Annealing-free cr-containing cold heading steel wire rod for 10.9-grade fastener and production method thereof

Abstract

The application relates to a non-annealing Cr-containing cold upsetting steel wire rod for 10.9-grade fasteners and a production method thereof. The wire rod is produced according to the process of rolling, wire laying, heat collecting and slow cooling in a tunnel furnace, is rolled by using a steel billet with a section of 240mm*240mm, has a total compression ratio of 183-510, has a wire laying temperature of 870-920 DEG C, is conveyed into a collecting process under heat preservation after wire laying, has a collecting temperature of 780-830 DEG C, has an entering temperature of 760-800 DEG C, has a residence time of 60-90 min in the coil in the tunnel furnace, has a cooling rate of less than or equal to 0.12 DEG C / s in the temperature range of 750-600 DEG C, has an exiting temperature of less than or equal to 500 DEG C, and can reach a tensile strength of less than or equal to 680 MPa, a section shrinkage of more than 50%, and a hardness of less than or equal to 90 HRB, thereby meeting the requirements of users for non-annealing, direct cold upsetting forming and production of 10.9-grade fasteners with small deformation, such as hexagonal bolts.

Description

Technical Field

[0001] This invention belongs to the technical field of cold heading steel wire rod, specifically relating to a Cr-containing cold heading steel wire rod for 10.9 grade fasteners that does not require annealing and its production method. Background Technology

[0002] ML40Cr is strengthened by Cr element, eliminating the need for adding high-priced alloying elements such as Mo and B. It has advantages such as stable element yield during smelting, controllable raw material costs, and easy tempering after cold heading. It is a traditional material for producing 10.9 grade bolts. However, due to its high strength and poor plasticity, for fasteners with small deformation, the wire rod as the base material must undergo spheroidizing annealing before cold heading, which prolongs the fastener production cycle and leads to increased energy consumption, carbon emissions, and costs for downstream users. Therefore, it is necessary to develop a Cr-containing cold heading steel wire rod for 10.9 grade fasteners that does not require annealing and its production method. This method can reduce the strength and hardness of the wire rod and improve its plasticity index to meet the user's demand for producing fasteners with small deformation, such as external hexagonal bolts, without annealing and through direct cold heading.

[0003] In existing technologies, ML40Cr is generally produced using a Stellmore air-cooling line. For example, patent CN117753777A discloses a method for producing ML40Cr hot-rolled wire rod. After the small-diameter square billet is rolled and wired, the 10mm wire is first slowly cooled and then strongly cooled using a STEM air-cooling line. The insulation cover is then closed, and the wire is then coiled and passed through an insulation corridor for further insulation. However, the following technical bottlenecks still exist in manufacturing wire rods with low strength and high plasticity:

[0004] While strong cooling via STEM air-cooling lines can reduce pearlite clusters and grains in the product's metallographic structure, it is limited by the steel alloy composition and the risk of C and Cr segregation at the center of small-sized billets. The hardenability is significantly improved, and during strong cooling, austenite tends to avoid pearlite and instead form hard and brittle bainite, causing cracks to appear along the bainite region when cold heading reaches 1 / 3 of the height. At the same time, as the wire rod size increases, the long residence time of the wire rod on the air-cooling line will further increase the temperature gradient from the wire rod surface to the core, resulting in high pearlite content and high hardness in the segregation zone. The surface first undergoes phase transformation to form a fine-grained hard structure, while the core undergoes phase transformation to form a coarse-grained soft structure. Subsequent cooling cannot completely eliminate this, resulting in high surface hardness and uneven structure and hardness, which affects the consistency of cold heading deformation and makes annealing-free production difficult.

[0005] Second, due to their small cross-sectional dimensions, small billets have short heat conduction paths and rapid cooling rates during solidification, resulting in limited compression ratios during subsequent rolling. Therefore, they require temperature control via a STEM air-cooling line to improve uniformity and product performance. While an insulation corridor is added after the STEM air-cooling line to slow down the cooling rate, Cr hinders carbon diffusion. The insulation corridor experiences high cooling rates after continuous cooling on the air-cooling line, leading to the superposition of thermal and structural stresses, making it difficult to achieve online softening of the microstructure. Furthermore, as the wire rod specifications increase, the wire rod strength and hardness become higher, increasing resistance to cold heading deformation. This increases the load on cold heading equipment and the risk of cold heading cracking, while also increasing die wear rates, making it difficult to meet the downstream users' demand for anneal-free production. Additionally, excessively long holding times after low-temperature furnace entry also affect production efficiency. 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 a 10.9 grade fastener non-annealed Cr-containing cold heading steel wire rod and its production method, which can reduce the strength and hardness of the wire rod, improve its plasticity index, take into account production efficiency, and meet the user's demand for 10.9 grade fastener products with small deformation such as external hexagon bolts that can be produced directly by cold heading without annealing.

[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 10.9 grade fastener-grade, chromium-containing, non-annealed cold-heading steel wire rod, wherein the chemical element composition of the cold-heading steel wire rod, by weight percentage, comprises: C: 0.38%~0.41%, Si: 0.15%~0.30%, Mn: 0.60%~0.90%, P: ≤0.015%, S: ≤0.010%, Cr: 0.90%~1.00%, Ni: ≤0.20%, Cu: ≤0.20%, Mo: ≤0.10%, Al: ≥0.02%, 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 870~920℃. After spinning, the wire is conveyed to the winding process under heat preservation.

[0011] During the hot coiling process, the coiling temperature is controlled at 780~830℃, and the wire rod coiling is a coil.

[0012] During slow cooling in the insulated tunnel furnace, the furnace inlet temperature is controlled at 760~800℃, 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 ≤0.12℃ / s, and the furnace outlet temperature is ≤500℃.

[0013] The aforementioned cold heading steel wire rod is designed with a high Cr content, which maintains the wire rod's hardenability and lowers the onset temperature of the austenite-ferrite-pearlite phase transformation. It eliminates the need for high-valence alloying elements such as Mo and B. The appropriate addition of Al refines the grains and reduces oxygen content, providing favorable conditions for suppressing grain growth and pearlite lamellar coarsening. Furthermore, the use of large-section billets and high compression ratio rolling overcomes the compression ratio limitations of smaller square billets. High compression ratio rolling eliminates metallurgical defects such as central porosity, micro-shrinkage cavities, and dendritic segregation within the billet, reduces compositional segregation to improve matrix continuity, and simultaneously breaks down initially coarse grains. Fine-grained structures have large grain boundary areas, allowing dislocations to slide between more grain boundaries. Grain boundaries also disperse stress, preventing localized stress concentration and improving matrix plasticity and uniformity. Combined with an appropriate coiling temperature, this avoids the problems of excessively high coiling temperatures (leading to coarse austenite grains due to reduced Cr resistance to grain growth) and coarsening of pearlite lamellae after slow cooling, which increases softening difficulty. Conversely, it avoids excessively low coiling temperatures, which could cause partial austenite transformation into ferrite during coiling, resulting in uneven microstructure and excessively high localized hardness. This lays the foundation for subsequent slow cooling and uniform phase transformation. The wire rod enters the hot coiling process directly under heat preservation without fan cooling.

[0014] Firstly, compared to the abnormal bainitic structure and localized hardening caused by the cooling effect of the Stellmore air-cooling line after coiling, the rapid entry of the wire rod into the hot coiling process under heat preservation can increase the coiling temperature. Especially as the wire rod specifications increase, it can reduce the surface cooling rate after stacking and reduce the initial temperature difference between the surface and the core, avoiding the sudden increase in hardness caused by abnormal bainitic brittleness induced by overcooling, thus ensuring the purity of the structure. Secondly, the wire rod is slowly cooled in the heat preservation tunnel furnace after coiling. The outer layer of the wire rod itself can form a heat insulation layer, and the residual heat can be used to quickly enter a more stable heat preservation environment, reducing production energy consumption. At the same time, the heat storage of stacking balances the cooling rate of the core, reducing the temperature difference between the wire rod surface and the core, so that pearlite is precipitated simultaneously on the surface and the core, avoiding the uneven structure and hardness caused by excessive pearlite content on the surface.

[0015] Second, compared to the difficulty in achieving online softening of the microstructure and the long online time due to the cooling effect of the Stellmore air-cooling line, on the one hand, the rapid entry of the wire rod into the furnace after hot coiling can appropriately increase the furnace temperature, which can avoid the delamination of the internal and external microstructure caused by premature phase transformation and inhibit the growth of austenite grains. At the same time, the low-speed cooling rate of the wire rod in the temperature range of 750~600℃ is more controllable, which can ensure that the wire rod achieves slow cooling in the holding tunnel furnace, prolongs the time of the wire rod in the phase transformation sensitive zone, and ensures that the austenite is completely transformed into pearlite and ferrite, with no residual austenite microstructure, reducing phase transformation stress and avoiding the low-temperature risk of bainite or martensite formation. On the other hand, the low-speed cooling in the high-temperature range can appropriately increase the carbon diffusion coefficient, so as to quickly achieve online softening of the microstructure, release thermal stress, further reduce the tensile strength and hardness of the wire rod, and improve its plasticity index. In particular, as the wire rod specifications increase, the residence time of the wire rod in the holding tunnel furnace can be reduced, taking into account production efficiency.

[0016] During the rolling process, an appropriate heating temperature can be selected to promote the solid solution of Cr carbides and inhibit the coarsening of austenite grains. An appropriate initial rolling temperature is selected to maintain high-temperature plasticity in order to reduce the risk of rolling cracks and to facilitate grain breakage under high pressure. An appropriate sizing temperature is selected to control the final austenite rolling structure and to connect the wire drawing and phase transformation. In the preferred technical solution, during the rolling process, the heating temperature is controlled at 1100~1150℃, the initial rolling temperature is 1000~1060℃, and the sizing temperature is 860~900℃.

[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] 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.

[0019] 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.

[0020] To further control the temperature drop rate, connect the spinning temperature and the winding and collecting temperature, avoid premature phase change leading to surface and core delamination, and reduce the frictional residence time of the wire rod on the roller conveyor, in the preferred technical solution, after spinning, the wire rod is conveyed by the roller conveyor through the heat preservation cover, the roller conveyor speed is ≥0.6m / s, and it enters the winding process within 1 minute.

[0021] To further control the rate of temperature drop, connect the coil collection temperature with the furnace entry temperature, and suppress austenite grain coarsening, in the preferred technical solution, the coiled coil enters the heat-preserving tunnel furnace within 1 minute after coiling.

[0022] To further control the cooling rate of the wire rod in the insulated tunnel furnace and promote online softening of the structure, in a preferred embodiment, the temperature of the atmosphere inside the insulated tunnel furnace is controlled at 450~650℃ during slow cooling.

[0023] In the preferred technical solution, a continuous casting process before rolling is included. The continuous casting process adopts full-process protective pouring and double electric stirring: the superheat is controlled at 20~30℃, and the preferred casting speed is 1.15m / s, which can balance fluidity and solidification structure; the electric stirring current of the crystallizer is 300~400A and the frequency is 2~5Hz. The crystallizer is the initial solidification zone of the billet. Since Cr promotes the growth of columnar crystals in Cr steel, the electric stirring of the crystallizer can be used to break up the columnar crystals and expand the equiaxed crystal zone; the final electric stirring current is 200~300A and the frequency is 8~10Hz. Cr-containing steel is prone to C and Cr center enrichment at the end of solidification. The final electric stirring drives the convection of molten steel through high-frequency stirring, which can break up the solute enrichment layer, suppress center segregation, and homogenize the composition.

[0024] To further compensate for internal porosity and reduce center segregation in Cr-containing steel, a light reduction and an appropriate total reduction can be adopted to provide a basis for high compression ratio rolling of large-section steel billets. In the preferred technical solution, the total reduction of the light reduction in the continuous casting process is 15-20 mm.

[0025] 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 ≤50 kg are used to adjust the slag fluidity, so as to deeply desulfurize and deoxidize, inhibit Cr oxidation, adsorb inclusions, and improve the purity of molten steel. After the white slag, calcium wire is fed in for calcium treatment. The amount of calcium wire fed in is ≤120 m, so as to optimize the morphology of inclusions, avoid hard and brittle inclusions and new defects caused by excessive calcium. At the same time, soft blowing of argon gas is started, and the soft blowing time is 15~25 min to promote the floating of inclusions and uniform steel composition and temperature.

[0026] 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% to balance temperature and cost and avoid abnormal composition. Preferably, the converter tapping process uses a double-plate sliding plate to effectively control the amount of slag and improve the purity of the molten steel. During tapping, deoxidizer, carburizer, alloying agent, and top slag are added sequentially to reduce oxidation and precisely control the composition. Preferably, aluminum blocks are selected as the deoxidizer, and low-nitrogen carburizers are selected. The alloys selected are silicon-manganese alloy, ferrosilicon alloy, high-carbon ferromanganese, and high-carbon ferrochrome. The top slag uses quicklime and calcium aluminate for preliminary desulfurization and impurity removal, laying the foundation for LF white slag. The carbon content at the converter endpoint is 0.08%~0.20% to control oxidation and reduce alloy burn-off. The P content at the converter endpoint is ≤0.011% to avoid the risk of cold brittleness caused by P. The converter tapping temperature is ≥1600℃ to provide a temperature basis for alloy dissolution and subsequent refining. Aluminum wire is prohibited from being fed at the argon blowing station to avoid excessive hard and brittle inclusions.

[0027] The second aspect of the present invention is to provide a 10.9 grade fastener non-annealed Cr-containing 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 non-annealed Cr-containing cold heading steel wire rods. The microstructure of the cold heading steel wire rod includes ferrite and pearlite, without hard and brittle phases such as bainite and martensite. The volume fraction of ferrite is ≥30%, the grain size is 8~10, dislocations can slide between more grain boundaries, and the grain boundaries can disperse the local stress during cold heading. It is qualified for 1 / 3 cold upsetting, the surface defect depth is ≤0.05mm, and the total decarburized layer depth is ≤1%D diameter, which meets the downstream users' requirements for producing non-annealed cold heading forming of external hexagonal bolts.

[0028] In the preferred technical solution, the diameter of the cold heading steel wire rod is 12~20mm, the tensile strength is ≤680MPa, the reduction of area is ≥50%, and the hardness is ≤90HRB. The lower tensile strength and hardness can reduce the resistance to cold heading deformation and reduce mold wear, while the higher reduction of area can reduce the risk of cold heading cracking, thus making it suitable for anneal-free cold heading.

[0029] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0030] This invention optimizes the composition of Cr-containing cold heading steel wire rod, eliminating the need for high-priced alloying elements such as Mo and B. Combined with large-section steel billets and high compression ratio rolling, it improves the plasticity and uniformity of the matrix. The hot-rolling process allows for rapid coil collection, and the residual heat of the coil can be used for slow cooling in a heat-insulating tunnel furnace. This promotes a uniform transformation of ferrite and pearlite structures, avoids abnormal bainitic structures and localized hardening, and further reduces the tensile strength and hardness of the wire rod, while improving its plasticity. This achieves online softening of the microstructure, balancing production efficiency. The wire rod can achieve a tensile strength ≤680MPa, a reduction of area ≥50%, and a hardness ≤90HRB. Testing shows it passes 1 / 3 cold upsetting with a surface defect depth ≤0.05mm. This meets the downstream user's demand for non-annealed production of 10.9 grade fasteners with low deformation, such as hexagonal bolts, thereby reducing user manufacturing costs and demonstrating good industrial adaptability and application prospects. Attached Figure Description

[0031] 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:

[0032] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention.

[0033] Figure 2 This is a metallographic diagram of Comparative Example 1 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, silicon-iron 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 the 10.9 grade fastener non-annealed Cr-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: 0.38%, Si: 0.30%, Mn: 0.60%, P: 0.015%, S: 0.003%, Cr: 0.90%, Ni: 0.20%, Cu: ≤0.10%, Mo: 0.03%, 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:

[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 alloys are silicon-manganese alloy, ferrosilicon 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 feeding is prohibited 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. The white slag is maintained for ≥20 minutes. After the white slag, calcium wire is fed in for calcium treatment. The amount of calcium wire fed in is ≤120m. 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 8Hz. The total reduction under light pressure is 15mm.

[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 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 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 860℃.

[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 870°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 780°C. After coiling, the coils are quickly fed into the heat preservation tunnel furnace via 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 entry temperature at 760℃, the furnace atmosphere temperature at 450~650℃, the residence time of the coil in the furnace at 60 minutes, the cooling rate within the 750~600℃ temperature range ≤0.12℃ / s, and the exit 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.

[0045] Example 2:

[0046] A preferred embodiment of the production method of the 10.9 grade fastener non-annealed Cr-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: 0.41%, Si: 0.15%, Mn: 0.90%, P: 0.010%, S: 0.010%, Cr: 1.00%, Ni: 0.10%, Cu: ≤0.20%, Mo: 0.10%, Al: 0.02%, 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 alloys are silicon-manganese alloy, ferrosilicon alloy, high-carbon ferromanganese, and high-carbon ferrochrome. The top slag is quicklime + calcium aluminate. The carbon content at the converter endpoint is 0.20%, the P content at the converter endpoint is ≤0.011%, the converter tapping temperature is ≥1600℃, and aluminum wire feeding is prohibited 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, calcium wire is fed in for calcium treatment. The amount of calcium wire fed in is ≤120m. 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 300A and the frequency is 10Hz. The total reduction under light pressure is 20mm.

[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 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, with the initial rolling temperature controlled at 1000℃. The rolling coefficient and deformation are rationally allocated, and 20mm diameter wire rod is obtained through 22 passes of rolling, 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 900℃.

[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 920°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 830°C. After coiling, the coils are quickly fed into the heat preservation tunnel furnace via roller conveyor within 1 minute.

[0054] The insulated tunnel furnace slow cooling process is used to feed the coil into the insulated tunnel furnace, control the furnace entry temperature to be 800℃, the furnace atmosphere temperature to be 450~650℃, the residence time of the coil in the insulated tunnel furnace to be 90min, the cooling rate in the temperature range of 750~600℃ to be ≤0.12℃ / s, and the furnace exit temperature to be 480℃. The residual heat of the coil is used to achieve slow cooling in the insulated tunnel furnace, realizing the online softening of the structure. Finally, after inspection, packaging, weighing, marking, and warehousing, the finished cold heading steel wire rod is obtained.

[0055] Comparative Example 1:

[0056] A method for producing Cr-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: 0.40%, Si: 0.25%, Mn: 0.85%, P: 0.016%, S: 0.012%, Cr: 1.05%, Ni: 0.10%, Cu: ≤0.10%, Mo: 0.10%, Al: 0.04%, 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 drawing → cooling on the Steyrmo cooling line. Specifically: the billet in the small square 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 145, and a wire rod with a diameter of 15mm is obtained; in the wire drawing process, the wire drawing temperature is 900℃, the roller speed on the Steyrmo cooling line is 0.20m / 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 2 As shown.

[0057] Comparative Example 2:

[0058] A method for producing Cr-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 82; 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.

[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 test - 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 examination 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 Cr-containing cold heading steel wire rods rolled in small-diameter square billets, the rapid cooling through the Stellmore cooling line results in poor slow cooling of the wire rods, leading to increased hardness and decreased plasticity. This invention, however, 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. This achieves a ferrite volume ratio ≥30% and a grain size of 8-10, avoiding abnormal bainite structures and... While locally hardening, the tensile strength and hardness of the wire rod are further reduced, while its plasticity index is improved, achieving online softening of the structure. As can be seen from the results of Examples 1 and 2, for Cr-containing cold heading steel wire rods with a diameter of 12-20mm, the tensile strength can be ≤680MPa, the reduction of area can be ≥50%, and the hardness can be ≤90HRB. The 1 / 3 cold upsetting test is qualified, which effectively solves the problem of high strength and poor plasticity of Cr-based cold heading steel without annealing. It can meet the needs of downstream users to produce external hexagonal bolts and other products that are directly cold-headed without annealing, so as to further reduce the annealing cost of users, improve production efficiency, and reduce carbon emissions.

[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 10.9 grade fasteners using chromium-containing cold-heading steel wire rod without annealing, characterized in that, The chemical element composition of the cold heading steel wire rod, by weight percentage, includes: C: 0.38%~0.41%, Si: 0.15%~0.30%, Mn: 0.60%~0.90%, P: ≤0.015%, S: ≤0.010%, Cr: 0.90%~1.00%, Ni: ≤0.20%, Cu: ≤0.20%, Mo: ≤0.10%, Al: ≥0.02%, 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 heat-insulating 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 870~920℃. After spinning, the wire rod is conveyed by roller conveyor through the heat preservation cover. The roller conveyor speed is ≥0.6m / s. The wire rod is conveyed into the winding process within 1 minute under heat preservation. During the hot coiling process, the coiling temperature is controlled at 780~830℃, the wire rod is coiled, and the coiled wire rod enters the heat preservation tunnel furnace within 1 minute after coiling. During slow cooling in the insulated tunnel furnace, the furnace inlet temperature is controlled at 760~800℃, the furnace atmosphere temperature is 450~650℃, 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 ≤0.12℃ / s, and the furnace outlet temperature is ≤500℃.

2. The method for producing 10.9 grade fastener non-annealed Cr-containing cold heading steel wire rod according to claim 1, characterized in that, During the rolling process, the heating temperature is controlled at 1100~1150℃, the initial rolling temperature is 1000~1060℃, the water descaling pressure is ≥22MPa, and the rolling process is carried out in 22~26 passes, with the sizing temperature at 860~900℃.

3. The method for producing 10.9 grade fastener non-annealed Cr-containing cold heading steel wire rod 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 10.9 grade fastener non-annealed Cr-containing cold heading steel wire rod 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 200~300A and the frequency is 8~10Hz; and the total reduction under light pressure is 15~20mm.

5. The method for producing 10.9 grade fastener non-annealed Cr-containing cold heading steel wire rod according to claim 4, 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 is added, calcium wire is fed in for calcium treatment, with a calcium wire feeding amount ≤120 m. At the same time, soft blowing of argon gas is started, and the soft blowing time is 15~25 min.

6. The method for producing 10.9 grade fastener non-annealed Cr-containing cold heading steel wire rod according to claim 5, 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 converter endpoint is 0.08%-0.20%, the P content at the converter endpoint is ≤0.011%, the converter tapping temperature is ≥1600℃, and aluminum wire feeding is prohibited at the argon blowing station.

7. A chromium-containing 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 10.9 grade fasteners using chromium-containing cold heading steel wire rod without annealing as described in any one of claims 1 to 6.

8. The 10.9 grade fastener non-annealed Cr-containing cold-heading steel wire rod according to claim 7, characterized in that, The diameter of the cold heading steel wire rod is 12~20mm, the microstructure includes ferrite and pearlite, the volume ratio of ferrite is ≥30%, the grain size is 8~10, the tensile strength is ≤680MPa, the reduction of area is ≥50%, and the hardness is ≤90HRB.

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