1. A1470 MPa-grade hot-dip galvanized steel sheet for cold stamping is characterized by comprising the following components in percentage by weight: c: 0.20-0.25%, Si: 0.5% -1.0%, Mn: 2.5% -3.0%, Ti: 0.02% -0.03%, Al: 0.05 to 0.8 percent, less than or equal to 0.01 percent of P, less than or equal to 0.005 percent of S, and the balance of Fe and inevitable impurities, wherein the Ceq is equal to C + Si/30+ Mn/20+2P +4S and less than or equal to 0.45 percent.

2. The 1470 MPa-grade galvanized steel sheet for cold stamping according to claim 1, further comprising Cr: less than or equal to 0.5 percent, Ni: less than or equal to 0.5 percent, Mo: less than or equal to 0.5 percent, Cu: less than or equal to 0.5 percent, Nb: less than or equal to 0.03 percent, less than or equal to 0.005 percent of Ca and less than or equal to 0.005 percent of B.

3. The 1470 MPa-grade galvanized steel sheet for cold stamping according to claim 1, wherein the microstructure of the steel sheet is ferrite + tempered martensite + retained austenite + bainite; wherein the microstructure comprises the following components in percentage by volume: ferrite is less than or equal to 10 percent, tempered martensite is 75 to 83 percent, and retained austenite is 8 to 12 percent.

4. The 1470 MPa-grade galvanized steel sheet for cold stamping according to claim 1, characterized in that the yield strength of the steel sheet is 900-1270 MPa, the tensile strength is 1470MPa or more, the elongation is 10% or more, and the hole expansion rate is 30% or more.

5. A method for preparing 1470MPa grade hot-dip galvanized steel sheet for cold stamping according to any one of claims 1 to 4, comprising smelting, casting, hot rolling, acid pickling, cold rolling, galvannealing and finishing; the method is characterized in that:

(1) hot rolling: the heating temperature is 1230-1280 ℃, the initial rolling temperature is 1100-1150 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 600-700 ℃;

(2) cold rolling: rolling reduction rate is 48% -52%;

(3) alloying galvanization annealing:

(a) heating and keeping constant temperature: heating the cold-rolled steel plate to 850-880 ℃ at the speed of 2-10 ℃/s, and keeping the temperature for 50-120 s;

(b) slow cooling: the slow cooling temperature is controlled to be 750-800 ℃;

(c) adjusting before quick cooling and galvanizing: rapidly cooling the slowly cooled steel plate to 250-300 ℃ at a cooling speed of more than 30 ℃/s, then heating to 450-470 ℃ at a heating speed of more than 10 ℃/s, and adjusting the time to 25-35 s before galvanization;

(d) then, the steel plate enters a zinc pot for galvanizing for 1-2 s, and then enters an alloying furnace for alloying galvanizing, wherein the alloying temperature is 550-600 ℃, and the weight percentage of Fe in the alloying coating is 7.0-11.5%;

(e) finishing: the finishing elongation is 0.1-0.4%.

6. The method for preparing 1470 MPa-grade galvanized steel sheet for cold stamping according to claim 5, comprising the following steps:

in the steps (3) - (c) of quick cooling and adjusting before galvanizing, the microstructure of the steel plate after quick cooling contains 75% -83% of martensite and 10% -15% of austenite.

Background

The high strength and the thinning of the automobile steel are main means for coping with the weight reduction of the automobile, and if 1500 MPa-grade steel plates are used for automobile body structural parts instead of the existing 780 MPa-grade steel plates, the thickness of the steel plates can be thinned by about 1 time, and the weight reduction effect is obvious. However, when the strength of the steel sheet is increased to 1500MPa, the plasticity is rapidly decreased, and the extremely low elongation indicates extremely poor drawing property of the material, so that cold press forming is hardly performed. In recent years, the selection of cold stamping and hot stamping for automobiles has been a focus of continuing component research and manufacturer's debate in high strength steel applications for automobiles, and the cold stamping forming application of steel sheets with higher strength (1500MPa or more) has been limited to poor elongation. Therefore, how to improve the plastic component of the 1500MPa steel plate advances the development of the cold-punching high-strength steel. Many researchers think that the current situation of poor cold stamping performance can be effectively solved by increasing the plasticity of the steel plate to more than 15%, because the plasticity of DP780 steel products widely applied in the market only reaches about 16%, and the plasticity of DP980 steel products only reaches 12%. However, the 1500MPa force can be used for characterizing the matrix structure of the alloy to be a substantially full martensite phase, and the plasticity enhancement necessarily depends on the introduction of residual austenite and the TRIP effect of metastable austenite in the deformation process at room temperature, so as to play a role in delaying necking.

The steel sheet disclosed in patent document "cold-rolled high-strength steel excellent in formability with a tensile strength of 1500MPa or more and a method for producing the same" (publication No. CN108018484B) comprises the following components in percentage by weight: c: 0.28 to 0.40%, Si: 1.5-2.5%, Mn: 2.0-3.0%, Al: 0.03-0.06 percent, (V: 0.01-0.2 percent, Nb: 0.01-0.1 percent, Ti: 0.01-0.05 percent), less than or equal to 3.8 percent of Mn + Cr + Mo, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, and the balance of Fe and other unavoidable impurities. The preparation method comprises the working procedures of smelting, hot rolling, pickling, cold rolling and continuous annealing, adopts the process idea of quenching distribution, and adopts martensite and austenite structures to form and manufacture the cold-rolled annealed steel plate with the tensile strength of more than 1500MPa and the elongation of more than 12 percent. Obviously, a large amount of noble alloys Cr and Mo are added into the alloy of the product, and micro alloy elements such as Nb, V, Ti and the like are selectively added, so that the alloy cost is infinitely and greatly improved; in addition, the content of C in the steel is already added to be more than 0.28 percent, which obviously and seriously reduces the resistance spot welding performance of the steel plate and influences the loading use of the steel plate.

The patent document "a low-carbon medium-manganese high-residual austenitic high-toughness steel and a heat treatment method thereof" (publication number: CN112063931A) discloses that the high-toughness steel comprises the following components: 0.10-0.25%, Mn: 4.0% -8.0%, Al: 1.0 to 2.5 percent, and the balance of Fe and inevitable impurities. The steel plate realizes the strengthening of work hardening by means of TRIP effect provided by a large amount of austenite in the deformation process, and then the 1500 MPa-grade steel plate is obtained. However, it is worth noting that the steel plate is added with a large amount of Mn (more than 4%) and Al (more than 1%), which seriously improves the smelting difficulty of the product, is only suitable for laboratory research, and has no industrialization prospect under the current conditions; meanwhile, the tensile curve has an obvious luders band, which seriously affects the surface quality of the steel plate and is not beneficial to stamping application.

Disclosure of Invention

The present invention has been made to overcome the above problems and disadvantages, and an object of the present invention is to provide a hot-dip galvanized steel sheet having a tensile strength of 1470MPa or more and suitable for press forming.

The purpose of the invention is realized as follows:

a1470 MPa-grade hot-dip galvanized steel sheet for cold stamping comprises the following components in percentage by weight: c: 0.20-0.25%, Si: 0.5% -1.0%, Mn: 2.5% -3.0%, Ti: 0.02% -0.03%, Al: 0.05 to 0.8 percent, less than or equal to 0.01 percent of P, less than or equal to 0.005 percent of S, and the balance of Fe and inevitable impurities, wherein the Ceq is equal to C + Si/30+ Mn/20+2P +4S and less than or equal to 0.45 percent.

Further, the steel sheet further contains Cr: less than or equal to 0.5 percent, Ni: less than or equal to 0.5 percent, Mo: less than or equal to 0.5 percent, Cu: less than or equal to 0.5 percent, Nb: less than or equal to 0.03 percent, less than or equal to 0.005 percent of Ca and less than or equal to 0.005 percent of B.

The microstructure of the steel plate is ferrite, tempered martensite, residual austenite and bainite; the microstructure of the steel plate is as follows according to volume percentage: ferrite is less than or equal to 10 percent, tempered martensite: 75-83% and residual austenite 8-12%.

The yield strength of the steel plate is more than 900-1270 MPa, the tensile strength is more than 1470MPa, the elongation is more than 10%, and the hole expansion rate is more than 30%.

The steel plate composition design reason is as follows:

c: c is an important element I in the invention. Interstitial solid solution atoms C cause certain lattice distortion in a matrix and play a role in solid solution strengthening. In the invention, the addition of C and Mn elements ensures the industrializable austenitizing temperature and plays a role in promoting the austenite stabilization in the critical region. In addition, for the steel containing the retained austenite in the room temperature structure, the addition of the C element effectively improves the C content in the retained austenite after the distribution, and ensures the phase stability of the retained austenite. In the invention, the final performance and application are influenced to a certain extent by the C content which is too low or too high. If the content of C is less than 0.2%, the formation of residual austenite at room temperature with enough content cannot be ensured, and the plasticity of the experimental steel is influenced; if the content of C is more than 0.25%, the subsequent spot welding performance is seriously affected.

Si: si element is one of important elements in the present invention. The Si is dissolved in the matrix in a solid solution mode, so that the strength of the matrix is improved. In addition, the main effect of the Si addition is that the addition of Si in a sufficient amount can suppress the formation of carbides at the overaging stage, and prevent the steel sheet from deteriorating the properties due to carbide precipitation. However, too high Si addition will result in poor surface quality of the galvanized sheet. Moreover, the product of the invention is a galvanized product, and the tempering resistance of the tempered martensite is good in a shorter overaging time, so that the invention comprehensively considers that the content of the Si element is controlled to be 0.5-1.0%.

Mn: the Mn element is one of important elements in the present invention. In the research, Mn guarantees the austenite stabilization of a critical zone and the stability of supercooled austenite in the cooling process, and inhibits the formation of pearlite; most importantly, the addition of sufficient Mn element improves the hardenability of the steel plate and ensures the transformation quantity of martensite in a quenching state. However, excessive Mn addition causes serious Mn segregation in the continuous casting process, and meanwhile, the slab continuous casting is easy to generate a hot cracking phenomenon; furthermore, the addition of high Mn will also cause an increase in carbon equivalent in the subsequent welding stage, thereby deteriorating the welding performance. Therefore, the Mn content in the steel of the invention is controlled to be 2.5-3.0%.

Al: al can inhibit carbide precipitation and is beneficial to retaining residual austenite, but excessive Al addition can bring difficulty to smelting and can reduce the strength of the steel plate. The Al content is controlled to be 0.05-0.8%.

Ti: ti is a microalloy strengthening element. Ti combines with impurity element N of steel grade to form TiN, free N atoms in the steel are present in the steel grade to deteriorate the toughness of the steel plate, so the formation of TiN plays a role in fixing N; in addition, Ti still forms Ti (C, N) with C, N, and plays a role in refining prior austenite grains. However, the Ti content is too high, which results in too large size of TiN and deterioration of the steel sheet properties. Therefore, the Ti content in the steel is 0.02 to 0.03 percent.

P: p element is impurity element in steel, is easy to be partially gathered in crystal boundary, and is easy to form Fe when P content in steel is high₂P particles, which reduce the plasticity and toughness of the steel, are preferably contained in a lower amount. In the invention, the content of the P element is controlled to be less than or equal to 0.02 percent.

S: the S element is an impurity element in steel, and is easily combined with Mn to form MnS inclusions, thereby deteriorating the plasticity of the steel plate, so that the lower the content is, the better the content is. In the invention, the content of the S element is controlled to be less than or equal to 0.005 percent.

Cr: the Cr element is used as a supplementary element of the Mn element in the invention, when the Mn content is added to be lower, the Cr element with a certain content is added to ensure the hardenability of the steel plate, however, the excessive Cr content can improve the martensite content in the quenching stage, and the retained austenite is influenced. Therefore, the Cr content is not preferably higher than 0.5%.

Ni, Mo and Cu: mo is a solid solution strengthening element, so that the hardenability of the material can be improved, the tempering brittleness can be prevented, and the fatigue performance of the material can be improved; ni improves the corrosion resistance of the steel plate to a certain extent. The Cu element itself is dissolved in austenite to improve the strength of the steel sheet. In the continuous annealing stage, the simple substance Cu is precipitated in austenite to play a certain precipitation strength role. The addition of Cu has a certain effect of improving the corrosion resistance of the steel plate. In addition, Ni and Cu are austenite stabilizing elements and can promote retained austenite. Mo, Ni and Cu are all more noble alloys, and the total addition content is controlled to be lower than 0.5 percent in consideration of the alloy cost.

Nb: the Nb element has an effect of supplementing the precipitation strengthening action as a supplementary element to the Ti element in the present invention. Nb is precipitated in a strain-induced precipitation mode in the hot rolling stage to form NbC or Nb (C, N), and plays a role in pinning original austenite grain boundaries so as to refine original austenite grains. However, too much Nb addition will result in an increase in the thickness of the hot-rolled sheet, resulting in an increase in the difficulty of cold rolling. Therefore, the Nb element is controlled to 0.03% in the present invention.

Ca: the quality of the casting blank steel plate can be improved by adding a proper amount of Ca to control the form of inclusions, so that the Ca content is less than or equal to 0.005 percent.

B: the addition of B in the present invention can supplement the hardenability of the steel sheet, and the formation of martensite at the rapid cooling stage in the continuous annealing process. Too much B will increase the brittleness of the steel plate and deteriorate the workability of the steel plate, therefore, the B of the invention is less than or equal to 0.005 percent.

The second technical scheme of the invention provides a preparation method of 1470 MPa-level hot-dip galvanized steel plate for cold stamping, which comprises smelting, casting, hot rolling, acid washing, cold rolling, alloying galvanization annealing and finishing;

(1) smelting and casting: smelting and casting the components into a wedge-shaped casting blank;

(2) hot rolling: the heating temperature is 1230-1280 ℃, the initial rolling temperature is 1100-1150 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 600-700 ℃; the thickness of the hot rolled coil is between 3.2 and 3.8 mm.

The heating temperature is 1230-1280 ℃: for high-strength steel containing Ti, the heating temperature is important to control, and in order to achieve a more ideal N fixation effect, the heating temperature is generally controlled to be above 1230 ℃. Thereby ensuring that fine TiN or Ti (C, N) is separated out in the heating and recrystallization zone rolling stages, and playing the effects of pinning grain boundaries and refining original austenite grains.

The initial rolling temperature is 1100-1150 ℃: in the rolling interval of the recrystallization zone corresponding to the initial rolling temperature, micro-alloy elements such as Nb, Ti and the like are subjected to strain-induced precipitation during the rolling interval, and a certain amount of Nb (C, N) or Ti (C, N) is formed to play a role in pinning original austenite grains and refining the original austenite grains.

The finishing temperature is above 900 ℃: the setting of the finish rolling temperature is mainly to ensure that the rough rolling and finish rolling stages are completed as soon as possible so as to prevent TiN precipitates from coarsening and further to prevent poor pinning effect of crystal boundary. Therefore, the finishing temperature of the invention should be controlled to be above 900 ℃.

Coiling temperature: the coiling temperature is set to be 600-700 ℃, and if the coiling temperature is too low in the product, hard phase structures such as bainite or martensite in the structure are formed, and the subsequent cold rolling load is increased; if the coiling temperature is too high, a considerable part of pro-eutectoid ferrite is formed in the controlled cooling stage, so that the steel plate is softened, and the phenomenon of coil collapse is easy to occur in the coiling process.

(3) Acid washing: FeO and Fe exist on the surface of the hot rolled and coiled steel₂O₃、Fe₃O₄And removing iron oxide in different existing forms after acid pickling.

(4) Cold rolling: the specification of a cold-rolled product is kept at 1.5-2.0 mm, the thickness of the cold-rolled product corresponds to the thickness of a target automobile part (1.6mm, 1.8mm and 2.0mm) corresponding to the product, the rolling reduction is controlled at 48-52%, and the low rolling reduction rate cannot ensure enough cold-rolling deformation energy storage, so that the ferrite recrystallization effect in a continuous annealing stage is insufficient; the load of the cold rolling mill is greatly increased under the condition of overhigh rolling reduction, and the realization of the target thickness cannot be ensured.

(5) Alloying galvanization annealing: the method specifically comprises the steps of heating isothermal, slow cooling, fast cooling, adjusting before galvanizing, galvanizing and alloying galvanizing;

(a) heating and keeping constant temperature: heating the cold-rolled steel plate to 850-880 ℃ at the speed of 2-10 ℃/s, and keeping the temperature for 50-120 s;

the heating temperature is 850-880 ℃, so that the full austenitizing behavior of the steel is ensured, and a martensitic matrix in a room-temperature structure is further ensured. If the temperature is too low, extra ferrite in a critical area is formed, and the strength of the steel plate is reduced; if the temperature is too high, austenite grains in the critical region are coarsened, and the strength of the steel plate is also reduced. The isothermal time was set as: the steel plate has the advantages that the steel plate has a structure which is subjected to a series of processes of ferrite recrystallization, austenite nucleation and austenite growth through heating and isothermicity, the isothermal phase structure of the complete austenitizing is guaranteed by the isothermal time of 50s, and the austenite grains are coarsened due to the fact that the time is too long. Therefore, the isothermal time is set to 50 to 120 seconds.

(b) Slow cooling: the slow cooling temperature is controlled to be 750-800 ℃, the selection of the slow cooling temperature range prevents excessive ferrite from being separated out in the slow cooling stage, reduces the strength of the steel plate,

(c) adjusting before quick cooling and galvanizing: rapidly cooling the slowly cooled steel plate to 250-300 ℃ at a cooling speed of more than 30 ℃/s, then heating to 450-470 ℃ at a heating speed of more than 10 ℃/s, and adjusting the time to 25-35 s before galvanization;

the microstructure of the steel plate after rapid cooling contains 75-83% of martensite and 10-15% of austenite.

The cooling speed of more than 30 ℃/s effectively prevents the formation of excessive epitaxial ferrite in the cooling process, reduces the strength of the steel plate, and controls the content of the ferrite in the steel plate to be less than 10 percent; the quenching temperature is 250-300 ℃, so that the steel plate contains more than 75% of martensite structures, and at the same time, not less than 15% of retained austenite is retained, so that the strength and the plasticity of the steel plate are ensured. Then the heating speed is more than 10 ℃/s, aiming at ensuring the isothermal time of the subsequent adjustment stage; before galvanization, the isothermal temperature of the stage is adjusted, so that the phenomenon of galvanization caused by too low temperature is prevented, and the phenomena of plating leakage and poor stability of a zinc layer caused by too high temperature are prevented; the adjustment time is controlled to be 25-35 s, and the aim is to promote the C-rich behavior of austenite in the stage, improve the phase stability of residual austenite at room temperature, and simultaneously prevent martensite tempering softening, carbide precipitation and deterioration of the steel plate performance caused by overlong time.

(d) And then, the steel plate enters a zinc pot for galvanizing at the galvanizing temperature of 460-470 ℃ for 1-2 s, and then enters an alloying furnace for alloying galvanizing at the alloying temperature of 550-600 ℃, wherein the weight percentage of Fe in the alloying coating is 7.0-11.5%.

(e) Finishing: the alloyed steel plate is finished by a finishing machine, and the finishing elongation is 0.1-0.4%.

The final organization constitutes: ferrite (less than or equal to 10 percent), tempered martensite (75-83 percent), residual austenite (8-13 percent) and the balance of bainite structure.

The yield strength of the steel is above 900-1270 MPa, the tensile strength is above 1470MPa, the elongation is 15%, and the content of residual austenite is 8-13%.

The invention has the beneficial effects that:

(1) compared with other 1500MPa high-strength steel, the invention has lower alloy cost and no or little addition of noble alloys such as Cr, Mo, Cu, Ni and the like. The low C content and C equivalent design is beneficial to obtaining good resistance spot welding performance.

(2) The invention adopts the process idea of quenching distribution, adopts the tissue combination of tempered martensite, residual austenite, a small amount of ferrite and a small amount of bainite, ensures the strength of the steel plate through the formation of a large amount of martensite, and ensures the plasticity of the steel plate through the introduction of more than 10 percent of residual austenite.

(3) The yield strength of the steel is more than 900-1270 MPa, the tensile strength is more than 1470MPa, and the elongation is 15%. The higher plasticity shows the excellent drawing performance (higher than DP980) of the steel plate, and is suitable for cold-pressed formed parts. Provides the possibility of cold stamping of 1500 MPa-grade steel plates.

Drawings

FIG. 1 is a microstructure of an embodiment of the present invention.

FIG. 2 is a typical austenite morphology map of example 1 of the present invention.

FIG. 3 is a typical austenite morphology map of example 1 of the present invention.

FIG. 4 is a typical austenite morphology map of example 1 of the present invention.

Detailed Description

The present invention is further illustrated by the following examples.

According to the embodiment of the invention, smelting, casting, hot rolling, acid washing, cold rolling, alloying galvanization annealing and finishing are carried out according to the component proportion of the technical scheme.

(1) Hot rolling: the heating temperature is 1230-1280 ℃, the initial rolling temperature is 1100-1150 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 600-700 ℃;

(2) cold rolling: rolling reduction rate is 48% -52%;

(3) alloying galvanization annealing:

(a) heating and keeping constant temperature: heating the cold-rolled steel plate to 850-880 ℃ at the speed of 2-10 ℃/s, and keeping the temperature for 50-120 s;

(b) slow cooling: the slow cooling temperature is controlled to be 750-800 ℃;

(c) adjusting before quick cooling and galvanizing: rapidly cooling the slowly cooled steel plate to 250-300 ℃ at a cooling speed of more than 30 ℃/s, then heating to 450-470 ℃ at a heating speed of more than 10 ℃/s, and adjusting the time to 25-35 s before galvanization;

(d) then, the steel plate enters a zinc pot for galvanizing for 1-2 s, and then enters an alloying furnace for alloying galvanizing, wherein the alloying temperature is 550-600 ℃, and the weight percentage of Fe in the alloying coating is 7.0-11.5%;

(e) finishing: the finishing elongation is 0.1-0.4%.

Furthermore, in the steps (3) - (c) of quick cooling and adjusting before galvanizing, the microstructure of the steel plate after quick cooling contains 75% -83% of martensite and 10% -15% of austenite.

The compositions of the steels of the examples of the invention are shown in table 1. The main process parameters of the rolling of the steel of the embodiment of the invention are shown in Table 2. The main process parameters of the hot galvanizing annealing of the steel in the embodiment of the invention are shown in the table 3. The microstructure of the steels of the examples of the invention is shown in Table 4. The properties of the steels of the examples of the invention are shown in Table 5.

TABLE 2 Main Process parameters for Steel Rolling in the examples of the present invention

TABLE 3 main process parameters of hot galvanizing annealing of steel of the embodiment of the invention

TABLE 4 microstructure of inventive example steels

Examples	Ferrite/% of	Tempered martensite/%)	Retained austenite/%	Bainite/% of
					1	9.3	76.6	9.2	4.9
2	8.4	79.5	8.9	3.2
					3	8.7	80.2	9.5	1.6
4	7.3	82.6	11.2	2.9
					5	6.7	75.9	8.9	8.5
6	2.9	81.7	10.5	4.9
					7	0	82.7	8.2	9.1
8	9.4	75.5	9.9	5.2
					9	9.1	75.8	11.7	3.4
10	5.6	80.6	9.9	3.9
					11	6.4	80.9	10.8	1.9
12	4.6	82.7	11.5	1.2
					13	3.6	75.8	12.9	7.7
14	0	82.8	12.3	4.9
					15	0	80.0	10.3	4.7

TABLE 5 Properties of steels of examples of the invention

Examples	Rp0.2/MPa	Rm/MPa	A80/％	λ/％
					1	1127	1494	11.7	33.6
2	1155	1498	11.5	34.5
					3	1203	1532	10.6	33.5
4	1174	1518	10.4	34.3
					5	1162	1522	11.5	31.3
6	1216	1524	9.3	32.9
					7	1231	1532	10.8	32.1
8	1211	1521	10.8	35.7
					9	1179	1513	10.5	38.7
10	1153	1515	8.6	31.9
					11	1177	1517	10.6	33.9
12	1196	1518	10.2	31.2
					13	1222	1533	8.9	37.6
14	1251	1526	9.8	32.5
					15	1237	1507	10.1	41.3

From the above, the yield strength is 900 to 1270MPa or more, the tensile strength is 1470MPa or more, and the elongation is 15%. The higher plasticity shows the excellent drawing performance (higher than DP980) of the steel plate, and is suitable for cold-pressed formed parts. Provides the possibility of cold stamping of 1500 MPa-grade steel plates.

In order to express the present invention, the above embodiments are properly and fully described by way of examples, and the above embodiments are only used for illustrating the present invention and not for limiting the present invention, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the present invention, and any modifications, equivalent substitutions, improvements, etc. made by the persons skilled in the relevant art should be included in the protection scope of the present invention, and the protection scope of the present invention should be defined by the claims.