A 890mpa grade very high strength marine steel plate and a manufacturing method thereof

Abstract

The application discloses a 890MPa-grade very high-strength marine steel plate and a manufacturing method thereof. The chemical components of the steel plate are as follows in percentage by mass: C: 0.12-0.16%, Si: 0.10-0.20%, Mn: 0.60-0.90%, P: <=0.012%, S <=0.002%, Ni: 1.60-2.00%, Cr: 0.40-0.70%, Nb: 0.015-0.035%, V: 0.05-0.09%, Mo: 0.50-0.70%, Ti: 0.007-0.02%, B: 0.0007-0.002%, Alt: 0.018-0.04%, O <=0.001%, N <=0.005%, and the balance of Fe and inevitable impurities. The steel plate is coupled and designed through an alloy component-deformation-heat treatment process, and has good comprehensive mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, specifically relating to an 890MPa grade ultra-high strength marine steel plate and its manufacturing method. Background Technology

[0002] In recent years, with the continuous exploration and development of the marine field, the research and development and production of steel required for supporting ships and marine engineering have become of paramount importance. With the increasing size of marine engineering equipment, the safety of ultra-deepwater operations on offshore platforms, and the lightweighting of marine engineering equipment, the strength requirements for steel used in marine engineering equipment are constantly increasing. Therefore, it is necessary to design thinner and lighter load-bearing structures to lower the platform's center of gravity, improve stability, reduce welding, and simultaneously increase the effective load of important equipment and improve equipment efficiency. In practical engineering, the demand for ultra-high strength steel with excellent low-temperature impact toughness and a yield strength of 890 MPa or higher is gradually increasing, such as for marine cranes, booms, new-generation ultra-deepwater semi-submersible platforms, deep-sea survey instruments, and deep-sea mining vehicles.

[0003] Currently, the low-temperature impact toughness of 890MPa grade steel plates designed and produced both domestically and internationally is generally poor, failing to meet the requirements of marine engineering equipment. Furthermore, the production of 890MPa ultra-high strength steel plates using TMCP or TMCP+T processes places extremely high demands on the capabilities of rolling mills and straightening equipment, and it is difficult to guarantee the uniformity of plate shape and performance.

[0004] The invention patent CN101451212B, entitled "A High-Strength Steel Plate and Its Preparation Method", produces a high-strength steel plate with a yield strength of not less than 890 MPa using quenching and tempering processes. However, its low-temperature impact toughness is not high, and the impact energy of the actual product at -40℃ is only 30 to 50 J, which cannot meet the requirements of low-temperature impact toughness for marine engineering steel.

[0005] The invention patent CN103898406B, entitled "A Low Weld Crack Sensitivity Steel Plate with a Yield Strength of 890MPa and its Manufacturing Method," employs controlled thermomechanical rolling and cooling technology without tempering. Its weld crack sensitivity index Pcm ≤ 0.25%, yield strength greater than 890MPa, tensile strength greater than 950MPa, Charpy impact energy Akv (-20℃) ≥ 120J, and plate thickness up to 60mm are all applicable. However, its thickness specification is relatively small and cannot meet the requirements for low-temperature impact toughness at -40℃ for marine engineering steel.

[0006] The invention patent CN109811254A, entitled "An Ultra-High Strength VL890TM Steel Plate and Its Production Method," is based on a low-alloy chemical composition design and employs controlled rolling and cooling + high compression ratio technology to provide an ultra-high strength VL890TM steel plate with a yield strength ≥890MPa, tensile strength 940~1100MPa, and transverse impact energy ≥46J at -20℃, as well as its production method. However, its maximum thickness is only 50mm, and it cannot meet the requirements for low-temperature impact toughness of marine engineering steel at -40℃. Summary of the Invention

[0007] To address the above shortcomings, this invention employs a coupled design of alloy composition design, smelting, controlled rolling and cooling, and LQT tempering heat treatment processes to obtain an 890MPa grade ultra-high strength marine engineering steel plate with excellent low-temperature performance and a maximum thickness of 80mm.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A type of 890MPa grade ultra-high strength marine engineering steel plate, wherein the chemical composition of the steel plate, by mass percentage, is: C: 0.12-0.16%, Si: 0.10-0.20%, Mn: 0.60-0.90%, P: ≤0.012%, S≤0.002%, Ni: 1.60-2.00%, Cr: 0.40-0.70%, Nb: 0.015-0.035%, V: 0.05-0.09%, Mo: 0.50-0.70%, Ti: 0.007-0.02%, B: 0.0007-0.002%, Alt: 0.018-0. 0.04%, O≤0.001%, N≤0.005%, balance is Fe and unavoidable impurities; and Ceq≤0.68%, CET≤0.40%, Bs-Ms≤180℃, where Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5, CET=C+(Mn+Mo) / 10+(Cr+Cu) / 20+Ni / 40, Bs(℃)=830-270C-90Mn-37Ni-70Cr-83Mo, Ms(℃)=561-474C-33Mn-17Ni-17Cr-21Mo.

[0010] The mechanisms of action of each alloy component in the steel of this invention are as follows:

[0011] C is an essential element for ensuring strength and hardenability. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, an increase in carbon content seriously affects the weldability and low-temperature toughness of steel. From the perspective of product performance, the C content in this invention is controlled at 0.12-0.16%.

[0012] Si: As a solid solution strengthening element, although Si is beneficial to improving the strength of steel plates and their oxidation resistance at high temperatures, Si promotes packet size coarsening, which seriously impairs the low-temperature toughness, elongation and weldability of ultra-high strength steel plates. Considering the economy and operability of steelmaking, the Si content in this invention is controlled at 0.10 to 0.20%.

[0013] Mn, as the most important alloying element in steel, not only improves the strength of steel plates but also expands the austenite phase region, lowers the Ar3 point temperature, and refines ferrite grains, thereby improving the low-temperature toughness of steel plates. However, when the Mn content is too high, Mn segregation will result in poor low-temperature toughness in the core of thick plates and a decrease in the performance of the weld heat-affected zone. Therefore, the Mn content in this invention is controlled at 0.60–0.90%.

[0014] P is an element that negatively impacts impact values, and can impair low-temperature toughness by segregating in the center of the slab and agglomerating at grain boundaries. Therefore, the P content in this invention is controlled to be no higher than 0.012%.

[0015] S: An element that negatively impacts impact values, it can form sulfide inclusions, becoming a crack initiation point. Therefore, the S content in this invention is controlled to be no higher than 0.002%.

[0016] Ni: Nickel dissolves in austenite, inhibiting austenite recrystallization, refining austenite grains, and improving the low-temperature toughness of steel plates. Ni can reduce the diffusion rate of various elements in steel, thus delaying the decomposition and transformation of austenite, thereby improving the steel's permeability. The simultaneous addition of Ni and Mo can improve the toughness of steel while increasing its strength. However, excessive Ni content will increase Ceq and CET, affecting weldability. Therefore, the Ni content in this invention is controlled at 1.60–2.00%.

[0017] Cr: It can improve the hardenability and strength of steel plates. The presence of Cr can enhance the tempering stability of steel and ensure that the steel obtains a tempered sorbite structure during high-temperature tempering. However, if the Cr content is too high, it will increase the tendency for temper brittleness and increase the difficulty of welding, while if the content is too low, it cannot effectively exert its strengthening effect. Therefore, the Cr content in this invention is controlled at 0.40-0.70%.

[0018] The addition of niobium (Nb) promotes grain refinement of the steel's rolled microstructure, simultaneously improving strength and toughness. Niobium effectively refines the microstructure during controlled rolling by inhibiting austenite recrystallization and strengthens the matrix through precipitation. The combined addition of Nb and boron (B) effectively inhibits austenite recrystallization and grain refinement during rolling while also suppressing carbon diffusion. This effectively inhibits the precipitation of M23(C,B)6 at austenite grain boundaries, ensuring effective segregation of boron at ferrite nucleation sites, improving hardenability, and resulting in higher strength and better low-temperature toughness. The addition of Nb and the formation of Nb(C,N) in the steel suppress the appearance of coarse M23(C,B)6 structures. This reduces carbon diffusion, improves boron utilization efficiency, and promotes microstructure homogenization. However, when the Nb content exceeds a certain range, MA islands will form in the welded HAZ, which is detrimental to toughness. Therefore, the Nb content in this invention is controlled at 0.015–0.035%.

[0019] Vitamin V (V): Adding vitamin V to steel can refine the grain structure, improve strength and toughness, and enhance the steel plate's resistance to tempering softening during tempering. Too low an addition yields little effect; too high a V content reduces the steel's toughness and weldability. With an appropriate nitrogen content, V can fully precipitate, significantly reducing the particle size and spacing in the steel, resulting in precipitation strengthening and thus increasing strength. Therefore, in this invention, the V content is controlled at 0.05%–0.09%.

[0020] Mo (Mo) can shift the C-curve of steel, thereby significantly improving its hardenability. It promotes the formation of martensite or bainite with numerous dislocations within the grains over a wider cooling range, resulting in phase transformation strengthening and dislocation strengthening effects, significantly improving the strength and microstructure uniformity of the steel. In this invention, the combined addition of Mo and Nb not only provides the strengthening effect of adding molybdenum or niobium alone, but also allows molybdenum to segregate at the NbC matrix interface, preventing the coarsening of NbC particles. The combined addition of Mo and B increases the content of effective (solid-dissolved) B in the steel, inhibiting the diffusion of C to grain boundaries, suppressing the precipitation and growth of M23(C,B)6 at grain boundaries, promoting the segregation of B at grain boundaries, reducing the nucleation sites of ferrite, and improving the hardenability of the steel. The combined addition of Mo and Ni can improve the toughness of steel while increasing its strength. However, with the increase of Mo, the amount of Mo-containing carbides also increases. Carbides distributed at grain boundaries disrupt the continuity of the matrix and deteriorate the toughness of the steel. Therefore, the Mo content in this invention is controlled at 0.50–0.70%.

[0021] Ti: Inhibits excessive austenite grain growth during slab heating, improving steel toughness. Simultaneously, Ti acts as a solidifier for nitrogen (N), ensuring a certain level of dissolved boron (B) in the steel and improving hardenability. However, excessive Ti content can lead to TiC precipitation as TiC on martensite / bainite laths and grain boundaries, severely degrading the low-temperature toughness of the steel. Therefore, the Ti content in this invention is controlled at 0.007%–0.02%.

[0022] B: While it can improve the hardenability and strength of steel plates, excessive B content can lead to the formation of coarse BN particles that are detrimental to hardenability and toughness, and also affect the weldability and surface quality of the steel plates. Therefore, the B content in this invention is controlled at 0.0007–0.002%.

[0023] Alt (Al): A deoxidizing and grain-refining element. At high temperatures, Al forms fine AlN precipitates, which inhibit austenite grain growth during the austenitization process of slabs / steel plates, thus refining the austenite grains and improving the toughness of the steel at low temperatures. Excessive Al content leads to the formation of larger Al oxides, reducing the low-temperature impact resistance of the steel plate, and also making the slab prone to edge and corner cracks during continuous casting. Therefore, this invention controls the Alt content to 0.018–0.04%.

[0024] O: An element that negatively impacts impact toughness, it combines with other elements in steel to form non-metallic inclusions, becoming crack initiation sites. Therefore, the O content in this invention is controlled to be no higher than 0.001%.

[0025] Nitrogen (N) reacts with boron (B) to form niobium (BN), and with alkanes (AL) to form coarse alkanes (ALN) that precipitate along the original austenite grain boundaries, affecting the hardenability and low-temperature impact toughness of the steel. Therefore, the N content in this invention is controlled to be no higher than 0.005%.

[0026] In the above technical solution, the steel plate further has a yield strength ≥890MPa, a tensile strength of 940~1100MPa, an elongation of section ≥14%, a reduction of area in the Z direction ≥50%, and a Charpy impact energy at -40℃ ≥100J.

[0027] A method for manufacturing the above-mentioned 890MPa grade ultra-high strength marine engineering steel plate, the method comprising the following steps:

[0028] (1) Smelting, continuous casting and slow cooling of billet: The continuous casting billet is produced by deep desulfurization of molten iron, converter smelting, ladle refining, vacuum treatment and continuous casting process. The superheat of molten steel in the tundish is 25-30°C. The casting is protected throughout the process. Electromagnetic stirring is turned on and a light reduction process is used. The light reduction is 6-9mm. After continuous casting, the continuous casting billet is obtained with a thickness of 250-360mm. The continuous casting billet is stacked and slow cooled after it is taken off the line.

[0029] (2) Rolling: The continuously cast billet is heated in four stages: a preheating stage, a heating stage 1, a heating stage 2, and a soaking stage. The heating temperature in the preheating stage is 720–820℃, and the heating time is 0.1–0.2 min / mm. The heating temperature in heating stage 1 is 900–1080℃, and the heating time is 0.2–0.4 min / mm. The heating temperature in heating stage 2 is 1100–1200℃, and the heating time is 0.1–0.3 min / mm. The heating temperature of the soaking zone is 1050-1130℃, and the heating time is 0.3-0.5 min / mm. After the continuously cast billet is heated and taken out of the furnace, it is descaled. The rolling adopts two-stage controlled rolling. The first stage rolling temperature is 950-1000℃. Except for the widening pass, the reduction rate of other single passes is ≥15%, and the thickness of the intermediate billet is 2-2.5 times the thickness of the finished product. The second stage rolling temperature is 800-850℃. The reduction rate of single passes is ≥10%, and the final rolling temperature is 770-830℃.

[0030] (3) Cooling: The steel plate is cooled directly after final rolling;

[0031] (4) Slow cooling: After controlled cooling is completed, immediately place the item in a slow cooling tank for slow cooling;

[0032] (5) Quenching and tempering heat treatment: The LQT process is used for quenching and tempering heat treatment. After exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate.

[0033] In the above technical solution, further, in step (1), the stacking temperature is ≥650℃, the stacking slow cooling rate is 5~6℃ / h, and the destacking temperature is ≤250℃.

[0034] In the above technical solution, further, in step (2), after the continuous casting billet is heated and taken out of the furnace, it is dephosphorized by high pressure water for 2 to 3 times to remove the iron oxide scale on the surface and reduce the temperature of the continuous casting billet.

[0035] In the above technical solution, further, in step (3), the cooling adopts a DQ+ACC rapid cooling system with an average cooling rate of ≥3℃ / s and a red temperature of 400~500℃.

[0036] In the above technical solution, further, in step (4), the temperature of the slow cooling tank is not lower than 350°C, and the slow cooling time is not less than 24 hours.

[0037] In the above technical solution, further, in step (5), the quenching and tempering heat treatment process is as follows: the first low-temperature quenching temperature is 720~760℃, and the furnace time is 1.8~2.1min / mm; the second high-temperature quenching temperature is 850~890℃, and the furnace time is 1.4~1.7min / mm; the steel plate after the second quenching is tempered, and the tempering temperature is 600~660℃, and the furnace time is 3~5min / mm.

[0038] The beneficial effects of this invention are as follows:

[0039] (1) The steel plate of the present invention has good comprehensive mechanical properties through the coupled design of alloy composition-deformation-heat treatment process, which solves the problem of difficulty in matching and harmonizing the strength, plasticity and low temperature toughness (-40℃) of 890MPa grade steel plate.

[0040] (2) This invention fully leverages the technical equipment advantages of the heavy plate rolling mill and heat treatment unit, and combines 250-360mm thick continuous casting slabs to develop ultra-high strength and high toughness marine engineering steel thick plates with a maximum finished product thickness of 80mm.

[0041] (3) The present invention adopts a reasonable rolling process and offline quenching and tempering process. Compared with online quenching of ultra-high strength steel of the same strength, offline quenching can accurately control the temperature at the start of quenching, and the temperature of the whole plate is uniform, resulting in more stable performance of the produced steel plate. Attached Figure Description

[0042] Figure 1 This is a 500x metallographic image of Example 1. Detailed Implementation

[0043] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0044] The chemical composition of the steel plates of Examples 1-12 of the present invention is shown in Table 1, and the carbon equivalent and Bs-Ms value of the steels of Examples 1-12 of the present invention are shown in Table 2.

[0045] Table 1 Chemical composition of steel plates

[0046]

[0047]

[0048] Table 2 Carbon equivalent and Bs-Ms value of steel plates

[0049] Example Ceq / % CET / % Bs-Ms / ℃ 1 0.64 0.35 157 2 0.64 0.35 150 3 0.62 0.35 161 4 0.62 0.34 150 5 0.62 0.35 168 6 0.66 0.35 132 7 0.59 0.32 153 8 0.62 0.34 150 9 0.58 0.33 164 10 0.63 0.35 151 11 0.60 0.34 166 12 0.63 0.35 158

[0050] The manufacturing method of the above-mentioned 890MPa grade ultra-high strength marine engineering steel plate includes the following steps:

[0051] (1) Smelting, continuous casting and slow cooling of billet: The continuous casting billet is produced by deep desulfurization of molten iron, converter smelting, ladle refining, vacuum treatment and continuous casting process. The superheat of molten steel in the tundish is 25-30℃. The casting is carried out under full protection. Electromagnetic stirring is turned on and a light reduction process is used. The light reduction is 6-9mm. After continuous casting, the continuous casting billet is obtained with a thickness of 250-360mm. After the continuous casting billet is removed from the line, it is stacked and slow cooled. The stacking temperature is ≥650℃, the stacking slow cooling rate is 5-6℃ / h, and the destacking temperature is ≤250℃. The smelting and stacking process parameters are shown in Table 3.

[0052] Table 3 Smelting and stacking process parameters for continuously cast billets

[0053]

[0054]

[0055] (2) Rolling: The continuously cast billet is heated in four stages: a preheating stage, heating stage 1, heating stage 2, and soaking stage. The preheating stage has a heating temperature of 720–820℃ and a heating time of 0.1–0.2 min / mm; heating stage 1 has a heating temperature of 900–1080℃ and a heating time of 0.2–0.4 min / mm; heating stage 2 has a heating temperature of 1100–1200℃ and a heating time of 0.1–0.3 min / mm; and the soaking stage has a heating temperature of 1050–113℃. The heating temperature is 0℃, and the heating time is 0.3~0.5min / mm. The heating process parameters are shown in Table 4. After the continuous casting billet is heated and taken out of the furnace, it is descaled 2~3 times to remove the iron oxide scale on the surface and reduce the temperature of the continuous casting billet. The rolling adopts two-stage controlled rolling. The first stage rolling temperature is 950~1000℃. Except for the widening pass, the reduction rate of other single passes is ≥15%, and the thickness of the intermediate billet is 2~2.5 times the thickness of the finished product. The second stage rolling temperature is 800~850℃. The reduction rate of single passes is ≥10%, and the final rolling temperature is 770~830℃.

[0056] (3) Cooling: After final rolling, the steel plate is directly cooled using a DQ+ACC rapid cooling system with an average cooling rate of ≥3℃ / s and a reddening temperature of 400~500℃; the steel plate rolling and cooling process is shown in Table 5.

[0057] Table 4 Heating process for continuously cast billets

[0058]

[0059]

[0060] Table 5 Steel Plate Rolling and Cooling Process

[0061]

[0062] (4) Slow cooling: After controlled cooling is completed, the product is immediately placed in a slow cooling tank. The temperature of the tank is not lower than 350℃ and the slow cooling time is not less than 24 hours.

[0063] (5) Quenching and tempering heat treatment: The LQT process is used for quenching and tempering heat treatment, which is carried out in two stages. The first low-temperature quenching temperature is 720-760℃ and the furnace time is 1.8-2.1 min / mm. The second high-temperature quenching temperature is 850-890℃ and the furnace time is 1.4-1.7 min / mm. After the second quenching, the steel plate is tempered at 600-660℃ and the furnace time is 3-5 min / mm. After being taken out of the furnace, it is air-cooled to obtain the finished steel plate. The quenching and tempering heat treatment process of the steel plate is shown in Table 6.

[0064] Table 6. Heat Treatment Process for Steel Plates

[0065]

[0066] like Figure 1 As shown, the microstructure of the steel plate in Example 1 is tempered martensite. The mechanical properties of the above-mentioned 890MPa grade ultra-high strength marine steel plate are tested and are shown in Table 7.

[0067] Table 7 Mechanical Properties of Steel Plates

[0068]

[0069]

[0070] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A type of 890MPa grade ultra-high strength marine engineering steel plate, characterized in that, The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.12~0.16%, Si: 0.10~0.20%, Mn: 0.60~0.90%, P: ≤0.012%, S≤0.002%, Ni: 1.60~2.00%, Cr: 0.40~0.70%, Nb: 0.015~0.035%, V: 0.05~0.09%, Mo: 0.50~0.70%, Ti: 0.007~0.02%, B: 0.0007~0.002%, Alt: 0.018~0.04%, O≤0. 0.001%, N≤0.005%, balance is Fe and unavoidable impurities; and Ceq≤0.68%, CET≤0.40%, Bs-Ms≤180℃, where, Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5, CET=C+(Mn+Mo) / 10+(Cr+Cu) / 20+Ni / 40, Bs(℃)=830-270C-90Mn-37Ni-70Cr-83Mo, Ms(℃)=561-474C-33Mn-17Ni-17Cr-21Mo; The method for preparing the steel plate includes the following steps: (1) Smelting, continuous casting and slow cooling of billet: The continuous casting billet is produced by using the process of deep desulfurization of molten iron, converter smelting, ladle refining, vacuum treatment and continuous casting. The superheat of molten steel in the tundish is 25~30℃. The casting is carried out under full protection. Electromagnetic stirring is turned on and a light reduction process is used. The light reduction is 6~9mm. After continuous casting, the continuous casting billet is obtained with a thickness of 250~360mm. After the continuous casting billet is removed from the line, it is stacked and slow cooled. (2) Rolling: The continuously cast billet is heated in four stages, namely a preheating stage, heating stage 1, heating stage 2, and soaking stage. The heating temperature of the preheating stage is 720~820℃ and the heating time is 0.1~0.2 min / mm. The heating temperature of heating stage 1 is 900~1080℃ and the heating time is 0.2~0.4 min / mm. The heating temperature of heating stage 2 is 1100~1200℃ and the heating time is 0.1~0.3 min / mm. The heating temperature of the soaking zone is 1050~1130℃, and the heating time is 0.3~0.5min / mm; after the continuously cast billet is heated and taken out of the furnace, it is descaled; the rolling adopts two-stage controlled rolling. The first stage rolling temperature is 950~1000℃, and except for the widening pass, the reduction rate of other single passes is ≥15%, and the thickness of the intermediate billet is 2~2.5 times the thickness of the finished product. The second stage rolling temperature is 800~850℃, the reduction rate of single passes is ≥10%, and the final rolling temperature is 770~830℃. (3) Cooling: The steel plate is cooled directly after final rolling; (4) Slow cooling: After controlled cooling is completed, immediately place the container in a slow cooling tank for slow cooling; (5) Quenching and tempering heat treatment: The LQT process is used for quenching and tempering heat treatment. After exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate. In step (5), the quenching and tempering heat treatment process is as follows: the first low-temperature quenching temperature is 720~760℃, and the furnace time is 1.8~2.1min / mm; the second high-temperature quenching temperature is 850~890℃, and the furnace time is 1.4~1.7min / mm; the steel plate after the second quenching is tempered at a temperature of 600~660℃ and a furnace time of 3~5min / mm.

2. The 890MPa grade ultra-high strength marine steel plate according to claim 1, characterized in that, The steel plate has a yield strength ≥890MPa, tensile strength 940~1100MPa, elongation at section ≥14%, reduction of area in the Z direction ≥50%, and Charpy impact energy at -40℃ ≥100J.

3. A method for manufacturing an 890MPa grade ultra-high strength marine steel plate according to any one of claims 1-2, the method comprising the following steps: (1) Smelting, continuous casting and slow cooling of billet: The continuous casting billet is produced by using the process of deep desulfurization of molten iron, converter smelting, ladle refining, vacuum treatment and continuous casting. The superheat of molten steel in the tundish is 25~30℃. The casting is carried out under full protection. Electromagnetic stirring is turned on and a light reduction process is used. The light reduction is 6~9mm. After continuous casting, the continuous casting billet is obtained with a thickness of 250~360mm. After the continuous casting billet is removed from the line, it is stacked and slow cooled. (2) Rolling: The continuously cast billet is heated in four stages, namely a preheating stage, heating stage 1, heating stage 2, and soaking stage. The heating temperature of the preheating stage is 720~820℃ and the heating time is 0.1~0.2 min / mm. The heating temperature of heating stage 1 is 900~1080℃ and the heating time is 0.2~0.4 min / mm. The heating temperature of heating stage 2 is 1100~1200℃ and the heating time is 0.1~0.3 min / mm. The heating temperature of the soaking zone is 1050~1130℃, and the heating time is 0.3~0.5min / mm; after the continuously cast billet is heated and taken out of the furnace, it is descaled; the rolling adopts two-stage controlled rolling. The first stage rolling temperature is 950~1000℃, and except for the widening pass, the reduction rate of other single passes is ≥15%, and the thickness of the intermediate billet is 2~2.5 times the thickness of the finished product. The second stage rolling temperature is 800~850℃, the reduction rate of single passes is ≥10%, and the final rolling temperature is 770~830℃. (3) Cooling: The steel plate is cooled directly after final rolling; (4) Slow cooling: After controlled cooling is completed, immediately place the container in a slow cooling tank for slow cooling; (5) Quenching and tempering heat treatment: The LQT process is used for quenching and tempering heat treatment. After exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate. In step (5), the quenching and tempering heat treatment process is as follows: the first low-temperature quenching temperature is 720~760℃, and the furnace time is 1.8~2.1min / mm; the second high-temperature quenching temperature is 850~890℃, and the furnace time is 1.4~1.7min / mm; the steel plate after the second quenching is tempered at a temperature of 600~660℃ and a furnace time of 3~5min / mm.

4. The preparation method according to claim 3, characterized in that, In step (1), the stacking temperature is ≥650℃, the slow cooling rate of the stacking is 5~6℃ / h, and the destacking temperature is ≤250℃.

5. The preparation method according to claim 3, characterized in that, In step (2), after the continuous casting billet is heated and taken out of the furnace, it is dephosphorized by high pressure water for 2 to 3 times to remove the iron oxide scale on the surface and reduce the temperature of the continuous casting billet.

6. The preparation method according to claim 3, characterized in that, In step (3), the cooling adopts the DQ+ACC rapid cooling system with an average cooling rate of ≥3℃ / s and a red temperature of 400~500℃.

7. The preparation method according to claim 3, characterized in that, In step (4), the temperature of the slow-cooling tank is not lower than 350°C, and the slow-cooling time is not less than 24 hours.

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