A method for producing oriented silicon steel by adopting a medium-thin slab continuous casting and rolling technology combined with nitriding

Abstract

The application discloses a method for producing oriented silicon steel by adopting a medium-thin slab continuous casting and rolling technology combined with a nitriding method and belongs to the field of steel metallurgy. 1.7 <1.00W / Kg, B8>1.90T; the application adopts an ASP continuous casting and rolling process, controls the size and quantity of AlN inherent inhibitors to be uniformly distributed in the slab, adopts a low-temperature slab short-time heating process, controls the hot-rolled structure to be uniform, adopts a two-stage normalizing process, adopts aging rolling in primary cold rolling, adjusts a decarburization annealing process, a high-temperature nitriding process and a suitable high-temperature annealing process, and thus the magnetic performance of the product is improved, the iron loss is reduced, the comprehensive yield is greatly improved, and the application prospect is very wide.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, and specifically relates to a method for producing oriented silicon steel using a combination of medium-thin slab continuous casting and rolling technology and nitriding. Background Technology

[0002] Grain-oriented silicon steel is a type of iron-based alloy with excellent soft magnetic properties, characterized by high magnetic induction, high resistivity, low iron loss, and low coercivity. It is primarily used to manufacture the cores of various transformers in power transmission and distribution systems, and also for the cores of large generators. Currently, the main inhibitor used in the preparation of grain-oriented silicon steel is AlN, which comprises both inherent and acquired inhibitors. Inherent AlN inhibitors are controlled using a combination of low-temperature slab heating and normalizing processes, while acquired AlN inhibitors are controlled using nitriding. Control of the inherent AlN inhibitor is generally achieved through production using both thick and thin slabs. When using thick slabs, the continuous casting process requires a slow cooling rate, resulting in uneven distribution of the size and quantity of second-phase particles precipitated in the columnar and equiaxed crystal structures of the slab. During hot rolling, the slab heating temperature needs to be increased to ensure the resolution of the second-phase particles, facilitating their precipitation during hot rolling and ensuring a perfect secondary recrystallization structure in the product. When using thin slabs, the slab microstructure is fine and uniform. However, due to the use of funnel-shaped crystallizers and high casting speeds, defects such as slag inclusions and slag entrapment are prone to appear on the slab surface, affecting the magnetic properties of the product. Invention patent CN104726670A discloses a method for preparing high-magnetic-induction grain-oriented silicon steel using thin slabs in a short-process method. This patent uses an LF ladle refining method to ensure the chemical composition of the grain-oriented silicon steel with MnS and AlN as inhibitors. The slab thickness is 100-200mm, the heating temperature is 1100-1250℃, the hot-rolled thickness is 1.8-2.5mm, and after normalizing at 1100-1180℃ and 850-980℃, it is cold-rolled to 0.26-0.32mm in a single cold rolling process. It is then decarburized and annealed at 750-890℃, followed by high-temperature annealing at a heating rate of 15-30℃ / h and a heating rate of 7-15℃. The resulting grain-oriented silicon steel has a high iron loss P... 1.7 <1.20W / Kg, magnetic induction B8>1.88T. However, to date, no research has been reported on methods for producing grain-oriented silicon steel using a combination of continuous casting and rolling technology with nitriding to further improve magnetic induction intensity and overall yield while reducing iron loss. Summary of the Invention

[0003] Therefore, the purpose of this invention is to provide a method for producing grain-oriented silicon steel using a combination of medium-thin slab continuous casting and rolling technology and nitriding. This invention uses AlN+Sn as the main inhibitor for composition design, and employs medium-thin slab continuous casting and rolling technology, as well as single-pass high-reduction cold rolling and high-temperature nitriding to produce high-quality grain-oriented silicon steel with high efficiency. The magnetic induction intensity and overall yield are significantly improved, while iron loss is effectively reduced.

[0004] The objective of this invention is achieved through the following means:

[0005] This invention provides a grain-oriented silicon steel with the following chemical composition and weight percentages: C: 0.04%–0.06%, Si: 3.0%–3.5%, Mn: 0.08%–1.20%, P: 0.01%–0.02%, S: 0.005%–0.007%, Al: 0.026%–0.030%, N: 0.006%–0.009%, Sn: 0.03%–0.05%, with the balance being iron and unavoidable impurities. The grain-oriented silicon steel contains P... 1.7 <1.00W / Kg, B8>1.90T.

[0006] The present invention also provides a method for producing the above-mentioned oriented silicon steel using a combination of medium-thin slab continuous casting and rolling technology and nitriding. The method includes hot metal pretreatment → converter smelting → RH refining → medium-thin slab continuous casting and rolling → normalizing → primary cold rolling → decarburizing and nitriding annealing → MgO coating → high-temperature annealing → hot stretching and leveling and coating with an insulating layer → rewinding and packaging.

[0007] In the continuous casting and rolling process of medium and thin slabs, the billet drawing speed is 2.0-3.0 m / min to ensure that the precipitation size of AlN second phase particles is small, and the thickness of the continuously cast slab is 120-150 mm; the heating temperature is 1100-1150℃, and the furnace time is 120-150 min; the roughing mill inlet temperature is 1050-1100℃, the roughing mill outlet temperature is 1000-1050℃, the intermediate slab is 45-55 mm, the finishing mill inlet temperature is 980-1020℃, the finishing mill outlet temperature is 930-950℃, the coiling temperature is controlled below 550℃, and the coil is held in the heat preservation box for 20-30 s to ensure the uniformity of temperature and structure of the whole coil and the beginning and end; the thickness of the hot-rolled plate is 2.3-2.6 mm.

[0008] The normalizing process adopts a two-stage normalizing treatment. The first stage of normalizing is held at 1050-1100℃ for 1.5-3 minutes, and the second stage of normalizing is held at 900-920℃ for 2-3 minutes. Due to the low-temperature heating of the medium and thin slab, the hot-rolled plate has no edge cracks, and the edges do not need to be trimmed after normalizing, which improves the yield of the normalizing process.

[0009] The cold rolling process is carried out using a 20-roll Sendzimir mill, with 5 to 6 passes, a total reduction of 88 to 92%, and a cold-rolled sheet thickness of 0.26 to 0.20 mm.

[0010] During the decarburizing and nitriding annealing process, decarburization is carried out at 830–850℃ for 3–4 minutes in a high-hydrogen, high-dew-point atmosphere to reduce carbon to below 20 ppm. The nitriding temperature is 850–950℃, the atmosphere is 50–70% H2 + 10–30% N2 + 10–30% NH3, and the nitriding time is 20–40 seconds. This ensures the decarburization of the steel plate and the formation of a good oxide layer on the surface of the steel plate, as well as the formation of well-infiltrated AlN second-phase particles from the surface layer to the center layer of the substrate.

[0011] The high-temperature annealing temperature is 1150~1250℃, and the holding time is 20~40h.

[0012] Based on the above technical solution, the slab is further subjected to hot delivery and hot charging, and the charging temperature is controlled above 500℃.

[0013] Based on the above technical solution, the roughing rolling adopts a 1+3 mode, that is, R1 is rolled in 1 pass and R2 is rolled in 3 passes.

[0014] Based on the above technical solution, the winding temperature is further set to 500–550°C.

[0015] Based on the above technical solution, furthermore, the aging temperature of the third pass is controlled at 180-220℃ during the cold rolling process.

[0016] Based on the above technical solution, the decarburization atmosphere is 80% H2 + 20% N2; the nitriding atmosphere is 60% H2 + 20% N2 + 20% NH3.

[0017] Based on the above technical solution, further, during the high-temperature annealing process, a heating rate of 15-20℃ / h is adopted before the secondary recrystallization begins from 650℃.

[0018] The advantages of this invention over the prior art are as follows:

[0019] This invention employs the ASP continuous casting and rolling process to improve production efficiency and yield, increasing the overall yield of the post-hot rolling process by more than 3%. It controls the uniform distribution of the size and quantity of AlN inherent inhibitors in the slab, uses a low-temperature slab short-time heating process to control the uniformity of the hot-rolled microstructure, adopts a two-stage normalizing process, uses aging rolling for the first cold rolling, and adjusts the decarburization annealing process, high-temperature nitriding process, and appropriate high-temperature annealing process to increase the magnetic induction to over 1.90T. This achieves the goal of improving the magnetic properties of the product, reducing iron loss, and significantly increasing the overall yield. Detailed Implementation

[0020] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0021] Example 1

[0022] A method for producing grain-oriented silicon steel using a combination of medium-thin slab continuous casting and rolling technology and nitriding is described below:

[0023] 1. Ingredients

[0024] The chemical composition and weight percentage of oriented silicon steel are shown in Table 1.

[0025] Table 1 Chemical composition (wt%)

[0026] C Si Mn P S Als N Sn 0.04 3.0 0.08 0.01 0.005 0.026 0.006 0.03

[0027] 2. Process

[0028] Hot metal pretreatment → converter smelting → RH refining → continuous casting and rolling of medium and thin slabs (ASP) → normalizing → primary cold rolling → decarburizing and nitriding annealing → MgO coating → high temperature annealing → hot stretching and leveling and coating with insulating layer → rewinding and packaging.

[0029] By adopting the ASP continuous casting and rolling process, the size and quantity of AlN inherent inhibitors are controlled to be evenly distributed in the slab. A low-temperature slab short-time heating process is used to control the uniformity of the hot-rolled microstructure. A two-stage normalizing process and an aging rolling process are adopted for the first cold rolling. The decarburizing annealing process, high-temperature nitriding process, and appropriate high-temperature annealing process are adjusted to improve the magnetic properties of the product, reduce iron loss, and significantly improve the overall yield.

[0030] Among them, the continuous casting and rolling of medium and thin slabs: the billet drawing speed is 2m / min to ensure that the precipitation size of AlN second phase particles is small, the thickness of the continuously cast slab is 120mm, the slab adopts hot delivery and hot charging, the furnace charging temperature is controlled above 500℃, the heating temperature is 1100℃, and the furnace time is 120min; the roughing rolling adopts the 1+3 mode (i.e., R1 rolling 1 pass, R2 rolling 3 passes), the roughing rolling inlet temperature is 1050℃, the roughing rolling outlet temperature is 1000℃, the intermediate slab is 45mm, the finishing rolling inlet temperature is 1000℃, the finishing rolling outlet temperature is 930℃, after being coiled in the hot coil box, it is turned around, the coiling temperature is 550℃, and it is held in the heat preservation box for 20s to ensure the uniformity of temperature and structure of the whole coil and the head and tail, and the thickness of the hot rolled plate is 2.3mm.

[0031] Normalizing: Select two coils of steel (1# and 2#) for normalizing. The normalizing process is 1050℃×2min+900℃×2min;

[0032] One-pass cold rolling: Cold rolling is carried out using a 20-roll Sendzimir mill, with 5-6 passes, resulting in a cold-rolled sheet thickness of 0.26 mm and a reduction rate of 88.7%; the aging temperature of the third pass is controlled at 180℃.

[0033] Decarburizing and nitriding annealing: Decarburization was carried out at 830℃ for 4 min, with a decarburizing atmosphere of 80% H2 + 20% N2, until the carbon content was reduced to 20 ppm; Nitriding temperature was 950℃, with an atmosphere of 60% H2 + 20% N2 + 20% NH3, and nitriding time was 30 s.

[0034] High-temperature annealing: The high-temperature annealing temperature is 1200℃, and the holding time is 30h. The annealing atmosphere is 20% H2 + 80% N2. The heating rate is 15℃ / h from 650 to 1050℃. Finally, after hot stretching, leveling and coating with an insulating layer, the yield rate is calculated and the magnetic properties are tested.

[0035] 3. Results

[0036] (1) Yield

[0037] The yield of the low-cost, high-performance oriented silicon steel prepared by this invention is shown in Table 2.

[0038] Table 2. Yield of materials per process (%)

[0039]

[0040] (2) Magnetic properties

[0041] The magnetic properties of the low-cost, high-performance oriented silicon steel prepared by this invention are shown in Table 3.

[0042] Table 3 Magnetic Properties

[0043]

[0044] Comparative Example 1

[0045] A method for producing grain-oriented silicon steel, the specific process of which is as follows:

[0046] 1. Ingredients

[0047] The chemical composition and weight percentage of oriented silicon steel are shown in Table 1.

[0048] 2. Process

[0049] A standard slab with a thickness of 230 mm was used. The heating temperature was 1150℃, and the furnace time was 200 min. Rough rolling adopted a 3+3 pattern, with an inlet temperature of 1100℃ and an outlet temperature of 1050℃. The finish rolling inlet temperature was 1000℃, and the outlet temperature was 930℃. The hot-rolled plate thickness was 2.3 mm, and the coiling temperature was 545℃. Two coils of steel (3# and 4#) were selected. The normalizing process was 1050℃×2 min + 900℃×2 min. A single cold rolling method was used, with a cold-rolled plate thickness of 0.26 mm and a reduction rate of 88.7%. Decarburization was performed at 830℃×4 min in an atmosphere of 80% H2 + 20% N2 to reduce carbon to 20 ppm. Nitriding temperature was 950℃ in an atmosphere of 60% H2 + 20% N2 + 20% NH3, and the nitriding time was 30 s. The high-temperature annealing temperature is 1200℃, the holding time is 30h, the annealing atmosphere is 20% H2 + 80% N2, the heating rate is 15℃ / h from 650 to 1050℃, and finally the steel coil is hot-stretched, leveled and coated with an insulating layer before the yield is calculated and the magnetic properties are tested.

[0050] 3. Results

[0051] (1) Yield

[0052] The yield of grain-oriented silicon steel produced using conventional slabs and nitriding methods is shown in Table 4.

[0053] Table 4. Yield of materials per process (%)

[0054] Process normalization cold rolling Decarburizing and nitriding annealing High temperature annealing Hot stretching flattening comprehensive yield 95 92 99 100 91 78.7

[0055] (2) Magnetic properties

[0056] The magnetic properties of oriented silicon steel produced using conventional slabs and nitriding methods are shown in Figure 5.

[0057] Table 5 Magnetic Properties

[0058]

[0059] 4. Comparison Results

[0060] (1) Yield

[0061] Table 6 shows a comparison of the yield of each process after hot rolling of oriented silicon steel prepared by the method of Example 1 and the conventional method of Comparative Example 1. It can be seen that the yield of oriented silicon steel prepared by the process of the present invention is 4.2% higher than that of the traditional thick slab method.

[0062] Table 6. Yield and Magnetic Performance Qualification Rate

[0063]

[0064]

[0065] (2) Magnetic properties

[0066] Table 7 compares the magnetic properties of oriented silicon steel prepared by the method of Example 1 and the conventional method of Comparative Example 1. The oriented silicon steel produced by the present invention has significantly improved magnetic properties, reduced iron loss by one grade, and increased magnetic induction by at least 0.05T, all reaching the HiB100 level.

[0067] Table 7 Comparison of Magnetic Properties

[0068]

[0069] Example 2

[0070] A method for producing grain-oriented silicon steel using a combination of medium-thin slab continuous casting and rolling technology and nitriding is described below:

[0071] 1. Ingredients

[0072] The chemical composition and weight percentage of oriented silicon steel are shown in Table 8.

[0073] Table 8 Chemical composition (wt%)

[0074] element C Si Mn P S Als N Sn content 0.06 3.5 1.20 0.02 0.007 0.030 0.009 0.05

[0075] 2. Process

[0076] Hot metal pretreatment → converter smelting → RH refining → continuous casting and rolling of medium and thin slabs (ASP) → normalizing → primary cold rolling → decarburizing and nitriding annealing → MgO coating → high temperature annealing → hot stretching and leveling and coating with insulating layer → rewinding and packaging.

[0077] By adopting the ASP continuous casting and rolling process, the size and quantity of AlN inherent inhibitors are controlled to be evenly distributed in the slab. A low-temperature slab short-time heating process is used to control the uniformity of the hot-rolled microstructure. A two-stage normalizing process and an aging rolling process are adopted for the first cold rolling. The decarburizing annealing process, high-temperature nitriding process, and appropriate high-temperature annealing process are adjusted to improve the magnetic properties of the product, reduce iron loss, and significantly improve the overall yield.

[0078] Among them, the continuous casting and rolling of medium and thin slabs: the billet drawing speed is 3m / min to ensure that the precipitation size of AlN second phase particles is small, the thickness of the continuously cast slab is 150mm, the slab adopts hot delivery and hot charging, the furnace charging temperature is controlled above 500℃, the heating temperature is 1150℃, and the furnace time is 150min; the roughing rolling adopts the 1+3 mode (i.e., R1 rolling 1 pass, R2 rolling 3 passes), the roughing rolling inlet temperature is 1100℃, the roughing rolling outlet temperature is 1050℃, the intermediate slab thickness is 55mm, the finishing rolling inlet temperature is 1000℃, the finishing rolling outlet temperature is 950℃, after being coiled in the hot coil box, it is turned around, the coiling temperature is 530℃, and it is held in the heat preservation box for 30s to ensure the uniformity of temperature and structure of the whole coil and the head and tail, and the thickness of the hot rolled plate is 2.6mm.

[0079] Normalizing: Select two rolls of raw materials (5# and 6#) for normalizing. The normalizing process is 1100℃×1.5min+920℃×2min.

[0080] One-pass cold rolling: Cold rolling is carried out using a 20-roll Sendzimir mill, with 5-6 passes, a cold rolling reduction rate of 91.5%, and the aging temperature of the third pass is controlled at 220℃. The thickness of the cold-rolled sheet is 0.23mm.

[0081] Decarburizing and nitriding annealing: Decarburization was carried out at 850℃ for 3 min in an atmosphere of 80% H2 + 20% N2 to reduce carbon to 10 ppm; nitriding temperature was 850℃ in an atmosphere of 60% H2 + 20% N2 + 20% NH3 for 30 s.

[0082] High-temperature annealing: The high-temperature annealing temperature is 1200℃, the holding time is 30h, the annealing atmosphere is 20% H2 + 80% N2, and the heating rate of the high-temperature annealing from 650 to 1050℃ is 20℃ / h; finally, after the steel coil is hot-stretched, leveled, and coated with an insulating layer, the yield is calculated and the magnetic properties are tested.

[0083] 3. Results

[0084] (1) Yield

[0085] The yield of low-cost, high-performance oriented silicon steel prepared by this invention is shown in Table 9.

[0086] Table 9. Yield of materials per process (%)

[0087] Process normalization cold rolling Decarburizing and nitriding annealing High temperature annealing Hot stretching flattening comprehensive yield 95 91 99 100 94 80.5

[0088] (2) Magnetic properties

[0089] The magnetic properties of the oriented silicon steel prepared by this invention are shown in Table 10.

[0090] Table 10 Magnetic Performance Indicators

[0091]

[0092] Comparative Example 2

[0093] A method for producing grain-oriented silicon steel, the specific process of which is as follows:

[0094] 1. Ingredients

[0095] The chemical composition and weight percentage of oriented silicon steel are shown in Table 8.

[0096] 2. Process

[0097] The standard slab is used with a thickness of 230 mm. The heating temperature is 1150℃ and the furnace time is 200 min. The roughing rolling adopts the 3+3 mode, with the roughing rolling inlet temperature of 1100℃ and the roughing rolling outlet temperature of 1050℃. The finishing rolling inlet temperature is 950℃ and the finishing rolling outlet temperature is 930℃. The hot-rolled plate thickness is 2.3 mm and the coiling temperature is 545℃. Two coils of steel (7# and 8#) were selected. The normalizing process was 1050℃×2min+900℃×2min. A single cold rolling method was used, with a cold-rolled plate thickness of 0.23mm and a reduction rate of 91.5%. Decarburization annealing was carried out at 850℃×3min in an atmosphere of 80% H2+20% N2 to remove carbon to 10ppm. Nitriding temperature was 950℃ in an atmosphere of 60% H2+20% N2+20% NH3 for 30s. High-temperature annealing temperature was 1200℃, held for 30h in an annealing atmosphere of 20% H2+80% N2, with a heating rate of 20℃ / h from 650-1050℃. Finally, the steel coils were hot-stretched, leveled, and coated with an insulating layer before yield calculation and magnetic property testing were performed.

[0098] 3. Results

[0099] (1) Yield

[0100] The yield of oriented silicon produced using conventional slabs and nitriding methods is shown in Table 11.

[0101] Table 11. Yield of materials per process (%)

[0102]

[0103] (2) Magnetic properties

[0104] The magnetic properties of oriented silicon steel produced using conventional slabs and nitriding methods are shown in Table 12.

[0105] Table 12 Magnetic Performance Indicators

[0106]

[0107] 4. Comparison Results

[0108] (1) Yield

[0109] The yield of oriented silicon steel after hot rolling using the method of Example 2 and the conventional method of Comparative Example 2 is shown in Table 13. It can be seen that the yield of oriented silicon steel prepared by the process of the present invention is 3.5% higher than that of the conventional thick slab method.

[0110] Table 13. Yield and Magnetic Performance Qualification Rate

[0111]

[0112] (2) Magnetic properties

[0113] Table 14 compares the magnetic properties of oriented silicon steel prepared by the method of Example 2 and the conventional method of Comparative Example 2. The oriented silicon steel produced by the preparation method of the present invention has significantly improved magnetic properties, reduced iron loss by one grade, and increased magnetic induction by at least 0.06T, all reaching the HiB090 level.

[0114] Table 14 Comparison of Magnetic Properties

[0115]

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A grain-oriented silicon steel, characterized in that, The chemical composition and weight percentage of the oriented silicon steel are as follows: C: 0.04%–0.06%, Si: 3.0%–3.5%, Mn: 0.08%–1.20%, P: 0.01%–0.02%, S: 0.005%–0.007%, Al: 0.026%–0.030%, N: 0.006%–0.009%, Sn: 0.03%–0.05%, with the balance being iron and unavoidable impurities. The P content of the oriented silicon steel is... 1.7 <1.00W / Kg, B8>1.90T.

2. A method for producing the oriented silicon steel of claim 1 using a combination of medium-thin slab continuous casting and rolling technology and nitriding, the method comprising: hot metal pretreatment → converter smelting → RH refining → medium-thin slab continuous casting and rolling → normalizing → primary cold rolling → decarburizing and nitriding annealing → MgO coating → high-temperature annealing → hot stretching and leveling and coating with an insulating layer → rewinding and packaging. in, In the continuous casting and rolling process of medium and thin slabs, the billet drawing speed is 2.0-3.0 m / min, the thickness of the continuously cast slab is 120-150 mm; the heating temperature is 1100-1150℃, the furnace time is 120-150 min; the roughing mill inlet temperature is 1050-1100℃, the roughing mill outlet temperature is 1000-1050℃, the intermediate slab thickness is 45-55 mm, the finishing mill inlet temperature is 980-1020℃, the finishing mill outlet temperature is 930-950℃, the coiling temperature is controlled below 550℃, and the heat treatment is carried out in the heat treatment box for 20-30 s; the hot-rolled plate thickness is 2.3-2.6 mm. The normalizing process adopts a two-stage normalizing treatment. The first stage of normalizing is held at 1050-1100℃ for 1.5-3 minutes, and the second stage of normalizing is held at 900-920℃ for 2-3 minutes. The cold rolling process involves 5 to 6 passes, with a total reduction rate of 88 to 92%, and the thickness of the cold-rolled sheet is 0.26 to 0.20 mm. During the decarburizing and nitriding annealing process, the decarburizing temperature is 830-850℃ for 3-4 minutes, and the decarburizing atmosphere is a high hydrogen and high dew point atmosphere, to reduce the carbon to below 20 ppm; the nitriding temperature is 850-950℃, the nitriding atmosphere is 50-70% H2 + 10-30% N2 + 10-30% NH3, and the nitriding time is 20-40 seconds. The high-temperature annealing temperature is 1150~1250℃, and the holding time is 20~40h.

3. The method according to claim 2, characterized in that, The slabs are hot-charged and delivered, with the furnace temperature controlled above 500℃.

4. The method according to claim 2, characterized in that, The roughing process adopts a 1+3 pattern, that is, R1 is rolled in 1 pass and R2 is rolled in 3 passes.

5. The method according to claim 2, characterized in that, The winding temperature is 500–550℃.

6. The method according to claim 2, characterized in that, During the cold rolling process, the aging temperature of the third pass is controlled at 180-220℃.

7. The method according to claim 2, characterized in that, The decarburization atmosphere is 80% H2 + 20% N2; the nitriding atmosphere is 60% H2 + 20% N2 + 20% NH3.

8. The method according to claim 2, characterized in that, During the high-temperature annealing process, the heating rate is 15-20℃ / h from 650℃ to the temperature at which secondary recrystallization begins.

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