A decarburization and nitriding process for high magnetic induction oriented silicon steel

Abstract

The present invention discloses a decarburization and nitriding process for high magnetic induction oriented silicon steel, belonging to the field of manufacturing oriented silicon steel. The decarburization and nitriding are carried out by an annealing furnace, which is successively divided into a preheating section - a decarburization section - a nitriding section and a cooling section. The process of decarburization first and then nitriding is adopted. Under a low-temperature wet atmosphere, the carbon is first decarburized to less than 0.0015%; subsequently, high-temperature short-time nitriding is carried out under a dry atmosphere. The present invention can stably decarburize the carbon to less than 0.0015%, control the nitriding amount within 200 - 220 ppm, control the average grain size of primary recrystallization within 15 - 20 μm, and the Goss grain deviation angle within 5 - 8°. The present invention helps to solve the problem of unstable magnetic properties caused by large fluctuations in the nitriding amount range.

Description

Technical Field

[0001] The present invention relates to the technical field of manufacturing oriented electrical steel, and more specifically, to a decarburization and nitriding process for high magnetic induction oriented electrical steel. Background Art

[0002] Due to its excellent magnetization performance in the rolling direction, oriented electrical steel is widely used in the industrial production of transformer cores. In the prior art, in the decarburization annealing and nitriding processes of oriented electrical steel, there are many process conditions that affect the final performance of the product. In particular, the magnetic properties are easily unstable due to large fluctuations in the nitriding amount range. Therefore, how to optimize the decarburization and nitriding processes and improve the product performance has attracted increasing attention in the industry.

[0003] After retrieval, the Chinese patent application number is: 2016110164685, and the invention creation name is: A decarburization and nitriding annealing method for producing low-temperature high magnetic induction oriented electrical steel. This application case includes smelting; heating the continuous casting billet after continuous casting; cold rolling to the finished thickness after conventional hot rolling and normalizing annealing; decarburization annealing after conventional alkali washing; nitriding annealing; online detecting the P1.3 / 50 value of the steel plate and calculating its revised value according to the formula; judging whether it simultaneously satisfies the relational expression according to the △N content after decarburization and nitriding; and conventionally performing subsequent processes. By adjusting the decarburization annealing and nitriding annealing, this application case makes the P1.3 / 50 value and the nitriding amount △N of the steel coil material simultaneously within the limited reasonable range, resulting in excellent magnetic properties of more than 1.92T for the products with a thickness not exceeding 0.30mm.

[0004] Another example is the Chinese patent application number: 2018106146768, and the invention creation name is: A production method and its product for nitriding annealing of high magnetic induction oriented electrical steel. The production method of this application case is: medium-temperature nitriding, the nitriding temperature is 800 - 850 °C, which is the same as the decarburization temperature, the nitriding time is 10 - 50 s, the nitriding medium is ammonia gas, the flow rate is 5 - 6m 3 / h, the pressure is 25 - 30 KPa, and at the same time, a nitrogen-hydrogen mixed protective gas is introduced into the furnace. The process standard range of the nitriding value is 150 - 210 ppm; this application case is used to produce finished products with low-temperature high magnetic induction oriented electrical steel and iron loss values reaching 0.90 - 0.95 W / kg. In summary, there are already a large number of public disclosures for the decarburization and nitriding processes of oriented electrical steel, and their R & D ideas and technical purposes are different. The industry is also constantly exploring new R & D paths. Summary of the Invention

[0005] 1. Technical Problems to be Solved by the Invention

[0006] The object of the present invention is to provide a decarburization and nitriding process for high magnetic induction oriented silicon steel to solve the problems in the prior art that after decarburization and nitriding of oriented silicon steel, the nitriding amount fluctuates greatly and the primary recrystallization grain size is large, which is not conducive to secondary recrystallization. The process can stably reduce the carbon content to less than 0.0015%, control the nitriding amount within 200 - 220 ppm, control the average primary recrystallization grain size within 15 - 20 μm, and the Goss grain deviation angle within 5 - 8°, providing a stable secondary recrystallization platform for the high-temperature annealing process.

[0007] 2. Technical Solution

[0008] To achieve the above object, the technical solution provided by the present invention is as follows:

[0009] A decarburization and nitriding process for high magnetic induction oriented silicon steel of the present invention includes preheating, decarburization, nitriding and cooling. Specifically, a annealing furnace is used for decarburization and nitriding. Correspondingly, the annealing furnace is successively divided into a preheating section - a decarburization section - a nitriding section and a cooling section. The present invention adopts a process of decarburization first and then nitriding. Under a low-temperature wet atmosphere, the carbon content is first reduced to less than 0.0015%; subsequently, high-temperature short-time nitriding is carried out under a dry atmosphere, with the nitriding amount being 200 - 220 ppm, the primary recrystallization grain size being 15 - 20 μm, and the Goss grain deviation angle being 5 - 8°.

[0010] Furthermore, the incoming strip thickness ≤ 0.23 mm.

[0011] Furthermore, the preheating section uses flame heating, and the strip is rapidly heated from room temperature to 830 - 840 °C, with a heating rate of 100 - 120 °C / S, and the furnace pressure is controlled at 10 - 15 pa.

[0012] Furthermore, the decarburization section uses resistance band heating, the temperature is stably controlled at 810 - 820 °C, the H2O dew point is controlled at 10 - 15 °C, the furnace pressure is controlled at 15 - 20 pa, and the strip traveling time in the decarburization section is controlled to be 90 - 120 s.

[0013] Furthermore, the decarburization section uses a protective atmosphere of hydrogen and oxygen, with the H2 content being 15 - 25% and the O2 content being 400 - 500 ppm.

[0014] Furthermore, the nitriding section uses resistance band heating, the temperature is controlled at 900 - 920 °C, the furnace pressure is controlled at 20 - 25 pa, ammonia decomposition is used for nitriding, and the ammonia flow rate is 100 - 130 m 3 / h, and the strip traveling time in the nitriding section is controlled to be 50 - 60 s.

[0015] Furthermore, a closed furnace throat is adopted between the decarburization section and the nitriding section to prevent the interference of atmospheres.

[0016] Furthermore, the cooling section is divided into a pre-cooling section and a rapid-cooling section. The pre-cooling section uses air cooling to cool the material to 550 - 500 °C; the rapid-cooling section uses mist cooling to cool the material to room temperature, and a squeezing roller is used to squeeze out the water.

[0017] In the present invention, the N content of the grain-oriented silicon steel determines the amount of the inhibitor AlN. The larger the range of the nitriding amount, the more unstable the amount of AlN becomes. It is very difficult to adjust it through high-temperature annealing, resulting in large fluctuations in magnetic properties and even failure to obtain abnormal-grown Goss texture. The decarburization and nitriding process is unstable and the range of the nitriding amount is large, so it cannot be controlled subsequently. Whether the nitriding amount is small or large, as long as the stability is high and the nitriding amount is stable, it can be remedied by adjusting the high-temperature annealing process. However, if the fluctuation range of the nitriding amount is large, exceeding 30 ppm, it will cause uneven distribution of AlN along the thickness and width of the plate, and at this time, it is impossible to remedy it by adjusting the subsequent processes. A stable nitriding amount is the prerequisite for obtaining stable magnetic properties. The large fluctuations in the magnetic properties of grain-oriented silicon steel are a direct manifestation of the instability of the inhibitor. There is a phenomenon of dividing into small coils during the production process because the magnetic properties are uneven, and it can only be coiled according to the magnetic properties, resulting in a large number of coils and increasing the workload. The stability of the nitriding process is the core process restricting high magnetic induction grain-oriented silicon steel. The present invention helps to solve the technical problem of unstable magnetic properties caused by large fluctuations in the range of the nitriding amount.

[0018] In addition, the smaller the primary recrystallized grain size, the greater the driving force for secondary recrystallization. However, in the current process, in order to better pursue the nitriding amount, most processes ignore the control of the primary recrystallized grain size, and even lose control, which also leads to unstable magnetism. This is why in some processes, the nitriding amount is as high as over 300 ppm, but the magnetic properties are average. Therefore, in the decarburization and nitriding process, while ensuring the nitriding amount, it is necessary to take into account the control of the primary recrystallized grain size.

[0019] The present invention adopts a process of decarburization first and then nitriding, and coordinates the process control of each process. First, decarburization is carried out in a low-temperature wet atmosphere to reduce the carbon content to less than 0.0015%. Since the temperature is low, the primary grains are fine; then high-temperature short-time nitriding is carried out in a dry atmosphere. High temperature is conducive to increasing the nitriding amount. Since the time is short, the grain growth is not obvious; due to the good decarburization quality in the previous stage, the temperature, time, and flow rate in the nitriding stage are effectively controlled, without the interference of the atmosphere, and the nitriding amount is stable with a small fluctuation range. The preheating section uses flame heating, with a fast heating rate, a high nucleation rate, and fine primary grains; in addition, a fast heating rate helps the nucleation and recrystallization of {111} grains, obtaining more favorable textures. In the cooling stage, it is first air-cooled to 550 - 500 °C and then water-cooled to room temperature. The air-cooling speed is slow, avoiding the too-fast water-cooling speed directly and increasing the stress of the steel strip; the method of first air-cooling and then water-cooling not only increases the cooling speed but also reduces the stress.

[0020] With the design concept of the present invention, for the incoming material with a thickness exceeding 0.23 mm, corresponding effects can be obtained by further adjusting the temperature or the running time of each section.

[0021] 3. Beneficial effects

[0022] Adopting the technical solution provided by the present invention, compared with the prior art, it has the following beneficial effects:

[0023] (1) The present invention adopts the process of decarburization first and then nitriding. Decarburization is first carried out in a low-temperature wet atmosphere, and the primary grains are fine, and the carbon is removed to less than 0.0015%; subsequently, high-temperature short-time nitriding is carried out in a dry atmosphere. High temperature is beneficial to the increase of the nitriding amount. Due to the short time, the grain growth is not obvious; due to the good decarburization quality in the previous stage, the temperature, time, and flow rate in the nitriding stage are effectively controlled, without the interference of the atmosphere, the nitriding amount is stable, and the fluctuation range is small. The nitriding amount is controlled at 200 - 220 ppm.

[0024] (2) The preheating section of the present invention uses flame heating. The faster the heating rate, the higher the nucleation rate, and the primary grains are fine; in addition, a fast heating rate helps the nucleation and recrystallization of {110} grains, obtaining more favorable textures; however, if the heating rate is too fast, the orientation of the retained {110} Gaussian grains is inaccurate, resulting in an increase in the deviation angle of the finished product; in the present invention, the heating rate is strictly controlled at 100 - 120 °C / s. Below 100 °C / s, the nucleation rate is slightly lower; above 120 °C / s, the orientation of the Gaussian nuclei is inaccurate; 100 - 120 °C / s ensures that the nucleation rate and the deviation angle of the Gaussian nuclei are within the best matching range.

[0025] (3) The temperature of the preheating section of the present invention is slightly higher than that of the decarburization section. The high temperature in the preheating section and the fast heating rate are to obtain more nucleation points, and the low temperature in the decarburization section is to inhibit the rapid growth of the nuclei formed in the previous section, so as to obtain fine primary recrystallized grains with a size of 15 - 20 μm, providing sufficient driving force for secondary recrystallization. Brief description of the drawings

[0026] Figure 1 Schematic diagram of the metallographic structure of the strip steel obtained in Example 1;

[0027] Figure 2 Schematic diagram of the metallographic structure of the strip steel obtained in Example 2;

[0028] Figure 3 Schematic diagram of the metallographic structure of the strip steel obtained in Comparative Example 1. Detailed implementation manners

[0029] To further understand the content of the present invention, the present invention will be described in detail with reference to the accompanying drawings.

[0030] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0031] The present invention will be further described below in conjunction with embodiments.

[0032] Embodiment 1

[0033] For the decarburization and nitriding process of high magnetic induction oriented silicon steel in this embodiment, the incoming material thickness is 0.23 mm, the temperature in the preheating section is raised to 840 °C, the heating rate is 120 °C / S, and the furnace pressure is 10 pa; the temperature in the decarburization section is 810 °C, the H2O dew point is controlled at 10 °C, the furnace pressure is controlled at 20 pa, and the passing time of the strip is 90 s; the H2 content in the atmosphere is 25%, and the O2 content is 400 ppm. The furnace pressure in the nitriding section is controlled at 20 pa, the ammonia flow rate is 100 m 3 / h, the temperature is controlled at 900 °C, and the passing time of the strip is 60 s; the pre-cooling section uses air cooling and is cooled to 550 °C; then it is cooled to room temperature by water mist and squeezed dry. The final decarburized annealing sheet has a carbon content of 0.0015%, a nitriding amount of 200 ppm, the primary recrystallized grain size is controlled at 15 μm, and the Goss grain deviation angle is 7°. The metallographic structure of the obtained strip is as Figure 1 shown.

[0034] Embodiment 2

[0035] For the decarburization and nitriding process of high magnetic induction oriented silicon steel in this embodiment, the incoming material thickness is 0.20 mm, the temperature in the preheating section is raised to 830 °C, the heating rate is 100 °C / S, and the furnace pressure is 15 pa; the temperature in the decarburization section is 820 °C, the H2O dew point is controlled at 15 °C, the furnace pressure is controlled at 20 pa, and the passing time of the strip is 120 s; the H2 content in the atmosphere is 15%, and the O2 content is 500 ppm. The furnace pressure in the nitriding section is controlled at 25 pa, the ammonia flow rate is 130 m 3 / h, the temperature is controlled at 920 °C, and the passing time of the strip is 50 s; the pre-cooling section uses air cooling and is cooled to 500 °C; then it is cooled to room temperature by water mist and squeezed dry. The final decarburized annealing sheet has a carbon content of 0.0011%, a nitriding amount of 220 ppm, the primary recrystallized grain size is controlled at 20 μm, and the Goss grain deviation angle is 5°. The metallographic structure of the obtained strip is as Figure 2 shown.

[0036] Embodiment 3

[0037] The decarburization and nitriding process of high magnetic induction oriented silicon steel in this embodiment has a feed thickness of 0.21 mm. The temperature in the preheating section is raised to 835 °C at a heating rate of 110 °C / S, and the furnace pressure is 15 Pa. The temperature in the decarburization section is 820 °C, the H2O dew point is controlled at 15 °C, the furnace pressure is controlled at 15 Pa, and the passing time of the strip is 105 s. The H2 content in the atmosphere is 20%, and the O2 content is 450 ppm. The furnace pressure in the nitriding section is controlled at 25 Pa, the ammonia flow rate is 130 m 3 / h, the temperature is controlled at 900 °C, and the passing time of the strip is 55 s. The pre-cooling section uses air cooling to cool to 540 °C; then it is cooled to room temperature by water mist and squeezed dry. The final decarburized and annealed sheet has a carbon content of 0.0013%, a nitriding amount of 210 ppm, the primary recrystallized grain size is controlled at 19 μm, and the Goss grain deviation angle is 6°.

[0038] Example 4

[0039] The decarburization and nitriding process of high magnetic induction oriented silicon steel in this embodiment has a feed thickness of 0.20 mm. The temperature in the preheating section is raised to 840 °C at a heating rate of 120 °C / S, and the furnace pressure is 12 Pa. The temperature in the decarburization section is 815 °C, the H2O dew point is controlled at 12 °C, the furnace pressure is controlled at 18 Pa, and the passing time of the strip is 105 s. The H2 content in the atmosphere is 20%, and the O2 content is 400 ppm. The furnace pressure in the nitriding section is controlled at 22 Pa, the ammonia flow rate is 120 m 3 / h, the temperature is controlled at 910 °C, and the passing time of the strip is 56 s. The pre-cooling section uses air cooling to cool to 520 °C; then it is cooled to room temperature by water mist and squeezed dry. The final decarburized and annealed sheet has a carbon content of 0.0014%, a nitriding amount of 205 ppm, the primary recrystallized grain size is controlled at 18 μm, and the Goss grain deviation angle is 8°.

[0040] Comparative Example 1

[0041] The decarburization and nitriding process of oriented silicon steel in this comparative example has a strip feed thickness of 0.20 mm. The temperature in the preheating section is 750 °C, and the furnace pressure is 15 Pa. The temperature in the decarburization section is 840 °C, the H2O dew point is controlled at 15 °C, the furnace pressure is controlled at 20 Pa, and the H2 content is 15%. The furnace pressure in the nitriding section is controlled at 25 Pa, the temperature is controlled at 980 °C, and the ammonia flow rate is 150 m 3 / h. The pre-cooling section uses air cooling to cool to 500 °C; then it is cooled to room temperature by water cooling and squeezed dry. The final decarburized and annealed sheet has a carbon content of 0.0013%, a nitriding amount of 260 ppm, the primary recrystallized grain size is controlled at 30 μm, the Goss grain deviation angle reaches 11°, and the grain uniformity is very poor.

[0042] The above has schematically described the present invention and its implementation manners. This description is not restrictive and is only one of the implementation manners of the present invention. In fact, it is not limited thereto. Therefore, if those of ordinary skill in the art are inspired by it and, without departing from the purpose of the present invention, design similar structural manners and embodiments to this technical solution without creative efforts, they shall fall within the protection scope of the present invention.

Claims

1. A decarburization and nitriding process for high magnetic induction oriented silicon steel, characterized in that: Decarburization and nitriding are carried out using an annealing furnace. The annealing furnace is sequentially divided into a preheating section, a decarburization section, a nitriding section, and a cooling section. The process of decarburization first and then nitriding is adopted. Under a low-temperature wet atmosphere, the carbon is first decarburized to less than 0.0015%. Subsequently, high-temperature short-time nitriding is carried out under a dry atmosphere. The preheating section uses flame heating. The strip is rapidly heated from room temperature to 830 - 840 °C, with a heating rate of 100 - 120 °C / S, and the furnace pressure is controlled at 10 - 15 Pa; the decarburization section uses resistance band heating, the temperature is stably controlled at 810 - 820 °C, the H2O dew point is controlled at 10 - 15 °C, the furnace pressure is controlled at 15 - 20 Pa, and the strip travel time in the decarburization section is controlled at 90 - 120 s; the decarburization section uses a protective atmosphere of hydrogen and oxygen, with a H2 content of 15 - 25% and an O2 content of 400 - 500 ppm; the nitriding section uses resistance band heating, the temperature is controlled at 900 - 920 °C, the furnace pressure is controlled at 20 - 25 Pa, and ammonia decomposition is used for nitriding, with an ammonia flow rate of 100 - 130 m 3 / h, and the strip travel time in the nitriding section is controlled at 50 - 60 s; The cooling section is divided into a pre-cooling section and a rapid cooling section. The pre-cooling section uses air cooling to cool to 550 - 500 °C; the rapid cooling section uses aerosol cooling to cool to room temperature, and a squeezing roller is used to squeeze dry. The nitriding amount of the strip steel is 200 - 220 ppm, the primary recrystallized grain size is 15 - 20 μm, the deviation angle of the Goss grains is 5 - 8°, and the carbon is decarburized to less than 0.0015%.

2. The decarburization and nitriding process for high magnetic induction oriented silicon steel according to claim 1, characterized in that: A sealed furnace throat is adopted between the decarburization section and the nitriding section, and the atmospheres do not interfere with each other.

3. The decarburization and nitriding process for high magnetic induction oriented silicon steel according to claim 1, characterized in that: The thickness of the incoming strip steel is ≤ 0.23 mm.

Images