High plasticity cord steel wire rod adapted to high speed drawing and manufacturing method thereof

Abstract

This invention relates to a high-ductility cord steel wire rod suitable for high-speed drawing and its manufacturing method. It employs a high-carbon composition design (C-Si-Mn-Cr-V) and includes the following steps: hot metal pretreatment → converter smelting → LF refining → continuous casting → grinding → heating → controlled rolling → controlled cooling. The LF refining process uses a double-slag method. During continuous casting, rectangular billets with a size ≥200mm×200mm are used. During controlled cooling, the wire rod's extrusion temperature is controlled at 860~885℃, the cooling rate before phase transformation is 10~15℃ / s, the cooling rate of the wire rod at 690~580℃ is 4.5~6℃ / s, and the cooling rate of the wire rod below 575℃ is ≤2℃ / s. This method can improve inclusions and network cementite defects, achieving a tensile strength of 1125~1235MPa and a reduction of area of ​​34%~37%, thus adapting to downstream applications in high-speed cord drawing production, reducing wire breakage rate, and improving production efficiency.

Description

Technical Field

[0001] This invention belongs to the field of cord steel wire rod, specifically relating to a high-plasticity cord steel wire rod adapted to high-speed drawing and its manufacturing method. Background Technology

[0002] As tires for new energy vehicles and engineering machinery develop towards lightweight and high-strength designs, it is necessary to improve driving range by reducing tire rolling resistance and withstand heavy loads by relying on the high-strength characteristics of tires. Therefore, it is necessary to use thinner and stronger cords to reduce the amount of cord used and reduce rolling resistance. However, existing cord steel wire rods generally use a high-carbon composition system to improve strength. Due to insufficient plasticity, they are difficult to match the requirements of high-speed drawing. During the large deformation process of high-speed drawing, stress concentration can easily cause wire breakage, resulting in frequent downtime and mold changes in downstream drawing processes. This leads to low processing efficiency and high costs. Therefore, it is necessary to develop a high-strength, high-plasticity cord steel wire rod that is suitable for high-speed drawing and its manufacturing method to meet users' needs for fine-gauge, high-strength, and high-stability cord processing and use.

[0003] Current production processes for cord steel wire rod generally involve electric furnace / converter smelting, LF refining, continuous casting, rolling, and temperature-controlled cooling in a Stellmore air-cooling line. However, the following technical bottlenecks remain in the manufacture of high-ductility cord steel wire rod:

[0004] 1. During converter smelting, lime and ferroalloys are added for deoxidation and alloying. However, the slag basicity is low, the dephosphorization time is short, and the P content in the tapped steel is high. Segregation at the grain boundaries leads to cold brittleness, which reduces the plasticity of the wire rod. To improve the brittleness problem, the amount of lime added is increased to improve the basicity of the converter slag, or lime is added again in the LF stage to increase the basicity. However, the slag system will deviate from the low melting point range, resulting in poor fluidity of the LF refining slag system. The contact area between the slag and the molten steel is reduced, and inclusions are difficult to be captured by the slag phase. Under high-intensity stirring, the molten steel flows violently, which will cause the inclusions to agglomerate again, forming large particles with a size ≥10μm. These inclusions cannot float to the surface and eventually remain in the steel, becoming the source of wire breakage during wire drawing, resulting in a decrease in plasticity.

[0005] Second, due to the high carbon content in steel grades, typically between 0.75% and 0.85%, the continuous casting of 140mm×140mm small square billets is characterized by short solidification time and high casting speed. The carbon-rich molten steel is encased in the center, and even with electromagnetic stirring, it is impossible to guarantee a low degree of segregation in the billet. During subsequent cooling, the supersaturated carbon will preferentially precipitate along the austenite grain boundaries to form a network of cementite, affecting the plasticity of the wire rod. Existing technologies generally reduce the level of network cementite by lowering the carbon content and increasing the air-cooling strength. However, carbon is the core strengthening element of high-carbon steel. Strengthening through solid solution not only makes the cementite lamellar distribution in the sorbite structure sparse, but also brings the risk of hard and brittle martensite due to localized rapid cooling, seriously affecting the plasticity of the structure. Although offline heat treatment can eliminate network cementite and uniform sorbite to improve plasticity, the offline treatment of reheating the wire rod after unwinding for austenitization adds an extra processing step, resulting in extended production cycles and increased energy consumption per ton of steel for downstream users. 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 high-plasticity cord steel wire rod adapted to high-speed drawing and its manufacturing method, which can improve inclusion and network cementite defects, take into account wire rod strength and improve wire rod plasticity, so as to adapt to downstream use for high-speed drawing production of cord, reduce wire breakage rate and improve production efficiency.

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

[0008] A method for manufacturing high-plasticity cord steel wire rod adapted to high-speed drawing, wherein the chemical composition of the cord steel wire rod, by mass percentage, includes: C: 0.86%~0.92%, Si: 0.20%~0.32%, Mn: 0.25%~0.35%, Cr: 0.12%~0.2%, S≤0.008%, P≤0.008%, V: 0.045%~0.075%, Al≤0.005%, Ti≤0.001%, O≤0.0015%, N≤0.003%, with the remainder being Fe and unavoidable impurity elements;

[0009] The manufacturing method includes the following steps: hot metal pretreatment → converter smelting → LF refining → continuous casting → grinding → heating → controlled rolling → controlled cooling; the LF refining process adopts a double-slag method. The first slag uses a CaO-Al2O3-SiO2 ternary slag system, with the slag basicity controlled at 1.05~1.55. The second slag uses low-alumina ferrosilicon with an Al mass content ≤1% to achieve a slag basicity of 0.4~0.8. During continuous casting, rectangular billets with a size ≥200mm×200mm are used. During controlled rolling, the initial rolling temperature is controlled at... The temperature ranges from 1065 to 1120℃. The single-pass reduction rate in roughing is 18% to 22%, with a cumulative reduction rate ≥70%. The single-pass reduction rate in intermediate rolling is 15% to 18%. The temperature before entering the finishing mill is 940 to 960℃, and the temperature before entering the sizing mill is 935 to 950℃. During controlled cooling, the wire rod extrusion temperature is controlled at 860 to 885℃. The cooling rate before phase transformation is 10 to 15℃ / s, the cooling rate of wire rod at 690 to 580℃ is 4.5 to 6℃ / s, and the cooling rate of wire rod below 575℃ is ≤2℃ / s.

[0010] The aforementioned cord steel wire rod adopts a high-carbon composition design of C-Si-Mn-Cr-V. C provides the carbide base, Cr refines the carbide size, and V refines the grains with carbonitrides. Combined with controlled rolling cumulative reduction rate and controlled cooling rate before phase transformation, it is possible to control the network cementite and sorbite ratio. The copper lock is appropriately designed with silicon, manganese, low aluminum and low control of harmful elements in a synergistic composition design. This provides favorable conditions for cooperating with the LF double slag method, reducing hard and brittle inclusions, controlling the size of longitudinal non-metallic inclusions, and improving the section reduction rate.

[0011] In its manufacturing method, targeting the low basicity and short dephosphorization time of the slag, a dual-slag method is adopted in the LF refining process. The first slag formation uses medium-high basicity slag, which can react with phosphorus in the molten steel to generate stable dephosphorization products, further reducing the phosphorus content in the molten steel and reducing phosphorus grain boundary segregation from the source. At the same time, the medium-high basicity slag has a moderate melting point, viscosity ≤0.8 Pa·s, and good fluidity. It can initially capture inclusions such as alumina and silica brought in by the converter through slag-steel interface reaction, and dissolve or transform them into a low-melting-point CaO-Al2O3-SiO2 composite phase, which is conducive to subsequent deep dephosphorization. High-alkalinity slag formation lays the foundation for high-alkalinity slag formation. If only one high-alkalinity slag is formed, the excessive CaO in the slag will cause the slag system to deviate from the low melting point range, resulting in increased slag viscosity, decreased fluidity, and difficulty in capturing inclusions in the slag phase. The second addition of low-alumina ferrosilicon can reduce the ratio of calcium oxide to silicon dioxide in the slag, bringing the slag basicity to 0.4~0.8. At this point, the slag system is in the low melting point range, the viscosity drops to ≤0.5 Pa·s, the fluidity is significantly improved, and the slag-steel interface is fully contacted, which can improve the inclusion capture efficiency. At the same time, it avoids the hard and brittle inclusions caused by excessive Al, and avoids them becoming a source of wire breakage during drawing.

[0012] During continuous casting, large rectangular billets can reduce bulging deformation and center segregation by increasing the cross-sectional size, thereby improving the consistency of the microstructure and suppressing the risk of network cementite caused by carbon segregation. At the same time, large billets can provide a greater compression ratio during rolling. Combined with the optimization of controlled rolling parameters, controlling the initial rolling temperature and the high pressure reduction rate can eliminate the original coarse grains. Controlling the intermediate rolling reduction rate and the finishing rolling temperature can elongate the grains and form high-density dislocation bands, which can further refine the grains and provide more nucleation sites for subsequent phase transformations, thereby increasing the sorbite content and reducing the pearlite lamellar spacing. This improves plasticity without excessively degrading the strength.

[0013] During controlled cooling, the coiling temperature is controlled within the austenite stable region, preventing the precipitation of proeutectoid ferrite or cementite. This avoids premature entry into the phase transformation region in some areas due to excessively rapid cooling, which would disrupt the uniformity of the microstructure. It also prevents excessively high temperatures, which could lead to coarsening of austenite grains due to prolonged high-temperature residence time. The coiling temperature, in conjunction with the pre-phase transformation cooling rate, allows for a higher cooling rate in the coil as it enters the phase transformation region, reducing the driving force for carbon diffusion. This prevents excessively slow cooling, which could cause carbon in the austenite to diffuse sufficiently and precipitate high-level proeutectoid cementite along grain boundaries. Simultaneously, it prevents austenite grain growth, providing a fine-grained matrix for the formation of fine lamellar sorbite, increasing the phase deformation nucleation rate, and avoiding excessively high cooling rates on the coil. Excessive temperature gradients or martensitic structures can form between the surface and core due to a large difference in cooling rates. 690~580℃ is the core temperature range for austenitic phase transformation in high-carbon steel. Controlling the cooling rate can stabilize the volume ratio of sorbite and ensure uniform lamellar distribution. It can prevent excessively rapid cooling from causing pearlite lamellars to become too fine or bainite transformation, which would result in a sharp increase in hardness but a decrease in plasticity. Conversely, it can prevent excessively slow cooling from forming coarse pearlite, which would lead to insufficient strength. Since internal stress is generated due to volume changes during phase transformation, the cooling rate below 575℃ is relatively low. Slow cooling can gradually relax the internal stress, making the carbide distribution more uniform and further improving plasticity to meet the plasticity requirements of high-speed drawing.

[0014] In the preferred technical solution, the molten iron pretreatment adopts molten iron and undergoes a pre-desulfurization process. 5~10kg / t of desulfurizing agent is added to the molten iron and stirred for 10~15min to further optimize desulfurization, control the S≤0.005% and Ti≤0.025% in the molten iron entering the furnace, and improve the purity of the molten iron.

[0015] In the preferred technical solution, the converter smelting controls the steel's carbon content to be ≥0.18%, high carbon content inhibits steel over-oxidation and reduces oxide inclusions; controls oxygen content to ≤0.045%, reducing inclusion sources and lowering refining load; controls phosphorus content to ≤0.013% and sulfur content to ≤0.0013%, extending the pretreatment effect and preventing the enrichment of harmful elements; the tapping temperature is ≥1592℃ to ensure steel fluidity and promote reaction and inclusion flotation; each furnace produces 90~110t of steel, and during the initial tapping stage (1 / 5~2 / 5 of the furnace), when the steel flow is vigorous, 150~200kg of silicon-manganese alloy is added for rapid dispersion. Silicon-manganese alloy acts as both an alloying agent and a pre-deoxidizer, reducing the amount of subsequent deoxidation and decreasing the formation of alumina inclusions. When the steel is tapped to 1 / 4 to 1 / 3 full, the molten steel flow rate is stable, and the slag can be evenly distributed in the ladle with the steel flow, avoiding the slag being impacted to the bottom of the ladle due to early addition and the insufficient dissolution due to late addition. Adding 200-360 kg of cleaning agent and 210-280 kg of lime allows the cleaning agent to quickly melt and form transition slag, adsorbing inclusions in the molten steel and promoting the agglomeration and growth of inclusions. Lime can quickly increase the slag basicity, laying a high basicity foundation for the dual-slag method of LF refining. The tapping process uses a sliding plate to block slag, preventing contamination of the converter's final slag.

[0016] In the preferred technical solution, the white slag time during the LF refining process is ≥23min, which preliminarily purifies the inclusions. During soft blowing, the argon flow rate is 62~65L / min, and the soft blowing time is ≥50min. The weak convection formed by soft blowing can promote the slag-steel reaction and avoid the collision and aggregation of inclusions, allowing small inclusions to float to the surface with sufficient time.

[0017] In the preferred technical solution, the superheat during continuous casting is controlled at 18~28℃, which can balance the solidification rate and crystal structure, suppress center segregation, and the casting speed is controlled at 1.15~1.2m / min, which can match the large billet size and optimize solidification uniformity. The current of the electromagnetic stirring in the crystallizer is 450~550A and the frequency is 3~6Hz, which can break up the initial columnar crystals and increase the equiaxed crystal ratio. The current of the electromagnetic stirring at the end is 360~420A and the frequency is 9~11Hz, which can suppress the center segregation at the end and ensure uniform solute distribution. The secondary cooling water ratio is controlled at 0.32~0.38L / kg, which can precisely control the cooling and avoid cracks and hard and brittle phases.

[0018] In the preferred technical solution, the continuous casting process involves light reduction at a rate of 0.5~1.0 mm / s and a total light reduction of 15~18 mm, which can further compensate for solidification shrinkage, refine grains, suppress segregation, eliminate central porosity and shrinkage cavities, and improve central density.

[0019] In the preferred technical solution, the temperature of the preheating section is <900℃ to balance thermal stress; the temperature of the heating section is 1080~1175℃ to initially homogenize alloying elements; the temperature of the soaking section is 1180~1200℃ to eliminate core temperature difference and deeply eliminate carbon segregation; the air-fuel ratio in the furnace is 0.45~0.55, and oxidation and decarburization are controlled by a slightly reducing atmosphere; the total heating time is 120~180min, matching the heating rhythm of the large billet to ensure sufficient homogenization.

[0020] In the preferred technical solution, the grinding process employs shot blasting and flaw detection. During controlled rolling, the billet undergoes high-pressure water descaling, 8 roughing rolling passes, 4 intermediate rolling passes, 8 pre-finishing rolling passes, 8 finishing rolling passes, and 4 sizing and reducing processes in sequence. By increasing the number of roughing and pre-finishing rolling passes, and utilizing the large reduction cross-section of the roughing rolling, the billet initially exhibits coarse grains. The pre-finishing rolling process induces grain refinement, and the deformation temperature is controlled. Combined with the finishing rolling process to refine the grains, the dimensional accuracy and microstructure uniformity are ultimately guaranteed, providing sufficient nucleation sites for sorbite phase transformation.

[0021] In the preferred technical solution, the descaling pressure is controlled at ≥21MPa during the controlled rolling process to remove iron oxide scale and avoid pressing defects.

[0022] In the preferred technical solution, during the controlled cooling process, the wire that has undergone the sizing and reducing process is cooled by water before entering the spinning machine to ensure it is connected to the target spinning temperature, thereby suppressing the precipitation of proeutectoid cementite at the austenite grain boundaries. The water pressure is controlled at 0.3~0.5MPa, and the water flow rate is 3~5m³. 3 / h, to avoid excessive water pressure and volume causing the surface temperature to drop too quickly and triggering the martensitic phase transformation, and to avoid excessively low water pressure and volume and uneven cooling leading to uneven thickness of subsequent pearlite lamellars, the wire rod lap density is 1.2~1.3 turns / m to reduce the concentrated heat transfer at the lap point and improve the uniformity of phase transformation.

[0023] In the preferred technical solution, during the controlled cooling process, the initial speed of the roller conveyor is 1~1.15m / s, which controls the residence time of the wire rod in the air-cooling section, and the rated air volume of the fan is 280,000 m³ / s. 3 / h, the first 3 fans have an air volume of 70%~80% of the rated air volume of the fans, which quickly passes through the sensitive area of ​​reticulated cementite precipitation. The air volume of the 4th and 5th fans is 55%~60% of the rated air volume of the fans, which stabilizes the pearlite transformation. The air volume of the 6th to 8th fans is successively reduced to 30%~35% of the rated air volume of the fans, which slowly cools the wire rod to avoid abnormal structure. The remaining fans are turned off. When the wire rod temperature drops below 575℃, an insulation cover is added to release phase transformation stress and homogenize the structure. The temperature of the wire rod exiting the cover is 300~350℃. After exiting the cover, the wire rod is air-cooled until it is coiled, so as to control the final hardness and winding performance.

[0024] A high-plasticity cord steel wire rod adapted to high-speed drawing, wherein the cord steel wire rod is obtained by any one of the above-described methods for manufacturing high-plasticity cord steel wire rod adapted to high-speed drawing.

[0025] In the preferred technical solution, the microstructure of the cord steel wire rod includes sorbite and pearlite, with a volume percentage of sorbite ≥ 59% and a volume percentage of pearlite ≥ 32%. A certain plastic buffer is used to avoid brittleness caused by an excessively high proportion of sorbite, so that the wire rod has both high strength and high plasticity.

[0026] In the preferred technical solution, the diameter of the cord steel wire rod is 5~7.5mm, the tensile strength is 1125~1235MPa, the reduction of area is 34%~37%, the grain size is 9~10, the network carburized material is ≤1.0, and the longitudinal non-metallic inclusion size is ≤8μm. It can be made into steel wire with a diameter of 0.15~0.38mm by multiple cold drawing. The synergistic optimization of tensile strength, reduction of area and longitudinal non-metallic inclusion size can achieve a balance between work hardening and plasticity reserve during the drawing process, eliminating the need for offline heat treatment and adapting to the requirements of high-speed drawing, and reducing the risk of brittle fracture during drawing.

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

[0028] This invention employs a vanadium-containing high-carbon composition system, combined with the LF refining dual-slag method, which optimizes slag fluidity, improves inclusion defects, and achieves a longitudinal non-metallic inclusion size ≤8μm, avoiding it as a source of wire breakage during drawing. The use of a large rectangular billet combined with optimized controlled rolling parameters further mitigates segregation caused by high carbon content, forming high-density dislocation bands and refining grains to provide more nucleation sites for subsequent phase transformation. The high cooling rate of the wire rod entering the phase transformation zone improves network cementite defects, achieving a network cementite level ≤1.0, meeting the requirements for high-carbon steel. The core temperature range of 690~580℃ for the martensite phase transformation controls the cooling rate, which can stabilize the volume ratio of sorbite and ensure uniform lamellar distribution. Then, slow cooling below 575℃ gradually relaxes the internal stress, making the carbide distribution more uniform and forming a microstructure including sorbite and pearlite. This can balance wire rod strength and improve wire rod plasticity, achieving a tensile strength of 1125~1235MPa and a section reduction rate of 34%~37%, so as to adapt to downstream applications in high-speed drawing production of cord, reduce wire breakage rate, and improve production efficiency. Detailed Implementation

[0029] 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 chemical reagents involved in the present invention, such as purification accelerators (wherein, the mass content of key effective components is: CaO 53.5%, SiO2 3.5%, Al2O3 34.3%, MgO 8.5%), are all purchased through commercial channels.

[0030] A method for manufacturing high-plasticity cord steel wire rod suitable for high-speed drawing, wherein the chemical composition of the cord steel wire rod, by mass percentage, includes: C: 0.86%~0.92%, Si: 0.20%~0.32%, Mn: 0.25%~0.35%, Cr: 0.12%~0.2%, S≤0.008%, P≤0.008%, V: 0.045%~0.075%, Al≤0.005%, Ti≤0.001%, O≤0.0015%, N≤0.003%, with the remainder being Fe and unavoidable impurity elements.

[0031] The design basis for the chemical composition (by mass percentage) of the above-mentioned steel cord wire includes:

[0032] (1) Carbon: C is the core strengthening element of high carbon steel. It is combined with the LF double slag method to promote uniform diffusion of C and avoid uneven structure caused by local carbon enrichment. It provides strength support through solid solution strengthening and carbide dispersion strengthening. It is also a necessary component for the formation of sorbite. However, if the carbon content is too high, the continuous casting large rectangular billet is prone to forming continuous network cementite due to central segregation. It is difficult to completely eliminate it in subsequent controlled cooling. At the same time, it will increase the hardenability of steel and easily precipitate martensite due to the strong stability of supercooled austenite. Therefore, in order to take into account the strength requirements of cord steel for wire rod, reduce the difficulty of controlling network cementite, and adapt to phase transformation regulation, the mass percentage of C is controlled at 0.86%~0.92%.

[0033] (2) Silicon: Si is the main deoxidizer and can react with O in steel to generate low-melting-point silicon dioxide. Combined with the low-alumina design of Al≤0.005%, the O content in steel can be controlled at ≤0.0015%. At the same time, it avoids wire breakage caused by excessively large size of hard and brittle alumina inclusions. It can also improve strength through solid solution strengthening. However, excessive content will significantly increase the cold brittleness of steel, resulting in a decrease in the reduction of area. Therefore, in order to ensure that the longitudinal non-metallic inclusions are ≤8μm and reduce the difficulty of plasticity improvement, the mass percentage of Si is controlled at 0.20%~0.32%.

[0034] (3) Manganese: Mn can form spherical MnS with S to withstand the single-pass reduction rate of 18~22% in rough rolling, avoid rolling cracks, and at the same time, the strength is finely adjusted through solid solution strengthening. However, Mn will increase the hardenability of steel and delay the pearlite transformation. It is necessary to avoid excessive content, which will cause martensite to precipitate due to excessive hardenability, and interfere with the formation of duplex sorbite and pearlite. Therefore, in order to cooperate with controlled rolling and controlled cooling, the mass percentage of Mn is controlled at 0.25%~0.35%.

[0035] (4) Chromium: Cr can form fine and dispersed carbides with Fe and C, avoiding the formation of continuous network cementite when the C content is high. At the same time, Cr can cooperate with the controlled rolling temperature to shrink the austenite grains, provide a fine grain basis for the formation of sorbite, increase the proportion of sorbite, and work with V to pin the grains, further improving the stability of the structure. However, if the Cr content is too high, it will increase the hardenability and affect the plasticity of the wire rod. Therefore, in order to balance the strength of the wire rod and adapt to the phase transformation control, Cr is added. The mass percentage of Cr is controlled at 0.12%~0.2%.

[0036] (5) Vanadium: During controlled rolling and controlled cooling, V will form VN dispersed precipitation with N. On the one hand, it pins the austenite grain boundaries and inhibits grain growth. On the other hand, VN, as a second phase particle, provides dispersion strengthening, which further improves the strength of the wire rod on the basis of high plasticity, and achieves the synergistic effect of strength improvement without sacrificing plasticity. It avoids the plasticity loss caused by simply relying on C to stack strength. However, if the V content is too high, the material cost will be high. Therefore, the mass percentage of V is controlled at 0.045%~0.075%.

[0037] (6) S / P / Al / Ti / O / N are harmful elements. To eliminate the hazards of hot brittleness and sulfide inclusions, S should be controlled to ≤0.008%; to suppress grain boundary segregation and cold brittleness, P should be controlled to ≤0.008%; to reduce wire breakage caused by hard and brittle alumina inclusions during drawing, Al should be controlled to ≤0.005%; to avoid wire breakage caused by hard and brittle titanium nitride inclusions during drawing, Ti should be controlled to ≤0.001%; ​​to control the total amount of oxide or nitride inclusions, O should be controlled to ≤0.0015% and N to ≤0.003%, so as to cooperate with the light reduction process of continuous casting of large rectangular billets, reduce segregation and defects, ensure the uniformity of the two-phase structure, and meet the requirements of high plasticity and low wire breakage rate for high-speed drawing.

[0038] The manufacturing method of the above-mentioned steel cord wire rod includes the following steps: hot metal pretreatment → converter smelting → LF refining → continuous casting → grinding → heating → controlled rolling → controlled cooling; specifically:

[0039] The molten iron pretreatment uses molten iron and undergoes a pre-desulfurization process, adding 5~10 kg / t of desulfurizing agent to the molten iron and stirring for 10~15 minutes to control the S≤0.005% and Ti≤0.025% in the molten iron entering the furnace.

[0040] The converter smelting is used to smelt pretreated molten iron and scrap steel into molten steel. The steel output is controlled with C≥0.18%, O≤0.045%, P≤0.013%, S≤0.0013%, and tapping temperature≥1592℃. Each furnace produces 90~110t of molten steel. When tapping, 150~200kg of silicon-manganese alloy is added at 1 / 5~2 / 5 of the tapping time. When tapping, 200~360kg of cleaning agent and 210~280kg of lime are added at 1 / 4~1 / 3 of the tapping time. Slag is blocked by a sliding plate during tapping.

[0041] The LF refining process is used to further improve the purity of molten steel after converter smelting. The process adopts a double slag method. The first slag is a CaO-Al2O3-SiO2 ternary slag system. The slag basicity is controlled at 1.05~1.55 and the white slag time is ≥23min to optimize the morphology of inclusions. The second slag is made by adding low-alumina ferrosilicon with an Al mass content ≤1% to change the slag basicity to 0.4~0.8. During soft blowing, the argon flow rate is 62~65L / min and the soft blowing time is ≥50min.

[0042] The continuous casting process is used to process molten steel refined by LF into billets. During the process, the superheat is controlled at 18~28℃. Rectangular billets with a size of ≥200mm×200mm are used in the continuous casting process. The casting speed is controlled at 1.15~1.2m / min. The current of the electromagnetic stirring in the crystallizer is 450~550A and the frequency is 3~6Hz. The current of the electromagnetic stirring at the end is 360~420A and the frequency is 9~11Hz. The secondary cooling water ratio is controlled at 0.32~0.38L / kg. Light reduction is applied at a rate of 0.5~1.0mm / s and a total light reduction of 15~18mm. The grinding process involves shot blasting and flaw detection to grind the billets.

[0043] The heating is used to heat the billet into a steel billet with rollability. The preheating section temperature is controlled at <900℃, the heating section temperature is 1080~1175℃, the soaking section temperature is 1180~1200℃, the air-fuel ratio in the furnace is 0.45~0.55, and the total heating time is 120~180min.

[0044] During the controlled rolling process, the steel billet undergoes high-pressure water descaling, 8 roughing rolling passes, 4 intermediate rolling passes, 8 pre-finishing rolling passes, 8 finishing rolling passes, and 4 sizing and reducing processes to produce wire rods with a diameter of 5~7.5mm. The descaling pressure is controlled at ≥21MPa. During controlled rolling, the initial rolling temperature is controlled at 1065~1120℃, the single-pass reduction rate of roughing is 18%~22%, the cumulative reduction rate is ≥70%, the single-pass reduction rate of intermediate rolling is 15%~18%, the temperature before finishing is 940~960℃, and the temperature before sizing and reducing is 935~950℃.

[0045] During the controlled cooling process, the wire that has undergone the sizing and reducing process is cooled by water and then enters the spinning machine to form coils. The water pressure is controlled at 0.3~0.5MPa and the water flow rate is 3~5m³. 3 / h, control the wire rod spinning temperature to 860~885℃, the wire rod spinning overlap density to 1.2~1.3 turns / m, the initial speed of the roller conveyor to 1~1.15m / s, and the rated air volume of the blower to 280000m³ / h. 3 / h, the air volume of the first 3 fans is 70%~85% of the rated air volume of the fans, the air volume of the 4th and 5th fans is 55%~60% of the rated air volume of the fans, and the air volume of the 6th to 8th fans is successively reduced to 30%~35% of the rated air volume of the fans. The remaining fans are turned off. When the temperature of the wire rod drops below 575℃, an insulation cover is added. The temperature of the wire rod exiting the cover is 300~350℃, so that the cooling rate before phase change is 10~15℃ / s, the cooling rate of the wire rod at 690~580℃ is 4.5~6℃ / s, and the cooling rate of the wire rod below 575℃ is ≤2℃ / s. After exiting the cover, the wire rod is air-cooled until it is coiled.

[0046] The chemical composition and mass percentage of the steel cord wire in each embodiment are shown in Table 1, with the balance being iron and unavoidable impurities.

[0047] Table 1. Chemical composition and weight percentage

[0048]

[0049] The difference between Comparative Examples 1 and 2 and Example 1 lies in the different process parameters of the smelting process from hot metal pretreatment → converter smelting → LF refining. In Comparative Example 1, quartz sand was added a second time to change the slag during the LF refining process, while Comparative Example 2 used the single slag method to adjust the slag during the LF refining process. The smelting process parameters of each example and comparative example of cord steel wire rod are shown in Table 2.

[0050] Table 2. Smelting process parameters

[0051]

[0052] The difference between Comparative Example 3 and Example 2 lies in the different process parameters of the continuous casting process. The process parameters of the continuous casting process of the cord steel wire rod in each example and comparative example are shown in Table 3.

[0053] Table 3. Continuous casting process parameters

[0054]

[0055] The difference between Comparative Example 4 and Example 3 lies in the different heating and controlled rolling process parameters. The heating and controlled rolling process parameters of the cord steel wire rod in each example and comparative example are shown in Table 4.

[0056] Table 4. Heating and Controlled Rolling Process Parameters

[0057]

[0058] The difference between Comparative Examples 5 and 6 and Example 4 lies in the different controlled cooling process parameters. The controlled cooling process parameters of the cord steel wire rod in each example are shown in Table 5.

[0059] Table 5. Cooling process parameters

[0060]

[0061] Chemical composition was tested according to GB / T223 standard; metallographic structure was tested according to GB / T13298 "Metallic materials - Microscopic examination method"; non-metallic inclusions in steel were tested according to GB / T10561 "Determination of non-metallic inclusion content in steel - Standard rating chart microscopic examination method"; mechanical properties of steel were determined according to GB / T228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature". The test data of each example and comparative example are shown in Table 6.

[0062] Table 6. Test data for different cord steel wire rods

[0063]

[0064] The comparison results between Example 1 and Comparative Examples 1 and 2 show that, compared with the single-slag method, the double-slag method is used in the LF refining process. The first slag formation uses medium-high basicity, and the slag basicity is controlled at 1.05~1.55. It can generate stable dephosphorization products by reacting with P in the molten steel. At the same time, the melting point of the medium-high basicity slag is moderate. Compared with the second slag transformation by adding quartz sand and improper process parameters in the LF refining process, the second addition of low-alumina ferrosilicon transforms the slag to a basicity of 0.4~0.8, which can reduce the proportion of calcium oxide and silicon dioxide in the slag, significantly improve fluidity, and ensure sufficient contact between slag and steel, thereby improving the inclusion capture efficiency.

[0065] The comparison results between Example 2 and Comparative Example 3 show that, compared with continuous casting of small square billets, continuous casting of large rectangular billets can reduce bulging deformation and center segregation by increasing the cross-sectional size, thereby improving the consistency of the microstructure and suppressing the risk of network cementite caused by carbon segregation. Meanwhile, the comparison results between Example 3 and Comparative Example 4 show that, compared with excessively high heating temperature and deviation of controlled rolling reduction rate, large billets can provide a greater compression ratio during rolling. Combined with the optimization of controlled rolling parameters, controlling the initial rolling temperature and high reduction rate to eliminate the original coarse grains, and controlling the intermediate rolling reduction rate and the finishing rolling temperature, the grains are elongated and form high-density dislocation bands, which can further refine the grains and provide more nucleation sites for subsequent phase transformation, thereby increasing the sorbite content.

[0066] The comparison results between Example 4 and Comparative Examples 5 and 6 show that, compared to insufficient water quenching and inadequate fan airflow, controlling the wire rod temperature during the controlled cooling process keeps it in the austenitic stable region, preventing the precipitation of proeutectoid ferrite or cementite. This avoids premature entry into the phase transformation region in some areas due to excessively rapid cooling, which would disrupt the uniformity of the microstructure. It allows for a higher cooling rate in the phase transformation region, reducing the driving force for carbon diffusion and preventing excessively slow cooling, which would lead to the precipitation of high-level proeutectoid cementite along grain boundaries due to sufficient carbon diffusion in the austenite. Simultaneously, it prevents austenite grain growth, providing a fine-grained matrix for the formation of fine lamellar sorbite, thus improving… Phase transformation nucleation rate; compared to the overlap density of the wire and the deviation of the insulation cover parameters, 690~580℃ is the core temperature range for the austenitic phase transformation of high carbon steel. Controlling the cooling rate can stabilize the volume ratio of sorbite and ensure uniform lamellar distribution. It avoids excessively fast cooling, which can lead to excessively fine pearlite lamellars or bainite transformation, resulting in a sharp increase in hardness but a decrease in plasticity. It also avoids excessively slow cooling, which can lead to coarse pearlite and insufficient strength. Since internal stress is generated due to volume change during the phase transformation, the cooling rate below 575℃ is relatively low. Slow cooling can gradually relax the internal stress, making the carbide distribution more uniform and further improving plasticity.

[0067] As can be seen from Examples 1-4, the present invention can improve inclusions and network cementite defects, while taking into account the strength and plasticity of the wire rod. The microstructure of the cord steel wire rod includes sorbite and pearlite, with a volume percentage of sorbite ≥59% and a volume percentage of pearlite ≥32%. The wire rod diameter is 5-7.5 mm, the tensile strength is 1125-1235 MPa, the reduction of area is 34%-37%, the grain size is 9-10, the network cementite is ≤1.0 grade, and the longitudinal non-metallic inclusion size is ≤8 μm, so as to adapt to downstream high-speed drawing production of cord, reduce wire breakage rate, and improve production efficiency.

[0068] 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 manufacturing high-plasticity cord steel wire rod adapted to high-speed drawing, characterized in that, The chemical composition of the aforementioned steel cord wire rod, by mass percentage, includes: C: 0.86%~0.92%, Si: 0.20%~0.32%, Mn: 0.25%~0.35%, Cr: 0.12%~0.2%, S≤0.008%, P≤0.008%, V: 0.045%~0.075%, Al≤0.005%, Ti≤0.001%, O≤0.0015%, N≤0.003%, with the remainder being Fe and unavoidable impurity elements; The manufacturing method includes the following steps: hot metal pretreatment → converter smelting → LF refining → continuous casting → grinding → heating → controlled rolling → controlled cooling; the LF refining process adopts a double-slag method. The first slag uses a CaO-Al2O3-SiO2 ternary slag system, with the slag basicity controlled at 1.05~1.

55. The second slag uses low-alumina ferrosilicon with an Al mass content ≤1% to achieve a slag basicity of 0.4~0.

8. During continuous casting, rectangular billets with a size ≥200mm×200mm are used. During controlled rolling, the initial rolling temperature is controlled at... The temperature ranges from 1065 to 1120℃. The single-pass reduction rate in roughing is 18% to 22%, with a cumulative reduction rate ≥70%. The single-pass reduction rate in intermediate rolling is 15% to 18%. The temperature before entering the finishing mill is 940 to 960℃, and the temperature before entering the sizing mill is 935 to 950℃. During controlled cooling, the wire rod extrusion temperature is controlled at 860 to 885℃. The cooling rate before phase transformation is 10 to 15℃ / s, the cooling rate of wire rod at 690 to 580℃ is 4.5 to 6℃ / s, and the cooling rate of wire rod below 575℃ is ≤2℃ / s.

2. The manufacturing method of high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 1, characterized in that, The molten iron pretreatment uses molten iron and undergoes a pre-desulfurization process, adding 5~10 kg / t of desulfurizing agent to the molten iron and stirring for 10~15 minutes to control the S≤0.005% and Ti≤0.025% in the molten iron entering the furnace.

3. The manufacturing method of high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 1, characterized in that, The converter smelting process controls the steel output C ≥ 0.18%, O ≤ 0.045%, P ≤ 0.013%, S ≤ 0.0013%, and the tapping temperature ≥ 1592℃. Each furnace produces 90~110t of molten steel. During tapping, 150~200kg of silicon-manganese alloy is added at 1 / 5~2 / 5 of the tapping time, and 200~360kg of purifying agent and 210~280kg of lime are added at 1 / 4~1 / 3 of the tapping time. Slag is blocked by a sliding plate during tapping. During the LF refining process, the white slag time is ≥ 23min, the argon flow rate during soft blowing is 62~65L / min, and the soft blowing time is ≥ 50min.

4. The manufacturing method of high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 1, characterized in that, During the continuous casting process, the superheat is controlled at 18~28℃, and the casting speed is controlled at 1.15~1.2m / min; the current of the electromagnetic stirring in the crystallizer is 450~550A, and the frequency is 3~6Hz; the current of the electromagnetic stirring at the end is 360~420A, and the frequency is 9~11Hz; the secondary cooling water volume is controlled at 0.32~0.38L / kg; the light reduction rate is 0.5~1.0mm / s, and the total light reduction is 15~18mm.

5. The method for manufacturing high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 1, characterized in that, The heating control preheating section temperature is <900℃, the heating section temperature is 1080~1175℃, the soaking section temperature is 1180~1200℃, the air-fuel ratio in the furnace is 0.45~0.55, and the total heating time is 120~180min.

6. The method for manufacturing high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 1, characterized in that, The grinding process employs shot blasting and flaw detection. During the controlled rolling process, the billet undergoes high-pressure water descaling, 8 roughing rolling passes, 4 intermediate rolling passes, 8 pre-finishing rolling passes, 8 finishing rolling passes, and 4 sizing and reducing processes in sequence. The descaling pressure is controlled to be ≥21MPa during the controlled rolling process.

7. The method for manufacturing high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 1, characterized in that, During the controlled cooling process, the wire that has undergone the sizing and reducing process is cooled by water before entering the spinning machine. The water pressure is controlled at 0.3~0.5MPa, and the water flow rate is 3~5m³. 3 / h, the overlap density of wire rod spinning is 1.2~1.3 turns / m.

8. The method for manufacturing high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 7, characterized in that, During the controlled cooling process, the initial speed of the roller conveyor is 1~1.15m / s, and the rated air volume of the fan is 280,000 m³ / s. 3 / h, the air volume of the first 3 fans is 70%~85% of the rated air volume of the fans, the air volume of the 4th and 5th fans is 55%~60% of the rated air volume of the fans, and the air volume of the 6th to 8th fans is successively reduced to 30%~35% of the rated air volume of the fans. The remaining fans are turned off. When the temperature of the wire rod drops below 575℃, an insulation cover is added. The temperature of the wire rod after exiting the cover is 300~350℃. After exiting the cover, the wire rod is air-cooled until it is coiled.

9. A high-plasticity cord steel wire rod adapted for high-speed drawing, characterized in that, The cord steel wire rod is obtained by the manufacturing method of high-plasticity cord steel wire rod adapted to high-speed drawing as described in any one of claims 1 to 8.

10. The high-plasticity cord steel wire rod adapted for high-speed drawing according to claim 9, characterized in that, The microstructure of the cord steel wire rod includes sorbite and pearlite, with a volume percentage of sorbite ≥59% and a volume percentage of pearlite ≥32%. The wire rod diameter is 5~7.5mm, the tensile strength is 1125~1235MPa, the reduction of area is 34%~37%, the grain size is grade 9~10, the network carburized material is ≤1.0 grade, and the longitudinal non-metallic inclusion size is ≤8μm.

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