Method for improving center density of S460NL steel continuous casting large round billet

Abstract

The invention belongs to the technical field of metallurgy, and relates to a method for improving the center density of an S460NL steel continuous casting large round billet, which comprises the following steps: (1) smelting to obtain molten steel meeting the standard; (2) the molten steel is subjected to continuous casting, the molten steel is crystallized in a crystallizer with the diameter phi being 800 mm, and a continuous casting large round billet is obtained; (3) according to the surface temperature T of the continuous casting large round billet, the solidification tail end of the continuous casting large round billet is dynamically pressed down; and (4) slowly cooling the continuous casting billet. According to the S460NL steel phi800mm continuous casting large round billet for the high-strength wind power flange prepared by adopting a continuous casting solidification tail end dynamic large reduction process, compared with a conventional tail end-free dynamic large reduction continuous casting billet, the low-power center porosity and the center shrinkage cavity are remarkably improved, and the flaw detection rejection rate of a large flange finished product is remarkably reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and relates to a method for improving the central density of a S460NL steel continuous casting large round billet, and in particular to a method for improving the central density of a S460NL steel continuous casting large round billet for a high-strength wind power flange. Background Art

[0002] With the rapid increase in wind turbine power and the need to reduce the weight of wind turbine towers, higher-strength wind turbine flanges are urgently needed to replace low-strength wind turbine flanges.

[0003] High-strength wind turbine flanges are mainly used to manufacture high-power offshore and onshore wind turbines. Their main function is to connect the tower to ensure that the tower has sufficient height. In the process of wind power generation, the flange, as a tower connector, is subjected to extremely complex forces, so the wind turbine flange is required to have good comprehensive mechanical properties, low temperature resistance and corrosion resistance. The larger the wind turbine, the larger the wind turbine flange.

[0004] In view of the fact that wind turbine flanges are subjected to tensile stress, compressive stress, bending load, torsion load or bending-torsion composite load during their service, and may be subjected to certain impacts, and the stress state is complex, the wind turbine flanges are required to have excellent mechanical properties, and at the same time, the mechanical properties are required to be isotropic, so the finished wind turbine flanges are required to be formed by integral forging, and welding is not allowed. Therefore, large-section continuous casting round billets or die casting round ingots must be used for forging. Compared with die casting, continuous casting has the characteristics of high metal recovery rate, relatively stable quality and low cost. Therefore, most of the wind turbine flanges are currently made of continuous casting round billets as raw materials through forging.

[0005] According to the characteristics of thermal shrinkage during the solidification process of metal, large-section continuous casting round billets are inevitably accompanied by relatively serious central shrinkage and central porosity during the solidification process. After being forged into large wind power flanges, the non-destructive ultrasonic flaw detection pass rate is low due to poor density, and there is a scrap rate of 2-3%. In order to reduce the scrap rate of large wind power flange products, it is necessary to ensure the central density of large-section continuous casting round billets as raw materials. Therefore, how to improve the central density of large-section continuous casting round billets has become the key to improving the flaw detection pass rate of large wind power flange products. Summary of the invention

[0006] In order to solve the problem of low qualified rate of ultrasonic non-destructive testing of finished large wind turbine flanges due to central looseness of S460NL steel Φ800mm large-section continuous casting billets for high-strength wind turbine flanges and central looseness, the present invention provides a method for improving the central density of S460NL steel continuous casting large round billets.

[0007] Specifically, the present invention is achieved through the following technical solutions:

[0008] A method for improving the center density of a S460NL steel continuous casting large round billet, comprising:

[0009] (1) Smelting to obtain molten steel that meets the standards;

[0010] (2) continuously casting the molten steel, and crystallizing the molten steel in a φ800 mm crystallizer to obtain a continuously cast large round billet;

[0011] (3) dynamically pressing down the solidification end of the continuous casting large round billet according to the surface temperature T of the continuous casting large round billet;

[0012] (4) Slowly cooling the continuously cast billet.

[0013] In the above-mentioned method for improving the center density of S460NL steel continuous casting large round billet, in step (3), when 830>T≥810℃, press down 20mm-25mm; when 810℃>T≥790℃, press down 18mm-23mm; when 790℃>T≥770℃, press down 15mm-22mm; when 770℃>T≥750℃, press down 12mm-20mm; when 750℃>T≥730℃, press down 8mm-15mm; when 730℃>T≥710℃, press down 4mm-10mm; when 710℃>T≥690℃, press down 3mm-5mm, and the overall press down amount at the end of solidification is controlled to be 80mm-120mm.

[0014] The above-mentioned method for improving the central density of S460NL steel continuous casting large round billet, in terms of weight percentage, the molten steel includes C 0.17-0.19%, Si 0.30-0.60%, Mn 1.55-1.65%, P≤0.015%, S≤0.005%, Nb0.03-0.06%, V 0.05-0.10%, Ti≤0.0050%, Cr≤0.30%, Ni≤0.8%, Cu≤0.10%, Mo≤0.10%, Al 0.018-0.033%, and the balance is Fe and unavoidable impurities.

[0015] In the above-mentioned method for improving the center density of S460NL steel continuous casting large round billet, the carbon equivalent CEV in the molten steel is controlled to be 0.44%≤CEV≤0.49%.

[0016] In the above method for improving the central density of S460NL steel continuous casting large round billet, the contents of H, O and N in the molten steel are controlled as follows: H≤2ppm, O≤20ppm, and N: 70-150ppm.

[0017] The above-mentioned method for improving the central density of the S460NL steel continuous casting large round billet is as follows: during the continuous casting process, three-stage electromagnetic stirring is set, and the stirring operation is limited as follows: the current of the crystallizer electromagnetic stirring is 210A-250A, the frequency is 2.4-2.8Hz, and the reversing time is 20-25s; the second cooling zone electromagnetic stirring is set at a position 10.5m-14.2m away from the meniscus, and the current of the second cooling zone electromagnetic stirring is 80-100A, the frequency is 1.8-2.0Hz, and the reversing time is 20-25s; the end electromagnetic stirring is set at 17.0m-18.5m away from the meniscus, and the current of the end electromagnetic stirring is 320-350A, the frequency is 1.0-1.2Hz, and the reversing time is 20-25s.

[0018] In the above method for improving the center density of the S460NL steel continuous casting large round billet, the superheat of the tundish is controlled to be 15°C-35°C during the continuous casting process.

[0019] In the above method for improving the central density of S460NL steel continuous casting large round billet, the casting speed during the continuous casting process is 0.17-0.19 m / min, and the secondary cooling water volume is controlled to be 0.12-0.14 L / kg.

[0020] In the above method for improving the central density of S460NL steel continuous casting large round billet, the slow cooling time is ≥36h; and the surface temperature of the round billet should be ≤200°C when leaving the slow cooling pit.

[0021] On the other hand, the present invention provides a S460NL steel continuous casting large round billet, which is prepared by the above method.

[0022] The technical solution of the present invention has the following beneficial effects:

[0023] The present invention adopts a continuous casting solidification end dynamic large pressure reduction process to prepare the S460NL steel Φ800mm continuous casting large round billet for high-strength wind power flange. Compared with the conventional continuous casting billet without end dynamic large pressure reduction, the low-multiple center looseness and center shrinkage cavity are significantly improved, and the flaw detection scrap rate of large flange finished products is significantly reduced. DETAILED DESCRIPTION

[0024] In order to fully understand the purpose, features and effects of the present invention, the present invention is described in detail through the following specific embodiments. Except for the following contents, the process method of the present invention adopts conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0025] When a numerical range is disclosed herein, the above range is considered to be continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0026] The main defect of large-scale wind turbine flange products is the low pass rate of non-destructive ultrasonic testing caused by poor density. The main reason is that the loose area in the center of the continuous casting billet is too large, resulting in different degrees of loose defects in the final finished flange. How to improve the center density of large-section continuous casting billets has become the key to improving the pass rate of large-scale wind turbine flange products. According to the characteristics of thermal shrinkage during the solidification process of metals, continuous casting billets will inevitably cause center looseness and center shrinkage during the solidification process, which is particularly serious for large-section continuous casting billets. Therefore, not only should the purpose of reducing center shrinkage and center looseness be achieved by controlling the solidification process, but also a process that can improve the center density of large-section continuous casting billets has been developed to solve this problem.

[0027] The present invention breaks through the technical barrier that traditional large round billets are difficult to implement large-scale pressing, and successfully implements the end multi-point dynamic large-scale pressing process on S460NL steel Φ800mm large round billets for large-scale high-strength wind power flanges.

[0028] Specifically, the terminal multi-point dynamic large pressing process adopted by the present invention is as follows: when the surface temperature T ranges from 830℃>T≥810℃, press down by 20mm-25mm; when the surface temperature ranges from 810℃>T≥790℃, press down by 18mm-23mm; when the surface temperature ranges from 790℃>T≥770℃, press down by 15mm-22mm; when the surface temperature ranges from 770℃>T≥750℃, press down by 12mm-20mm; when the surface temperature ranges from 750℃>T≥730℃, press down by 8mm-15mm; when the surface temperature ranges from 730℃>T≥710℃, press down by 4mm-10mm; when the surface temperature ranges from 710℃>T≥690℃, press down by 3mm-5mm, and the overall pressing amount at the end of solidification is controlled to be 80mm-120mm.

[0029] The large round billet is processed according to the terminal multi-point dynamic large reduction process of the present invention, which significantly reduces the central looseness and central shrinkage of the continuous casting large round billet, thereby improving the flaw detection pass rate of large-sized wind power flanges.

[0030] In addition, the present invention also controls the composition and process of the continuous casting large round billet, and further improves the central density of the large-section continuous casting billet by improving the uniformity of the composition of the cross-section of the billet.

[0031] In some preferred embodiments, the chemical composition of the continuous casting large round billet of the present invention is as follows, by weight percentage: C 0.17-0.19%, Si 0.30-0.60%, Mn 1.55-1.65%, P≤0.015%, S≤0.005%, Nb 0.03-0.06%, V 0.05-0.10%, Ti≤0.0050%, Cr≤0.30%, Ni≤0.8%, Cu≤0.10%, Mo≤0.10%, Al 0.018~0.033%, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 is controlled as 0.44%≤CEV≤0.49%, the contents of H, O and N in steel are controlled as: H≤2ppm, O≤20ppm, N:70-150ppm, the remainder is Fe and unavoidable impurities.

[0032] The present invention controls the composition of molten steel within the above range, thereby ensuring that the mechanical properties of the finished large-size wind turbine flange meet the mechanical property requirements of 460MPa-level high-strength wind turbine flanges.

[0033] In order to effectively control the central shrinkage cavity, central looseness and macro segregation, so as to improve the flaw detection pass rate and mechanical property stability of large-size wind power flanges, the present invention has developed a dedicated continuous casting process for high-strength wind power flanges based on a full study of the characteristics of the steel grade.

[0034] Specifically, the proprietary continuous casting process of the high-strength wind power flange includes: crystallizing the molten steel in a φ800mm crystallizer, setting up three-stage electromagnetic stirring during the continuous casting process, and the stirring operation is limited as follows: the current of the crystallizer electromagnetic stirring is 210A-250A, the frequency is 2.4-2.8Hz, and the reversing time is 20-25s; the second cooling zone electromagnetic stirring is set at a position 10.5m-14.2m away from the meniscus, and the current of the second cooling zone electromagnetic stirring is 80-100A, the frequency is 1.8-2.0Hz, and the reversing time is 20-25s; the end electromagnetic stirring is set at 17.0m-18.5m away from the meniscus, and the current of the end electromagnetic stirring is 320-350A, the frequency is 1.0-1.2Hz, and the reversing time is 20-25s.

[0035] In order to reduce the central looseness and shrinkage of large-sized continuous casting billets, a lower superheat should be used as much as possible, and in order to ensure that the continuous casting process proceeds smoothly without interruption of casting, the superheat of the tundish is controlled to be 15°C-35°C during the continuous casting process of the present invention.

[0036] Since the drawing speed and the water ratio of the secondary cooling water have a great influence on the solidification structure, center quality and surface quality of the ingot, preferably, the drawing speed in the continuous casting process is 0.17-0.19m / min, and the water ratio of the secondary cooling water is controlled to be 0.12-0.14L / kg. The center looseness, shrinkage cavity, center crack, middle crack, subcutaneous crack and subcutaneous bubble on the acid-leached low-magnification test piece of the round ingot cross section are evaluated according to YB / T 4149, and the rating chart is implemented according to YB / T153, all ≤2 level.

[0037] During the research process, the inventor found that the internal stress concentration of the directly air-cooled ingot causes problems such as cracking during subsequent processing by users. Therefore, the continuous casting ingot cannot be directly air-cooled but needs to be slowly cooled.

[0038] In some preferred embodiments, the slow cooling time of the present invention is ≥36h; and the surface temperature of the round billet when leaving the slow cooling pit should be ≤200°C. Slow cooling of the billet according to the method of the present invention can diffuse and evenly distribute the segregation structure (distributed in the surface and core areas) in the billet.

[0039] Through practice, the technical solution of the present invention significantly improves the low-multiple quality of S460NL steel continuous casting large round billets, and the central shrinkage and central looseness of the billets are significantly improved. The final forging of large-size high-strength wind power flange finished products has a significantly reduced flaw detection scrap rate.

[0040] Example

[0041] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods without specific conditions in the following examples are based on conventional methods and conditions.

[0042] Example 1

[0043] The S460NL steel for large wind turbine flanges is smelted by "electric furnace + LF furnace refining + VD vacuum refining". The chemical composition of the steel by mass percentage is as follows: C 0.18%, Si 0.38%, Mn 1.60%, P0.0075%, S 0.0017%, Nb0.035%, V 0.06%, Ti 0.0025%, Cr 0.05%, Ni 0.02%, Cu 0.01%, Mo 0.003%, Al0.030%, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 is 0.46%, and the contents of H, O and N in the steel are controlled as: H 0.5ppm, O 16ppm, N 120ppm, and the remainder is Fe and unavoidable impurities.

[0044] Φ800mm large round billet continuous casting is adopted, and the continuous casting process parameters are as follows: the superheat of the tundish is 30℃, the pulling speed is 0.18m / min, the secondary cooling water volume is 0.12L / kg, and a three-stage electromagnetic stirring is set in the continuous casting process, including crystallizer electromagnetic stirring, secondary cooling zone electromagnetic stirring set at a position 10.5m away from the meniscus, and terminal electromagnetic stirring set at a position 17.0m away from the meniscus. Among them, the current of the crystallizer electromagnetic stirring is controlled to 240A, the stirring frequency is controlled to 2.6Hz, and the reversing time is controlled to 22s; the current of the secondary cooling zone electromagnetic stirring is controlled to 90A, the stirring frequency is controlled to 2.0Hz, and the reversing time is controlled to 23s; the current of the terminal electromagnetic stirring is controlled to 330A, the stirring frequency is controlled to 1.2Hz, and the reversing time is controlled to 22s.

[0045] A dynamic large reduction process is implemented on the continuous casting large round billet at the end of solidification, wherein the solidification end of the continuous casting large round billet is dynamically pressed down according to the surface temperature of the continuous casting large round billet, and the overall reduction at the end of solidification is controlled to be 80 mm, when the surface temperature T of the continuous casting large round billet is 822°C, the reduction is 20 mm; when the surface temperature T of the continuous casting large round billet is 803°C, the reduction is 18 mm; when the surface temperature T of the continuous casting large round billet is 782°C, the reduction is 15 mm; when the surface temperature T of the continuous casting large round billet is 756°C, the reduction is 12 mm; when the surface temperature T of the continuous casting large round billet is 740°C, the reduction is 8 mm; when the surface temperature T of the continuous casting large round billet is 721°C, the reduction is 4 mm; when the surface temperature T of the continuous casting large round billet is 695°C, the reduction is 3 mm.

[0046] The continuous casting billet is cooled in the slow cooling pit for 48 hours, and the surface temperature of the round billet is 186℃ when it leaves the slow cooling pit.

[0047] The S460NL steel continuous casting large round billet for the Φ800mm high-strength wind turbine flange was subjected to macroscopic inspection, and the macroscopic quality was graded according to YB / T 4149: the level of central porosity and central shrinkage cavity was ≤0.5. The qualified rate of large wind turbine flange products forged from continuous casting billets was 100%.

[0048] Example 2

[0049] The S460NL steel for large wind turbine flanges is smelted by "electric furnace + LF furnace refining + VD vacuum refining". The chemical composition of the steel by mass percentage is as follows: C 0.18%, Si 0.52%, Mn 1.58%, P0.0085%, S 0.0023%, Nb0.04%, V 0.06%, Ti 0.0035%, Cr 0.08%, Ni 0.04%, Cu 0.02%, Mo 0.004%, Al0.028%, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 is 0.48%, and the contents of H, O and N in the steel are controlled as: H 0.5ppm, O 15ppm, N 118ppm, and the remainder is Fe and unavoidable impurities.

[0050] Φ800mm large round billet continuous casting is adopted, and the continuous casting process parameters are as follows: the superheat of the tundish is 15℃, the pulling speed is 0.19m / min, the secondary cooling water volume is 0.14L / kg, and a three-stage electromagnetic stirring is set in the continuous casting process, including crystallizer electromagnetic stirring, secondary cooling zone electromagnetic stirring set at a position 14.2m away from the meniscus, and terminal electromagnetic stirring set at a position 18.5m away from the meniscus. Among them, the current of the crystallizer electromagnetic stirring is controlled to 230A, the stirring frequency is controlled to 2.8Hz, and the reversing time is controlled to 25s; the current of the secondary cooling zone electromagnetic stirring is controlled to 100A, the stirring frequency is controlled to 1.8Hz, and the reversing time is controlled to 20s; the current of the terminal electromagnetic stirring is controlled to 350A, the stirring frequency is controlled to 1.0Hz, and the reversing time is controlled to 25s.

[0051] A dynamic large reduction process is implemented on the continuous casting large round billet at the end of solidification, wherein the solidification end of the continuous casting large round billet is dynamically pressed down according to the surface temperature of the continuous casting large round billet, and the overall reduction at the end of solidification is controlled to be 120 mm, when the surface temperature T of the continuous casting large round billet is 828°C, the reduction is 25 mm; when the surface temperature T of the continuous casting large round billet is 805°C, the reduction is 23 mm; when the surface temperature T of the continuous casting large round billet is 785°C, the reduction is 22 mm; when the surface temperature T of the continuous casting large round billet is 768°C, the reduction is 20 mm; when the surface temperature T of the continuous casting large round billet is 745°C, the reduction is 15 mm; when the surface temperature T of the continuous casting large round billet is 726°C, the reduction is 10 mm; when the surface temperature T of the continuous casting large round billet is 705°C, the reduction is 5 mm.

[0052] The continuous casting billet is cooled in the slow cooling pit for 50 hours, and the surface temperature of the round billet is 185℃ when it leaves the slow cooling pit.

[0053] The S460NL steel continuous casting large round billet for the Φ800mm high-strength wind turbine flange was subjected to macroscopic inspection, and the macroscopic quality was graded according to YB / T 4149: the level of central porosity and central shrinkage cavity was ≤0.5. The qualified rate of large wind turbine flange products forged from continuous casting billets was 100%.

[0054] Comparative Example 1

[0055] The S460NL steel for large wind turbine flanges is smelted by "electric furnace + LF furnace refining + VD vacuum refining". The chemical composition of the steel by mass percentage is as follows: C 0.18%, Si 0.42%, Mn 1.62%, P0.0065%, S 0.0020%, Nb0.035%, V 0.06%, Ti 0.0025%, Cr 0.05%, Ni 0.02%, Cu 0.01%, Mo 0.003%, Al0.030%, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 is 0.46%, and the contents of H, O and N in the steel are controlled as: H 0.5ppm, O 17ppm, N 122ppm, and the remainder is Fe and unavoidable impurities.

[0056] Φ800mm large round billet continuous casting is adopted, and the continuous casting process parameters are as follows: the superheat of the tundish is 30℃, the pulling speed is 0.18m / min, the secondary cooling water volume is 0.12L / kg, and a three-stage electromagnetic stirring is set in the continuous casting process, including crystallizer electromagnetic stirring, secondary cooling zone electromagnetic stirring set at a position 10.5m away from the meniscus, and terminal electromagnetic stirring set at a position 17.0m away from the meniscus. Among them, the current of the crystallizer electromagnetic stirring is controlled to 240A, the stirring frequency is controlled to 2.6Hz, and the reversing time is controlled to 22s; the current of the secondary cooling zone electromagnetic stirring is controlled to 90A, the stirring frequency is controlled to 2.0Hz, and the reversing time is controlled to 23s; the current of the terminal electromagnetic stirring is controlled to 330A, the stirring frequency is controlled to 1.2Hz, and the reversing time is controlled to 22s.

[0057] The continuous casting billet is cooled in the slow cooling pit for 48 hours, and the surface temperature of the round billet is 186℃ when it leaves the slow cooling pit.

[0058] The S460NL steel continuous casting large round billet for Φ800mm high-strength wind turbine flange was subjected to macroscopic microstructure inspection, and the macroscopic quality was graded according to YB / T 4149: the level of central porosity and central shrinkage cavity ≤ 0.5. The qualified rate of large wind turbine flange products forged from continuous casting billets was 96.8%.

[0059] Comparative Example 2

[0060] The S460NL steel for large wind turbine flanges is smelted by "electric furnace + LF furnace refining + VD vacuum refining". The chemical composition of the steel by mass percentage is as follows: C 0.18%, Si 0.46%, Mn 1.59%, P0.0083%, S 0.0023%, Nb0.04%, V 0.06%, Ti 0.0034%, Cr 0.08%, Ni 0.04%, Cu 0.02%, Mo 0.004%, Al0.029%, carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 is 0.48%, and the contents of H, O and N in the steel are controlled as: H 0.5ppm, O 15ppm, N 116ppm, and the remainder is Fe and unavoidable impurities.

[0061] Φ800mm large round billet continuous casting is adopted, and the continuous casting process parameters are as follows: the superheat of the tundish is 15℃, the pulling speed is 0.19m / min, the secondary cooling water volume is 0.14L / kg, and a three-stage electromagnetic stirring is set in the continuous casting process, including crystallizer electromagnetic stirring, secondary cooling zone electromagnetic stirring set at a position 14.2m away from the meniscus, and terminal electromagnetic stirring set at a position 18.5m away from the meniscus. Among them, the current of the crystallizer electromagnetic stirring is controlled to 230A, the stirring frequency is controlled to 2.8Hz, and the reversing time is controlled to 25s; the current of the secondary cooling zone electromagnetic stirring is controlled to 100A, the stirring frequency is controlled to 1.8Hz, and the reversing time is controlled to 20s; the current of the terminal electromagnetic stirring is controlled to 350A, the stirring frequency is controlled to 1.0Hz, and the reversing time is controlled to 25s.

[0062] The continuous casting billet is cooled in the slow cooling pit for 50 hours, and the surface temperature of the round billet is 185℃ when it leaves the slow cooling pit.

[0063] The S460NL steel continuous casting large round billet for Φ800mm high-strength wind turbine flange was subjected to macroscopic microstructure inspection, and the macroscopic quality was graded according to YB / T 4149: the level of central porosity and central shrinkage cavity was ≤0.5. The qualified rate of large wind turbine flange products forged from continuous casting billets was 97.6%.

[0064] The present invention has been disclosed above with preferred embodiments, but those skilled in the art should understand that these embodiments are only used to describe the present invention and should not be understood as limiting the scope of the present invention. It should be noted that all changes and substitutions equivalent to these embodiments should be deemed to be included in the scope of the claims of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined in the claims.

Claims

1. A method for improving the center density of S460NL steel continuous casting large round billet, characterized in that: include: (1) Smelting to obtain molten steel that meets the standards; (2) continuously casting the molten steel, and crystallizing the molten steel in a φ800 mm crystallizer to obtain a continuously cast large round billet; (3) dynamically pressing down the solidification end of the continuous casting large round billet according to the surface temperature T of the continuous casting large round billet; (4) Slowly cooling the continuously cast billet.

2. The method according to claim 1, characterized in that In step (3), when 830>T≥810℃, press down 20mm-25mm; when 810℃>T≥790℃, press down 18mm-23mm; when 790℃>T≥770℃, press down 15mm-22mm; when 770℃>T≥750℃, press down 12mm-20mm; when 750℃>T≥730℃, press down 8mm-15mm; when 730℃>T≥710℃, press down 4mm-10mm; when 710℃>T≥690℃, press down 3mm-5mm, and the overall press down amount at the end of solidification is controlled to be 80mm-120mm.

3. The preparation method according to claim 1, characterized in that: Measured by weight percentage, the molten steel includes C0.17-0.19%, Si 0.30-0.60%, Mn 1.55-1.65%, P≤0.015%, S≤0.005%, Nb 0.03-0.06%, V 0.05-0.10%, Ti≤0.0050%, Cr≤0.30%, Ni≤0.8%, Cu≤0.10%, Mo≤0.10%, Al 0.018-0.033%, and the balance is Fe and unavoidable impurities.

4. The preparation method according to claim 3, characterized in that: The carbon equivalent CEV in the molten steel is controlled to be 0.44%≤CEV≤0.49%.

5. The preparation method according to claim 3, characterized in that: The contents of H, O and N in the molten steel are controlled as follows: H≤2ppm, O≤20ppm, N: 70-150ppm.

6. The preparation method according to claim 1, characterized in that: During the continuous casting process, three-stage electromagnetic stirring is set, and the stirring operation is limited as follows: the current of the crystallizer electromagnetic stirring is 210A-250A, the frequency is 2.4-2.8Hz, and the switching time is 20-25s; the second cooling zone electromagnetic stirring is set at a distance of 10.5m-14.2m from the meniscus, and the current of the second cooling zone electromagnetic stirring is 80-100A, the frequency is 1.8-2.0Hz, and the switching time is 20-25s; the end electromagnetic stirring is set at a distance of 17.0m-18.5m from the meniscus, and the current of the end electromagnetic stirring is 320-350A, the frequency is 1.0-1.2Hz, and the switching time is 20-25s.

7. The preparation method according to claim 1, characterized in that: During the continuous casting process, the superheat of the tundish is controlled at 15℃-35℃.

8. The preparation method according to claim 1, characterized in that: The casting speed during continuous casting is 0.17-0.19 m / min, and the secondary cooling water volume is controlled at 0.12-0.14 L / kg.

9. The preparation method according to claim 1, characterized in that: The slow cooling time is ≥36h; and the surface temperature of the round billet should be ≤200°C when it comes out of the slow cooling pit.

10. A S460NL steel continuous casting large round billet, characterized in that: The method is prepared by any one of claims 1 to 9.