High-magnetic-induction oriented silicon steel and production method thereof

Abstract

The invention discloses high magnetic induction oriented silicon steel and a production method. The high magnetic induction oriented silicon steel comprises the following components in percentage by weight: 0.032 to 0.065 percent of C, 2.58 to 4.02 percent of Si, 0.014 to 0.036 percent of Als, 0.021 to 0.043 percent of Mn, 0.0015 to 0.014 percent of Nb, 0.0015 to 0.0084 percent of B, 0.0037 to 0.0081 percent of N and 0.004 to 0.015 percent of S. A large amount of B element is added, a proper amount of BN and other congenital inhibitors can be provided, the situation that primary grains are too large is effectively avoided, meanwhile, boron oxide, boric acid and borate are added into the magnesium oxide additive, an acquired inhibitor can be generated with N, an AlN + BN double-inhibitor system is formed, and the stability of magnetic performance is guaranteed; and the temperature is kept at 910-1005 DEG C for 18-25 hours, so that B is fully diffused and forms a new BN inhibitor with N, and the stability of the magnetic performance is effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of high magnetic induction oriented silicon steel processing and manufacturing, and particularly relates to high magnetic induction oriented silicon steel and a production method. Background Art

[0002] High-induction oriented silicon steel is a soft magnetic material with excellent magnetic properties. Due to its high permeability and low loss, it is widely used in power transmission and transformation products such as large transformers, including the iron cores within these transformers. The key to its performance lies in controlling the growth of grains with different orientations through the use of inhibitors, resulting in a highly uniform Goss texture. The more precisely the Goss texture orientation of the finished grains in the oriented silicon steel, the higher the magnetic induction intensity and the lower the iron loss. Therefore, inhibitors are crucial for improving the magnetic properties of high-induction oriented silicon steel. Currently, there are two common methods for producing high-induction oriented silicon steel: one using MnS or CuS as the primary inhibitor. These inhibitors have a high solution temperature and generally require heating the ingot to above 1300°C before rolling; the other using AlN as the primary inhibitor. This significantly lowers the heating temperature, but requires extensive nitriding in the subsequent process. Because the former method suffers from high energy consumption and low yield in the hot rolling process, the latter method, which produces high-induction oriented silicon steel at lower temperatures, is more widely used.

[0003] The manufacturing process of low-temperature and high-magnetic induction oriented silicon steel mainly includes: steelmaking → continuous casting → hot rolling → pickling and normalizing → cold rolling → decarburization annealing → nitriding → coating of magnesium oxide isolation agent → high-temperature annealing → stretching and leveling annealing → coating of insulating coating → finishing and other steps.

[0004] Grain growth during decarburization annealing using low-temperature slab technology is called "primary recrystallization," characterized by predominantly equiaxed ferrite grains. Further grain growth during high-temperature annealing is called "secondary recrystallization." Precipitates formed in the steel matrix during steelmaking and alloying can hinder the growth of primary recrystallized grains. Inhibitors are generally divided into innate and acquired inhibitors. Innate inhibitors are primarily added during steelmaking, typically during hot rolling or normalizing, and primarily affect primary recrystallized grain size. Acquired inhibitors, primarily introduced during nitriding or magnesium oxide, primarily affect secondary recrystallized grain size. Because silicon steel compositions using AlN as the primary inhibitor contain fewer precipitates in the steel matrix than traditional high-temperature steels, the ability to inhibit primary recrystallization growth is weakened. Consequently, the grain size of low-temperature, high-magnetic-induction oriented silicon steel during decarburization annealing is larger than that of traditional high-temperature steels, resulting in a higher number of coarse grains. Uneven primary grains will lead to uneven secondary recrystallization. At the same time, larger, coarsened primary grains are difficult to be absorbed by abnormally grown Gossian grains, ultimately forming an uneven and imperfect Gossian texture and poor product magnetic properties. Therefore, it is necessary to appropriately add innate inhibitors to adjust the size of the primary grains without affecting the formation of acquired inhibitors.

[0005] The current production process primarily utilizes AlN as an inhibitor. This compositional system requires high slab heating temperatures, while a single inhibitor can easily lead to unstable magnetic properties. During decarburization annealing, primary recrystallization growth and microstructure uniformity are difficult to control, resulting in poor magnetic stability. During high-temperature annealing, inhibitors often oxidize on the surface, weakening their inhibitory capacity and hindering further improvement in the magnetic properties of grain-oriented silicon steel. Summary of the Invention

[0006] The present invention aims to overcome the deficiencies in the prior art and provides a high magnetic induction oriented silicon steel and a production method thereof.

[0007] To achieve the above-mentioned object, the present invention provides a high magnetic induction oriented silicon steel, wherein the components of the high magnetic induction oriented silicon steel include, by weight percentage, C: 0.032-0.065%, Si: 2.58-4.02%, Als: 0.014-0.036%, Mn: 0.021-0.043%, Nb: 0.0015-0.014%, B: 0.0015-0.0084%, N: 0.0037-0.0081%, S: 0.004-0.015%, and the remainder is Fe and unavoidable impurities.

[0008] The functions of each component in silicon steel are as follows: The main function of C is to maintain a certain amount of γ phase during high-temperature normalization of the hot-rolled plate, allowing for the production of a large amount of fine AlN during rapid cooling. This is primarily because the solid solubility in the γ phase is 10 times greater than that in the α phase. However, excessive carbon content can make decarburization difficult, hindering subsequent high-temperature annealing. When the C content is less than 0.032%, the proportion of γ phase is insufficient. When the C content exceeds 0.065%, it is likely to cause the subsequent C content to exceed the standard, resulting in deterioration of magnetic properties. Therefore, the C content is set between 0.032% and 0.065%.

[0009] Si is an extremely effective element for increasing the resistance of oriented silicon steel and reducing eddy current losses. Increasing the silicon content can effectively reduce iron loss. However, Si tends to segregate at grain boundaries during the hot rolling phase transformation process, making it prone to cracking. When the silicon content is less than 2.58%, the iron loss reduction effect is not significant, and when the Si content exceeds 4.02%, the processing performance is reduced. Therefore, the Si content is set between 2.58% and 4.02%.

[0010] AlS is an important element in the formation of aluminum nitride inhibitors. When the AlS content is less than 0.014%, insufficient aluminum nitride is precipitated after normalization, resulting in insufficient inhibitor strength. On the other hand, when the AlS content exceeds 0.036%, the AlN becomes coarser, reducing inhibitor strength. Therefore, the AlS content is set between 0.014% and 0.036%.

[0011] Mn can react with S to form MnS, which is an important supplementary inhibitor in oriented silicon steel. During the primary recrystallization process, it inhibits grain growth. Mn also prevents cracks during hot rolling. When the manganese content is less than 0.021%, it cannot fully exert its effect. On the other hand, when the manganese content exceeds 0.043%, the magnetic flux density of the oriented silicon steel decreases. Therefore, the manganese content is set to 0.021% to 0.043%. The S content is set to 0.004% to 0.015%.

[0012] Nb reacts with nitrogen to form NbN, which acts as an auxiliary inhibitor to improve the overall inhibitory ability. The Nb content is generally controlled within the range of 0.0015-0.014%.

[0013] B has a stronger ability to combine with nitrogen than aluminum and a faster diffusion rate. BN is generated preferentially during hot rolling. At the same time, the solid solution temperature of BN is low, and a large amount of fine BN can be precipitated in the normalizing stage. It can effectively make up for the problem of insufficient inherent inhibitory ability caused by insufficient solid solution of AlN in the normalizing stage. When the B content is less than 0.0015%, the role of BN inhibitor cannot be fully exerted. On the other hand, when the B content is greater than 0.0084%, the BN size is too large and loses its inhibitory ability. Therefore, the B content is set to 0.0038% to 0.0084%.

[0014] Nitrogen is an important element in the formation of aluminum nitride. However, AlN in grain-oriented silicon steel is generally formed primarily through subsequent nitriding. Nitrogen in the steel matrix can react with boron to form fine BN after normalization, effectively improving the inherent inhibition capability. The nitrogen content range is set to 0.0037% to 0.0081%.

[0015] Furthermore, the weight percentage contents of Als, Mn, B, Nb and N satisfy formula (1) and formula (2): 0.71≤18.71×[Als]+6.12×[Mn]+ 21.3×[B]+7.25×[Nb]≤1.12(1) 0.83≤[B] / 10.81:[N] / 14≤1.41 (2) Wherein, [Als] is the numerical portion of the weight percentage of Als, [Mn] is the numerical portion of the weight percentage of Mn, [B] is the numerical portion of the weight percentage of B, [Nb] is the numerical portion of the weight percentage of Nb, and [N] is the numerical portion of the weight percentage of N.

[0016] A production method for the high magnetic induction oriented silicon steel as described above is also provided, and the process flow of the production method is: steelmaking and continuous casting → hot rolling → pickling and cold rolling → decarburization and nitriding continuous annealing → coating of magnesium oxide isolation agent → high temperature annealing → stretching and flattening annealing → coating of insulating coating → finishing.

[0017] The raw material slab is hot rolled to form a hot rolled coil with a thickness H of 1.75-2.86 mm; After hot rolling, the hot rolled coil is subjected to a two-stage heating and normalizing process under dry pure N2 conditions, which includes a high-temperature normalizing and soaking section and a low-temperature normalizing and soaking section. First, the hot-rolled plate is heated to the high-temperature normalized soaking section temperature T1 at a heating rate v of 112-458°C / s. The high-temperature normalized soaking section temperature T1 satisfies formula (1), and the plate is kept at T1 for 39-98 seconds. T1=1125-3562×([Als]-1.929×[N])-11251×[B]+42×H (3) Wherein, H is the thickness of hot rolled coil, in mm; After the hot rolled coil is kept warm in the high temperature normalizing and soaking section, the low temperature normalizing and soaking stage begins, and the temperature of the hot rolled coil is reduced to T2, T2 is 885-923℃, and kept warm at T2 temperature for 41-83s.

[0018] Furthermore, the normalized steel coil is cold-rolled to a finished thickness through a single reduction process, with a cold-rolling reduction rate of 86.45-92.36%.

[0019] Furthermore, in the decarburization and nitriding continuous annealing process, the continuous annealing temperature is 810-848°C, and after continuous annealing, the carbon content of the steel strip is not higher than 10ppm, the nitrogen content is 174-235ppm, and the oxygen content is 522-971ppm; at the same time, the initial grain size satisfies the coefficient of variation CV less than 13%.

[0020] Furthermore, in the step of coating the magnesium oxide separator, water is added to the magnesium oxide separator mixing tank and boric acid, boron oxide or borates accounting for 2.6-4.1% of the total weight of the magnesium oxide are added, and then titanium dioxide is added to the magnesium oxide separator, stirred until uniform, MgO is added, stirring is continued for 1-1.5 hours, and the steel plate is coated and dried; and the titanium dioxide content accounts for 2.5-11.5% of the total weight of the magnesium oxide.

[0021] Furthermore, in the high-temperature annealing process, the heating stage I adopts rapid heating with a heating rate of 61-77°C / h; the holding stage I is set to 622-723°C and kept at a low temperature for 10-21h; the heating stage II adopts rapid heating to 910-1005°C with a heating rate of 32-56°C / h; the holding stage II is set to 910-1005°C and kept at a temperature of 18-25h; the heating stage III is heated to 1100-1120°C at 6-15°C / h, and the holding stage III is set to 1100-1120°C and kept at a temperature of 14-29h; the heating stage IV is heated to 1150-1250°C at 11-24°C / h and enters the holding stage IV, followed by heat preservation and purification for 14-28h; Among them, N2 atmosphere is used before the insulation section I, 55-63% N2 and the rest H2 is used in the insulation section II, 22-35% N2 and the rest H2 is used in the insulation section III, and pure atmosphere is used after entering the insulation section IV.

[0022] Generally, inhibitors are divided into innate inhibitors and acquired inhibitors. Innate inhibitors are mainly added during the steelmaking process and are generally formed during hot rolling or normalizing. They mainly affect the size of the primary recrystallized grains. Acquired inhibitors are mainly added during nitriding or magnesium oxide and mainly affect the size of the secondary recrystallized grains. Generally, during high-temperature annealing, secondary recrystallization usually begins in the surface and subsurface layers. The main reason is that as the temperature rises, the inhibitors in the surface layer decompose first, resulting in a weakening of the inhibitory ability of the surface layer.

[0023] The present invention also incorporates a significant amount of elemental boron during the steelmaking process. This boron combines with nitrogen to form a significant amount of innate inhibitors, such as BN. This effectively prevents excessive initial grain size during decarburization annealing. BN also forms during nitriding, acting as a subsequent auxiliary inhibitor and effectively enhancing the steel coil's inhibitory capacity. To prevent insufficient acquired inhibitors due to low nitriding levels, the present invention also incorporates a certain amount of boron oxide and other boron-containing compounds into the magnesium oxide separator. This allows for the formation of new BN inhibitors during high-temperature annealing by adjusting the atmosphere and temperature, further enhancing inhibitory capacity.

[0024] The present invention adopts a two-stage normalization process. The first high-temperature holding stage is mainly to ensure the high solid solution of the main inhibitors AlN and BN. The present invention controls the temperature of the normalization high-temperature stage according to formula (3) for different hot-rolled raw materials and key components. This is to ensure that the inhibitors of oriented silicon steel are highly dissolved during the normalization high-temperature heating process, but not to overheat and cause the inhibitors to be too large in size, affecting the inhibitory effect. In addition, a suitable normalization temperature is proposed according to the different thicknesses of hot-rolled coils to ensure that there is no fluctuation in magnetic properties caused by hot-rolled raw materials. In the present invention, rapid heating is used to shorten the heating time, which can promote the material to enter the temperature range with the largest austenite proportion in the shortest time, reducing the problem of the coarse size of the precipitated phase caused by slow heating, thereby losing the ability to control the primary recrystallization.

[0025] The reason why using a composite annealing separator containing boron compounds significantly improves magnetic properties is that boron oxide decomposes into active boron atoms at high temperatures (above 850°C), which diffuse rapidly through grain boundaries and dislocations. Simultaneously, when combined with a suitable high-temperature annealing atmosphere, a new BN inhibitor is formed, effectively enhancing the inhibitory capacity of the surface and subsurface layers.

[0026] The present invention utilizes a four-stage holding process during high-temperature annealing. Low-temperature holding stage I primarily serves to remove moisture from the magnesium oxide, preventing oxidation of boron during subsequent annealing and the formation of a BN inhibitor. Holding stage II, designed to ensure effective boron addition and inhibitor production, requires a high-temperature annealing stage at 910-1005°C for 18-25 hours. This ensures boron addition while also ensuring uniform temperature control of the coil. The annular furnace atmosphere during this stage must avoid oxidizing to effectively generate BN and prevent oxidation of boron. However, it must also be highly reducing to prevent boron deposition as inert BN, which would compromise the inhibitor's ability to resist annealing. Therefore, the annular furnace atmosphere uses 55-63% nitrogen (the remainder being hydrogen). Holding stage III shifts the traditional heating process to a stable, moderate temperature-controlled holding process for secondary recrystallization, utilizing a mixed gas atmosphere that stabilizes the inhibitor. This results in more complete secondary recrystallization and improved magnetic stability. Holding stage IV primarily conducts conventional annealing to remove elements such as nitrogen and sulfur, achieving the desired steel quality.

[0027] Compared with the prior art, the present invention has the following beneficial effects: the present invention adds a relatively large amount of B element during the steelmaking process, thereby providing an appropriate amount of innate inhibitors such as BN, effectively preventing the initial grains from being too large; at the same time, the addition of boron oxide, boric acid, and borate to the magnesium oxide additive can react with N to form acquired inhibitors, forming an AlN+BN dual inhibitor system, thereby ensuring the stability of magnetic properties; in addition, the high-temperature annealing adopts a four-stage heat preservation, wherein the heat preservation is carried out at 910-1005°C for 18-25 hours to allow B to fully diffuse and form a new BN inhibitor with N, thereby effectively improving the stability of magnetic properties. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to specific embodiments.

[0029] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention; Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention; Table 3 is a list of magnetic properties of products of various embodiments of the present invention and comparative examples.

[0030] The production process of grain-oriented silicon steel includes the following steps: steelmaking and continuous casting → hot rolling → pickling and normalizing → cold rolling → decarburization and nitriding continuous annealing → coating with magnesium oxide separator → high temperature annealing → stretching and leveling annealing → coating with insulating coating → finishing. The measures to achieve the above objectives are as follows: (1) The components of high magnetic induction oriented silicon steel include by weight percentage: C: 0.032-0.065%, Si: 2.58-4.02%, Als: 0.014-0.036%, Mn: 0.021-0.043%, Nb: 0.0015-0.014%, B: 0.0015-0.0084%, N: 0.0037-0.0081%, S: 0.004-0.015%, and the rest is Fe and unavoidable impurities.

[0031] The weight percentage contents of Als, Mn, B, Nb and N satisfy formula (1) and formula (2): 0.71≤18.71×[Als]+6.12×[Mn]+ 21.3×[B]+7.25×[Nb]≤1.12(1) 0.83≤[B] / 10.81:[N] / 14≤1.41 (2) Among them, [Als] is the numerical part of the weight percentage of Als, [Mn] is the numerical part of the weight percentage of Mn, [B] is the numerical part of the weight percentage of B, [Nb] is the numerical part of the weight percentage of Nb, and [N] is the numerical part of the weight percentage of N, that is, the numerical part of the weight percentage is taken without the percentage sign.

[0032] (2) The raw material slab is hot rolled to form a hot rolled coil with a thickness H of 1.75-2.86 mm; (3) After hot rolling, the hot rolled coil is subjected to a two-stage heating and normalizing process under dry pure N2 conditions, which includes a high temperature normalizing and soaking section and a low temperature normalizing and soaking section; First, the hot-rolled plate is heated to the high-temperature normalized soaking section temperature T1 at a heating rate v of 112-458°C / s. The high-temperature normalized soaking section temperature T1 satisfies formula (1), and the plate is kept at T1 for 39-98 seconds. T1=1125-3562×([Als]-1.929×[N])-11251×[B]+42×H (3) Wherein, H is the thickness of hot rolled coil, in mm; After the hot rolled coil is kept warm in the high temperature normalizing and soaking section, the low temperature normalizing and soaking stage begins, and the temperature of the hot rolled coil is reduced to T2, T2 is 885-923℃, and kept warm at T2 temperature for 41-83s.

[0033] (4) The normalized steel coil is cold rolled to the finished thickness with a cold rolling reduction rate of 86.45-92.36%.

[0034] (5) Decarburization and nitriding continuous annealing process, the continuous annealing temperature is 810-848℃, the carbon content of the steel strip after continuous annealing is not higher than 10ppm, the nitrogen content is 174-235ppm, and the oxygen content is 522-971ppm; at the same time, the initial grain size meets the coefficient of variation CV less than 13%.

[0035] (6) The step of coating the magnesium oxide separator is to add water to the magnesium oxide separator mixing tank and add boric acid, boron oxide or borates accounting for 2.6-4.1% of the total weight of the magnesium oxide, then add titanium dioxide to the magnesium oxide separator, stir until uniform, add MgO, continue stirring for 1-1.5 hours, coat on the steel plate and dry; and the titanium dioxide content accounts for 2.5-11.5% of the total weight of the magnesium oxide.

[0036] (7) High temperature annealing process, heating stage I adopts rapid heating, heating rate is 61-77℃ / h; holding stage I is set to 622-723℃ low temperature for 10-21h; heating stage II adopts rapid heating to 910-1005℃, heating rate is 32-56℃ / h; holding stage II is set to 910-1005℃, holding for 18-25h; heating stage III is heated to 1100-1120℃ at 6-15℃ / h, holding stage III is set to 1100-1120℃, holding for 14-29h; heating stage IV is heated to 1150-1250℃ at 11-24℃ / h and enters holding stage IV, followed by heat preservation and purification for 14-28h; Among them, N2 atmosphere is used before the insulation section I, 55-63% N2 and the rest H2 is used in the insulation section II, 22-35% N2 and the rest H2 is used in the insulation section III, and pure atmosphere is used after entering the insulation section IV.

[0037] (8) Stretching, flattening, annealing, and applying insulating coating.

[0038] Table 1. List of ingredients of various embodiments of the present invention and comparative examples (wt%) Table 2. Main process control list of each embodiment of the present invention and comparative example Table 3. Material parameters and product magnetic properties of various embodiments and comparative examples of the invention As can be seen from Table 3, the low-temperature high magnetic induction oriented silicon steel products produced by the embodiment of the method of the present invention have the same specifications and magnetic properties after the magnetic domain refinement and magnetic induction B 800 The average value reaches above 1.920T, which is higher than the thickness of the finished products of the same specifications.

[0039] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A high magnetic induction oriented silicon steel, characterized by: The components of high magnetic induction oriented silicon steel include, by weight percentage, C: 0.032-0.065%, Si: 2.58-4.02%, Als: 0.014-0.036%, Mn: 0.021-0.043%, Nb: 0.0015-0.014%, B: 0.0015-0.0084%, N: 0.0037-0.0081%, S: 0.004-0.015%, and the rest are Fe and unavoidable impurities.

2. The high magnetic induction oriented silicon steel according to claim 1, characterized in that: The weight percentage contents of Als, Mn, B, Nb and N satisfy formula (1) and formula (2): 0.71≤18.71×[Als]+6.12×[Mn]+ 21.3×[B]+7.25×[Nb]≤1.12(1) 0.83≤[B] / 10.81:[N] / 14≤1.41 (2) Wherein, [Als] is the numerical portion of the weight percentage of Als, [Mn] is the numerical portion of the weight percentage of Mn, [B] is the numerical portion of the weight percentage of B, [Nb] is the numerical portion of the weight percentage of Nb, and [N] is the numerical portion of the weight percentage of N.

3. A method for producing high magnetic induction oriented silicon steel as claimed in claim 1, characterized in that: The process flow of the production method is: steelmaking and continuous casting → hot rolling → pickling and normalizing → cold rolling → decarburization and nitriding continuous annealing → coating of magnesium oxide isolation agent → high temperature annealing → stretching and leveling annealing → coating of insulation coating → finishing; After hot rolling, the hot rolled coil is subjected to a two-stage heating and normalizing process under dry pure N2 conditions, which includes a high-temperature normalizing and soaking section and a low-temperature normalizing and soaking section. First, the hot-rolled plate is heated to the high-temperature normalized soaking section temperature T1 at a heating rate v of 112-458°C / s. The high-temperature normalized soaking section temperature T1 satisfies formula (1), and the plate is kept at T1 for 39-98 seconds. T1=1125-3562×([Als]-1.929×[N])-11251×[B]+42×H (3) Wherein, H is the thickness of hot rolled coil, in mm; After the hot rolled coil is kept warm in the high temperature normalizing and soaking section, the low temperature normalizing and soaking stage begins, and the temperature of the hot rolled coil is reduced to T2, T2 is 885-923℃, and kept warm at T2 temperature for 41-83s.

4. The method for producing high magnetic induction oriented silicon steel according to claim 3, wherein: The thickness H of the hot rolled coil produced by hot rolling is 1.75-2.86 mm.

5. The method for producing high magnetic induction oriented silicon steel according to claim 3, wherein: The normalized steel coil is cold-rolled to a finished thickness through a single reduction process, with a cold-rolling reduction rate of 86.45-92.36%.

6. The method for producing high magnetic induction oriented silicon steel according to claim 3, characterized in that: The decarburization and nitriding continuous annealing process has a continuous annealing temperature of 810-848° C., and after continuous annealing, the carbon content of the steel strip is no more than 10 ppm, the nitrogen content is 174-235 ppm, and the oxygen content is 522-971 ppm; at the same time, the initial grain size satisfies the coefficient of variation CV of less than 13%.

7. The method for producing high magnetic induction oriented silicon steel according to claim 3, wherein: The magnesium oxide release agent coating step comprises adding water to a magnesium oxide release agent mixing tank and adding boric acid, boron oxide or borates accounting for 2.6-4.1% of the total weight of the magnesium oxide, then adding titanium dioxide to the magnesium oxide release agent, stirring until uniform, adding MgO, continuing stirring for 1-1.5 hours, coating on a steel plate and drying; and the titanium dioxide content accounts for 2.5-11.5% of the total weight of the magnesium oxide.

8. The method for producing high magnetic induction oriented silicon steel according to claim 3, wherein: In the high-temperature annealing process, the heating stage I adopts rapid heating with a heating rate of 61-77°C / h; the holding stage I is set to 622-723°C with a low temperature holding for 10-21 hours; the heating stage II adopts rapid heating to 910-1005°C with a heating rate of 32-56°C / h; the holding stage II is set to 910-1005°C and kept warm for 18-25 hours; the heating stage III is heated to 1100-1120°C at 6-15°C / h, and the holding stage III is set to 1100-1120°C and kept warm for 14-29 hours; the heating stage IV is heated to 1150-1250°C at 11-24°C / h and enters the holding stage IV, followed by heat preservation and purification for 14-28 hours; Among them, N2 atmosphere is used before the insulation section I, 55-63% N2 and the rest H2 is used in the insulation section II, 22-35% N2 and the rest H2 is used in the insulation section III, and pure atmosphere is used after entering the insulation section IV.

Images