A flexible production method of a low alloy high strength steel with yield strength 355 / 390 MPa

Abstract

The application relates to a flexible production method of a yield strength 355 / 390MPa grade low-alloy high-strength steel, the chemical components of the steel are as follows: 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%-0.03%; the balance is Fe and impurities; the production process comprises converter smelting, LF refining, continuous casting, heating, rolling, post-rolling controlled cooling and straightening. Through coupling design of smelting, continuous casting, low-temperature heating, low-temperature rolling and controlled cooling processes, the comprehensive actions of fine-grain strengthening, precipitation strengthening and phase change strengthening of high-quality continuous casting billets in low-temperature rolling are fully utilized, and flexible and high-efficiency production of the yield strength 355MPa grade and 390MPa grade steel plates is realized.

Description

Technical Field

[0001] This invention relates to the field of low-alloy high-strength steel production technology, and in particular to a flexible and efficient production method for low-alloy high-strength steel with a yield strength of 355 / 390MPa. Background Technology

[0002] Low-alloy high-strength steel with yield strengths of 355MPa and 390MPa possesses excellent mechanical and weldability 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 type of medium and heavy plate product. However, with changing market conditions, the profitability of low-alloy high-strength steel with yield strengths of 355MPa and 390MPa is declining, necessitating cost reduction in production to improve its efficiency.

[0003] Currently, the main production mode for low-alloy high-strength steel with a thickness of 10-40mm and a yield strength of 355MPa or 390MPa is controlled rolling and controlled cooling. In order to ensure the performance of the steel plate, the initial rolling temperature and the final rolling temperature are usually required to be controlled within a narrow temperature range, which results in a slow production pace and stringent process requirements, which is not conducive to efficient production.

[0004] Chinese patent application CN 102965575 A discloses "An ultra-fast cooling preparation method for 355MPa grade ship plate steel". The method involves first smelting steel according to a set chemical composition, casting the molten steel into billets, heating and rolling the billets into continuously cast slabs, then heating and rough rolling the continuously cast slabs, followed by finish rolling. The finish rolling temperature is 950–1000℃, the final rolling temperature is 900–950℃, and the reduction rate is ≥50%, resulting in steel plates. The steel plates are then subjected to an ultra-fast cooling process, cooling to 700–750℃ at a rate of 40–50℃ / s, followed by laminar flow cooling at a rate of 8–15℃ / s to 600–650℃, and finally air cooling to room temperature. This yields 355MPa grade ship plate steel with a yield strength ≥355MPa, tensile strength 490–630MPa, Charpy impact energy ≥34J at -40℃, and elongation of the standard tensile sample ≥22%. It achieves high-temperature controlled rolling through ultra-fast cooling and laminar flow cooling after rolling, which improves rolling efficiency, reduces mill load, and better realizes reduced production.

[0005] Chinese invention patent CN 102912229 B discloses "a low-cost hot-rolled structural steel plate of 390MPa grade and its manufacturing method", with the following composition: 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%, Ca 0.0025%~0.0060%, 0.12≤Ca / Al≤0.2, and the balance being Fe. The method includes converter smelting, ladle refining, continuous casting, and rolling. Refining involves Al deoxidation and Ti nitrogen fixation to achieve O ≤ 0.002% and N ≤ 0.006%. Boron alloying is added before the end of refining, and Si-Ca wire is fed. The hot charging temperature of the continuously cast billet is greater than 840℃. The slab is heated to 1100–1180℃ and held for 2–3 hours. Rough rolling begins at 1020–1100℃, with an intermediate billet thickness / finished product thickness > 5. Finish rolling begins at 950–1000℃, with a cumulative deformation greater than 50%. The final rolling temperature is 850–900℃. After rolling, the slab is cooled at 6–20℃ / s to 500–600℃ and coiled, followed by air cooling. This process is simple, low-cost, and produces products with good toughness, plasticity, and weldability.

[0006] Compared with existing technologies, this invention obtains high-quality continuously cast billets through high-purity smelting and heavy-pressure continuous casting processes. At the same time, it reduces the heating temperature, ensuring sufficient re-dissolution of the second-phase particles while reducing the original austenite grain size. During rolling, low-temperature rapid large deformation rolling achieves the dispersed precipitation of the second-phase particles. Rapid cooling after rolling reduces the release of internal stored energy, making full use of precipitation strengthening, fine grain strengthening, and phase transformation strengthening effects. This enables flexible and efficient production of steel plates with a thickness of 10-40mm, significantly improving steel plate production efficiency while ensuring sufficient performance margin. It also significantly reduces the energy consumption of the heating furnace and enhances the efficiency of low-alloy high-strength steel with yield strength of 355MPa and 390MPa. Summary of the Invention

[0007] This 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, it makes full use of the combined effects of fine grain strengthening, precipitation strengthening and phase transformation strengthening of high-quality continuous casting billets during low-temperature rolling, and realizes flexible and efficient production of steel plates with yield strengths of 355MPa and 390MPa.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390 MPa is disclosed. The steel's chemical composition, by weight percentage, contains: 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%–0.03%. The finished steel plate is a 355 MPa or 390 MPa yield strength steel plate; the balance is Fe and unavoidable impurities. The production process includes converter smelting, LF refining, continuous casting, heating, rolling, post-rolling controlled cooling, and straightening.

[0010] 1) Converter smelting and LF refining;

[0011] The raw materials for smelting are pretreated with molten iron, and the converter smelting adopts top and bottom blowing to ensure that P ≤ 0.02% when the steel is tapped from the converter.

[0012] During the LF furnace refining process, white slag is produced by using aluminum-iron deoxidation to ensure that the oxygen content is below 20 ppm. MnS inclusions are modified by feeding calcium and the soft blowing time is guaranteed to be ≥5 min. The total LF refining processing time is 30-50 min, and S ≤0.005% is achieved at the end of LF refining.

[0013] 2) Continuous casting;

[0014] The superheat of continuous casting is controlled at 20-25℃, the electromagnetic stirring current intensity in the secondary cooling zone is 500-700A, and the stirring is continuous for 20-50s; the casting speed is controlled at 1.10-1.20m / min, and the solidification end of the billet adopts a dynamic heavy pressure reduction process with a total reduction of 20-25mm; the center segregation of the billet is rated to C0.5 level according to YB / T4003 "Low-magnification microstructure defect rating chart of continuously cast steel slab";

[0015] 3) Heating;

[0016] After the billet is removed from the production line, it is hot-charged into a heating furnace for heating. The furnace temperature is not lower than 500℃. The preheating temperature is 550-850℃ and the preheating time is 0.5-1h. The heating temperature is 1180-1220℃ and the heating time is 1-1.5h. The soaking temperature is 1130-1170℃ and the soaking time is 0.5-1h. The total time in the furnace is 2-4h. When the billet is removed from the furnace, the temperature of the continuous casting billet 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, using a controlled rolling mode. The initial rolling temperature for roughing is 950–1000℃. The widening stage of roughing is completed in 2–4 passes, with a reduction rate of 8%–18% per pass, a pass interval of 2–8 s, and a rolling speed of 1.0–1.5 m / s. In the longitudinal rolling stage of roughing, the reduction rate for the first 3 passes is 25%–30%, the rolling speed is 0.5–1.0 m / s, and the pass interval is 10–15 s. The reduction rate for the remaining passes of roughing is 16%–25%, the rolling speed is 1.0–2.0 m / s, and the pass interval is 5–15 s.

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

[0020] 5) Cooling;

[0021] The rolled steel plates are cooled immediately at a rate of 8–15℃ / s. The red-hot temperature of the steel plate with a yield strength of 355MPa is 630–670℃, and the red-hot temperature of the steel plate with a yield strength of 390MPa is 580–620℃.

[0022] Furthermore, the thickness of the cast billet is 200-250 mm, the thickness of the intermediate billet 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, after the billet exits the heating furnace, it undergoes at least two descaling processes with a descaling water pressure of 20–25 MPa.

[0024] Furthermore, for steel plates with a thickness of 10mm to 20mm, a single-plate controlled rolling method is adopted, and the intermediate billet is not cooled before finishing. Finish rolling is carried out immediately after rough rolling. For steel plates with a thickness of 20mm to 40mm, a multi-plate controlled rolling method is adopted, that is, a single-stand cross rolling is adopted. After the last steel plate in the rough rolling stage is finished, the first steel plate in the finishing rolling stage is immediately finished. 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 last steel plate is finished.

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

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

[0027] Furthermore, the finished product properties of the 390MPa yield strength grade steel plate are as follows: yield strength ≥390MPa, tensile strength 490~650MPa, elongation ≥22%, Charpy impact energy at -20℃ ≥150J; the mechanical properties meet the requirements of Q390 grade in GB / T1591 "Low Alloy High Strength Structural Steel" standard.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) Based on the original Q355MPa grade steel plate composition, high-quality continuous casting billets are obtained through high-purity 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 significantly 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, resulting in low alloy cost and ensuring the product's profitability.

[0031] (3) Under the condition of unchanged composition, it can be guaranteed that the performance of the original yield strength 355MPa grade steel plate can meet the performance requirements of the yield strength 390MPa grade steel plate.

[0032] (4) The present invention does not limit the finishing rolling temperature of steel plates, the production process is simple, the process window is small, which is conducive to the production of steel plates with complex nominal dimensions, and the first-time performance qualification rate after actual application is as high as 99.5% or more.

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

[0034] (6) By adopting the low-temperature flexible rolling process, the same batch or even the same furnace of molten steel (with the same composition) 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. Attached Figure Description

[0035] Figure 1 These are metallographic images of steel plates produced using conventional processes.

[0036] Figure 2 These are metallographic images of steel plates produced using the process described in this invention (Example 1). Detailed Implementation

[0037] The present invention discloses a flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390 MPa, which can produce steel plates with a yield strength of 355 MPa or 390 MPa using the same chemical composition. 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%–0.03%; with the balance being Fe and unavoidable impurities.

[0038] The present invention discloses a flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390MPa. The production process includes: converter smelting—LF refining—continuous casting—heating—rolling—controlled cooling after rolling—straightening—cooling bed—shearing—warehousing. The specific control process is as follows:

[0039] 1) Converter smelting and LF refining:

[0040] The raw materials for smelting undergo hot metal pretreatment. The converter smelting employs top and bottom blowing to ensure that P ≤ 0.02% at the time of tapping. During the LF furnace refining process, white slag is created using aluminum-iron deoxidation to ensure an oxygen content below 20 ppm. MnS inclusions are modified by calcium feeding, ensuring a soft blowing time ≥ 5 min. The total LF refining time is 30–50 min, and S ≤ 0.005% is ensured at the end of LF refining.

[0041] 2) Continuous casting:

[0042] During continuous casting, a low superheat is adopted, with the superheat controlled at 20-25℃. Electromagnetic stirring in the secondary cooling zone is used, with a current intensity of 500-700A and continuous stirring for 20-50s. The casting speed is controlled at 1.10-1.20m / min. Dynamic heavy reduction process is adopted at the end of the billet solidification, with the preferred heavy reduction positions being sections 10, 11, and 12, and the total reduction being 20-25mm. This minimizes the proportion of columnar crystals, improves the density of the continuously cast billet, and the center segregation of the billet can reach grade C0.5 according to YB / T4003 "Low-magnification microstructure defect rating chart of continuously cast steel slab".

[0043] 3) Heating:

[0044] After the continuously cast slab comes off the production line, it is hot-charged into a walking beam furnace for heating. The slab's temperature upon entering the furnace is not lower than 500℃. It then goes through a preheating section, a heating section, and a soaking section before exiting the furnace. The temperature range of the preheating section is 550–850℃, and the preheating time is 0.5–1 hour. The temperature range of the heating section is 1200±20℃, and the heating time is 1–1.5 hours. The temperature range of the soaking section is 1150±20℃, and the soaking time is 0.5–1 hour. The total time in the furnace is 2–4 hours. The temperature of the continuously cast slab when exiting the furnace is 1080–1130℃, and the temperature difference between the surface and the core of the slab is guaranteed to be no more than 25℃. The above heating process control ensures both the full solid solution of microalloying elements and the reduction of the original austenite grain size.

[0045] After the continuously cast billet exits the heating furnace, it undergoes at least two descaling processes with a descaling water pressure of 20–25 MPa to ensure that there is no primary iron oxide scale residue on the surface of the steel plate.

[0046] 4) Rolling:

[0047] For steel plates with a thickness of 10–40 mm, a controlled rolling production mode is adopted. Among them, steel plates with a thickness of 10–20 mm adopt a single-piece controlled rolling strategy, and the intermediate billet is not cooled before finishing rolling. Steel plates with a thickness of 20–40 mm (excluding 20 mm) adopt a cross-rolling strategy, i.e., multi-piece controlled rolling. Since it is a single stand, when the last steel plate in the roughing stage is finished, the first steel plate in the finishing stage is immediately finished, and the second steel plate in the finishing stage is immediately finished after the first steel plate in the finishing stage is finished, and so on until the last steel plate is finished.

[0048] The continuously cast billet is rolled immediately after descaling, with an initial rolling temperature of 950–1000℃. The roughing and widening stage is completed in 2–4 passes, with a reduction rate of 8%–18% per pass, a pass interval of 2–8 s, and a rolling speed of 1.0–1.5 m / s. In the roughing and longitudinal rolling stage, the first 3 passes have a reduction rate of 25%–30%, a rolling speed of 0.5–1.0 m / s, and a pass interval of 10–15 s. The remaining passes of the roughing stage have a reduction rate of 16%–25%, a rolling speed of 1.0–2.0 m / s, and a pass interval of 5–15 s.

[0049] By increasing the reduction at low temperature and appropriately increasing the interval between passes, the ratio of dynamic to static recrystallization is increased, ensuring complete recrystallization during the roughing 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 in the finishing rolling is 10% to 20%, the rolling speed is 2.5 to 3.3 m / s, and the interval between passes is 3 to 8 s. By rolling with a large reduction, the internal energy storage of the rolled steel plate is increased, and the grains are refined; there is no requirement for the final rolling temperature.

[0051] 5) Cooling:

[0052] After rolling, the steel plates are rapidly cooled in a cooling system at a rate of 8–15 °C / s. The red-hot temperature of the steel plate with a yield strength of 355 MPa is 630–670 °C, and that of the steel plate with a yield strength of 390 MPa is 580–620 °C. This rapid cooling reduces the release of internal energy stored in the steel plate after rolling, thereby refining the grain size and improving low-temperature impact resistance at -20 °C.

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

[0054] After the steel plate leaves the cooling system, it is straightened to ensure good plate shape. After straightening, it is cooled on a cooling bed. Once the temperature of the steel plate is below 100℃, it is removed from the cooling bed and sheared before being put into storage.

[0055] Conventional processes employ controlled rolling production. After roughing, the steel plate needs to be air-cooled to reach the finishing rolling temperature (usually requiring 1-4 minutes), followed by laminar flow cooling after rolling. The production method described in this invention is applicable to the production of steel plates with a thickness of 10-40mm. For 10-20mm thick steel plates, a single-plate controlled rolling mode is used without intermediate billet cooling; finishing rolling begins immediately after roughing. For 20-40mm (excluding 20mm) thick steel plates, multiple plates are simultaneously controlled rolled. In multi-plate controlled rolling, the first finishing plate is rolled immediately after the last roughing plate is finished, and the second finishing plate is rolled immediately after the first finishing plate is finished, and so on, ensuring the rolling mill operates continuously and thus improving rolling efficiency.

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

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

[0058] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection 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 in each embodiment, Table 3 shows the heating process parameters in each embodiment, Table 4 shows the rolling and cooling process parameters in each embodiment, and Table 5 shows the properties of the finished steel plates in 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 / °C 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 reduction at the end of solidification / mm 25 23 20 21 23 24 20 21 Continuous casting billet thickness / mm 227 229 233 232 231 228 234 232 Center segregation grade of continuous casting billet 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 represent the roughing and widening stage; the bolded text indicates the finishing and opening passes in each embodiment.

[0071] Table 5 Properties of Finished Steel Plates

[0072]

[0073] Figure 1 Metallographic images of steel plates produced using conventional processes. Figure 2 The image shows the metallographic structure 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 improved by 3 levels.

[0074] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within 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 355 / 390 MPa, characterized in that, The chemical composition of the steel, by weight percentage, contains: C: 0.16%–0.18%; Si: 0.20%–0.35%; Mn: 0.8%–0.86%; P≤0.02%; S≤0.005%; Nb: 0.005%–0.01%. Als: 0.01%~0.03%; balance is Fe and unavoidable impurities; The finished steel plates are grade 355MPa or 390MPa steel plates; 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 raw materials for smelting undergo hot metal pretreatment, and the converter smelting adopts top and bottom blowing to ensure that P ≤ 0.02% when the steel is tapped from the converter. During the LF furnace refining process, white slag is produced using aluminum-iron deoxidation to ensure an oxygen content below 20 ppm. Calcium feeding is used to modify MnS inclusions, and soft blowing time is guaranteed to be ≥5 min. The total LF refining processing time is 30–50 min, and S ≤0.005% is achieved at the end of LF refining. 2) Continuous casting; The superheat of continuous casting is controlled at 20-25℃, the electromagnetic stirring current intensity in the secondary cooling zone is 500-700A, and the stirring is continuous for 20-50s. The casting speed is controlled at 1.10-1.20 m / min. The solidification end of the billet adopts a dynamic heavy pressure reduction process with a total reduction of 20-25 mm. The thickness of the billet is 200-250 mm. The center segregation of the billet is rated to C0.5 level according to YB / T4003 "Low-magnification microstructure defect rating chart of continuous casting steel slab". 3) Heating; After the billet is removed from the production line, it is hot-charged into a heating furnace for heating. The furnace temperature is not lower than 500℃. The preheating temperature is 550-850℃ and the preheating time is 0.5-1h. The heating temperature is 1180-1220℃ and the heating time is 1-1.5h. The soaking temperature is 1130-1170℃ and the soaking time is 0.5-1h. The total time in the furnace is 2-4h. When the billet is removed from the furnace, the temperature of the continuous casting billet is 1080-1130℃, and the temperature difference between the surface and the core of the billet does not exceed 25℃. 4) Rolling; After the billet exits the heating furnace, it undergoes at least two descaling passes at a descaling water pressure of 20–25 MPa. The billet is rolled immediately after descaling using a controlled rolling method. For steel plates with a thickness of 10 mm ≤ 20 mm, a single-plate controlled rolling method is used, with no intermediate cooling required; finishing rolling begins immediately after rough rolling. For steel plates with a thickness of 20 mm < 40 mm, a multi-plate controlled rolling method is used, i.e., single-stand cross-rolling. After the last steel plate in the rough rolling stage is finished, the first steel plate in the finishing rolling stage is immediately finished; 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 last steel plate is finished. The initial rolling temperature for roughing is 950–1000℃; the widening stage of roughing is completed in 2–4 passes, with a reduction rate of 8%–18%, a pass interval of 2–8 s, and a rolling speed of 1.0–1.5 m / s; the first 3 passes of the longitudinal rolling stage of roughing have a reduction rate of 25%–30%, a rolling speed of 0.5–1.0 m / s, and a pass interval of 10–15 s; the remaining passes of roughing have a reduction rate of 16%–25%, a rolling speed of 1.0–2.0 m / s, and a pass interval of 5–15 s; the thickness of the intermediate slab is 1.5–2.5 times the thickness of the finished steel plate. The reduction rate for each pass of the finishing rolling is 10% to 20%, the rolling speed is 2.5 to 3.3 m / s, and the interval between passes is 3 to 8 s; the thickness of the finished steel plate is 10 to 40 mm. 5) Cooling; The rolled steel plates are cooled immediately at a rate of 8–15℃ / s. The red-hot temperature of the steel plate with a yield strength of 355MPa is 630–670℃, and the red-hot temperature of the steel plate with a yield strength of 390MPa is 580–620℃.

2. The flexible production method for 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 grade 9 to 11.

3. The flexible production method for low-alloy high-strength steel with a yield strength of 355 / 390 MPa according to claim 1, characterized in that, The finished product properties of steel plates with a yield strength of 355MPa are as follows: yield strength ≥355MPa, tensile strength 470~630MPa, elongation ≥23%, Charpy impact energy at -20℃ ≥200J; the mechanical properties meet the requirements of Q355 grade in GB / T1591 "Low Alloy High Strength Structural Steel" standard.

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

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