Flexible production method of low-alloy high-strength steel with yield strength of 355 / 390MPa

Abstract

The invention relates to a flexible production method of low-alloy and high-strength steel with yield strength of 355 / 390 MPa. The steel comprises the following chemical components: 0.13-0.18% of C; 0.20% to 0.35% of Si; 0.8% to 1.10% of Mn; p is less than or equal to 0.02%; s < = 0.005%; 0.005% to 0.01% of Nb; 0.01%-0.03% of Als; the balance of Fe and impurities; the production process comprises the steps of converter smelting, LF refining, continuous casting, heating, rolling, controlled cooling after rolling and straightening. Through the coupling design of smelting, continuous casting, low-temperature heating, low-temperature rolling and controlled cooling processes, the comprehensive effect of fine grain strengthening, precipitation strengthening and phase change strengthening of a high-quality continuous casting blank during low-temperature rolling is fully utilized, and flexible and efficient production of steel plates with the yield strength of 355MPa and 390MPa is achieved.

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 efficient production method for low alloy high strength steel with a yield strength of 355 / 390 MPa. Background Art

[0002] Yield strength 355MPa and 390MPa low alloy high strength steel has good mechanical and welding properties and is widely used in machinery, construction, wind power, vehicles, ships, pressure vessels, chemical equipment and other fields. It is the most widely used variety of medium and thick plate products. With the changes in the market situation, the profitability of yield strength 355MPa and 390MPa low alloy high strength steel is getting lower and lower, and it is urgently needed to improve its profit-making ability through cost-reducing 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 355 MPa or 390 MPa is controlled rolling and controlled cooling. In order to ensure the performance of the steel plate, it is usually required that the start temperature of the finishing rolling and the final rolling temperature be controlled within a narrow temperature range, resulting in a slow pace of steel plate production and strict process requirements, which is not conducive to efficient production.

[0004] The Chinese patent application with application publication number CN 102965575 A discloses "an ultra-fast cooling preparation method for 355MPa grade ship plate steel". First, molten steel is smelted according to the set chemical composition, the molten steel is cast into a billet, and the billet is heated and rolled into a continuous casting slab. The continuous casting slab is heated for rough rolling, and then fine rolling is performed. The start rolling temperature of fine rolling is 950-1000°C, the final rolling temperature is 900-950°C, and the reduction rate is ≥50% to obtain a steel plate. The steel plate is subjected to an ultra-fast cooling process to cool to 700-750°C at 40-50°C / s, and then laminar cooling is adopted to cool to 600-650°C at a speed of 8-15°C / s, and finally air-cooled to room temperature to obtain a 355MPa grade ship plate steel with a yield strength ≥355MPa, a tensile strength of 490-630MPa, a Charpy impact energy at -40°C ≥34J, and an elongation of a standard tensile specimen ≥22%. It achieves high-temperature controlled rolling through ultra-fast cooling and laminar cooling after rolling, improves rolling efficiency, reduces rolling mill load, and better realizes reduced production.

[0005] The Chinese invention patent with authorization announcement number CN 102912229 B discloses "A 390MPa grade low-cost hot-rolled structural steel plate and its manufacturing method", with ingredients: C 0.15%~0.20%, Si 0.05%~0.20%, Mn 0.1%~0.5%, B 0.0005%~0.004%, N≤0.0060%, Al 0.02%~0.05%, Ti 0.015%~0.025%, Ca0.0025%~0.0060%, 0.12≤Ca / Al≤0.2, and the rest is Fe. The method includes converter smelting, refining outside the furnace, continuous casting, rolling, adding Al deoxidation during refining, adding Ti to fix nitrogen, so that O≤0.002%, N≤0.006%, adding boron alloying before the end of refining, feeding Si-Ca wire; the hot charging temperature of the continuous casting billet is greater than 840℃; the slab is heated to 1100-1180℃, kept warm for 2-3 hours, rough rolling is started at 1020-1100℃, the intermediate billet thickness / finished product thickness>5, finishing rolling is started at 950-1000℃, the cumulative deformation is greater than 50%, the final rolling temperature is 850-900℃, and after rolling, it is cooled to 500-600℃ at 6-20℃ / s for coiling, and then air-cooled. The process is simple, the cost is low, and the product has good toughness and plasticity and welding properties.

[0006] Compared with the prior art, the present invention obtains high-quality continuous casting billets through high-purity smelting and continuous casting process under heavy pressure, and at the same time reduces the heating temperature to ensure that the second phase particles are fully dissolved back while reducing the size of the original austenite grains. During rolling, the dispersion precipitation of the second phase particles is achieved through low-temperature rapid large deformation rolling. Rapid cooling after rolling reduces the release of internal energy storage, and makes full use of precipitation strengthening, fine grain strengthening and phase transformation strengthening to achieve flexible and efficient production of steel plates with a thickness of 10 to 40 mm. While greatly improving the production efficiency of steel plates, sufficient performance margin is guaranteed, the energy consumption of the heating furnace is greatly reduced, and the efficiency of the low-alloy high-strength steel with a yield strength of 355MPa and 390MPa is improved. Summary of the invention

[0007] The present invention provides a flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390MPa. Through the coupled design of smelting, continuous casting, low-temperature heating, low-temperature rolling and controlled cooling processes, the comprehensive effects of fine grain strengthening, precipitation strengthening and phase transformation strengthening of high-quality continuous casting billets during low-temperature rolling are fully utilized, thereby achieving flexible and efficient production of steel plates with a yield strength of 355MPa and 390MPa.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390 MPa, wherein the chemical composition of the steel contains, by weight percentage, C: 0.13% to 0.18%; Si: 0.20% to 0.35%; Mn: 0.8% to 1.10%; P≤0.02%; S≤0.005%; Nb: 0.005% to 0.01%; Als: 0.01% to 0.03%; the finished steel plate is a steel plate with a yield strength of 355 MPa or a steel plate with a yield strength of 390 MPa; the remainder is Fe and unavoidable impurities; the production process includes converter smelting, LF refining, continuous casting, heating, rolling, controlled cooling after rolling and straightening; wherein:

[0010] 1) Converter smelting and LF refining;

[0011] The smelting raw materials are pre-treated with molten iron, and the converter smelting adopts top and bottom double blowing to ensure that P ≤ 0.02% when the converter taps steel;

[0012] During the LF furnace refining process, white slag is made, aluminum iron is used for deoxidation to ensure that the oxygen content is less than 20ppm, and calcium is fed to modify the MnS inclusions to ensure that the soft blowing time is ≥5min; the total treatment time of LF refining is 30-50min, and S is ≤0.005% at the end of LF refining;

[0013] 2) Continuous casting;

[0014] The continuous casting superheat is controlled at 20-25℃, the electromagnetic stirring current intensity in the secondary cooling zone is 500-700A, and the continuous stirring is 20-50s; the pulling speed is controlled at 1.10-1.20m / min, and the dynamic heavy pressure reduction process is adopted at the end of the solidification of the ingot, with a total pressure reduction of 20-25mm; the center segregation of the ingot is rated at C0.5 level according to YB / T4003 "Continuous Casting Steel Slab Macrostructure Defect Rating Chart";

[0015] 3) heating;

[0016] After the billet comes off the line, it is hot-charged and sent to the heating furnace for heating. The temperature entering the furnace is not less than 500℃; the temperature of the preheating section is 550~850℃, and the preheating time is 0.5~1h; the temperature of the heating section is 1180~1220℃, and the heating time is 1~1.5h; the temperature of the soaking section is 1130~1170℃, and the soaking time is 0.5~1h; the total time in the furnace is 2~4h, and the temperature of the continuous casting billet when it comes out of the furnace is 1080~1130℃, and the temperature difference between the surface and the core of the billet does not exceed 25℃;

[0017] 4) rolling;

[0018] The billet is rolled immediately after descaling, and the controlled rolling mode is adopted; the rough rolling start temperature is 950-1000℃; the rough rolling widening stage is completed in 2-4 passes, the pass reduction rate is 8%-18%, the pass interval time is 2-8s, and the rolling rate is 1.0-1.5m / s; the first three passes of the rough rolling longitudinal rolling stage have a reduction rate of 25%-30%, a rolling speed of 0.5-1.0m / s, and a pass interval time of 10-15s; the reduction rate of the remaining passes of the rough rolling is 16%-25%, the rolling speed is 1.0-2.0m / s, and the pass interval time is 5-15s;

[0019] The reduction rate of each pass of finishing rolling is 10% to 20%, the rolling speed is 2.5 to 3.3 m / s, and the interval time between passes is 3 to 8 s;

[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 355MPa grade steel plate is 630-670°C, and the red-return temperature of the yield strength 390MPa grade steel plate is 580-620°C.

[0022] Furthermore, the thickness of the ingot is 200-250 mm, the thickness of the intermediate ingot is 1.5-2.5 times the thickness of the finished steel plate, and the thickness of the finished steel plate is 10-40 mm.

[0023] Furthermore, the ingot is descaled at least twice after exiting the heating furnace, and the descaling water pressure is 20-25 MPa.

[0024] Furthermore, for steel plates with a thickness of 10 mm ≤ ≤ 20 mm, a single-block controlled rolling method is adopted, the intermediate billet is not cooled to room temperature, and finishing rolling is carried out immediately after the rough rolling is completed; for steel plates with a thickness of 20 mm < ≤ 40 mm, a multi-block controlled rolling method is adopted, that is, a single-frame cross-rolling is adopted, 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 with finishing rolling, 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 with finishing rolling, and so on until the finish rolling of the last steel plate is completed.

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

[0026] Furthermore, the finished product performance of the yield strength 355MPa grade steel plate is: yield strength ≥355MPa, tensile strength 470~630MPa, elongation ≥23%, -20℃ Charpy impact energy ≥200J; the mechanical properties meet the requirements of Q355 level in GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0027] Furthermore, the finished product performance of the yield strength 390MPa grade steel plate is: yield strength ≥390MPa, tensile strength 490~650MPa, elongation ≥22%, -20℃ Charpy impact energy ≥150J; the mechanical properties meet the requirements of Q390 level in 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 Q355MPa grade steel plate composition, high-quality continuous casting billets are obtained through high-cleanliness smelting and heavy pressure continuous casting process optimization; under low-temperature heating and rolling process conditions, the outstanding contributions of precipitation strengthening, fine grain strengthening and phase transformation strengthening are fully utilized to ensure the performance of 10-40mm thick steel plates while greatly improving production efficiency (the number of blocks per hour is increased by more than 2 blocks / h);

[0030] (2) No precious metal elements such as V, Cu, and rare earth are added to the steel, so the alloy cost is low, ensuring the product's ability to create benefits;

[0031] (3) Under the condition of unchanged composition, the performance of the original 355MPa grade steel plate can be guaranteed to meet the performance requirements of the 390MPa grade steel plate;

[0032] (4) The present invention does not limit the finishing rolling temperature of the steel plate, the production process is simple, the process window is less restricted, and it 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 99.5% or more;

[0033] (5) Effectively reduce the burning loss of slabs, improve the yield rate and obtain 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 355MPa and 390MPa by adjusting the post-rolling cooling process parameters, which greatly improves the production efficiency of steelmaking. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a metallographic photograph of a steel plate produced using conventional technology.

[0036] Figure 2 This is a metallographic photograph of a steel plate produced using the process of the present invention (Example 1). DETAILED DESCRIPTION

[0037] The flexible production method of the low-alloy high-strength steel with a yield strength of 355 / 390MPa described in the present invention can produce a steel plate with a yield strength of 355MPa or a steel plate with a yield strength of 390MPa by using the same chemical composition; the chemical composition in the steel includes C: 0.13% to 0.18% by weight; Si: 0.20% to 0.35%; Mn: 0.8% to 1.10%; P≤0.02%; S≤0.005%; Nb: 0.005% to 0.01%; Als: 0.01% to 0.03%; the balance is Fe and unavoidable impurities.

[0038] The flexible production method of the yield strength 355 / 390MPa grade low alloy high strength steel of the present invention comprises the following production process: converter smelting - LF refining - continuous casting - heating - rolling - controlled cooling after rolling - straightening - cooling bed - shearing - storage, and specifically controls the following process:

[0039] 1) Converter smelting and LF refining:

[0040] The smelting raw materials are pre-treated with molten iron, and the converter smelting adopts top and bottom combined blowing to ensure that P ≤ 0.02% when the converter taps the steel. During the LF furnace refining process, white slag is made, aluminum iron is 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 ≥ 5min; the total treatment time of LF refining is 30 to 50min, and S ≤ 0.005% is ensured at the end of LF refining;

[0041] 2) Continuous casting:

[0042] During continuous casting, low superheat is used, and the superheat is controlled at 20-25°C. Electromagnetic stirring is used in the secondary cooling zone, the current intensity is 500-700A, and continuous stirring is performed for 20-50s. The pulling speed is controlled at 1.10-1.20m / min, and a dynamic heavy pressure reduction process is adopted at the end of solidification of the ingot. The preferred heavy pressure reduction positions are 10, 11 and 12, and the total reduction is 20-25mm. The proportion of columnar crystals is minimized to the greatest extent, and the density of the continuous casting ingot is improved. The center segregation of the ingot can reach C0.5 level according to YB / T4003 "Continuous Casting Steel Slab Low-Magnification Structure Defect Rating Chart".

[0043] 3) Heating:

[0044] After the continuous casting slab comes off the line, it is hot-charged and sent to a walking-beam heating furnace for heating. The temperature of the casting entering the furnace is not less than 500°C, and it goes through the preheating section, heating section and soaking section before coming out of the furnace. The temperature range of the preheating section is 550-850°C, and the preheating time is 0.5-1h; the temperature range of the heating section is 1200±20°C, and the heating time is 1-1.5h; the temperature range of the soaking section is 1150±20°C, 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 it comes out of the furnace is 1080-1130°C, and the temperature difference between the surface and the core of the casting slab is guaranteed not to exceed 25°C; the above heating process control not only ensures the full solid solution of the micro-alloy elements, but also reduces the size of the original austenite grains.

[0045] 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 no primary iron oxide scale remains on the surface of the steel plate.

[0046] 4) Rolling:

[0047] For steel plates with a thickness of 10 to 40 mm, a controlled rolling production mode is adopted. Among them, a single-block controlled rolling strategy is adopted for steel plates with a thickness of 10 to 20 mm. The intermediate billet is not cooled to a temperature, and finishing rolling is carried out immediately after the rough rolling is completed. A cross-rolling strategy, i.e., multi-block controlled rolling, is adopted for steel plates with a thickness of 20 to 40 mm (excluding 20 mm). Since it is a single stand, after the rough rolling of the last steel plate in the rough rolling stage is completed during cross rolling, the first steel plate in the finishing rolling stage is immediately finished, and after the first steel plate in the finishing rolling stage is finished, 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.

[0048] The continuous casting billet is rolled immediately after descaling, and the starting rolling temperature is 950-1000°C; the rough rolling widening stage is completed in 2-4 passes, the pass reduction rate is 8%-18%, the pass interval time is 2-8s, and the rolling rate is 1.0-1.5m / s; the first 3 passes of the rough rolling longitudinal rolling stage have a reduction rate of 25%-30%, a rolling speed of 0.5-1.0m / s, and a pass interval time of 10-15s; the remaining passes of the rough rolling have a reduction rate of 16%-25%, a rolling speed of 1.0-2.0m / s, and a pass interval time of 5-15s.

[0049] By means of large reduction at low temperature and appropriate increase in the interval between passes, the dynamic and static recrystallization ratios are increased respectively to ensure complete recrystallization in the rough rolling stage.

[0050] The thickness of the intermediate billet is 1.5 to 2.5 times the thickness of the finished steel plate, the reduction rate of each pass of finishing rolling is 10% to 20%, the rolling speed is 2.5 to 3.3 m / s, and the interval time between passes is 3 to 8 s. Through rapid and large reduction rolling, the internal energy storage of the steel plate after rolling is increased and the grains are refined; there is no requirement for the final rolling temperature.

[0051] 5) Cooling:

[0052] After rolling, the steel plate quickly enters the cooling system for cooling, with a cooling rate of 8-15°C / s. The red-return temperature of the yield strength 355MPa grade steel plate is 630-670°C, and the red-return temperature of the yield strength 390MPa grade steel plate is 580-620°C. Through rapid cooling, the release of internal energy storage after rolling of the steel plate is reduced, thereby refining the grain size and improving the -20°C low-temperature impact performance.

[0053] 6) Straightening, cooling bed, shearing and storage;

[0054] After the steel plate leaves the cooling system, it is straightened to ensure that the plate shape is good. After straightening, it is cooled on a cooling bed. When the temperature of the steel plate is lower than 100℃, it is sheared on the cooling bed and can be delivered to the warehouse.

[0055] Conventional processes use a controlled rolling production mode. After the rough rolling, the steel plate needs to be air-cooled to the start temperature of the finishing rolling (usually takes 1 to 4 minutes), and laminar cooling is performed after the rolling is completed. The production method of the present invention is suitable for the production of steel plates with a thickness of 10 to 40 mm. When rolling 10 to 20 mm thick steel plates, a single block controlled rolling mode is adopted but the intermediate billet is not cooled, and finishing rolling is performed immediately after the rough rolling is completed; when rolling 20 to 40 mm (excluding 20 mm) thick steel plates, multiple steel plates are controlled and rolled at the same time. When multiple controlled rolling is performed, the rough rolling of the last steel plate is completed and the finishing rolling of the first steel plate is immediately rolled. After the finishing rolling of the first steel plate is completed, the second steel plate is immediately finished and so on to ensure that the rolling mill does not stop, thereby improving the rolling efficiency.

[0056] The finished product performance of the 355MPa yield strength steel plate produced by the method of the present invention is: yield strength ≥355MPa, tensile strength 470-630MPa, elongation ≥23%, -20℃ Charpy impact energy ≥200J; the mechanical properties meet the requirements of Q355 level in GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0057] The finished product performance of the 390MPa yield strength steel plate produced by the method of the present invention is: yield strength ≥390MPa, tensile strength 490-650MPa, elongation ≥22%, -20℃ Charpy impact energy ≥150J; the mechanical properties meet the Q390 level requirements of GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0058] 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 protection scope 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 protection scope of the present invention.

[0059] [Example]

[0060] 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.

[0061] Table 1 Chemical composition of steel (wt%)

[0062] Example C Si Mn P S Nb Als 1 0.17 0.21 1.08 0.018 0.0039 0.008 0.025 2 0.18 0.32 0.82 0.015 0.0048 0.01 0.02 3 0.15 0.35 1.10 0.013 0.0042 0.01 0.022 4 0.14 0.28 0.92 0.016 0.0046 0.009 0.023 5 0.16 0.33 1.06 0.018 0.0032 0.007 0.018 6 0.13 0.29 0.86 0.013 0.0036 0.007 0.03 7 0.15 0.25 1.02 0.018 0.0028 0.009 0.027 8 0.17 0.28 0.98 0.016 0.0033 0.008 0.019

[0063] Table 2 Smelting and continuous casting process parameters

[0064] Example 1 2 3 4 5 6 7 8 LF processing time / min 36 35 43 40 33 50 48 45 Oxygen content in molten steel after LF refining / ppm 13 15 14 18 14 16 20 17 LF refining soft blowing time / min 8 5 9 7 6 5 8 6 Electromagnetic stirring current intensity / A 600 600 600 600 600 600 600 600 Electromagnetic stirring time / s 30 45 40 45 35 50 30 30 Superheat / ℃ 21 25 22 22 23 25 20 22 Pulling speed m / min 1.12 1.18 1.15 1.15 1.12 1.18 1.10 1.16 Total pressure reduction at the end of solidification / mm 25 23 20 21 23 24 20 21 Continuous casting slab thickness / mm 227 229 233 232 231 228 234 232 Center segregation level of continuous casting slab C0.5 C0.5 C0.5 C0.5 C0.5 C0.5 C0.5 C0.5

[0065] Table 3 Heating process parameters

[0066]

[0067] Table 4 Rolling and cooling process parameters

[0068]

[0069]

[0070] Note: The first three passes in the table are the rough rolling and widening stages; the bold characters are the finishing rolling passes in each embodiment.

[0071] Table 5 Finished steel plate properties

[0072]

[0073] Figure 1 This is a metallographic photograph of a steel plate produced using conventional technology. Figure 2 This is a metallographic structure photograph of the steel plate produced in Example 1. The thickness of the steel plate is the same, but the grain size of the steel plate produced in Example 1 is increased by 3 levels.

[0074] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390 MPa, characterized in that: The chemical composition of the steel by weight percentage includes C: 0.13% ~ 0.18%; Si: 0.20% ~ 0.35%; Mn: 0.8% ~ 1.10%; P ≤ 0.02%; S ≤ 0.005%; Nb: 0.005% ~ 0.01%; Als: 0.01% to 0.03%: the balance is Fe and unavoidable impurities; The finished steel plate is a 355MPa yield strength steel plate or a 390MPa yield strength steel plate; the production process includes converter smelting, LF refining, continuous casting, heating, rolling, post-rolling controlled cooling and straightening; among which: 1) Converter smelting and LF refining; The smelting raw materials are pre-treated with molten iron, and the converter smelting adopts top and bottom double blowing to ensure that P ≤ 0.02% when the converter taps steel; During the LF furnace refining process, white slag is made, aluminum iron is used for deoxidation to ensure that the oxygen content is less than 20ppm, and calcium is fed to modify the MnS inclusions to ensure that the soft blowing time is ≥5min; the total treatment time of LF refining is 30-50min, and S is ≤0.005% at the end of LF refining; 2) Continuous casting; The continuous casting superheat is controlled at 20-25°C, the electromagnetic stirring current intensity in the secondary cooling zone is 500-700A, and the continuous stirring is 20-50s; The drawing speed is controlled at 1.10-1.20 m / min, and the dynamic heavy pressure reduction process is adopted at the end of the solidification of the ingot, with a total pressure reduction of 20-25 mm; the center segregation of the ingot is rated at C0.5 according to YB / T4003 "Continuous Casting Steel Slab Macrostructure Defect Rating Chart"; 3) heating; After the billet comes off the line, it is hot-charged and sent to the heating furnace for heating. The temperature entering the furnace is not less than 500℃; the temperature of the preheating section is 550~850℃, and the preheating time is 0.5~1h; the temperature of the heating section is 1180~1220℃, and the heating time is 1~1.5h; the temperature of the soaking section is 1130~1170℃, and the soaking time is 0.5~1h; the total time in the furnace is 2~4h, and the temperature of the continuous casting billet when it comes out of the furnace is 1080~1130℃, and the temperature difference between the surface and the core of the billet does not exceed 25℃; 4) rolling; The billet is rolled immediately after descaling, and the controlled rolling mode is adopted; the rough rolling start temperature is 950-1000℃; the rough rolling widening stage is completed in 2-4 passes, the pass reduction rate is 8%-18%, the pass interval time is 2-8s, and the rolling rate is 1.0-1.5m / s; the first three passes of the rough rolling longitudinal rolling stage have a reduction rate of 25%-30%, a rolling speed of 0.5-1.0m / s, and a pass interval time of 10-15s; the reduction rate of the remaining passes of the rough rolling is 16%-25%, the rolling speed is 1.0-2.0m / s, and the pass interval time is 5-15s; The reduction rate of each pass of finishing rolling is 10% to 20%, the rolling speed is 2.5 to 3.3 m / s, and the interval time between passes is 3 to 8 s; 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 355MPa grade steel plate is 630-670°C, and the red-return temperature of the yield strength 390MPa grade steel plate is 580-620°C.

2. The flexible production method of low alloy high strength steel with a yield strength of 355 / 390 MPa according to claim 1, characterized in that: The thickness of the ingot is 200-250 mm, the thickness of the intermediate ingot is 1.5-2.5 times the thickness of the finished steel plate, and the thickness of the finished steel plate is 10-40 mm.

3. The flexible production method of low alloy high strength steel with a yield strength of 355 / 390 MPa according to claim 1, characterized in that: The ingot is descaled at least twice after exiting the heating furnace, and the descaling water pressure is 20-25 MPa.

4. The flexible production method of low alloy high strength steel with a yield strength of 355 / 390 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, and the intermediate billet is not cooled to room temperature. 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, a single-stand cross-rolling is adopted. 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 with finish rolling. 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 with finish rolling, and so on until the finish rolling of the last steel plate is completed.

5. The flexible production method of low alloy high strength steel with a yield strength of 355 / 390 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 355 / 390 MPa according to claim 1, characterized in that: The finished product performance of the yield strength 355MPa grade steel plate is: yield strength ≥355MPa, tensile strength 470~630MPa, elongation ≥23%, -20℃ Charpy impact energy ≥200J; the mechanical properties meet the requirements of Q355 grade in 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 355 / 390 MPa according to claim 1, characterized in that: The finished product performance of the yield strength 390MPa grade steel plate is: yield strength ≥390MPa, tensile strength 490~650MPa, elongation ≥22%, -20℃ Charpy impact energy ≥150J; the mechanical properties meet the requirements of Q390 grade in GB / T1591 "Low Alloy High Strength Structural Steel" standard.

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