A kind of extra-thick high-strength and tough rack steel for offshore platform and its manufacturing method

Abstract

The present invention relates to the technical field of steel materials and their preparation, and in particular to a super-thick high-strength and tough rack steel for marine platforms and a manufacturing method thereof, wherein the sum of the mass percentage of V element and the mass percentage of Nb element in the element composition of the steel plate is in the range of 0.09-0.12%; the mass percentage of V element: the mass percentage of Nb element is controlled at 2:1. A heat treatment process of two quenchings and high-temperature tempering after controlled rolling is adopted, and a Ni-Cr-Mo alloying component system is used to obtain a steel material with a tempered martensitic structure, with a maximum thickness of up to 210 mm, and finally the yield strength of the steel plate is greater than 880 MPa, the tensile strength is greater than 1000 MPa, the Charpy V-notch impact energy at -60°C is greater than 170 J, and the elongation after fracture is greater than 17%. The steel plate of the present invention has a simple production process, low cost, good uniformity of structure in the thickness direction, excellent strength-toughness matching, and can be used to manufacture racks for marine platforms.

Description

Technical Field

[0001] The invention relates to the technical field of steel materials and preparation thereof, and in particular to a special thick high-strength and tough rack steel for an offshore platform and a manufacturing method thereof. Background Art

[0002] As a key facility for offshore operations and life, self-elevating offshore platforms play a vital role in marine energy exploitation. Among them, rack steel is the core component of the lifting legs of self-elevating offshore platforms. It not only needs to support the weight of the entire platform, but also has to withstand harsh environments such as offshore wind and waves, undercurrents, and atmospheric erosion. Therefore, in the construction of offshore platforms, the quality of rack steel is crucial, requiring it to have a large thickness, high strength and toughness, as well as good seawater corrosion resistance and welding performance.

[0003] As the thickness of gear steel gradually increases, the strength-toughness matching problem becomes more complicated, especially at 1 / 2 of the thickness direction. This not only puts forward requirements on the composition of extra-thick rack steel, but also requires special attention to its smelting, rolling and heat treatment processes. These processes have an important influence on the uniformity of the structure of the steel thickness section and the control of the gradient structure.

[0004] At present, the domestic steel for extra-thick gears mainly adopts the Ni-Cr-Mo alloying component system and adopts the traditional quenching + tempering heat treatment process. However, this method may lead to the following problems: (1) The highest strength grade steel currently used in self-elevating platforms - 690MPa grade high-toughness, corrosion-resistant and easy-to-weld offshore platform steel, is only produced by some manufacturers in Japan, the United States and Germany, which seriously restricts the development of my country's offshore engineering equipment. (2) During the quenching process, Ni-Cr-Mo alloy steel is prone to structural inheritance, resulting in coarse grains, uneven structure, and mixed crystals in the steel plate. (3) Due to the internal segregation of the continuous casting billet and the limitation of the compression ratio, the thickness of the offshore platform rack steel plate currently produced is generally less than 150mm. When the thickness exceeds 150mm, problems such as unstable low-temperature impact energy, poor Z-direction performance and uneven thickness direction performance often occur. Therefore, it is necessary to more effectively control the structure of the steel plate thickness section during the heat treatment process to ensure that the steel plate can obtain excellent strength and toughness. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, a super thick high strength rack steel for offshore platforms and a manufacturing method thereof are proposed, which utilizes the Ni-Cr-Mo alloying composition system, controls the ratio of V and Nb elements, and adopts double quenching + high temperature tempering after controlled rolling. ofThe heat treatment process obtains a tempered martensitic steel with a maximum thickness of 256 mm, and the final yield strength of the steel plate is greater than 880 MPa, the tensile strength is greater than 1000 MPa, the Charpy V-notch impact energy at -60°C is greater than 170 J, and the elongation after fracture is greater than 17%. The steel plate of the present invention has a simple production process, low cost, good uniformity of structure in the thickness direction, and excellent strength and toughness matching, and can be used to manufacture racks for offshore platforms.

[0006] In order to achieve the above-mentioned purpose, the steel plate of the present invention has been systematically tested in terms of composition design, rolling process parameters, microstructure control, etc. The technical solution of the present invention is as follows:

[0007] A special thick high-strength and tough rack steel for an offshore platform, wherein the mass percentages of the elements are as follows: C: 0.09-0.16, Si: 0.1-0.3, Mn: 0.3-0.8, Ni: 3.3-4.3, Cr: 0.8-1.5, Mo: 0.8-1.5, Cu: 0.2-0.5, B: 0.001-0.002, S≤0.005%, P≤0.003%, further containing V and Nb, and the balance being Fe and other inevitable impurities; the sum of the mass percentage of the V element and the mass percentage of the Nb element is in the range of 0.09-0.12%;

[0008] The microstructure of the extra-thick high-strength and tough rack steel for the offshore platform is a tempered martensite structure and a carbide precipitation phase, and the thickness of the extra-thick high-strength and tough rack steel for the offshore platform is 256 mm.

[0009] The mass percentage of the V element: the mass percentage of the Nb element are controlled at 2:1.

[0010] The yield strength of the extra-thick high-strength and tough rack steel for the offshore platform is greater than 880 MPa, the tensile strength is greater than 1000 MPa, the Charpy V-notch impact energy at -60°C is greater than 170 J, and the elongation after fracture is greater than 17%.

[0011] A method for preparing a particularly thick and high-strength rack steel for an offshore platform comprises the following steps:

[0012] S1. Smelting: According to the mass percentage ratio of each element, the steel ingot is obtained by smelting and pouring, and the steel ingot is treated by LF and RH refining furnaces. The superheat of the molten steel in the tundish is ≤20°C, and the casting is protected throughout the whole process;

[0013] S2, continuous casting; the molten steel prepared in S1 flows into the casting mold of the continuous casting machine through the pouring system, gradually solidifies under the action of cooling water to form a steel billet, and further cools in the secondary cooling zone to obtain a continuously cast billet;

[0014] S3, rolling: heating the continuous casting billet prepared in S2 to 1100-1200°C and keeping the temperature for 5-8h, and then performing controlled rolling to obtain a hot-rolled plate, with the roller speed controlled at 1.0-1.5m / s, the pressure value ≥3000KN·m, and ensuring 2-4 passes during rolling, with a single pass reduction rate of more than 8%;

[0015] S4, slow cooling; hydrogen expansion and slow cooling of the hot-rolled plate prepared in step S3 to obtain a hot-rolled steel plate, the hydrogen expansion temperature is ≥300°C, and the time is >14min / mm;

[0016] S5, quenching; the hot-rolled steel plate in step S4 is completely austenitized, and then water-quenched to room temperature after heat preservation to obtain a quenched steel plate;

[0017] S6, tempering; the quenched steel plate in step S5 is subjected to high temperature tempering, and then air-cooled to room temperature after heat preservation to obtain a final steel plate.

[0018] In step S1, the melting temperature is 1600-1700°C, the treatment time of LF and RH refining furnace is 10-30 minutes, and the superheat temperature of molten steel in the tundish is 10-20°C.

[0019] In step S2, the cooling water flow rate of the mold cooling is 20~30L / min·m², the pulling speed during the solidification process is 0.6~0.8m / min, and the solidification speed is 2~4mm / min; the water flow rate in the secondary cooling zone is 50~70L / min·m², the cooling intensity is 3~5℃ / min, and the continuous casting billet is cooled to 900~950℃ and then heated to the rolling temperature.

[0020] In step S3, the continuous casting billet is subjected to two-stage rolling. In the first stage, the rough rolling temperature is 1100-1200°C, the rough rolling is performed for 3-5 passes, and the reduction rate of a single pass is between 6-10%. After the rough rolling, the temperature is raised to 820-870°C for the second stage of finishing rolling, the finishing rolling is performed for 2-4 passes, and the reduction rate of a single pass is between 6-10%. After rolling, the billet is air-cooled.

[0021] In step S4, the slow cooling time is ≥ 48 h.

[0022] In step S5, a furnace heating method is adopted, the heating rate is ≤120°C / h, and a two-time quenching process is used. The first quenching temperature is 900~950°C, the average temperature holding time coefficient is 1.5~2.0min / mm, and a fast laminar cooling system with an average cooling rate greater than 15°C / s is used to cool to room temperature; the second quenching temperature is 820~870°C, the average temperature holding time coefficient is 1.0~1.5min / mm, and other conditions are the same.

[0023] In step S6, the temperature range of high temperature tempering is 600-650°C, the uniform temperature holding time coefficient is 3.0-4.0 min / mm, and air cooling is performed to room temperature.

[0024] The principle of the present invention is as follows:

[0025] V and Nb are added in combination. NbC mainly plays a role in grain refinement, while VC mainly plays a role in precipitation strengthening. The combination of the two elements can achieve both grain refinement and precipitation strengthening. In this composition system, V:Nb is controlled at 2:1, which can achieve the best grain refinement and precipitation strengthening effects. When the combination of V and Nb is in the range of 0.09~0.12%, the thickness can reach 256mm.

[0026] In the preparation method, step S2 ensures the uniformity of the structure of the core and surface of the continuous casting billet by controlling the solidification process, temperature distribution and cooling rate of the billet to avoid cracks, deformation or internal defects; after cooling to 900-950°C and then heating to the rolling temperature, the internal temperature and the surface temperature tend to be uniform. Within this temperature range, the ratio of V:Nb is 2:1, which can ensure that NbC and VC will not be excessively dissolved and retain a certain precipitation strengthening power.

[0027] The finishing rolling temperature range in step S3 is 820~870℃; if it is greater than 870℃, sufficient precipitation strengthening cannot be achieved, and the fine grain strengthening effect of NbC will not be reflected, which is not conducive to strength and toughness. If it is less than 820℃, excessive precipitation of NbC and VC will occur, which is not conducive to the plasticity of the steel plate and the plate shape will also deteriorate. It is appropriate to control the ratio of V:Nb to 2:1, and finish rolling in this temperature range is appropriate.

[0028] In step S5, two quenching processes are used. The purpose of the first quenching is to obtain a martensitic structure by rapid cooling, significantly improving the hardness and strength of the steel. The purpose of the second quenching is to optimize the performance of the martensitic structure, eliminate some internal stress, and further improve the toughness of the steel.

[0029] In step S6, the high temperature tempering temperature range is 600-650°C, which can completely remove the residual stress of the steel plate itself, improve the toughness of the material, and retain the precipitation effect of NbC and VC. The precipitates are mainly VC, and some (Cr, Mo)C precipitates will provide hardening. NbC is less and mainly precipitates at high temperature to provide fine grain strengthening effect.

[0030] The beneficial effects of the present invention are as follows: the maximum thickness of the rack steel prepared by the present invention can reach 256 mm, and the final yield strength of the steel plate is greater than 880 MPa, the tensile strength is greater than 1000 MPa, the Charpy V-notch impact energy at -60°C is greater than 170 J, and the elongation after fracture is greater than 17%.

[0031] (1) The steel plate of the present invention reduces the Mn content and increases the Cr and Mo contents on the basis of conventional rack steel, thereby reducing the segregation phenomenon of the core structure, which is beneficial to the uniformity of the structure in the thickness direction; at the same time, by utilizing the Ni-Cr-Mo alloying component system and controlling the Nb and V content ratio, utilizing the fine grain strengthening effect of NbC at high temperature and utilizing the precipitation strengthening effect of VC during tempering, an excellent strength-toughness match can be achieved.

[0032] (2) The steel plate of the present invention adopts a two-stage rolling process of low speed and high pressure, which reduces the surface temperature, improves the core rolling force penetration, forms a large number of deformation bands and dislocations inside the austenite grains, increases the nucleation position, promotes the refinement of austenite grains, and ensures the performance requirements of extra-thick rack steel.

[0033] (3) The steel plate of the present invention controls the solidification process, temperature distribution and cooling rate of the steel billet during the continuous casting process, and reduces the heating rate during the quenching process, thereby ensuring the uniformity of the structure in the thickness direction, and further refines the original austenite grains by using the double quenching process, thereby promoting the fine grain strengthening effect and precipitation strengthening effect, which is beneficial to the improvement of the overall strength and toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a transmission electron microscope image of the finished steel plate obtained in Example 2 of the present invention at 1 / 2 thickness. DETAILED DESCRIPTION

[0035] Example 1 (Low Carbon High Alloy)

[0036] A special thick high-strength rack steel for offshore platform is prepared. The thickness of the finished steel plate is 256mm. The process steps are as follows:

[0037] According to the chemical cost of the extra-thick high-strength and tough rack steel for an offshore platform, the chemical composition is recorded in weight percentage as follows: C: 0.09, Si: 0.3, Mn: 0.8, Ni: 4.3, Cr: 1.5, Mo: 1.5, Cu: 0.5, B: 0.002, V: 0.08%, Nb: 0.04%, Ti: 0.01%, S≤0.005%, P≤0.003%, and the balance is Fe and other inevitable impurities; the smelting temperature is 1700°C, the LF and RH refining furnace treatment time is 20 minutes, the temperature of the tundish steel superheat is 15°C; the cooling water flow rate for mold cooling is 25L / min·m², the pulling speed during the solidification process is 0.7m / min, the solidification speed is 3mm / min, the water flow in the secondary cooling zone is 60L / min·m², the cooling intensity is 4℃ / min, and the continuous casting billet is cooled to 900℃ and then heated to the rolling temperature; the 370mm thick continuous casting billet is heated to 1200℃, kept warm for 6h, and then hot-rolled into a 256mm thick hot-rolled steel plate through 5 passes, rough rolling 3 passes, the starting rolling temperature is 1100℃, and the single pass reduction rate is between 9~14%; finish rolling 2 passes, the starting rolling temperature is 820℃, and the single pass reduction rate is between 6~10%; after hot rolling, air cool to room temperature;

[0038] Subsequently, the hot-rolled steel plate after hydrogen expansion and slow cooling was heated to 940°C and kept warm for 7 hours. The heating rate was 100°C / h and cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15°C / s. The second quenching temperature was 840°C and kept warm for 4 hours. The uniform temperature holding time coefficient was 1.0min / mm, and other conditions were the same. The quenched steel plate was heated to 600°C in the furnace and kept warm for 11 hours, then air-cooled to room temperature to obtain a finished plate.

[0039] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 891MPa, the tensile strength is 1023MPa, the Charpy V-notch impact energy at -60℃ is 183J, and the elongation after fracture is 18.1%.

[0040] Example 2 (Medium Carbon Medium Alloy)

[0041] A special thick high-strength rack steel for offshore platform is prepared. The thickness of the finished steel plate is 256mm. The process steps are as follows:

[0042] According to the chemical cost of the extra-thick high-strength and tough rack steel for an offshore platform, the chemical composition is recorded in weight percentage as follows: C: 0.12, Si: 0.2, Mn: 0.4, Ni: 3.7, Cr: 1.2, Mo: 0.8, Cu: 0.3, B: 0.002, V: 0.06%, Nb: 0.03%, Ti: 0.01%, S≤0.005%, P≤0.003%, and the balance is Fe and other inevitable impurities; the smelting temperature is 1700°C, the treatment time of LF and RH refining furnaces is 20min, the superheat temperature of the molten steel in the tundish is 15°C; the cooling water flow rate of the mold cooling is 25L / min·m², the pull-out speed during the solidification process is 0.7m / min, the solidification speed is 3mm / min, the water flow rate in the secondary cooling zone is 60L / min·m², and the cooling intensity is 4°C / mi n, after the continuous casting billet is cooled to 900℃, it is heated to the rolling temperature; the 370mm thick continuous casting billet is heated to 1200℃, kept warm for 6h, and then hot-rolled into a 256mm thick hot-rolled steel plate through 5 passes, rough rolling is performed for 3 passes, the starting rolling temperature is 1150℃, and the single pass reduction rate is between 9~14%; finishing rolling is performed for 2 passes, the starting rolling temperature is 860℃, and the single pass reduction rate is between 6~10%; after hot rolling, it is air-cooled to room temperature; then the hot-rolled steel plate after hydrogen expansion and slow cooling is heated to 940℃ and kept warm for 7h, the heating rate is 100℃ / h, and it is cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15℃ / s, the second quenching temperature is 840℃, kept warm for 4h, the average temperature holding time coefficient is 1.0min / mm, and other conditions are the same; the quenched steel plate is heated to 630℃ with the furnace and kept warm for 11h, and then air-cooled to room temperature to obtain a finished plate.

[0043] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 915MPa, the tensile strength is 1056MPa, the Charpy V-notch impact energy at -60℃ is 176J, and the elongation after fracture is 17.6%.

[0044] Example 3 (High Carbon Low Alloy)

[0045] A special thick high-strength rack steel for offshore platform is prepared. The thickness of the finished steel plate is 256mm. The process steps are as follows:

[0046] According to the chemical cost of the extra-thick high-strength and tough rack steel for an offshore platform, the chemical composition is recorded in weight percentage as follows: C: 0.16, Si: 0.1, Mn: 0.3, Ni: 3.3, Cr: 0.8, Mo: 0.8, Cu: 0.2, B: 0.001, V: 0.06%, Nb: 0.03%, Ti: 0.01%, S≤0.005%, P≤0.003%, and the balance is Fe and other inevitable impurities; the smelting temperature is 1700°C, the treatment time of LF and RH refining furnaces is 20min, the superheat temperature of the molten steel in the tundish is 15°C; the cooling water flow rate of the mold cooling is 25L / min·m², the pull-out speed during the solidification process is 0.7m / min, the solidification speed is 3mm / min, the water flow rate in the secondary cooling zone is 60L / min·m², and the cooling intensity is 4°C / mi n, after the continuous casting billet is cooled to 900℃, it is heated to the rolling temperature; the 370mm thick continuous casting billet is heated to 1200℃, kept warm for 6h, and then hot-rolled into a 256mm thick hot-rolled steel plate through 5 passes, rough rolling is performed for 3 passes, the starting rolling temperature is 1200℃, and the single pass reduction rate is between 9~14%; finishing rolling is performed for 2 passes, the starting rolling temperature is 870℃, and the single pass reduction rate is between 6~10%; after hot rolling, it is air-cooled to room temperature; then the hot-rolled steel plate after hydrogen expansion and slow cooling is heated to 940℃ and kept warm for 7h, the heating temperature is 100℃ / h, and it is cooled to room temperature using a rapid laminar cooling system with an average cooling rate greater than 15℃ / s, the second quenching temperature is 840℃, kept warm for 4h, the average temperature holding time coefficient is 1.0min / mm, and other conditions are the same; the quenched steel plate is heated to 650℃ with the furnace and kept warm for 11h, and then air-cooled to room temperature to obtain a finished plate.

[0047] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 954MPa, the tensile strength is 1171MPa, the Charpy V-notch impact energy at -60℃ is 172J, and the elongation after fracture is 17.3%.

[0048] Comparative Example 1 (the sum of V and Nb contents is outside 0.09-0.12%)

[0049] The components of Comparative Example 1 are compared with those of Example 2, except for the differences in the contents of V and Nb elements. The V content of Comparative Example 1 is 0.02%, and the Nb content is 0.01%, and other conditions are the same.

[0050] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 835MPa, the tensile strength is 986MPa, the Charpy V-notch impact energy at -60℃ is 157J, and the elongation after fracture is 17.9%.

[0051] Comparative Example 2 (V:Nb is not 2:1)

[0052] The composition of Comparative Example 2 is different from that of Example 2 only in the content of V and Nb elements. The V content of Comparative Example 2 is 0.03%, and the Nb content is 0.06%. Other conditions are the same.

[0053] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 951MPa, the tensile strength is 1124MPa, the Charpy V-notch impact energy at -60℃ is 146J, and the elongation after fracture is 15.5%. Although the strength is improved, the impact energy is obviously insufficient.

[0054] Comparative Example 3 (S3 rolling temperature is outside the range)

[0055] The composition of Comparative Example 3 is consistent with that of Example 2. In the rolling process, the rough rolling temperature of Comparative Example 3 is 1050° C., the finishing rolling temperature is 900° C., and other conditions are consistent.

[0056] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 769MPa, the tensile strength is 882MPa, the Charpy V-notch impact energy at -60℃ is 169J, and the elongation after fracture is 16.4%.

[0057] Comparative Example 4 (S6 temperature outside the range)

[0058] The composition of Comparative Example 4 is consistent with that of Example 2. During the tempering process, the tempering temperature of Comparative Example 4 is 550° C., and other conditions are consistent.

[0059] After testing, the structure of the steel plate is tempered martensite structure and carbide precipitation phase, the yield strength is 967MPa, the tensile strength is 1186MPa, the Charpy V-notch impact energy at -60℃ is 143J, and the elongation after fracture is 15.8%. Although the strength is improved, the impact energy is obviously insufficient.

[0060] The mechanical properties of the embodiments and comparative examples are shown in Table 1.

[0061] Table 1

[0062]

[0063] In addition to the above embodiments, the process parameters and component contents can be adjusted according to the content of the present invention to achieve the preparation of a super thick high-strength and tough rack steel for an offshore platform of the present invention, and exhibit performance basically consistent with the present invention. The above is an exemplary description of the present invention. It should be noted that any simple deformation, modification or equivalent replacement that can be made by other technicians in this field without spending creative labor falls within the protection scope of the present invention without departing from the core of the present invention.

Claims

1. A special thick high-strength rack steel for offshore platforms, characterized in that: The mass percentage of each element is as follows: C: 0.09~0.16, Si: 0.1~0.3, Mn: 0.3~0.8, Ni: 3.3~4.3, Cr: 0.8~1.5, Mo: 0.8~1.5, Cu: 0.2~0.5, B: 0.001~0.002, S≤0.005%, P≤0.003%, V and Nb are also contained, and the balance is Fe and other inevitable impurities; the sum of the mass percentage of V element and the mass percentage of Nb element is in the range of 0.09~0.12%; The microstructure of the extra-thick high-strength and tough rack steel for offshore platforms is a tempered martensite structure and a carbide precipitation phase, and the thickness of the extra-thick high-strength and tough rack steel for offshore platforms is 256 mm; The mass percentage of the V element: the mass percentage of the Nb element are controlled at 2:

1.

2. The extra-thick high-strength and tough rack steel for an offshore platform according to claim 1, characterized in that: The yield strength of the extra-thick high-strength and tough rack steel for the offshore platform is greater than 880 MPa, the tensile strength is greater than 1000 MPa, the Charpy V-notch impact energy at -60°C is greater than 170 J, and the elongation after fracture is greater than 17%.

3. A method for preparing the extra-thick high-strength and tough rack steel for an offshore platform as claimed in claim 2, characterized in that: The steps include: S1. Smelting: According to the mass percentage ratio of each element, molten steel is obtained by smelting, and LF and RH refining furnaces are used for treatment. The superheat of the molten steel in the tundish is ≤20°C, and the casting is protected throughout the process; S2, continuous casting; the molten steel prepared in S1 flows into the casting mold of the continuous casting machine through the pouring system, gradually solidifies under the action of cooling water to form a steel billet, and further cools in the secondary cooling zone to obtain a continuously cast billet; S3, rolling: heating the continuous casting billet prepared in S2 to 1100-1200°C and keeping the temperature for 5-8h, and then performing controlled rolling to obtain a hot-rolled plate, with the roller speed controlled at 1.0-1.5m / s, the pressure value ≥3000KN·m, and ensuring 2-4 passes during rolling, with a single pass reduction rate of more than 8%; S4, slow cooling; hydrogen expansion and slow cooling of the hot-rolled plate prepared in step S3 to obtain a hot-rolled steel plate, the hydrogen expansion temperature is ≥300°C, and the time is >14min / mm; S5, quenching; the hot-rolled steel plate in step S4 is completely austenitized, and then water-quenched to room temperature after heat preservation to obtain a quenched steel plate; S6, tempering; subjecting the quenched steel plate in step S5 to high temperature tempering, keeping the temperature and then air cooling to room temperature to obtain a final steel plate; In step S2, the cooling water flow rate of the mold cooling is 20~30L / min·m², the pulling speed during the solidification process is 0.6~0.8m / min, and the solidification speed is 2~4mm / min; the water flow rate in the secondary cooling zone is 50~70L / min·m², the cooling intensity is 3~5℃ / min, and the continuous casting billet is cooled to 900~950℃ and then heated to the rolling temperature; In step S3, the continuous casting billet is subjected to two-stage rolling, wherein the rough rolling temperature of the first stage is 1100-1200°C, the rough rolling is performed for 3-5 passes, and the reduction rate of a single pass is between 9-14%; after the rough rolling is completed, the temperature is raised to 820-870°C for the second stage of finishing rolling, the finishing rolling is performed for 2-4 passes, and the reduction rate of a single pass is between 6-10%, and air cooling is performed after rolling; In step S5, a furnace heating method is adopted, the heating rate is ≤120°C / h, and a two-time quenching process is used. The first quenching temperature is 900~950°C, the average temperature holding time coefficient is 1.5~2.0min / mm, and a fast laminar cooling system with an average cooling rate greater than 15°C / s is used to cool to room temperature; the second quenching temperature is 820~870°C, the average temperature holding time coefficient is 1.0~1.5min / mm, and other conditions are the same.

4. The preparation method according to claim 3, characterized in that: In step S1, the melting temperature is 1600-1700°C, the treatment time of LF and RH refining furnace is 10-30 minutes, and the superheat temperature of molten steel in the tundish is 10-20°C.

5. The preparation method according to claim 3, characterized in that: In step S4, the slow cooling start temperature is ≥300°C, and the slow cooling time is ≥48h.

6. The preparation method according to claim 3, characterized in that: In step S6, the temperature range of high temperature tempering is 600-650°C, the uniform temperature holding time coefficient is 3.0-4.0 min / mm, and air cooling is performed to room temperature.

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