Online detection and compensation method for local decarburization of high-strength spring steel band

Abstract

The invention provides an online detection and compensation method for local decarburization of a high-strength spring steel band, which comprises the following steps: arranging an oxygen content sensor in a continuous annealing furnace to monitor oxygen concentration in real time, and marking a potential decarburization risk section; scanning the risk section steel strip by adopting infrared thermal imaging, and generating a carbon distribution map to position a decarburization area; pulse laser heating is conducted on the decarburization area, and methane-argon mixed gas is introduced to achieve in-situ carburization; and finally, local thermal stress is eliminated through high-frequency induction heating. According to the method, decarburization risk dynamic early warning, defect accurate positioning, local efficient carbon supplementation and stress homogenization control are achieved, and the surface quality and fatigue performance of the steel strip are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength spring steel strip preparation, in particular to an online detection and compensation method for local decarburization of a high-strength spring steel strip. Background Art

[0002] In the steel manufacturing industry, high-strength spring steel strip (such as 60Si2MnA) is widely used in key components such as automotive suspensions and mechanical valves due to its excellent elasticity and fatigue properties. The production of this type of steel strip typically requires heat treatment processes such as continuous annealing, quenching, and tempering to ensure that its mechanical properties meet the required standards. However, during the continuous annealing process, the steel strip surface is prone to decarburization, especially when subjected to prolonged exposure to high temperatures (800-900°C). Carbon on the strip surface reacts with residual oxygen or water vapor in the furnace, reducing the surface carbon content and forming a decarburized layer. The presence of this decarburized layer not only reduces the surface hardness and wear resistance of the steel strip, but also causes microcracks during the subsequent quenching process due to the differential microstructural transformation between the surface and core, seriously affecting the fatigue life and safety of the spring.

[0003] Currently, the industry generally uses protective atmosphere annealing (such as nitrogen or hydrogen-nitrogen mixture) to reduce the risk of decarburization, and combines sampling metallographic testing or hardness testing to monitor decarburization. However, these methods still have obvious drawbacks: First, while protective atmosphere annealing can reduce the overall probability of decarburization, local fluctuations in the furnace atmosphere (such as a transient increase in residual oxygen content) during continuous production can still cause intermittent decarburization in certain sections of the steel strip. Traditional sampling inspections cannot cover the entire surface of the steel strip, resulting in delayed discovery of the problem. Second, even if decarburization is detected, existing compensatory measures (such as overall secondary annealing or carburizing) are often inefficient and may affect other properties of the steel strip due to excessive treatment.

[0004] Therefore, how to achieve accurate detection and efficient repair of local decarburization has become a key problem in the production of spring steel strips. Summary of the Invention

[0005] To this end, the technical problem to be solved by the present invention is to overcome the defect in the prior art that decarburization of the steel strip is difficult to detect and accurately repair in real time due to local atmosphere fluctuations in the continuous annealing furnace, and to provide an online detection and compensation method for local decarburization of high-strength spring steel strips, which can achieve dynamic early warning of decarburization risks, precise positioning of defective areas, local efficient carbon replenishment, and stress uniformization control, thereby significantly improving the surface quality and fatigue performance of the spring steel strips.

[0006] To solve the above technical problems, the present invention provides an online detection and compensation method for local decarburization of a high-strength spring steel strip, comprising the following steps: Multiple groups of oxygen content sensors are placed in the soaking and slow cooling sections of the continuous annealing furnace to monitor the oxygen concentration in each area of ​​the furnace in real time. When the oxygen concentration of any group of oxygen content sensors exceeds the threshold, the coordinates of the steel strip position in that area are recorded and marked as a potential decarburization risk section. Perform infrared thermal imaging scanning on the surface of the steel strip in the potential decarburization risk zone after leaving the furnace. Utilizing the difference in thermal radiation between the decarburized and non-decarburized areas, a surface carbon distribution map is generated to locate the actual position and range of the decarburized area. The identified decarburized area is locally heated to the austenitizing temperature using a pulsed laser beam, and a methane-argon mixed gas is introduced at the same time to cause in-situ carburizing reaction in the decarburized area, restoring the surface carbon content to the level of the steel strip matrix; The carbon-replenished steel strip is quickly preheated as a whole by a high-frequency induction heating device to eliminate the local thermal stress caused by laser carbon refilling.

[0007] In one embodiment of the present invention, the oxygen content sensor is arranged as follows: The spacing between the multiple groups of oxygen content sensors is determined according to the length of each section of the annealing furnace. The spacing between two adjacent groups of oxygen content sensors arranged in the soaking section shall not exceed 1 / 5 of the length of the section, and the spacing between two adjacent groups of oxygen content sensors arranged in the slow cooling section shall not exceed 1 / 3 of the length of the section. The detection range of each set of oxygen content sensors covers the entire width of the steel strip, and at least three detection points are set in the width direction; When two or more adjacent detection points in the width direction detect that the oxygen concentration exceeds the threshold at the same time, the marking area is extended in the length direction to the area 1 / 8 of the distance upstream and downstream of the detection range of the group of oxygen content sensors, and the steel strip at the corresponding position in the area is marked as a potential decarburization risk section.

[0008] In one embodiment of the present invention, after the potential decarburization risk section is determined, a color code laser coder is used to mark the starting position of the risk section on the edge of the steel strip.

[0009] In one embodiment of the present invention, the specific implementation of the infrared thermal imaging scanning includes: Multiple infrared thermal imager arrays are set up in the running direction of the steel belt. Each array contains three thermal imagers at different angles, which perform synchronous scanning from directly above the steel belt, 45 degrees to the left, and 45 degrees to the right. The data collected by each group of thermal imagers is fused in three dimensions to generate a surface carbon distribution map, and the thermal radiation difference is determined based on the surface carbon distribution map; A model for the relationship between thermal radiation difference and decarburization depth is established. When the thermal radiation difference exceeds the set threshold, it is marked as a decarburization area. The area and depth of the decarburization area are calculated to determine the decarburization level. The marked decarburization area is subjected to edge recognition and contour extraction to generate an accurate decarburization area contour map.

[0010] In one embodiment of the present invention, determining the ratio of methane-argon mixed gas according to the decarburization level includes the following steps: The decarburization level is divided into three levels, including: mild decarburization: decarburization depth ≤ 0.1mm, moderate decarburization: 0.1mm < decarburization depth ≤ 0.15mm, severe decarburization: decarburization depth > 0.15mm; According to different decarburization levels, the methane-argon mixing ratio is adjusted: mild decarburization uses a methane-argon mixing ratio of 1:9, moderate decarburization uses a methane-argon mixing ratio of 1:4, and severe decarburization uses a methane-argon mixing ratio of 1:2.

[0011] In one embodiment of the present invention, the laser spot diameter and power density are adjusted according to the area and depth of the decarburized zone, wherein: For decarburization areas less than 10mm², a small spot with a diameter of 1-2mm and high power density mode is used; For decarburization areas of 10-50 mm², a medium spot scanning mode with a diameter of 3-5 mm is used; For decarburization areas larger than 50mm², a regional multi-spot collaborative processing mode is adopted.

[0012] In one embodiment of the present invention, the temperature of the pulsed laser beam is set to 50 to 80°C above the austenitizing temperature, the real-time temperature of the steel strip during heating is monitored, and the flow rate of the methane-argon mixed gas introduced is adjusted according to the real-time temperature ratio of the steel strip. When the steel strip is heated to the austenitizing temperature, the flow rate of the methane-argon mixed gas reaches a maximum value.

[0013] In one embodiment of the present invention, during the in-situ carburization reaction in the decarburization zone, an online spectrum analyzer is used to monitor the surface carbon concentration changes. When the carbon concentration reaches 95% of the matrix level, the process enters the heat preservation stage, and the pulsed laser beam local heating and the supply of methane-argon mixed gas are stopped. The holding time is calculated based on the decarburization area at 5s / mm², and the surface carbon concentration change value is continuously detected until the surface carbon content reaches the level of the steel strip matrix.

[0014] In one embodiment of the present invention, when the supply of the methane-argon mixed gas is stopped, the gas supply is stopped by gradually reducing the gas flow rate, and the ratio of the methane-argon mixed gas is gradually adjusted to maintain argon protection until the temperature drops to a safe range.

[0015] In one embodiment of the present invention, the specific process steps of performing overall rapid preheating by a high-frequency induction heating device include: Multiple sets of induction coils are arranged in the width direction of the steel strip. The power of each set of coils is independently adjustable. The lateral temperature distribution of the steel strip is monitored in real time by an infrared thermal imager. When the temperature deviation exceeds ±15°C, the power of the coil at the corresponding position is adjusted. In the initial stage, the temperature is raised to 300-350°C at a rate of 30-50°C / s, in the middle stage, the temperature is raised to 450-500°C at a rate of 15-20°C / s, and in the final stage, the temperature is raised to the target temperature of 550-600°C at a rate of 5-10°C / s; Maintain the target temperature for 30-60 seconds to fully release the thermal stress, and monitor the stress changes in real time through a laser ultrasonic detection device. When the residual stress drops below the safety threshold, enter the cooling process. During the cooling process, maintain the tension state and stretch the steel strip.

[0016] The above technical method of the present invention has the following advantages over the prior art: The core of the online detection and compensation method for local decarburization of high-strength spring steel strip described in the present invention lies in the organic combination of dynamic monitoring, infrared imaging, laser carbon replenishment and high-frequency induction homogenization processes to form a closed-loop control.

[0017] First, oxygen content sensors are placed in the soaking and slow cooling sections of the continuous annealing furnace to capture furnace atmosphere fluctuations in real time and identify areas at risk for decarburization, providing precise location information for subsequent inspections. This step overcomes the randomness and lag inherent in traditional spot-based inspections. Subsequently, infrared thermal imaging technology is used to scan the surface of the steel strip in these risky areas. Based on the thermal radiation differences between the decarburized areas (primarily ferrite) and the non-decarburized areas (primarily pearlite), a carbon distribution map is rapidly generated, enabling non-destructive localization of decarburized micro-areas. Compared to destructive metallographic inspection, this method is not only more efficient but also covers the entire length of the strip, preventing missed inspections. During the repair phase, this technology eliminates traditional bulk carburizing or secondary annealing in favor of localized heating of the decarburized areas with a pulsed laser beam. Simultaneously, a methane-argon mixture is introduced to in-situ carbon infiltration into the strip surface. The high energy density of the laser ensures efficient carburizing, while localized treatment avoids the energy waste and microstructural property fluctuations associated with bulk heating. Finally, high-frequency induction heating is used to quickly preheat the carbonized area, homogenizing the thermal stress introduced by the laser treatment and preventing cracks caused by stress concentration during subsequent quenching. The synergistic effect of this series of steps not only enables the real-time detection and precise repair of decarburization defects, but also significantly improves the overall quality stability of the steel strip. From a technical perspective, this method's beneficial effects are primarily reflected in three key areas: First, by integrating oxygen sensors with infrared imaging, early warning and precise localization of decarburization risks are achieved, avoiding the blind spots of traditional methods. Second, the localized laser carburization approach balances efficiency and energy consumption, repairing only the defective area without affecting other properties of the steel strip. Third, the high-frequency induction homogenization process eliminates localized thermal stresses, ensuring process stability during subsequent quenching. These improvements, combined, systematically address the decarburization issue in high-strength spring steel strip, thereby improving the fatigue life and reliability of the final product and meeting the demanding requirements of high-end applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings, wherein: Figure 1 It is a flowchart of the steps of the on-line detection and compensation method for local decarburization of a high-strength spring steel strip of the present invention; Figure 2 is a flow chart of the steps of arranging the oxygen content sensor of the present invention; Figure 3 It is a flowchart of the steps of a specific embodiment of infrared thermal imaging scanning of the present invention; Figure 4 This is the surface carbon distribution map obtained by the present invention through three-dimensional fusion processing; Figure 5 is a flow chart of the steps for determining the ratio of methane-argon mixed gas according to the decarburization level of the present invention; Figure 6 This is a flow chart of the steps of adjusting the laser spot diameter and power density according to the area and depth of the decarburization zone of the present invention; Figure 7 It is a flowchart of the specific process of the present invention for performing overall rapid preheating by a high-frequency induction heating device. DETAILED DESCRIPTION

[0019] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0020] Reference Figure 1 As shown, an online detection and compensation method for local decarburization of a high-strength spring steel strip according to the present invention comprises the following steps: Multiple groups of oxygen content sensors are placed in the soaking and slow cooling sections of the continuous annealing furnace to monitor the oxygen concentration in each area of ​​the furnace in real time. When the oxygen concentration of any group of oxygen content sensors exceeds the threshold, the coordinates of the steel strip position in that area are recorded and marked as a potential decarburization risk section. It should be noted that because the decarburization of steel strips is mainly caused by the oxidation reaction between carbon and oxygen in a high-temperature environment, when the oxygen concentration exceeds the threshold, it indicates that there is an oxidizing atmosphere in the area, which will accelerate the loss of carbon elements on the surface of the steel strip. Therefore, it is necessary to record the coordinates of the steel strip at this location and mark it as a potential decarburization risk section; this real-time monitoring method can provide early warning of decarburization risks, provide a basis for subsequent targeted testing, and avoid the randomness and lag problems of traditional sampling testing. At the same time, determining the decarburization area of ​​the steel strip can narrow the decarburization range of the steel strip and reduce subsequent workload.

[0021] Specifically, the determination of the oxygen concentration threshold is fundamentally based on the coupling of materials science and heat treatment processes. For high-strength spring steel strip (such as 60Si2MnA), the critical decarburization oxygen concentration threshold is determined using an experimentally established process database. Specifically, in a laboratory environment simulating the actual operating conditions of an annealing furnace, the variation in surface decarburization depth over time (1-30 minutes) at varying oxygen concentrations (5-500 ppm gradient) was measured. Combined with metallographic analysis and microhardness testing, the critical oxygen concentration that results in significant decarburization (>0.05 mm) was determined.

[0022] This threshold is not a fixed value, but is dynamically associated with three key parameters: one is the annealing furnace temperature zone where the steel strip is currently located (the threshold value of the soaking section is set 20-30% lower than that of the slow cooling section), the second is the running speed of the steel strip (for every 10m / min increase in speed, the threshold value is lowered by 5%), and the third is the thickness of the steel strip (for every 0.1mm increase in thickness, the threshold value is increased by 3%).

[0023] Perform infrared thermal imaging scanning on the surface of the steel strip in the potential decarburization risk zone after leaving the furnace. Utilizing the difference in thermal radiation between the decarburized and non-decarburized areas, a surface carbon distribution map is generated to locate the actual position and range of the decarburized area. It should be noted that since the main component of the decarburization zone is ferrite, its thermal emissivity is lower than that of the normal area dominated by pearlite. This difference can be clearly presented through infrared imaging, thereby generating an accurate surface carbon distribution map. This method can not only accurately locate the position and range of the actual decarburization zone, but also realize non-contact, full-surface detection. Compared with destructive metallographic testing, this method is not only more efficient, but also can cover the entire length of the potential decarburization risk section to avoid missed detection.

[0024] The identified decarburized area is locally heated to the austenitizing temperature using a pulsed laser beam, and a methane-argon mixed gas is introduced at the same time to cause in-situ carburizing reaction in the decarburized area, restoring the surface carbon content to the level of the steel strip matrix; It should be noted that this technology abandons traditional overall carburizing or secondary annealing and instead uses a pulsed laser beam to locally heat the decarburized area. At the same time, a methane-argon gas mixture is introduced to allow carbon to penetrate the surface of the steel strip in situ. The high energy density of the laser ensures carburizing efficiency, while local treatment avoids energy waste and fluctuations in microstructure and properties caused by overall heating. The austenitizing temperature is the temperature range in which the solubility of carbon in steel is the highest. At this temperature, the crystal structure of the steel is most conducive to the diffusion and solid solution of carbon atoms. In the heat treatment of steel strip, the specific value of the austenitizing temperature depends on the chemical composition of the steel (especially the carbon content and alloying elements). Taking a typical high-strength spring steel (such as 60Si2MnA) as an example: its austenitizing temperature range is usually 850-920℃ (the specific value needs to be adjusted according to the steel type).

[0025] The activated carbon atoms produced by the decomposition of methane at high temperature can quickly penetrate into the surface of the steel strip, while argon as a protective gas can prevent secondary oxidation; this local treatment method realizes in-situ carburizing repair of the decarburized area, which not only ensures the carbon replenishment effect, but also avoids the energy waste and fluctuations in organizational properties caused by the overall treatment.

[0026] The carbon-replenished steel strip is quickly preheated as a whole by a high-frequency induction heating device to eliminate the local thermal stress caused by laser carbon replenishment; It should be noted that since laser carbon filling will form a temperature gradient in local areas and cause concentrated thermal stress, high-frequency induction heating uses the principle of electromagnetic induction to uniformly heat the entire steel strip, prompting the internal stress to be redistributed to reach a balanced state. This step effectively eliminates local thermal stress, prevents the risk of cracking caused by uneven stress during subsequent quenching, and ensures the quality stability of the final product.

[0027] The method of this embodiment realizes the full process control from early warning, detection to repair through the organic coordination of various steps, and significantly improves the surface quality and fatigue performance of the spring steel strip.

[0028] Reference Figure 2 As shown, according to the actual use requirements, in order to solve the detection blind spot problem and misjudgment risk that may exist in the traditional arrangement, this embodiment optimizes the design of the oxygen content sensor arrangement: in the specific implementation process, first, differentiated arrangements are made according to the structural characteristics of each section of the annealing furnace. Since the soaking section is a high-incidence area of ​​decarburization, the distance between two adjacent groups of oxygen content sensors is controlled to no more than 1 / 5 of the length of the section. This design ensures that a continuous detection coverage area can be formed in the running direction of the steel strip to avoid monitoring gaps; in the slow cooling section, since the decarburization risk is relatively lower, the spacing is relaxed to no more than 1 / 3 of the length of the section, thereby optimizing the number of sensor configurations while ensuring the detection effect.

[0029] Each group of oxygen content sensors is equipped with at least three detection points in the width direction of the steel strip. This multi-point arrangement can effectively capture the uneven distribution of oxygen concentration in the transverse direction of the steel strip, overcoming the defect that single-point detection may miss the edge area.

[0030] When two or more adjacent inspection points along the width simultaneously detect excessive oxygen concentrations, the marking area is expanded longitudinally. This design leverages the physical property that abnormal oxygen concentrations typically diffuse along the strip's travel direction under the influence of airflow. By expanding the markings at 1 / 8 spacing upstream and downstream, it ensures complete coverage of the potential risk area while avoiding resource waste caused by excessive marking. This intelligent area expansion algorithm is particularly suitable for dynamic inspection scenarios on continuous production lines. By establishing a spatial correlation judgment mechanism, it significantly improves the accuracy of risk marking.

[0031] The oxygen content sensor arrangement method disclosed in this embodiment combines the fine division of the spatial dimension with the real-time monitoring of the time dimension to achieve all-round, no-dead-angle monitoring of the oxygen concentration distribution in the annealing furnace, providing a reliable early warning basis for subsequent decarburization detection and compensation; from the technical effect point of view, this arrangement method ensures the continuity of detection through reasonable spacing setting on the one hand, and improves the reliability of detection through multi-point detection and intelligent expansion mechanism on the other hand, and ultimately achieves high-precision positioning of potential decarburization risk sections, providing accurate data support for the subsequent steps of the entire method.

[0032] Specifically, when the method of this embodiment detects the potential decarburization risk section of the steel strip, the steel strip is continuously running in the annealing furnace. In order to mark the detected potential decarburization risk section for subsequent precise positioning, when the oxygen content sensor detects an abnormality and determines the potential decarburization risk section, it immediately triggers the color code laser coder to mark the edge of the steel strip. This marking method utilizes the principle of interaction between the laser and the surface material of the steel strip. By precisely controlling the laser parameters, a stable and lasting color mark is formed on the surface without affecting the performance of the steel strip itself. Compared with traditional inkjet or mechanical scratching markings, laser coding has higher accuracy and environmental adaptability, can work stably on high-temperature and high-speed production lines, and the mark will not disappear due to processing in subsequent processes.

[0033] The marking position at the edge of the steel strip is based on two important considerations: first, to avoid affecting the main use area of ​​the steel strip, and second, to facilitate the rapid location of potential decarburization risk sections in subsequent processes. Among them, the color code is selected considering the high contrast with the steel strip substrate to ensure accurate identification under various lighting conditions.

[0034] From a technical perspective, this marking method uses non-contact laser technology to change the oxidation state of the steel strip surface at a microscopic level, thereby producing color differences. This method neither introduces foreign contaminants nor causes mechanical damage, fully meeting the cleanliness requirements of steel production. The entire marking method is data-linked with other detection equipment on the production line. When the oxygen content sensor detects an anomaly, the position coordinates can be transmitted to the coding and marking device in real time, ensuring the timeliness and accuracy of the marking and establishing a reliable position reference for the entire decarburization detection and compensation process.

[0035] Reference Figure 3 As shown, on the basis of the above embodiments, the present invention further deepens and refines the infrared thermal imaging scanning steps, constructs a complete method for identifying decarburization areas using infrared thermal imaging scanning, and realizes accurate detection and quantitative evaluation of decarburization defects on the surface of steel strips.

[0036] In the specific implementation process, multiple groups of specially configured infrared thermal imager arrays are first arranged in the running direction of the steel strip. Each array consists of three thermal imagers with a specific spatial distribution, which synchronously collect thermal radiation data of the steel strip surface from three directions: directly above, 45 degrees to the left, and 45 degrees to the right. The design of this multi-angle synchronous scanning is based on an important material science phenomenon: the decarburized area and the non-decarburized area will show different thermal radiation characteristics at different observation angles. This multi-dimensional data can effectively eliminate the interference caused by the oxide film, slight warping or contaminants on the steel strip surface during single-angle detection; refer to Figure 4 As shown, the collected multi-source data will be transmitted to the central processing system in real time for three-dimensional fusion processing. The thermal images at different angles will be accurately superimposed through the spatial registration algorithm to generate a high-resolution surface carbon distribution map, in which: the decarburized area is a ferrite-rich area with low thermal emissivity, presenting a banded low-temperature dark area; the non-decarburized area is a mixture of pearlite and austenite with high emissivity and a uniform bright background.

[0037] After obtaining the carbon distribution map, the thermal radiation difference values ​​of each area are calculated based on the inherent relationship between the material's thermophysical properties and carbon content. This step utilizes the essential differences in specific heat capacity, thermal conductivity, and thermal emissivity between the decarburized zone (predominantly ferrite) and the normal zone (predominantly pearlite). By establishing a corresponding model between thermal radiation difference and decarburization depth, the transition from qualitative detection to quantitative analysis is achieved. This model, trained with extensive experimental data, accurately infers the decarburization depth based on the value of thermal radiation difference. When the detection value exceeds a set threshold, it is marked as a decarburized zone, and key parameters such as the decarburization area percentage and maximum depth are calculated to determine the decarburization level, providing an accurate quantitative basis for subsequent compensation processing.

[0038] In this example, standard specimens with different decarburization depths (gradient of 0.01-0.5 mm) were prepared in the laboratory, and a quantitative relationship between decarburization depth and mechanical properties was established using microhardness testing and electron probe analysis. The experiments found that when the decarburization depth exceeded 0.08 mm, the fatigue life of 60Si2MnA spring steel decreased by more than 30%. This critical value was determined to be the minimum detection threshold.

[0039] After the decarburization area is identified and graded, the edge recognition algorithm is activated to extract the contours of the decarburization area. This step uses advanced image processing technology to accurately distinguish the boundaries between the decarburization area and the normal area, and generate a decarburization area contour map with sub-millimeter accuracy. This precise contour information is crucial for the subsequent laser carburization process, as it ensures that the carburization treatment can completely cover the decarburization area while minimizing the impact on the normal area.

[0040] The entire method achieves comprehensive, high-precision detection of decarburization defects on the steel strip surface through the organic combination of multi-angle data acquisition, three-dimensional information fusion, quantitative model analysis, and precise contour extraction, providing reliable technical support for the entire online decarburization detection and compensation method. From a technical perspective, this multi-angle 3D scanning method has significant advantages over traditional single-angle detection: first, the spatial multi-source data fusion effectively eliminates the limitations of a single detection perspective; second, the quantitative analysis model based on the thermal properties of the material greatly improves the accuracy of the detection results; finally, precise contour extraction creates the necessary conditions for subsequent local compensation processing, thereby ensuring the optimal processing accuracy and efficiency of the entire method.

[0041] Reference Figure 5 As shown, after determining the decarburization grade according to the decarburization area ratio and the maximum depth, the present invention further establishes a scientific correspondence between the decarburization grade and the carburizing gas ratio, and optimizes the carbon compensation efficiency through a graded treatment mechanism.

[0042] During specific implementation, defects are first divided into three process levels based on the determined decarburization depth data: when the decarburization depth does not exceed 0.1mm, it is classified as mild decarburization. At this time, the carbon loss of the steel strip matrix is ​​relatively small, and a methane-argon mixing ratio of 1:9 is used. This low-concentration carburizing gas can not only ensure the recovery of surface carbon content, but also avoid the formation of brittle phases caused by excessive carburization; for moderate decarburization of 0.1-0.15mm, switch to a medium-concentration ratio of 1:4, and accelerate the diffusion of carbon atoms by increasing the methane ratio to compensate for the deeper carbon loss layer; when a severe defect with a decarburization depth exceeding 0.15mm is detected, a high-concentration ratio of 1:2 is used. At this time, argon mainly plays a carrier gas and protective role, and the active carbon atoms produced by the decomposition of high-density methane can quickly fill the deep decarburization area.

[0043] This hierarchical control mechanism is based on the principles of carbon diffusion kinetics. Different ratios correspond to different carbon potential gradients: Mild decarburization requires a low carbon flux, and the use of a rarefied gas allows for precise control of the carburized layer. Moderate decarburization requires a moderate carbon concentration gradient to ensure that carbon atoms can penetrate the ferrite layer. Severe decarburization requires a steep carbon potential gradient to drive carbon atoms to diffuse deeper. During implementation, the temperature field distribution in the laser heating zone is monitored in real time, and the gas flow rate is dynamically adjusted to maintain a balance between the methane decomposition rate and the carbon absorption capacity of the austenite, avoiding the formation of carbon black or insufficient carburization.

[0044] From the technical perspective, this intelligent proportioning method has achieved three major breakthroughs: First, through the precise mapping of defect levels and process parameters, the process risks of "over-processing" or "under-processing" are avoided; second, the graded gas proportioning significantly improves the carbon atom utilization rate and reduces gas consumption compared to the traditional fixed proportioning process; third, the dynamic adjustment mechanism ensures that different depths of decarbonization zones can obtain uniform and consistent carbon compensation.

[0045] Reference Figure 6 As shown, on the basis of determining the geometric characteristics of the decarburization zone, the present invention also provides an adaptive laser processing method, which realizes the intelligent matching of laser parameters and the morphology of the decarburization zone. By establishing a dynamic response mechanism between the decarburization zone area and the laser beam parameters, the problem of uneven energy distribution or low processing efficiency in traditional laser processing is solved.

[0046] In specific implementation, the generated decarburization zone contour map is first analyzed, the projected area and maximum depth of the decarburization zone are accurately calculated, and the optimal processing mode is automatically selected according to the area threshold: when a micro decarburization zone with an area less than 10mm² is detected, a small spot with a diameter of 1-2mm is enabled in combination with a high power density mode. This configuration can concentrate energy in a small area and quickly establish austenite phase transformation conditions through short-term high temperature action. At the same time, high energy density can ensure that carbon atoms are fully diffused within a limited action time, avoiding the heat-affected zone from diffusing into the normal area; for medium-sized decarburization zones of 10-50mm², it is switched to 3-5m In the m-spot scanning mode, the laser beam fully covers the decarburization area along a specific trajectory, maintaining a stable temperature field by controlling the scanning speed and overlap rate. This mode not only ensures processing efficiency but also makes the energy distribution more uniform. When encountering a large-scale decarburization area exceeding 50mm², the multi-spot collaborative working mode is activated, dividing the target area into several sub-areas, which are processed synchronously by multiple laser heads. A 10-15% overlap band is set in the active area of ​​each spot to ensure seamless connection. This distributed processing strategy effectively overcomes the temperature unevenness problem caused by edge cooling when a single-spot system processes a large area.

[0047] From a technical perspective, this hierarchical adjustment method is based on three key characteristics of the interaction between lasers and materials: First, the spot size determines the energy range, which needs to be matched to the geometric dimensions of the decarburization zone to avoid energy waste or insufficient coverage; second, the power density affects the austenitization rate and depth, requiring dynamic adjustment based on the decarburization depth; and third, the coordinated use of multiple spots through phase control can achieve an interference enhancement effect, improving energy utilization over large areas. During implementation, the melt pool morphology and temperature distribution are monitored in real time, and the laser parameters are dynamically fine-tuned through closed-loop feedback to ensure consistent treatment quality across decarburization zones of varying sizes.

[0048] The technical advantages of this method are reflected in three aspects: first, adaptive parameter matching enables the laser energy to act precisely on the target area, minimizing the heat-affected zone; second, the multi-mode switching mechanism takes into account both processing accuracy and efficiency, allowing both small-area repair and large-area processing to maintain optimal speed; finally, the collaborative processing mode avoids the power density drop caused by traditional single-beam expansion through spatial energy distribution, perfectly achieving the process goal of "precise repair with minimal intervention" and providing reliable guarantee for the uniformity of steel strip quality.

[0049] Specifically, when using a pulsed laser beam to heat the steel strip, the target temperature of the pulsed laser beam is first set to a superheat range of 50 to 80°C above the austenitizing temperature. The setting of this temperature window is based on two key scientific principles: on the one hand, moderate overheating can accelerate the austenite phase transformation process and shorten the structural transformation time; on the other hand, retaining a margin of 50 to 80°C can compensate for the heat loss caused by heat conduction during the laser heating process, ensuring that the actual working temperature is always maintained above the austenitizing critical point.

[0050] After the laser begins operating, a high-precision infrared temperature sensor monitors the temperature curve of the steel strip's heating zone in real time with millisecond sampling frequency. When the temperature reaches 70% of the austenitization temperature, a gradual introduction of a methane-argon mixture is initiated. The gas flow rate increases linearly with the temperature rise. This proactive gas supply strategy ensures a uniform gas blanket forms on the steel strip's surface immediately upon reaching the austenitization temperature. As the temperature continues to rise toward the austenitization critical point, the gas flow rate simultaneously reaches a preset maximum value. At this point, the austenite lattice on the steel strip's surface fully expands, carbon solubility peaks, and the active carbon atoms generated by the decomposition of the high-flow gas mixture rapidly dissolve into the austenite lattice. This temperature-flow control mechanism is based on the diffusion kinetics of carbon in austenite. During the heating phase, the carbon diffusion coefficient increases exponentially with temperature. Increasing the flow rate ensures that the carbon supply rate matches the diffusion capacity. During the austenitization plateau, the maximum flow rate ensures saturation of the surface carbon potential, forming a concentration gradient toward the center that drives diffusion.

[0051] From the technical perspective, this dynamic control method has achieved three breakthroughs: first, the superheat design ensures the rapid start-up of the austenitization process; second, the mechanism of proportional adjustment of flow rate with temperature avoids ineffective gas supply in the low-temperature stage and delayed gas supply in the high-temperature stage; finally, the precise synchronization of maximum flow rate and austenitization temperature ensures the full absorption of carbon atoms in the most active stage of the lattice.

[0052] During the specific implementation process, after the laser-induced carburizing reaction is initiated, a high-sensitivity online spectrometer is used to monitor the carbon concentration gradient in the treatment area at a sampling frequency of 20 times per second. Its working principle is to use the difference in emission intensity of steels with different carbon contents in the characteristic spectral band to convert the spectral signal into a real-time carbon concentration value through a calibration curve. When the surface carbon concentration is detected to have recovered to the critical point of 95% of the matrix level, the process conversion program is immediately triggered: first, the energy supply of the pulsed laser beam is cut off to stop the austenitization process; then, the methane-argon mixed gas valve is simultaneously closed to terminate the carbon source input. This dual shut-off mechanism is based on the precise control of the material phase transformation dynamics. The 95% threshold setting not only ensures the full recovery of the main carbon content, but also retains an appropriate margin for carbon homogenization in the subsequent holding stage.

[0053] After entering the holding stage, the holding time is automatically calculated based on the projected area of ​​the decarburization zone and dynamically adjusted at a ratio of 5s / mm². This parameter is derived from a carbon diffusion model established based on a large amount of experimental data, which can ensure that carbon atoms in decarburization zones of different sizes have sufficient time to complete redistribution. During the holding period, the spectrometer operates continuously, tracking the changes in carbon concentration in real time until it reaches the matrix level. This continuous monitoring mechanism effectively solves the problem of uneven carbon distribution caused by local temperature fluctuations.

[0054] From a technical perspective, this method offers three scientific advantages: First, online spectral monitoring overcomes the lag inherent in traditional post-process testing, enabling real-time closed-loop control of the process. Second, intelligent determination based on the 95% threshold avoids energy waste caused by blindly extending the main carburizing stage. Finally, the area-proportional holding time design conforms to Fick's second law of diffusion, ensuring optimal homogenization of defects of varying sizes. In practical applications, precise holding time control has successfully eliminated the "edge carbon depletion" phenomenon common in traditional processes, particularly for large decarburized areas (>50 mm²), providing reliable assurance for consistent quality of high-strength spring steel strip.

[0055] On the basis of the above embodiments, the present invention further optimizes the control process of the termination stage of the carburizing process. By establishing a gradual exit mechanism for gas supply, it effectively solves the problem of surface quality defects caused by traditional sudden gas interruption. The core of this technical method is to construct a coordinated attenuation curve of gas flow and temperature field to achieve a smooth transition of process parameters.

[0056] During the specific implementation process, when the online spectrometer detects that the carbon concentration has reached the conversion condition of 95% of the matrix level, the gas valve will not be closed immediately. Instead, an intelligent gas reduction program will be initiated: first, the total gas flow rate will be gradually reduced at a rate of 5% of the initial flow rate per second. This stage lasts for 8-10 seconds to complete the 90% flow reduction. At the same time, the ratio of methane to argon in the mixed gas is synchronously adjusted, and the methane ratio is linearly reduced from the working concentration to zero in the first 5 seconds, and pure argon is maintained for the next 3-5 seconds. This phased exit strategy is based on the coupling principle of gas dynamics and surface reaction: the gradual flow reduction avoids the disturbance of the molten pool caused by sudden changes in gas pressure and maintains the stability of the surface reaction; the preferential reduction of the methane ratio ensures that the supply of active carbon atoms precedes the withdrawal of the shielding gas, preventing non-equilibrium carburization in the late stage; and the pure argon maintenance stage effectively suppresses the secondary oxidation of the high-temperature steel strip surface through inert gas coverage.

[0057] In terms of temperature control, the cooling curve is monitored in real time. Argon protection is only fully terminated when the temperature drops below the safety threshold of 600°C (below the onset of significant oxidation of the steel strip). Throughout this process, the rate of change in gas flow is dynamically matched to the rate of temperature drop, with a PID control algorithm ensuring optimal coordination between the two.

[0058] From a technical perspective, this intelligent exit mechanism has achieved three breakthroughs: first, it avoids fluctuations in surface carbon concentration caused by sudden gas outages; second, the methane-first exit strategy eliminates free carbon deposition caused by the decomposition of residual hydrocarbons; finally, the design of argon protection to a safe temperature can control the formation of a surface oxide layer, greatly reducing the workload of subsequent cleaning processes.

[0059] Reference Figure 7 As shown in the figure, in the stress elimination stage, a set of precisely controllable gradient heating and stress control methods were constructed. Through multi-stage temperature control and real-time feedback adjustment, precise microstructure control of the steel strip after carbon filling heat treatment was achieved. This technical method combines zoned induction heating, gradient heating and dynamic stress monitoring technology to solve the problems of temperature unevenness and stress concentration caused by traditional overall heating.

[0060] During implementation, multiple independently temperature-controlled induction coil arrays are arranged across the width of the steel strip. Each coil is equipped with a dedicated power regulation module. A high-resolution infrared thermal imager scans the transverse temperature distribution of the steel strip at a frequency of 50 frames per second. If the temperature in any area deviates by more than ±15°C from the set value, the temperature gradient is immediately calculated and the power output of the coil at the corresponding location is adjusted. This real-time compensation mechanism, based on the skin effect of electromagnetic induction heating, precisely controls the heat penetration depth of different areas by varying the frequency and power, thereby maintaining transverse temperature uniformity of the steel strip within ±10°C.

[0061] The heating process utilizes a three-stage gradient heating strategy: Initially, the temperature is rapidly increased at a rate of 30-50°C / s to quickly move the steel strip through the low-temperature brittle zone, avoiding the risk of hydrogen embrittlement caused by prolonged dwelling below 300°C. The heating rate is then reduced to 15-20°C / s in the intermediate temperature zone, where the strip's plasticity increases. Properly controlling the heating rate reduces thermal stress accumulation. Finally, the target temperature is approached at a slower rate of 5-10°C / s to ensure smooth microstructural transformation. This variable heating curve design conforms to the temperature-dependent variation of steel's thermal expansion coefficient, ensuring that thermal stress development in different temperature zones remains under control.

[0062] When the temperature reaches the target range of 550-600℃, it enters the insulation stage. At this time, the laser ultrasonic detection device starts working, and the internal stress state is inverted in real time by measuring the changes in the surface acoustic wave propagation velocity. The insulation time is dynamically adjusted according to the stress attenuation curve. Usually, the residual stress can be reduced to below the safety threshold (<50MPa) within 30-60 seconds. This process utilizes the principle of high-temperature creep of metals and achieves stress relaxation through atomic diffusion and recombination.

[0063] When entering the cooling process, a micro-tension of 10-15N / mm² is applied to both ends of the steel strip. This controlled tensile state can effectively offset the shrinkage stress during the cooling process and prevent warping deformation. The entire cooling process can be carried out by segmented air cooling. First, the wind speed is quickly passed through the pearlite transformation zone, and then the cooling rate is reduced to pass through the bainite transformation zone, so that the structure obtains an ideal proportion.

[0064] From the perspective of technical mechanism, this method has three core advantages: first, the combination of zoned induction heating and real-time temperature control solves the problem of uneven heat treatment caused by the edge effect of wide steel strips; second, the gradient heating strategy matches the changes in the thermophysical properties of the material in different temperature ranges through rate optimization; finally, the synergistic effect of stress monitoring and tension control ensures the dimensional stability of the steel strip throughout the entire heat treatment process.

[0065] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. An online detection and compensation method for local decarburization of high-strength spring steel strip, characterized in that: The following steps are involved: Multiple groups of oxygen content sensors are placed in the soaking and slow cooling sections of the continuous annealing furnace to monitor the oxygen concentration in each area of ​​the furnace in real time. When the oxygen concentration of any group of oxygen content sensors exceeds the threshold, the coordinates of the steel strip position in that area are recorded and marked as a potential decarburization risk section. Perform infrared thermal imaging scanning on the surface of the steel strip in the potential decarburization risk zone after leaving the furnace. Utilizing the difference in thermal radiation between the decarburized and non-decarburized areas, a surface carbon distribution map is generated to locate the actual position and range of the decarburized area. The identified decarburized area is locally heated to the austenitizing temperature using a pulsed laser beam, and a methane-argon mixed gas is introduced at the same time to cause in-situ carburizing reaction in the decarburized area, restoring the surface carbon content to the level of the steel strip matrix; The carbon-replenished steel strip is quickly preheated as a whole by a high-frequency induction heating device to eliminate the local thermal stress caused by laser carbon refilling.

2. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 1, characterized in that: The arrangement of the oxygen content sensor is as follows: The spacing between the multiple groups of oxygen content sensors is determined according to the length of each section of the annealing furnace. The spacing between two adjacent groups of oxygen content sensors arranged in the soaking section shall not exceed 1 / 5 of the length of the section, and the spacing between two adjacent groups of oxygen content sensors arranged in the slow cooling section shall not exceed 1 / 3 of the length of the section. The detection range of each set of oxygen content sensors covers the entire width of the steel strip, and at least three detection points are set in the width direction; When two or more adjacent detection points in the width direction detect that the oxygen concentration exceeds the threshold at the same time, the marking area is extended in the length direction to the area 1 / 8 of the distance upstream and downstream of the detection range of the group of oxygen content sensors, and the steel strip at the corresponding position in the area is marked as a potential decarburization risk section.

3. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 2, characterized in that: After determining the potential decarburization risk section, a color laser coder is used to mark the starting position of the risk section on the edge of the steel strip.

4. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 1, characterized in that: The specific implementation of the infrared thermal imaging scanning includes: Multiple infrared thermal imager arrays are set up in the running direction of the steel belt. Each array contains three thermal imagers at different angles, which perform synchronous scanning from directly above the steel belt, 45 degrees to the left, and 45 degrees to the right. The data collected by each group of thermal imagers is fused in three dimensions to generate a surface carbon distribution map, and the thermal radiation difference is determined based on the surface carbon distribution map; A model for the relationship between thermal radiation difference and decarburization depth is established. When the thermal radiation difference exceeds the set threshold, it is marked as a decarburization area. The area and depth of the decarburization area are calculated to determine the decarburization level. The marked decarburization area is subjected to edge recognition and contour extraction to generate an accurate decarburization area contour map.

5. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 4, characterized in that: Determining the ratio of methane-argon mixed gas according to the decarburization level includes the following steps: The decarburization level is divided into three levels, including: mild decarburization: decarburization depth ≤ 0.1mm, moderate decarburization: 0.1mm < decarburization depth ≤ 0.15mm, severe decarburization: decarburization depth > 0.15mm; According to different decarburization levels, the methane-argon mixing ratio is adjusted: mild decarburization adopts a methane-argon mixing ratio of 1:9, moderate decarburization adopts a methane-argon mixing ratio of 1:4, and severe decarburization adopts.

6. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 4, characterized in that: According to the area and depth of the decarburization zone, adjust the laser spot diameter and power density, where: For decarburization areas less than 10mm², a small spot with a diameter of 1-2mm and high power density mode is used; For decarburization areas of 10-50 mm², a medium spot scanning mode with a diameter of 3-5 mm is used; For decarburization areas larger than 50mm², a regional multi-spot collaborative processing mode is adopted.

7. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 1, characterized in that: The temperature of the pulsed laser beam is set to 50 to 80°C above the austenitizing temperature. The real-time temperature of the steel strip during heating is monitored, and the flow rate of the methane-argon mixed gas introduced is adjusted according to the real-time temperature ratio of the steel strip. When the steel strip is heated to the austenitizing temperature, the flow rate of the methane-argon mixed gas reaches its maximum value.

8. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 1, characterized in that: During the in-situ carburizing reaction in the decarburized zone, an online spectrum analyzer was used to monitor the surface carbon concentration changes. When the carbon concentration reached 95% of the matrix level, the system entered the heat preservation stage, stopping the pulsed laser beam local heating and the supply of methane-argon mixed gas. The holding time is calculated based on the decarburization area at 5s / mm², and the surface carbon concentration change value is continuously detected until the surface carbon content reaches the level of the steel strip matrix.

9. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 8, characterized in that: When stopping the supply of methane-argon mixed gas, the gas supply is stopped by gradually reducing the gas flow rate, and the ratio of the methane-argon mixed gas is gradually adjusted to maintain argon protection until the temperature drops to a safe range.

10. The method for online detection and compensation of local decarburization of a high-strength spring steel strip according to claim 1, characterized in that: The specific process steps for overall rapid preheating by high-frequency induction heating device include: Multiple sets of induction coils are arranged in the width direction of the steel strip. The power of each set of coils is independently adjustable. The lateral temperature distribution of the steel strip is monitored in real time by an infrared thermal imager. When the temperature deviation exceeds ±15°C, the power of the coil at the corresponding position is adjusted. In the initial stage, the temperature is raised to 300-350°C at a rate of 30-50°C / s, in the middle stage, the temperature is raised to 450-500°C at a rate of 15-20°C / s, and in the final stage, the temperature is raised to the target temperature of 550-600°C at a rate of 5-10°C / s; Maintain the target temperature for 30-60 seconds to fully release the thermal stress, and monitor the stress changes in real time through a laser ultrasonic detection device. When the residual stress drops below the safety threshold, enter the cooling process. During the cooling process, maintain the tension state and stretch the steel strip.

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