High-toughness extra-thick steel plate and preparation method thereof

Abstract

The invention relates to a preparation method of a high-strength and high-toughness super-thick steel plate, the preparation method adopts a certain chemical component proportion for smelting, continuous casting and rolling to obtain the super-thick steel plate with the thickness of more than or equal to 80mm, and the super-thick steel plate comprises the following chemical components in percentage by mass: 0.15-0.18% of C, 0.6-0.8% of Mn, 0.04-0.09% of V, 0.001-0.01% of Ti, 2.5-3.0% of Ni, 0.2-0.4% of Mo, 0.001-0.002% of B and the balance of Fe and inevitable impurities. And heating the super-thick steel plate to 850-880 DEG C along with a furnace, preserving heat for 90-120 minutes, and then carrying out primary water quenching and secondary water quenching treatment to obtain the high-toughness super-thick steel plate with the austenite grain structure. Through low-cost alloy design, two times of water quenching and other heat treatment processes, the mechanical properties of the super-thick steel plate are greatly improved, specifically, the yield strength is larger than 960 MPa, the tensile strength is larger than 1050 MPa, the ductility is larger than 12%, the yield ratio is smaller than or equal to 0.95, the transverse impact energy at the temperature of-60 DEG C is larger than or equal to 100 J, and the longitudinal impact energy at the temperature of-60 DEG C is larger than or equal to 150 J.

Description

Technical Field

[0001] The invention relates to the technical field of iron and steel metallurgy, and in particular to a high-strength and toughness extra-thick steel plate and a preparation method thereof. Background Art

[0002] As marine engineering equipment continues to develop in the direction of large-scale, the thickness of steel plates continues to increase. In order to ensure the service safety of marine engineering equipment, the market has also put forward higher requirements for the strength of extra-thick plates, which need to meet low yield strength ratio, good low-temperature toughness, excellent tensile properties and welding properties. Offshore platforms need to support hundreds of tons of drilling equipment, and the complex marine environment in which they are located determines the high strength and large thickness of offshore platform steel. Therefore, the demand for this type of steel in the marine engineering industry is becoming increasingly strong.

[0003] In recent years, my country has made great progress in the production of steel for marine engineering equipment. Domestic high-strength ship plates and offshore platform steel plates are widely used in my country's offshore oil projects. However, China still relies on imports for thick and high-strength steel plates, which limits my country's ability to independently exploit marine resources. At present, there are two major problems with 960MPa-grade high-strength and tough steel plates: high content of Ni, Mo, and Cu alloy elements, which leads to increased overall costs, and in order to ensure strength and hardenability, more Mn elements are added, which will cause segregation problems in thick plates; and many 960MPa-grade high-strength and tough steel plates are not thick enough, and as the thickness increases, their low-temperature impact toughness is poor. 960MPa-grade steel plates with both low cost and high strength and toughness still need to be further developed.

[0004] In the prior art, patent CN116676541A discloses a low-cost 960MPa ultra-high strength steel plate, which improves toughness through specific cost design and heat treatment process, and the impact energy at -40℃ is ≥150J in the horizontal direction and ≥200J in the vertical direction. The thickness of the steel plate produced by this patented technology is 6 to 12mm, which is far from meeting the requirements of extra-thick steel plates. Patent CN110863143A discloses a method for manufacturing 960MPa ultra-high strength steel with excellent low-temperature toughness, and its -60℃ Charpy impact energy can reach 230J, but the thickness of the patented steel plate is limited by the composition and production conditions, and is only 10mm, and the Mn element content is 3.0%, which does not meet the index requirements of 960MPa marine engineering steel. Patent CN114134419A discloses a high-strength steel plate with a yield strength of 960MPa and a manufacturing method thereof, with a yield strength of 980-1060MPa and a lateral Charpy impact energy of 60-180J at -40℃. However, the thickness of the steel plate in the patent is limited by the composition and production conditions and is only 10-30mm. None of the above patents proposes a targeted solution for high-strength and tough extra-thick steel plates.

[0005] In view of the above problems, the present invention is proposed. Summary of the invention

[0006] The invention discloses a high-strength and toughness extra-thick steel plate and a preparation method thereof, aiming to solve the technical problems existing in the prior art.

[0007] The present invention adopts the following technical solutions:

[0008] In one aspect, the present invention provides a method for preparing a high-strength and tough extra-thick steel plate, comprising the following steps:

[0009] (1) The chemical composition ratio of a high-strength and tough extra-thick steel plate is used for smelting, continuous casting and rolling to obtain an extra-thick steel plate with a thickness of ≥80 mm. The chemical composition of the extra-thick steel plate, in terms of mass%, contains C: 0.15-0.18%, Mn: 0.6-0.8%, V: 0.04-0.09%, Ti: 0.001-0.01%, Ni: 2.5-3.0%, Mo: 0.2-0.4%, B: 0.001-0.002%, and the rest is Fe and unavoidable impurities;

[0010] (2) performing a heat treatment process on the extra-thick steel plate in step (1), including heating the extra-thick steel plate to 850° C. to 880° C. in a furnace, keeping the temperature for 90 min to 120 min, and then performing a primary water quenching and a secondary water quenching to obtain a high-strength and tough extra-thick steel plate;

[0011] The water inlet temperature of the primary water quenching is 850℃~880℃, and the final cooling temperature is 400℃~450℃;

[0012] The water inlet temperature of the secondary water quenching is 500℃~550℃, and the final cooling temperature is room temperature;

[0013] High strength and toughness extra thick steel plate has an austenite grain structure.

[0014] As a preferred technical solution, the heat treatment process also includes tempering treatment, the tempering treatment temperature is 500°C to 620°C, and the insulation time is ≤40min.

[0015] As a preferred technical solution, the tempering treatment is performed after the secondary water quenching treatment.

[0016] As a preferred technical solution, the high-strength and toughness extra-thick steel plate has a microstructure of nano-cementite and VC, and the nano-cementite and VC are dispersedly distributed.

[0017] As a preferred technical solution, the size of nano cementite is less than 90 nm, and the size of VC is less than 10 nm.

[0018] As a preferred technical solution, the mechanical properties of the high-strength and tough extra-thick steel plate meet the following requirements: yield strength > 960MPa, tensile strength > 1050MPa, elongation > 12%, yield strength ratio ≤ 0.95, -60°C transverse impact energy ≥ 100J, -60°C longitudinal impact energy ≥ 150J.

[0019] As a preferred technical solution, the thickness of the continuous casting embryo obtained after continuous casting is ≥230mm.

[0020] On the other hand, the present invention also provides a high-strength and toughness extra-thick steel plate, which is prepared using the above-mentioned preparation method of the high-strength and toughness extra-thick steel plate.

[0021] As a preferred technical solution, the average grain size of austenite in the high-strength and toughness extra-thick steel plate is less than 15 μm.

[0022] The technical solution adopted by the present invention can achieve at least one of the following beneficial effects:

[0023] 1. The present invention obtains fine austenite grains (average size <15 μm) through low-cost alloy design and heat treatment processes such as double water quenching, and simultaneously obtains 100 nm cementite and less than 10 nm VC precipitation, thereby preparing a high-strength and tough extra-thick steel plate with a thickness of ≥80 mm, and its mechanical properties meet the following requirements: yield strength>960 MPa, tensile strength>1050 MPa, elongation>12%, yield strength ratio≤0.95, -60°C transverse impact energy≥100 J, -60°C longitudinal impact energy≥150 J.

[0024] 2. The steel plate of the present invention has a simple chemical composition and low cost. The addition of alloy elements such as Ni ensures toughness and improves hardenability. The low C and low Mn composition design reduces the segregation phenomenon in the center of the extra-thick plate. The composition adjustment by removing the Cr element refines the size of the carbide after tempering.

[0025] 3. The present invention adopts a quenching and tempering heat treatment process, which is simple and easy to implement. After the heat treatment, the carbides inside the steel plate are small in size and dispersedly distributed, and the overall cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments, which constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions explain the present invention and do not constitute improper limitations on the present invention. In the drawings:

[0027] Figure 1 EBSD diagram of Example 1 of the present invention;

[0028] Figure 2 This is a TEM image of Example 1 of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention and the corresponding drawings. In the description of the present invention, it should be noted that the term "or" is usually used in the sense of including "and / or", unless the content clearly indicates otherwise.

[0030] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] The object of the present invention is to provide a high-strength and toughness extra-thick steel plate, in particular to a FH960 grade high-strength and toughness extra-thick plate, which can not only meet the performance requirements of steel for marine engineering equipment, but also reduce the production cost of the material.

[0032] In order to achieve the above-mentioned purpose, the present invention proposes an innovative technical solution, including the use of Ni-containing, low C, low Mn and Cr-free composition design and a specific heat treatment process.

[0033] Among them, the reasons for the composition design (in mass %) are as follows:

[0034] C: 0.15~0.18%

[0035] Carbon is the main element that plays a role in solid solution strengthening. It can significantly improve hardenability and strength. During the tempering process, carbon can form carbides with other alloy elements to improve strength through precipitation strengthening. When the carbon content is low, the hardenability and strength of the steel plate are difficult to guarantee. However, when the carbon content is too high, the toughness of the steel plate is difficult to guarantee, and too much carbon will also cause segregation, leading to the formation of banded structure. Therefore, the composition design is considered to be low in carbon.

[0036] Mn: 0.6~0.8%

[0037] The main function of manganese is to improve hardenability. In addition, adding manganese to steel can improve the strength of steel by solid solution strengthening. However, if the manganese content is too high, the cracking tendency of the slab will increase, and high concentrations of manganese will easily lead to segregation and produce banded structures, while lower manganese content will contribute less to strength. And Mn has little effect on carbides.

[0038] V: 0.04~0.09%

[0039] On the one hand, V hinders the growth of austenite grains during heating and plays a role in refining grains; on the other hand, V will be dispersed in the steel in the form of carbides to improve the strength and toughness of the steel. When the addition amount is less than 0.04%, the precipitation strengthening effect cannot meet the mechanical property requirements. In order to refine the austenite grain size, it is controlled at 0.04-0.09%.

[0040] Ti: 0.001~0.01%

[0041] Ti is a strong carbide-forming element. Precipitates containing titanium have strong bonding strength, are stable, and are not easy to decompose. For example, the dissolution temperature of TiC is very high, and it will exist in the steel in the form of a precipitate phase, pinning the grain boundary and inhibiting the growth of austenite. In order to refine the austenite grain size, the Ti content of the present invention is controlled at 0.001-0.01%.

[0042] Ni: 2.5-3.0%

[0043] The main function of Ni is to improve the toughness of the matrix, especially the low-temperature toughness of steel. Since nickel exists in the iron matrix in the form of substitutional solid solution, it plays a certain role in solid solution strengthening of steel, which improves the yield strength of steel. Ni has little effect on carbides and toughness. In order to ensure low-temperature toughness, the Ni content is controlled at 2.5-3.0%.

[0044] Mo: 0.2~0.4%

[0045] Mo can play a role in solid solution strengthening in steel, and can improve the stability of carbides, thereby improving the strength of steel after tempering. The presence of Mo element can significantly improve the hardenability of steel, and Mo element is a medium-strong carbide forming element. It will precipitate in the form of carbides during the tempering process, thereby improving the strength of steel after tempering. And Mo can also inhibit the precipitation of impurity elements such as P, S and some precipitated phases at the grain boundary, thereby improving the toughness of steel. However, due to the consideration of removing the Cr element in the composition design, and the high Mo content will lead to increased costs, so the Mo content is controlled at 0.2-0.4%.

[0046] B: 0.001~0.002%

[0047] The B element can significantly improve the hardenability of the extra thick plate. The solubility of B in austenite is not high, and it is generally enriched at the austenite grain boundary, which reduces the free energy at the grain boundary, reduces the position of ferrite nucleation at the grain boundary, and inhibits ferrite formation, thereby improving hardenability. In general, as long as a small amount of B element (about 5-30ppm) is added, the hardenability of the steel can be greatly improved. However, too much B content will form carbides and nitrides at the grain boundary, deteriorating performance. Therefore, in order to improve hardenability, the B content is limited to 0.001-0.002% in the present invention.

[0048] Cr removal treatment

[0049] Chromium is a strong carbide-forming element. Therefore, in addition to playing a certain role in solid solution strengthening and improving hardenability, the addition of chromium to steel can also be dissolved in Fe3C to form alloy cementite during tempering, and also in the form of M7C3 and M 23 C6 carbides are precipitated to ensure the strength after tempering. However, too much Cr will accelerate the coarsening of internal M3C carbides, affecting low-temperature impact toughness. Generally, chromium is added to FH960 grade marine steel, but the present invention attempts to remove Cr to reduce the deterioration of large-sized carbides on low-temperature impact toughness. (Compared with steel containing Cr, the adverse effects of Cr are emphasized)

[0050] Embodiment 1:

[0051] This example was prepared according to the following steps:

[0052] (1) Smelting, continuous casting and rolling are performed according to a certain chemical composition ratio to obtain an extra-thick steel plate with a thickness of 80 mm, wherein the thickness of the continuous casting blank obtained after continuous casting is 230 mm;

[0053] The chemical composition of the above-mentioned extra thick steel plate, measured by mass%, contains C: 0.17%, Mn: 0.7%, V: 0.04%, Ti: 0.008%, Ni: 2.9%, Mo: 0.3%, B: 0.001%, and the rest is Fe and unavoidable impurities.

[0054] (2) heat treating the extra thick steel plate in step (1), including heating the extra thick steel plate to 860°C and keeping it at this temperature for 180 minutes. The main purpose of keeping it at this temperature is to obtain fine original austenite grains, and the average grain size of the original austenite is 13 μm. Subsequently, water quenching once, water quenching twice, and tempering after water quenching are performed to obtain the high-strength and tough extra thick steel plate;

[0055] Among them, the water inlet temperature of the first water quenching is 860℃, and the final cooling temperature is 430℃; the water inlet temperature of the second water quenching is 520℃, and the final cooling temperature is room temperature;

[0056] Tempering treatment: the tempering temperature is 500℃ and the holding time is 30min. The main purpose of this process is to improve toughness by tempering while obtaining cementite of 62.6nm and VC precipitation of less than 7.2nm, and obtaining high strength and high toughness.

[0057] The production process parameters of this embodiment are shown in the table:

[0058]

[0059] Sampling tests were carried out on the above steel plates, and the mechanical properties test results are shown in the following table:

[0060]

[0061] Embodiment 2:

[0062] This example was prepared according to the following steps:

[0063] (1) Smelting, continuous casting and rolling are performed according to a certain chemical composition ratio to obtain an extra-thick steel plate with a thickness of 80 mm, wherein the thickness of the continuous casting blank obtained after continuous casting is 230 mm;

[0064] The chemical composition of the above-mentioned extra thick steel plate, measured by mass%, contains C: 0.17%, Mn: 0.7%, V: 0.04%, Ti: 0.008%, Ni: 2.9%, Mo: 0.3%, B: 0.001%, and the rest is Fe and unavoidable impurities.

[0065] (2) heat treating the extra thick steel plate in step (1), including heating the extra thick steel plate to 860°C in a furnace and keeping it at that temperature for 180 minutes. The main purpose of keeping it at that temperature is to obtain fine original austenite grains, and the average grain size of the original austenite is 13.5 μm. Then, performing a first water quenching, a second water quenching and a tempering after the water quenching to obtain the high strength and toughness extra thick steel plate;

[0066] Among them, the water inlet temperature of the first water quenching is 860℃, and the final cooling temperature is 430℃; the water inlet temperature of the second water quenching is 520℃, and the final cooling temperature is room temperature;

[0067] Tempering treatment: the tempering temperature is 600℃ and the holding time is 30min. The main purpose of this process is to improve toughness by tempering while obtaining 75.6nm cementite and 6.4nm VC precipitation, and to obtain high strength and high toughness.

[0068] The production process parameters of this embodiment are shown in the table:

[0069]

[0070] Sampling tests were carried out on the above steel plates, and the mechanical properties test results are shown in the following table:

[0071]

[0072] Embodiment 3:

[0073] This example was prepared according to the following steps:

[0074] (1) Smelting, continuous casting and rolling are performed according to a certain chemical composition ratio to obtain an extra-thick steel plate with a thickness of 100 mm, wherein the thickness of the continuous casting blank obtained after continuous casting is 250 mm;

[0075] The chemical composition of the above-mentioned extra thick steel plate, measured by mass%, contains C: 0.15%, Mn: 0.6%, V: 0.09%, Ti: 0.004%, Ni: 2.6%, Mo: 0.2%, B: 0.002%, and the rest is Fe and unavoidable impurities.

[0076] (2) heat treating the extra thick steel plate in step (1), including heating the extra thick steel plate to 880°C and keeping it at this temperature for 180 minutes. The main purpose of keeping it at this temperature is to obtain fine original austenite grains, and the average grain size of the original austenite is 12.6 μm. Subsequently, a first water quenching, a second water quenching and a tempering treatment after the water quenching are performed to obtain the high-strength and tough extra thick steel plate;

[0077] The water inlet temperature of the first water quenching is 880℃, and the final cooling temperature is 450℃; the water inlet temperature of the second water quenching is 500℃, and the final cooling temperature is room temperature;

[0078] Tempering treatment: the tempering temperature is 500℃ and the holding time is 40min. The main purpose of this process is to improve toughness by tempering while obtaining 72.4nm cementite and 7.7nm VC precipitation, and to obtain high strength and high toughness.

[0079] The production process parameters of this embodiment are shown in the table:

[0080]

[0081] Sampling tests were carried out on the above steel plates, and the mechanical properties test results are shown in the following table:

[0082]

[0083] Embodiment 4:

[0084] This example was prepared according to the following steps:

[0085] (1) Smelting, continuous casting and rolling are performed according to a certain chemical composition ratio to obtain an extra-thick steel plate with a thickness of 100 mm, wherein the thickness of the continuous casting blank obtained after continuous casting is 250 mm;

[0086] The chemical composition of the above-mentioned extra thick steel plate, measured by mass%, contains C: 0.15%, Mn: 0.6%, V: 0.09%, Ti: 0.004%, Ni: 2.6%, Mo: 0.2%, B: 0.002%, and the rest is Fe and unavoidable impurities.

[0087] (2) heat treating the extra thick steel plate in step (1), including heating the extra thick steel plate to 880°C in a furnace and keeping it at that temperature for 180 minutes. The main purpose of keeping it at that temperature is to obtain fine original austenite grains, and the average grain size of the original austenite is 14 μm. Then, performing a first water quenching, a second water quenching and a tempering after the water quenching to obtain the high strength and toughness extra thick steel plate;

[0088] The water inlet temperature of the first water quenching is 880℃, and the final cooling temperature is 450℃; the water inlet temperature of the second water quenching is 500℃, and the final cooling temperature is room temperature;

[0089] Tempering treatment: the tempering temperature is 600℃ and the holding time is 40min. The main purpose of this process is to improve toughness by tempering while obtaining 69.6nm cementite and 5.9nm VC precipitation, and to obtain high strength and high toughness.

[0090] The production process parameters of this embodiment are shown in the table:

[0091]

[0092] Sampling tests were carried out on the above steel plates, and the mechanical properties test results are shown in the following table:

[0093]

[0094] Comparative Example 1:

[0095] The main difference between this comparative example and Examples 1-4 is that the chemical composition contains Cr, and the preparation steps are as follows:

[0096] (1) Smelting, continuous casting and rolling are performed according to a certain chemical composition ratio to obtain steel plates;

[0097] The chemical composition of the above steel plate, measured by mass%, contains C: 0.15%, Mn: 0.7%, V: 0.06%, Ti: 0.006%, Ni: 2.5%, Mo: 0.4%, B: 0.001%, Cr: 0.5%, and the rest is Fe and unavoidable impurities.

[0098] (2) performing a heat treatment process on the steel plate in step (1), including heating the steel plate to 860° C. in a furnace, keeping the temperature for 180 min, and then performing a primary water quenching treatment, a secondary water quenching treatment, and a tempering treatment after the water quenching;

[0099] Among them, the water inlet temperature of the first water quenching is 860℃, and the final cooling temperature is 430℃; the water inlet temperature of the second water quenching is 520℃, and the final cooling temperature is room temperature;

[0100] Tempering treatment: the tempering temperature is 500℃ and the holding time is 30min.

[0101] The production process parameters of this embodiment are shown in the table:

[0102]

[0103] Sampling tests were carried out on the above steel plates, and the mechanical properties test results are shown in the following table:

[0104]

[0105] The -60°C impact energy of the comparative example 1 is significantly lower than that of the examples 1-4 not containing chromium. The composition design by removing the chromium element improves the low-temperature impact toughness after tempering at 500°C by about 70J.

[0106] Comparative Example 2:

[0107] The main difference between this comparative example and Examples 1-4 is that the chemical composition contains Cr, and the preparation steps are as follows:

[0108] (1) Smelting, continuous casting and rolling are performed according to a certain chemical composition ratio to obtain steel plates;

[0109] The chemical composition of the above steel plate, measured by mass%, contains C: 0.15%, Mn: 0.7%, V: 0.06%, Ti: 0.006%, Ni: 2.5%, Mo: 0.4%, B: 0.001%, Cr: 0.5%, and the rest is Fe and unavoidable impurities.

[0110] (2) performing a heat treatment process on the steel plate in step (1), including heating the steel plate to 860° C. in a furnace, keeping the temperature for 180 min, and then performing a primary water quenching treatment, a secondary water quenching treatment, and a tempering treatment after the water quenching;

[0111] Among them, the water inlet temperature of the first water quenching is 860℃, and the final cooling temperature is 430℃; the water inlet temperature of the second water quenching is 520℃, and the final cooling temperature is room temperature;

[0112] Tempering treatment: the tempering temperature is 600℃ and the holding time is 30min.

[0113] The production process parameters of this embodiment are shown in the table:

[0114]

[0115] Sampling tests were carried out on the above steel plates, and the mechanical properties test results are shown in the following table:

[0116]

[0117] The -60°C impact energy of Comparative Example 2 is significantly lower than that of Examples 1-4 without chromium. The composition design of removing the chromium element improves the low-temperature impact toughness after tempering at 600°C by about 70J.

[0118] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation modes, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are within the protection of the present invention.

Claims

1. A method for preparing a high-strength and tough extra-thick steel plate, characterized in that: The following steps are involved: (1) using the chemical composition ratio of high-strength and tough extra-thick steel plates for smelting, continuous casting and rolling to obtain extra-thick steel plates with a thickness of ≥80 mm, wherein the chemical composition of the extra-thick steel plates, in terms of mass %, comprises C: 0.15-0.18%, Mn: 0.6-0.8%, V: 0.04-0.09%, Ti: 0.001-0.01%, Ni: 2.5-3.0%, Mo: 0.2-0.4%, B: 0.001-0.002%, and the remainder is Fe and unavoidable impurities; (2) performing a heat treatment process on the extra-thick steel plate in step (1), comprising heating the extra-thick steel plate to 850° C. to 880° C. in a furnace, keeping the temperature for 90 min to 120 min, and then performing a primary water quenching and a secondary water quenching to obtain the high-strength and tough extra-thick steel plate; The water inlet temperature of the primary water quenching is 850°C to 880°C, and the final cooling temperature is 400°C to 450°C; The water inlet temperature of the secondary water quenching is 500°C to 550°C, and the final cooling temperature is room temperature; The high-strength and toughness extra-thick steel plate has an austenite grain structure.

2. The method for preparing a high-strength and tough extra-thick steel plate according to claim 1, characterized in that: The heat treatment process also includes tempering treatment, the tempering treatment temperature is 500° C. to 620° C., and the insulation time is ≤40 minutes.

3. The method for preparing a high-strength and tough extra-thick steel plate according to claim 2, characterized in that: The tempering treatment is performed after the secondary water quenching treatment.

4. The method for preparing a high-strength and tough extra-thick steel plate according to claim 2 or 3, characterized in that: The high-strength and toughness extra-thick steel plate has a microstructure of nano cementite and VC, and the nano cementite and VC are dispersedly distributed.

5. The method for preparing a high-strength and tough extra-thick steel plate according to claim 4, characterized in that: The size of the nano cementite is less than 90 nm, and the size of the VC is less than 10 nm.

6. The method for preparing a high-strength and tough extra-thick steel plate according to any one of claims 1 to 5, characterized in that: The mechanical properties of the high-strength and tough extra-thick steel plate meet the following requirements: yield strength>960MPa, tensile strength>1050MPa, elongation>12%, yield strength ratio≤0.95, -60°C transverse impact energy≥100J, -60°C longitudinal impact energy≥150J.

7. The method for preparing a high-strength and tough extra-thick steel plate according to any one of claims 1 to 5, characterized in that: The thickness of the continuous casting embryo obtained after the continuous casting is ≥230 mm.

8. A high-strength and tough extra-thick steel plate, characterized in that: It is prepared using the method for preparing a high-strength and tough extra-thick steel plate according to any one of claims 1 to 7.

9. The high-strength and tough extra-thick steel plate according to claim 8, characterized in that: The average grain size of austenite in the high-strength and toughness extra-thick steel plate is less than 15 μm.

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