High plasticity super high strength cord steel wire rod and production process thereof

Abstract

The application relates to a high-plasticity super-high-strength cord steel wire rod and a production process thereof. The chemical components of the cord steel wire rod include the following in percentage by mass: C: 0.90%-0.94%, Si: 0.17%-0.30%, Mn: 0.30%-0.48%, Cr: 0.17%-0.50%, P: <=0.012%, S: <=0.008%, Ni: <=0.05%, Cu: <=0.05%, Al: <=0.0030%, Ti: <=0.0020%, O: <=0.0020%, and N: <=0.0045%. Based on the medium rectangular billet heavy pressing, one-fire straight rolling and grading controlled cooling process, the application can effectively improve the segregation of the casting billet, avoid the production cost increase caused by the second-fire material forming, inhibit the formation of abnormal net-shaped carbide and martensite structure and the like, shorten the natural aging period, has excellent deep drawing processing performance and can be used for the production of 90-grade and 92-grade super-high-strength cord steel wire rods.

Description

Technical Field

[0001] This invention belongs to the field of cord steel wire rod technology, specifically relating to a high-plasticity, ultra-high-strength cord steel wire rod based on rectangular billet under heavy pressure and graded controlled cooling, and its one-fire direct rolling production process. Background Technology

[0002] Steel cord is a high-tech product among wire products, with expensive production equipment and complex processes, making it one of the wire products with the most stringent quality requirements. During processing, steel cord undergoes a series of deformations including tension, torsion, and bending. During tire service, it withstands combined alternating loads of bending and tension, as well as impact loads. Therefore, the quality of the raw material wire rod plays a decisive role in the processing and performance of steel cord. With the development of lightweight and high-performance automotive tires, grade 90 and grade 92 ultra-high strength cord steel wire rods have attracted widespread attention due to their ability to produce ultra-high strength steel cord. However, the development of grade 90 and grade 92 cord steel wire rods with good product quality and drawing performance still faces the following technical bottlenecks:

[0003] I. In existing technologies, the one-fire casting process for small square billets of cord steel wire rod generally uses continuous casting of small square billets with cross-sectional dimensions of 140mm×140mm or 160mm×160mm. After heating, the billets are directly rolled into wire rods. The continuous casting process employs electromagnetic stirring in the crystallizer and at the end, with weak or strong cooling for the secondary cooling. The advantages of this process are short flow and low cost. However, due to the small billet size, for 90-grade and 92-grade cord steel wire rods with a C content of 0.90% or higher, the carbon content is even higher. Improper control during continuous casting can easily lead to severe center segregation in the billet. Even with electromagnetic stirring and light reduction technology, it is difficult to control the center segregation index below 1.10, and the center segregation problem cannot be effectively solved. The center segregation zone has severe C and Mn element agglomeration, and its content is far higher than the matrix average. The composition of this region is such that it is prone to forming central network carbides that degrade the plasticity and toughness of the wire rod after rolling. Although existing technologies generally employ post-rolling forced cooling processes to suppress network carbides, such as the network carbide control method for ultra-high carbon cold-drawn steel wire rod disclosed in patent CN111850400A, which mainly controls network carbides through high-temperature diffusion, reducing rolling speed, increasing rolling temperature and wire drawing temperature, and increasing cooling rate, it does not specifically address the central segregation problem at the source of continuous casting billets. Therefore, for billets with severe segregation, its effect is limited and it is difficult to stabilize production. At the same time, forced cooling processes bring the risk of martensitic transformation. For example, the 16mm specification prestressed steel strand wire rod XYL82B and its production method disclosed in patent CN118880168A mainly uses 180mm 2The continuous casting of small billets in one firing and the controlled cooling process of Stellmore air cooling with insulation corridor have limited ability to control the center segregation of high carbon steel with C≥0.90% under the continuous casting of small billets. Under the strong cooling process, it is difficult to coordinate the phase transformation process, which will result in a sorbite ratio of only about 85% and the risk of martensite structure. Abnormalities such as network carbides and martensite structure will seriously affect the drawing performance, resulting in a significant increase in the wire breakage rate in the wet drawing and stranding stages of subsequent cord steel manufacturing.

[0004] Second, in the existing technology, the continuous casting of large square billets of cord steel wire rod generally adopts a two-fire forming process. For example, the high carbon steel wire rod and its production method disclosed in patent CN115055654A use light reduction treatment for 300mm×400mm billets. After heating the continuous casting billet, it is rolled to obtain the billet material, and then heated and rolled and air-cooled. However, this two-fire forming process will lead to a longer production cycle and an increase in energy consumption per unit product, resulting in higher production costs. Furthermore, large square billets or medium rectangular billets will directly enter the rolling line after being rolled, which will lead to severe impact and high wear on the rolling line. At the same time, for 90-grade and 92-grade cord steel wire rod, there is still a risk of network carbides under low temperature wire drawing and strong cooling, and the rapid cooling rate will cause martensitic structure, affecting the sorbite ratio and microstructure properties, and the natural aging period is long. Summary of the Invention

[0005] This invention aims to at least partially solve one of the aforementioned technical problems. It provides a high-plasticity, ultra-high-strength cord steel wire rod and its production process. Based on a medium-sized rectangular billet under heavy pressure, single-fire direct rolling, and staged controlled cooling process, it effectively improves billet segregation, avoids increased production costs due to second-fired processing, suppresses the formation of abnormalities such as network carbides and martensite, and shortens the natural aging period. This results in cord steel wire rods with good product quality and excellent deep-drawing processing performance, suitable for the efficient and stable production of 90-grade and 92-grade ultra-high-strength cord steel wire rods.

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

[0007] The first aspect of this invention is to provide a production process for high-plasticity, ultra-high-strength cord steel wire rod, wherein the chemical composition of the cord steel wire rod, by mass percentage, includes: C: 0.90%~0.94%, Si: 0.17%~0.30%, Mn: 0.30%~0.48%, Cr: 0.17%~0.50%, P≤0.012%, S≤0.008%, Ni≤0.05%, Cu≤0.05%, Al≤0.0030%, Ti≤0.0020%, O≤0.0020%, N≤0.0045%, with the remainder being Fe and unavoidable impurity elements; the process includes sequential continuous casting, heating, rolling, and controlled cooling steps.

[0008] When the billet in the continuous casting process passes through the straightening and pressing rollers, it is subjected to heavy pressing: the initial pressing amount is 0.7~1.3 mm, and the pressing amount of each subsequent pass increases by 0.2~1.4 mm, with a total pressing amount of 21.4~25.6 mm, resulting in a rectangular billet with a width × length of (172.5~180) × (240~253) mm;

[0009] The rolling process adopts one-fire direct rolling: first, the rectangular billet is rolled into an intermediate billet through the initial rolling pass, and then the intermediate billet is rolled into wire rod through roughing, intermediate rolling, pre-finishing rolling, finishing rolling and sizing passes. The elongation coefficient of each pass is controlled between 1.10 and 1.45.

[0010] The controlled cooling process spins the wire into coils, controlling the spinning temperature to ≥920℃. The cooling rate from the time the coil is spun to the start of the phase change is controlled to be 20~25℃ / s. The cooling rate during the phase change process is 5~15℃ / s. When the coil temperature drops below 580℃, an insulation cover is added for slow cooling. After the phase change, the cooling rate is controlled to be 0.5~1.5℃ / s, and the temperature after exiting the cover is 400~500℃.

[0011] In the preferred technical solution, the internal dimensions of the copper tube in the crystallizer of the continuous casting process are a rectangular billet cross-section of 200mm×240mm, the continuous casting speed is controlled at 1.20~1.45m / min, and the superheat is controlled at 15~25℃.

[0012] In the preferred technical solution, the current of the electromagnetic stirring of the crystallizer in the continuous casting process is 500~700A and the frequency is 2~5Hz; the current of the electromagnetic stirring at the end is 400~600A and the frequency is 8~15Hz.

[0013] In the preferred technical solution, the secondary cooling process of the continuous casting process adopts a weak cooling process, and the specific water content is controlled at 0.30~0.38L / kg.

[0014] In the preferred technical solution, the billet in the continuous casting process passes through 10 sets of tension straightening and pressing rollers. The pressing amount of each tension straightening and pressing roller along the billet conveying direction is as follows: 0 / 0.7~1.3 / 1.2~1.8 / 2.2~2.8 / 2.7~3.3 / 4.2~4.8 / 5.2~5.8 / 5.2~5.8 / 0 / 0mm.

[0015] In the preferred technical solution, the rectangular billet has no intermediate cracks, the area of ​​the equiaxed crystal region is 70%~78%, the central shrinkage cavity is ≤0.5 grade, and the carbon segregation index is ≤1.05.

[0016] In the preferred technical solution, the heating process controls the preheating section temperature to be 900~980℃, the heating section temperature to be 1110~1150℃, the soaking section temperature to be 1160~1200℃, and the total heating time to be 180~240min.

[0017] In the preferred technical solution, the heated rectangular billet is descaled by high-pressure water with a pressure ≥20MPa.

[0018] In the preferred technical solution, the rectangular billet in the rolling process sequentially passes through a 4-pass shunting mill, a 14-pass roughing and intermediate rolling mill, a 4-pass pre-finishing rolling mill, an 8-pass finishing rolling mill, and a 4-pass sizing mill. According to the rolling direction, the elongation coefficient of the first to the 14th passes is 1.2 to 1.37, the elongation coefficient of the 15th to the 24th passes is 1.2 to 1.31, and the elongation coefficient of the 25th to the 34th passes gradually decreases to below 1.03.

[0019] In a preferred embodiment, the rolling process rolls the rectangular billet into an intermediate billet of 160×160mm.

[0020] In the preferred technical solution, the rolling process controls the initial rolling temperature to be 1040~1080℃, the finishing rolling temperature to be 920~960℃, and the sizing temperature to be 920~945℃; the wire drawing temperature is 920~940℃.

[0021] In the preferred technical solution, the rated air volume of the fan in the controlled cooling process is 300,000 m³ / h. 3 / h, the opening volume of fans 1 to 8 is 70%~85% / 75%~90% / 75%~90% / 65%~85% / 60%~80% / 40%~50% / 30%~40% / 30%~40% respectively from front to back, and the opening volume follows the general trend of first increasing and then decreasing, while the remaining fans are turned off.

[0022] A second aspect of the present invention is to provide a high-plasticity, ultra-high-strength cord steel wire rod, wherein the cord steel wire rod is produced by the production process of the high-plasticity, ultra-high-strength cord steel wire rod described in any one of the above claims.

[0023] In the preferred technical solution, the specifications of the cord steel wire rod are 5.5mm, free of martensite, with a grain boundary cementite grade ≤1.0, a sorbite ratio ≥94%, a tensile strength ≥1200MPa, and a reduction of area ≥45%.

[0024] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0025] (1) The continuous casting process of this invention adopts medium rectangular billet heavy reduction, with a small initial reduction and a gradually increasing reduction, to avoid the formation of central shrinkage cavity and central component segregation zone from the source of billet casting. This allows the carbon segregation index of the rectangular billet to be stably controlled at 0.98~1.05, providing a foundation for the subsequent stability and effective control of network cementite and martensite. This solves the problem that the existing process path of light reduction of small square billet is difficult to solve for the problem of the difficulty in controlling the central segregation of 90-grade and 92-grade ultra-high strength cord steel wire rod below 1.10, which seriously leads to the network cementite and martensite structure in the center of the wire rod after rolling. It can be further combined with low superheat control, secondary cooling weak cooling process, appropriate casting speed, crystallizer electromagnetic stirring and end electromagnetic stirring control, which can not only prevent the generation of intermediate cracks during billet straightening and reduction, but also homogenize the chemical composition of the central paste zone, and maximize the dispersion and feeding of the central molten steel.

[0026] (2) Under the premise of ensuring that the center segregation of the billet is effectively improved, the present invention adopts one-fire direct rolling and de-head rolling technology, which solves the problem of long process flow and high cost caused by the two-fire forming process after the continuous casting of large square billets. In the one-fire direct rolling, the elongation coefficient of each pass is controlled, which solves the problem of severe impact and high wear caused by the direct entry of medium rectangular billets into the traditional roughing mill, while avoiding internal micro-cracks or coarsening of the central structure.

[0027] (3) The present invention adopts a staged cooling process. After the wire rod is spun and before the phase transformation, it is rapidly cooled to suppress the precipitation of carbon in the network. During the phase transformation, the cooling rate is appropriately reduced to ensure the uniform transformation of the sorbite structure and avoid the formation of martensite. When the wire rod temperature drops below 580℃, a heat insulation cover is added for slow cooling to fully release the phase transformation stress and shorten the natural aging period. This can suppress network carbides and avoid the formation of martensite. It can achieve no martensite, grain boundary cementite ≤1.0 grade, sorbite ratio ≥94%, tensile strength ≥1200MPa, and section reduction rate ≥45%, ensuring that the wire rod has good deep drawing processing performance. It solves the problem that it is difficult to balance the high plasticity and ultra-high strength of the cord steel wire rod. It can be used for the efficient and stable production of 90 grade and 92 grade ultra-high strength cord steel wire rod. Attached Figure Description

[0028] 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:

[0029] Figure 1 This is a metallographic diagram of the cord steel wire rod of Embodiment 1 of the present invention; in the figure, a is a low magnification longitudinal section of the wire rod, and b is a 500x metallographic diagram of the wire rod.

[0030] Figure 2 This is a metallographic diagram of the cord steel wire rod of Embodiment 2 of the present invention; in the figure, a is a low magnification longitudinal section of the wire rod, and b is a 500x metallographic diagram of the wire rod.

[0031] Figure 3This is a metallographic diagram of the cord steel wire rod of Embodiment 3 of the present invention; in the figure, a is a low magnification longitudinal section of the wire rod, and b is a 500x metallographic diagram of the wire rod.

[0032] Figure 4 This is a metallographic diagram of the cord steel wire rod of Comparative Example 1 of the present invention; in the figure, a is a low magnification longitudinal section of the wire rod, b is a network carbide metallographic structure, and c is a martensitic structure.

[0033] Figure 5 This is an EPMA scan of the cord steel in Comparative Example 2 of this invention; in the figure, a is the metallographic structure of the wire rod, b is the line scan of C content, c is the line scan of Si content, d is the line scan of Mn content, and e is the line scan of P content.

[0034] Figure 6 This is a metallographic diagram of the cord steel wire rod of Comparative Example 3 of the present invention; in the figure, a is a low magnification longitudinal section of the wire rod, b is the martensitic structure, and c is the martensitic structure.

[0035] Figure 7 This is a metallographic diagram of the cord steel wire rod of Comparative Example 4 of the present invention; in the figure, a is a low magnification longitudinal section of the wire rod, and b is a 500x metallographic diagram of the wire rod. Detailed Implementation

[0036] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0037] To address the challenges of controlling center segregation below 1.10 in 90-grade and 92-grade ultra-high strength cord steel wire rods due to the difficulty of existing small-bulk light-pressing processes, which severely leads to the formation of network carbides and martensite in the center of the rolled wire rod, resulting in insufficient drawing performance and a high wire breakage rate, and the problems of long process flow and high cost associated with the two-fire rolling process after large-bulk continuous casting, and the issue that directly feeding large-bulk or medium-sized rectangular billets into a traditional roughing mill would cause severe impact and high wear on the rolling line, this invention employs a medium-sized rectangular billet heavy-pressing, one-fire direct rolling, and staged controlled cooling technology to control the source of center segregation in continuous casting, suppress network carbides and martensite, and balance production efficiency and cost control. This invention discloses a preferred embodiment of the production process for high-plasticity, ultra-high-strength cord steel wire rod. The chemical composition of the cord steel wire rod, by mass percentage, includes: C: 0.90%~0.94%, Si: 0.17%~0.30%, Mn: 0.30%~0.48%, Cr: 0.17%~0.50%, P≤0.012%, S≤0.008%, Ni≤0.05%, Cu≤0.05%, Al≤0.0030%, Ti≤0.0020%, O≤0.0020%, N≤0.0045%, with the remainder being Fe and unavoidable impurity elements. The process includes sequential continuous casting, heating, rolling, and controlled cooling steps.

[0038] When the billet in the continuous casting process passes through the straightening and pressing rollers, it is subjected to heavy pressing: the initial pressing amount is 0.7~1.3 mm, and the pressing amount of each subsequent pass increases by 0.2~1.4 mm, with a total pressing amount of 21.4~25.6 mm, resulting in a rectangular billet with a width × length of (172.5~180) × (240~253) mm;

[0039] The rolling process adopts one-fire direct rolling: first, the rectangular billet is rolled into an intermediate billet through the initial rolling pass, and then the intermediate billet is rolled into wire rod through roughing, intermediate rolling, pre-finishing rolling, finishing rolling and sizing passes. The elongation coefficient of each pass is controlled between 1.10 and 1.45.

[0040] The controlled cooling process spins the wire into coils, controlling the spinning temperature to ≥920℃. The cooling rate from the time the coil is spun to the start of the phase change is controlled to be 20~25℃ / s. The cooling rate during the phase change process is 5~15℃ / s. When the coil temperature drops below 580℃, an insulation cover is added for slow cooling. After the phase change, the cooling rate is controlled to be 0.5~1.5℃ / s, and the temperature after exiting the cover is 400~500℃.

[0041] The continuous casting process described above employs a medium-sized rectangular billet with heavy reduction. The initial reduction is 0.7–1.3 mm, allowing for initial deformation of the billet with a small reduction, preventing internal cracks caused by sudden large reductions. Subsequent reductions increase by 0.2–1.4 mm per pass, gradually increasing the reduction in the middle stage to accommodate the gradual solidification of the central mushy region, achieving continuous feeding and compositional homogenization of the molten steel. The reduction continues to gradually increase until solidification is complete, resulting in a medium-sized rectangular billet with dimensions of (172.5–180) × (240–253) mm. This reduction regime is directly related to an increased proportion of equiaxed crystal areas and the absence of central shrinkage cavities. A reasonable incremental reduction can promote the columnar crystal orientation of the medium-sized rectangular billet towards equiaxed crystals. The transformation expands the equiaxed crystal region and effectively closes the central shrinkage cavity, avoiding the formation of the central segregation zone. The one-fire forming process of smaller square billets can be achieved by progressively pressing medium-sized rectangular billets. This can prevent the formation of intermediate cracks during the straightening and pressing of the billet, and homogenize the chemical composition of the central mushy zone. It maximizes the dispersion and feeding of the central molten steel, thereby avoiding the formation of central shrinkage cavities and central compositional segregation zones from the source of the billet. This allows the carbon segregation index of the rectangular billet to be stably controlled at 0.98~1.05, providing a basis for the subsequent stable and effective control of network carbides and martensite. It also avoids the defects of needing two-fire forming or severe wear of the rolling line due to the excessive size of the rectangular billet.

[0042] Based on this, a single-fire direct rolling technology is adopted, adding a billet opening pass before the traditional roughing mill. The medium rectangular billet, after being subjected to heavy pressure, is directly rolled into an intermediate billet, which can then directly enter the subsequent rolling passes. Compared with the two-fire forming process, it does not require secondary heating, significantly reducing energy consumption and production costs. At the same time, it fully considers that the medium rectangular billet will suffer severe impact and high wear when directly entering the traditional roughing mill. Adding a billet opening pass can smoothly roll the rectangle into a smaller intermediate billet, protecting the mill units in the subsequent rolling passes. In single-fire direct rolling, the elongation coefficient of each pass is controlled between 1.10 and 1.45. This avoids excessive elongation coefficient, which would lead to excessive tensile stress in the center and generate internal microcracks, and avoids insufficient deformation and coarsening of the central structure due to excessive elongation coefficient, which would be detrimental to carbide breakage and thus affect the microstructure and properties after subsequent phase transformation.

[0043] The controlled cooling process maintains a wire drawing temperature of ≥920℃ to prevent premature precipitation of network carbides due to excessively low temperatures. A staged cooling process is employed: rapid cooling before phase transformation, controlling the cooling rate from wire drawing to the start of phase transformation at 20~25℃ / s, suitable for systems with higher carbon content and suppressing the precipitation of proeutectoid network carbides; slow cooling during phase transformation, with a cooling rate of 5~15℃ / s, ensuring uniform sorbite lamellars and preventing martensite formation. Compared to other technologies that often employ a single cooling rate, easily leading to overcooling and martensite formation in the later stages of phase transformation, slow cooling after phase transformation is used. When the wire temperature drops below 580℃, an insulation cover is added for slow cooling, controlling the post-phase transformation cooling rate at 0.5~1.5℃ / s, with an outlet temperature of 400~500℃. This fully releases phase transformation stress, shortening the natural aging period from >15 days to ≤7 days.

[0044] In summary, this invention employs a medium-sized rectangular billet combined with a progressively increasing reduction process for the cast billet. While ensuring effective improvement of center segregation in the cast billet, it utilizes a single-fire direct rolling and de-rolling technology to avoid the increased costs associated with two-fire production. Simultaneously, by optimizing the post-rolling controlled cooling process, the cooling rates before, during, and after phase transformation are precisely controlled, which both suppresses the precipitation of network carbides and avoids the formation of martensite, thereby obtaining 90-grade and 92-grade ultra-high strength cord steel wire rods with excellent deep drawing performance.

[0045] Furthermore, the internal dimensions of the copper tube in the crystallizer of the continuous casting process are a rectangular billet cross-section of 200mm×240mm. The continuous casting speed is controlled at 1.20~1.45m / min, and the superheat is controlled at 15~25℃. For example, when using integral gate casting and a medium-sized rectangular billet cross-section of 200mm×240mm, a continuous casting speed of 1.35m / min can be preferred. By controlling the superheat and using appropriate casting speed control in conjunction with heavy pressure technology, the dispersion and feeding of the central molten steel can be further promoted, and the generation of intermediate cracks can be avoided.

[0046] Furthermore, the current of the electromagnetic stirring in the crystallizer during the continuous casting process is 500~700A, and the frequency is 2~5Hz; the current of the electromagnetic stirring at the end is 400~600A, and the frequency is 8~15Hz. For example, the continuous casting process uses electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification combined with heavy pressure technology. Preferably, the current of the electromagnetic stirring in the crystallizer is 600A, and the frequency is 3Hz. The high-current, low-frequency electromagnetic stirring in the crystallizer breaks up the primary columnar crystals, promotes uniform shell growth, and reduces subcutaneous segregation. Preferably, the current of the electromagnetic stirring at the end is 500A, and the frequency is 12Hz. The high-frequency electromagnetic stirring at the end further increases the proportion of equiaxed crystals and improves central solidification feeding and segregation.

[0047] Furthermore, the secondary cooling process in the continuous casting process adopts a weak cooling process, with the specific water volume controlled at 0.30~0.38L / kg. The secondary cooling zone is the area where the billet continues to cool in the fan-shaped section after leaving the crystallizer. The specific water volume is the amount of secondary cooling water consumed per kilogram of billet. By controlling the low superheat, the weak secondary cooling process, and the appropriate casting speed, the solidification rate of the billet is further controlled, preventing intermediate cracks and improving central feeding.

[0048] Furthermore, in the continuous casting process, the billet passes through 10 sets of straightening and pressing rollers. Along the billet conveying direction, the reduction amount of each pass of the straightening and pressing rollers is as follows: 0 / 0.7~1.3 / 1.2~1.8 / 2.2~2.8 / 2.7~3.3 / 4.2~4.8 / 5.2~5.8 / 5.2~5.8 / 0 / 0 mm. Initially, a small reduction of 0.7~1.3 mm is used to avoid excessive reduction in the early stages, which could lead to internal pressing cracks and significant element segregation at the cracks, resulting in abnormal microstructure. In the middle stage, the reduction is gradually increased to 5.2~5.8 mm to avoid excessive solidification stress from a single large reduction, achieving continuous feeding and compositional homogenization of the central molten steel. In the later stage, a large reduction of 5.2~5.8 mm is maintained until solidification is complete, ultimately reaching a total reduction of 21.4~25.6 mm, which promotes complete welding of the central shrinkage cavity.

[0049] Furthermore, the rectangular billet is free of intermediate cracks, with an equiaxed crystal region area ratio of 70%~78%, a central shrinkage cavity ≤0.5 grade, and a carbon segregation index ≤1.05. For example, when inspected by the drilling method, the carbon segregation index is stably controlled at 0.98~1.05, which is suitable for the characteristics of one-fire direct rolling without reheating. The absence of intermediate cracks can avoid crack propagation and promote uniform deformation in subsequent multi-pass rolling. The high proportion of equiaxed crystal region area can reduce the anisotropy of rolling deformation and ensure density. The carbon segregation index is at a low level, which can reduce the basis for martensite formation from the source.

[0050] Furthermore, the heating process controls the preheating section temperature to be 900~980℃, the heating section temperature to be 1110~1150℃, the soaking section temperature to be 1160~1200℃, and the total heating time to be 180~240min. The rectangular billet can be heated into a rollable high-temperature steel billet through the preheating section, heating section and soaking section of the heating furnace, thereby austenitizing the structure and further alleviating segregation.

[0051] Furthermore, the heated rectangular billet is descaled by high-pressure water with a pressure ≥20MPa; this removes the iron oxide scale from the surface of the rectangular billet, preventing the iron oxide scale from being rolled into the wire rod and affecting the surface quality of the wire rod.

[0052] Furthermore, the rectangular billet in the rolling process sequentially passes through a 4-pass detachable billet mill, a 14-pass roughing and intermediate rolling mill, a 4-pass pre-finishing rolling mill, an 8-pass finishing rolling mill, and a 4-pass sizing mill. According to the rolling direction, the elongation coefficient for passes 1-14 is 1.2-1.37, for passes 15-24 it is 1.2-1.31, and for passes 25-34 it gradually decreases to below 1.03. For example, in a rolling line arrangement where high-pressure water descaling is followed by a sequential 4-pass detachable billet mill, a 14-pass roughing and intermediate rolling mill, a 4-pass pre-finishing rolling mill, an 8-pass finishing rolling mill, and a 4-pass sizing mill, totaling 34 passes, the billet is rolled into a 5.5mm wire rod. The R-Factor for each pass represents the pass elongation coefficient, i.e., the exit cross-sectional area / entry cross-sectional area of ​​the rolled piece. The passes are numbered sequentially according to the rolling direction using the number #. In this design, the elongation coefficient of the first 1-14 passes during the roughing and intermediate rolling stages is 1.2-1.37. Preferably, the rolling process rolls the rectangular billet into an intermediate billet of 160×160mm. The larger elongation coefficient enables rapid surface reduction, improves rolling efficiency while ensuring stable bite, reduces subsequent rolling load, and avoids severe impact and high wear caused by medium-sized rectangular billets directly entering the traditional roughing mill with a small design entry, resulting in large deformation and fragmented as-cast structure. In the intermediate stages, the elongation coefficient of the later roughing and intermediate rolling stages (passes 15-24), pre-finishing rolling, and initial secondary finishing rolling is stable at 1.2-1.3, ensuring uniform deformation penetration of the rolled piece. In the later stages, the elongation coefficient of the finishing rolling and sizing stages (passes 25-34) gradually decreases to below 1.03, further enabling finishing rolling and ensuring the stability of high-speed rolling.

[0053] Furthermore, the rolling process controls the initial rolling temperature to 1040~1080℃, which can reduce deformation resistance, reduce mill load and roll wear; the finishing rolling temperature is 920~960℃, which can promote grain refinement; the sizing temperature is 920~945℃, which can promote uniform deformation and precise dimensional control of the rolled piece during the sizing stage; and the wire drawing temperature is 920~940℃, which further avoids the risk of grain coarsening.

[0054] Furthermore, the rated air volume of the fan in the controlled cooling process is 300,000 m³ / h.3 / h, the operating rates of fans 1-8, from front to back, are 70%~85% / 75%~90% / 75%~90% / 65%~85% / 60%~80% / 40%~50% / 30%~40% / 30%~40% of the rated air volume, and the operating rates follow a general trend of first increasing and then decreasing, while the remaining fans are turned off; For further optimization, the coils in the controlled cooling process are conveyed along the roller conveyor, with each fan numbered # in the conveying direction. The operating rates of fans 1-3 are 75%~90% and gradually increase, while the operating rates of fans 4-8 are 30%~85% and gradually decrease. For example, if the rated air volume of the fans is 300,000 m³ / h... 3 / h, the operating rates of fans 1 to 8 are 75% / 80% / 80% / 75% / 70% / 50% / 40% / 40% respectively, and the remaining fans are turned off; the initial roller speed is 1.1m / s, and then the roller speed gradually increases to 1.4m / s, so that the wire rod is rapidly cooled after spinning to suppress the precipitation of carbon in the wire mesh. Compared with the controlled cooling process of gradually decreasing high air volume, it can avoid the local martensite formation. During the phase transformation of the wire rod, the temperature range is 620~580℃, and the cooling rate is appropriately reduced. This stage mainly occurs between fans 4 to 8. The roller length is 15.5m, which can be controlled at 5~15℃ / s to ensure uniform transformation of sorbite structure while avoiding the formation of martensite.

[0055] To further illustrate the present invention, specific embodiments are described below through examples and comparative examples. The chemical composition of the cord steel wire rods in each example and comparative example is shown in Table 1 by weight percentage, with the remainder being Fe and unavoidable impurities.

[0056] Table 1. Chemical composition of the steel cord wire rods in each embodiment and comparative example.

[0057]

[0058] The production process of the above-mentioned steel cord wire rod includes the following steps:

[0059] Step 1, Continuous Casting: Molten steel conforming to the chemical composition of cord steel wire rod in Table 1 is cast into a billet using a continuous casting machine. The continuous casting machine adopts an integral gate casting method. In Examples 1-3, Comparative Examples 3, and Comparative Examples 4, the internal dimensions of the copper tube in the crystallizer of the continuous casting machine are rectangular billets with a width × length of 200mm × 240mm. The continuous casting process adopts electromagnetic stirring in the crystallizer, electromagnetic stirring at the end of solidification, and heavy reduction technology. The secondary cooling adopts a weak cooling process. When the billet passes through 10 sets of tensioning and reducing rollers, heavy reduction is used: the initial reduction is 0.7~1.3. mm, and then the reduction amount for each pass increases by 0.2~1.4 mm, with a total reduction amount of 21.4~25.6 mm; in Comparative Example 1, the internal dimensions of the copper tube in the crystallizer of the continuous casting process are 160 mm wide × 160 mm rectangular billet cross-section, and the secondary cooling adopts a strong cooling process without heavy reduction; the difference between Comparative Example 2 and Example 1 is that the initial reduction amount is larger when the billet passes through 10 sets of tensioning and pressing rolls; a rectangular billet is obtained, and the rectangular billet is inspected at low magnification: the central crack of the rectangular billet, the area ratio of the equiaxed crystal zone, the central shrinkage cavity, and the carbon segregation index is inspected by drilling method.

[0060] The process parameters and rectangular billet inspection results for each embodiment and comparative example in the continuous casting process are shown in Table 2 below:

[0061] Table 2. Continuous casting process parameters and rectangular billet inspection results for each embodiment and comparative example of cord steel wire rod.

[0062]

[0063] Step 2, Heating: The rectangular billet obtained in Step 1 is sent into the heating furnace and passes through the preheating section, heating section and soaking section in sequence. The temperature of the preheating section is 970℃, the temperature of the heating section is 1150℃, and the temperature of the soaking section is 1190℃. The total heating time is 210 minutes to obtain a high-temperature steel billet. After heating, the high-temperature steel billet, i.e., the rectangular billet, is descaled by high-pressure water to remove the iron oxide scale on the surface. The pressure of the high-pressure water is ≥20MPa.

[0064] Step 3, Rolling: Examples 1-3 and Comparative Examples 2-4 use single-fire direct rolling. First, the rectangular billet after high-pressure water descaling is rolled into an intermediate billet with a cross-sectional size of 160×160mm through the billet opening pass. Then, the intermediate billet is rolled into wire rod with a diameter of 5.5mm through roughing and intermediate rolling, pre-finishing rolling, finishing rolling and sizing passes. The rolling line layout includes: after high-pressure water descaling, it enters the billet opening mill in sequence for 4 passes, roughing and intermediate rolling for 14 passes, pre-finishing rolling for 4 passes, finishing rolling for 8 passes and sizing for 4 passes, for a total of 34 passes. The exit sizing speed, i.e. the finished product speed, is 98m / s. Comparative Example 1 uses a single-fire direct rolling process. The rectangular billet is rolled into wire rod through roughing and intermediate rolling, pre-finishing rolling, finishing rolling, and sizing reduction passes. The rolling line arrangement includes 16 passes of roughing and intermediate rolling, 4 passes of pre-finishing rolling, 8 passes of finishing rolling, and 4 passes of sizing reduction after high-pressure water descaling, for a total of 32 passes. The exit sizing reduction speed, i.e. the finished product speed, is 98 m / s. The rectangular billet after high-pressure water descaling is rolled into wire rod with a diameter of 5.5 mm.

[0065] According to the rolling direction, the elongation coefficient R-Factor of each pass in the rolling process of Examples 1-3, Comparative Examples 2-4 and Comparative Example 1 is shown in Table 3 below. In Table 3, 1# to 34# of Examples 1-3 and Comparative Examples 2-4 are 4 passes of detached billet mill, 14 passes of roughing and intermediate rolling, 4 passes of pre-finishing rolling, 8 passes of finishing rolling and 4 passes of sizing, a total of 34 passes numbered in sequence; In Table 3, 1# and 2# of Comparative Example 1 have an elongation coefficient of 1, indicating no reduction, and 3# to 34# are 16 passes of roughing and intermediate rolling, 4 passes of pre-finishing rolling, 8 passes of finishing rolling and 4 passes of sizing, a total of 32 passes numbered in sequence.

[0066] Table 3. Elongation coefficients of each embodiment and comparative example in each pass of the rolling process.

[0067]

[0068] Step 4, Controlled Cooling: The wire obtained in Step 3 is fed into a spinning machine to be spun into coils. The coils are then conveyed along a roller conveyor to the air-cooling line. The roller conveyor speeds for each embodiment and comparative example on the air-cooling line are shown in Table 4 below:

[0069] Table 4. Roller speeds of various embodiments and comparative examples on the air-cooled line

[0070]

[0071] The rated air volume of the fan on the air-cooled line is 300,000 m³ / h. 3 / h, each fan is turned on at a percentage of its rated air volume, and the remaining fans are turned off. In Examples 1-3 and Comparative Examples 1-3, when the wire rod temperature drops below 580℃, an insulation cover is added for slow cooling. The cooling rate after phase change is the same as the cooling rate of the wire rod inside the insulation cover. In Comparative Example 4, when the wire rod temperature drops below 580℃, no insulation cover is added, and the wire rod is naturally cooled to below 200℃ after passing through the fan section. The cooling rate after phase change is about 4℃ / s. Then, natural aging is performed to finally obtain the cord steel wire rod. The controlled cooling process parameters of each example and comparative example are shown in Table 5 below:

[0072] Table 5. Cooling process parameters for each embodiment and comparative example

[0073]

[0074] The cord steel wire rods obtained in each embodiment and comparative example were tested. Microstructure was examined according to standard GB / T13298, the martensite ratio was determined according to YB / T4411, the center segregation ratio was determined according to YB / T4413, the grain boundary cementite ratio was determined according to YB / T4412, the sorbite ratio was determined according to YB / T169, and the mechanical properties were tested according to standard GB / T228.1, tensile testing of metallic materials—part 1: testing at room temperature. The test results are shown in Table 6 below.

[0075] Table 6. Test results of the steel cord wire rods in each embodiment and comparative example

[0076]

[0077] The metallographic structures of the steel cord wire rods in Examples 1-3 are shown below. Figures 1-3 As shown, the metallographic structure of the cord steel wire rod in Comparative Example 2 is as follows: Figure 4As shown, the comparison results between Examples 1-3 and Comparative Example 1 show that, compared to the small square billet one-fire forming process of Comparative Example 1, although no intermediate cracks were found after low-magnification inspection of the cast billet, the carbon segregation index of the continuously cast rectangular billet was 1.18, and the center segregation index was difficult to control below 1.10. Obvious martensite appeared in the center of the wire rod, with network carbides at grade 2.5, resulting in severe wire breakage during drawing. The continuous casting process of this invention uses a medium-sized rectangular billet with a width × length of (172.5~180) × (240~253) mm under heavy pressure. It can be further combined with electromagnetic stirring in the crystallizer and electromagnetic stirring at the end. Through low superheat control, secondary cooling weak cooling process and appropriate drawing speed control, the pressure of billet straightening can be prevented. The process generates intermediate cracks and homogenizes the chemical composition of the central paste-like region, maximizing the dispersion and feeding of the central molten steel. This avoids the formation of central shrinkage cavities and central component segregation zones, reducing the foundation for martensite formation from the source. While ensuring effective improvement of central segregation in the billet, a single-fire direct rolling and de-rolling technology is adopted to avoid the increased costs associated with two-fire production. During single-fire direct rolling, the elongation coefficient of each pass is controlled between 1.10 and 1.45. This avoids excessive elongation coefficient leading to excessive central tensile stress and internal microcracks, and also avoids insufficient deformation and coarsening of the central structure due to insufficient elongation coefficient, which is not conducive to carbide breakage and thus affects the microstructure and properties after subsequent phase transformation.

[0078] The comparison results between Examples 1-3 and Comparative Example 2 show that, compared to Comparative Example 2, the medium rectangular billet under heavy pressing had excessive early-stage pressing, with the first pass pressing down by 3.3 mm leading to internal pressing cracks. Figure 5 As shown in Comparative Example 2, EPMA scanning of the cord steel reveals significant elemental segregation at the crack, leading to abnormal microstructure. The initial reduction of 0.7–1.3 mm allows for initial billet deformation with a small reduction, preventing internal cracks caused by sudden large reductions. Subsequent reductions increase by 0.2–1.4 mm per pass, gradually increasing the reduction in the middle stage to accommodate the gradual solidification of the central mushy zone, achieving continuous feeding and compositional homogenization of the central molten steel. Further gradual increases in reduction until solidification complete allow for stable control of the carbon segregation index of the rectangular billet at 0.98–1.05, providing a foundation for the stable and effective control of subsequent network carbides and martensite. This also avoids defects such as the need for two-pass finishing or severe wear on the rolling line due to excessively large rectangular billet dimensions.

[0079] Metallographic diagram of the steel cord wire rod in Comparative Example 3 is shown below. Figure 6As shown in the comparison results between Examples 1-3 and Comparative Example 3, compared with Comparative Example 3, the excessively rapid cooling rate during the phase transformation resulted in the formation of martensite in the center of the wire rod. The martensite in the center of the wire rod accounted for approximately 1.5% of the area, had a high hardness, a reduction in area of ​​17%, and caused brittle fracture, significantly increasing the early wire breakage rate during drawing. In this invention, rapid cooling before phase transformation, controlling the cooling rate from wire drawing to the start of phase transformation to 20-25°C / s, is applicable to systems with higher carbon content and suppresses the precipitation of proeutectoid network carbides. Slow cooling during phase transformation, with a cooling rate of 5-15°C / s during the phase transformation process, ensures uniform sorbite lamellars and avoids martensite formation.

[0080] Metallographic diagram of the steel cord wire rod in Comparative Example 4 is shown below. Figure 7 As shown in the comparison results between Examples 1-3 and Comparative Example 4, compared to Comparative Example 4 which did not have an insulation cover, the wire rod naturally cooled to below 200°C after passing through the fan section. The cooling rate after phase transformation was approximately 4°C / s. The wire rod lacked martensite, but had high residual stress, a natural aging period of >15 days, and a high wire breakage rate during the initial use (first 7 days). This invention employs slow cooling after phase transformation. When the wire rod temperature drops below 580°C, an insulation cover is added for slow cooling, and the cooling rate after phase transformation is controlled at 0.5°C / s. At a speed of 1.5℃ / s and an outlet temperature of 400~500℃, phase transformation stress is fully released, shortening the natural aging period from >15 days to ≤7 days. By optimizing the post-rolling controlled cooling process, the cooling rate before, during, and after phase transformation is precisely controlled, which inhibits the precipitation of network carbides and avoids the formation of martensite. Ultimately, the desired results are achieved: no martensite, grain boundary cementite ≤1.0 grade, sorbite ratio ≥94%, tensile strength ≥1200MPa, and reduction of area ≥45%.

[0081] In summary, the medium-sized rectangular billet under heavy pressure, single-fire direct rolling, and staged controlled cooling technology of this invention has significant improvements and advantages in segregation control accuracy, martensite suppression capability, production cost control, and compatibility with 90-grade and 92-grade cord steel products. It can effectively improve billet segregation, avoid the increase in production costs caused by second-fired processing, suppress the formation of abnormalities such as network carbides and martensite structure, and shorten the natural aging period, so that 90-grade and 92-grade cord steel wire rods have good product quality and excellent deep drawing processing performance.

[0082] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. For example, molten iron pretreatment, converter smelting, and LF refining processes can be used to obtain molten steel for continuous casting. 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 production process for high-plasticity, ultra-high-strength steel cord wire rod, characterized in that, The chemical composition of the aforementioned steel cord wire rod, by mass percentage, includes: C: 0.90%~0.94%, Si: 0.17%~0.30%, Mn: 0.30%~0.48%, Cr: 0.17%~0.50%, P≤0.012%, S≤0.008%, Ni≤0.05%, Cu≤0.05%, Al≤0.0030%, Ti≤0.0020%, O≤0.0020%, N≤0.0045%, with the remainder being Fe and unavoidable impurity elements; its process includes sequential continuous casting, heating, rolling, and controlled cooling steps. When the billet in the continuous casting process passes through the straightening and pressing rollers, it is subjected to heavy pressing: the initial pressing amount is 0.7~1.3 mm, and the pressing amount of each subsequent pass increases by 0.2~1.4 mm, with a total pressing amount of 21.4~25.6 mm, resulting in a rectangular billet with a width × length of (172.5~180) × (240~253) mm; The rolling process adopts a single-fire direct rolling: first, the rectangular billet is rolled into an intermediate billet through 4 opening passes, and then the intermediate billet is rolled into wire rod through 14 roughing and intermediate rolling passes, 4 pre-finishing rolling passes, 8 finishing rolling passes, and 4 sizing reduction passes. According to the rolling direction, the elongation coefficient of the first to the 14th passes is 1.2 to 1.37, the elongation coefficient of the 15th to the 24th passes is 1.2 to 1.31, and the elongation coefficient of the 25th to the 34th passes gradually decreases to below 1.

03. The controlled cooling process spins the wire into coils, controlling the spinning temperature to ≥920℃. The cooling rate from the time the coil is spun to the start of the phase change is controlled to be 20~25℃ / s. The cooling rate during the phase change process is 5~15℃ / s. When the coil temperature drops below 580℃, an insulation cover is added for slow cooling. After the phase change, the cooling rate is controlled to be 0.5~1.5℃ / s, and the temperature after exiting the cover is 400~500℃.

2. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The internal dimensions of the copper tube in the crystallizer of the continuous casting process are a rectangular billet cross-section of 200mm×240mm. The continuous casting speed is controlled at 1.20~1.45m / min, and the superheat is controlled at 15~25℃.

3. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The current of the electromagnetic stirring in the crystallizer of the continuous casting process is 500~700A and the frequency is 2~5Hz; the current of the electromagnetic stirring at the end is 400~600A and the frequency is 8~15Hz; the secondary cooling adopts a weak cooling process, and the specific water volume is controlled at 0.30~0.38L / kg.

4. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The billet in the continuous casting process passes through 10 sets of tensioning and pressing rollers. Along the billet conveying direction, the pressing amount of each tensioning and pressing roller is as follows: 0 / 0.7~1.3 / 1.2~1.8 / 2.2~2.8 / 2.7~3.3 / 4.2~4.8 / 5.2~5.8 / 5.2~5.8 / 0 / 0 mm. The rectangular billet has no intermediate cracks, the area ratio of equiaxed crystal regions is 70%~78%, the central shrinkage cavity is ≤0.5 grade, and the carbon segregation index is ≤1.

05.

5. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The heating process controls the preheating section temperature to be 900~980℃, the heating section temperature to be 1110~1150℃, the soaking section temperature to be 1160~1200℃, and the total heating time to be 180~240min.

6. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The heated rectangular billet is descaled by high-pressure water with a pressure ≥20MPa.

7. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The rolling process controls the initial rolling temperature to be 1040~1080℃, the finishing rolling temperature to be 920~960℃, and the sizing temperature to be 920~945℃; the wire drawing temperature is 920~940℃.

8. The production process of high-plasticity ultra-high-strength cord steel wire rod according to claim 1, characterized in that, The rated air volume of the fan in the controlled cooling process is 300,000 m³ / h. 3 / h, the opening volume of fans 1 to 8 is 70%~85% / 75%~90% / 75%~90% / 65%~85% / 60%~80% / 40%~50% / 30%~40% / 30%~40% respectively from front to back, and the opening volume follows the general trend of first increasing and then decreasing, while the remaining fans are turned off.

9. A high-plasticity, ultra-high-strength cord steel wire rod, characterized in that, The cord steel wire rod is produced by the production process of high plasticity ultra-high strength cord steel wire rod as described in any one of claims 1 to 8.

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