A high-r-value ultra-high strength steel plate and its manufacturing method

Abstract

This invention discloses a high r-value ultra-high strength steel plate, which comprises an upper surface layer, an intermediate layer, and a lower surface layer in the thickness direction. The microstructure of the upper and lower surface layers is predominantly ferrite, with a carbon content ≤0.025% in both layers. The mass percentage of chemical elements in the upper and lower surface layers respectively satisfies: Ti-3.42N-3.98C≥0. The microstructure of the intermediate layer is predominantly tempered martensite and bainite, at least one of these. Correspondingly, this invention also discloses a method for manufacturing the high r-value ultra-high strength steel plate. The steel plate of this invention possesses both ultra-high strength and a high r-value. Furthermore, the thickness reduction during deformation is more slight, resulting in better formability in both global tensile deformation and localized deformation.

Description

Technical Field

[0001] This invention relates to a steel plate and its manufacturing method, and more particularly to a high-strength steel plate and its manufacturing method. Background Technology

[0002] The r-value represents the steel sheet's resistance to thinning during the drawing process. For high-formability deep-drawing steel sheets (such as interstitial steel) with a tensile strength below 350 MPa, the r-value is a crucial technical indicator; a high r-value signifies superior drawing performance.

[0003] However, existing high-strength steel plates, since they are not used for deep drawing, have not focused on the r-value of the steel plate and the properties it represents.

[0004] For example, Chinese patent document CN101768695A, published on July 7, 2010, entitled "1000MPa Grade Ti Microalloyed Ultrafine Grain Cold-Rolled Duplex Steel and Its Preparation Process," discloses a chemical composition of: 0.03-0.2% C, 0.2-0.8% Si, 1.2-2.0% Mn, Ti: 0.03-0.15%, ≤0.02% P, S≤0.015%, 0.02-0.15% Al, with the remainder being Fe and unavoidable impurities. After hot rolling and cold rolling, annealing is performed in the critical zone at a cooling rate of less than 50℃ / s to obtain a cold-rolled duplex steel with a strength exceeding 980MPa. It can be seen that this high-strength steel does not focus on the r-value of the steel plate.

[0005] For example, Chinese patent document CN101363099A, published on February 11, 2009, entitled "A Cold-Rolled Duplex Steel Sheet with Tensile Strength of 1000MPa and its Preparation Method," discloses the following: C: 0.14-0.21%, Si: 0.4-0.9%, Mn: 1.5-2.1%, P: ≤0.02%, S≤0.01%, Nb: 0.001-0.05%, V: 0.001-0.02%. After hot rolling and cold rolling, the steel is held at 760-820℃ with a cooling rate of 40-50℃ / s, and then aged at 240-320℃ for 180-300s. This high-strength steel does not focus on the r-value of the steel sheet.

[0006] In their paper "Forming Properties of High-Strength Duplex Steel" (Metal Heat Treatment, Vol. 46, No. 5, 2021), Xiong Ziliu et al. introduced a duplex steel with a tensile strength of 980 MPa, but its r-value is less than 0.9.

[0007] In their paper "Maximum Bending Angle of 1000MPa Grade Ultra-High Strength Martensitic Steel Based on Three-Point Bending" (Journal of Plasticity Engineering, Vol. 21, No. 4, 2014), Ge Delong et al. introduced martensitic steel with a strength of 1000-1400MPa, whose r-value is less than 0.7. Summary of the Invention

[0008] One of the objectives of this invention is to provide a high r-value ultra-high strength steel plate. This steel plate has both ultra-high strength and a high r-value. The thickness reduction of this high r-value ultra-high strength steel plate is more slight during the deformation process, thus exhibiting better formability in both global tensile deformation and local deformation.

[0009] In the high r-value ultra-high strength steel plate described in this invention, the r-value represents the plate's resistance to thickness reduction during the stretching and forming deformation process. Steel plates with a high r-value are less prone to thickness reduction during tensile deformation, thus reducing the likelihood of fracture. For ultra-high strength steel plates, a high r-value reduces localized thickness reduction during severe deformation, mitigating the necking process and improving the plate's formability.

[0010] To achieve the above objectives, this invention proposes a high r-value ultra-high strength steel plate, which includes an upper surface layer, an intermediate layer, and a lower surface layer in the thickness direction; wherein the main microstructure of the upper and lower surface layers is ferrite, the carbon content of the upper and lower surface layers is ≤0.025%, and the mass percentage of chemical elements in the upper and lower surface layers respectively satisfies: Ti-3.42N-3.98C≥0; the microstructure of the intermediate layer includes ferrite and tempered martensite.

[0011] One of the core aspects of this invention lies in achieving a high r-value by ensuring that the mass percentage of chemical elements in both the upper and lower surface layers satisfies Ti-3.42N-3.98C≥0, and that the carbon content in both the upper and lower surface layers is ≤0.025%. Meanwhile, the overall high strength of the steel plate is primarily guaranteed by the microstructure of the intermediate layer.

[0012] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the average grain diameter of ferrite in the upper and lower surface layers is ≤15 micrometers.

[0013] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the volume ratio of ferrite in the upper and lower surface layers is ≥97%.

[0014] Furthermore, in the high r-value ultra-high strength steel plate described in this invention, the microstructure of the intermediate layer may also include bainite.

[0015] Furthermore, in the high r-value ultra-high strength steel plate described in this invention, regardless of whether the intermediate layer contains bainite, the volume ratio of tempered martensite + bainite is ≥40%.

[0016] Furthermore, in the high r-value ultra-high strength steel plate of the present invention, the ferrite in the upper and lower surface layers contains carbides, and the types of carbides are Ti(C,N) and Nb(C,N).

[0017] Furthermore, in the high r-value ultra-high strength steel plate described in this invention, the thickness of both the upper and lower surface layers is 100-200 micrometers.

[0018] Furthermore, in the high r-value ultra-high strength steel plate described in this invention, the sum of the thicknesses of its upper and lower surface layers does not exceed 40% of the total thickness of the steel plate.

[0019] Furthermore, the thickness of the high r-value ultra-high strength steel plate described in this invention is 0.8-2.5 mm.

[0020] Furthermore, the high r-value ultra-high strength steel plate of the present invention contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0021] C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; N≤0.005%.

[0022] Furthermore, in the high r-value ultra-high strength steel plate described in this invention, the mass percentage of each chemical element is as follows:

[0023] C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; N≤0.005%; balance is Fe and unavoidable impurities.

[0024] In some embodiments, the design principles of each chemical element in the high r-value ultra-high strength steel plate of the present invention are specifically described as follows:

[0025] C: In the high-r-value high-strength steel plate described in this invention, carbon (C) can improve the strength of the steel by affecting the martensitic hardness. If the carbon content in the steel is too low, the strength of the steel after quenching will be low; while the higher the carbon content in the steel, the higher the strength of the steel after quenching. However, excessive carbon is not conducive to obtaining a high r-value. Therefore, in the high-r-value ultra-high-strength steel plate described in this invention, the carbon content is controlled between 0.08% and 0.20%.

[0026] Si: In the high-r-value high-strength steel plate described in this invention, Si can play a role in solid solution strengthening, inhibiting Fe3C precipitation, and promoting the formation of retained austenite. Based on this, the Si content is controlled to be 0.01-1.7%.

[0027] Mn: Mn is the main element for improving the hardenability of steel, and its content should be matched with the cooling capacity of the cooling method used. When the Mn content is too low, the steel plate cannot be hardened, thus failing to achieve high strength; conversely, if the Mn content is too high, it is detrimental to the carbon equivalent. Therefore, in the high r-value ultra-high strength steel plate described in this invention, the Mn content is controlled between 0.7% and 2.7%.

[0028] Ti: In the high-r-value high-strength steel plate described in this invention, the Ti content is closely related to the r-value. If the Ti content is too low, it will not be sufficient to fix all the C and N in the upper and lower surface layers, resulting in a low r-value; if the Ti content is too high, Ti will be excessive. Therefore, in the high-r-value ultra-high-strength steel plate described in this invention, the Ti element is controlled between 0.16% and 0.26%.

[0029] Nb: In the high r-value high-strength steel plate described in this invention, Nb plays an auxiliary role in fixing C and N, forming Nb(C, N), ensuring that C and N in the upper and lower surface regions are not in a solid solution state. Therefore, in the high r-value ultra-high-strength steel plate described in this invention, the Nb element is controlled between 0.015-0.05%.

[0030] Al: Its main function in this invention is deoxidation during the smelting process, and its content ranges from 0.02% to 0.06%.

[0031] N: In this invention, N is not conducive to forming a composition ratio of Ti-3.42N-3.98C≥0 in the upper and lower surface layers, and is not conducive to forming a state without interstitial atoms in local areas. Therefore, it is necessary to control N≤0.005%.

[0032] In this invention, the unavoidable impurities are mainly S and P, and their content is expected to be as low as possible when the process conditions permit.

[0033] It can be understood that the basic composition of the high-r-value high-strength steel plate described in this invention is: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%. This basic composition forms the basis of the steel plate's strength, but a high r-value cannot be obtained based solely on these basic components. To obtain a high r-value, the steel plate described in this invention also contains: Ti: 0.16-0.26%, Nb: 0.015-0.05%, and N≤0.005%.

[0034] Furthermore, the high r-value ultra-high strength steel plate of the present invention also contains at least one of Cr, Mo, and B; wherein B≤0.005% and Cr+Mo≤0.5%.

[0035] In the high r-value ultra-high strength steel plate described in this invention, Cr, Mo, and B alloying elements can effectively work with manganese to improve hardenability, so as to match the hardenability and rapid cooling capacity of the steel, and further avoid the inability to harden or excessive carbon equivalent.

[0036] Furthermore, the high r-value ultra-high strength steel plate of the present invention has a tensile strength ≥980MPa, and its r-value r 90 ≥1.2, the microhardness HV of the upper and lower surface layers is ≤130, and the microhardness HV of the middle layer is ≥330.

[0037] Accordingly, another objective of the present invention is to provide a method for manufacturing high r-value ultra-high strength steel plates, which can produce high r-value ultra-high strength steel plates by optimizing the process.

[0038] To achieve the above objectives, the present invention also proposes a method for manufacturing high r-value ultra-high strength steel plates as described above, comprising the steps of: smelting and casting; hot rolling; pickling followed by cold rolling; annealing; wherein the annealing step includes:

[0039] High-temperature humidification and decarburization process: Water vapor is sprayed into the annealing furnace for humidification, while the heating temperature of the steel plate is controlled at 880-950℃ and maintained for 100-300s, and the dew point in the annealing furnace is controlled to be above 0℃.

[0040] Normal slow cooling process: The steel plate is cooled from the high temperature and humidity to between 730-770℃ at a cooling rate of 3-10℃ / s;

[0041] Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770℃ to 680-720℃, with the cooling temperature range controlled between 40-60℃ and the cooling rate between 0.03-0.1℃ / s, ensuring that the slow cooling process lasts for more than 500 seconds;

[0042] Rapid cooling process: The steel plate is rapidly cooled to below 300℃ at a cooling rate of ≥50℃ / s;

[0043] Over-aging process: The steel plate is tempered at 240-320℃ for 150-500 seconds.

[0044] The manufacturing process described in this invention, especially the unique annealing process, is crucial for obtaining a high r-value for the ultra-high strength steel plate described in this invention.

[0045] The decarburization method involves injecting water vapor into the furnace for humidification. At high temperatures, the following reactions occur, promoting the decarburization of the steel plate: C + H₂O = CO + H₂; C + 2H₂O = CO₂ + 2H₂. The steel plate is heated to between 880 and 950°C and maintained for 100-300 seconds. The amount of water used for humidification is controlled based on the weight of the strip passing through per unit time and the required thickness of the decarburized layer, ensuring the dew point inside the annealing furnace remains above 0°C. This high-temperature humidification decarburization process results in preliminary decarburized layers of 100-200 micrometers thick on both the upper and lower surfaces of the steel plate (i.e., both the upper and lower surfaces are decarburized layers). In this process, if the steel plate heating temperature is too low, insufficient decarburization will occur; if the heating temperature is too high, excessive grain growth will occur. Therefore, controlling the steel plate heating temperature to 880-950°C during the high-temperature humidification decarburization process is beneficial to the decarburization reaction of the steel plate. The reason for choosing a furnace dew point above 0°C is that if the furnace dew point is low, the decarburization reaction of the steel plate will be slow and insufficient to obtain a decarburized layer of sufficient thickness.

[0046] After the aforementioned high-temperature humidification and decarburization process, the carbon content of the upper and lower surfaces of the steel plate decreased significantly, reaching ≤0.05%. However, the carbon content of the upper and lower surfaces was still too high, and the direct cooling of austenite to ferrite would still result in Fe3C precipitation. To further reduce the carbon content of the upper and lower surfaces of the steel plate, a quasi-static slow cooling process was performed below the Ac1 temperature of the iron-carbon phase diagram. In this quasi-static slow cooling process, the steel plate was slowly cooled from 730-770℃ to 680-720℃ at a rate of 0.03-0.1℃ / s, with the temperature range controlled between 40-60℃ throughout the entire process, and the duration ≥500s. Within this temperature range, the austenite in the upper and lower surfaces of the steel plate, after preliminary decarburization, decomposes first to form ferrite. During the slow cooling process, carbon in the ferrite diffuses into the austenite in the high-carbon region (intermediate layer) that has not undergone transformation, thereby further promoting the reduction of carbon content in the upper and lower surfaces. With sufficient slow cooling time, the carbon content in the upper and lower surfaces can be reduced to a level close to that of industrial pure iron, thus enabling the upper and lower surface regions of the invented steel to satisfy Ti-3.42N-3.98C≥0.

[0047] In the manufacturing method described in this invention, the slow cooling from 730-770℃ to 680-720℃ is crucial. After decarburization, the austenite decarburization content in the surface layer decreases, and Ac3 increases, thus initiating the austenite-ferrite transformation during the slow cooling process from 730-770℃ to 680-720℃. When the local carbon content in the upper and lower surface layers of the steel plate of this invention is reduced to ≤0.025%, the Ti, C, and N contents in the steel plate can meet the level of Ti-3.42N-3.98C≥0, thereby essentially forming a structure without interstitial atoms in the upper and lower surface layers, greatly increasing the r-value of the upper and lower surface layers, and thus improving the overall r-value of the steel plate.

[0048] After slow cooling, the steel plate is rapidly cooled to below 300°C at a cooling rate of ≥50°C / s, and then tempered at 240-320°C for 150-500s. During this cooling process, the austenite in the intermediate layer transforms into martensite, and in some embodiments, it may also contain a small amount of bainite and retained austenite, thereby achieving ultra-high strength, such as a tensile strength of ≥980MPa.

[0049] The smelting, casting, hot rolling and cold rolling processes in the manufacturing method described in this invention can all be conventional processes, and this invention does not make any special modifications to these processes.

[0050] The high-r-value high-strength steel plate and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0051] This invention ensures ultra-high strength by controlling the microstructure of the intermediate layer, and achieves a high r-value by controlling the mass percentage content of chemical elements in the upper and lower surface layers to meet the requirements of Ti-3.42N-3.98C≥0. This results in ultra-high strength steel plates with high r-values.

[0052] From the perspective of composition design, this invention, by adding an appropriate amount of Ti and assisting in fixing N and C with Nb, can substantially achieve a solid solution state without interstitial atoms in the upper and lower surfaces, thereby greatly improving the r value of the surface layer, which in turn affects the overall r value of the steel plate, enabling the r value of the 980MPa cold-rolled high-strength steel plate to reach more than 1.2, thus greatly reducing the thinning rate during deformation and obtaining better formability.

[0053] From a manufacturing process perspective, the high r-value high-strength steel plate of the present invention employs a unique annealing process, undergoes a preliminary high-temperature humidification and decarburization process, and a long-term slow cooling austenite to ferrite transformation, which redistributes the carbon content, thereby reducing the carbon content of the upper and lower surfaces of the ultra-high strength steel plate to below 0.025%. Attached Figure Description

[0054] Figure 1 The annealing process curve of the manufacturing method of the high r-value high-strength steel plate of the present invention is schematically shown in one embodiment. Detailed Implementation

[0055] The high-r-value high-strength steel plate and its manufacturing method described in this invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0056] The high-r-value high-strength steel plates of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2 were prepared by the following steps:

[0057] (1) Smelting: Molten iron is smelted in a converter and its composition is further optimized through a refining process;

[0058] (2) Slabs were obtained by continuous casting. The composition of the slabs in each embodiment and comparative example is shown in Table 1.

[0059] (3) Hot rolling: The conventional hot rolling process is adopted. For example, in some embodiments, the furnace heating temperature of the steel plate is controlled at 1220-1280℃, the final rolling temperature is 870-930℃, and the coiling temperature is 570-630℃.

[0060] (4) Cold rolling after pickling: In some embodiments, the cold rolling reduction rate is controlled to be 40-65%;

[0061] (5) Annealing Figure 1 The diagram schematically illustrates the annealing process curve of one embodiment of the manufacturing method for high-r-value high-strength steel plates according to the present invention, specifically including:

[0062] High-temperature humidification and decarburization process: Water vapor is sprayed into the annealing furnace for humidification, while the heating temperature of the steel plate is controlled at 880-950℃ and maintained for 100-300s, and the dew point in the annealing furnace is controlled to be above 0℃.

[0063] Normal slow cooling process: The steel plate is cooled from the high temperature and humidity to between 730-770℃ at a cooling rate of 3-10℃ / s;

[0064] Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770℃ to 680-720℃, with the cooling temperature range controlled between 40-60℃ and the cooling rate between 0.03-0.1℃ / s, ensuring that the slow cooling process lasts for more than 500 seconds;

[0065] Rapid cooling process: The steel plate is rapidly cooled to below 300℃ at a cooling rate of ≥50℃ / s;

[0066] Over-aging process: The steel plate is tempered at 240-320℃ for 150-500 seconds.

[0067] The annealing process parameters for Examples 1-6 and Comparative Example 1 are listed in Table 2-1. Comparative Example 2 did not use the annealing process of the present invention, but instead used a conventional quenching and tempering process. The specific process parameters for Comparative Example 2 are listed in Table 2-2.

[0068] Table 1 lists the mass percentage of each chemical element in Examples 1-6 and Comparative Examples 1-2 of the present invention.

[0069] Table 1. (wt%, balance Fe and other unavoidable impurities besides S, P, and N)

[0070]

[0071]

[0072] Table 2-1 lists the specific process parameters for the annealing steps of the high-r-value high-strength steel plates of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0073] Table 2-1

[0074]

[0075] Table 2-2

[0076]

[0077] Steel plates from Examples 1-6 and Comparative Examples 1-2 were sampled, and the carbon content and other component content of the upper and lower surfaces of the steel plates were detected by GDS (Glow Discharge Spectrometer). The microstructure was observed and detected by metallographic microscope, and the observation and detection results are listed in Table 3.

[0078] Table 3.

[0079]

[0080] Note: When the Ti-3.42N-3.98C value in the upper and lower surfaces is greater than 0, it indicates that there are no interstitial atoms in the upper and lower surfaces; when the Ti-3.42N-3.98C value is less than 0, it indicates that there are interstitial atoms in the upper and lower surfaces. The two columns of data in the table represent the values ​​of the upper surface and the lower surface, respectively.

[0081] To verify the effectiveness of the present invention, samples were taken from the steel plates of Examples 1-6 and Comparative Examples 1-2, and their performance was tested. The test results are listed in Table 4. Wherein:

[0082] Mechanical property testing is performed on the steel plate in a tensile direction perpendicular to the rolling direction. JIS 5# standard plate tensile specimens with a gauge length of 50 mm and a gauge width of 25 mm are used for testing on a tensile testing machine. During the testing process, data on strength, elongation, and r90 value can be obtained. The r90 value is taken from 4-6% of the tensile strain range.

[0083] Hardness testing was performed using a microhardness tester to measure HV hardness, with a load of 150g and a loading time of 15s.

[0084] Table 4 lists the relevant performance parameters of the high-r-value high-strength steel plates of Examples 1-6 and the comparative steel plates of Comparative Examples 1-2.

[0085] Table 4.

[0086]

[0087] Note: The table shows two columns of data for the hardness of the upper and lower surfaces, representing the hardness values ​​of the upper and lower surfaces respectively.

[0088] As can be seen from Table 4, the tensile strength of the high-strength steel plates in Examples 1-6 of the present invention is ≥985MPa, and their r values ​​are r 90 The microhardness of the upper and lower surface layers is ≥1.2, and the microhardness of the middle layer is ≥330. This indicates that the high-strength steel plate described in this invention can have both high strength and high formability.

[0089] The amount of Ti added in Comparative Example 1 does not meet the requirements of this invention, and even if it adopts the process of this invention, it cannot achieve a high r value.

[0090] Although Comparative Example 2 used the same components as Example 2 of the present invention, it did not use the annealing process of the present invention, but instead used a conventional quenching and tempering process, and therefore could not obtain a high r value.

[0091] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0092] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high r-value ultra-high strength steel plate, characterized in that, The structure comprises an upper surface layer, an intermediate layer, and a lower surface layer in the thickness direction; the main microstructure of the upper and lower surface layers is ferrite, and the carbon content of both the upper and lower surface layers is ≤0.025%; the mass percentage of chemical elements in the upper and lower surface layers respectively satisfies: Ti-3.42N-3.98C≥0; the microstructure of the intermediate layer includes ferrite and tempered martensite; The mass percentage of each chemical element in the high r-value ultra-high strength steel plate is: C: 0.08-0.20%; Si: 0.01-1.7%; Mn: 0.7-2.7%; Ti: 0.16-0.26%; Nb: 0.015-0.05%; Al: 0.02-0.06%; N≤0.005%; balance Fe and unavoidable impurities; The high r-value ultra-high strength steel plate has a tensile strength ≥980MPa, an r-value r90 ≥1.2, a microhardness HV ≤130 for the upper and lower surface layers, and a microhardness HV ≥330 for the middle layer.

2. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, The average grain diameter of ferrite in the upper and lower surface layers is ≤15 micrometers.

3. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, The volume ratio of ferrite in the upper and lower surface layers is ≥97%.

4. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, The microstructure of the intermediate layer also includes bainite.

5. The high r-value ultra-high strength steel plate as described in claim 4, characterized in that, The volume ratio of tempered martensite to bainite in the intermediate layer is ≥40%.

6. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, The ferrite in the upper and lower surface layers contains carbides of the type Ti(C,N) and Nb(C,N).

7. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, The thickness of its upper and lower surfaces is 100-200 micrometers, respectively.

8. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, Its thickness is 0.8-2.5mm.

9. The high r-value ultra-high strength steel plate as described in claim 1, characterized in that, Its chemical elements also include at least one of Cr, Mo, and B; wherein B ≤ 0.005% and Cr + Mo ≤ 0.5%.

10. A method for manufacturing a high r-value ultra-high strength steel plate as described in any one of claims 1-9, characterized in that, The steps include: smelting and casting; hot rolling; and cold rolling after pickling. annealing; The annealing step includes: High-temperature humidification and decarburization process: Water vapor is sprayed into the annealing furnace for humidification, while the heating temperature of the steel plate is controlled at 880-950℃ and maintained for 100-300s, and the dew point in the annealing furnace is controlled to be above 0℃. Normal slow cooling process: The steel plate is cooled from the high temperature and humidity to between 730-770℃ at a cooling rate of 3-10℃ / s; Quasi-static slow cooling process: The steel plate is slowly cooled from 730-770℃ to 680-720℃, with the cooling temperature range controlled between 40-60℃ and the cooling rate between 0.03-0.1℃ / s, ensuring that the slow cooling process lasts for more than 500 seconds; Rapid cooling process: The steel plate is rapidly cooled to below 300℃ at a cooling rate of ≥50℃ / s; Over-aging process: The steel plate is tempered at 240-320℃ for 150-500 seconds.

Images