CAST 5A DUPLEX STAINLESS STEEL AND METHOD FOR PREPARING LARGE CASTING OF CAST 5A DUPLEX STAINLESS STEEL

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of Chinese Patent Application No. 202411262987.4 filed on Sep. 10, 2024, the contents of which are incorporated herein by reference in their entirety.

FIELD OF TECHNOLOGY

The present disclosure belongs to the technical field of metal smelting, and specifically relates to a cast 5A duplex stainless steel and a method for preparing a large casting of the cast 5A duplex stainless steel.

BACKGROUND

Currently, fast reactors are preferred reactor types for fourth-generation advanced nuclear energy systems in the world, and have the advantage of being capable of directly utilizing discarded uranium isotopes. For example, 600 MW fast reactor circulating water pump systems adopt seawater as a heat transfer medium to cool three-loop light water. These water pump systems, including components and parts such as pump bodies, valves and impellers, have the characteristics of large dimensions (up to tens of meters), complex shapes, significant differences in cross-sectional dimensions of parts, etc., which are difficult to produce by machining methods and are generally formed integrally by adopting casting methods. In addition, since the seawater has strong corrosiveness, the water pump systems are also confronted with impacts such as fluid corrosion, erosion corrosion and cavitation corrosion. Therefore, materials of the relevant components and parts such as the pump bodies are required to have high strength, high compactness and no casting defects, and also have strong resistance to chloride corrosion and fluid erosion corrosion.

Super duplex stainless steel, as a third generation of duplex stainless steel, is characterized by ultra-low carbon, high chromium, high nickel, high molybdenum and high nitrogen, and has excellent mechanical properties and corrosion resistance, so that the super duplex stainless steel has become an ideal choice for applications under harsh working conditions. ASTM A890 5A, as a representative of the third generation of duplex stainless steel, includes the following ranges of components: ≤0.030% of C, ≤1.0% of Si, ≤1.5% of Mn, ≤0.040% of P, ≤0.040% of S, 24.0-26.0% of Cr, 6.0-8.0% of Ni, 4.0-5.0% of Mo, and 0.10-0.30% of N, and has excellent comprehensive mechanical properties and cost performance.

However, with increase of alloy contents, the difficulty in casting and forming 5A super duplex stainless steel is obviously increased. During a production and manufacturing process, both brittle second phases, represented by a σ phase, easily produced in a stage at 600-950° C. and ferrite undergoing brittle decomposition in a stage at 400-500° C. are likely to cause cracking of large castings. In addition, the castings with complex structures, numerous casting hot spots, large differences in wall thickness in local areas and uneven wall thicknesses are likely to generate casting defects such as stress concentration, shrinkage porosity and air holes. An overall casting process is extremely difficult, and a scrap rate of the large castings is as high as 70-90% or above.

Existing preparation processes for casting and forming of the 5A material duplex stainless steel generally include: obtaining molten steel with standard components by melting through a medium-frequency induction furnace using an argon oxygen decarburization/vacuum oxygen decarburization (AOD/VOD) duplex melting process, then adjusting a temperature of the molten steel to 1,550-1,580° C., pouring the same into a sand mold until, performing the operation of hot mold opening and shakeout after the molten steel is solidified and the sand mold is cooled to about 900-1,000° C., and cutting off a gating system, then rapidly hoisting a casting for water cooling or cold water spraying to room temperature, and finally, performing solid solution treatment at 1,120-1,150° C., followed by water cooling to adjust a dual-phase structure of the casting. However, the hot mold opening and shakeout have high risks for the large castings, and have high working intensity and low production efficiency. Even though, the scrap rate of the large castings is still as high as 50% or above.

Currently, mainstream solutions focus on optimizing the design of a sand casting and cooling system. For example, in CN110242781A, an optimized sand core and a suitable chill partition are adopted to avoid macroscopic internal stress caused by significant differences in cooling rates at different sites of the parts as much as possible. It is also taken into consideration that a small amount of pure rare earth Ce (CN110242781A) or an alloy containing mixed rare earth elements La and Ce (CN112410675A) is added into basic alloy components to achieve better mechanical properties as much as possible. However, all these improved casting processes still require the operation of hot mold opening and cannot fundamentally solve the problem of brittle cracking caused by precipitation of a large amount of the σ phase during a cooling process of casting structures. Moreover, mutual coupling effects and proportional coordination between added alloy elements are not taken into consideration in previous technologies.

In summary, to better improve safe production of the large castings of the 5A duplex stainless steel, reduce the production cost and increase finished product rates of the large castings, it is urgently required to develop a preparation method suitable for casting and forming of the 5A duplex stainless steel and capable of eliminating the hot mold opening and preventing the casting cracking.

SUMMARY

To solve the above technical problems, an objective of the present disclosure is to provide a cast 5A duplex stainless steel and a method for preparing a large casting of the cast 5A duplex stainless steel, which can not only reduce casting defects of the large casting of the 5A duplex stainless steel and decrease a scrap rate, but also increase a yield and a finished product rate of the large casting, improve production efficiency and reduce production cost and safety risks.

To achieve the above invention objective, technical solutions adopted by the present disclosure are as follows.

In a first aspect of the present disclosure, the present disclosure provides a cast 5A duplex stainless steel, which includes the following components: by weight percentage, ≤0.03% of C, ≤1% of Si, ≤1.5% of Mn, ≤0.04% of P, ≤0.02% of S, 24.0-24.5% of Cr, 6.0-7.0% of Ni, 4.0-4.5% of Mo, 0.20-0.30% of N, 0.20-0.30% of W, 0.20-0.30% of Nb, 0.02-0.035% of Re, and the balance of Fe, wherein a PREN value, namely % Cr+3.3% Mo+16% N, is ≥40.

Preferably, a mass ratio of the W to the Nb is 1:1.

Preferably, the Re is a La+Ce mixed rare earth, wherein a weight ratio of the La to the Ce is 1:(4-5).

In a second aspect of the present disclosure, the present disclosure provides a method for preparing a large casting of the cast 5A duplex stainless steel, which includes:

• • (S1) preparing raw materials according to mass percentages of various elements in the cast 5A duplex stainless steel, subjecting the remaining raw materials excluding rare earth elements to melting through a medium-frequency induction furnace to obtain molten steel, detecting alloy components of the sampled molten steel through a spectrometer, and performing adjustment based on detection results until the alloy components meet designed component requirements;
  • (S2) subjecting the molten steel to purification treatment through an AOD duplex refining process;
  • (S3) adding rare earth elements into the molten steel before tapping of the molten steel or in a pouring ladle, and pouring the molten steel into a sand mold;
  • (S4) air cooling the molten steel to room temperature, performing mold opening and shakeout, cutting off a gating system, and taking out a casting;
  • (S5) transferring the casting into a heat treatment furnace for solid solution treatment; and
  • (S6) cooling the furnace to 1,045-1,080° C. after heat treatment, and taking the casting out of the furnace for water cooling.

Preferably, in the step S1, a melting temperature is 1,650-1,680° C.

Preferably, in the step S1, the W and Nb elements are added in the form of a W—Nb intermediate alloy.

Preferably, in the step (S3), the rare earth elements are pressed into the molten steel in the form of a rare earth ferrosilicon alloy, and a mass percentage of the rare earth elements in the rare earth ferrosilicon alloy is 25-30%.

Preferably, in the step (S3), a pouring temperature is 1,530-1,580° C.

Preferably, in the step (S5), the solid solution treatment includes: loading the casting into the heat treatment furnace with a furnace temperature of 1,000° C., performing heat preservation for 30 min, raising the temperature to 1,130-1,160° C. at a rate of 80-150° C./h, and performing heat preservation for 3-3.5 h.

Preferably, in the step (S6), a water cooling manner includes cooling with circulating water, wherein a temperature of the circulating water is ≤80° C.

Beneficial Effects

According to the present disclosure, through component design, the precipitation of an intermetallic σ phase can be completely avoided when the 5A duplex stainless steel is slowly cooled to room temperature from a liquid phase, and the casting has a fine structure, so that the structure and macroscopic stress of the large casting are significantly reduced, a cracking tendency of the large casting is improved, and the finished product rate of the casting can be increased to 95% or above.

According to the present disclosure, since hot mold opening and shakeout and a high-temperature cutting gating system are not required during a casting process, safety risks during a production process are greatly reduced, and labor intensity of workers is reduced. With increase of the yield and implementation of a hot furnace loading process, the casting production efficiency is substantially improved, and the cost is substantially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a metallographic structure of a casting after pouring and slow cooling in Example 1 of the present disclosure;

FIG. 2 shows a scanning electron microstructure of the casting after pouring and slow cooling in Example 1 of the present disclosure;

FIG. 3 shows a metallographic structure of the casting after solid solution treatment in Example 1 of the present disclosure;

FIG. 4 shows a metallographic structure of a casting after pouring and slow cooling in Example 2 of the present disclosure;

FIG. 5 shows a scanning electron microstructure of the casting after the pouring and slow cooling in Example 2 of the present disclosure;

FIG. 6 shows a metallographic structure of a casting after pouring and slow cooling in Comparative Example 1 of the present disclosure; and

FIG. 7 shows a metallographic structure of a casting after pouring and slow cooling in Comparative Example 2 of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

To more clearly illustrate the technical solutions in examples of the present disclosure or in the prior art, specific embodiments of the present disclosure are described below with reference to the accompanying drawings. Obviously, the drawings described below are merely some examples of the present disclosure, and for those of ordinary skill in the art, other drawings and other embodiments can also be obtained according to these drawings without exerting creative efforts.

The present disclosure provides a cast 5A duplex stainless steel, which includes the following components: by weight percentage, ≤0.03% of carbon (C), ≤1% of silicon (Si), ≤1.5% of manganese (Mn), ≤0.04% of phosphorus (P), ≤0.02% of sulfur(S), 24.0-24.5% of chromium (Cr), 6.0-7.0% of nickel (Ni), 4.0-4.5% of molybdenum (Mo), 0.20-0.30% of nitrogen (N), 0.20-0.30% of tungsten (W), 0.20-0.30% of niobium (Nb), 0.02-0.035% of rare earth elements (Re), and the balance of iron (Fe), wherein a PREN value, namely % Cr+3.3% Mo+16% N, is ≥40.

Preferably, a mass ratio of the W to the Nb is 1:1. More preferably, the W and Nb elements are added in the form of a W—Nb intermediate alloy.

Preferably, the Re is a La+Ce mixed rare earth, wherein a weight ratio of the La to the Ce is 1:(4-5). More preferably, the rare earth elements are added in the form of a rare earth ferrosilicon alloy, wherein a mass percentage of the rare earth elements in the rare earth ferrosilicon alloy is 25-30%.

The chromium is an important element in the stainless steel, and corrosion resistance of the stainless steel is improved with increase of the content of the chromium. However, the chromium is also a ferrite forming element, and the addition of chromium into the steel can promote the formation of ferrite and increase a tendency to form an intermetallic σ phase in the stainless steel. The molybdenum can improve pitting resistance and crevice corrosion resistance of the stainless steel, and especially in a chloride ion environment, the corrosion resistance is particularly prominent. Moreover, the molybdenum is also a ferrite forming element and can also increase the tendency to form the intermetallic σ phase in the stainless steel. In the present disclosure, the content of the chromium element is controlled at a lower limit of 24.0-24.5%, and the content of the molybdenum element is controlled at a lower limit of 4.0-4.5%.

The W and the Nb are obvious elements commonly used for grain refinement and microalloy strengthening, which can improve the toughness and cracking tendency of a casting. However, due to high melting points, the W and the Nb easily form carbides with carbon to generate inclusions. Therefore, their contents need to be low. The W and the Nb can form an infinite solid solution, thereby facilitating entrance into the ferrite. In the present disclosure, the mass ratio of the W to the Nb is controlled at 1:1, which can significantly inhibit the precipitation of the intermetallic o-phase. In the present disclosure, the W and Nb components are designed at the lower content of 0.20-0.30% and are added in the form of the W—Nb intermediate alloy.

Most importantly, the rare earth elements Ce and La are added in the present disclosure. The Ce and the La have surface activity and have the effect of refining structural elements, and the effect of the Ce is higher than that of the La. In the present disclosure, the mixed rare earth ferrosilicon alloy with a lower price is adopted. The Ce and the La, enriched at a grain boundary, can refine grains, change a state of the grain boundary and improve the plasticity and toughness of the steel and the cracking tendency of the casting. Meanwhile, the Ce and the La also have the effects of purification and microalloying, which can not only achieve desulfurization and degassing, but also change morphology, size and distribution of oxides and sulfides in the steel by forming sulfur oxides, thereby generating extremely favorable impacts on properties of the steel. However, a pure rare earth alloy is prone to oxidation and difficult to enter a matrix by solid solution, and large inclusions are formed. Therefore, no effect is achieved when the rare earth content is too low, and many inclusions are likely to be produced when the content is too high. In the present disclosure, the La and Ce components are designed at the content of 0.020-0.035% with a La to Ce ratio of 1:(4-5) and are pressed into molten steel or a pouring ladle in the rare earth ferrosilicon alloy. Thus, the rare earth elements enter the ferrite through the solid solution more easily, thereby significantly inhibiting the precipitation of the σ phase.

Through the design of various important elements in the present disclosure, the precipitation of the intermetallic σ phase can be completely avoided when the molten steel is slowly cooled to room temperature from a liquid phase, and the casting has a fine structure, so that the structure and macroscopic stress of a large casting are significantly reduced, and a cracking tendency of the large casting is improved.

The present disclosure further provides a method for preparing the large casting based on the aforementioned cast 5A duplex stainless steel, which includes the steps of primary smelting through a medium-frequency induction furnace, AOD duplex refining, addition of a rare earth ferrosilicon alloy, pouring, air cooling, solid solution treatment, and water cooling, etc. The method specifically includes the following steps:

• • (S1) preparing raw materials according to mass percentages of various elements in the cast 5A duplex stainless steel, subjecting the remaining raw materials excluding rare earth elements to melting through the medium-frequency induction furnace to obtain molten steel, detecting alloy components of the sampled molten steel through a spectrometer, and performing adjustment based on detection results until the alloy components meet designed component requirements;
  • (S2) subjecting the molten steel to purification treatment through an AOD duplex refining process;
  • (S3) pressing the rare earth ferrosilicon alloy into the molten steel before tapping of the molten steel or in a pouring ladle, and pouring the molten steel into a sand mold;
  • (S4) air cooling the molten steel to room temperature, performing mold opening and shakeout, cutting off a gating system, and taking out a casting;
  • (S5) transferring the casting into a heat treatment furnace for solid solution treatment; and
  • (S6) cooling the furnace to 1,045-1,080° C. after heat treatment, and taking the casting out of the furnace for water cooling.

Preferably, in the step (S1), a melting temperature is 1,650-1,680° C.

Preferably, in the step (S3), a pouring temperature is 1,530-1,580° C.

Preferably, in the step (S5), the solid solution treatment includes: loading the casting into the heat treatment furnace with a furnace temperature of 1,000° C., performing heat preservation for 30 min, raising the temperature to 1,130-1,160° C. at a rate of 80-150° C./h, and performing heat preservation for 3-3.5 h.

Preferably, in the step (S6), a water cooling manner includes cooling with circulating water, wherein a temperature of the circulating water is ≤80° C.

The technical solutions of the present disclosure are described in detail below through specific examples.

Example 1

In the present example, chemical components were designed as follows by weight percentage: ≤0.03% of C, ≤1% of Si, ≤1.5% of Mn, ≤0.04% of P, ≤0.02% of S, 24.0-24.5% of Cr, 6.0-7.0% of Ni, 4.0-4.5% of Mo, 0.20-0.30% of N, 0.27% of W, 0.27% of Nb, 0.03% of Re, and the balance of Fe, wherein a PREN value, namely % Cr+3.3% Mo+16% N, was 42.1.

Basic raw materials were prepared according to the chemical components, wherein the W and the Nb were added in the form of an intermediate alloy (with a W to Nb weight ratio of 1:1). The remaining raw materials excluding a rare earth ferrosilicon alloy was melted by adopting a medium-frequency induction furnace at 1,680° C. to obtain molten steel. The preliminary alloy components were adjusted by using a spectrometer until designed component requirements were met.

The molten steel was subjected to purification treatment through an AOD duplex refining process.

Before tapping of the molten steel or in a pouring ladle, a rare earth ferrosilicon alloy (with a Ce to La ratio of 4.5:1 and an addition amount of 6.4 kg/t molten steel) was pressed into the molten steel, and the molten steel was poured into a sand mold at a temperature of 1,550° C.

Air cooling was performed to room temperature, mold opening and shakeout were performed, a gating system was cut off, a casting was taken out, and it was observed that the casting had no macroscopic large cracks and fine cracks and had a dense surface. A small sample was cut from the casting, the alloy components were analyzed by using a spectrometer, and rare earth elements La and Ce in the alloy were analyzed by adopting an inductively coupled plasma emission spectroscopy method. Results are shown in Table 1, the components meet standard requirements, and the PREN value is 42.1. A metallographic structure is shown in FIG. 1. From the figure, it can be seen that the casting has a fine as-cast structure and only includes austenite and ferrite structures, precipitation of a σ phase is basically not observed, an extremely small amount of the σ phase can be observed only through magnified scanning electron microscopy (as shown in FIG. 2), and it is analyzed that a content of the σ phase is only about 0.5%, indicating that 5A steel with the components can completely inhibit the precipitation of the σ phase during a slow cooling process, thereby avoiding cracking of the casting.

The casting was transferred and loaded into a heat treatment furnace with a furnace temperature of 1,000° C., heat preservation was performed for 30 min, the temperature was raised to 1,150° C. at 150° C./h, and heat preservation was performed for 3.5 h.

After the heat treatment was completed, the furnace was cool to 1,080° C., and the casting was taken out of the furnace for water cooling, wherein the water cooling adopted cooling with circulating water with a water temperature of ≤80° C. The produced impeller casting has a dense structure and meets requirements of ASME VIII, DIV.1, APP.7 based on penetration inspection. A small sample was cut from the large casting for metallographic structure observation. As shown in FIG. 3, the casting structurally consists of two phases including austenite and ferrite, wherein a content of the ferrite is 54.4%. A mechanical property test was carried out on the sample according to an ASTM A370 method. The casting has a tensile strength of 767 MPa, an elongation of 31%, a low-temperature (−46° C.) impact resistance of 107 AKV/J, and an average hardness value of 247 HB, thus having excellent mechanical properties. A crevice corrosion test was carried out in a 6% FeCl₃ solution at 50° C. for 24 h according to ASTM G48-3 Method A. The casting has a weight loss of 0.0016 g, no pitting corrosion, and a corrosion rate of 0.471 g/m², thus having excellent corrosion resistance.

Example 2

In the present example, chemical components were designed as follows by weight percentage: ≤0.03% of C, ≤1% of Si, ≤1.5% of Mn, ≤0.04% of P, ≤0.02% of S, 24.0-24.5% of Cr, 6.0-7.0% of Ni, 4.0-4.5% of Mo, 0.20-0.30% of N, 0.22% of W, 0.22% of Nb, 0.02% of Re, and the balance of Fe.

Basic raw materials were prepared according to the chemical components, wherein the W and the Nb were added in the form of an intermediate alloy (with a W to Nb weight ratio of 1:1). The remaining raw materials excluding a rare earth ferrosilicon alloy was melted by adopting a medium-frequency induction furnace at 1,650° C. to obtain molten steel. The preliminary alloy components were adjusted by using a spectrometer until designed component requirements were met.

The molten steel was subjected to purification treatment through an AOD duplex refining process.

Before tapping of the molten steel or in a pouring ladle, a rare earth ferrosilicon alloy (with a Ce to La ratio of 5:1 and an addition amount of 4.2 kg/t molten steel) was pressed into the molten steel, and the molten steel was poured into a sand mold at a temperature of 1,560° C.

Air cooling was performed to room temperature, mold opening and shakeout were performed, a gating system was cut off, a casting was taken out, and it was observed that the casting had no macroscopic large cracks and fine cracks and had a dense surface. A small sample was cut from the casting, the alloy components were analyzed by using a spectrometer, and rare earth elements La and Ce in the alloy were analyzed by adopting an inductively coupled plasma emission spectroscopy method. Results are shown in Table 1, the components meet standard requirements, and a PREN value is 41.7. A metallographic structure is shown in FIG. 4. From the figure, it can be seen that the casing has a fine as-cast structure and basically only includes austenite and ferrite structures, an extremely small amount of a σ phase is observed, the small amount of the σ phase can be observed only through magnified scanning electron microscopy (as shown in FIG. 5), and it is analyzed that a content of the σ phase is only about 1%, indicating that 5A steel with the components can completely inhibit the precipitation of the σ phase during a slow cooling process, thereby avoiding cracking of the casting.

The casting was transferred and loaded into a heat treatment furnace with a furnace temperature of 1,000° C., heat preservation was performed for 30 min, the temperature was raised to 1,130° C. at 80° C./h, and heat preservation was performed for 3 h.

After the heat treatment was completed, the furnace was cool to 1,080° C., and the casting was taken out of the furnace for water cooling, wherein the water cooling adopted cooling with circulating water with a water temperature of ≤80° C. The produced casting has a dense structure and excellent mechanical properties and corrosion resistance. A small sample was cut from the large casting. The casting structurally consists of two phases including austenite and ferrite, wherein a content of the ferrite is 55.1%.

Comparative Example 1

In the present comparative example, chemical components were designed as follows by weight percentage: ≤0.03% of C, ≤1% of Si, ≤1.5% of Mn, ≤0.04% of P, ≤0.02% of S, 24.0-24.5% of Cr, 6.0-7.0% of Ni, 4.0-4.5% of Mo, 0.20-0.30% of N, and the balance of Fe.

Basic raw materials were prepared according to the chemical components, melting was performed by adopting a medium-frequency induction furnace at 1,680° C. to obtain molten steel, and the preliminary alloy components were adjusted by using a spectrometer until designed component requirements were met.

The molten steel was subjected to purification treatment through an AOD duplex refining process.

The molten steel was poured into a sand mold at a temperature of 1,580° C.

Air cooling was performed to room temperature, mold opening and shakeout were performed, a gating system was cut off, a casting was taken out, and it was observed that the casting had macroscopic open cracks. The casting was unqualified, and a subsequent heat treatment process was not performed. A small sample was cut from an uncracked area of the casting, the alloy components were analyzed by using a spectrometer, and rare earth elements La and Ce in an alloy were analyzed by adopting an inductively coupled plasma emission spectroscopy method. Results are shown in Table 1, the components meet standard requirements, and a PREN value is 41.1. A metallographic structure is shown in FIG. 6. From the figure, it can be seen that the casting has a coarse as-cast structure and has a large amount of a σ precipitation phase at a grain boundary that forms a network, and a content of the σ precipitation phase is as high as 28.5%. The metallographic structure is similar to that of a general 5A duplex stainless steel casting.

Comparative Example 2

In the present comparative example, chemical components were designed as follows by weight percentage: ≤0.03% of C, ≤1% of Si, ≤1.5% of Mn, ≤0.04% of P, ≤0.02% of S, 24.0-24.5% of Cr, 6.0-7.0% of Ni, 4.0-4.5% of Mo, 0.20-0.30% of N, 0.5% of W, 0.20% of Nb, 0.012% of Re, and the balance of Fe.

Basic raw materials were prepared according to the chemical components, wherein the W and the Nb were added in the form of an intermediate alloy (with a W to Nb weight ratio of 2.5:1). Melting was performed by adopting a medium-frequency induction furnace at 1,650° C. to obtain molten steel. The preliminary alloy components were adjusted by using a spectrometer until designed component requirements were met.

The molten steel was subjected to purification treatment through an AOD duplex refining process.

The molten steel was poured into a sand mold at a temperature of 1,550° C.

Air cooling was performed to room temperature, mold opening and shakeout were performed, a gating system was cut off, a casting was taken out, and it was observed that the casting had macroscopic open cracks. The casting was unqualified, and a subsequent heat treatment process was not performed. A small sample was cut from an uncracked area of the casting, the alloy components were analyzed by using a spectrometer, and rare earth elements La and Ce in an alloy were analyzed by adopting an inductively coupled plasma emission spectroscopy method. Results are shown in Table 1, the components meet standard requirements, and a PREN value is 42.3. A metallographic structure is shown in FIG. 7. From the figure, it can be seen that the casting has a relatively fine as-cast structure, but has a large amount of a blocky σ precipitation phase, and a content of the σ precipitation phase is as high as 24.7%, which are primary causes of the macroscopic open cracks of the casting.

The examples provided by the present disclosure are elaborated in detail above. The principles and embodiments of the present disclosure are set forth using specific examples herein, and the description of the above examples is only used to facilitate understanding of core ideas of the present disclosure. It should be pointed out that for those of ordinary skill in the art, various improvements and modifications of the present disclosure can also be made without departing from the principles of the present disclosure, and all the improvements and modifications also fall within the scope of protection of the claims of the present disclosure.