo: 0.15%, Al: 0.01%, 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:

[0037] 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-slide plate to effectively control the amount of slag and improve the purity of the molten steel. During tapping, deoxidizer → recarburizer → alloy → top slag are added in sequence. The deoxidizer is aluminum block, the recarburizer is a low-nitrogen recarburizer, and the alloy is silicon-manganese alloy, high-carbon ferromanganese, and high-carbon ferrochrome. The top slag is quicklime + calcium aluminate. The carbon content at the converter endpoint is 0.08%, the P content at the converter endpoint is ≤0.011%, the converter tapping temperature is ≥1600℃, and aluminum wire is prohibited from being fed at the argon blowing station.

[0038] 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 ≥20 minutes. After the white slag, ferromolybdenum alloy is added, and calcium wire is fed in for calcium treatment. The amount of calcium wire fed in is ≤100m. At the same time, soft blowing of argon gas is started, and the soft blowing time is 25 minutes.

[0039] 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 30℃ 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 6Hz. The total reduction under light pressure is 20mm.

[0040] 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.

[0041] The rolling process involves placing the finished steel billet in a heating furnace, controlling the heating temperature at 1170℃, 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 1080℃. The rolling coefficient and deformation are rationally allocated, and 26 passes are used to obtain 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, with a single-pass compression ratio of 1.25~1.30. The sizing temperature is controlled at 890℃.

[0042] 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 910°C. After spinning, the coils are fully covered by the Stellmore cooling line insulation cover. The coils are conveyed by roller conveyors through the insulation cover at a speed of 0.6 m / s, so that the coils can quickly pass through 5 sections of the Stellmore cooling line within 1 minute. The coils are then conveyed into the coiling process under insulation.

[0043] The hot coiling process is used to coil the wire rod into coils, and the coiling temperature is controlled at 810°C. After coiling, the coils are quickly fed into the heat preservation tunnel furnace through the roller conveyor within 1 minute.

[0044] The slow cooling process in the insulated tunnel furnace is used to feed the coil into the furnace, controlling the furnace inlet temperature at 750℃, the furnace atmosphere temperature at 400~600℃, the residence time of the coil in the furnace at 90 minutes, the cooling rate within the 750~600℃ temperature range ≤0.1℃ / s, and the outlet temperature at 450℃. The residual heat of the coil is used to achieve slow cooling in the furnace, resulting in 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, the microstructure is a uniform mixture of ferrite and pearlite.

[0045] Example 2:

[0046] A preferred embodiment of the production method of CrMo cold heading steel wire rod for 10.9 grade fasteners according to the present invention, wherein the chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.35%, Si: 0.15%, Mn: 0.70%, P: 0.010%, S: 0.003%, Cr: 1.10%, Ni: 0.20%, Cu: 0.10%, Mo: 0.25%, Al: 0.03%, 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:

[0047] 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-slide plate to effectively control the amount of slag and improve the purity of the molten steel. During tapping, deoxidizer → carburizer → alloy → top slag are added in sequence. The deoxidizer is aluminum block, the carburizer is a low-nitrogen carburizer, and the alloy is silicon-manganese alloy, high-carbon ferromanganese, and high-carbon ferrochrome. The top slag is quicklime + calcium aluminate. The carbon content at the converter endpoint is 0.17%, the P content at the converter endpoint is ≤0.011%, the converter tapping temperature is ≥1600℃, and aluminum wire is prohibited from being fed at the argon blowing station.

[0048] 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. The white slag is maintained for ≥20 minutes. After the white slag, ferromolybdenum alloy is added, and calcium wire is fed in for calcium treatment. The amount of calcium wire fed in is ≤100m. At the same time, soft blowing of argon gas is started, and the soft blowing time is 15 minutes.

[0049] 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 20℃ 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 250A and the frequency is 8Hz. The total reduction under light pressure is 15mm.

[0050] 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.

[0051] The rolling process involves placing the finished steel billet in a heating furnace, controlling the heating temperature at 1120℃, 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 1020℃. The rolling coefficient and deformation are reasonably allocated, and a wire rod with a diameter of 20mm is obtained through 26 rolling passes, with a total compression ratio of 183. The roughing stage uses an 850mm ultra-heavy-duty rolling mill to increase the roughing compression ratio, with a single-pass compression ratio of 1.25~1.30, and the sizing temperature is controlled at 950℃.

[0052] 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 960°C. After spinning, the coils are fully covered by the Stellmore cooling line insulation cover. The coils are conveyed by roller conveyors through the insulation cover at a speed of 0.6 m / s, so that the coils can quickly pass through 5 sections of the Stellmore cooling line within 1 minute. The coils are then conveyed into the coiling process under insulation.

[0053] The hot coiling process is used to coil the wire rod into coils, and the coiling temperature is controlled at 860°C. After coiling, the coils are quickly fed into the heat preservation tunnel furnace via roller conveyor within 1 minute.

[0054] The slow cooling process in the insulated tunnel furnace is used to feed the coil into the furnace, controlling the furnace inlet temperature at 790℃, the furnace atmosphere temperature at 400~600℃, the residence time of the coil in the furnace at 60 minutes, the cooling rate ≤0.1℃ / s within the 750~600℃ temperature range, and the outlet temperature at 480℃. The residual heat of the coil is used for slow cooling in the furnace, 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 2 As shown, the microstructure is a uniform mixture of ferrite and pearlite.

[0055] Comparative Example 1:

[0056] A method for producing CrMo 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: 0.35%, Si: 0.25%, Mn: 0.81%, P: 0.016%, S: 0.012%, Cr: 1.00%, Ni: 0.10%, Cu: 0.10%, Mo: 0.22%, Al: 0.02%, with the remainder being Fe and unavoidable impurities; the production method follows the sequence of converter smelting → LF refining → small billet continuous casting → The production process involves billet finishing → heating → rolling → wire rod casting → cooling on the Steyrmo cooling line. Specifically: the billet in the billet continuous casting process is a small square billet with a cross-sectional length × width of 160mm × 160mm; in the rolling process, the sizing temperature is 920℃, the total compression ratio is 193, and a wire rod with a diameter of 13mm is obtained; in the wire rod casting process, the casting temperature is 920℃, the Steyrmo cooling line roller speed is 0.14m / s, the fans are all off, the insulation cover is fully covered, and the wire rod cooling rate is 0.2~0.3℃ / s. Its metallographic structure is shown in the figure. Figure 3 As shown, the microstructure consists of bainite, a small amount of ferrite, and a small amount of pearlite.

[0057] Comparative Example 2:

[0058] A method for producing CrMo cold heading steel wire rod differs from Example 2 in that its production method follows a process flow of converter smelting → LF refining → small billet continuous casting → billet finishing → heating → rolling → wire drawing → Steyrmo cooling line cooling. Specifically, the billet in the billet continuous casting process is a small billet with a cross-sectional length × width of 160mm × 160mm; the rolled wire rod has a diameter of 20mm and a total compression ratio of 82. The Steyrmo cooling line has all fans turned off and the insulation cover fully installed. The wire rod cooling rate is 0.2~0.3℃ / s. Its metallographic structure is shown in the figure below. Figure 4 As shown, the microstructure consists of bainite, a small amount of ferrite, and a small amount of pearlite.

[0059] 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 microstructure was tested according to GB / T13298 "Metallic materials - Microstructure testing method"; and a 1 / 3 cold upsetting test was performed according to YB / T5293 "Metallic materials - Upsetting test method". The test results are shown in Table 1 below:

[0060] Table 1. Test results of technical properties of different cold heading steel wire rods

[0061]

[0062] The comparison results between Example 1 and Comparative Example 1, and between Example 2 and Comparative Example 2, show that compared to CrMo cold heading steel wire rods rolled in small billets, the Steyrmo cooling line has a fast cooling speed but poor slow cooling effect. Even if the Steyrmo line roller speed is reduced, the generation of martensite or bainite cannot be completely avoided, resulting in increased wire rod hardness, decreased plasticity, and significant differences in uniformity. This invention combines large-section steel billets with high compression ratio rolling to improve matrix plasticity and uniformity. Combined with hot-rolling for rapid coil collection, the residual heat of the coils can be used for slow cooling in a heat-insulating tunnel furnace, promoting a uniform transformation of ferrite and pearlite structures, resulting in a uniform ferrite and pearlite structure with a pearlite volume percentage ≤65%, eliminating bainite formation. Abnormal structures such as strontium and martensite further reduce the tensile strength and hardness of the wire rod, while improving its plasticity. This overcomes the limitations of low-temperature rolling and enables online softening of the microstructure. As shown in the results of Examples 1 and 2, for medium and large-sized CrMo cold heading steel wire rods with diameters of 12-20mm, tensile strength ≤780MPa, tensile strength difference within the same ring ≤100MPa, hardness ≤92HRB, and reduction of area ≥40% can be achieved. After testing, 1 / 3 cold upsetting is qualified. While meeting the user's one-ball two-drawing process, it can achieve rough drawing production with a large reduction rate of 20%~30%, and at the same time reduce the user's subsequent annealing time. After testing, the annealing time can be reduced from 32h to 24h, thereby reducing energy consumption and costs.

[0063] 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 CrMo cold heading steel wire rod for 10.9 grade fasteners, characterized in that, The chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.33%~0.35%, Si: 0.15%~0.30%, Mn: 0.60%~0.70%, P: ≤0.015%, S: ≤0.010%, Cr: 0.90%~1.10%, Ni: ≤0.20%, Cu: ≤0.20%, Mo: 0.15%~0.25%, Al: 0.01%~0.03%, with the remainder being Fe and unavoidable impurities; its production method follows a process of rolling, wire drawing, hot coiling, and slow cooling in a holding tunnel furnace. During the rolling process, a steel billet with a cross-section of 240mm×240mm is used, and the total compression ratio is 183~510; During the spinning process, the spinning temperature is controlled at 910~960℃. After spinning, the wire is conveyed to the winding process under heat preservation. During the hot winding process, the winding collection temperature is controlled at 810~860℃, and the wire rod winding is a coil. During slow cooling in the insulated tunnel furnace, the furnace inlet temperature is controlled at 750~790℃, the residence time of the coil in the insulated tunnel furnace is 60~90min, the cooling rate in the temperature range of 750~600℃ is ≤0.1℃ / s, and the furnace outlet temperature is ≤500℃.

2. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 1, characterized in that, During the rolling process, the heating temperature is controlled at 1120~1170℃, the initial rolling temperature is 1020~1080℃, the water descaling pressure is ≥22MPa, and the rolling process is carried out in 22~26 passes, with the sizing temperature at 890~950℃.

3. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 2, characterized in that, Before rolling, shot blasting and magnetic particle testing are used to finish the steel billet.

4. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 1, characterized in that, After spinning, the wire rod is conveyed by roller conveyor through the heat insulation cover. The roller conveyor speed is ≥0.6m / s, and it enters the winding process within 1 minute.

5. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 1, characterized in that, After being collected and coiled, the coil is placed into the heat-insulating tunnel oven within 1 minute; During the slow cooling process of the insulated tunnel furnace, the temperature of the atmosphere inside the furnace is controlled at 400~600℃, and the furnace exit temperature is >400℃.

6. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 1, characterized in that, The process includes a continuous casting process before rolling, wherein the continuous casting process adopts full-process protective casting and double electric stirring: the superheat is controlled at 20~30℃; the electric stirring current of the crystallizer is 300~400A and the frequency is 2~5Hz; the electric stirring current at the end is 200~250A and the frequency is 6~8Hz; and the total reduction under light pressure is 15~20mm.

7. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 6, characterized in that, The process includes an LF refining process before continuous casting, wherein the white slag holding time is controlled to be ≥20 min. After the white slag, ferromolybdenum alloy is added and calcium wire is fed in for calcium treatment. The amount of calcium wire fed in is ≤100 m. At the same time, soft blowing of argon gas is started, and the soft blowing time is 15~25 min.

8. The method for producing CrMo cold heading steel wire rod for grade 10.9 fasteners according to claim 7, characterized in that, The process includes a converter smelting process before the LF refining process. The converter smelting process uses a 150-200 ton converter, with the scrap steel ratio controlled at 15%-25%. During tapping, deoxidizer, carbon raiser, alloy and 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%-0.17%, the P content at the end of the converter is ≤0.011%, the tapping temperature of the converter is ≥1600℃, and aluminum wire feeding is prohibited at the argon blowing station.

9. A CrMo cold-heading steel wire rod for 10.9 grade fasteners, characterized in that, The cold heading steel wire rod is produced by the production method of CrMo cold heading steel wire rod for 10.9 grade fasteners as described in any one of claims 1 to 8.

10. The CrMo cold-heading steel wire rod for 10.9 grade fasteners according to claim 9, characterized in that, The cold heading steel wire rod has a diameter of 12~20mm, a microstructure of ferrite and pearlite, a tensile strength of ≤780MPa, a tensile strength difference of ≤100MPa within the same ring, a hardness of ≤92HRB, and a reduction of area of ​​≥40%.

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