An annealing method for improving surface defects of grain-oriented silicon steel

Abstract

The application belongs to the technical field of oriented silicon steel production and discloses an annealing method for improving the surface quality of oriented silicon steel, which comprises the following steps: decarburization annealing, nitriding annealing, high-temperature annealing and tensile flattening annealing, wherein the decarburization annealing section is divided into three sections according to the furnace length, the protective gas is non-uniformly introduced through segmented regulation and control, and a group of detection points are arranged in the furnace, so that the water-hydrogen partial pressure ratio of any detection point in the furnace satisfies the formula: 0.096i-0.00353i 2 +0.02651≤P i ≤0.099i-0.00396i 2 +0.3, wherein i represents the number of the detection point, P i represents the water-hydrogen partial pressure ratio measured by the detection point i. According to the application, the water-hydrogen partial pressure ratio of different parts in the furnace during the decarburization annealing process is regulated and controlled to be distributed according to a specific rule, so that a suitable oxidation layer structure is formed, thereby reducing the generation of surface defects such as water marks and crystal dew on the surface of the finished oriented silicon steel.

Description

Technical Field

[0001] This invention belongs to the field of grain-oriented silicon steel production technology, and specifically relates to an annealing method for improving the surface quality of grain-oriented silicon steel. Background Technology

[0002] Grain-oriented silicon steel is an excellent soft magnetic material, widely used in the industrial production of transformer cores. Currently, the common technique involves using lower hot-rolled slab heating temperatures, significantly reducing the production cost of grain-oriented silicon steel. However, due to the lower hot-rolling temperature, inhibitors need to be added in subsequent processes, leading to stricter requirements for process control in these processes. The process window required to obtain magnetically stable and high-quality finished products is smaller, and surface defects such as watermarks and crystal exposure are easily generated during production.

[0003] These defects are mainly caused by temperature differences between the upper and lower surfaces and the inside and outside of the steel coil during high-temperature annealing, as well as differences in the atmosphere of the inner casing and the atmosphere and fluidity between the layers of the steel coil. These factors lead to significant differences in the formation environment of the bottom layer on the upper surface or outer surface of the steel coil compared to the formation environment of the bottom layer in the middle of the steel coil. Therefore, the formation time of the bottom layer in different parts of the steel coil varies during high-temperature annealing. If the oxide layer structure formed during decarburization annealing is unreasonable, it will result in better bottom layer formation in some parts and poorer formation in others, leading to problems such as watermarks and crystal exposure. Therefore, how to ensure good bottom layer formation in different parts of the steel coil and reduce the occurrence of surface defects such as watermarks and crystal exposure has always been a challenge in the production of grain-oriented silicon steel. Summary of the Invention

[0004] The technical problem to be solved by this invention is to address the shortcomings of existing technologies by providing an annealing method for improving the surface quality of grain-oriented silicon steel. This method involves controlling the water-hydrogen partial pressure ratio P at different locations within the furnace during the decarburization annealing process. H2O / P H2 This allows the oxide layer to be distributed according to a specific pattern, forming a suitable oxide layer structure, thereby reducing surface defects such as watermarks and crystal exposure on the surface of oriented silicon steel products.

[0005] To address the technical problem proposed in this invention, this invention provides an annealing method for improving the surface quality of grain-oriented silicon steel, the process flow of which includes: decarburization annealing → nitriding annealing → high-temperature annealing → tensile leveling annealing.

[0006] In the above scheme, the chemical composition of the oriented silicon steel by weight percentage is as follows: Si: 3.31~3.6%, C: 0.05~0.075%, Mn: 0.05~0.12%, P: 0.01~0.05%, S: 0.005~0.01%, N: 0.005~0.009%, Cr: 0.02~0.2%, Cu: 0.1~0.2%, Al: 0.0306~0.0330%, with the balance being Fe and unavoidable impurities.

[0007] In the above scheme, the decarburization annealing is carried out at a constant temperature of 800~900℃ for a duration of 1~3 minutes.

[0008] In the above scheme, the protective gas for decarburization annealing is a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:(1~3) and a total flow rate of 400~600 m³ / s. 3 / h.

[0009] In the above scheme, the decarburization annealing section is divided into three sections according to the furnace length. The first section is the front 1 / 2 to 2 / 3 of the furnace length, the second section is the 1 / 4 of the furnace length after the first section, and the third section is the remaining 1 / 12 to 1 / 4 of the furnace length. The protective gas is introduced non-uniformly in each section.

[0010] Furthermore, the protective gas inlets are not uniformly distributed, with the first and third sections arranged at a spacing of 3-4m, and the second section arranged at a spacing of 1.5-2m.

[0011] Furthermore, the flow rate of the protective gas is not uniformly distributed. The first section receives 1 / 4 to 3 / 8 of the total flow rate, the second section receives 1 / 2 to 2 / 3 of the total flow rate, and the third section receives 1 / 12 to 1 / 8 of the total flow rate. However, the flow rate at each inlet within each section is uniformly distributed.

[0012] Furthermore, the protective gas is non-uniformly humidified, with the first humidification temperature being 5~15℃, the second humidification temperature being 67~77℃, and the third humidification temperature being 40~60℃.

[0013] In the above scheme, a set of detection points, numbered from 1 to n, are set up inside the annealing furnace from the front to the back. The detection points start 2-5m from the front of the furnace and are then set up at fixed intervals of 5-10m, satisfying 16 ≤ n ≤ 20. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: 0.096i - 0.00353i 2 +0.02651≤P i ≤0.099i-0.00396i 2 +0.3, where i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i.

[0014] In the above scheme, the O content in the steel after decarburization annealing is 500~1300ppm.

[0015] The main technical concept of this invention is as follows:

[0016] During the high-temperature annealing process of steel coils, the bottom layer is formed by the reaction of the oxide layer and the magnesium oxide coated on the surface. The reaction sequence of MgO and SiO2 is that MgO reacts first with the surface of the oxide layer and then gradually moves inward until all the oxide layers are completely reacted, forming a Mg2SiO4 bottom layer covering the substrate surface. Due to the significant temperature difference between the inner and outer rings during the high-temperature annealing process, the outer ring has a higher temperature, and the interlayer atmosphere is similar to that inside the inner ring. Conversely, the inner ring has a lower temperature, and the interlayer atmosphere and the atmosphere inside the inner ring are less likely to circulate. Therefore, the bottom layer inevitably forms from the outside in, with the outer ring bottom layer forming first and the inner ring bottom layer forming last. This results in the inability of a single partial pressure ratio atmosphere to meet the conditions required for good bottom layer formation in different parts of the coil.

[0017] When the protective gas for decarburization annealing is introduced non-uniformly, the steel plate passes through areas with different partial pressure ratios in the furnace. The oxide layer formed on the surface of the steel strip exhibits different structural characteristics. The main reason is that the proportions of SiO2 and Fe2SiO4 in the oxide layer formed under different partial pressure ratios are different. SiO2 is more likely to form under low partial pressure ratios, while Fe2SiO4 is more likely to form under high partial pressure ratios. Moreover, the morphology of SiO2 formed under different conditions is also different. A suitable oxide layer structure can simultaneously satisfy the overflow of N in the outer and inner rings and the formation of a good bottom layer, thereby achieving the absence of oxide color and crystal defects.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] This invention controls the water-hydrogen partial pressure ratio in different parts of the furnace during decarburization annealing to achieve a specific distribution of the water-hydrogen partial pressure ratio. This results in the formation of a suitable oxide layer structure during decarburization annealing, ensuring good bottom layer formation in all parts of the steel coil. The incidence of surface defects such as watermarks and crystallization is significantly reduced. The length of crystallization defects accounts for less than 7% of the total coil length, and the length of watermark defects accounts for less than 6% of the total coil length. Detailed Implementation

[0020] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0021] Each embodiment of the present invention is manufactured according to the following process, including the following steps:

[0022] 1) Smelting and continuous casting

[0023] According to the chemical composition weight percentage of oriented silicon steel: Si: 3.31~3.6%, C: 0.05~0.075%, Mn: 0.05~0.12%, P: 0.01~0.05%, S: 0.005~0.01%, N: 0.005~0.009%, Cr: 0.02~0.2%, Cu: 0.1~0.2%, Al: 0.0306~0.0330%, with the balance being Fe and unavoidable impurities, smelting and continuous casting are carried out to obtain slabs;

[0024] 2) Hot rolling, pickling and normalizing, and cold rolling

[0025] The slab is heated in a furnace at a temperature of 1120~1190℃; it is hot-rolled to a thickness of 1.8~2.8mm; during pickling and normalizing, a two-stage normalizing process is adopted, with the first stage normalizing at a temperature of 1100~1150℃ for 10~30s, and the second stage normalizing at a temperature of 850~950℃ for 10~30s; cold rolling is carried out in one pass, with intermediate passes performing effective rolling at 180~250℃ to control the finished product thickness to 0.18~0.3mm;

[0026] 3) Decarburization annealing

[0027] The cold-rolled steel strip undergoes decarburization annealing in an annealing furnace at a constant temperature of 800-900℃ for 1-3 minutes. The protective gas for decarburization annealing is a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:(1-3), and a total flow rate of 400-600 m³ / h. 3 / h;

[0028] The decarburization annealing section is divided into three segments according to the furnace length. The first segment is the front 1 / 2 to 2 / 3 of the furnace length, the second segment is the rear 1 / 4 of the furnace length, and the third segment is the remaining 1 / 12 to 1 / 4 of the furnace length. The protective gas is introduced non-uniformly in each segment.

[0029] a. The protective gas inlets are not uniformly distributed; the first and third sections are distributed at intervals of 3-4m, and the second section is distributed at intervals of 1.5-2m.

[0030] b. The flow rate of the protective gas is not uniformly distributed. The first section is filled with 1 / 4 to 3 / 8 of the total flow rate, the second section is filled with 1 / 2 to 2 / 3 of the total flow rate, and the third section is filled with 1 / 12 to 1 / 8 of the total flow rate. However, the flow rate at each inlet within each section is uniformly distributed.

[0031] c. The protective gas is humidified non-uniformly, with the first humidification temperature being 5~15℃, the second humidification temperature being 67~77℃, and the third humidification temperature being 40~60℃;

[0032] Inside the annealing furnace, a set of detection points, numbered from 1 to n, is set up from the front to the back of the furnace. The detection points begin 2-5 meters from the front of the furnace and are then spaced at fixed intervals of 5-10 meters, ensuring that 16 ≤ n ≤ 20. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: In the formula, i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i;

[0033] After decarburization annealing, the oxygen content in the steel is 500~1300 ppm;

[0034] 4) Nitriding annealing

[0035] After decarburization annealing, nitriding is performed at a temperature of 770~900℃ for 15~30s. The nitriding atmosphere is a mixture of N2+H2+NH3 with a dew point of -10~20℃, where the volume ratio of N2 to H2 is 1:(2~3) and the flow rate of NH3 is 3~20m³. 3 / h, then coated with MgO and dried;

[0036] 5) High-temperature annealing

[0037] First, rapidly heat the gas to 700-750℃ at a rate of 50℃ / h or higher, then introduce a N2 and H2 mixture with a volume ratio of 1:(1-3) and hold for 15-20 hours; then slowly heat the gas to 1170-1210℃ at a rate of 15-17℃ / h, and hold for 20-30 hours in a 100% H2 atmosphere; finally, cool the gas to 300-500℃ at a rate of 12-30℃ / h, and then air cool to room temperature.

[0038] 6) Stretching and leveling annealing

[0039] After leveling and annealing at 800~900℃, an insulating coating is applied to obtain the finished grain-oriented silicon steel.

[0040] Example 1

[0041] The chemical composition of the oriented silicon steel in this embodiment is as follows (by weight percentage): Si: 3.31%, C: 0.059%, Mn: 0.09%, P: 0.01%, S: 0.005%, N: 0.008%, Cr: 0.05%, Cu: 0.1%, Al: 0.0310%, with the balance being Fe and unavoidable impurities.

[0042] According to the chemical composition of grain-oriented silicon steel, slabs are smelted and continuously cast to obtain slabs. The slabs are heated in a furnace at 1130℃ and then hot-rolled to a thickness of 2.6mm. Subsequently, pickling and normalizing are performed using a two-stage normalizing process: the first stage at 1120℃ for 30 seconds, and the second stage at 950℃ for 20 seconds. Finally, cold rolling is performed in a single pass with intermediate passes at 195℃ to obtain a cold-rolled steel strip with a finished thickness of 0.27mm. This cold-rolled steel strip is then annealed using the following steps:

[0043] 1) Decarburization annealing

[0044] The cold-rolled steel strip underwent decarburization annealing in an annealing furnace at a constant temperature of 860℃ for 1.5 minutes. The protective gas for decarburization annealing was a humidified mixture of N2 and H2 with a volume ratio of 1:2 and a total flow rate of 400 m³ / s. 3 / h;

[0045] The decarburization annealing section is divided into three segments: the first segment is the front 5 / 8 of the furnace length; the second segment is the rear 1 / 4 of the furnace length; and the third segment is the remaining 1 / 8 of the furnace length. The protective gas is introduced non-uniformly in each segment.

[0046] a. The protective gas inlets are not uniformly distributed; the first and third sections are arranged at 3m intervals, and the second section is arranged at 1.5m intervals.

[0047] b. The flow rate of the protective gas is not uniformly distributed. The first section is filled with 3 / 10 of the total flow rate, the second section is filled with 3 / 5 of the total flow rate, and the third section is filled with 1 / 10 of the total flow rate. The flow rate of each inlet in each section is uniformly distributed.

[0048] c. The protective gas is humidified non-uniformly, with the first humidification temperature at 5℃, the second humidification temperature at 74℃, and the third humidification temperature at 50℃.

[0049] Inside the annealing furnace, a set of detection points, numbered 1 to 16, is set up from the front to the back of the furnace. The detection points begin 2 meters from the front of the furnace and are set up at 8-meter intervals thereafter. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: In the formula, i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i;

[0050] The measured results of the partial pressure ratio of water and hydrogen at each detection point are as follows:

[0051]

[0052] After decarburization annealing, the oxygen content in the steel was 756 ppm.

[0053] 2) Nitriding annealing

[0054] After decarburization annealing, nitriding was performed at a temperature of 860℃ for 20 seconds. The nitriding atmosphere was a mixture of N2, H2, and NH3 with a dew point of 20℃, wherein the volume ratio of N2 to H2 was 1:3, and the flow rate of NH3 was 12 m³ / s. 3 / h, then coated with MgO and dried;

[0055] 3) High-temperature annealing

[0056] First, the temperature is rapidly increased to 700℃ at a rate of 50℃ / h, and a N2 and H2 mixture with a volume ratio of 1:3 is introduced and held for 15h; then, the temperature is slowly increased to 1200℃ at a rate of 17℃ / h, and held for 25h in a 100% H2 atmosphere; then, the temperature is decreased to 500℃ at a rate of 20℃ / h, and then air-cooled to room temperature.

[0057] 4) Stretching and leveling annealing

[0058] The product is oriented silicon steel by performing leveling annealing at 870℃ and applying an insulating coating.

[0059] Comparative Example 1

[0060] Comparative Example 1 is the same as Example 1 except for the decarburization annealing, except that:

[0061] The decarburization annealing temperature was 860℃, and the duration was 1.5 min. The protective gas for decarburization annealing was a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:2 and a total flow rate of 400 m³ / min. 3 / h, with nozzles evenly arranged at 3m intervals; after humidifying the mixed gas, it is introduced, and a set of detection points numbered from 1 to 16 are set up from the front of the furnace to the back of the furnace. The detection points start from 2m at the front end of the furnace and are set up every 8m thereafter. The measured partial pressure ratio is 0.45±0.05; after decarburization, the O content in the steel is measured to be 710ppm.

[0062] Example 2

[0063] The chemical composition of the oriented silicon steel in this embodiment is as follows (by weight percentage): Si: 3.5%, C: 0.07%, Mn: 0.09%, P: 0.05%, S: 0.005%, N: 0.007%, Cr: 0.15%, Cu: 0.12%, Al: 0.0320%, with the balance being Fe and unavoidable impurities.

[0064] According to the chemical composition of grain-oriented silicon steel, slabs are smelted and continuously cast to obtain slabs. The slabs are heated in a furnace at 1150℃ and then hot-rolled to a thickness of 2.8mm. Subsequently, pickling and normalizing are performed using a two-stage normalizing process: the first stage at 1130℃ for 30 seconds, and the second stage at 910℃ for 20 seconds. Finally, cold rolling is performed in a single pass with intermediate passes at 205℃ to obtain a cold-rolled steel strip with a finished thickness of 0.30mm. This cold-rolled steel strip is then annealed using the following steps:

[0065] 1) Decarburization annealing

[0066] The cold-rolled steel strip underwent decarburization annealing in an annealing furnace at a constant temperature of 840℃ for 2 minutes. The protective gas for decarburization annealing was a humidified mixture of N2 and H2 with a volume ratio of N2 to H2 of 1:3 and a total flow rate of 600 m³ / s. 3 / h;

[0067] The decarburization annealing section is divided into three sections: the first section is the front half of the furnace length; the second section is the rear quarter of the furnace length; and the third section is the remaining quarter of the furnace length. The protective gas is introduced non-uniformly in each section.

[0068] a. The protective gas inlets are not uniformly distributed; the first and third sections are arranged at 4m intervals, and the second section is arranged at 2m intervals.

[0069] b. The flow rate of the protective gas is not uniformly distributed. The first section is filled with 1 / 4 of the total flow rate, the second section is filled with 5 / 8 of the total flow rate, and the third section is filled with 1 / 8 of the total flow rate. The flow rate of each inlet in each section is uniformly distributed.

[0070] c. The protective gas is humidified non-uniformly, with the first humidification temperature at 10℃, the second humidification temperature at 76℃, and the third humidification temperature at 65℃.

[0071] Inside the annealing furnace, a set of detection points, numbered 1 to 20, is set up from the front to the back of the furnace. The detection points begin 5 meters from the front of the furnace and are set up every 10 meters thereafter. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: In the formula, i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i;

[0072] The measured results of the partial pressure ratio of water and hydrogen at each detection point are as follows:

[0073]

[0074] After decarburization annealing, the oxygen content in the steel is 655 ppm;

[0075] 2) Nitriding annealing

[0076] After decarburization annealing, nitriding is performed at a temperature of 840℃ for 15 seconds. The nitriding atmosphere is a mixture of N2, H2, and NH3 with a dew point of 20℃, where the volume ratio of N2 to H2 is 1:3 and the flow rate of NH3 is 5 m³ / s. 3 / h, then coated with MgO and dried;

[0077] 3) High-temperature annealing

[0078] First, the temperature is rapidly increased to 750℃ at a rate of 70℃ / h, and a N2 and H2 mixture with a volume ratio of 1:2 is introduced and held for 20h; then, the temperature is slowly increased to 1190℃ at a rate of 16℃ / h, and held for 30h in a 100% H2 atmosphere; then, the temperature is decreased to 500℃ at a rate of 18℃ / h, and then air-cooled to room temperature.

[0079] 4) Stretching and leveling annealing

[0080] The steel is annealed at 900℃ and then coated with an insulating coating to obtain the finished grain-oriented silicon steel.

[0081] Comparative Example 2

[0082] Comparative Example 2 is the same as Example 2 except for the decarburization annealing, except that:

[0083] The decarburization annealing temperature was 840℃, and the duration was 2 minutes. The protective gas for decarburization annealing was a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:3 and a total flow rate of 600 m³ / s. 3 / h, with nozzles evenly arranged at 4m intervals; after humidifying the mixed gas, it is introduced, and a set of detection points numbered from 1 to 20 are set up from the front of the furnace to the back of the furnace. The detection points start from 5m at the front end of the furnace and are set up every 10m thereafter. The partial pressure ratio is measured to be 0.35±0.05; after decarburization is completed, the O content in the steel is measured to be 620ppm.

[0084] Example 3

[0085] The chemical composition of the oriented silicon steel in this embodiment is as follows (by weight percentage): Si: 3.4%, C: 0.06%, Mn: 0.05%, P: 0.01%, S: 0.005%, N: 0.0065%, Cr: 0.02%, Cu: 0.2%, Al: 0.0330%, with the balance being Fe and unavoidable impurities.

[0086] According to the chemical composition of grain-oriented silicon steel, slabs are smelted and continuously cast to obtain slabs. The slabs are heated in a furnace at 1180℃ and then hot-rolled to a thickness of 2.4mm. Subsequently, pickling and normalizing are performed using a two-stage normalizing process: the first stage at 1100℃ for 30 seconds, and the second stage at 880℃ for 30 seconds. Finally, cold rolling is performed in a single pass with intermediate passes at 230℃ to obtain a cold-rolled steel strip with a finished thickness of 0.23mm. This cold-rolled steel strip is then annealed using the following steps:

[0087] 1) Decarburization annealing

[0088] The cold-rolled steel strip underwent decarburization annealing in an annealing furnace at a constant temperature of 830℃ for 2.5 minutes. The protective gas for decarburization annealing was a humidified mixture of N2 and H2 with a volume ratio of 1:1 and a total flow rate of 600 m³ / s. 3 / h;

[0089] The decarburization annealing section is divided into three sections: the first section covers the front 2 / 3 of the furnace length, the second section covers the rear 1 / 4 of the furnace length, and the third section covers the remaining 1 / 12 of the furnace length. The protective gas is introduced non-uniformly in each section.

[0090] a. The protective gas inlets are not uniformly distributed; the first and third sections are arranged at 3m intervals, and the second section is arranged at 1.5m intervals.

[0091] b. The flow rate of the protective gas is not uniformly distributed. The first section is filled with 1 / 4 of the total flow rate, the second section is filled with 2 / 3 of the total flow rate, and the third section is filled with 1 / 12 of the total flow rate. The flow rate of each inlet in each section is uniformly distributed.

[0092] c. The protective gas is humidified non-uniformly, with the first humidification temperature at 15℃, the second humidification temperature at 69℃, and the third humidification temperature at 60℃.

[0093] Inside the annealing furnace, a set of detection points, numbered 1 to 17, is set up from the front to the back of the furnace. The detection points begin 2 meters from the front of the furnace and are set up every 5 meters thereafter. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: In the formula, i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i;

[0094] The measured results of the partial pressure ratio of water and hydrogen at each detection point are as follows:

[0095]

[0096] After decarburization annealing, the oxygen content in the steel is 805 ppm;

[0097] 2) Nitriding annealing

[0098] After decarburization annealing, nitriding is performed at a temperature of 900℃ for 30 seconds. The nitriding atmosphere is a mixture of N2, H2, and NH3 with a dew point of -10℃, where the volume ratio of N2 to H2 is 1:3 and the flow rate of NH3 is 20 m³ / s. 3 / h, then coated with MgO and dried;

[0099] 3) High-temperature annealing

[0100] First, the temperature is rapidly increased to 700℃ at a rate of 60℃ / h, and a N2 and H2 mixture with a volume ratio of 1:3 is introduced and held for 20h; then, the temperature is slowly increased to 1210℃ at a rate of 17℃ / h, and held for 20h in a 100% H2 atmosphere; then, the temperature is decreased to 300℃ at a rate of 15℃ / h, and then air-cooled to room temperature.

[0101] 4) Stretching and leveling annealing

[0102] The steel is annealed at 850℃ and then coated with an insulating coating to obtain the finished grain-oriented silicon steel.

[0103] Comparative Example 3

[0104] Comparative Example 3 is the same as Example 3 except for the decarburization annealing, except that:

[0105] The decarburization annealing temperature was 830℃, and the duration was 2.5 min. The protective gas for decarburization annealing was a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:1 and a total flow rate of 600 m³ / s. 3 / h, with nozzles evenly arranged at 3m intervals; after humidifying the mixed gas, it is introduced, and a set of detection points numbered from 1 to 17 are set up from the front of the furnace to the back of the furnace. The detection points start from 2m at the front end of the furnace and are set up every 5m thereafter. The partial pressure ratio is measured to be 0.6±0.05; after decarburization is completed, the O content in the steel is measured to be 852ppm.

[0106] Example 4

[0107] The chemical composition of the oriented silicon steel in this embodiment is as follows (by weight percentage): Si: 3.35%, C: 0.055%, Mn: 0.08%, P: 0.05%, S: 0.005%, N: 0.008%, Cr: 0.05%, Cu: 0.2%, Al: 0.0320%, with the balance being Fe and unavoidable impurities.

[0108] According to the chemical composition of grain-oriented silicon steel, slabs are smelted and continuously cast to obtain slabs. The slabs are heated in a furnace at 1120℃ and then hot-rolled to a thickness of 2.2mm. Subsequently, pickling and normalizing are performed using a two-stage normalizing process: the first stage at 1150℃ for 15 seconds, and the second stage at 900℃ for 15 seconds. Then, cold rolling is performed in a single pass with intermediate passes at 210℃ to obtain a cold-rolled steel strip with a finished thickness of 0.2mm. This cold-rolled steel strip is then annealed using the following steps:

[0109] 1) Decarburization annealing

[0110] The cold-rolled steel strip underwent decarburization annealing in an annealing furnace at a constant temperature of 820℃ for 3 minutes. The protective gas for decarburization annealing was a humidified mixture of N2 and H2 with a volume ratio of N2 to H2 of 1:2 and a total flow rate of 500 m³ / s. 3 / h;

[0111] The decarburization annealing section is divided into three sections: the first section is the front half of the furnace length; the second section is the rear quarter of the furnace length; and the third section is the remaining quarter of the furnace length. The protective gas is introduced non-uniformly in each section.

[0112] a. The protective gas inlets are not uniformly distributed; the first and third sections are arranged at 4m intervals, and the second section is arranged at 2m intervals.

[0113] b. The flow rate of the protective gas is not uniformly distributed. The first section is filled with 3 / 8 of the total flow rate, the second section is filled with 1 / 2 of the total flow rate, and the third section is filled with 1 / 8 of the total flow rate. The flow rate at each inlet within each section is uniformly distributed.

[0114] c. The protective gas is humidified non-uniformly, with the first humidification temperature at 8℃, the second humidification temperature at 72℃, and the third humidification temperature at 50℃.

[0115] Inside the annealing furnace, a set of detection points, numbered 1 to 20, is set up from the front to the back of the furnace. The detection points begin 5 meters from the front of the furnace and are set up every 10 meters thereafter. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: In the formula, i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i;

[0116] The measured results of the partial pressure ratio of water and hydrogen at each detection point are as follows:

[0117]

[0118] After decarburization annealing, the oxygen content in the steel is 1150 ppm;

[0119] 2) Nitriding annealing

[0120] After decarburization annealing, nitriding is performed at a temperature of 850℃ for 20 seconds. The nitriding atmosphere is a mixture of N2, H2, and NH3 with a dew point of 10℃, wherein the volume ratio of N2 to H2 is 1:3, and the flow rate of NH3 is 7 m³ / s. 3 / h, then coated with MgO and dried;

[0121] 3) High-temperature annealing

[0122] First, the temperature is rapidly increased to 750℃ at a rate of 50℃ / h, and a N2 and H2 mixture with a volume ratio of 1:3 is introduced and held for 20h; then, the temperature is slowly increased to 1170℃ at a rate of 17℃ / h, and held for 20h in a 100% H2 atmosphere; then, the temperature is decreased to 300℃ at a rate of 30℃ / h, and then air-cooled to room temperature.

[0123] 4) Stretching and leveling annealing

[0124] The steel is annealed at 880℃ and then coated with an insulating coating to obtain the finished grain-oriented silicon steel.

[0125] Comparative Example 4

[0126] Comparative Example 4 is the same as Example 4 except for the decarburization annealing, except that:

[0127] The decarburization annealing temperature was 820℃, and the duration was 3 minutes. The protective gas for decarburization annealing was a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:2 and a total flow rate of 500 m³ / s. 3 / h, with nozzles evenly arranged at 4m intervals; after humidifying the mixed gas, it is introduced, and a set of detection points numbered from 1 to 20 is set up from the front of the furnace to the back of the furnace. The detection points start from 5m at the front end of the furnace and are set up every 10m thereafter. The partial pressure ratio is measured to be 0.65±0.05; after decarburization, the O content in the steel is measured to be 1230ppm.

[0128] The occurrence of defects on the surface of the oriented silicon steel products obtained in each embodiment and comparative example was tracked, and the results are as follows:

[0129]

[0130] The above data shows that by adopting the annealing method of the present invention, the occurrence rate of surface defects such as watermarks and crystal exposure can be significantly reduced. The proportion of crystal exposure defects to the total roll length is less than 7%, and the proportion of watermark defects to the total roll length is less than 6%.

[0131] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An annealing method for improving the surface quality of grain-oriented silicon steel, the process comprising: The process is characterized by decarburization annealing → nitriding annealing → high-temperature annealing → stretching and leveling annealing, wherein the decarburization annealing section is divided into three sections according to the furnace length. The first section is the front 1 / 2 to 2 / 3 of the furnace length, the second section is the rear 1 / 4 of the furnace length, and the third section is the remaining 1 / 12 to 1 / 4 of the furnace length. The protective gas is introduced non-uniformly in each section. Inside the decarburization annealing furnace, a set of detection points, numbered from 1 to n, is set up, where 16 ≤ n ≤ 20. The detection points start 2-5m from the front of the furnace and are then spaced at fixed intervals of 5-10m. Through segmented control, the water-hydrogen partial pressure ratio at any detection point inside the furnace satisfies the formula: 0.096i - 0.00353i. 2 +0.02651≤P i ≤0.099i-0.00396i 2 +0.3, where i represents the detection point number, P i This represents the water-hydrogen partial pressure ratio measured at detection point i.

2. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, The protective gas inlets are not uniformly distributed. The first and third sections are arranged at a spacing of 3 to 4 meters, and the second section is arranged at a spacing of 1.5 to 2 meters.

3. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, The flow rate of the protective gas is not uniformly distributed. The first section receives 1 / 4 to 3 / 8 of the total flow rate, the second section receives 1 / 2 to 2 / 3 of the total flow rate, and the third section receives 1 / 12 to 1 / 8 of the total flow rate. However, the flow rate at each inlet within each section is uniformly distributed.

4. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, The protective gas is humidified non-uniformly, with the first humidification temperature being 5~15℃, the second humidification temperature being 67~77℃, and the third humidification temperature being 40~60℃.

5. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, The protective gas for the decarburization annealing is a humidified mixture of N2 and H2, with a volume ratio of N2 to H2 of 1:(1~3), and a total flow rate of 400~600 m³ / s. 3 / h.

6. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, The decarburization annealing is carried out at a constant temperature of 800~900℃ for 1~3 minutes.

7. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, After the decarburization annealing is completed, the oxygen content in the steel is 500~1300ppm.

8. The annealing method for improving the surface quality of grain-oriented silicon steel according to claim 1, characterized in that, The chemical composition of the oriented silicon steel, by weight percentage, is as follows: Si: 3.31~3.6%, C: 0.05~0.075%, Mn: 0.05~0.12%, P: 0.01~0.05%, S: 0.005~0.01%, N: 0.005~0.009%, Cr: 0.02~0.2%, Cu: 0.1~0.2%, Al: 0.0306~0.0330%, with the balance being Fe and unavoidable impurities.

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