1. A high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate is characterized in that the steel plate comprises the following components by weight percent: c: 0.16-0.25%, Mn: 1.8% -2.6%, Si: 0.2-0.8%, Al: 0.50-1.5%, 0.10-0.60% of Cr, 0.10-0.70% of Cu, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.003% of N, Nb: 0.005% -0.15%, V: 0.005-0.15%, Ti: 0.005 to 0.15 percent, and the balance of Fe and inevitable impurities.

2. The high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate according to claim 1, wherein the steel plate contains 1.0% to 2.0% of Si + Al, and Al and Si (1-3): 1.

3. The high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate according to claim 1, wherein the microstructure of the steel plate is ferrite + martensite + residual austenite + bainite, and the steel plate structure comprises the following components in percentage by volume: 10 to 35 percent of ferrite, 40 to 80 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite; the residual austenite is blocky, the grain size is between 0.05 and 0.5 mu m, and the grain size is distributed at the interface of martensite/ferrite and in the ferrite.

4. The high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate according to claim 1, wherein the yield strength of the steel plate is 850-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after A80 fracture is more than or equal to 14%, and the hole expansion rate is more than or equal to 20%; the fatigue limit strength is more than or equal to 600 MPa.

5. A method for preparing the high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate of any one of claims 1 to 4, comprising smelting, medium thin slab continuous casting, hot continuous rolling, pickling cold rolling and continuous hot dip galvanizing; the method is characterized in that:

(1) smelting: the temperature of the converter smelting molten steel is between 1600 and 1700 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1550-1600 ℃, and the thickness of the continuous casting billet is 110-150 mm;

(3) hot continuous rolling: the charging temperature of the casting blank is 450-700 ℃, the heating temperature is 1170-1300 ℃, the initial rolling temperature is 1020-1160 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 600-700 ℃;

(4) acid pickling and cold rolling: the cold rolling reduction rate is 40-70%;

(5) continuous hot galvanizing: the preheating temperature is controlled to be 300-550 ℃, the annealing temperature is 780-880 ℃, the annealing time is 30-300 s, and the annealing is slowly cooled to 700-760 ℃; then rapidly cooling, wherein the rapid cooling rate is more than 30 ℃/s, the rapid cooling temperature is 450-470 ℃, the galvanizing temperature is 450-470 ℃, the strip steel is cooled to 400-420 ℃ after galvanizing is finished, then air cooling is adopted, and the temperature of a top roller of a cooling tower is controlled to be 250-300 ℃;

(6) finishing: the finishing elongation is 0.3-0.7%.

6. The method for preparing the high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate according to the claim 4, wherein in the step (3) of hot continuous rolling, the microstructure of the hot-rolled steel plate is ferrite + pearlite + bainite + cementite, and the microstructure of the steel plate is calculated by the volume percentage: 30-60% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite; the grain size of the hot rolled product is more than 6.0 grade.

Background

In recent years, with the higher and higher requirements of the automotive industry on the formability index of materials, the traditional dual-phase steel has difficulty in meeting the requirements of complex stamping parts on high ductility, and the TRIP steel has limited the wide use due to the expensive production cost caused by high alloy content. The existing DH steel introduces a certain amount of residual austenite into the traditional dual-phase steel, so that the material shows excellent forming performance through the TRIP effect, the defects of DP steel and TRIP steel in the application process can be obviously overcome, and the DH steel is just one of the research hotspots in the field of automobile steel development at present. However, the ultrahigh-strength steel key parts are often subjected to cyclic loading during service, so that the local load is higher than the yield strength of the material, and the fatigue failure phenomenon occurs. Since this phenomenon seriously affects the normal service of parts, the fatigue failure problem of ultra-high strength steel is of great concern to manufacturers and users. Based on the current research situation, the problems of poor formability and fatigue failure of the ultrahigh-strength automobile steel are urgently needed to be solved, so that the invention aims to develop the novel 1.2 GPa-grade ultrahigh-strength cold-rolled hot-galvanized automobile steel with high plasticity and fatigue resistance.

Patent document CN 108642379 a discloses a cold-rolled dual-phase steel with 1200 MPa-level tensile strength and a preparation method thereof, and the steel comprises the following main chemical components: 0.09-0.13% of C, 0.1-0.4% of Si, 2.0-2.6% of Mn, P: less than or equal to 0.01 percent, S: less than or equal to 0.01 percent, Al: 0.02-0.06%, Cr: 0.3-0.6%, Mo: 0.1-0.3%, Nb: 0.01-0.04%, Ti: 0.01-0.04%, B: 0.001-0.003%, and the balance of Fe and inevitable impurities. The invention adopts a cold rolling-continuous annealing production process to produce the cold-rolled dual-phase steel with the strength level of 1200MPa, the ductility of the product is very poor, the elongation after fracture is only about 7 percent, and the forming problems such as stamping cracking and the like and the service risks such as fatigue failure and the like are very easy to occur in the practical application process.

Patent document CN 109207841 a discloses a low-cost high-formability 1180MPa grade cold-rolled annealed dual-phase steel plate and a manufacturing method thereof, wherein the main chemical components are as follows: c: 0.1% -0.125%, Si: 0.4-0.8%, Mn: 2.6% -2.9%, Al: 0.01 to 0.05 percent, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and the balance of Fe and other unavoidable impurities. The cold-rolled dual-phase steel with the strength level of 1200MPa produced by the cold-rolling-continuous annealing production process has poor plasticity, and is difficult to meet the requirements of stamping and forming of complex parts, long service life, high reliability, service and the like.

Disclosure of Invention

In view of the above problems in the prior art, the present invention is directed to developing a cold-rolled hot-dip galvanized DH1180 steel sheet having excellent overall performance and high plastic fatigue resistance, and a method for manufacturing the same.

The purpose of the invention is realized as follows:

a high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate comprises the following components in percentage by weight: c: 0.16-0.25%, Mn: 1.8% -2.6%, Si: 0.2-0.8%, Al: 0.50-1.5%, 0.10-0.60% of Cr, 0.10-0.70% of Cu, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.003% of N, Nb: 0.005% -0.15%, V: 0.005-0.15%, Ti: 0.005 to 0.15 percent, and the balance of Fe and inevitable impurities.

Furthermore, the steel plate contains 1.0% to 2.0% of Si and Al, and the ratio of Al to Si is (1-3) to 1.

Further, the microstructure of the steel plate is a ferrite structure, a martensite structure, a retained austenite structure and a small amount of bainite structure, and the steel plate structure comprises the following components in percentage by volume: 10 to 35 percent of ferrite, 40 to 80 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite; the residual austenite is blocky, the grain size is between 0.05 and 0.5 mu m, and the grain size is distributed at the interface of martensite/ferrite and in the ferrite.

The yield strength of the steel is 850-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after breakage of A80 is more than or equal to 14%, and the hole expansion rate is more than or equal to 20%; the fatigue limit strength is more than or equal to 600 MPa; the requirements of high strength, high plasticity, high hole expansion and fatigue resistance of the automobile are met.

The reason for the alloy design of the present invention is as follows:

c: the carbon element guarantees the strength requirement of the steel through solid solution strengthening, and free carbon can play a good stabilizing effect on austenite, so that the forming performance of the steel is improved. The content of the element C is too low, so that a proper amount of residual austenite is difficult to obtain, and the mechanical property index of the steel in the invention cannot be met; too high a content can embrittle the steel, not only increasing the cost, but also being prone to fatigue failure risks. Therefore, the content of the C element is controlled to be 0.16-0.25 percent in the invention.

Mn: manganese is an austenite stabilizing element in steel, can expand an austenite phase region, reduce the critical quenching speed of the steel, and can refine grains, thereby being beneficial to solid solution strengthening to improve the strength. The content of Mn element is too low, the super-cooled austenite is not stable enough, and the plasticity, the toughness and other processing performances of the steel plate are reduced; the excessively high content of the Mn element causes deterioration in the weldability of the steel sheet, and increases in the production cost, which is not favorable for industrial production. Therefore, the content of the Mn element is controlled to be 1.8-2.6% in the invention, so that the steel of the invention contains the retained austenite with the proportion not higher than 10%.

Si: the silicon element has a certain solid solution strengthening effect in ferrite, so that the steel has enough strength, and meanwhile, the Si can inhibit the decomposition of residual austenite and the precipitation of carbide, thereby reducing the inclusion in the steel. The Si element content is too low to play a role in strengthening; the Si element content is too high, and compact Mn is easily formed on the surface of the steel plate₂SiO₄An oxide layer, which reduces the galvanized surface quality of the steel sheet. Therefore, the content of the Si element is controlled to be 0.2 to 0.8 percent in the invention.

Al: the aluminum element contributes to deoxidation of molten steel and can also suppress decomposition of residual austenite and precipitation of carbide. Too high content of Al element not only increases production cost, but also causes difficulties in continuous casting production, etc. Therefore, the content of the Al element is controlled within the range of 0.50 to 1.5 percent in the invention. In addition, the ductility and the fatigue resistance of the product are obviously improved through the synergistic effect between Si and Al, on one hand, the percentage content of Si and Al is controlled, and the percentage content of Si and Al is controlled to be 1.0-2.0 percent; on the other hand, the mass percentage ratio of Al/Si is limited as follows: si is 1: 1-3: 1.

Cr: the chromium element can increase the hardenability of the steel to ensure the strength of the steel and can stabilize the retained austenite to improve the forming and the ductility of the product, the hardenability of the steel is influenced by too low Cr content, and the production cost is increased by too high Cr content. Therefore, the content of Cr element is controlled within the range of 0.10-0.60% in the present invention.

Cu: the Cu element is a solid solution strengthening element, so that the hardenability of steel can be improved, the thermodynamic stability of austenite can be effectively improved, the stable residual austenite at room temperature can be formed, and the plasticity and fatigue resistance of the material can be improved; however, considering that the cost of the copper element is high, the content of the Cu element is controlled to be 0.10-0.70 percent.

P: the P element is a harmful element in steel, seriously reduces the plasticity and the deformability of the steel, and the lower the content, the better the content. In the invention, the content of the P element is controlled to be less than or equal to 0.01 percent in consideration of the cost.

S: the S element is a harmful element in steel, seriously affects the formability of steel, and the lower the content, the better the formability. In the invention, the content of the S element is controlled to be less than or equal to 0.01 percent in consideration of the cost.

N: the N element is easy to react with Ti to separate out TiN large particles, and the TiN large particles serve as crack sources in the deformation process and are unfavorable for resisting hydrogen brittleness, so that the content of the N element in the steel needs to be strictly controlled. The invention controls the content of N to be less than or equal to 0.003 percent.

Nb: the microalloy element Nb forms a compound with carbon and nitrogen, is favorable for delaying recrystallization of the material in the hot rolling process, has the functions of refining the grain size, and obviously improving the obdurability and the anti-fatigue failure effect of the material, and in the invention, the content of the Nb element is controlled to be 0.005-0.15 percent.

V: the microalloying element vanadium mainly exists in a VC form, the strength and the fatigue resistance of the material are improved through fine grain strengthening and dispersion strengthening, and undissolved VC particles can pin ferrite grain boundaries in the hot galvanizing annealing heating process, so that the effect of refining grains is achieved; when the annealing temperature is increased to a two-phase region, the VC is low in dissolving temperature, so that the VC is fully dissolved in a matrix, and solid-solution C atoms are enriched into austenite to improve the stability of the VC; during the annealing process, VC in the ferrite is separated out again, thereby producing obvious precipitation strengthening. Therefore, in the present invention, the content of V element is controlled to be 0.005% to 0.15%.

Ti: the small amount of Ti element can refine the grain size, the precipitate can pin dislocation to delay the expansion of crack source and improve the toughness of the material obviously, and in the invention, the Ti element content is controlled at 0.005-0.15%.

The invention also provides a preparation method of the high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate, which is characterized by comprising the following steps: smelting, continuous casting of medium and thin slabs, hot continuous rolling, acid pickling and cold rolling, and continuous hot galvanizing. The preparation process comprises the following specific steps:

(1) smelting: smelting by a converter to obtain molten steel meeting the following component requirements in percentage by mass, C: 0.16-0.25%, Mn: 1.8% -2.6%, Si: 0.2-0.8%, Al: 0.50-1.5%, 0.10-0.60% of Cr, 0.10-0.70% of Cu, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.003% of N, Nb: 0.005% -0.15%, V: 0.005-0.15%, Ti: 0.005-0.15 percent, and the balance of Fe and inevitable impurities, wherein the temperature of molten steel is between 1600 and 1700 ℃.

(2) Continuous casting of medium and thin slabs: the casting temperature is 1550-1600 ℃, and the thickness of the continuous casting billet is 110-150 mm.

(3) Hot continuous rolling: the charging temperature of a casting blank is 450-700 ℃, the heating temperature is 1170-1300 ℃, the initial rolling temperature is 1020-1160 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 600-700 ℃; the thickness of the hot rolled coil is between 2 and 4 mm.

The microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite, and the steel plate structure comprises the following components in percentage by volume: 30-60% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite; the grain size of the hot rolled product is more than 6.0 grade.

(4) Acid pickling and cold rolling: the iron scale on the surface of the steel coil is removed by acid liquor before cold rolling, and the cold rolling reduction rate is 40-70%. The rolling reduction is too high, so that the deformation resistance is too high, and the rolling is difficult to reach the target thickness; the reduction ratio is too low, resulting in a decrease in the elongation of the cold-rolled steel sheet.

(5) Hot galvanizing: the preheating temperature is controlled to be 300-550 ℃, the annealing temperature is 780-880 ℃, the annealing time is 30-300 s, and the annealing is slowly cooled to 700-760 ℃; then rapidly cooling, wherein the rapid cooling rate is more than 30 ℃/s, the rapid cooling temperature is 450-470 ℃, the galvanizing temperature is 450-470 ℃, the strip steel is subjected to air knife cooling to 400-420 ℃ after the galvanizing is finished, then air cooling is adopted, and the temperature of a top roller of a cooling tower is controlled to be 250-300 ℃;

the annealing temperature is 780-880 ℃, and if the annealing temperature is too high, the ductility of the steel is reduced due to complete austenitizing and insufficient ferrite proportion; if the annealing temperature is too low, the proportion of soft phase ferrite in the final material is too high, which may significantly reduce the strength of the material. The annealing time is 30-300 s, if the annealing time is too long, the grains of the steel plate are coarse, the annealing time is too short, and the elongation of the steel plate is reduced because the steel plate does not finish the annealing and recrystallization processes in an urgent manner; the rapid cooling temperature is 450-470 ℃, the temperature close to the temperature of the zinc pot is convenient for galvanizing, the strip steel is cooled to 400-420 ℃ after being taken out of the zinc pot, the normal solidification of the surface of a zinc layer is ensured, and the flowing of zinc liquid is prevented; the temperature of the cooling tower top roller is controlled to be 250-300 ℃, so that the zinc layer can be completely solidified, and the influence on the quality of a coating layer caused by roller adhesion is avoided.

(6) Finishing: the finishing elongation is controlled within the range of 0.3-0.7%.

The microstructure of the cold-rolled hot-dip galvanized product is ferrite, martensite, residual austenite and a small amount of bainite, and the steel plate structure is as follows according to volume percentage: 10 to 35 percent of ferrite, 40 to 80 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite; meanwhile, the retained austenite in the product is blocky, the grain size is between 0.05 and 0.5 mu m, and the retained austenite is mainly distributed at the interface of martensite/ferrite and in the ferrite.

The yield strength of 850-1100 MPa, the tensile strength of 1180-1300 MPa, the elongation after breakage of A80 is more than or equal to 14%, and the hole expansion rate is more than or equal to 20%; a cold-rolled hot-galvanized DH1180 steel plate with the fatigue limit strength of more than or equal to 600 MPa.

The invention has the beneficial effects that:

(1) the invention adopts the production process of converter smelting, medium and thin slab continuous casting, hot continuous rolling, acid pickling and cold rolling and continuous hot dip galvanizing, can realize the industrial production of DH1180 hot dip galvanizing products on the traditional cold rolling hot dip galvanizing dual-phase steel production line, and has the advantages of low cost, no need of adding new production equipment and stable production process.

(2) The DH1180 hot-dip galvanized product produced by the method introduces residual austenite and a small amount of bainite on the basis of the traditional cold-rolled hot-dip galvanized dual-phase steel, and realizes the characteristics of excellent welding performance, high strength and high plasticity under the coupling action of transformation induced plasticity (TRIP) effect assistance and bainite coordinated deformation.

(3) The DH1180 hot-dip galvanized product produced by the method can realize longitudinal sampling yield strength of 850-1100 MPa, tensile strength of 1180-1300 MPa, elongation after breakage of A80 of more than or equal to 14%, hole expansion rate of more than or equal to 20%, thickness specification of 0.8-1.6 mm and fatigue limit strength of more than or equal to 600 MPa.

(4) The structure of the finished steel plate comprises 10-35% of ferrite, 40-80% of martensite, 3-12% of residual austenite and 3-15% of bainite according to volume percentage.

(5) The retained austenite in the product of the invention is blocky, the grain size is between 0.05um and 0.5um, and the retained austenite is mainly distributed at the martensite/ferrite interface and in the ferrite.

Drawings

FIG. 1 is a metallographic microstructure of a steel sheet of example 1;

FIG. 2 is an engineering stress-strain curve of example 1.

Detailed Description

The present invention is further illustrated by the following examples.

According to the embodiment of the invention, smelting, medium and thin slab continuous casting, hot continuous rolling, acid pickling and cold rolling and continuous hot galvanizing are carried out according to the component proportion of the technical scheme.

(1) Smelting: the temperature of the converter smelting molten steel is between 1600 and 1700 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1550-1600 ℃, and the thickness of the continuous casting billet is 110-150 mm;

(3) hot continuous rolling: the charging temperature of the casting blank is 450-700 ℃, the heating temperature is 1170-1300 ℃, the initial rolling temperature is 1020-1160 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 600-700 ℃; the microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite, and the steel plate structure comprises the following components in percentage by volume: 30-60% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite; the grain size of the hot rolled product is more than 6.0 grade.

(4) Acid pickling and cold rolling: the cold rolling reduction rate is 40-70%;

(5) hot galvanizing: the preheating temperature is controlled to be 300-550 ℃, the annealing temperature is 780-880 ℃, the annealing time is 30-300 s, and the annealing is slowly cooled to 700-760 ℃; then rapidly cooling, wherein the rapid cooling rate is more than 30 ℃/s, the rapid cooling temperature is 450-470 ℃, the galvanizing temperature is 450-470 ℃, the strip steel is cooled to 400-420 ℃ after galvanizing is finished, then air cooling is adopted, and the temperature of a top roller of a cooling tower is controlled to be 250-300 ℃;

(6) finishing: the finishing elongation is 0.3-0.7%.

The compositions of the steels of the examples of the invention are shown in table 1. The main process parameters of the steel continuous casting and hot rolling of the embodiment of the invention are shown in Table 2. The main process parameters of the steel continuous hot galvanizing of the embodiment of the invention are shown in the table 3. The structure of the steel of the examples of the present invention is shown in Table 4. The overall properties of the examples of the invention are shown in Table 5.

TABLE 1 composition (wt%) of steels of examples of the present invention

Examples	C	Mn	Si	Al	Cr	Cu	P
								1	0.21	2.35	0.48	0.57	0.26	0.35	0.004
2	0.22	2.16	0.36	0.84	0.36	0.22	0.003
								3	0.19	2.44	0.65	0.76	0.26	0.51	0.004
4	0.20	2.18	0.35	0.83	0.45	0.18	0.005
								5	0.24	2.06	0.29	0.78	0.16	0.63	0.005
6	0.16	2.35	0.46	1.15	0.56	0.38	0.004
								7	0.25	1.85	0.35	0.88	0.15	0.24	0.006
8	0.18	2.54	0.78	0.85	0.37	0.62	0.002
								9	0.19	2.47	0.62	0.66	0.28	0.37	0.004
10	0.21	2.22	0.65	0.72	0.45	0.22	0.001
								11	0.20	2.31	0.58	0.87	0.18	0.30	0.003
12	0.17	1.98	0.64	1.34	0.25	0.24	0.004
								13	0.23	2.02	0.41	1.08	0.14	0.42	0.001
14	0.18	2.48	0.62	1.16	0.25	0.55	0.003
								15	0.19	2.30	0.74	1.22	0.52	0.17	0.004
Examples	S	N	Nb	V	Ti	Si+Al	Al:Si
								1	0.003	0.001	0.012	0.065	0.005	1.05	1.19
2	0.002	0.002	0.086	0.025	0.013	1.20	2.33
								3	0.001	0.001	0.022	0.008	0.122	1.41	1.17
4	0.003	0002	0.016	0.116	0.023	1.18	2.37
								5	0.003	0.002	0.015	0.055	0.111	1.07	2.69
6	0.005	0.001	0.095	0.007	0.024	1.41	2.50
								7	0.002	0.001	0.013	0.127	0.008	1.23	2.51
8	0.003	0.002	0.105	0.044	0.052	1.63	1.09
								9	0.003	0.001	0.016	0.020	0.044	1.28	1.06
10	0.002	0.001	0.008	0.009	0.120	1.37	1.11
								11	0.005	0.001	0.106	0.024	0.042	1.45	1.50
12	0.004	0.002	0.038	0.066	0.034	1.68	2.09
								13	0.005	0.001	0.079	0.007	0.076	1.49	2.63
14	0.003	0.002	0.054	0.028	0.022	1.78	1.87
								15	0.005	0.002	0.035	0.088	0.008	1.96	1.65

TABLE 2 Main Process parameters for continuous casting and Hot Rolling of steels according to examples of the invention

TABLE 3 Main Process parameters of the continuous hot galvanizing of the steel of the examples of the present invention

TABLE 4 Structure of inventive example steels

TABLE 5 comprehensive Properties of examples of the invention

Note: 1180MPa grade high-strength steel fatigue strength sigma_-1(yield strength + tensile strength) × 0.23 ═ 850+1180 ═ 0.23 ═ 467 MPa; if the actually measured high cycle fatigue limit strength of the high-strength steel is far higher than that of the high-strength steelσ_-1The material is excellent in fatigue resistance.

The embodiment shows that the cold-rolled hot-galvanized DH1180 steel plate has the performance that the yield strength is 850-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after breakage of A80 is more than or equal to 14%, the hole expansion rate is more than or equal to 20%, the thickness specification is 0.8-1.6 mm, and the fatigue limit strength is more than or equal to 600 MPa; meet the performance requirements of high strength, high plasticity, high hole expansion rate, fatigue resistance and the like of the steel for the automobile.

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.