A low carbon equivalent ultra-high strength steel sheet and a method for manufacturing the same

Abstract

The present application relates to a kind of low carbon equivalent ultra-high strength steel plate and its preparation method, belong to ultra-high strength steel technical field.The low carbon equivalent ultra-high strength steel plate includes C:0.32-0.40%, Si:0.10-0.50%, Mn:0.3-1.5%, P≤0.010%, S≤0.005%, Cr≤1.0%, Mo:0.20-0.50%, Ti:0.04-0.10%, Nb≤0.06%, Al:0.01-0.04%, N≤0.005%, B:0.001-0.004%, and satisfy 1.0%≤Mn+Cr≤1.5%, 0.05%≤Nb+Ti≤0.10%, the rest is Fe.The carbon equivalent CEV of obtained steel plate is ≤0.70%, tensile strength is ≥1900MPa, yield strength is ≥1600MPa, low temperature impact absorption energy KV2 (-40 DEG C) is ≥32J.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength steel technology, and in particular to a low-carbon equivalent ultra-high strength steel plate and its preparation method. Background Technology

[0002] With the increasing demand for energy conservation and emission reduction, lightweighting through the application of ultra-high strength steel plates has become one of the main trends in the automotive, machinery, and other fields. Currently, the most widely used ultra-high strength steel plates domestically and internationally are grade 450 and 500, with strengths reaching 1500MPa and 1700MPa respectively, while grade 550 and higher strength steel plates are less commonly used. As is well known, for steel materials, strength and toughness are mutually restrictive; as strength increases, the toughness of the steel plate decreases, resulting in insufficient impact resistance, which limits the development and application of ultra-high strength steel plates.

[0003] Currently, ultra-high strength steel plates are mainly composed of low-temperature tempered martensitic structure. They typically achieve good toughness reserves while improving strength by adding large amounts of alloying elements such as Cr, Ni, and Mo, especially Ni, which is generally added at levels above 2%. However, excessive alloying elements increase the carbon equivalent of the steel plate, reducing its weldability and increasing production costs, thus hindering its widespread application. Therefore, the development of 550-grade ultra-high strength steel plates with low carbon equivalent remains a challenge for the steel industry.

[0004] To address the aforementioned issues, this invention proposes an ultra-high strength steel plate with low carbon equivalent and its preparation process, achieving excellent strength and toughness matching under a low carbon equivalent composition system. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a low-carbon equivalent ultra-high strength steel plate and its preparation method to solve the problem of mismatch between strength and toughness in existing steel plates.

[0006] On the one hand, the present invention provides a low-carbon equivalent ultra-high strength steel plate, which, by mass percentage, comprises C: 0.32-0.40%, Si: 0.10-0.50%, Mn: 0.3-1.5%, P≤0.010%, S≤0.005%, Cr≤1.0%, Mo: 0.20-0.50%, Ti: 0.04-0.10%, Nb≤0.06%, Al: 0.01-0.04%, N≤0.005%, B: 0.001-0.004%, and satisfies 1.0%≤Mn+Cr≤1.5%, 0.05%≤Nb+Ti≤0.10%, with the remainder being Fe and unavoidable impurities.

[0007] Furthermore, by mass percentage, it includes C: 0.34-0.39%, Si: 0.20-0.30%, Mn: 0.8-1.3%, P≤0.009%, S≤0.003%, Cr≤1.0%, Mo: 0.25-0.35%, Ti: 0.05-0.07%, Nb≤0.05%, Al: 0.02-0.03%, N≤0.005%, B: 0.0015-0.0025%, and satisfies 1.0%≤Mn+Cr≤1.5%, 0.06%≤Nb+Ti≤0.10%, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, the steel plate has a carbon equivalent (CEV) ≤ 0.70%, tensile strength ≥ 1900 MPa, yield strength ≥ 1600 MPa, and low-temperature impact absorption energy (KV2 (-40℃)) ≥ 32 J.

[0009] On the other hand, the present invention provides a method for preparing low-carbon equivalent ultra-high strength steel plate, comprising the following steps:

[0010] S1: Smelting and casting: The raw materials are smelted according to the composition ratio and then cast to obtain steel billets;

[0011] S2: Rolling, the steel billet is heated to 1250-1280℃ and then subjected to rough rolling and finish rolling. The final rolling temperature is below 860℃. When the temperature drops to 500-550℃, it is coiled to obtain a hot-rolled plate.

[0012] S3: Heat treatment, which involves heating, holding, cooling and tempering the hot-rolled plate to obtain the finished steel plate.

[0013] Furthermore, in step S2, during the roughing stage, the roughing temperature is 1150℃-1250℃, the total compression ratio is greater than 60%, and the final roughing temperature is lower than 1100℃.

[0014] Furthermore, in step S2, the finishing rolling stage is divided into recrystallization zone rolling and non-recrystallization zone rolling, wherein the temperature of recrystallization zone rolling is 920-980℃; and the temperature of non-recrystallization zone rolling is 820-920℃.

[0015] Furthermore, the deformation amount of the non-recrystallized rolling is 15-30% of the total deformation amount in the finishing rolling stage.

[0016] Furthermore, in step S3, the heating temperature of the hot-rolled plate is 880-920℃, and the holding time is 2-3 min / mm.

[0017] Furthermore, the cooling adopts a staged cooling method. After the hot-rolled plate is heated and kept at a certain temperature, it is first cooled to 50-100°C above Ms point at a cooling rate of 50-200°C / s, and then cooled to below Mf point at an ultra-fast cooling rate of 500-2000°C / s.

[0018] Furthermore, the cooled steel plate undergoes tempering treatment at a temperature of 150-180℃ for a holding time of 30-240 minutes.

[0019] Furthermore, the obtained steel plate contains more than 98% martensite with a grain size of less than 5.0 μm.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] 1. This invention, through the combination of chemical elements and the regulation of the ratio between chemical elements, can obtain a steel plate with a combination of strength and toughness. The steel plate has a carbon equivalent (CEV) ≤ 0.70%, tensile strength ≥ 1900 MPa, yield strength ≥ 1600 MPa, low-temperature impact absorption energy (KV2) (-40℃) ≥ 32 J, and grain size less than 5.0 μm.

[0022] 2. The preparation of steel plates in this invention mainly includes smelting and casting, rolling and heat treatment. During smelting and casting, the content of harmful elements such as S, P and N in the steel is strictly controlled to reduce the content of large-size inclusions in the steel and ensure the plasticity and toughness of the finished steel plate. During the rolling process, large deformation rough rolling is adopted in the high-temperature zone to refine the austenite grains by dynamic recrystallization. In the finishing rolling zone, the distribution ratio of the recrystallized zone and the non-recrystallized zone is controlled to ensure that the deformation of the non-recrystallized zone accounts for 15-30% of the total deformation of the finishing rolling, thereby obtaining a flattened grain structure. This allows the rolled steel plate to have high deformation energy storage and promotes the dispersed precipitation of small-size microalloyed second-phase particles.

[0023] 3. In the heat treatment process of this invention, the steel plate obtains a high-strength and high-toughness martensitic structure through tempering. The quenching adopts staged cooling. After the hot-rolled plate is heated and held at a temperature, it is first cooled to 50-100°C above the Ms point at a cooling rate of 50-200°C / s, and then cooled to below the Mf point at an ultra-fast cooling rate of 500-2000°C / s, thereby obtaining a martensitic structure of more than 98%, while obtaining good strength and toughness, and the grain size is relatively small.

[0024] 4. In this invention, the steel plate achieves refined microstructure through optimized microalloying composition design and precise control of nanoscale precipitates. At the same time, it is matched with strengthening heat treatment technology, controlled by dislocation substructure, and has excellent cold bending and welding performance, which can meet the lightweight design and manufacturing requirements of vehicle equipment.

[0025] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0026] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0027] Figure 1 EBSD image of the microstructure of Example 1;

[0028] Figure 2 This is a view of the steel plate obtained in Example 1 after bending.

[0029] Figure 3 The image shows the microstructure EBSD plot of Comparative Example 1. Detailed Implementation

[0030] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] With the continuous development and progress of society, the requirements for steel plate performance are also constantly increasing. Currently, ultra-high strength steel plates are mainly of grade 450 and 500, with strengths of 1500MPa and 1700MPa respectively, which can no longer meet higher requirements. In addition, for steel materials, if one blindly pursues higher strength, it will affect other properties, such as toughness.

[0032] Therefore, the present invention provides a low-carbon equivalent ultra-high strength steel plate, comprising, by mass percentage: C: 0.32-0.40%, Si: 0.10-0.50%, Mn: 0.3-1.5%, P≤0.010%, S≤0.005%, Cr≤1.0%, Mo: 0.20-0.50%, Ti: 0.04-0.10%, Nb≤0.06%, Al: 0.01-0.04%, N≤0.005%, B: 0.001-0.004%, and satisfying 1.0%≤Mn+Cr≤1.5%, 0.05%≤Nb+Ti≤0.10%, with the remainder being Fe and unavoidable impurities.

[0033] Compared with existing technologies, this invention, by controlling the combination of alloying elements and strictly controlling the content of each element, produces steel plates with higher strength, toughness, and weldability. The resulting steel plates have a carbon equivalent of less than 0.70%, a tensile strength ≥1900MPa, a yield strength ≥1600MPa, and a low-temperature impact absorption energy KV2 (-40℃) ≥32J.

[0034] C: In this invention, carbon, as the main interstitial solid solution strengthening element, can combine with Ti and Nb to form TiC and NbC, which can improve the strength and other properties of the steel plate. Meanwhile, to ensure the steel plate has superior weldability, the carbon equivalent needs to be controlled to be less than 0.70%. According to the carbon equivalent calculation formula CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, while considering both the strength and weldability of the steel plate, the C content is controlled between 0.32% and 0.40%.

[0035] Si: The main deoxidizing and solid solution element in steel, but excessive Si will cause decarburization on the surface of the steel plate. Therefore, it should be controlled at 0.10-0.50%.

[0036] Mn, Cr, and Mo: In this invention, manganese, chromium, and molybdenum primarily improve the hardenability and strength of the steel plate, but they also increase its carbon equivalent. Excessive Mn content exacerbates banded structure formation and reduces weldability. Therefore, the Mn content in the steel plate of this invention is controlled between 0.3% and 1.5%. Cr has a similar effect to Mn and can replace some Mn, but its content should not exceed 1.0%. To ensure hardenability, the requirement of 1.0% ≤ Mn + Cr ≤ 1.5% must also be met. Mo can combine with elements such as Ti and Nb to precipitate MC-type carbides, increasing precipitation strengthening and inhibiting the growth of precipitated phases; however, excessive Mo can negatively impact weldability and increase cost. Therefore, the Mo content in the steel of this invention is controlled between 0.20% and 0.50%.

[0037] Ti and Nb are both strong carbide-nitride forming elements, capable of forming fine, dispersed MC-type carbides or nitrides, resulting in precipitation strengthening and refining austenite grains, thus improving the strength and toughness of steel. Adding too little has little effect, while adding too much easily leads to the formation of large-sized nitrides, affecting the toughness of the steel. Therefore, the Ti content in the invented steel is controlled at 0.04-0.10%, and the Nb content is controlled below 0.06%, satisfying the condition 0.05% ≤ Nb + Ti ≤ 0.10%.

[0038] P and S: Harmful elements in steel that can form harmful inclusions and segregate at grain boundaries, severely reducing the ductility and toughness of steel. Therefore, they should be eliminated as much as possible, with P ≤ 0.010% and S ≤ 0.005%.

[0039] B: Boron can significantly improve the hardenability of steel plates and purify grain boundaries. When the boron content is below 0.001%, the effect on improving hardenability is not obvious, and when the boron content is above 0.004%, the increase in effect is not significant. Therefore, the boron content should be controlled within the range of 0.001-0.004%.

[0040] Nitrogen (N) combines with elements such as Al, Ti, and Nb to form nitrides, which refine the austenite grains. However, excessive N content can lead to the formation of large-sized TiN, reducing the toughness of the steel. Therefore, the N content should be controlled below 0.005%.

[0041] Preferably, by mass percentage, it includes C: 0.34-0.39%, Si: 0.22-0.30%, Mn: 0.8-1.3%, P≤0.009%, S≤0.003%, Cr≤1.0%, Mo: 0.25-0.35%, Ti: 0.05-0.07%, Nb≤0.05%, Al: 0.02-0.03%, N≤0.005%, B: 0.0015-0.0025%, and satisfies 1.0%≤Mn+Cr≤1.5%, 0.06%≤Nb+Ti≤0.10%, with the remainder being Fe and unavoidable impurities.

[0042] This invention provides a method for preparing low-carbon equivalent ultra-high strength steel plates, comprising the following steps:

[0043] S1: Smelting and casting: The raw materials are smelted and cast according to the composition ratio to obtain steel billets;

[0044] S2: Rolling, the steel billet is heated to 1250-1280℃ and then subjected to rough rolling and finish rolling. The final rolling temperature is below 860℃. When the temperature drops to 500-550℃, it is coiled to obtain a hot-rolled plate.

[0045] S3: Heat treatment, which involves heating, holding, cooling and tempering the hot-rolled plate to obtain the finished steel plate.

[0046] Compared with the prior art, the present invention uses smelting and casting, hot rolling and heat treatment. Through the combination of rolling and heat treatment, the steel plate contains more than 98% martensitic structure, and at the same time obtains good strength and toughness.

[0047] Specifically, strictly controlling the content of harmful elements such as S, P, and N in the steel plate during the smelting and casting processes can reduce the content of large-sized inclusions and ensure the plasticity and toughness of the steel plate. At the same time, using a protective atmosphere during the casting process can achieve low-content control of harmful elements in the steel billet.

[0048] Specifically, the smelting and casting method is converter + ladle refining + vacuum degassing + continuous casting. Other smelting and casting processes may also be used while ensuring metallurgical quality.

[0049] Specifically, in step S2, during the roughing stage, the roughing temperature is 1150-1250℃, the total compression ratio is greater than 60%, and the final roughing temperature is lower than 1100℃.

[0050] When the roughing temperature is below 1150℃, the slab temperature is too low, the rolling deformation resistance increases, and the difficulty of subsequent finishing rolling increases; when the roughing temperature is above 1250℃, the slab temperature is too high, the grains are prone to grow rapidly, which is not conducive to microstructure refinement.

[0051] The roughing compression ratio is a key process for breaking down coarse grains and refining recrystallized grains in the high-temperature zone. When the compression ratio is low, the deformation is insufficient, the grains are not thoroughly broken down, and the grain refinement is inadequate. Similarly, a high final rolling temperature can lead to coarsening of recrystallized grains. Lowering the final rolling temperature helps to suppress grain growth.

[0052] Specifically, in the roughing stage, there are a total of 3-5 rolling passes. The deformation per pass is 10-20% in the first 1-2 passes and 20-40% in the last 2-3 passes, so as to achieve the refinement and homogenization of the slab structure. At the same time, the finishing rolling temperature of roughing is 1150-1250℃, which is beneficial to suppress the coarsening and growth of recrystallized grains.

[0053] Specifically, in step S2, the finishing rolling stage is divided into recrystallization zone rolling and non-recrystallization zone rolling, wherein the temperature of recrystallization zone rolling is 920-980℃; and the temperature of non-recrystallization zone rolling is 820-920℃.

[0054] Specifically, the deformation amount of the non-recrystallized rolling is 15-30% of the total deformation amount in the finishing rolling stage. After rolling, a flattened structure is obtained, which is conducive to the accumulation of deformation energy storage and further refinement of grain structure in subsequent heat treatment processes.

[0055] Specifically, in the finishing rolling stage, the total number of rolling passes is 5-7, of which 3-4 passes are in the recrystallization zone and 2-3 passes are in the non-recrystallization zone. The recrystallization zone rolling adopts large deformation rolling, with a deformation amount of 20-60% per pass, to quickly reduce the slab thickness; the non-recrystallization zone rolling adopts smaller deformation rolling, with a deformation amount of 10-20% per pass.

[0056] Specifically, in step S3, the heating temperature of the hot-rolled plate is 880-920℃, and the holding time is 2-3 min / mm.

[0057] The hot-rolled plate is heated and kept at a constant temperature for a duration determined by its thickness, in order to make its internal structure more uniform and to inhibit grain coarsening and growth.

[0058] Specifically, the cooling adopts a staged cooling method. After the hot-rolled plate is heated and kept at a temperature, it is first cooled at a cooling rate of 50-200℃ / s to 50-100℃ above the Ms point, and then cooled at an ultra-fast cooling rate of 500-2000℃ / s to below the Mf point.

[0059] During the cooling process, the combination of rapid and ultra-rapid cooling promotes a significant increase in dislocation multiplication during the martensitic transformation, further enhancing the strength of the steel plate. The resulting steel plate contains over 98% martensite with a grain size of less than 5 μm.

[0060] Specifically, the tempering temperature is 150-180℃, and the holding time is 30-240 minutes.

[0061] To more clearly describe the present invention, the following embodiments and comparative examples are provided for further illustration.

[0062] Example

[0063] The composition of the steel plate in the example is shown in Table 1. The specific preparation includes the following steps:

[0064] S1: The steel billet with a thickness of 80mm × width of 200mm × length is obtained by smelting in a vacuum induction furnace, followed by casting and forging.

[0065] S2: The steel billet is heated to 1250-1280℃ and held for 2 hours before rough rolling, completing 4 passes with a total deformation of 65%. The final rolling temperature is controlled below 1080℃, resulting in a billet thickness of 30mm. Then, 7 passes of finish rolling are performed, including 4 passes in the recrystallization zone (920-980℃) and 3 passes in the non-recrystallization zone (820-920℃). The final rolling temperature is below 860℃, ensuring that the total deformation is controlled to 15-25% for the non-recrystallization zone. The final steel plate thickness is 6mm. When the temperature drops to 500-550℃, simulated coiling (using a heating furnace to simulate the coiling cooling rate) is performed to obtain the hot-rolled plate.

[0066] S3: The hot-rolled plate is heated to 880-920℃ and held for 15 minutes. Then it is rapidly cooled to 450℃ at a cooling rate of 50-200℃ / s. Subsequently, it is cooled to room temperature directly by ultra-fast cooling with high-pressure water jet at a cooling rate of more than 500℃ / s. Finally, it is tempered at a tempering temperature of 150-180℃ and held for 120 minutes to obtain the steel plate.

[0067] The specific process parameters for Examples 1-4 are shown in Table 2. Comparative Examples 1-3 were prepared using conventional processes, and their process parameters are shown in Table 2.

[0068] Table 1 Chemical composition of Examples 1-4 and Comparative Examples 1-3

[0069] Group Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 C 0.35 0.32 0.39 0.34 0.36 0.42 0.44 Si 0.22 0.19 0.27 0.25 0.3 0.4 0.3 Mn 1.5 1.3 1.1 0.8 1.4 0.8 0.8 Cr — — — 0.6 — 1.2 0.8 Mo 0.25 0.35 0.33 0.31 0.37 0.5 0.63 P 0.008 0.009 0.006 0.008 0.009 0.008 0.009 S 0.003 0.001 0.002 0.002 0.001 0.002 0.001 Ti 0.08 0.06 0.05 0.07 0.06 — — Nb — — 0.04 — — — — Al 0.015 0.02 0.028 0.022 0.03 — — N 0.003 0.004 0.003 0.004 0.004 — — B 0.0015 0.0025 0.0031 0.0024 0.0019 — — Ni — — — — 1.8 1.2

[0070] *In Table 1, “—” indicates that it does not contain.

[0071] Table 2. Process parameters during the preparation of Examples 1-4 and Comparative Examples 1-3.

[0072]

[0073]

[0074] Performance testing

[0075] The above Examples 1-4 and Comparative Examples 1-3 were subjected to performance tests, mainly including the calculation of carbon equivalent, yield strength, tensile strength, and low-temperature impact absorption energy (the thickness of the steel plate in this invention is 6mm, so the prepared sample is a half sample with a size of 10mm×10mm×5mm), etc. The specific test results are shown in Table 3.

[0076] Table 3 Detection Results

[0077]

[0078] Referring to Examples 1-4 and Comparative Examples 1-3, and in conjunction with Table 3 and... Figure 1-3 It can be seen that when using the component ratio and preparation method provided by the present invention, the carbon equivalent CEV of the obtained steel plate is ≤0.70%, with good strength and toughness, tensile strength ≥1900MPa, yield strength ≥1600MPa, and low-temperature impact absorption energy KV2(-40℃) of the half sample ≥16J. Therefore, the low-temperature impact absorption energy KV2(-40℃) of the steel plate is ≥32J, and the grain size is less than 5.0μm. The 90° cold bending radius R / plate thickness d is 2-2.5.

[0079] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-carbon equivalent ultra-high strength steel plate, characterized in that, Based on mass percentage, it includes C: 0.32-0.40%, Si: 0.10-0.50%, Mn: 0.3-1.5%, P≤0.010%, S≤0.005%, Cr≤1.0%, Mo: 0.20-0.50%, Ti: 0.04-0.10%, Nb≤0.06%, Al: 0.01-0.04%, N≤0.005%, B: 0.001-0.004%, and satisfies 1.0%≤Mn+Cr≤1.5%, 0.05%≤Nb+Ti≤0.10%, with the remainder being Fe and unavoidable impurities; The steel plate is obtained through smelting, casting, rolling and heat treatment; The rolling process includes roughing and finishing. In the roughing stage, the roughing temperature is 1150-1250℃, the total compression ratio is greater than 60%, and the finishing temperature is less than 1100℃. The total number of rolling passes is 3-5, with a single-pass deformation of 10-20% in the first 1-2 passes and 20-40% in the last 2-3 passes. The finishing rolling stage is divided into recrystallization zone rolling and non-recrystallization zone rolling, with a total of 5-7 rolling passes, of which 3-4 passes are in the recrystallization zone and 2-3 passes are in the non-recrystallization zone. The rolling temperature in the recrystallization zone is 920-980℃, and the deformation per pass is 20-60%. The rolling temperature in the non-recrystallization zone is 820-920℃, and the deformation per pass is 10-20%. During the heat treatment stage, the heating temperature is 880-920℃ and the holding time is 2-3 min / mm. After heating and holding, the temperature is first cooled to 50-100℃ above Ms point at a cooling rate of 50-200℃ / s, and then cooled to below Mf point at an ultra-fast cooling rate of 500-2000℃ / s. Then, tempering treatment is carried out at 150-180℃ for 30-240 min. The steel plate has a carbon equivalent (CEV) of ≤0.70%, a tensile strength of ≥1900MPa, a yield strength of ≥1600MPa, a low-temperature impact absorption energy (KV2) (-40℃) of ≥32J, and a martensitic structure of more than 98% with a grain size of less than 5μm.

2. The low-carbon equivalent ultra-high strength steel plate according to claim 1, characterized in that, Based on mass percentage, it includes C: 0.34-0.39%, Si: 0.20-0.30%, Mn: 0.8-1.3%, P≤0.009%, S≤0.003%, Cr≤1.0%, Mo: 0.25-0.35%, Ti: 0.05-0.07%, Nb≤0.05%, Al: 0.02-0.03%, N≤0.005%, B: 0.0015-0.0025%, and satisfies 1.0%≤Mn+Cr≤1.5%, 0.06%≤Nb+Ti≤0.10%, with the remainder being Fe and unavoidable impurities.

3. A method for preparing a low-carbon equivalent ultra-high-strength steel plate as described in any one of claims 1-2, characterized in that, Includes the following steps: S1: Smelting and casting: The raw materials are smelted according to the composition ratio and then cast to obtain steel billets; S2: Rolling, the steel billet is heated to 1250-1280℃ and then subjected to rough rolling and finish rolling. The final rolling temperature is below 860℃. When the temperature drops to 500-550℃, it is coiled to obtain a hot-rolled plate. S3: Heat treatment, which involves heating, holding, cooling and tempering the hot-rolled plate to obtain a steel plate.

4. The method for preparing low-carbon equivalent ultra-high-strength steel plate according to claim 3, characterized in that, In step S2, during the roughing stage, the roughing temperature is 1150-1250℃, the total compression ratio is greater than 60%, and the final roughing temperature is lower than 1100℃.

5. The method for preparing low-carbon equivalent ultra-high-strength steel plate according to claim 3, characterized in that, In step S2, the finishing rolling stage is divided into recrystallization zone rolling and non-recrystallization zone rolling, wherein the temperature of recrystallization zone rolling is 920-980℃; and the temperature of non-recrystallization zone rolling is 820-920℃.

6. The method for preparing low-carbon equivalent ultra-high-strength steel plate according to claim 5, characterized in that, The deformation amount in the non-recrystallized rolling is 15-30% of the total deformation in the finishing rolling stage.

7. The method for preparing low-carbon equivalent ultra-high-strength steel plate according to claim 3, characterized in that, In step S3, the heating temperature of the hot-rolled plate is 880-920℃, and the holding time is 2-3 min / mm.

8. The method for preparing low-carbon equivalent ultra-high-strength steel plate according to claim 3, characterized in that, The cooling process employs a staged cooling method. After the hot-rolled plate is heated and kept at a constant temperature, it is first cooled at a cooling rate of 50-200℃ / s to 50-100℃ above the Ms point, and then cooled at an ultra-fast cooling rate of 500-2000℃ / s to below the Mf point. Subsequently, it undergoes a tempering treatment at 150-180℃ for 30-240 minutes.

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