Oriented silicon steel decarburization and nitriding integrated process and surface nitrogen concentration gradient control method

Abstract

The invention discloses an oriented silicon steel decarburization and nitriding integrated process and a surface nitrogen concentration gradient control method, and belongs to the technical field of oriented silicon steel manufacturing. The method comprises the following steps: melting an oriented silicon steel raw material into molten steel, and carrying out primary treatment; carrying out decarburization annealing treatment; nitriding treatment is carried out; and performing high-temperature annealing treatment and the like. According to the method, gradient diffusion of nitrogen atoms is accurately realized through regional gradient regulation and control of pulse plasma parameters and treatment time, so that the surface nitrogen concentration and depth gradient reach the optimal control range, and the method has the advantages of effectively avoiding the magnetic aging phenomenon, improving the uniformity of an inhibitor and optimizing the high-temperature annealing stability.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of oriented silicon steel manufacturing, in particular to an oriented silicon steel decarburization and nitriding integrated process and a surface nitrogen concentration gradient control method. BACKGROUND

[0002] As the core material of electrical equipment such as power transformers, the magnetic properties of oriented silicon steel directly affect the energy efficiency and operation stability of the equipment. At present, the low-temperature high-magnetic-induction oriented silicon steel manufacturing process is mainly used in industrial production. This process forms inhibitors through nitriding treatment after decarburization annealing, and has the advantages of low energy consumption and high material yield. However, the existing process faces many technical bottlenecks in realizing decarburization and nitriding integrated treatment.

[0003] In the decarburization annealing link, the traditional process is difficult to stably control the carbon content in the steel below 30ppm, which leads to the easy occurrence of magnetic aging phenomenon in the finished product. More importantly, there are obvious deficiencies in the control of nitrogen concentration during the nitriding process, mainly manifested as uneven distribution of nitrogen concentration in the surface layer and the central region, affecting the formation effect of the inhibitors. Although the existing technology attempts to adjust the nitrogen concentration gradient through a multi-stage plasma nitriding doping method, it is still difficult to achieve precise control in actual production process, especially under the complex working conditions of simultaneous decarburization and nitriding.

[0004] There are also technical problems in the high-temperature annealing stage. Due to inaccurate temperature gradient control, the strip steel edge performance is often abnormal, showing fluctuations in magnetic properties and a decline in shape quality. In addition, the existing nitriding process does not have fine enough coordination control of temperature field and plasma parameters, which affects the nitriding efficiency and makes it difficult to ensure the gradient distribution of nitrogen atoms in the material.

[0005] In view of the above problems, the existing technology needs to be improved. SUMMARY

[0006] The purpose of the present application is to provide an oriented silicon steel decarburization and nitriding integrated process and a surface nitrogen concentration gradient control method, which has the advantages of precise control of nitrogen concentration gradient distribution, improvement of magnetic property stability and improvement of shape quality.

[0007] The application provides an oriented silicon steel decarburization and nitriding integrated process and a surface nitrogen concentration gradient control method, and the technical scheme is as follows:

[0008] The oriented silicon steel raw material is melted into a molten steel, and is subjected to preliminary treatment;

[0009] decarburization annealing treatment is performed;

[0010] nitriding treatment is performed;

[0011] Step 3 includes:

[0012] The steel billet enters the decarburization zone with a set temperature of 900-950℃ and a power density of 5-7W / cm 2 Decarburization is carried out using a pulsed plasma with a frequency of 50-100 Hz and a pulse duty cycle of 20-30%;

[0013] The steel billet enters the deep nitriding zone with a set temperature of 850-750℃ and a power density of 3-5W / cm 2 Deep nitriding with pulsed plasma at a frequency of 100-500 Hz and a pulse duty cycle of 30-40%;

[0014] The billet enters the surface control zone with a set temperature of 750-650℃ and a power density of 1-3W / cm 2 Fine adjustment of surface nitrogen concentration using pulsed plasma at a frequency of 500-1000 Hz and a pulse duty cycle of 40-50%;

[0015] By precisely controlling the pulse parameters and treatment time of each area, gradient diffusion of nitrogen atoms is achieved, so that the surface nitrogen concentration is controlled within the range of 2.8-3.2×10^20cm^-3, and the nitrogen concentration gradient per 50μm depth is controlled within the range of 0.2-0.25×10^20cm^-3;

[0016] Perform high temperature annealing treatment.

[0017] Furthermore, the present application also proposes that in the deep nitriding zone, the pulse frequency is gradually reduced by reducing 50 Hz every 10 minutes to create a smoother nitrogen concentration transition.

[0018] Furthermore, the present application also proposes that, in the surface control area, the nitrogen concentration distribution within 100 μm of the surface is precisely controlled by dynamically adjusting the pulse duty cycle to 20-50%.

[0019] Furthermore, the present application also proposes that in steps 301 , 302 and 303 , 0.5-1% hydrogen is added to the plasma.

[0020] Furthermore, the present application also proposes that in steps 301 , 302 and 303 , independently controlled pulse plasma generators are used respectively.

[0021] Furthermore, the present application also proposes that step 1 includes:

[0022] melting the oriented silicon steel raw material into molten steel;

[0023] The molten steel is dusted by means of a blowing device;

[0024] Check the temperature of the molten steel and adjust it to 1400-1450℃.

[0025] Furthermore, the present application also proposes that step 2 includes:

[0026] Lower the temperature of molten steel to 1200-1250℃;

[0027] Control the annealing temperature curve to reduce the carbon content in the steel to below 30ppm;

[0028] Perform primary recrystallization treatment.

[0029] Furthermore, the present application also proposes that step 4 includes:

[0030] The temperature was increased in the range of 1000-1050°C at a rate of 15-40°C / h, with a nitrogen flow rate of 80-100 ml / min;

[0031] The temperature was increased in the range of 1050-1100°C at a rate of 5-15°C / h, with a nitrogen flow rate of 30-50 ml / min;

[0032] The secondary recrystallization treatment is carried out in the temperature range of 1100-1150°C, with a gas flow rate of 50-100 ml / min, a temperature of 900-950°C, and a holding time of 30-60 minutes.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The present invention adopts gradient temperature field control technology and sets multiple temperature zones in the integrated decarburization and nitriding furnace, achieving gradient diffusion of nitrogen atoms. This effectively solves the problem of insufficient precision in controlling surface nitrogen concentration in traditional methods and significantly improves the magnetic permeability and iron loss performance of grain-oriented silicon steel.

[0035] 2. This invention achieves more precise control of nitrogen concentration gradients by applying pulsed plasma with different parameters in different temperature zones. Specifically, the furnace body is divided into a decarburization zone, a deep nitriding zone, and a surface control zone. Independently controlled pulsed plasma generators are used in each zone. By adjusting the pulse frequency, duty cycle, and power density, the energy and concentration of nitrogen ions are precisely controlled, achieving high-precision control of surface nitrogen concentration.

[0036] 3. The present invention promotes the decarburization process by adding hydrogen to the plasma without affecting the nitriding effect, achieving good synergy between the decarburization and nitriding processes and effectively solving the problem of the difficulty in effectively reducing the carbon content in steel during decarburization annealing in traditional methods.

[0037] 4. This invention uses multi-stage pulsed plasma technology to create a smoother nitrogen concentration transition by gradually reducing the pulse frequency and dynamically adjusting the pulse duty cycle. This effectively avoids the problem of high surface concentration and low center concentration in traditional methods, thereby improving the overall performance of grain-oriented silicon steel.

[0038] 5. The gradient temperature field-assisted nitrogen concentration gradient control method of the present invention realizes the precise control of surface nitrogen concentration and the gradient diffusion of nitrogen atoms, effectively solving the problem of inaccurate nitriding temperature and dew point control in traditional methods, and significantly improving nitriding efficiency and product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a flow chart of the steps of an integrated decarburization and nitriding process for grain-oriented silicon steel and a method for controlling surface nitrogen concentration gradient according to the present invention;

[0040] Figure 2 This is a flowchart of step 1 of the integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling the surface nitrogen concentration gradient of the present invention;

[0041] Figure 3 This is a flowchart of step 2 of the integrated process for decarburization and nitriding of grain-oriented silicon steel and the method for controlling the surface nitrogen concentration gradient of the present invention;

[0042] Figure 4 This is a flowchart of step 3 of the integrated process for decarburization and nitriding of grain-oriented silicon steel and the method for controlling the surface nitrogen concentration gradient of the present invention;

[0043] Figure 5 This is a flow chart of step 4 of the integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling the surface nitrogen concentration gradient according to the present invention. DETAILED DESCRIPTION

[0044] The technical solutions of the present invention will be described clearly and completely below with reference to the accompanying drawings and embodiments. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are also within the scope of protection of the present invention.

[0045] In the existing technology, the production process of oriented silicon steel generally has the problem of difficulty in coordinating the decarburization and nitriding processes. When the traditional method adopts a single temperature field treatment, it is difficult to balance the decarburization efficiency and the nitriding accuracy, resulting in surface enrichment and gradient mutation of the nitrogen concentration distribution inside the material. Especially in the manufacturing process of low-temperature and high-magnetic induction oriented silicon steel, carbon residue after decarburization can easily cause magnetic aging defects, and the excessively high surface nitrogen concentration in the nitriding stage will affect the subsequent recrystallization quality. Although the existing plasma treatment technology can achieve nitriding doping, it is limited by fixed process parameters and cannot construct effective nitrogen atom diffusion kinetic conditions.

[0046] To address these issues, the inventors discovered that decarburization and nitriding processes differ in temperature sensitivity: decarburization requires higher temperatures to promote carbon atom diffusion, while nitriding requires a temperature gradient to control the penetration depth of nitrogen atoms. By analyzing the coupling relationship between plasma energy distribution and material surface reactions, they proposed dividing the processing area into functionally differentiated temperature ranges. Based on the influence of plasma frequency on ion kinetic energy, they designed a progressive treatment scheme from high frequency to low frequency. They also recognized the regulatory effect of pulse duty cycle on the activation state of the material surface and established a dynamic parameter adjustment mechanism.

[0047] Therefore, please refer to the attached Figure 1-5 As shown, this application proposes an integrated decarburization and nitriding process for grain-oriented silicon steel and a method for controlling the surface nitrogen concentration gradient. The process includes sequentially passing the steel billet through a decarburization zone, a deep nitriding zone, and a surface control zone for staged treatment. The decarburization zone uses a treatment temperature of, for example, 900 to 950°C with a plasma frequency of 50 to 100 Hz, the deep nitriding zone uses a decreasing temperature of, for example, 850 to 750°C with a plasma frequency of 100 to 500 Hz, and the surface control zone uses a treatment temperature of, for example, 750 to 650°C with a plasma frequency of 500 to 1000 Hz. By setting the temperature gradient and plasma parameters in different zones, the nitrogen atomic diffusion process can be precisely controlled.

[0048] The temperature setting in the decarburization zone places the material in the optimal temperature window for promoting the carbon-oxygen reaction, which accelerates the migration of carbon atoms to the surface. The temperature gradient in the deep nitriding zone is achieved by gradually lowering the treatment temperature to create a thermodynamic driving potential, guiding nitrogen atoms to penetrate the material. High-frequency plasma in the surface control zone utilizes a low-energy ion beam generated by a high-frequency electric field to achieve micron-level regulation of surface nitrogen concentration. Pulse duty cycle regulation controls the surface activation state of the material by varying the effective duration of the plasma action cycle.

[0049] Specifically, the steel billet undergoes a carbon-oxygen reaction-dominated stage in the decarburization zone, where higher temperatures promote the rapid diffusion of carbon atoms. After entering the deep nitriding zone, the thermal gradient formed by the decreasing temperature works synergistically with the medium-frequency plasma, allowing nitrogen atoms to obtain sufficient kinetic energy to penetrate the surface. In the surface control zone, the low-temperature environment inhibits excessive diffusion of nitrogen atoms, and the high-frequency plasma fine-tunes the amount of nitrogen adsorption on the surface. The temperature gradients of each treatment zone form a matching relationship with the plasma parameters. The decarburization zone prioritizes the removal of carbon elements to create a clean surface for nitriding. The deep nitriding zone constructs the basic distribution of nitrogen concentration, and the surface control zone completes the final concentration calibration.

[0050] Compared with existing technologies, traditional methods use a single processing temperature, which results in mutual constraints between decarburization and nitriding process parameters. However, this application achieves process decoupling through staged temperature control. Existing plasma processing technologies mostly use fixed frequencies, which makes it difficult to balance penetration depth and surface accuracy. The progressive frequency adjustment strategy of this application meets the requirements of deep penetration and surface control at the same time. The fixed duty cycle design of traditional processes cannot adapt to changes in surface conditions at different processing stages. The dynamic adjustment mechanism of this application can optimize the plasma action intensity in real time.

[0051] Through the above technical solution, the present application achieves the coordinated optimization of the decarburization and nitriding processes and effectively controls the surface nitrogen concentration distribution gradient. A gentle concentration transition layer is formed in the deep nitriding stage, avoiding the concentration mutation phenomenon of the traditional process. The high-frequency treatment in the surface control stage can accurately correct the local concentration deviation to ensure that the final nitrogen concentration distribution meets the design requirements. The staged temperature setting not only ensures the decarburization efficiency, but also creates suitable thermodynamic conditions for the gradient diffusion of nitrogen atoms, solving the technical problem of incomplete decarburization and uneven nitriding in the traditional process.

[0052] The present application further proposes to gradually reduce the pulse frequency by 50 Hz every 10 minutes in the deep nitriding zone to create a smoother nitrogen concentration transition.

[0053] Gradually reducing the pulse frequency refers to adjusting the plasma excitation frequency in stages during the nitriding process. This can be achieved using a programmable frequency controller. This parameter adjustment method helps to control the energy distribution of nitrogen ions. The deep nitriding zone refers to the process area with a treatment temperature of 850-750°C, and a gradient temperature field is used to promote the diffusion of nitrogen atoms from the surface to the interior. The pulse frequency reduction step is set to 50Hz every 10 minutes. This value is determined based on the matching relationship between the nitrogen atom diffusion rate and the plasma energy decay rate, which can avoid concentration fluctuations caused by energy mutations.

[0054] Specifically, a linearly decreasing pulse frequency strategy is implemented during the deep nitriding stage. The high-frequency pulses in the initial stage generate high-energy nitrogen ions, which effectively penetrate the material's surface and form an initial penetration channel. As the frequency is gradually reduced in predetermined steps, the kinetic energy of the nitrogen ions in the plasma decays in a step-by-step manner, causing the nitrogen atoms to form a progressive diffusion pattern within the material. This dynamic adjustment mechanism maintains the driving force for surface diffusion while providing a continuous energy supply for deep penetration. By matching the gradient distribution of the temperature field, the nitrogen atoms form a continuous and smooth concentration curve across the thickness of the material, effectively solving the problem of surface concentration accumulation caused by traditional fixed-frequency treatments.

[0055] Compared with the prior art, the conventional nitriding process adopts a fixed frequency of plasma treatment, resulting in excessive accumulation of high-energy ions on the surface layer of the material to form a steep concentration gradient. The present scheme gradually attenuates the frequency, so that the energy of nitrogen ions dynamically adapts to the diffusion requirements at different depths of the material, avoiding over-saturation of nitrogen concentration on the surface layer, and ensuring deep penetration efficiency. Compared with the fixed nitriding parameter treatment method in patents such as CN110218853A, the present technical scheme reduces the nitrogen concentration gradient change rate by about 40% while maintaining the processing efficiency.

[0056] Through the above technical scheme, the present application realizes the smooth diffusion of nitrogen atoms in the thickness direction of the material, eliminating the problem of excessive difference in concentration between the surface layer and the core in the conventional process. The frequency adjustment mechanism controls the nitrogen concentration gradient change rate in the material to be within 50 microns , ensuring that the oriented silicon steel obtains uniform inhibitor distribution during subsequent high-temperature annealing, thereby improving the uniformity of magnetic domain structure and the stability of magnetic properties.

[0057] The present application further proposes to precisely control the nitrogen concentration distribution within 100 microns of the surface layer in the surface control zone by dynamically adjusting the pulse duty cycle.

[0058] Wherein, dynamically adjusting the pulse duty cycle means adjusting the ratio of high-energy pulse to intermittent time within the plasma action period, changing the penetration rate and energy distribution of nitrogen ions. Specifically, a closed-loop control system combined with real-time concentration monitoring data can be used to achieve this, by adjusting the output waveform parameters of the power supply to change the duty cycle. The surface control zone refers to the processing area with a temperature setting of 750-650°C, which is beneficial to the diffusion kinetics balance of nitrogen atoms on the surface layer. The nitrogen concentration distribution within 100 microns of the surface layer refers to the variation trend of nitrogen atom concentration within a depth range of 100 microns from the surface of the material, and the concentration gradient at this depth directly affects the stability of the magnetic domain structure.

[0059] Specifically, in the surface control zone processing stage, the dynamic adjustment range of the pulse duty cycle is set to 20% to 50%. When the surface layer nitrogen concentration is too high, the duty cycle is adjusted to the 20%-30% range, at which time the plasma action time is shortened and the intermittent time is extended, promoting the diffusion of nitrogen atoms that have penetrated to a deeper layer; when the surface layer concentration is insufficient, the duty cycle is adjusted to the 40%-50% range, extending the high-energy pulse action time to increase the enrichment of nitrogen atoms on the surface layer. The adjustment process is linked with the nitrogen concentration detection device through a feedback control system, real-time matching the diffusion requirements of different depth regions, and ensuring the formation of a smooth concentration gradient curve within 100 microns of the surface layer.

[0060] Compared with the prior art, the conventional method usually adopts a fixed duty cycle parameter for surface treatment, resulting in a sudden drop in nitrogen concentration between the surface layer and the subsurface layer. The CN110218853A patent disclosed in the prior art only controls the overall nitrogen concentration by adjusting the plasma power, and cannot achieve dynamic adjustment of the duty cycle and concentration gradient. The present scheme overcomes the concentration gradient mutation problem caused by fixed parameter processing through a dynamic duty cycle adjustment mechanism.

[0061] Through the above technical scheme, the present application effectively eliminates the phenomenon of sudden drop in nitrogen concentration at the interface between the surface layer and the subsurface layer, and reduces the nitrogen concentration gradient change rate in the surface layer 100 μm range to one third of the original technology. The technical scheme can accurately control the diffusion dynamics process of nitrogen atoms in the surface layer region, avoid lattice distortion and magnetic domain movement caused by concentration mutation, and thus improve the uniformity of the magnetic properties of oriented silicon steel.

[0062] The present application further proposes adding hydrogen gas as an auxiliary gas in the plasma treatment process in the decarburization zone, deep layer nitriding zone and surface control zone, and the hydrogen gas volume concentration is controlled in the range of 0.5-1%.

[0063] Among them, the addition of hydrogen gas in the plasma refers to mixing hydrogen gas component into the working gas during the plasma generation process, which can be specifically mixed with nitrogen and argon in a certain proportion by using a mass flow controller and then introduced into the plasma reaction chamber. This concentration range can form a moderate reducing atmosphere to promote the decarburization reaction, and can also avoid excessive hydrogen atom penetration leading to lattice distortion. In the decarburization stage, hydrogen molecules preferentially react with carbon oxides to generate volatile products, accelerating the removal of carbon elements from the steel matrix. In the nitriding process, hydrogen ions remove the oxide film through surface cleaning to expose the fresh metal surface to promote the adsorption of nitrogen atoms. The continuous introduction of hydrogen gas in the surface control stage can adjust the surface nitrogen atom activity and inhibit the formation of brittle phases in the nitriding layer.

[0064] Specifically, in the high-temperature environment of the decarburization zone, hydrogen molecules and CO gas generated by the decomposition of iron carbide have a reduction reaction to generate water vapor and methane to be discharged from the system, which significantly accelerates the removal rate of carbon elements. In the deep layer nitriding zone, hydrogen ions remove the passivation layer on the surface of the billet through sputtering, making it easier for nitrogen ions to diffuse into the matrix. In the surface control zone, the adsorption equilibrium of hydrogen atoms on the metal surface reduces the supersaturation state of nitrogen atoms, achieving accurate regulation of nitrogen concentration. The three treatment stages continuously maintain a hydrogen gas ratio of 0.5-1%, forming a dynamically balanced composite plasma environment that ensures decarburization efficiency without reducing nitriding quality.

[0065] Compared with existing technologies, traditional processes use a single nitrogen or nitrogen-argon mixed atmosphere for decarburization and nitriding. The oxide film formed during the decarburization process hinders subsequent nitrogen atom penetration. Simply increasing the nitrogen concentration, while improving nitriding efficiency, inhibits the decarburization reaction. This solution introduces a controlled proportion of hydrogen to create a reducing atmosphere during the decarburization phase to promote carbon removal, maintain surface activity during the nitriding phase to promote nitrogen atom diffusion, and adjust the nitrogen atom distribution during the surface control phase. This effectively resolves the technical contradictions between the decarburization and nitriding process parameters.

[0066] Through the above technical solution, this application achieves the coordinated optimization of the decarburization and nitriding processes, ensuring that the carbon content is effectively reduced to the process requirements while maintaining the diffusion and penetration efficiency of nitrogen atoms in the steel matrix. This composite plasma environment can simultaneously complete the triple functions of surface purification, deep nitriding, and concentration control, avoiding the efficiency loss caused by the step-by-step processing required in traditional processes and significantly improving the integration level of the grain-oriented silicon steel preparation process.

[0067] The present application further proposes using independently controlled pulse plasma generators in the decarburization zone, deep nitriding zone and surface control zone respectively.

[0068] Independently controlled pulsed plasma generators are devices with independent parameter adjustment for different processing zones. This is achieved using a split high-frequency power supply with an independent gas circulation system, with each zone equipped with its own power module and frequency controller. This feature eliminates electromagnetic interference between zones through physical isolation, ensuring that the pulse energy inputs of each processing stage do not affect each other.

[0069] Mutual interference in pulse parameter adjustments refers to the superposition of harmonics generated by high-frequency pulse signals when using a shared plasma generator. This manifests as a coupling effect between power density and frequency adjustments in different processing zones. This feature, through independent device architectures, disconnects these parameter linkages, enabling decoupled adjustment of power density, frequency, and duty cycle parameters in each processing zone.

[0070] Specifically, the decarbonization zone is equipped with a high power density independent generator, which outputs 5-7W / cm 2 High power density and 50-100Hz low-frequency pulses ensure the effective removal of carbon elements. The deep nitriding zone uses a medium-power independent generator, which achieves deep penetration of nitrogen atoms through a pulse sequence of gradually decreasing frequency. The surface control zone is driven by a low-power independent generator, which uses high-frequency pulses to adjust the surface nitrogen concentration at the micron level. The independent generators in these three areas achieve parameter synchronization through a collaborative control system, maintaining process continuity while avoiding electromagnetic interference. The power modules of each generator use a shielded circuit design to prevent high-frequency signals from propagating across zones, ensuring the purity of the pulse waveform in each treatment zone.

[0071] Compared to existing technologies, traditional methods use a single plasma generator to simultaneously handle multiple process stages, resulting in signal crosstalk during parameter adjustment, affecting the linear transition of the nitrogen concentration gradient. This solution achieves physical isolation of the processing zones through a split equipment architecture, ensuring that power fluctuations in deep nitriding do not interfere with the high-frequency pulse stability of the surface control zone. Furthermore, the high energy input in the decarburization zone does not cause temperature anomalies in the nitriding zone.

[0072] Through the above technical solution, this application solves the parameter coupling problem caused by sharing plasma equipment in multiple process stages, ensures the independent optimization capability of pulse parameters in the three stages of decarburization, deep nitriding and surface control, enables the diffusion process of nitrogen atoms in the gradient temperature field to be precisely controlled, and effectively improves the uniformity of nitrogen concentration distribution within the surface range of 100μm.

[0073] The present application further proposes a method for melting oriented silicon steel raw materials into molten steel and performing preliminary treatment, specifically comprising melting the raw materials into molten steel, performing dust removal treatment on the molten steel through a blowing device, detecting the temperature of the molten steel and adjusting it to 1400-1450°C.

[0074] Dust removal from molten steel using a high-pressure airflow physically flushes the molten steel to remove slag and non-metallic inclusions. Specifically, argon or nitrogen can be used as the injection medium, injected from the bottom of the ladle through a multi-hole lance. This step reduces impurities in the molten steel, preventing grain boundary defects caused by inclusions in subsequent processing.

[0075] Detecting the molten steel temperature and adjusting it to 1400-1450°C involves real-time monitoring of the melt state using thermocouples or infrared temperature measuring devices, combined with temperature compensation using a heating device, such as electromagnetic induction heating or resistance heating. This temperature range ensures optimal decarburization activity in the molten steel while preventing abnormal grain growth caused by overheating.

[0076] Specifically, after the molten steel is melted to form a uniform liquid matrix, gas dynamics are used during the injection phase to float low-density oxide particles to the surface of the molten steel, forming a slag layer. Slagging is then used to separate impurities. During temperature control, the molten steel is continuously monitored and maintained within a specific range. If the temperature deviates from the set point, the heating power is adjusted or preheated raw materials are added to stabilize the temperature. This combined operation controls the material's purity and thermodynamic state from the metallurgical source, providing the foundation for a uniformly composed steel billet for subsequent gradient nitriding.

[0077] Compared with existing technologies, traditional processes for treating molten steel often rely on a single static sedimentation method for dust removal, resulting in low impurity removal efficiency and a long time consumption. This application, however, enhances impurity separation through dynamic injection. Regarding temperature control, conventional methods rely on manual intermittent temperature measurement, making precise regulation difficult. This application utilizes continuous monitoring and automatic feedback regulation, effectively addressing the issue of unstable decarburization efficiency caused by temperature fluctuations.

[0078] Through the above technical solution, the present application realizes efficient removal of impurities and precise temperature control in the molten steel treatment stage, avoids the interference of non-metallic inclusions on nitriding uniformity, ensures the consistency of chemical composition of the steel billet before decarburization annealing, and provides a stable material basis for subsequent gradient nitrogen concentration control.

[0079] The present application further proposes that the decarburization annealing treatment stage includes three consecutive steps: lowering the temperature of the molten steel to 1200-1250°C; controlling the annealing temperature curve to reduce the carbon content in the steel to below 30ppm; and performing primary recrystallization treatment.

[0080] Among them, the temperature of the molten steel is reduced to 1200-1250℃, which means that the temperature of the molten steel is rapidly cooled from the initial molten state to the critical temperature range of the solid phase transformation through a segmented temperature control device. Specifically, it can be achieved by a multi-stage water-cooled roller and mist cooling composite system. This temperature range is the starting point for the transformation from austenite to ferrite, ensuring the phase structure basis for the steel billet to enter the subsequent decarburization treatment. Among them, the annealing temperature curve control refers to the dynamic adjustment of the temperature distribution and time parameters of each area in the annealing furnace through a programmable temperature controller. Specifically, it can be achieved by infrared radiation heating and a closed-loop feedback system, and the diffusion of carbon atoms is promoted through the synergistic effect of temperature gradient and time parameters. Among them, the primary recrystallization treatment refers to the formation of uniform equiaxed grains inside the steel body through a specific temperature field environment after decarburization is completed. Specifically, it can be achieved by a process combining gradient heating and isothermal holding, providing a stable crystal structure basis for the subsequent nitriding process.

[0081] Specifically, a progressive decarburization system is formed through three-stage temperature control: first, the solidification and phase transformation of the molten steel are completed in the 1200-1250°C range, forming a microstructure suitable for carbon diffusion. Subsequently, by dynamically adjusting the annealing temperature curve, carbon atoms in the ferrite matrix migrate to the surface at a maximized diffusion rate, ensuring that the carbon content precisely meets the target. Finally, primary recrystallization is triggered at the critical temperature, eliminating the stress concentration and lattice distortion caused by the decarburization process and forming uniform, fine, equiaxed grains. This technical solution systematically solves the difficult problem of coordinated optimization of decarburization efficiency and crystal structure through coupled control of temperature, time, and phase transformation.

[0082] Compared to existing technologies, traditional decarburization annealing processes typically use a single static temperature range, resulting in insufficient carbon diffusion dynamics and uncontrollable grain growth. This solution uses stepped temperature control and dynamic curve adjustment to precisely control the crystal structure while ensuring carbon content meets standards. This avoids the problems of incomplete decarburization and abnormal grain growth caused by temperature fluctuations in traditional methods.

[0083] Through the above technical solution, the present application realizes the effective control of carbon content and the simultaneous optimization of crystal structure during the decarburization process of the steel billet, ensuring that the carbon content is stably reduced to below 30ppm, while forming a uniform and fine primary recrystallization structure, providing ideal matrix conditions for subsequent nitriding treatment, and fundamentally avoiding the negative effects of magnetic aging and grain boundary anomalies on magnetic properties.

[0084] The present application further proposes a method of using a staged gradient to control the heating rate and gas flow rate in the high-temperature annealing stage, which specifically includes three steps: heating at a heating rate of 15-40°C / h in the temperature range of 1000-1050°C, and a nitrogen flow rate of 80-100ml / min; heating at a heating rate of 5-15°C / h in the temperature range of 1050-1100°C, and a nitrogen flow rate of 30-50ml / min; performing secondary recrystallization treatment in the temperature range of 1100-1150°C, with a gas flow rate of 50-100ml / min, a temperature of 900-950°C, and a holding time of 30-60 minutes.

[0085] Among them, the staged gradient control of the heating rate refers to dividing the high-temperature annealing process into multiple temperature intervals, and using differentiated heating rates in each interval. For example, the heating rate in the first stage can be 40°C / h, and the second stage can be 10°C / h. This method reduces the heating difference between the edge and center areas of the strip by matching the heat conduction characteristics of different temperature intervals. Among them, the dynamic adjustment of gas flow refers to the synchronous change of the nitrogen supply during the heating process. For example, high-flow nitrogen is used to maintain surface protection in the initial stage of the high-temperature zone, and the flow rate is gradually reduced in subsequent stages to adapt to the nitrogen concentration gradient requirements. This combined control method can coordinate the relationship between the temperature field and the atmosphere environment and suppress abnormal grain boundary migration.

[0086] Specifically, in the first heating stage, a higher heating rate and nitrogen flow rate are used to enable the steel plate as a whole to quickly enter a thermal equilibrium state, while maintaining the stability of the surface oxide layer through sufficient nitrogen. In the second heating stage, the heating rate is reduced and the nitrogen flow rate is reduced to mitigate the risk of thermal stress accumulation caused by temperature changes, while forming a dynamic balance between the diffusion rate of nitrogen atoms in the steel matrix and the surface concentration gradient. In the secondary recrystallization treatment stage, the combination of specific temperature ranges and gas flow rates is controlled to allow the secondary recrystallized grains to fully grow in a low-stress environment. For example, keeping the temperature at 900°C for 50 minutes promotes a uniform grain size distribution. The parameters of each stage work synergistically to effectively eliminate the differences in heat and thermal expansion coefficients between the edge and center areas of the strip.

[0087] Compared with the existing technology, the existing high-temperature annealing process usually adopts a single heating rate or a fixed gas flow rate. For example, CN110218853A only controls the overall heating rate to 20°C / h, without considering the difference in heat conduction in different temperature ranges. The present application solves the problem of edge thermal stress concentration caused by sudden changes in temperature gradient in the traditional method by regulating the combined parameters of heating rate and gas flow in stages. In the existing technology, the secondary recrystallization treatment is usually carried out at a constant temperature, while the present application effectively avoids the abnormal grain growth phenomenon through the coordinated control of temperature range and holding time.

[0088] Through the above technical solution, the present application achieves a dynamic balance between the thermal states of the strip's edges and center, eliminating grain boundary stress concentration caused by differential thermal expansion and significantly improving the uniformity of secondary recrystallized grain distribution. This resolves the issues of magnetic property fluctuations and unstable plate quality at the strip's edges during high-temperature annealing, keeping the difference in magnetic permeability between the edges and center within 5% and reducing plate waviness to less than 1 mm / m.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. An integrated process for decarburization and nitriding of oriented silicon steel and a method for controlling surface nitrogen concentration gradient, characterized in that: The method comprises: Step 1: melting the oriented silicon steel raw material into molten steel and performing preliminary treatment; Step 2: performing decarburization annealing treatment; Step 3, performing nitriding treatment; The step 3 includes: Step 301: Place the steel billet in a decarburization zone with a set temperature of 900-950°C and a power density of 5-7 W / cm 2 Decarburization is carried out using a pulsed plasma with a frequency of 50-100 Hz and a pulse duty cycle of 20-30%; Step 302: The steel billet enters the deep nitriding zone with a set temperature of 850-750°C and a power density of 3-5W / cm 2 Deep nitriding with pulsed plasma at a frequency of 100-500 Hz and a pulse duty cycle of 30-40%; Step 303: The billet is placed in a surface control zone with a set temperature of 750-650°C and a power density of 1-3 W / cm 2 Fine adjustment of surface nitrogen concentration using pulsed plasma at a frequency of 500-1000 Hz and a pulse duty cycle of 40-50%; Step 304: By precisely controlling the pulse parameters and treatment time of each region, gradient diffusion of nitrogen atoms is achieved, so that the surface nitrogen concentration is controlled within the range of 2.8-3.2×10^20cm^-3, and the nitrogen concentration gradient per 50μm depth is controlled within the range of 0.2-0.25×10^20cm^-3; Step 4: perform high temperature annealing treatment.

2. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: In step 302 , in the deep nitriding zone, the pulse frequency is gradually reduced by 50 Hz every 10 minutes to create a smoother nitrogen concentration transition.

3. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: In step 303, in the surface control area, the pulse duty cycle is dynamically adjusted to 20-50% to precisely control the nitrogen concentration distribution within 100 μm of the surface.

4. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: In steps 301 , 302 and 303 , 0.5-1% of hydrogen is added to the plasma.

5. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: In steps 301 , 302 and 303 , independently controlled pulse plasma generators are used respectively.

6. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: The step 1 includes: Step 101: Melting the oriented silicon steel raw material into molten steel; Step 102: performing dust removal on the molten steel by using a blowing device; Step 103: Detect the temperature of the molten steel and adjust it to 1400-1450°C.

7. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: The step 2 includes: Step 201: lowering the temperature of the molten steel to 1200-1250° C. Step 202: Control the annealing temperature curve to reduce the carbon content in the steel to below 30 ppm; Step 203: Perform primary recrystallization treatment.

8. The integrated decarburization and nitriding process for grain-oriented silicon steel and the method for controlling surface nitrogen concentration gradient according to claim 1, characterized in that: The step 4 includes: Step 401: heating the temperature in the range of 1000-1050°C at a heating rate of 15-40°C / h, with a nitrogen flow rate of 80-100 ml / min; Step 402: heating the temperature in the range of 1050-1100°C at a heating rate of 5-15°C / h, with a nitrogen flow rate of 30-50 ml / min; Step 403: Perform secondary recrystallization treatment in the temperature range of 1100-1150° C., with a gas flow rate of 50-100 ml / min, a temperature of 900-950° C., and a holding time of 30-60 minutes.

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