Thick steel plate for high-efficiency heat treatment and high-performance hydrogen storage equipment and its manufacturing method

Abstract

The present invention provides a thick steel plate for high-performance hydrogen storage equipment with efficient heat treatment and a manufacturing method thereof. The steel plate comprises the following components by weight: C: 0.06% to 0.09%, Si: 0.37% to 0.49%, Mn: 1.45% to 1.57%, P: ≤ 0.010%, S: ≤ 0.005%, Ni: 1.2% to 1.4%, V: 0.03% to 0.05%, Ti: 0.005% to 0.01%, Alt: 0.020% to 0.045%, V / Ti = 3 to 10, with the balance being Fe and unavoidable inclusions. The steel plate production method includes smelting, continuous casting, slab heating, rolling, hot straightening, slow cooling, and heat treatment. At room temperature, the steel plate's T / 4 position is: 885MPa≤Rm≤970MPa, 755MPa≤Rel≤780MPa; at T / 2 position, the steel plate's KV2 is: 890MPa≤Rm≤970MPa, 750MPa≤Rel≤770MPa. At -60°C, the steel plate's KV2 is ≥130J. The steel plate's microstructure is ferrite + sorbite, with a ferrite to sorbite volume ratio of 3.5 to 4.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and in particular relates to a thick steel plate for high-efficiency heat treatment and high-performance hydrogen storage equipment and a manufacturing method thereof. Background Art

[0002] Hydrogen energy boasts a wide range of sources, convenient storage and transportation, efficient utilization, and a clean, environmentally friendly footprint. It is both a clean energy source and a key means of supporting the clean transformation of fossil energy and the large-scale deployment of renewable energy. Judging from the development trends of hydrogen energy, hydrogen technology has become a disruptive energy technology leading the next generation of industrial transformation.

[0003] The development of steel plates for hydrogen energy storage and transportation equipment focuses on a good match of strength and toughness to ensure the low-temperature service performance of the steel plates. With the upgrading of domestic pressure-bearing manufacturing equipment, domestic petrochemical projects are developing in the direction of large-scale and high-performance. The demand for large-thickness steel plates is increasing, and it is particularly important to solve the uniformity of grains in the thickness direction of large-thickness steel plates. The performance of some traditional pressure-bearing equipment steel and the corresponding manufacturing methods can no longer meet the requirements of the development trend of hydrogen energy storage and transportation. In addition, although some grades can meet the project design requirements at this stage, they still need to be upgraded in the long run. Therefore, it is urgent to develop an efficient production method to produce supporting high-performance steel plates for hydrogen storage equipment.

[0004] The public invention patent "A low temper brittle Cr-Mo series high temperature resistant pressure equipment steel plate" (CN104805380A) describes a steel plate composed of the following components in weight percentage: C: 0.05% to 0.17%, Si: 0.30%, Mn: 0.40% to 0.65%, S: 0.035%, P: 0.015%, Cr: 0.80% to 1.15%, Mo: 0.45% to 0.60%, Sn: 0.005%, Sb: 00025‰, As: 0.005‰; under the above composition, a normalizing + tempering heat treatment method is adopted to produce steel plates with a thickness specification of 20-40mm. No research has been conducted on steel plates with a thickness of less than 20mm and more than 40mm, and the research content needs to be improved urgently for full thickness steel plates.

[0005] The disclosed invention patent "A steel plate for pressure vessels with a thickness greater than 200-250 mm and resistant to hydrogen-induced cracking" (CN113278878A) describes a steel plate composed of the following components in weight percentage: C: 0.10-0.20%, Si: 0.15-0.40%, Mn: 0.95-1.35%, P: ≤0.005%, S: ≤0.0008%, Cr: 0.10-0.30%, Ni: 0.25-0.40%, Mo: 0.08-0.12%, Alt: 0.02-0.05%, Nb: 0.01-0.02%, V: 0.01-0.03%, Ti: 0.01-0.02%, B: ≤0.0005%, and the remainder is Fe and unavoidable impurity elements. The above-mentioned components are combined with the normalizing + water accelerated cooling + tempering heat treatment process, which is only suitable for the production of steel plates with a thickness specification of 200-250mm. The total heat treatment time is more than 12 hours, which seriously occupies heat treatment resources and does not conform to the idea of ​​efficient production.

[0006] The published invention patent, "A Method for Producing Thin-Gauge, High-Toughness 12Cr1MoVR Pressure Vessel Steel Plate" (CN114855057A), describes a steel plate composed of the following components by weight: C: 0.12-0.14%, Si: 0.25-0.35%, Mn: 0.50-0.65%, P: ≤0.015%, S: ≤0.005%, Nb: 0.025-0.035%, V: 0.15-0.30%, Cr: 0.90-1.20%, Mo: 0.25-0.35%, Al: 0.15-0.32%, H ≤1.7ppm; O: ≤0.0036%; N: ≤0.0044%. The remainder is iron and unavoidable impurities. The steel plate is produced in thicknesses of 10-40mm. No research has been conducted on thicknesses above 40mm, and only room-temperature impact values ​​are given. The lack of a series of impact studies does not meet the design requirements for steel plates used in hydrogen storage equipment. Summary of the Invention

[0007] To address the shortcomings of the existing technology, the present invention aims to produce a thick steel plate with efficient heat treatment for high-performance hydrogen storage equipment. Through a novel chemical composition design, efficient heat treatment, and specialized manufacturing processes, the plate achieves: 885MPa≤Rm≤970MPa, 755MPa≤Rel≤780MPa at T / 4; 890MPa≤Rm≤970MPa, 750MPa≤Rel≤770MPa at T / 2; and KV2≥130J at -60°C. This results in a (65-110)mm thick steel plate with excellent strength, low-temperature toughness, formability, and shape, meeting the manufacturing and application requirements of high-performance hydrogen storage equipment.

[0008] The object of the invention is achieved like this:

[0009] A thick steel plate for high-performance hydrogen storage equipment with efficient heat treatment has a chemical composition range of: C: 0.06% to 0.09%, Si: 0.37% to 0.49%, Mn: 1.45% to 1.57%, P: ≤ 0.010%, S: ≤ 0.005%, Ni: 1.2% to 1.4%, V: 0.03% to 0.05%, Ti: 0.005% to 0.01%, Alt: 0.020% to 0.045%, and the balance is Fe and unavoidable inclusions.

[0010] Furthermore, the V / Ti=3-10.

[0011] Furthermore, at room temperature, the steel plate has a strength of 885 MPa ≤ R at T / 4. m ≤970MPa、755MPa≤R el ≤780MPa, Steel plate T / 2: 890MPa≤R m ≤970MPa、750MPa≤R el ≤770MPa; KV2≥130J under -60℃ condition.

[0012] Furthermore, the steel plate has a ferrite+sorbite structure, and the volume ratio of ferrite to sorbite is 3.5-4.

[0013] Furthermore, the thickness of the steel plate is 65 to 110 mm.

[0014] The reasons for the composition design of the present invention are as follows:

[0015] C:0.06%~0.09%

[0016] Carbon is a major element in steel, combining with strong carbide alloying elements to create precipitation strengthening. However, excessive carbon content can affect the steel's machinability, and oversaturated carbides can also affect the steel's low-temperature toughness. Therefore, the present invention sets the carbon content range to 0.06% to 0.09%.

[0017] Si: 0.37%~0.49%

[0018] Silicon (Si) acts as a reducing agent and deoxidizer during steelmaking. Si is an inexpensive alloying element. Adding an appropriate amount of Si to steel can improve the hardness and strength of the ferrite. However, when the Si content exceeds 0.49%, it tends to form hard phase compounds, affecting the plasticity and toughness of the steel sheet and potentially causing cracks during subsequent processing. Therefore, the Si content in this invention is set within a range of 0.37% to 0.49%.

[0019] Mn: 1.45%~1.57%

[0020] Mn is a strong austenite-stabilizing element, lowering the lower critical point of steel, increasing the degree of undercooling during austenite cooling, and refining the pearlite structure to improve the mechanical properties of the steel plate. However, excessive Mn content increases the tendency of grain coarsening in the steel. Therefore, the present invention sets the Mn content range to 1.45% to 1.57%.

[0021] P:≤0.010%, S:≤0.005%

[0022] S and P are harmful elements in steel and must be strictly controlled to ensure the purity and plasticity of the steel. Therefore, the present invention limits the content of S to P≤0.010% and S≤0.005%.

[0023] Ni: 1.2%~1.4%

[0024] The lattice constant of the Ni element is similar to that of gamma iron, so it can form a continuous solid solution, which can lower the critical point and increase the stability of austenite. It is an alloying element that strengthens the austenite region and has the effect of refining ferrite grains. At the same time, it can reduce the quenching temperature, increase the hardenability of the steel plate, and ensure a comprehensive improvement in the strength, plasticity and toughness of the steel, especially the low-temperature toughness. Due to the use requirements, the steel plates used in hydrogen storage equipment also need to have good corrosion resistance. Ni can form a dense oxide film on the surface of the steel plate, thereby improving the corrosion resistance of the steel plate. However, considering the overall cost, the present invention sets the Ni content range to 1.2% to 1.4%.

[0025] V: 0.03%~0.05%

[0026] V is a strong carbide-forming element, primarily present in steel as carbides. It acts as precipitation strengthening, refines the steel's structure, and reduces grain size, thereby improving the steel's strength, yield ratio, and low-temperature toughness after normalizing. However, excessive V content can negatively impact the steel's hardenability. Therefore, the present invention sets the V content range to 0.03% to 0.05%.

[0027] Ti: 0.005~0.01%

[0028] Ti has a strong affinity for nitrogen, oxygen, and carbon, and its affinity for sulfur is stronger than that of iron. It is a good deoxidizer and degasser, and an effective element for fixing nitrogen and carbon. It easily combines with carbon in steel, exerting a precipitation strengthening effect. Simultaneously, with appropriate production processes, it is finely and dispersedly distributed in the steel plate matrix, thereby improving strength without reducing the toughness of the steel plate. However, when the Ti content is too high, the resulting carbides are too large, affecting the toughness and hardenability of the steel plate. Therefore, the present invention increases Ti content to 0.005-0.01%. When Ti and V are added simultaneously to the steel plate, the two work together to form a multi-component composite precipitate phase (Ti, V)X (X=C, N), forming fine and dispersed carbon and nitrogen precipitates that pin grain boundaries and exert a precipitation strengthening effect. Furthermore, TiC particles that remain stable even when heated in high-temperature ranges can effectively improve low-temperature toughness, so the V / Ti ratio is controlled within the range of 3-10.

[0029] Alt: 0.020%~0.045%

[0030] Alt is a commonly used deoxidizer in steel. Adding a small amount of aluminum can refine the grain size and improve the steel's strength and impact toughness. Excessive amounts can affect the steel's hot working, welding, and machinability. The present invention limits the Alt content to 0.020% to 0.045%.

[0031] The second technical solution of this invention provides a method for manufacturing thick steel plates for high-performance hydrogen storage equipment with efficient heat treatment, including smelting, continuous casting, slab heating, rolling, hot straightening, slow cooling, and heat treatment. This method utilizes a short normalizing heat treatment combined with a two-stage controlled rolling process to produce container steel plates with a thickness of 65-110 mm. The produced steel plates exhibit uniform microstructure throughout their thickness, good strength-toughness matching at 1 / 4 and 1 / 2 the thickness, and excellent low-temperature performance.

[0032] 1. Smelting Process: Molten steel is smelted in a converter, using high-quality scrap steel and molten iron as raw materials. The molten iron content is controlled at above 78%, ensuring steel purity and reducing the difficulty of subsequent processes. Strict control is exercised over the converter dephosphorization and decarburization smelting parameters. To effectively reduce the harmful element P content, oxygen blowing for dephosphorization is controlled for 8-12 minutes, and for decarburization for 7-9 minutes, keeping the phosphorus content in the molten steel below 0.005%. Deep desulfurization is further performed in an LF refining furnace, controlling the sulfur content to below 0.003%. Degassing is completed in a VD furnace, with a net cycle time of 7-12 minutes and a 5-7 minute cooling period before pouring.

[0033] 2. Casting Process: After breaking vacuum, casting is performed using a slab continuous casting machine, with a key focus on controlling the casting temperature. The tundish molten steel pouring temperature is 1495-1506°C, with a superheat setting of 10-20°C. The casting speed during casting is 1.0-1.2 m / min. Low-temperature casting is preferred to refine the original as-cast structure. To control centerline segregation and porosity in the continuous casting slab, electromagnetic stirring or soft reduction is employed, with a soft reduction rate of 8-10%. The slabs are stacked for slow cooling after leaving the production line, with a slow cooling time of 24-36 hours.

[0034] 3. Heating process: The continuous casting slab is sent to the heating furnace for heating. The slab is heated through the preheating section, heating section and soaking section before being taken out of the furnace. The temperature range of the preheating section is 790-835℃, the temperature range of the heating section is 1185-1210℃, and the temperature range of the soaking section is 1235-1265℃. The soaking time of the slab is controlled at 4.6-5.4h. When the heating temperature is lower than 1235℃, the coarse precipitates in the continuous casting slab cannot be dissolved, the austenitization of the steel plate is incomplete, and the final rolling temperature of the first stage cannot be guaranteed. When the heating temperature is higher than 1250℃, the fine precipitates in the continuous casting slab are easily re-dissolved and the grains are excessively grown.

[0035] 4. Rolling process:

[0036] The rolling adopts a three-stage controlled rolling method. The starting rolling temperature of the recrystallization zone in the first stage is 1150-1160℃, and the ending rolling temperature of the recrystallization zone is ≥1115℃. The original austenite structure is fully refined, and the total deformation rate is controlled at more than 50%. Large deformation and rapid rolling are adopted, and the rolling speed is 3.7-4.2m / s. The starting rolling temperature of the non-recrystallization zone is 900-950℃, and the final rolling temperature is 830-860℃. However, the reduction is controlled at 3-5% at this stage. Small deformation and multiple rapid rolling are carried out at a rolling speed of 3.5 to 3.8 m / s. At this time, the austenite grains are further flattened and elongated. As the grain boundary area increases, the ferrite nucleation rate increases in the subsequent phase transformation process, and the grains are fully refined; three-stage homogeneous rolling, the starting rolling temperature is 760 to 790 ° C, the ending rolling temperature is 672 to 680 ° C, and the reduction is controlled at 1 to 3%. At this stage, the surface quality of the steel plate is further adjusted to improve the service performance of the steel plate.

[0037] 5. Heat treatment process: Since elements such as C, Mn, Ni, V, and Ti are added to the steel, the steel plate can obtain a ferrite + sorbite structure with excellent strength and toughness after rolling. However, the grain size distribution of the steel plate is uneven, and there is concentration of structural stress and thermal stress, which easily leads to delayed cracking during flame cutting. Therefore, heat treatment should be used in time to soften and eliminate stress. Heating for too long will cause secondary growth and increase production costs. Therefore, the present invention adopts a short-time normalizing heat treatment to ensure that the strength of the hydrogen storage steel plate is not lost, while making the steel plate have suitable plasticity and toughness, as well as good processing performance. Therefore, the normalizing heat treatment temperature of the steel is 870-890°C, and a short-time heat preservation system of 10-15 minutes is adopted, the heating rate is controlled at 0.9-1.1 min / mm, and the cooling rate is 23-26°C / s.

[0038] The beneficial effects of the present invention are:

[0039] (1) Based on the strengthening elements of C, Si, and Mn, by adding appropriate amounts of Ni, V, and Ti alloying elements, while strictly controlling the content of harmful elements P and S, combined with production process optimization, a refined ferrite + sorbite structure is obtained, with a volume ratio of ferrite to sorbite of 3.5 to 4. Second-phase carbide particles with a size of less than 10nm are evenly dispersed, ensuring the strong plasticity and low-temperature toughness of the steel plate.

[0040] (2) The mechanical properties of the steel plates for hydrogen storage equipment obtained through a unique production process are as follows: at room temperature, at T / 4 of the steel plate: 885MPa≤Rm≤970MPa, 755MPa≤Rel≤780MPa; at T / 2 of the steel plate: 890MPa≤Rm≤970MPa, 750MPa≤Rel≤770MPa; at -60°C, KV2≥130J.

[0041] (3) The present invention obtains a high-performance steel plate for hydrogen storage equipment with an excellent plate shape and a thickness of (65 to 110) mm. DETAILED DESCRIPTION

[0042] The present invention will be further described below by way of examples.

[0043] The embodiment of the present invention performs smelting, continuous casting, slab heating, rolling, hot straightening, slow cooling, and heat treatment according to the component ratio of the technical solution, and is characterized in that:

[0044] Slab heating

[0045] The temperature range of the preheating section is 790-835°C, the temperature range of the heating section is 1185-1210°C, the temperature range of the soaking section is 1235-1265°C, and the soaking time of the slab is controlled at 4.6-5.4h;

[0046] Rolling process

[0047] The rolling adopts a three-stage controlled rolling method. The starting rolling temperature of the recrystallization zone in the first stage is 1150-1160℃, the ending rolling temperature of the recrystallization zone is ≥1115℃, the total deformation rate is above 50%, large deformation rapid rolling is adopted, and the rolling speed is 3.7-4.2m / s; the starting rolling temperature of the non-recrystallization zone in the second stage is 900-950℃, the final rolling temperature is 830-860℃, but the reduction is controlled at 3-5% at this stage, small deformation multi-pass rapid rolling is adopted, and the rolling speed is 3.5-3.8m / s; the three-stage homogenization rolling has a starting rolling temperature of 760-790℃, an ending rolling temperature of 672-680℃, and a controlled reduction of 1-3%.

[0048] Heat treatment

[0049] The normalizing heat treatment temperature is 870-890°C, and a short-time heat preservation system of 10-15 minutes is adopted, the heating rate is controlled at 0.9-1.1 min / mm, and the cooling rate is 23-26°C / s.

[0050] Furthermore, in the smelting process, the dephosphorization oxygen blowing is controlled within 8 to 12 minutes, and the decarburization oxygen blowing is controlled within 7 to 9 minutes, so that the mass fraction of phosphorus in the molten steel is controlled to be within 0.005%; the LF refining furnace is further used for deep desulfurization treatment, and the sulfur content is controlled below 0.003%; the degassing is completed in the VD furnace, with a net cycle time of 7 to 12 minutes, and a calming time before pouring of 5 to 7 minutes.

[0051] Furthermore, the continuous casting process adopts a slab continuous casting machine for casting, focusing on controlling the casting temperature, the tundish molten steel casting temperature is 1495-1506°C, the superheat is set to 10-20°C, and the casting rate during casting is 1.0-1.2m / min. Low-temperature casting is better to refine the original cast structure. In order to control the center segregation and looseness of the continuous casting billet, electromagnetic stirring or continuous casting billet light reduction process is adopted, wherein the light reduction rate is controlled at 8-10%, the billet is stacked and slowly cooled after it comes off the line, and the enclosure stacking is adopted, the number of enclosure surfaces is controlled at 2-3, and the stacking slow cooling time is 24-36h, which improves the internal quality of the steel plate and reduces the occurrence of defects such as cracks.

[0052] The present invention is described in detail in the following examples. These examples are intended only to provide a general description of the present invention and are not intended to limit the present invention. Table 1 shows the chemical composition of the examples, Table 2 shows the smelting and continuous casting process parameters for the steels used in the examples, Table 3 shows the rolling parameters for the steels used in the examples, Table 4 shows the heat treatment parameters for the steels used in the examples, and Table 5 shows the final mechanical properties of the examples.

[0053] Table 1 Chemical composition of the examples (wt, %)

[0054]

[0055] Table 2 Smelting and continuous casting process parameters of the embodiment

[0056]

[0057] Table 3 shows the rolling parameters of the example steel

[0058]

[0059] Table 4 Heat treatment process parameters of example steel

[0060]

[0061] Table 5 Final results of the organization and mechanical properties of the examples

[0062]

[0063] As can be seen from the above, the mechanical properties of the steel plate for producing (65-110) mm thick and excellent plate shape for high-performance hydrogen storage equipment provided by the present invention are as follows: at room temperature (0-25°C), the steel plate T / 4: 885MPa≤R m ≤970MPa、755MPa≤R el ≤780MPa, Steel plate T / 2: 890MPa≤R m ≤970MPa、750MPa≤R el ≤770MPa; KV2≥130J under -60℃ condition.

[0064] In order to describe the present invention, the present invention has been appropriately and fully illustrated through the examples above. The above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the scope of protection of the present invention, and the patent protection scope of the present invention should be defined by the claims.

Claims

1. A method for manufacturing thick steel plates for high-performance hydrogen storage equipment with high efficiency heat treatment, comprising smelting, continuous casting, slab heating, rolling, hot straightening, slow cooling, and heat treatment, characterized in that: Slab heating The temperature range of the preheating section is 790~835℃, the temperature range of the heating section is 1185~1210℃, the temperature range of the soaking section is 1235~1265℃, and the soaking time of the slab is controlled at 4.6~5.4h; Rolling process The rolling adopts a three-stage controlled rolling method. The starting rolling temperature of the recrystallization zone in the first stage is 1150~1160℃, the ending rolling temperature of the recrystallization zone is ≥1115℃, the total deformation rate is above 50%, large deformation rapid rolling is adopted, and the rolling speed is 3.7~4.2m / s; the starting rolling temperature of the non-recrystallization zone in the second stage is 900~950℃, the finishing rolling temperature is 830~860℃, the single-pass reduction is controlled at 3~5%, and small deformation multi-pass rapid rolling is adopted at a rolling speed of 3.5~3.8m / s; the three-stage homogenization rolling has a starting rolling temperature of 760~790℃, an ending rolling temperature of 672~680℃, and a controlled reduction of 1~3%; Heat treatment The normalizing heat treatment temperature is 870~890℃, and a short-term holding system of 10~15min is adopted, the heating rate is controlled at 0.9~1.1min / mm, and the cooling rate is 23~26℃ / s; A thick steel plate for high-performance hydrogen storage equipment with efficient heat treatment comprises the following components, calculated by weight percentage: C: 0.06%-0.09%, Si: 0.37%-0.49%, Mn: 1.45%-1.57%, P: ≤0.010%, S: ≤0.005%, Ni: 1.2%-1.4%, V: 0.03%-0.05%, Ti: 0.005-0.01%, Alt: 0.020%-0.045%, and the balance being Fe and unavoidable inclusions.

2. The method for manufacturing a thick steel plate for high-performance hydrogen storage equipment with high-efficiency heat treatment according to claim 1, characterized in that: V / Ti = 3~10 。 3. The method for manufacturing a thick steel plate for high-performance hydrogen storage equipment with high-efficiency heat treatment according to claim 1, characterized in that: The steel plate is at room temperature, at T / 4 of the steel plate: 885MPa≤R m ≤970MPa、755MPa≤R el ≤780MPa, steel plate T / 2: 890MPa≤R m ≤970MPa、750MPa≤R el ≤770MPa; KV2≥130J under -60℃ condition.

4. The method for manufacturing a thick steel plate for high-performance hydrogen storage equipment with high-efficiency heat treatment according to claim 1, characterized in that: The steel plate has a microstructure of ferrite + sorbite, with a volume ratio of ferrite to sorbite of 3.5-4. The steel plate also includes second-phase particles TiC and VC, with sizes of 4-6 nm and 7-9 nm, respectively.

5. The method for manufacturing a thick steel plate for high-performance hydrogen storage equipment with high-efficiency heat treatment according to claim 1, characterized in that: The thickness of the steel plate is 65-110 mm.

6. The method for manufacturing a thick steel plate for high-performance hydrogen storage equipment with high-efficiency heat treatment according to claim 1, characterized in that: In the smelting process, oxygen blowing for dephosphorization is controlled within 8 to 12 minutes, and oxygen blowing for decarburization is controlled within 7 to 9 minutes; deep desulfurization treatment is further performed using an LF refining furnace; degassing is completed in a VD furnace, with a net cycle time of 7 to 12 minutes, and a calming time of 5 to 7 minutes before pouring.

7. The method for manufacturing a thick steel plate for high-performance hydrogen storage equipment with high-efficiency heat treatment according to claim 1, characterized in that: The continuous casting process adopts a slab continuous casting machine for casting, focusing on controlling the casting temperature, the tundish molten steel casting temperature is 1495~1506℃, the superheat is set to 10~20℃, and the casting rate during casting is 1.0~1.2m / min; low temperature casting is used to refine the original cast structure; in order to control the center segregation and looseness of the continuous casting billet, electromagnetic stirring or continuous casting billet light reduction process is adopted, wherein the light reduction rate is controlled at 8~10%, the billet is stacked for slow cooling after it comes off the line, and enclosure stacking is adopted, the number of enclosure faces is controlled at 2~3, and the stacking slow cooling time is 24~36h.

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