A controlled atmosphere heat treatment method for preventing decarburization of a wire rod surface

Abstract

This invention discloses a controlled atmosphere heat treatment method for preventing decarburization on the surface of wire, belonging to the field of metal wire heat treatment technology. The method includes: preparing an atmosphere by mixing methanol cracking gas with nitrogen to obtain a reference gas, and mixing nitrogen and hydrogen to obtain a reducing gas; placing the wire in a preheating zone at 500 to 700°C, and passing the reducing gas to reduce surface oxides; then placing it in a holding zone at 800 to 1050°C, passing the reference gas, and maintaining a slight positive pressure; during this period, closed-loop control is performed, and real-time adjustments are made to ensure that the absolute deviation of the mass fraction of the actual carbon potential from the target carbon potential is controlled within ±0.02%; then cooling to 500°C or below, and injecting nitrogen to dilute carbon monoxide to simultaneously reduce the carbon potential; after cooling, the wire is removed via a water seal or nitrogen curtain. This invention, through preheating and micro-reduction treatment of the wire, effectively reduces trace oxides on the wire surface, providing excellent surface conditions for subsequent high-temperature heat treatment.

Description

Technical Field

[0001] This invention relates to the field of heat treatment of metal wires, specifically a controlled atmosphere heat treatment method to prevent decarburization of wire surfaces. Background Technology

[0002] Medium and high carbon steel, alloy spring steel, and bearing steel wire rods are widely used in various fields of machinery manufacturing. Existing heat treatment processes are developing towards continuous heat treatment with high precision carbon control and non-destructive surface treatment, while ensuring basic mechanical properties. Among them, the anti-decarburization treatment of wire rod surface is receiving increasing attention, especially in the high-end equipment industry, where the requirements for micro and macro indicators such as wire rod hardness uniformity and fatigue life are becoming increasingly stringent. The heating and cooling process is the key link in wire rod heat treatment, and the carbon potential of the atmosphere in each process section of the furnace directly affects the surface microstructure and final comprehensive performance of the wire rod. High-alloy and medium-to-high carbon wires are generally used in high-strength core components subjected to alternating stress and are widely used in precision machinery. Therefore, the control of the ferrite softening layer and decarburization depth on the surface of this type of wire is relatively strict. Reducing the depth of the fully decarburized layer and the partially decarburized layer to below the detection limit of detectable methods is the development trend and basic requirement of this type of wire. However, due to the thermodynamic unsteady-state characteristics of the gas-solid interface and the limitations of traditional atmosphere control conditions, this type of wire has previously mostly been treated with a single high-purity nitrogen gas for protection or a fixed carbon potential mixed gas. Its dynamic correction and atmosphere intervention methods are limited. Traditional solutions cannot adapt to the fluctuations in oxygen partial pressure at the inlet and outlet and are prone to kinetic mismatch due to the sharp drop in carbon solid solubility during the cooling stage, which can lead to problems such as secondary decarburization, abnormal carburizing and carbon deposition, or precipitation of network carbides, thus limiting its application range in the field of high-end wire heat treatment. Summary of the Invention

[0003] The purpose of this invention is to provide a controlled atmosphere heat treatment method to prevent decarburization on the surface of wires, so as to solve the problems mentioned in the background art.

[0004] The technical solution of the present invention includes: S1, preparing an atmosphere: methanol is cracked at 900°C to 1050°C under the action of a nickel-based catalyst to obtain methanol cracked gas; methanol cracked gas is mixed with high-purity nitrogen to obtain a reference protective atmosphere; and high-purity nitrogen is mixed with hydrogen to obtain a reducing mixed atmosphere. S2. Preheating and Micro-reduction: The wire is sent into the preheating zone of the continuous heat treatment furnace through the nitrogen curtain sealing port. The temperature of the preheating zone is controlled at 500℃ to 700℃. A reducing mixed atmosphere is independently introduced into the preheating zone and the dew point of the preheating zone is controlled at -40℃ to -30℃ to reduce the oxides on the surface of the wire. S3, Insulation and other carbon potential locking: The preheated wire is sent into the insulation zone of a continuous heat treatment furnace at 800℃ to 1050℃, and a reference protective atmosphere is introduced into the insulation zone to maintain the gas pressure in the insulation zone at 150Pa to 300Pa. S4. Carbon potential closed-loop control: By collecting data from the zirconia oxygen probe, dew point meter and infrared gas analyzer set in the insulation zone in real time, the carbon potential is calculated in real time. The target carbon potential is set according to the nominal carbon content of the wire matrix and the temperature. The absolute deviation of the mass fraction between the actual carbon potential and the target carbon potential is controlled within ±0.02% by pulse injection of enriching gas or dilution gas. S5. Gradient carbon potential reduction control: The heat-insulated wire is sent into the cooling zone and gradually cooled to 500℃ and below. At the same time, high-purity nitrogen is injected at multiple points along the longitudinal direction of the cooling zone to dilute the carbon monoxide in the atmosphere of the cooling zone, reduce the carbon potential of the furnace gas, and make the carbon potential decrease synchronously with the temperature gradient. S6. Furnace exit treatment: The cooled wire is removed from the continuous heat treatment furnace through a water seal or nitrogen curtain.

[0005] Furthermore, in step S1, the volume fraction of hydrogen in the reducing mixed atmosphere is controlled between 5% and 10%.

[0006] Further, in step S1, the reference protective atmosphere is formed by mixing methanol cracking gas and high-purity nitrogen gas in a volume ratio of 1:0.8 to 1:1.2.

[0007] Furthermore, in step S4, the infrared gas analyzer is used to detect the partial pressures of carbon monoxide, carbon dioxide, and methane.

[0008] Furthermore, in step S4, the enrichment gas is methane or propane; the dilution gas is high-purity nitrogen.

[0009] Furthermore, in step S5, the decreasing curve of the atmospheric carbon potential in the gradient carbon potential control is matched with the decreasing curve of carbon solid solubility when the wire transforms from austenite to ferrite or pearlite at the corresponding temperature, so as to prevent carbon deposition or secondary decarburization on the surface of the wire during the cooling process.

[0010] Furthermore, in step S5, the cooling zone is divided into multiple temperature segments along the longitudinal direction. As the wire passes through different temperature segments in sequence, the injection volume of high-purity nitrogen is gradually increased to achieve a parabolic gradient carbon reduction potential control.

[0011] Furthermore, the wire is made of medium-high carbon steel, alloy spring steel, or bearing steel.

[0012] Compared with the prior art, the present invention has the following improvements and advantages: 1. By independently introducing a reducing mixed atmosphere made of high-purity nitrogen and hydrogen during the preheating stage and strictly controlling the dew point in a specific area of ​​the preheating zone, the wire can be preheated and micro-reduced. This process effectively reduces the trace oxides on the surface of the wire, providing excellent surface conditions for subsequent high-temperature heat treatment. 2. A multi-parameter closed-loop control system was introduced during the heat preservation stage. The system can accurately set the target carbon potential according to the nominal carbon content of the wire matrix and the temperature. When the deviation is made, enrichment gas or dilution gas is injected in a pulse manner to strictly control the carbon potential mass fraction deviation within ±0.02 percent, thereby achieving precise carbon balance and effectively preventing surface decarburization or abnormal carburization during high-temperature heat preservation. 3. During the cooling stage, a gradient carbon potential reduction control process is adopted. High-purity nitrogen is injected at multiple points along the longitudinal direction of the cooling zone to dilute the carbon monoxide concentration, so that the local atmosphere carbon potential gradually decreases in a parabolic shape. This decrease curve is precisely matched with the carbon solid solubility decrease curve during the transformation of austenite to ferrite or pearlite in the wire, so as to control the change of carbon potential during the cooling process and reduce the risk of surface carbon accumulation or secondary decarburization. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0014] Example 1: A controlled atmosphere heat treatment method for preventing decarburization on the surface of wire includes the following steps: S1. Preparation of the treatment atmosphere: After vaporizing liquid methanol, it is subjected to a cracking reaction under heating to 900℃ to 1050℃ and the action of a nickel-based catalyst to obtain methanol cracking gas containing carbon monoxide and hydrogen; the methanol cracking gas is mixed with high-purity nitrogen to obtain a reference protective atmosphere; high-purity nitrogen is mixed with hydrogen to obtain a reducing mixed atmosphere. S2. Preheating and micro-reduction treatment of wire: The wire to be treated is sent into the preheating zone of the continuous heat treatment furnace through the nitrogen curtain sealing port. The temperature of the preheating zone is controlled at 500℃ to 700℃. A reducing mixed atmosphere is independently introduced into the preheating zone, and the dew point of the preheating zone is controlled between -40℃ and -30℃ to reduce the trace oxides on the surface of the wire. S3. Perform heat preservation and carbon potential locking treatment on the wire: send the preheated wire into the heat preservation zone of the continuous heat treatment furnace, control the temperature of the heat preservation zone to 800℃ to 1050℃, introduce a reference protective atmosphere into the heat preservation zone, and maintain the gas pressure in the heat preservation zone at a slight positive pressure of 150Pa to 300Pa. S4. Closed-loop control of carbon potential in the insulation zone: A zirconia oxygen probe, dew point meter, and infrared gas analyzer are installed in the insulation zone to collect oxygen potential signals, dew point, and infrared gas partial pressure in real time, and calculate the real-time carbon potential; a target carbon potential is set according to the nominal carbon content of the wire matrix and the temperature of the insulation zone. When the real-time carbon potential deviates from the target carbon potential, enrichment gas or dilution gas is injected in a pulse manner to control the absolute deviation of the mass fraction of the actual carbon potential in the insulation zone from the target carbon potential within ±0.02%. S5. Gradient carbon potential reduction control treatment for wire: After heat preservation, the wire is sent into the cooling zone of the continuous heat treatment furnace, so that the temperature of the cooling zone drops from the temperature gradient of the heat preservation zone to 500℃ and below. At the same time, high-purity nitrogen is injected along the longitudinal direction of the cooling zone in multiple jets to dilute the carbon monoxide concentration in the atmosphere of the cooling zone, reduce the carbon potential of the furnace gas, and make the carbon potential drop synchronously with the temperature gradient and match the carbon solid solubility of the wire at this temperature, so as to avoid secondary decarburization or carbon deposition on the surface of the wire. S6. Furnace unloading process: The cooled wire is removed from the continuous heat treatment furnace through the water seal or nitrogen curtain at the discharge port. In this embodiment, alloy spring steel wire with a nominal carbon content of 0.60% is used as the treatment substrate. The cracking reaction temperature after methanol liquid vaporization is set at 900℃. Under this low-temperature critical thermodynamic condition, a nickel-based catalyst is used to promote the generation of high-purity methanol cracking gas and suppress the synthesis of by-products. The methanol liquid feed vaporization rate is set at 2.0L / h, and the specific loading amount of the nickel-based catalyst is 6kg. The nickel-based catalyst uses γ-Al₂O₃ as a support, and the active component is... The methanol cracking catalyst, for example, the PR-7 methanol cracking catalyst, and the catalyst contains The mass loading is 15% to 20%, and the specific surface area of ​​the carrier is 150 m² / g to 250 m² / g. The conveying speed of the wire in the continuous heat treatment furnace is set to 1.0 m / min. The temperature of the preheating zone is precisely locked at 500℃, and the holding time of the wire in the preheating zone is controlled at 18 min, while maintaining the dew point at -40℃. This low dew point parameter can drive the reducing mixed atmosphere to produce a physicochemical reduction reaction at the gas-solid interface. The hydrogen gas integral is set to 5%, which effectively reduces the micro oxide film attached to the surface of the alloy spring steel wire, providing good surface conditions for subsequent carbon atom exchange. After the wire enters the insulation zone, the insulation time is controlled at 50 minutes, the temperature of the insulation zone is set at 860℃, and the air pressure is maintained at a slight positive pressure of 150Pa. The reference protective atmosphere is mixed at a volume ratio of 1:0.8 to resist the disturbance of external micro-environment fluctuations. The closed-loop control system sets the target carbon potential based on the nominal carbon content and calculates the real-time carbon potential. The mathematical model is as follows: in, The oxygen potential signal measured by the zirconia oxygen probe. The absolute temperature of the insulation zone. and The figures are the partial pressures of carbon monoxide and carbon dioxide gases measured by an infrared gas analyzer, respectively. , , , Based on the empirical constants of the equipment calibration, the specific calibration method is as follows: Under a standard atmosphere with known carbon potential, multiple steady-state tests are conducted by adjusting the temperature, oxygen potential, and gas partial pressure respectively. The least squares method is then used to evaluate the results of multiple tests. , , , , The data was obtained by performing multiple linear regression fitting. In this embodiment, the specific values ​​of the constants determined after calibration for the continuous heat treatment furnace are as follows: , , , Target carbon potential The setting is based on the nominal carbon content of the wire matrix. and the temperature of the insulation area Confirm, set the correspondence as follows: In a preferred embodiment, set When faced with specific operating conditions requiring higher surface carbon concentration, set... .

[0015] In this embodiment, the target carbon potential is set to 0.65%; the partial pressures of carbon monoxide, carbon dioxide and methane are detected in real time using an infrared gas analyzer; the detected methane partial pressure data is used to monitor the cracking state of the enriched gas in the furnace in real time and to serve as a feedforward compensation reference when injecting enriched gas in a pulsed manner. When a carbon potential deviation is detected, methane is used as the enrichment gas or high-purity nitrogen is used as the dilution gas for pulsed injection to control the actual carbon potential mass fraction deviation within the range of ±0.01%, achieving dynamic equilibrium of carbon atoms at the gas-solid interface. In the subsequent gradient carbon potential reduction control stage, the temperature of the cooling zone is reduced to 450℃, and the residence time of the wire in the cooling zone is controlled to 25 minutes. By utilizing the hydrodynamic dilution effect of high-purity nitrogen injected at multiple points, the atmospheric carbon potential decreases synchronously with the phase transformation thermodynamic curve of the alloy spring steel wire, achieving a parabolic gradient carbon potential reduction control. This prevents the secondary decarburization and the potential precipitation of surface network carbides caused by the sharp decrease in carbon solid solubility during the cooling stage.

[0016] Example 2: This embodiment focuses on the processing conditions of medium- and high-carbon steel wire. To verify the process adaptability under moderate thermodynamic driving forces, the nominal carbon content of the matrix is ​​used. Using 0.55% medium-high carbon steel wire as the treatment substrate, the methanol liquid feed vaporization rate was set to 2.5 L / h, the nickel-based catalyst loading amount was 7 kg, and the wire conveying speed was controlled at 1.2 m / min; the preheating zone temperature was increased to 600℃, so that the wire residence time in the preheating zone was 18 min, and the dew point was set to -35℃. Under these environmental parameters, the hydrogen gas fraction in the reducing mixed atmosphere was precisely adjusted to 7.5%. This stoichiometric ratio ensured sufficient in-situ reduction motive power while avoiding the risk of hydrogen embrittlement. After the wire entered the insulation zone, the residence time was controlled at 50 minutes. The ratio of the reference protective atmosphere was 1:1.0, using an equal-volume mixing strategy to construct a fluid medium with a moderate carbon transfer coefficient. Subsequently, the wire entered the cooling zone, and the residence time was controlled at 25 minutes, verifying the robustness of this technical solution under conventional process parameters.

[0017] Example 3: For bearing steel wire with high carbon content, this embodiment verifies the system's operational stability under high temperature and high activity boundary conditions. The methanol cracking reaction temperature is pushed up to the upper threshold of 1050°C, the temperature of the insulation zone is simultaneously set to 1050°C, and a relatively high micro-positive pressure of 300 Pa is applied to enhance the furnace gas penetration. To match the reaction kinetic rate at high temperature, the hydrogen gas integral in the reducing mixed atmosphere is increased to 10% of the full load, and the reference protective atmosphere is mixed in a ratio of 1:1.2. This method of accelerating the interfacial reaction rate using a cascade mechanism establishes a robust isocarbon potential lock-in state in the high-temperature austenite region, demonstrating the system's ability to control thermal hysteresis effects when processing highly alloyed wire.

[0018] Example 4: This embodiment focuses on examining the corrective performance of a dynamic adaptive gradient carbon potential atmosphere system under unsteady-state disturbances. Propane is selected as the enrichment gas, and high-purity nitrogen with an input flow rate of 0.5 L / min is used as the dilution gas. Utilizing the high carbon potential energy of propane and the consumption characteristics of free oxygen in the air, carbon potential reconstruction is achieved during local oxygen partial pressure fluctuations. In the gradient carbon potential reduction control stage, this embodiment uses the nominal carbon content of the matrix. For high-carbon steel wire rod with a carbon content of 0.70%, the cooling zone is precisely divided into three characteristic temperature ranges: 780℃, 650℃, and 500℃; the control system calculates the critical threshold for phase transformation. For hypoeutectoid steel wire with a carbon mass fraction below 0.77%, the calculation formula is as follows: Substituting into this embodiment Calculation The critical temperature for phase transformation is 793℃; for hypereutectoid steel wire with a carbon mass fraction higher than 0.77%, the critical threshold for phase transformation is... The calculation formula is based on an empirical model of the temperature curve of austenite precipitating secondary cementite. The calculation formula is as follows: in, The slope constant is determined based on specific alloying elements of the wire; the critical threshold for phase transformation is calculated. The first characteristic temperature range of the cooling zone is set slightly below The temperature was set so that carbon monoxide could be diluted at the beginning of the phase transformation, and then the subsequent characteristic temperature ranges were set to 650℃ and 500℃ respectively according to the cooling rate requirements of the austenite to pearlite transformation. High-purity nitrogen was injected in an increasing manner across different temperature ranges: at 780℃, the injection flow rate of high-purity nitrogen was set to 2.0 m³ / s. 3 The flow rate was controlled at 5 m / s per hour, maintaining a local pressure of 180 Pa; at a temperature of 650℃, the injection flow rate was increased to 3.5 m³ / h. 3 The flow rate was controlled at 8 m / s, and the local pressure was maintained at 200 Pa; at a temperature of 500℃, the injection flow rate was further increased to 5.0 m³ / h. 3 / h, flow rate controlled at 12m / s, local air pressure maintained at 220Pa; As a comparison, if the high-purity nitrogen injection flow rate and velocity are lower than the above-mentioned set values, the dilution effect is insufficient, and the decrease in local atmosphere carbon potential lags behind the decrease in carbon solid solubility curve of the wire. Slight carbon deposition and network carbide precipitation can be observed on the wire surface. Conversely, if the injection flow rate and velocity are higher than the above-mentioned set values, excessive dilution leads to a sharp drop in local carbon potential, and secondary decarburization can be observed, with a significant increase in the hardness difference between the surface and the core. By controlling the above-mentioned specific parameter settings, the matching of atmosphere carbon potential with the austenite-to-pearlite transformation process of the wire is achieved, reducing the risk of carbon deposition caused by interfacial kinetic mismatch.

[0019] Example 5: In this embodiment, medium-high carbon steel wire rod was selected as the test carrier. The preheating zone temperature was set to 650℃, the dew point was controlled at -38℃, and the heat preservation zone temperature was set to 900℃. This parameter matrix aims to simulate the high heat load state in actual production. The appropriate preheating temperature helps the reducing atmosphere to initially wet and spread on the surface of the wire rod, accelerating the in-situ decomposition of micro-oxides. Although the higher thermodynamic base increases the initial tendency of carbon atom escape, the dynamic balance of carbon exchange at the gas-solid interface was still achieved by relying on the strong reducing surface pretreatment and the subsequent precise carbon potential matching. No ferrite softening layer was detected on the surface of the wire rod after it came out of the furnace.

[0020] Comparative Example 1: This comparative example uses a traditional high-purity nitrogen-protected heat treatment process, in which a fixed flow rate of high-purity nitrogen is introduced into the furnace to replace the air throughout the process; no preheating micro-reduction zone is set up, no dynamic carbon potential closed-loop control is performed in the heat preservation zone, and there is no gradient carbon potential reduction control dilution operation during the cooling stage; this scheme relies on unidirectional physical isolation and cannot respond to the drastic fluctuations in local oxygen partial pressure caused by the introduction of a small amount of air at the inlet and outlet.

[0021] Comparative Example 2: This comparative example uses a conventional endothermic atmosphere fixed carbon potential control process, with the entire furnace circulated with an endothermic mixed gas set at a fixed carbon potential. The control system maintains a single target carbon potential only for the heat preservation zone. High-purity nitrogen is not introduced for multi-point jet dilution during the cooling stage. The carbon monoxide concentration in the atmosphere remains constant during the cooling process, resulting in the actual carbon potential of the cooling zone being much higher than the carbon solid solubility of the wire at that temperature.

[0022] Verification experiment: To verify the actual effect of the technical solution of the present invention, the heat-treated wires prepared in Examples 1 to 5 and the wires treated in Comparative Examples 1 and 2 were systematically evaluated in terms of mechanical and microstructure. The experiment focused on examining the carbon concentration gradient distribution, microhardness variation characteristics and fatigue life performance under alternating stress of the wire surface, so as to comprehensively measure the technical effect of the dynamic adaptive gradient carbon potential atmosphere system on eliminating ferrite softening layer and carbon deposit defects.

[0023] Testing standards: The determination of metallographic structure and decarburized layer depth strictly follows the standard GB / T224-2019 "Determination of Decarburized Layer Depth of Steel"; the micro Vickers hardness test is performed in accordance with the standard GB / T4340.1-2009 "Vickers Hardness Test of Metallic Materials"; the rotational bending fatigue test is performed in accordance with the standard GB / T4337-2015 "Rotational Bending Method for Fatigue Test of Metallic Materials".

[0024] Specific testing process: The specific specifications of each batch of wire processed in this test were all 12mm diameter wire rods, which met the sampling and processing allowance requirements of the standard sample. Cylindrical samples with a length of 20mm were cut from the middle section of each batch of wire. After cold mounting, progressive water grinding and polishing, they were etched with 4% nitric acid alcohol solution. The microstructure from the edge of the cross section to the core was observed using an optical metallographic microscope, and the physical depth of the fully decarburized layer and the partially decarburized layer was measured. Using a micro Vickers hardness tester, the microhardness at a distance of 0.1 mm from the surface and the core substrate were measured under a load of 200 g, and the range between the two was calculated. Then, 12 mm diameter wires from the same batch were machined into standard fatigue specimens with a working section diameter of 6 mm. Room temperature rotary bending fatigue tests were conducted under the conditions of stress ratio R=-1 and frequency of 50 Hz, and the total number of cycles when the specimen fractured was recorded. Table 1 Performance test data of Examples 1-5 and Comparative Examples 1-2 Test data shows that Examples 1 to 5 all achieved absolute zero values ​​for both fully decarburized and partially decarburized layers under different parameter boundaries. The hardness difference between the surface and the core was strictly controlled within 15 HV, and the fatigue life was significantly improved compared with the traditional process. In contrast, Comparative Example 1, due to the lack of an active carbon potential intervention mechanism, resulted in severe softening of the surface ferrite, with a hardness difference as high as 65 HV, and fatigue cracks were very likely to initiate in the softened layer. Although Comparative Example 2 suppressed decarburization, the kinetic mismatch during the cooling stage led to secondary carburization and carbon deposition on the surface, resulting in abnormal surface hardening and deterioration of the material's overall fatigue resistance. This invention, through a cascade mechanism of in-situ construction of reduction channels, closed-loop locking, and gradient carbon potential control, can control the influence of thermal hysteresis on the microstructure and maintain process stability within a certain industrial operating window.

[0025] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A controlled atmosphere heat treatment method for preventing decarburization on the surface of wire, characterized in that, Includes the following methods and steps: S1. Preparation atmosphere: Methanol is cracked at 900℃ to 1050℃ in the presence of a nickel-based catalyst to obtain methanol cracked gas; methanol cracked gas is mixed with high-purity nitrogen to obtain a reference protective atmosphere; high-purity nitrogen is mixed with hydrogen to obtain a reducing mixed atmosphere. S2. Preheating and Micro-reduction: The wire is sent into the preheating zone of the continuous heat treatment furnace through the nitrogen curtain sealing port. The temperature of the preheating zone is controlled at 500℃ to 700℃. A reducing mixed atmosphere is independently introduced into the preheating zone and the dew point of the preheating zone is controlled at -40℃ to -30℃ to reduce the oxides on the surface of the wire. S3, Insulation and other carbon potential locking: The preheated wire is sent into the insulation zone of a continuous heat treatment furnace at 800℃ to 1050℃, and a reference protective atmosphere is introduced into the insulation zone to maintain the gas pressure in the insulation zone at 150Pa to 300Pa. S4. Carbon potential closed-loop control: By collecting data from the zirconia oxygen probe, dew point meter and infrared gas analyzer set in the insulation zone in real time, the carbon potential is calculated in real time. The target carbon potential is set according to the nominal carbon content of the wire matrix and the temperature. The absolute deviation of the mass fraction between the actual carbon potential and the target carbon potential is controlled within ±0.02% by pulse injection of enriching gas or dilution gas. S5. Gradient carbon potential reduction control: The heat-insulated wire is sent into the cooling zone and gradually cooled to 500℃ and below. At the same time, high-purity nitrogen is injected at multiple points along the longitudinal direction of the cooling zone to dilute the carbon monoxide in the atmosphere of the cooling zone, reduce the carbon potential of the furnace gas, and make the carbon potential decrease synchronously with the temperature gradient. S6. Furnace exit treatment: The cooled wire is removed from the continuous heat treatment furnace through a water seal or nitrogen curtain.

2. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: In step S1, the volume fraction of hydrogen in the reducing mixed atmosphere is controlled between 5% and 10%.

3. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: In step S1, the reference protective atmosphere is formed by mixing methanol cracking gas and high-purity nitrogen gas in a volume ratio of 1:0.8 to 1:1.

2.

4. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: In step S4, the infrared gas analyzer is used to detect the partial pressures of carbon monoxide, carbon dioxide, and methane.

5. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: In step S4, the enrichment gas is methane or propane; the dilution gas is high-purity nitrogen.

6. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: In step S5, the decreasing curve of the atmospheric carbon potential in the gradient carbon potential control is matched with the decreasing curve of carbon solid solubility when the wire transforms from austenite to ferrite or pearlite at the corresponding temperature, so as to prevent carbon deposition or secondary decarburization on the surface of the wire during the cooling process.

7. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: In step S5, the cooling zone is divided into multiple temperature segments along the longitudinal direction. As the wire passes through different temperature segments in sequence, the injection volume of high-purity nitrogen is gradually increased to achieve a parabolic gradient carbon reduction potential control.

8. The controlled atmosphere heat treatment method for preventing decarburization of wire surface according to claim 1, characterized in that: The wire rod is made of medium-high carbon steel, alloy spring steel, or bearing steel.

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