Method for reducing the overall strength fluctuation of salt bath heat treated wire rod and application thereof

Abstract

This invention relates to the field of heat treatment technology for metallic materials, specifically to a method and application for reducing the overall strength fluctuation of wire rods subjected to salt bath heat treatment. The method includes: first, austenitizing heating of the wire rod, with 5-7 temperature zones along the direction of movement, the temperature first rising from 895-905℃ to 955-965℃, then decreasing to 915-925℃, for a total time of 9-12 minutes; then, salt bath treatment, dividing the salt bath tank into edge and middle zones along the perpendicular direction of movement, setting the traction speed according to the zones, wherein molten salt pumps are installed on both sides of the salt bath tank, operating at a frequency of 45-50Hz and a flow rate of 90-100 m³ / s. 3 The liquid level is 32-35 cm deep, and the traction speed at the edge is 3.5-5.2 m / min, which is greater than that in the middle region (3.0-4.7 m / min). This invention effectively reduces strength fluctuations when multiple wire rods are heat-treated simultaneously through the synergistic effect of austenitizing heating and zoned speed control.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, specifically to a method and application for reducing the overall strength fluctuation of wire rods subjected to salt bath heat treatment. Background Technology

[0002] As the core load-bearing component of long-span bridges, bridge cables directly determine the stability and lifespan of the bridge. With the development of bridges towards longer spans, requirements for cable wire rods have increased, demanding ultra-high strength, high torsional performance, and fracture resistance. Therefore, extremely high requirements are placed on the strength and microstructure uniformity of the steel wire raw materials. Salt bath heat treatment, as a process for precisely controlling the microstructure and properties of wire, is widely used in the production of high-strength bridge cable wire rods.

[0003] However, in actual production, multiple wire rods are often heat-treated simultaneously in the same salt bath, leading to fluctuations in tensile strength among the rods and making it difficult to guarantee consistent product performance. Existing technologies primarily optimize process parameters for single wire rods, but have not yet provided an effective solution to the strength fluctuation problem when multiple wire rods are heat-treated simultaneously. Therefore, how to reduce strength fluctuations and improve product performance stability during simultaneous heat treatment of multiple wire rods is a pressing technical problem in this field. Summary of the Invention

[0004] This invention provides a method and application for reducing the overall strength fluctuation of wire rods subjected to salt bath heat treatment, in order to solve the problem of large fluctuations in tensile strength and insufficient product stability among multiple wire rods when they are simultaneously heat-treated in the same salt bath.

[0005] In a first aspect, the present invention provides a method for reducing strength fluctuations in wire rods subjected to salt bath heat treatment, comprising: Austenitizing heating steps: The wire rod is continuously heated in the heating furnace. Along the direction of wire rod movement, there are 5-7 temperature zones set in the heating furnace. The temperature first rises in stages from 895℃~905℃ to 955℃~965℃, and then drops in stages from 955℃~965℃ to 915℃~925℃. The total heating time of the wire rod is 9~12 minutes. Salt bath treatment steps: The austenitized and heated wire rods are sent into a salt bath tank for salt bath treatment, wherein: The salt bath tank is divided into at least a side region and a middle region in a horizontal plane along a direction perpendicular to the movement direction of the wire rod. The side region corresponds to the two sides of the salt bath tank, and the middle region corresponds to the middle of the salt bath tank. The traction speed of the wire rod is set according to the area of ​​the wire rod in the salt bath tank; The wire rod traction speed in the edge region is greater than that in the middle region; the wire rod traction speed in the edge region is 3.5~5.2m / min, and the wire rod traction speed in the middle region is 3.0~4.7m / min. The salt bath tank is equipped with molten salt pumps on both sides, with an operating frequency of 45-50Hz and a flow rate of 90-100m³ / h. 3 / h, the liquid level depth in the salt bath is 32~35cm.

[0006] In one optional embodiment, the salt bath is divided into three equal regions in the horizontal plane along the direction perpendicular to the movement of the wire rod, namely the first side region, the middle region, and the second side region. And / or, two to four molten salt pumps are provided on each side of the salt bath tank.

[0007] In one optional embodiment, the parameters of the salt bath treatment are set according to the wire rod diameter as follows: When the wire rod diameter is 11mm~12.5mm, the salt bath temperature is 540~560℃, the number of wire rods processed simultaneously in each zone is 5~6, the wire rod traction speed in the edge zone is 4.8~5.2m / min, and the wire rod traction speed in the middle zone is 4.3~4.7m / min; Alternatively, when the wire rod diameter is 13mm~14mm, the salt bath temperature is 520~540℃, the number of wire rods processed simultaneously in each zone is 4~5, the wire rod traction speed in the edge zone is 4.1~4.5m / min, and the wire rod traction speed in the middle zone is 3.6~4.0m / min; Alternatively, when the wire rod diameter is 15mm~16mm, the salt bath temperature is 500~520℃, the number of wire rods processed simultaneously in each zone is 2~3, the wire rod traction speed in the edge zone is 3.5~3.9m / min, and the wire rod traction speed in the middle zone is 3.0~3.4m / min.

[0008] In one optional embodiment, the salt bath treatment uses a mixed molten salt composed of potassium nitrate and sodium nitrate as the quenching medium; optionally, the potassium nitrate and sodium nitrate in the mixed molten salt are mixed in a weight ratio of (1~1.5):(1~1.5).

[0009] In one optional embodiment, the kinematic viscosity of the mixed molten salt is 20-25 mm. 2 / s, with a dynamic viscosity of 0.04~0.05 Pa·s.

[0010] In one optional embodiment, the heating furnace is provided with 7 temperature zones in sequence, and the temperature control range of each temperature zone is as follows: temperature zone 1 895℃~905℃, temperature zone 2 935℃~945℃, temperature zone 3 955℃~965℃, temperature zone 4 955℃~965℃, temperature zone 5 955℃~965℃, temperature zone 6 935℃~945℃, and temperature zone 7 915℃~925℃, wherein the heating time of the wire rod from temperature zone 3 to temperature zone 5 is 4~6 minutes.

[0011] In one optional embodiment, the austenitizing heating step employs a natural gas self-preheating pulse burner; optionally, the burners are installed on the upper and lower parts of both sides of the heating furnace cavity.

[0012] In one optional embodiment, before the austenitizing heating step, the wire rod is further subjected to shot blasting; optionally, the shot blasting time is 5 to 8 minutes; and the wire is fed out using a horizontal wire feeding unit and straightened by a straightening machine.

[0013] In one alternative embodiment, the salt bath treatment further includes surface cleaning, which includes air blowing and water washing.

[0014] In one optional embodiment, the air-blowing nozzles are distributed in one row above and one row below the wire rod, 6-10 mm away from the wire rod, with a flow rate of 10-15 L / min for each nozzle, and the blowing direction is opposite to the direction of wire rod movement, forming an angle of 130-140°. And / or, the temperature of the water washing treatment is 40~45℃ and the water pressure is 10~15 bar.

[0015] In one optional embodiment, the air blowing treatment is performed 1 to 3 times; And / or, the water washing process is performed 1 to 3 times.

[0016] In one alternative embodiment, the wire rod is a high-carbon steel wire rod for bridge cables, and optionally, the wire rod grade includes 92Si.

[0017] In one optional embodiment, the salt bath tank has a cuboid structure, with its length direction aligned with the wire rod's movement direction and its width direction perpendicular to the wire rod's movement direction. Optionally, the salt bath tank has a depth of 40-45 cm, a width of 2.0-2.5 m, and a length of 10-11 m.

[0018] Secondly, the present invention also provides the application of the above-mentioned method for reducing the strength fluctuation of wire rods subjected to salt bath heat treatment in the production of high-strength bridge cable wire rods.

[0019] The technical solution of this invention has the following advantages: 1. The method for reducing the overall strength fluctuation of wire rods after salt bath heat treatment provided by this invention firstly involves specific multi-temperature zone gradient austenitizing heating (setting 5-7 temperature zones along the wire rod movement direction, with the temperature first rising in stages to 955℃-965℃, then decreasing in stages to 915℃-925℃, for a total time of 9-12 minutes) to obtain austenitic structure with uniform composition and appropriate grain size in the wire rod, allowing the wire rod to enter the salt bath in a uniform austenitic state; then, by dividing the salt bath into zones along the wire rod arrangement direction and setting differentiated traction speeds, the traction speed of the wire rod in the edge zone is greater than that in the middle zone, while simultaneously coordinating the operation frequency of the molten salt pumps on both sides of the salt bath at 45-50Hz and the flow rate at 90-100m³ / h. 3 The process conditions of 32-35 cm liquid level per hour directly compensate for the uneven flow field caused by the molten salt pumps on both sides of the salt bath, resulting in a faster flow velocity at the edges and a slower flow velocity in the middle. This invention, through the synergistic effect of austenitizing heating for microstructure homogenization pretreatment and the flow field compensation mechanism of salt bath zone velocity control, can control the strength fluctuation of multiple wire rods during simultaneous heat treatment within 30 MPa without structural modifications to existing equipment, significantly improving the consistency of product performance.

[0020] 2. This invention provides matching salt bath temperature, number of coils to be processed, and traction speed parameters according to different coil diameters, which can provide appropriate cooling intensity for coils with different heat capacities, thereby further reducing the strength fluctuation of coils of different specifications.

[0021] 3. This invention uses potassium nitrate and sodium nitrate mixed at a weight ratio of (1~1.5):(1~1.5), which can maintain the kinematic viscosity of the mixed molten salt at 20~25 mm. 2 Within a range of / s, this ratio helps improve the fluidity of molten salt and promotes temperature uniformity in the salt bath. At the same time, this ratio can provide a wide adjustable temperature range, making it easy to adapt to the heat treatment requirements of different intensity levels.

[0022] 4. The present invention performs shot blasting before austenitizing heating, which can remove the oxide scale on the surface of the wire rod, helping to avoid oxide scale contamination of the molten salt or affecting the heating uniformity; after salt bath treatment, air blowing and water washing can control the residual salt content on the surface to a low level (below 0.26 kg / t), which is beneficial to improve the rust resistance of the wire rod during storage, maintain the stability of the wire rod surface, and thus maintain the overall stability of the wire rod, providing better surface quality and superior performance for subsequent drawing and galvanizing processes. Attached Figure Description

[0023] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0024] Figure 1 This is a SEM image depicting the microstructure of wire rod after salt bath heat treatment according to Example 1 of the present invention; Figure 2 The image shows the surface condition of wire rod treated in Example 13 with a surface residual salt content > 0.26 kg / t after being left for 1 month. Figure 3 The image shows the surface condition of wire rods treated in Example 13 with a surface residual salt content > 0.26 kg / t after being left for 3 months. Figure 4 This is a surface condition diagram of the wire rod treated in Example 1 after one month of storage; Figure 5 This is a surface condition diagram of the wire rod treated in Example 1 after being stored for 3 months; Figure 6 This is the SEM image of the processed wire rod 2 in Experiment Example 2; Figure 7 This is a schematic diagram showing the angle formed between the purging direction and the wire rod movement direction in an embodiment of the present invention. Detailed Implementation

[0025] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0026] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0027] The salt bath used in this embodiment of the invention has the following specifications: depth: 40-45cm, width: 2.0-2.5m, length: 10-11m. In practice, any salt bath within these specifications will meet the requirements of this invention.

[0028] Example 1 92Si hot-rolled wire rod with a diameter of 12.5 mm is subjected to salt bath heat treatment through the following steps: 1) Shot blast the wire rod for 6 minutes. The surface cleanliness of the wire rod after shot blasting is Sa2.5. 2) The shot-blasted wire rods are fed into a horizontal wire feeding unit and straightened by a straightening machine before being heated in the next step; 3) Austenitizing heating is achieved using natural gas self-preheating pulse burners, which are installed on the upper and lower sides of the furnace chamber. The furnace is divided into 7 temperature control zones, with heating temperatures of 900℃ for zone 1, 940℃ for zone 2, 960℃ for zone 3, 960℃ for zone 4, 960℃ for zone 5, 940℃ for zone 6, and 920℃ for zone 7. The temperature control accuracy for each zone is ±1℃. The heating time for the wire rod in zones 3-5 is 5 minutes, with a total heating time of 9 minutes. 4) A molten salt mixture of potassium nitrate and sodium nitrate in a 1:1 weight ratio was used as the quenching medium. The kinematic viscosity of the molten salt mixture was 23 mm. 2 The dynamic viscosity is 0.045 Pa·s. The wire rod exiting the heating furnace is subjected to heat treatment. Three salt pumps are evenly distributed on each side of the salt bath. During heat treatment, all salt pumps are turned on simultaneously, pumping salt solution into the salt bath. The salt pump flow rate is 92 m³ / s. 3 The operating frequency is 46Hz, and the liquid level is 34cm. The salt pumps are located on both sides of the salt bath. The flow velocity in the salt bath is higher at the positions closer to the salt pumps (both sides) than at the positions further away from the salt pumps (middle). The salt bath is divided into three equal parts in the direction of the vertical wire rod movement: the first side region, the middle region, and the second side region. Each region has 5 wire rods. The salt bath temperature is 558℃. The traction speed in the first and second side regions is 5.0m / min, and the traction speed in the middle region is 4.6m / min. 5) Perform surface air blowing and water washing on the wire rods exiting the salt bath. The blowing nozzles are distributed in one row above and one row below the wire rod, 8mm from the wire rod, with a single nozzle flow rate of 12L / min. Blow in the opposite direction, with the blowing direction at an angle to the wire rod's movement direction (see...). Figure 7 The salt on the wire rod was blown back into the salt bath at a 130° angle. The water washing temperature was 42℃ and the water pressure was 13 bar. After washing, the residual salt content on the wire rod surface was 0.21 kg / t. 6) Take in the heat-treated steel wire.

[0029] Results: The maximum tensile strength of the 15 wire rods subjected to simultaneous heat treatment was 1546 MPa, the minimum was 1525 MPa, and the range was 21 MPa; the sorbite content was 96%; there was no martensite or network carbides in the core (please provide detailed test methods). See Table 1 for details.

[0030] Tensile strength test: The tensile strength was tested according to GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature"; Sorbite content test: Sorbite content was tested according to YB / T169-2014 "Metallographic Test Method for Sorbite Content of High Carbon Steel Wire Rod".

[0031] Martensite and network carbides were rated according to YB / T-4411-2014 "Evaluation Method for Central Martensite in High Carbon Steel Wire Rod" and YB / T-4412-2014 "Evaluation Method for Network Cementite in High Carbon Steel Wire Rod", respectively.

[0032] Example 2 92Si hot-rolled wire rod with a diameter of 13mm undergoes salt bath heat treatment through the following steps: Example 2 follows the same steps as Example 1, but the process parameters are adjusted. The differences are as follows: Shot blasting for 8 minutes (Sa=3 grade); Austenitizing heating: The heating furnace is divided into 7 temperature control zones. The heating temperatures of each temperature control zone are as follows: Zone 1 905℃ / Zone 2 945℃ / Zone 3 965℃ / Zone 4 965℃ / Zone 5 965℃ / Zone 6 935℃ / Zone 7 915℃. The temperature control accuracy of each temperature control zone is ±1℃. The heating time for Zones 3-5 is 6 minutes, and the total heating time is 12 minutes. Salt bath: potassium nitrate to sodium nitrate weight ratio is 1:1.5, temperature is 538℃, 5 rods per zone, first edge zone / second edge zone 4.2m / min, middle zone 3.8m / min, salt pump flow rate 98m³ / h, operating frequency 47Hz, liquid surface depth 33cm; Air blowing and cleaning: The blowing nozzle is 9mm away from the wire rod, the flow rate of a single nozzle is 15L / min, and the angle between the blowing direction and the wire rod movement direction is 135°. The water washing temperature is 44℃ and the water pressure is 14bar. The residual salt content on the wire rod surface after cleaning is 0.17kg / t. Results: The tensile strength range of the 15 wire rods that underwent simultaneous heat treatment was 25 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0033] Example 3 92Si hot-rolled wire rod with a diameter of 16mm undergoes salt bath heat treatment through the following steps: Example 3 follows the same steps as Example 1, but the process parameters are adjusted. The differences are as follows: Shot blasting for 8 minutes (Sa=3 grade); Austenitizing heating: 6 min in temperature control zones 3-5, total 11 min; Salt bath: potassium nitrate to sodium nitrate weight ratio is 1.5:1, temperature is 503℃, 2 rods per zone, first edge zone / second edge zone 3.6m / min, middle zone 3.0m / min, salt pump flow rate 100m³ / h, operating frequency 49Hz, liquid level 35cm; Air blowing and cleaning: The blowing nozzle is 7mm away from the wire rod, the flow rate of a single nozzle is 15L / min, the angle between the blowing direction and the wire rod movement direction is 140°, the water washing temperature is 41℃, and the water pressure is 15bar. The residual salt content on the wire rod surface after cleaning is 0.15kg / t; Results: The tensile strength range of the six wire rods subjected to simultaneous heat treatment was 27 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0034] Example 4 92Si hot-rolled wire rod with a diameter of 12mm undergoes salt bath heat treatment through the following steps: Example 4 follows the same steps as Example 1, but the process parameters are adjusted. The differences are as follows: Shot blasting for 5 minutes (Sa=2.5 grade); Austenitizing heating: 4 min in temperature control zones 3-5, total 12 min; Salt bath: potassium nitrate to sodium nitrate weight ratio is 1.2:1, temperature is 560℃, 6 rods per zone, first edge zone / second edge zone 5.2m / min, middle zone 4.7m / min, salt pump flow rate 100m³ / h, operating frequency 50Hz, liquid level 35cm; Air blowing and cleaning: The blowing nozzle is 8mm away from the wire rod, the flow rate of a single nozzle is 14L / min, and the angle between the blowing direction and the wire rod movement direction is 133°. The water washing temperature is 43℃ ​​and the water pressure is 15bar. The residual salt content on the wire rod surface after cleaning is 0.18kg / t. Results: The tensile strength range of the 18 wire rods that underwent simultaneous heat treatment was 25 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0035] Example 5 92Si hot-rolled wire rod with a diameter of 11mm is subjected to salt bath heat treatment through the following steps: Example 5 follows the same steps as Example 1, but the process parameters are adjusted. The differences are as follows: Shot blasting for 7 minutes (Sa=3 grade); Austenitizing heating: 5 min in temperature control zones 3-5, total 12 min; Salt bath: potassium nitrate to sodium nitrate weight ratio is 1:1.3, temperature is 540℃, 6 rods per zone, first edge zone / second edge zone 4.8m / min, middle zone 4.3m / min, salt pump flow rate 90m³ / h, operating frequency 45Hz, liquid level 32cm; Air blowing and cleaning: The blowing nozzle is 6mm away from the wire rod, the flow rate of a single nozzle is 10L / t, and the angle between the blowing direction and the wire rod movement direction is 138°. The water washing temperature is 40℃ and the water pressure is 10bar. The residual salt content on the wire rod surface after cleaning is 0.15kg / t. Results: The tensile strength range of the 18 wire rods that underwent simultaneous heat treatment was 25 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0036] Example 6 92Si hot-rolled wire rod with a diameter of 13.5 mm is subjected to salt bath heat treatment through the following steps: Example 6 follows the same steps as Example 1, but the process parameters are adjusted. The differences are as follows: Shot blasting for 7 minutes (Sa=3 grade); Austenitizing heating: 6 min in temperature control zones 3-5, total 11 min; Salt bath: potassium nitrate to sodium nitrate weight ratio is 1.4:1, temperature is 540℃, 5 rods per zone, first edge zone / second edge zone 4.5m / min, middle zone 4.0m / min, salt pump flow rate 100m³ / h, operating frequency 50Hz, liquid level 35cm; Air blowing and cleaning: The blowing nozzle is 10mm away from the wire rod, the flow rate of a single nozzle is 12L / min, and the angle between the blowing direction and the wire rod movement direction is 136°. The water washing temperature is 45℃ and the water pressure is 10bar. The residual salt content on the wire rod surface after cleaning is 0.16kg / t. Results: The tensile strength range of the 15 wire rods that underwent simultaneous heat treatment was 23 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0037] Example 7 92Si hot-rolled wire rod with a diameter of 14mm undergoes salt bath heat treatment through the following steps: Example 7 follows the same steps as Example 1, but the process parameters are adjusted as follows: Shot blasting for 7 minutes (Sa=2.8 grade); Austenitizing heating: The heating furnace is divided into 7 temperature control zones. The heating temperatures of each temperature control zone are as follows: Zone 1 895℃ / Zone 2 935℃ / Zone 3 955℃ / Zone 4 955℃ / Zone 5 955℃ / Zone 6 945℃ / Zone 7 925℃. The temperature control accuracy of each temperature control zone is ±1℃. The heating time for Zones 3-5 is 5 minutes, and the total heating time is 9 minutes. Salt bath: potassium nitrate to sodium nitrate weight ratio is 1.1:1, temperature is 538℃, 4 rods per zone, first edge zone / second edge zone 4.1m / min, middle zone 3.6m / min, salt pump flow rate 95m³ / h, operating frequency 50Hz, liquid level 34cm; Air blowing and cleaning: The blowing nozzle is 6mm away from the wire rod, the flow rate of a single nozzle is 11L / min, and the angle between the blowing direction and the wire rod movement direction is 134°. The water washing temperature is 43℃ ​​and the water pressure is 13bar. The residual salt content on the wire rod surface after cleaning is 0.16kg / t. Results: The tensile strength range of the 12 wire rods that underwent simultaneous heat treatment was 24 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0038] Example 8 92Si hot-rolled wire rod with a diameter of 15mm undergoes salt bath heat treatment through the following steps: Example 8 follows the same steps as Example 1, but the process parameters are adjusted. The differences are as follows: Shot blasting for 7 minutes (Sa=3 grade); Austenitizing heating: 6 minutes in temperature control zones 3-5, total 10 minutes; Salt bath: Potassium nitrate to sodium nitrate weight ratio 1:1.3, temperature 520℃, 3 rods per zone, flow rate 3.9 m / min in the first / second edge zone, 3.4 m / min in the middle zone, salt pump flow rate 97 m³ / min. 3 / h, operating frequency 49Hz, liquid level 33cm; Air blowing and cleaning: The blowing nozzle is 10mm away from the wire rod, the flow rate of a single nozzle is 12L / min, and the angle between the blowing direction and the wire rod movement direction is 137°. The water washing temperature is 40℃ and the water pressure is 13bar. The residual salt content on the wire rod surface after cleaning is 0.15kg / t. Results: The tensile strength range of the nine wire rods subjected to simultaneous heat treatment was 25 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0039] Example 9 92Si hot-rolled wire rod with a diameter of 15.5 mm is subjected to salt bath heat treatment through the following steps: Example 9 follows the same steps as Example 1, but the process parameters are adjusted as follows: Shot blasting for 6 minutes (Sa=2.5 grade); Austenitizing heating: 7 min in temperature control zones 3-5, total 12 min; Salt bath: potassium nitrate to sodium nitrate weight ratio is 1:1, temperature is 500℃, 2 rods per zone, first edge zone / second edge zone 3.5m / min, middle zone 3.0m / min, salt pump flow rate 99m³ / h, operating frequency 48Hz, liquid level 33cm; Air blowing and cleaning: The blowing nozzle is 6mm away from the wire rod, the flow rate of a single nozzle is 10L / min, and the angle between the blowing direction and the wire rod movement direction is 139°. The water washing temperature is 43℃ ​​and the water pressure is 12bar. The residual salt content on the wire rod surface after cleaning is 0.17kg / t. Results: The tensile strength range of the six wire rods subjected to simultaneous heat treatment was 26 MPa, and no martensite or network carbides were found in the core. See Table 1 for details.

[0040] Example 10 Each zone processes 7 wire rods, and the rest is the same as in Example 1.

[0041] Results: The tensile strength range of the 21 wire rods that underwent simultaneous heat treatment was 30 MPa. No martensite or network carbides were found in the core. The residual salt content on the surface of the wire rods after cleaning was 0.20 kg / t. See Table 1 for details.

[0042] Example 11 Each zone processes 6 wire rods, and the rest is the same as in Example 2.

[0043] Results: The tensile strength range of the 18 wire rods subjected to simultaneous heat treatment was 29 MPa. No martensite or network carbides were found in the core. The residual salt content on the surface of the wire rods after cleaning was 0.18 kg / t. See Table 1 for details.

[0044] Example 12 Each zone processes 4 wire rods, and the rest is the same as in Example 3.

[0045] Results: The tensile strength range of the 12 wire rods that underwent simultaneous heat treatment was 28 MPa. No martensite or network carbides were found in the core. The residual salt content on the surface of the wire rods after cleaning was 0.19 kg / t. See Table 1 for details.

[0046] Example 13 Example 13 is the same as Example 1, except that the steps of air blowing and water washing after salt bath treatment are omitted.

[0047] Results: The tensile strength range of the five wire rods heat-treated simultaneously in each zone was 28 MPa. No martensite or network carbides were found in the core, and the residual salt content on the wire rod surface was 0.91 kg / t. See Table 1 for details.

[0048] Example 14 The austenitizing heating process for wire rod is divided into 5 zones, and the rest is the same as in Example 1.

[0049] The specific zones are as follows: The heating furnace is divided into 5 temperature control zones. The heating temperatures of each zone are 900℃ for zone 1, 935℃ for zone 2, 960℃ for zone 3, 945℃ for zone 4, and 925℃ for zone 5. The temperature control accuracy of each zone is ±1℃. The heating time of the wire rod in zone 3 is 6 minutes, and the total heating time is 11 minutes. Results: The tensile strength range of the 15 wire rods subjected to simultaneous heat treatment was 27 MPa. No martensite or network carbides were found in the core. The residual salt content on the surface of the wire rods after cleaning was 0.21 kg / t. See Table 1 for details.

[0050] Example 15 In the step of heating the wire rod to austenitize it, the temperatures of the six temperature control zones are different, but the rest is the same as in Example 1.

[0051] The specific zones are as follows: The heating furnace is divided into 6 temperature control zones. The heating temperatures of each zone are 900℃ for zone 1, 935℃ for zone 2, 960℃ for zone 3, 960℃ for zone 4, 935℃ for zone 5, and 920℃ for zone 6. The temperature control accuracy of each zone is ±1℃. The heating time for the wire rod in zone 3-4 is 4 minutes, and the total heating time is 9 minutes. Results: The tensile strength range of the 15 wire rods subjected to simultaneous heat treatment was 26 MPa. No martensite or network carbides were found in the core. The residual salt content on the surface of the wire rods after cleaning was 0.22 kg / t. See Table 1 for details.

[0052] Comparative Example 1 Comparative Example 1 is identical to Example 1 in that it is not divided into sections and has the same traction speed, and everything else is the same: Comparative Example 1: All traction speeds during the salt bath were 4.8 m / min (regardless of whether it was the edge or the middle area). Results: The tensile strength range of the 15 wire rods subjected to simultaneous heat treatment was 83 MPa (1579-1496 MPa). The core of the 5 wire rods in the middle region contained grade 2.5 martensite, and the residual salt content on the wire rod surface after cleaning was 0.21 kg / t. This is because the phase transformation time was insufficient in the middle region due to the higher heat rate, preventing the core from forming sorbite and causing it to transform into martensite. See Table 1 for details.

[0053] Comparative Example 2 Compared with Example 1, Comparative Example 2 had different salt pump parameters, but all other parameters were the same.

[0054] The parameters of the salt pump in Comparative Example 2 are: Salt pump flow rate 60m 3 / h, operating frequency 35Hz, liquid level 30cm; Results: The tensile strength range of the 15 wire rods that underwent simultaneous heat treatment was 53 MPa (1510-1457 MPa). No martensite or network carbides were found in the core. The residual salt content on the surface of the wire rods after cleaning was 0.22 kg / t. See Table 1 for details.

[0055] Comparative Example 3 Compared with Example 1, Comparative Example 3 did not use multi-temperature zone austenitizing heating before salt bath treatment. The heating temperature at all locations was the same, which was 960°C, and everything else was the same.

[0056] Results: The tensile strength of the four wire rods that were heat-treated at the same time had a range of 85 MPa, and the core contained grade 3 martensite. The residual salt content on the surface of the wire rods after cleaning was 0.23 kg / t.

[0057] Table 1. Comparison of tensile strength of heat-treated wire rods in Examples 1-15 and Comparative Examples 1-3.

[0058] Comparative Results Analysis: Comparing Example 1 and Comparative Example 1, it can be found that Example 1, by dividing the salt bath tank into sections and individually controlling the wire rod traction speed of each section, can reduce the strength fluctuation range among multiple wire rods in Example 1 by 74% compared to Comparative Example 1, effectively improving the stability of product performance. Comparing Example 1 and Comparative Example 2, it can be found that if the salt pump flow rate and salt inrush frequency are low during the salt bath process, the liquid level in the salt bath tank will be low, resulting in poor wire rod cooling efficiency. At the same time, the uniformity of the flow field and temperature field in the salt bath tank is poor, and the cooling of wire rods in different positions is inconsistent. Therefore, when multiple wire rods are heat-treated simultaneously, the overall strength is low and the strength fluctuation is large. Comparing Example 1 and Example 13, it can be found that the steps of air blowing and water washing after salt bath treatment can effectively reduce the residual salt on the wire rod surface, which is beneficial to improving the rust resistance of the wire rod during storage, maintaining the stability of the wire rod surface, and providing better surface quality and superior performance for subsequent drawing and galvanizing processes. Comparing Example 1 and Comparative Example 3, it can be found that using seven-zone austenitizing heating before salt bath treatment can eliminate problems such as differences in surface tissue, network carbides, or residual abnormal tissue caused by uneven heating or insufficient austenitization. This lays the microstructure foundation for subsequent uniform sorbitic phase transformation from the source, which is beneficial for obtaining wire rods with excellent internal quality.

[0059] Experimental Example 1 Repeatability data from multiple batches: For wire rods with a diameter of 12.5 mm, five independent repeated experiments were conducted using the exact same method as in Example 1 to test their tensile strength and make comparisons, as shown in Table 2.

[0060] Table 2 shows the tensile strength test results of wire rod obtained from five independent repeated experiments in Example 1.

[0061] Table 2 provides the maximum, minimum, and range values ​​of tensile strength, showing a strength fluctuation range of 21-23 MPa and a standard deviation of <2 MPa, demonstrating the stability of the process of this invention.

[0062] Experiment Example 2 Extended temperature testing: The salt bath temperature of Example 1 was adjusted to 563°C, while other conditions remained unchanged, and the treatment of wire rod 1 was repeated four times.

[0063] The salt bath temperature of Example 3 was adjusted to 495°C, while other conditions remained unchanged, and the treatment of wire rod 2 was repeated four times.

[0064] Because the salt bath temperature of treated wire rod 1 was too high, the phase transformation temperature was relatively high at 600℃, the sorbitization rate was <90%, and the tensile strength and shrinkage were low (as shown in Table 3); the salt bath temperature of treated wire rod 2 was too low, resulting in the appearance of abnormal bainite and martensite structures in the microstructure, and the tensile strength and shrinkage of the wire rod fluctuated greatly (as shown in Table 3 and...). Figure 2 (As shown). The SEM image of the processed wire rod 2 in Experiment Example 2 is shown below. Figure 6 .

[0065] Table 3 Performance test results of wire rod 1 and wire rod 2

[0066] Experimental Example 3 Design of different molten salt ratios: experiments with potassium nitrate:sodium nitrate at a weight ratio of 2:1 or 1:2. With a potassium nitrate to sodium nitrate weight ratio of 1:2, the molten salt operating temperature decreases to 480-540℃, exceeding the 500-560℃ range and failing to meet the salt temperature requirements for heat treatment. Simultaneously, the lower temperature increases the kinematic viscosity to 28-33 mmHg. 2 / s, the dynamic viscosity is 0.06-0.07 Pa·s, and the fluidity of molten salt deteriorates; if the ratio of potassium nitrate to sodium nitrate is 2:1, the kinematic viscosity decreases to 15-18 mm. 2 / s, with a dynamic viscosity of 0.02-0.03 Pa·s, which is beneficial for temperature homogenization, but the adjustable temperature range of the molten salt shifts upward to 530-585℃, which also exceeds the required salt temperature range; Fluidity and temperature uniformity are positively correlated. Better fluidity leads to more uniform salt temperature. This invention limits the molten salt formulation to a mixture of potassium nitrate and sodium nitrate in a weight ratio of 1.5-1:1-1.5. This formulation has a wide adjustable temperature range (500-600℃) to meet the heat treatment requirements of wire rods of different strength grades; the kinematic viscosity of this salt formulation is 20-25 mm. 2 With a dynamic viscosity of 0.04-0.05 Pa·s, it exhibits good fluidity, which is beneficial for temperature uniformity within the salt bath.

[0067] Experiment Example 4 Detailed impact of salt pump parameters: Changes in salt pump flow rate and operating frequency (80m) for simultaneous heat treatment of 14mm 92Si wire rods. 3 / h, 110m 3 Data on temperature uniformity and intensity fluctuations at / h, 40Hz, and 55Hz, showcasing the core parameter range (90-100m). 3 The optimization points (Hz, 45-50Hz) are shown in Table 4.

[0068] Table 4. Changes in flow rate, operating frequency, and corresponding parameters of the salt pump for simultaneous heat treatment of 14mm 92Si wire rod.

[0069] The flow rate and operating frequency of the salt pump affect the liquid level (cooling effect and the number of cooling coils) and temperature uniformity (intensity fluctuations). The minimum flow rate is 80 m³ / h. 3 The flow rate is 110 m³ / h, with a shallow liquid level of 25 cm, allowing for the simultaneous heat treatment of up to 7 14mm 92Si tubes; however, the low operating frequency of 40 Hz and a liquid level of 27 cm make uniform mixing difficult. 3 If the operating frequency ( / h) and operating frequency (55Hz) are too high, the molten salt will overflow from the side, affecting safety.

[0070] Experimental Example 5 Long-term storage data: When wire rods with a residual salt content >0.26 kg / t after heat treatment in Example 13 were stored for 1 month and 3 months, the changes in rust ratio and intensity were periodically tested. If the rust ratio was ≥15%, corrosion pits would appear on the surface, resulting in poor surface quality of the drawn steel wire and incomplete galvanizing during the galvanizing process. See [link to relevant documentation]. Figure 2 and Figure 3 In Example 1, the residual salt content on the surface of the treated wire rod was ≤0.26 kg / t. After storage for 1 month and 3 months, the changes in rust ratio and intensity were periodically monitored. The surface rust ratio was 0% and 5% respectively, with a rust ratio <15% and no corrosion pits. (See attached image) Figure 4 and Figure 5 It meets the usage requirements and can be used normally.

[0071] Microstructure: SEM analysis of the wire rod treated in Example 1 showed a sorbite content ≥95%, with no martensite / network carbides in the core (see attached). Figure 1 ).

[0072] In summary, according to the method and parameter requirements provided by this invention, multiple wire rods can be heat-treated simultaneously, and the strength fluctuation of all wire rods between the frames can be controlled within 30 MPa.

[0073] Conclusion: This invention effectively reduces strength fluctuations when multiple wire rods are heat-treated simultaneously by matching traction speed through a partitioned salt bath process, optimizing austenitizing heating and surface cleaning, thereby improving product stability and yield. It is suitable for the production of high-strength bridge cable wire rods.

[0074] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for reducing strength fluctuations in wire rods subjected to salt bath heat treatment, characterized in that, include: Austenitizing heating steps: The wire rod is continuously heated in the heating furnace. Along the direction of wire rod movement, there are 5-7 temperature zones set in the heating furnace. The temperature first rises in stages from 895℃~905℃ to 955℃~965℃, and then drops in stages from 955℃~965℃ to 915℃~925℃. The total heating time of the wire rod is 9~12 minutes. Salt bath treatment steps: The austenitized and heated wire rods are sent into a salt bath tank for salt bath treatment, wherein: The salt bath tank is divided into at least a side region and a middle region in a horizontal plane along a direction perpendicular to the movement direction of the wire rod. The side region corresponds to the two sides of the salt bath tank, and the middle region corresponds to the middle of the salt bath tank. The traction speed of the wire rod is set according to the area of ​​the wire rod in the salt bath tank; The wire rod traction speed in the edge region is greater than that in the middle region; the wire rod traction speed in the edge region is 3.5~5.2m / min, and the wire rod traction speed in the middle region is 3.0~4.7m / min. The salt bath tank is equipped with molten salt pumps on both sides, with an operating frequency of 45-50Hz and a flow rate of 90-100m³ / h. 3 / h, the liquid level depth in the salt bath is 32~35cm.

2. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 1, characterized in that, The salt bath tank is divided into three equal regions in the horizontal plane along the direction perpendicular to the movement of the wire rod: the first side region, the middle region, and the second side region. And / or, two to four molten salt pumps are provided on each side of the salt bath tank.

3. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 2, characterized in that, The parameters for the salt bath treatment are set according to the wire rod diameter as follows: When the wire rod diameter is 11mm~12.5mm, the salt bath temperature is 540~560℃, the number of wire rods processed simultaneously in each zone is 5~6, the wire rod traction speed in the edge zone is 4.8~5.2m / min, and the wire rod traction speed in the middle zone is 4.3~4.7m / min; Alternatively, when the wire rod diameter is 13mm~14mm, the salt bath temperature is 520~540℃, the number of wire rods processed simultaneously in each zone is 4~5, the wire rod traction speed in the edge zone is 4.1~4.5m / min, and the wire rod traction speed in the middle zone is 3.6~4.0m / min; Alternatively, when the wire rod diameter is 15mm~16mm, the salt bath temperature is 500~520℃, the number of wire rods processed simultaneously in each zone is 2~3, the wire rod traction speed in the edge zone is 3.5~3.9m / min, and the wire rod traction speed in the middle zone is 3.0~3.4m / min.

4. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 3, characterized in that, The salt bath treatment uses a mixed molten salt composed of potassium nitrate and sodium nitrate as the quenching medium. In the mixed molten salt, potassium nitrate and sodium nitrate are mixed in a weight ratio of (1~1.5):(1~1.5).

5. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 4, characterized in that, The kinematic viscosity of the mixed molten salt is 20~25 mm. 2 / s, with a dynamic viscosity of 0.04~0.05 Pa·s.

6. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 1, characterized in that, The heating furnace has seven temperature zones arranged sequentially, with the following temperature control ranges: Zone 1: 895℃~905℃, Zone 2: 935℃~945℃, Zone 3: 955℃~965℃, Zone 4: 955℃~965℃, Zone 5: 955℃~965℃, Zone 6: 935℃~945℃, and Zone 7: 915℃~925℃. The heating time for the wire rod from Zone 3 to Zone 5 is 4~6 minutes.

7. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 1, characterized in that, The austenitizing heating step employs natural gas self-preheating pulse burners; the burners are respectively installed on the upper and lower parts of both sides of the heating furnace cavity.

8. The method for reducing strength fluctuations of wire rods subjected to salt bath heat treatment according to claim 1, characterized in that, Before the austenitizing heating step, the process also includes a shot blasting treatment of the wire rod, the shot blasting treatment time being 5-8 minutes; and a step of feeding the wire using a horizontal feeding unit and straightening it using a straightening machine.

9. The method for reducing strength fluctuation of wire rods after salt bath heat treatment according to any one of claims 1-8, characterized in that, The salt bath treatment also includes surface cleaning, which includes air blowing and water washing. The air-blowing nozzles are distributed in one row above and one row below the wire rod, 6-10 mm away from the wire rod. The flow rate of a single nozzle is 10-15 L / min. The blowing direction is opposite to the direction of wire rod movement and forms an angle of 130-140°. And / or, the temperature of the water washing treatment is 40~45℃ and the water pressure is 10~15 bar.

10. The application of the method for reducing strength fluctuation of salt bath heat-treated wire rod as described in any one of claims 1 to 9 in the production of high-strength bridge cable wire rod.

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