A method for controlling color defects in a continuous annealing and oxidizing of a steel strip

Abstract

This invention discloses a method for controlling oxidation color defects in continuous annealing of strip steel, relating to the field of strip steel heat treatment technology. The method comprises an online oxygen content detection device, an online dew point detection device, an online oxidation color detector, and a fan power control system. The online dew point detection device is used to detect the dew point in the air ducts of the slow cooling section, rapid cooling section, aging section, and final cooling section; the online oxidation color detector is used to detect the oxidation color on the strip steel surface in real time. Finally, the fan power feedback control system uses the above detection results to control the fan power, achieving online control of strip steel oxidation color. It can automatically control oxidation color defects and automatically alarm for abnormalities, shortening the time for leak point investigation and effectively controlling the amount of oxidation color generation, greatly improving the automation level of the unit.

Description

Technical Field

[0001] This invention relates to the field of strip steel heat treatment technology, specifically to a method for controlling oxidation color defects in continuous annealing of strip steel. Background Technology

[0002] In the continuous annealing process of cold rolling, annealing oxidation discoloration has always been a prominent problem, especially for ultra-high strength steel. Because high strength steel contains a large amount of strengthening alloying elements such as Si and Mn, these alloying elements diffuse and accumulate on the surface of the steel plate during annealing and undergo oxidation, resulting in a faster oxidation rate than ordinary carbon steel. The main methods to control oxidation discoloration are to reduce the oxygen content and dew point in the furnace and increase the hydrogen content. Although increasing the hydrogen content can reduce the oxidized strip steel to a certain extent, this is mainly applicable to the high-temperature section and has a poor effect on the low-temperature section. At the same time, it will also increase the consumption of hydrogen. Therefore, reducing the dew point and oxygen content in the furnace, especially reducing the oxygen content and dew point in the air ducts of the slow cooling section, fast cooling section, aging section and final cooling section, is currently the main means to control oxidation discoloration.

[0003] Patent CN105648178B discloses a method for controlling the oxidation color of duplex steel in a cold rolling continuous annealing process. This method mainly controls the oxidation color of duplex steel by reducing the temperature of the heating and final cooling sections, limiting the power of the cooling fan in the rapid cooling section to within 80%, controlling the pH value and conductivity of the water quenching tank, and increasing the hydrogen content in the furnace section.

[0004] The current methods for controlling the oxidation color of duplex steel in the cold rolling continuous annealing process have the following shortcomings:

[0005] Regarding the fan power, this method mechanically keeps the cooling fan power of the fast cooling section below 80%. On the one hand, it cannot quantify whether the oxygen content and dew point under 80% fan power can guarantee the normal surface of the strip steel. On the other hand, it cannot automatically control the oxidation color defect according to the continuous deterioration of the fan condition. Moreover, if air or water leakage occurs in other sections of the fan, it cannot be detected in time, so it cannot control the oxidation color in time, nor can it remind the operators to pay attention to the abnormality of the fan in that area.

[0006] Therefore, in view of this, we studied and improved the existing structure and its defects, and proposed a method for controlling oxidation color defects in strip steel during continuous annealing. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for controlling oxidation color defects in continuous annealing of strip steel, thus solving the problems mentioned in the background section.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for controlling oxidation color defects in continuous annealing of strip steel, wherein the method comprises an online oxygen content detection device, an online dew point detection device, an online oxidation color detector, and a fan power control system, and the control method includes the following operating steps:

[0009] S1. Oxygen content and dew point detection:

[0010] Online oxygen content detection device and online dew point detection device are used for online real-time cyclic detection of ppm-level oxygen content and dew point in each air duct of the slow cooling section, fast cooling section, aging section and final cooling section. The oxygen content detection point and dew point detection point are set on the air duct on the side of the fan outlet. The oxygen content and dew point detection results are fed back to the fan power control system.

[0011] S2, Oxidation color defect detection:

[0012] An online oxidation color detector is installed near the quality inspection station at the unit's outlet to quantitatively detect oxidation color defects on the upper and lower surfaces of the strip. This online oxidation color detector has a built-in stable high-intensity illumination system and uses a high-speed camera to detect the grayscale of the upper and lower surfaces of the strip. The grayscale value is used to characterize the degree of oxidation color defects on the strip surface. The fan power control system also selects the fan with the smallest difference between the grayscale values ​​detected by the online oxidation color detector and the preset limits for oxygen content and dew point. The system performs point-to-point detection and limits the power of the fan in the area where the fan with the smallest difference is located. When the difference between the oxygen content or dew point in this section and the preset limit value of oxygen content and the preset limit value of dew point is not the smallest, it automatically switches to the new fan with the smallest difference. The system performs point-to-point detection and limits the power of the fan in the area where the new fan with the smallest difference is located until the oxidation gray value meets the release standard.

[0013] S3, Automatic fan power limit:

[0014] Oxygen content measuring instruments and dew point detectors are installed in the air ducts of the slow cooling section, fast cooling section, aging section and final cooling section. Based on the oxygen content and dew point measurement results, the maximum power of the fans in the slow cooling section, fast cooling section, aging section and final cooling section is automatically limited by comparing the oxygen content and dew point limit values ​​with the table lookup method.

[0015] S4. Set up the oxygen content and dew point correspondence table:

[0016] A table showing the correspondence between the limit color difference grayscale value and oxygen content and dew point is set up, and the table showing the correspondence between the limit color difference grayscale value and oxygen content and dew point limit value for each steel grade is maintained in the online oxidation colorimeter computer database system.

[0017] Furthermore, in step S1, the fan power control system presets oxygen content limits and dew point limits for each fan. It compares the real-time detected oxygen content and dew point with the preset limits and performs fan power limit feedback control. That is, when the real-time detected oxygen content and dew point exceed their respective limits, the cyclic detection is automatically changed to fixed-point detection in the duct where the detection point exceeds the limit. The fan speed corresponding to the duct where the detection point is located is automatically reduced until the detection result is qualified, and then the cyclic detection and judgment continue.

[0018] Furthermore, when automatically reducing the fan speed corresponding to the air duct where the detection point is located, if the fans are arranged symmetrically, the two fans need to reduce their speed synchronously to maintain pressure balance.

[0019] Furthermore, in step S3, the oxygen content and dew point limits are differentiated according to steel grade, and the maximum power of the fans in the slow cooling section, fast cooling section, aging section, and final cooling section is limited according to the respective reference tables for each steel grade.

[0020] Furthermore, in step S3, a grayscale value table for the color difference limit of each steel grade is set in advance, and the maximum power of the fan in the slow cooling section, fast cooling section, aging section and final cooling section is distinguished by using the grayscale detection results of the online oxidation colorimeter through the table lookup method.

[0021] Furthermore, in step S4, when a new steel grade appears, the limit color difference gray value can be automatically compared and the oxygen content and dew point limit value of the steel grade can be updated.

[0022] Furthermore, in addition to carbon seals, the shafts of the fans in the slow cooling section and the fast cooling section are also sealed with high-pressure nitrogen gas with a pressure of 3 kg or more.

[0023] Furthermore, all conical inlets after the fan heat exchangers adopt a double-layer sealing structure and are sealed with high-pressure nitrogen gas with a pressure greater than or equal to 3 kg.

[0024] Furthermore, the dew point inside the furnace in the slow cooling section, rapid cooling section, aging section, and final cooling section is controlled below -40°C.

[0025] This invention provides a method for controlling oxidation color defects in continuous annealing of strip steel, which has the following beneficial effects:

[0026] This method for controlling oxidation color defects in continuous annealing strip steel includes an online oxygen content detection device to monitor the oxygen content in each air duct of the slow cooling, rapid cooling, aging, and final cooling sections; an online dew point detection device to monitor the dew point in the air ducts of the slow cooling, rapid cooling, aging, and final cooling sections; and an online oxidation color detector to monitor the oxidation color on the strip steel surface in real time. Finally, a fan power feedback control system uses the above detection results to control the fan power, achieving online control of strip steel oxidation color. This system can automatically control oxidation color defects and automatically alarm for abnormalities, shortening the time for leak detection and effectively controlling the amount of oxidation color, greatly improving the automation level of the unit. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the online adjustment of oxide color in a method for controlling oxide color defects during continuous annealing of strip steel according to the present invention.

[0028] Figure 2 This is a schematic diagram of the implementation location of the method for controlling oxidation color defects in continuous annealing of strip steel according to the present invention in the continuous annealing unit;

[0029] Figure 3 This is a schematic diagram of the overall process of a method for controlling oxidation color defects in continuous annealing of strip steel according to the present invention. Detailed Implementation

[0030] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0031] like Figures 1-3 As shown, the present invention provides a technical solution: a method for controlling oxidation color defects in continuous annealing of strip steel, comprising an online oxygen content detection device, an online dew point detection device, an online oxidation color detector, and a fan power control system. The control method includes the following operating steps:

[0032] S1. Oxygen content and dew point detection:

[0033] Online oxygen content detection device and online dew point detection device are used for online real-time cyclic detection of ppm-level oxygen content and dew point in each air duct of the slow cooling section, fast cooling section, aging section and final cooling section. The oxygen content detection point and dew point detection point are set on the air duct on the side of the fan outlet. The oxygen content and dew point detection results are fed back to the fan power control system.

[0034] S2, Oxidation color defect detection:

[0035] An online oxidation color detector is installed near the quality inspection station at the unit's outlet to quantitatively detect oxidation color defects on the upper and lower surfaces of the strip. This online oxidation color detector has a built-in stable high-intensity illumination system and uses a high-speed camera to detect the grayscale of the upper and lower surfaces of the strip. The grayscale value is used to characterize the degree of oxidation color defects on the strip surface. The fan power control system also selects the fan with the smallest difference between the grayscale values ​​detected by the online oxidation color detector and the preset limits for oxygen content and dew point. The system performs point-to-point detection and limits the power of the fan in the area where the fan with the smallest difference is located. When the difference between the oxygen content or dew point in this section and the preset limit value of oxygen content and the preset limit value of dew point is not the smallest, it automatically switches to the new fan with the smallest difference. The system performs point-to-point detection and limits the power of the fan in the area where the new fan with the smallest difference is located until the oxidation gray value meets the release standard.

[0036] S3, Automatic fan power limit:

[0037] Oxygen content measuring instruments and dew point detectors are installed in the air ducts of the slow cooling section, fast cooling section, aging section and final cooling section. Based on the oxygen content and dew point measurement results, the maximum power of the fans in the slow cooling section, fast cooling section, aging section and final cooling section is automatically limited by comparing the oxygen content and dew point limit values ​​with the table lookup method.

[0038] S4. Set up the oxygen content and dew point correspondence table:

[0039] A table showing the correspondence between the limit color difference grayscale value and oxygen content and dew point is set up, and the table showing the correspondence between the limit color difference grayscale value and oxygen content and dew point limit value for each steel grade is maintained in the online oxidation colorimeter computer database system.

[0040] Furthermore, in step S1, the fan power control system presets oxygen content limits and dew point limits for each fan. It compares the real-time detected oxygen content and dew point with the preset limits and performs fan power limit feedback control. That is, when the real-time detected oxygen content and dew point exceed their respective limits, the cyclic detection is automatically changed to fixed-point detection in the duct where the detection point exceeds the limit. The fan speed corresponding to the duct where the detection point is located is automatically reduced until the detection result is qualified, and then the cyclic detection and judgment continue.

[0041] Furthermore, when automatically reducing the fan speed corresponding to the air duct where the detection point is located, if the fans are arranged symmetrically, the two fans need to reduce their speed synchronously to maintain pressure balance.

[0042] Furthermore, in step S3, the oxygen content and dew point limits are differentiated according to steel grade, and the maximum power of the fans in the slow cooling section, fast cooling section, aging section, and final cooling section is limited according to the respective reference tables for each steel grade.

[0043] Furthermore, in step S3, a grayscale value table for the color difference limit of each steel grade is set in advance, and the maximum power of the fan in the slow cooling section, fast cooling section, aging section and final cooling section is distinguished by using the grayscale detection results of the online oxidation colorimeter through the table lookup method.

[0044] Furthermore, in step S4, when a new steel grade appears, the limit color difference gray value can be automatically compared and the oxygen content and dew point limit value of the steel grade can be updated.

[0045] Furthermore, in addition to carbon seals, the shafts of the fans in the slow cooling section and the fast cooling section are also sealed with high-pressure nitrogen gas with a pressure of 3 kg or more.

[0046] Furthermore, all conical inlets after the fan heat exchangers adopt a double-layer sealing structure and are sealed with high-pressure nitrogen gas with a pressure greater than or equal to 3 kg.

[0047] Furthermore, the dew point inside the furnace in the slow cooling section, rapid cooling section, aging section, and final cooling section is controlled below -40°C.

[0048] Example:

[0049] Figure 2 The components are: 1. Slow cooling section; 2. Rapid cooling section; 3. Online gas analyzer; 5. Over-aging section; 6. Final cooling section; 7. Leveling machine; 8. Online colorimeter.

[0050] like Figures 1-3As shown, sampling points are set at the fan outlets of the slow cooling section 1, fast cooling section 2, over-aging section 4, and final cooling section 5 in the central section of the continuous annealing unit. Gas samples taken from these sampling points are led through sampling pipes to the gas detection device 3 for oxygen content and dew point analysis. This allows for online real-time cyclic monitoring of oxygen content and dew point in each section of the duct. The fan power control system compares the preset oxygen content and dew point limits to perform fan power limitation feedback control. Specifically, if the real-time detected oxygen content and dew point exceed their respective limits, the cyclic monitoring is automatically switched to fixed-point monitoring at that location, automatically reducing the fan speed in that area (if the fans are symmetrically arranged, both fans must reduce speed synchronously to maintain pressure balance) until the test results are satisfactory, then the cyclic monitoring and judgment process continues. After leaving the annealing furnace, the strip steel passes through leveling machine 6 and exits to the online oxidation color difference detector 7. This online oxidation colorimeter detects the current oxidation color value of the strip steel. It features a stable, high-intensity illumination system and uses a high-speed camera to detect the grayscale of the upper and lower surfaces of the strip steel. The grayscale value characterizes the degree of oxidation color defect on the strip steel surface. If the actual color difference value exceeds the limit, the online oxidation colorimeter computer checks the oxygen content and dew point. If both oxygen content and dew point exceed the limit, the fan power control system reduces the fan speed based on the color difference limit and the oxygen content and dew point limits until the oxygen content and dew point reach within the target values, and then automatically alarms. Finally, the strip steel passes through the horizontal inspection platform 8, where quality inspectors can reconfirm using a limit template. If the oxidation colorimeter's judgment is incorrect, it can be manually modified and saved, correcting the color difference limit value corresponding to the oxidation color of that steel grade.

[0051] Specific example 1 is as follows: An annealing unit produces 780DP products. The strip steel leaves the annealing furnace after exiting the final cooling section 5 and passing through the leveling machine 6. It then reaches the oxidation color difference detector 7, which detects an actual color difference value of 35, which is lower than the limit sample color difference value of 40. This value is fed back to the database. The database searches for the limit color difference values, oxygen content, and dew point limits for each steel grade. The oxygen content and dew point limits for the current steel grade are found to be 30ppm and -35℃, respectively. These are used as the preset limit values ​​for oxygen content and dew point in the current fan duct. At the same time, the database is searched and it is found that the oxygen content and dew point of the No. 1 fan duct in the rapid cooling section are 40ppm and -40℃, respectively. Therefore, it is determined that there is a leak in the duct, and an alarm is triggered. The operator can then conduct on-site troubleshooting based on the alarm. At the same time, the fan is automatically limited. The fan power automatically decreases, and the oxygen content decreases accordingly. When the oxygen content drops to 20 ppm (with a margin that can be set), the corresponding fan power is used as the current limiting power. If a leak is found in the fan expansion joint, after repairing it by welding, applying silicone sealant, or attaching a silicone sealant, the oxygen content will drop to 10 ppm, and the fan power limit will be automatically increased.

[0052] Specific example 2 is as follows: An annealing unit produces 980DP products. The strip steel leaves the annealing furnace after exiting the final cooling section 5 and passing through the leveling machine 6. It then reaches the oxidation color difference detector 7, which detects an actual color difference value of 38, which is lower than the limit sample color difference value of 45. This value is fed back to the database. The database searches for the limit color difference values, oxygen content, and dew point limits for each steel grade. The oxygen content and dew point limits for the current steel grade are 25ppm and -35℃, respectively. These are used as the preset limit values ​​for oxygen content and dew point in the current fan duct. At the same time, the database is searched and it is found that the oxygen content and dew point of the No. 1 fan duct in the slow cooling section are 10ppm and -25℃, respectively. Therefore, it is determined that the duct is leaking water, and an alarm is triggered. The operator can then conduct on-site troubleshooting based on the alarm. At the same time, the fan is automatically limited. The fan power automatically decreases, and the dew point decreases accordingly. When the dew point drops to -38℃ (with a margin for decrease, which can be set), the corresponding fan power is used as the current limiting power. If a leak is detected in the fan heat exchanger, after handling the issue by shutting off the water supply, activating protective gas, or replacing the heat exchanger, and the dew point drops to 40ppm, the fan power limit will be automatically increased.

[0053] Specific example 3 is as follows: An annealing unit produces 1180DP products. The strip steel leaves the annealing furnace after exiting the final cooling section 5 and passing through the leveling machine 6. It then reaches the oxide color detector 7, which detects that the actual color difference value is 42, which is lower than the limit sample color difference value of 45. However, when comparing with the limit sample on the horizontal inspection table 8, it is found that the actual value is better than the limit sample. At this time, it is necessary to modify and save the limit color difference value of this steel grade, and correct the color difference limit value of the oxide color corresponding to this steel grade to 40. If the steel grade subsequently exhibits a color difference value less than 40, this is reported to the database. The database is then used to find the current steel grade's oxygen content and dew point limits, which are 20 ppm and -40°C, respectively. These limits are set as the current preset limits for oxygen content and dew point in the fan duct. Simultaneously, the database shows that the current effective section #1 fan duct has an oxygen content of 25 ppm and a dew point of -35°C, indicating both water and air leakage. An alarm is triggered, allowing operators to conduct on-site troubleshooting. The fan is automatically limited. The fan power automatically decreases, and the dew point decreases accordingly. When the oxygen content drops to 20 ppm and the dew point to -42°C, the corresponding fan power is used as the current power limit.

[0054] Based on the above embodiments and case descriptions, this application can detect leaking fans online and perform automatic amplitude limiting control, which can reduce oxidation color difference loss, improve product surface qualification rate, improve work efficiency, quickly locate abnormal fans, and shorten the time for leak point investigation.

[0055] The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described in order to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and to design various embodiments with various modifications suitable for a particular purpose.

Claims

1. A method for controlling oxidation color defects in continuous annealing of strip steel, characterized in that: The method for controlling oxidation color defects in continuous annealing of strip steel consists of an online oxygen content detection device, an online dew point detection device, an online oxidation color detector, and a fan power control system. The control method includes the following operating steps: S1. Oxygen content and dew point detection: Online oxygen content detection device and online dew point detection device are used for online real-time cyclic detection of ppm-level oxygen content and dew point in each air duct of the slow cooling section, fast cooling section, aging section and final cooling section. The oxygen content detection point and dew point detection point are set on the air duct on the side of the fan outlet. The oxygen content and dew point detection results are fed back to the fan power control system. S2, Oxidation color defect detection: An online oxidation color detector is installed near the quality inspection station at the unit outlet to quantitatively detect oxidation color defects on the upper and lower surfaces of the strip. This online oxidation color detector has a stable high-intensity illumination system and uses a high-speed camera to detect the grayscale of the upper and lower surfaces of the strip. The grayscale value is used to characterize the degree of oxidation color defects on the strip surface. The fan power control system also selects the fan with the smallest difference based on the grayscale value of the upper and lower surfaces of the strip detected by the online oxidation color detector, combined with the difference between the oxygen content and dew point of each fan and the preset limits of oxygen content and dew point. The system performs point-to-point detection on the air duct where the fan with the smallest difference is located and limits the fan power in the area where the fan with the smallest difference is located. When the difference between the oxygen content or dew point of this section and the preset limits of oxygen content and dew point are not the smallest, the system automatically switches to a new fan with the smallest difference. The system performs point-to-point detection on the air duct where the new fan with the smallest difference is located and limits the fan power in the area where the new fan with the smallest difference is located until the oxidation color grayscale value meets the release standard. S3, Automatic fan power limit: Oxygen content measuring instruments and dew point detectors are installed in the air ducts of the slow cooling section, fast cooling section, aging section and final cooling section. Based on the oxygen content and dew point measurement results, the maximum power of the fans in the slow cooling section, fast cooling section, aging section and final cooling section is automatically limited by comparing the oxygen content and dew point limit values ​​with the table lookup method. S4. Set up the oxygen content and dew point correspondence table: A table showing the correspondence between the limit color difference grayscale value and oxygen content and dew point is set up, and the table showing the correspondence between the limit color difference grayscale value and oxygen content and dew point limit value for each steel grade is maintained in the online oxidation colorimeter computer database system.

2. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: In step S1, the fan power control system presets oxygen content limits and dew point limits for each fan. It compares the real-time detected oxygen content and dew point with the preset limits and performs fan power limit feedback control. That is, when the real-time detected oxygen content and dew point exceed their respective limits, the cyclic detection is automatically changed to fixed-point detection in the duct where the detection point exceeds the limit. The fan speed corresponding to the duct where the detection point is located is automatically reduced until the detection result is qualified, and then the cyclic detection and judgment continue.

3. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 2, characterized in that: When automatically reducing the fan speed corresponding to the air duct where the detection point is located, if the fans are arranged symmetrically, the two fans need to reduce their speed synchronously to maintain pressure balance.

4. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: In step S3, the oxygen content and dew point limits are differentiated according to steel grade, and the maximum power of the fans in the slow cooling section, fast cooling section, aging section and final cooling section is limited according to the respective reference tables for each steel grade.

5. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: In step S3, a grayscale value table for the color difference limit of each steel grade is set in advance, and the maximum power of the fan in the slow cooling section, fast cooling section, aging section and final cooling section is distinguished by using the grayscale detection results of the online oxidation colorimeter through the table lookup method.

6. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: In step S4, when a new steel grade appears, the limit color difference gray value is automatically compared and the oxygen content and dew point limit value of the steel grade are updated.

7. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: In addition to carbon seals, the shafts of the fans in the slow cooling section and the fast cooling section are also sealed with high-pressure nitrogen gas with a pressure of 3 kg or more.

8. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: All conical inlets after the fan heat exchangers adopt a double-layer sealing structure and are sealed with high-pressure nitrogen gas with a pressure of 3 kg or more.

9. The method for controlling oxidation color defects in continuous annealing of strip steel according to claim 1, characterized in that: The dew point inside the furnace in the slow cooling section, rapid cooling section, aging section, and final cooling section is controlled below -40℃.

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