A flexible production method for low-alloy high-strength steel with a yield strength of 420 / 460 MPa

Abstract

The present invention relates to a flexible production method for low-alloy high-strength steel with a yield strength of 420 / 460 MPa. The chemical composition of the steel is C: 0.12% to 0.16%; Si: 0.20% to 0.3%; Mn: 1.2% to 1.5%; P≤0.02%; S≤0.005%; Nb: 0.02% to 0.03%; V: 0.02% to 0.03%; Als: 0.01% to 0.03%; the balance being Fe and impurities. The production process includes converter smelting, LF refining, continuous casting, heating, rolling, post-rolling controlled cooling, and straightening. The present invention utilizes the combined effects of grain refinement, precipitation strengthening, and phase transformation strengthening during rolling of high-quality continuous casting billets to achieve flexible and efficient production of low-alloy high-strength steel with yield strengths of 420 MPa and 460 MPa.

Description

Technical Field

[0001] The present invention relates to the technical field of low-alloy high-strength steel production, and in particular to a flexible and high-efficiency production method for low-alloy high-strength steel with a yield strength of 420 MPa and 460 MPa. Background Art

[0002] Low-alloy high-strength steels with yield strengths of 420MPa and 460MPa offer excellent mechanical and weldability and are widely used in machinery, construction, wind power, vehicles, ships, chemical equipment, and other fields. They are a widely used medium and thick plate product. However, with changing market conditions, the profitability of low-alloy high-strength steels with yield strengths of 420MPa and 460MPa has declined, necessitating an urgent need to improve their profitability through cost-effective production.

[0003] At present, the main production mode of low-alloy high-strength steel with a thickness of 10 to 40 mm and a yield strength of 420 MPa or 460 MPa is controlled rolling and controlled cooling. In order to ensure the performance of the steel plate, the finishing rolling start temperature and the final rolling temperature are usually required to be controlled within a narrow range, resulting in a slow pace of steel plate production and strict process requirements, which is not conducive to efficient production.

[0004] Chinese invention patent publication number CN 112553437 B discloses a "method for controlling yield strength fluctuations in 420 MPa-grade hot-dip galvanized high-strength steel." The method includes steelmaking, continuous casting, heating the ingot, controlled rolling, controlled cooling, coiling, autotempering and slow cooling, pickling, and galvannealing. The controlled rolling process has a final rolling temperature of 880±14°C, and the coiling process has a coiling temperature of 530±17°C. The cold-based hot-dip galvanized low-alloy high-strength steel disclosed in this invention exhibits a low yield strength ratio, a good balance between strength and ductility, and stable coiling performance. The average yield strength ratio is 0.83, and the yield strength difference between the beginning, middle, and end of the coil is ≤20 MPa.

[0005] The Chinese invention patent with authorization publication number CN 102400053 B discloses "a steel plate for building structures with a yield strength of 460 MPa and a manufacturing method thereof." The chemical composition of the steel plate is as follows by weight: C 0.05% to 0.10%, Si 0.2% to 0.3%, Mn 1.40% to 1.60%, Nb 0.025% to 0.060%, Ti 0.015% to 0.030%, Als 0.015% to 0.045%, and also contains one of Cr 0.2% to 0.4%, Mo 0.10% to 0.30%, and B 0.0010% to 0.0020%. The balance is Fe and unavoidable impurities. The impurity elements are controlled to be P ≤ 0.015%, S ≤ 0.005%, [N] ≤ 0.0040%, and [O] ≤ 0.0020%. The manufacturing method comprises the following steps: heating the steel billet to a temperature of 1180 to 1220°C, starting the rolling process at a temperature of 1040 to 1080°C in the first stage, finishing the rolling process at a temperature of 950 to 1000°C, and starting the rolling process at a temperature of 920 to 870°C in the second stage; controlling the finishing temperature at 800 to 850°C, and increasing the cumulative deformation of the recrystallization zone in the second stage by more than 60%; waiting for a temperature of more than 15 seconds after hot rolling, and then water cooling; the starting temperature of the water cooling process is 680 to 750°C, the cooling rate is 5 to 9°C / s, and the red-returning temperature is 450 to 620°C.

[0006] In addition, the designed compositions of steel plates of different thicknesses are usually different, resulting in more steel tapping marks, which affects production efficiency during smelting and continuous casting.

[0007] The present invention is optimized on the basis of the original Q420 grade steel composition, and obtains high-quality continuous casting billets through high-purity smelting and continuous casting process optimization. At the same time, the heating temperature is reduced to ensure that the second phase particles are fully dissolved back while reducing the original austenite grain size. During rolling, the dispersion precipitation of the second phase particles is achieved through low-temperature large deformation rolling. Rapid cooling after rolling reduces the release of internal stored energy, and fully utilizes the effects of precipitation strengthening, fine grain strengthening and phase transformation strengthening to ensure flexible and efficient production of steel plates with a thickness of 10 to 40 mm and a yield strength of 420 MPa and 460 MPa. The production efficiency of the steel plates is greatly improved while ensuring sufficient performance margin, and the energy consumption of the heating furnace is greatly reduced while improving the performance-creating ability of low-alloy high-strength steels with a yield strength of 420 MPa and 460 MPa. Summary of the Invention

[0008] The present invention provides a flexible production method for low-alloy high-strength steel with a yield strength of 420 / 460 MPa. Through the coupled design of smelting, continuous casting, low-temperature heating, low-temperature rolling and controlled cooling processes, the combined effects of fine grain strengthening, precipitation strengthening and phase transformation strengthening during the rolling of high-quality continuous casting billets are fully utilized to achieve flexible and efficient production of steel plates.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A flexible production method for low-alloy high-strength steel with a yield strength of 420 / 460 MPa. The chemical composition of the steel, by weight percentage, is as follows: C: 0.12% to 0.16%; Si: 0.20% to 0.3%; Mn: 1.2% to 1.5%; P≤0.02%; S≤0.005%; Nb: 0.02% to 0.03%; V: 0.02% to 0.03%; Als: 0.01% to 0.03%; the balance being Fe and unavoidable impurities. The finished steel plate has a yield strength of 420 MPa or 460 MPa. The production process includes converter smelting, LF refining, continuous casting, heating, rolling, post-rolling controlled cooling, and straightening. Specifically, the following processes are controlled:

[0011] 1) Converter smelting and LF refining;

[0012] The smelting raw materials are pre-treated with molten iron, and the converter smelting adopts top and bottom blowing to ensure that P ≤ 0.02% when the converter is tapped;

[0013] During the LF furnace refining process, white slag is produced, aluminum and iron are used for deoxidation to ensure that the oxygen content is less than 20ppm, and calcium is fed to modify MnS inclusions to ensure that the soft blowing time is ≥8min. The total treatment time of LF refining is 35-50min, and S is ≤0.005% at the end of LF refining.

[0014] 2) continuous casting;

[0015] The continuous casting superheat is controlled at 17-23°C, the electromagnetic stirring current intensity in the secondary cooling zone is 700-900A, and the stirring is carried out in alternating forward and reverse directions for 30-60 seconds. The casting speed is controlled at 1.05-1.15m / min, and a heavy reduction process is used at the end of the solidification of the slab, with a total reduction of 15-20mm. The center segregation of the slab is rated at C0.5 according to YB / T4003 "Grading Chart for Macrostructure Defects of Continuously Cast Steel Slabs";

[0016] 3) heating;

[0017] After the billet comes off the production line, it is hot-charged and sent to the heating furnace for heating. The furnace entry temperature is not less than 550℃, the preheating section temperature is 700-950℃, and the preheating section time is 0.5-1h; the heating section temperature is 1210±20℃, and the heating section time is 1-1.5h; the soaking section temperature is 1170±20℃, and the soaking section time is 0.5-1h; the total time in the furnace is 2-4h; the billet temperature when it comes out of the furnace is 1150-1180℃, and the temperature difference between the billet surface and the core does not exceed 25℃;

[0018] 4) rolling;

[0019] The continuous casting slab is rolled immediately after descaling. The starting temperature of rough rolling is 980-1030°C. The rough rolling widening stage is completed in 2-4 passes, with a pass reduction rate of 8%-18%, a pass interval time of 4-10s, and a rolling speed of 0.8-1.2m / s. The reduction rate of at least the first three passes of the rough rolling longitudinal rolling stage is 23%-28%, a rolling speed of 0.5-1.0m / s, and a pass interval time of 10-15s. The reduction rate of the remaining rough rolling passes is 16%-23%, a rolling speed of 1.0-2.0m / s, and a pass interval time of 5-10s. The reduction rate of each pass of the finishing rolling is 10%-20%, a rolling speed of 2.5-3.3m / s, and a pass interval time of 3-8s.

[0020] 5) cooling;

[0021] The steel plate is cooled immediately after rolling, and the cooling rate is 8-15°C / s. The red-return temperature of the yield strength 420MPa grade steel plate is 610-650°C, and the red-return temperature of the yield strength 460MPa grade steel plate is 570-610°C.

[0022] Furthermore, the continuous casting billet is subjected to at least two descaling passes after exiting the heating furnace, and the descaling water pressure is 20-25 MPa.

[0023] Furthermore, for steel plates with a thickness of 10 mm or less and a thickness of 20 mm or less, a single-block controlled rolling method is adopted, the intermediate billet is not cooled, and finishing rolling is carried out immediately after the rough rolling is completed; for steel plates with a thickness of 20 mm or less and a thickness of 40 mm or less, a multi-block controlled rolling method is adopted, that is, single-stand cross rolling is carried out, and after the rough rolling of the last steel plate in the rough rolling stage is completed, the first steel plate in the finishing rolling stage is immediately finished, and after the finish rolling of the first steel plate in the finishing rolling stage is completed, the second steel plate in the finishing rolling stage is immediately finished, and so on until the finish rolling of the last steel plate is completed.

[0024] Furthermore, the thickness of the cast billet is 200-250 mm, the thickness of the intermediate billet is 2-3 times the thickness of the finished steel plate, and the thickness of the finished steel plate is 10-40 mm.

[0025] Furthermore, the ferrite grain size of the finished steel plate is 9 to 11 levels.

[0026] Furthermore, the finished product performance of the 420MPa yield strength steel plate is: yield strength ≥420MPa, tensile strength 520~680MPa, elongation ≥19%, -20℃ Charpy impact energy ≥180J; the mechanical properties meet the Q420 grade requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0027] Furthermore, the finished product performance of the 460MPa yield strength steel plate is: yield strength ≥460MPa, tensile strength 540~720MPa, elongation ≥17%, -20℃ Charpy impact energy ≥150J; the mechanical properties meet the Q460 level requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) Based on the original Q420 grade steel composition, high-quality continuous casting billets are obtained through smelting and continuous casting process optimization. At the same time, the heating temperature is reduced to ensure that the second phase particles are fully dissolved while reducing the original austenite grain size. During rolling, the dispersion and precipitation of the second phase particles are achieved through low-temperature large deformation rolling. Rapid cooling after rolling reduces the release of internal stored energy, making full use of the combined effects of precipitation strengthening, grain refinement strengthening and phase transformation strengthening, ensuring flexible and efficient production of steel plates with a thickness of 10 to 40 mm and a yield strength of 420 MPa and 460 MPa (the number of blocks per hour is increased by 1.6 blocks / h);

[0030] (2) No precious metal elements such as Ti, Cu, Cr, Mo, Ni and rare earth are added, and the alloy cost is extremely low, ensuring the product's ability to create effects.

[0031] (3) Under a lower alloy composition system, the performance of the original yield strength 420MPa grade steel plate can be guaranteed to meet the performance requirements of the yield strength 460MPa grade steel plate.

[0032] (4) The present invention does not limit the final rolling temperature of the steel plate, the production process is simple, and the process window is less restricted, which is conducive to the production of steel plates with complex nominal sizes. In addition, the first-time performance qualification rate after actual application is as high as over 99%.

[0033] (5) It effectively reduces the burning loss of slabs, improves the yield rate and obtains high-quality steel plate surface.

[0034] (6) By adopting the low-temperature flexible rolling process, the molten steel (with the same composition) from the same casting or even the same furnace can produce a series of products with yield strength of 420MPa and 460MPa by adjusting the post-rolling cooling process parameters, which greatly improves the production efficiency of steelmaking. DETAILED DESCRIPTION

[0035] The present invention discloses a flexible production method for low-alloy high-strength steel with a yield strength of 420 / 460 MPa. Steel plates with a yield strength of 420 MPa or 460 MPa can be produced using the same chemical composition. The chemical composition of the steel is as follows by weight: C: 0.12% to 0.16%; Si: 0.20% to 0.3%; Mn: 1.2% to 1.5%; P≤0.02%; S≤0.005%; Nb: 0.02% to 0.03%; V: 0.02% to 0.03%; Als: 0.01% to 0.03%; the balance being Fe and unavoidable impurities.

[0036] The present invention discloses a flexible and efficient production method for low-alloy steel with a yield strength of 420-460 MPa. The production process includes: converter smelting - LF refining - continuous casting - heating - rolling - controlled cooling after rolling - straightening - cooling bed - shearing, etc. The specific control process is as follows:

[0037] 1) Converter smelting and LF refining;

[0038] The smelting raw materials are pretreated with molten iron, and the converter smelting adopts top and bottom blowing to ensure that P ≤ 0.02% when the converter is tapping steel.

[0039] During the LF furnace refining process, white slag is produced, aluminum and iron are used for deoxidation to ensure that the oxygen content is less than 20ppm, and calcium is fed to modify MnS inclusions. The soft blowing time is guaranteed to be ≥8 minutes to promote the floating of inclusions. The total LF refining treatment time is 35 to 50 minutes, and the S content of LF is guaranteed to be ≤0.005% when it leaves the station.

[0040] 2) continuous casting;

[0041] Low superheat and strong electromagnetic stirring technology are used to reduce central segregation, the superheat is controlled at 17-23°C, the electromagnetic stirring current intensity in the secondary cooling zone is 700-900A, and positive and reverse alternating stirring is adopted, and continuous stirring is 30-60s; the pulling speed is controlled at 1.05-1.15m / min, and the heavy reduction process is adopted at the end of the solidification of the ingot. The heavy reduction positions are preferably 10, 11 and 12 sections, and the total reduction is 15-20mm; the proportion of columnar crystals is minimized to the greatest extent, the density of the continuous casting ingot is improved, and the central segregation of the ingot is ensured to reach C0.5 level according to YB / T4003 "Continuous Casting Steel Slab Macrostructure Defect Rating Chart".

[0042] 3) heating;

[0043] After the continuous casting slab comes off the line, it is hot-charged into a walking beam heating furnace for heating. The temperature of the slab entering the furnace is not lower than 550℃. It passes through the preheating section, heating section and soaking section in sequence before being discharged from the furnace. The temperature range of the preheating section is 700-950℃, and the preheating time is 0.5-1h; the temperature range of the heating section is 1210±20℃, and the heating time is 1-1.5h; the temperature range of the soaking section is 1170±20℃, and the soaking time is 0.5-1h; the total time in the furnace is 2-4h; the temperature of the continuous casting slab when discharged from the furnace is 1150-1180℃, and the temperature difference between the surface and the core of the slab is guaranteed to be no more than 25℃, which not only ensures the full solid solution of micro-alloy elements, but also reduces the size of the original austenite grains.

[0044] After the continuous casting billet leaves the heating furnace, it shall be descaled at least twice with the descaling water pressure of 20-25MPa to ensure that there is no primary iron oxide scale remaining on the surface of the steel plate.

[0045] 4) rolling;

[0046] The present invention adopts a controlled rolling production mode for steel plates with a thickness of 10 to 40 mm. Specifically, a single-block controlled rolling strategy is used for steel plates with a thickness of 10 to 20 mm. The intermediate billet is not cooled, and finish rolling is performed immediately after rough rolling. A cross-rolling strategy, i.e., multi-block controlled rolling, is used for steel plates with a thickness of 20 to 40 mm. Due to the single-stand cross-rolling, after the rough rolling of the last steel plate in the roughing stage is completed, the first steel plate in the finish rolling stage is immediately finished. After the finish rolling of the first steel plate in the finish rolling stage is completed, the second steel plate is immediately finished, and so on, until the finish rolling of the last steel plate is completed.

[0047] The continuous casting slab is rolled immediately after descaling, the starting rolling temperature of rough rolling is 980-1030°C, the rough rolling widening stage is completed in 2-4 passes, the pass reduction rate is 8%-18%, the pass interval time is 4-10s, and the rolling speed is 0.8-1.2m / s; the reduction rate of at least the first three passes of the rough rolling longitudinal rolling stage is 23%-28%, the rolling speed is 0.5-1.0m / s, the pass interval time is 10-15s, and the reduction rate of the remaining rough rolling passes is 16%-23%, the rolling speed is 1.0-2.0m / s, and the pass interval time is 5-10s. The dynamic and static recrystallization ratios are respectively increased by means of large reduction at low temperature and appropriate increase in pass interval time to ensure complete recrystallization in the rough rolling stage. The thickness of the intermediate billet is 2 to 3 times the thickness of the finished steel plate, the reduction rate of each finishing rolling pass is 10% to 20%, the rolling speed is 2.5 to 3.3 m / s, and the pass interval time is 3 to 8 s. The purpose is to increase the internal energy storage of the rolled steel plate and refine the grains through rapid and large reduction rolling; the present invention has no requirements for the final rolling temperature of finishing rolling.

[0048] 5) cooling;

[0049] After rolling, the steel plates are rapidly cooled in a cooling system at a rate of 8-15°C / s. The red-hot temperature for Q420-grade steel plates is 610-650°C, and for Q460-grade steel plates, 570-610°C. Rapid cooling reduces the release of stored energy after rolling, thereby refining the grain size, improving elongation, and improving low-temperature impact resistance at -20°C.

[0050] 6) Straightening, cooling bed and shearing, etc.;

[0051] After rapid cooling, the steel plate is straightened to ensure the plate shape. After straightening, it is placed on a cooling bed for cooling. When the temperature of the steel plate is lower than 100℃, it is placed on the cooling bed for online shearing and finally delivered to the warehouse.

[0052] In the present invention, the thickness of the continuous casting billet is 200-250 mm, the thickness of the finished steel plate is 10-40 mm, the ferrite grain size of the finished steel plate is 9-11, and the grains are uniform and fine.

[0053] When rolling using conventional processes, the steel plate needs to be air-cooled after rough rolling to the finishing rolling start temperature (usually takes 1 to 4 minutes), and laminar cooling is performed after rolling. The production method of the present invention is suitable for the production of steel plates with a thickness of 10-40 mm. When rolling steel plates with a thickness of 10 to 20 mm, a single-block controlled rolling mode is adopted without intermediate billet cooling (because the fully automatic rolling mill controlled rolling mode has two more passes than the hot rolling mode), and finishing rolling is performed immediately after rough rolling. When rolling steel plates with a thickness of 20 to 40 mm (excluding 20 mm), multiple steel plates are controlled and rolled at the same time. When multiple controlled rolling is performed, the first steel plate in the finishing rolling stage is immediately finished after the rough rolling of the last steel plate. After the finishing rolling of the first steel plate in the finishing rolling stage is completed, the second steel plate is immediately finished rolling. This process is repeated to ensure that the rolling mill does not stop, thereby improving rolling efficiency.

[0054] The finished product performance of the 420MPa yield strength steel plate produced by the method of the present invention is: yield strength ≥420MPa, tensile strength 520-680MPa, elongation ≥19%, -20℃ Charpy impact energy ≥180J; the mechanical properties meet the Q420 level requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0055] The finished product performance of the 460MPa yield strength steel plate produced by the method of the present invention is: yield strength ≥460MPa, tensile strength 540-720MPa, elongation ≥17%, -20℃ Charpy impact energy ≥150J; the mechanical properties meet the Q460 level requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0056] In order to more intuitively embody the present invention, the embodiments of the present invention are further described in conjunction with examples. The following examples are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solution that can be obviously obtained by a person skilled in the art within the technical scope disclosed in the present invention, including simple changes or equivalent replacements, is within the scope of protection of the present invention.

[0057] [Example]

[0058] Table 1 shows the chemical composition of the steel in each embodiment, Table 2 shows the smelting and continuous casting process parameters of each embodiment, Table 3 shows the heating process parameters of each embodiment, Table 4 shows the rolling and cooling process parameters of each embodiment, and Table 5 shows the properties of the finished steel plates of each embodiment.

[0059] Table 1 Chemical composition of steel (wt%)

[0060] Example C Si Mn P S Nb V Als 1 0.156 0.25 1.48 0.016 0.0035 0.028 0.021 0.015 2 0.132 0.22 1.28 0.014 0.0038 0.025 0.023 0.025 3 0.142 0.25 1.40 0.012 0.0048 0.024 0.025 0.022 4 0.160 0.27 1.42 0.018 0.0046 0.021 0.028 0.023 5 0.126 0.23 1.36 0.018 0.0042 0.025 0.022 0.028 6 0.158 0.28 1.46 0.014 0.0036 0.027 0.026 0.019 7 0.131 0.25 1.22 0.016 0.0038 0.029 0.025 0.023 8 0.156 0.29 1.48 0.013 0.0043 0.028 0.028 0.021

[0061] Table 2 Steel smelting and continuous casting process parameters

[0062] Example 1 2 3 4 5 6 7 8 Phosphorus content of converter steel / wt% 0.016 0.014 0.012 0.018 0.018 0.014 0.016 0.013 Oxygen content in molten steel after LF refining / ppm 16 16 15 14 14 15 20 15 LF refining soft blowing time / min 8 10 9 11 12 10 13 9 LF refining total time / min 41 38 36 45 42 39 40 36 Electromagnetic stirring current intensity / A 800 800 800 800 800 800 800 800 Electromagnetic stirring time / s 30 45 40 45 35 50 55 48 Superheat / ℃ 21 17 22 22 23 18 18 22 Casting speed m / min 1.12 1.08 1.05 1.15 1.05 1.08 1.10 1.15 Total reduction at the end of solidification / mm 15 18 20 17 18 18 20 20 Continuous casting slab thickness / mm 236 234 233 235 233 223 231 232 Center segregation grade of continuous casting slab C0.5 C0.5 C0.5 C0.5 C0.5 C0.5 C0.5 C0.5

[0063] Table 3 Billet heating process parameters

[0064]

[0065] Table 4 Steel plate rolling and cooling process parameters

[0066]

[0067]

[0068] Note: The first three passes in the table are the roughing and widening stages; the bold characters are the finishing and opening passes in each embodiment.

[0069] Table 5 Finished steel plate properties

[0070]

[0071] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A flexible production method for low-alloy high-strength steel with a yield strength of 420 / 460 MPa, characterized in that: The chemical composition of the steel is calculated by weight as follows: C: 0.12% to 0.16%; Si: 0.20% to 0.3%; Mn: 1.2% to 1.5%; P ≤ 0.02%; S ≤ 0.005%; Nb: 0.02% to 0.03%; V：0.02％～0.03％； Als: 0.01% to 0.03%: the balance is Fe and unavoidable impurities; The finished steel plate is a 420MPa yield strength steel plate or a 460MPa yield strength steel plate. The production process includes converter smelting, LF refining, continuous casting, heating, rolling, post-rolling controlled cooling and straightening. The specific control process is as follows: 1) Converter smelting and LF refining; The smelting raw materials are pre-treated with molten iron, and the converter smelting adopts top and bottom blowing to ensure that P ≤ 0.02% when the converter is tapped; During the LF furnace refining process, white slag is produced, aluminum and iron are used for deoxidation to ensure that the oxygen content is less than 20ppm, and calcium is fed to modify MnS inclusions to ensure that the soft blowing time is ≥8min. The total treatment time of LF refining is 35-50min, and S is ≤0.005% at the end of LF refining. 2) continuous casting; The continuous casting superheat is controlled at 17-23°C, the electromagnetic stirring current intensity in the secondary cooling zone is 700-900A, and the stirring is carried out in alternating forward and reverse directions for 30-60 seconds. The casting speed is controlled at 1.05-1.15m / min, and a heavy reduction process is used at the end of the solidification of the slab, with a total reduction of 15-20mm. The center segregation of the slab is rated at C0.5 according to YB / T4003 "Grading Chart for Macrostructure Defects of Continuously Cast Steel Slabs"; 3) heating; After the billet comes off the production line, it is hot-charged and sent to the heating furnace for heating. The furnace entry temperature is not less than 550℃, the preheating section temperature is 700-950℃, and the preheating section time is 0.5-1h; the heating section temperature is 1210±20℃, and the heating section time is 1-1.5h; the soaking section temperature is 1170±20℃, and the soaking section time is 0.5-1h; the total time in the furnace is 2-4h; the billet temperature when it comes out of the furnace is 1150-1180℃, and the temperature difference between the billet surface and the core does not exceed 25℃; 4) rolling; The continuous casting slab is rolled immediately after descaling. The starting temperature of rough rolling is 980-1030°C. The rough rolling widening stage is completed in 2-4 passes, with a pass reduction rate of 8%-18%, a pass interval time of 4-10s, and a rolling speed of 0.8-1.2m / s. The reduction rate of at least the first three passes of the rough rolling longitudinal rolling stage is 23%-28%, a rolling speed of 0.5-1.0m / s, and a pass interval time of 10-15s. The reduction rate of the remaining rough rolling passes is 16%-23%, a rolling speed of 1.0-2.0m / s, and a pass interval time of 5-10s. The reduction rate of each pass of the finishing rolling is 10%-20%, a rolling speed of 2.5-3.3m / s, and a pass interval time of 3-8s. 5) cooling; The steel plate is cooled immediately after rolling, and the cooling rate is 8-15°C / s. The red-return temperature of the yield strength 420MPa grade steel plate is 610-650°C, and the red-return temperature of the yield strength 460MPa grade steel plate is 570-610°C.

2. The flexible production method of a low-alloy high-strength steel with a yield strength of 420 / 460 MPa according to claim 1, characterized in that: The continuous casting billet is subjected to at least two descaling passes after exiting the heating furnace, and the descaling water pressure is 20-25 MPa.

3. The flexible production method of low-alloy high-strength steel with a yield strength of 420 / 460 MPa according to claim 1, characterized in that: For steel plates with a thickness of 10mm≤≤20mm, a single-block controlled rolling method is adopted. The intermediate billet is not cooled and finishing rolling is carried out immediately after rough rolling. For steel plates with a thickness of 20mm<≤40mm, a multi-block controlled rolling method is adopted, that is, single-stand cross rolling is carried out. After the rough rolling of the last steel plate in the rough rolling stage is completed, the first steel plate in the finishing rolling stage is immediately finished. After the finishing rolling of the first steel plate in the finishing rolling stage is completed, the second steel plate in the finishing rolling stage is immediately finished, and so on until the finishing rolling of the last steel plate is completed.

4. The flexible production method of a yield strength 420 / 460 MPa grade low alloy high strength steel according to claim 1, characterized in that: The thickness of the cast billet is 200-250 mm, the thickness of the intermediate billet is 2-3 times the thickness of the finished steel plate, and the thickness of the finished steel plate is 10-40 mm.

5. The flexible production method of low-alloy high-strength steel with a yield strength of 420 / 460 MPa according to claim 1, characterized in that: The ferrite grain size of the finished steel plate is 9 to 11 levels.

6. The flexible production method of low-alloy high-strength steel with a yield strength of 420 / 460 MPa according to claim 1, characterized in that: The finished product performance of the yield strength 420MPa grade steel plate is: yield strength ≥420MPa, tensile strength 520~680MPa, elongation ≥19%, -20℃ Charpy impact energy ≥180J; the mechanical properties meet the Q420 grade requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

7. The flexible production method of low-alloy high-strength steel with a yield strength of 420 / 460 MPa according to claim 1, characterized in that: The finished product performance of the yield strength 460MPa grade steel plate is: yield strength ≥460MPa, tensile strength 540~720MPa, elongation ≥17%, -20℃ Charpy impact energy ≥150J; the mechanical properties meet the Q460 grade requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

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