An ultra-high strength structural steel with good comprehensive performance and a production method thereof

Abstract

The application discloses a kind of comprehensive performance good super high-strength structural steel and its production method, and its component weight percentage is as follows: C 0.15~0.21%, Si≤0.50%, Mn 0.60~1.60%, Ti 0.051~0.15%, V 0.040~0.12%, RE 0.0005~0.0030%, Mg 0~0.003%, Cr 0.20~1.20%, B 0.0005~0.0030%, Al 0.02~0.06%, Ca 0.0005~0.004%, N≤0.004%, P≤0.020%, S≤0.0030%, O≤0.0025%, 5≤(Mo+W+2.3*Cr) / (Ti+V)≤26, Ti+V≥0.11%, the rest is Fe and inevitable impurity;A, B, C, D and DS five kinds of inclusions level are all controlled below 1.0 level, and the total sum of non-metallic inclusion rating is controlled below 3.0.The yield strength of the super high-strength structural steel is ≥960MPa, the tensile strength is ≥1000MPa, the cold bending performance meets D=3a, 90 degrees, and the elongation A 50 ≥21%, and the impact energy at-60 DEG C is ≥120J.

Description

Technical Field

[0001] This invention relates to the field of structural steel technology, and in particular to an ultra-high strength structural steel with excellent comprehensive performance and its production method. Background Technology

[0002] 960MPa grade hot-rolled ultra-high-strength steel for engineering machinery is mainly used in the manufacture of components such as crane booms and concrete pump truck placing booms. This requires high strength, plasticity, low-temperature toughness, and fatigue performance of the steel plate. Traditional 960MPa ultra-high-strength steel is generally produced using offline quenching and tempering heat treatment or online quenching and tempering processes to obtain tempered martensite. Tempered martensite has low plasticity, making it prone to cracking during bending, hole expansion, and other processing. For example, 960MPa ultra-high-strength steel produced by traditional processes can only meet the cold bending performance requirements of D=5-6a, 90 degrees, and elongation ≤16%. Tempering at temperatures above 600℃ causes partial recrystallization of martensite, effectively improving the plasticity of the ultra-high-strength steel, but significantly reducing its strength.

[0003] Chinese patent CN103014538B developed an ultra-high strength steel with a yield strength of 960MPa through online quenching and tempering at 510-550℃, and the microstructure is tempered martensite.

[0004] Chinese patent CN102134680A introduces a production method for high-strength steel with a yield strength of 960MPa. It adopts a low carbon content design and a high Cr content, with C: 0.07% to 0.09% and Cr: 1.05% to 1.15%. This patent does not contain Nb, Ti, or V microalloying elements and has a high Cr content.

[0005] Chinese patent CN102560274A developed an ultra-high strength steel with a yield strength of 1000MPa through offline heat treatment. The microstructure is tempered martensite, and its main components are Cr: 0.30-0.50%; Mo: 0.30-0.50%; Ni: 0.20-0.50%; V: 0.030-0.050%.

[0006] Chinese patent CN102505096A describes a tempered martensitic ultra-high strength steel obtained through online quenching followed by tempering at 460-520℃. Summary of the Invention

[0007] The purpose of this invention is to provide an ultra-high strength structural steel with excellent comprehensive performance and its production method. The microstructure of this high-strength steel is tempered sorbite with a large number of nano-precipitates. Its yield strength is ≥960MPa, tensile strength is ≥1000MPa, cold bending performance meets D=3a, 90 degrees, and elongation A… 50 >21%, impact energy at -60℃ >120J.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A high-strength structural steel with excellent comprehensive performance has the following composition by weight percentage: C: 0.15–0.21%, Si ≤ 0.50%, Mn: 0.60–1.60%, Ti: 0.051–0.15%, V: 0.040–0.12%, RE: 0.0005–0.0030%, Cr: 0.20–1.20%, B: 0.0005–0.0030%, Al: 0.02–0.06%, Ca: 0.0005–0.004%, N ≤ 0.004%, P ≤ 0.020%, S ≤ 0.0030%, O ≤ 0.0025%, with the balance including Fe and other unavoidable impurities; and must simultaneously satisfy the following:

[0010] The nanoprecipitate control index (NPI) is 5–26, and NPI = (Mo + W + 2.3 * Cr) / (Ti + V).

[0011] Ti+V≥0.11%;

[0012] The levels of all five types of inclusions (A, B, C, D, and DS) in the ultra-high strength structural steel are controlled below grade 1.0, and the total rating of all non-metallic inclusions is controlled below 3.0.

[0013] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0014] Furthermore, it also contains one or more of the following elements: Nb: 0–0.060%, Mg: 0–0.003%, Mo: 0–0.40%, W: 0–0.30%, Ni: 0–0.60%, and Cu: 0–0.40%.

[0015] The microstructure of the ultra-high strength structure described in this invention is a martensitic recrystallized structure plus micro- and nano precipitates. The grain size of the martensitic recrystallized structure is 6 to 12 micrometers, and the size of the TiC and VC precipitates is 3 to 6 nm.

[0016] The ultra-high strength structure described in this invention has a yield strength ≥ 960 MPa, a tensile strength ≥ 1000 MPa, and cold bending performance satisfying D = 3a, 90 degrees, and an elongation A. 50 ≥21%, impact energy at -60℃ ≥120J.

[0017] In the composition design of the 1000MPa grade high-strength steel for engineering machinery described in this invention:

[0018] Carbon: Solid solution strengthening, adjusting the strength and toughness of martensitic structure. Through experiments, the tensile strength of low-carbon martensite after reheating and quenching is related to the following C content: Rm = 2510C (%) + 790 (MPa). After quenching, the strength, plasticity and toughness are further adjusted by tempering. A higher C content will lead to an increase in the overall C equivalent, which makes it easy to crack during welding. The C content range of this invention is 0.15-0.21%.

[0019] Silicon: A certain amount of Si can play a good deoxidation role, and at the same time, it can inhibit the precipitation of carbides and improve the toughness of steel during tempering. Too much Si can easily produce red iron scale. The silicon content range of this invention is ≤0.50%.

[0020] Manganese: Mn content above 0.8% can improve the hardenability of steel. Mn content exceeding 1.6% is prone to segregation and inclusions such as MnS, which deteriorates the toughness of martensitic high-strength steel. The Mn content range of this invention is 0.60 to 1.60%.

[0021] Titanium (Ti) is a microalloying element that, through controlled rolling and cooling, can form a large number of nanoscale precipitates with elements such as C and N. During heat treatment, it strongly inhibits the growth of austenite grains and retains a large amount of these precipitates after heat treatment, thus playing a precipitation strengthening role. The titanium content in this invention ranges from 0.051% to 0.15%.

[0022] Vanadium (V) is a microalloying element that forms nanoscale precipitates with carbon (C). During heat treatment and tempering, a large amount of nanoscale VC precipitates are generated. The vanadium content in this invention ranges from 0.040% to 0.12%.

[0023] Niobium: Nb is a microalloying element that forms nanoscale precipitates with C, which inhibits the growth of austenite grains during hot rolling, thereby refining the microstructure after phase transformation; the niobium content in this invention ranges from 0 to 0.060%.

[0024] Rare Earth Elements (RE): Specifically, La or Ce are used. These elements purify molten steel. During steelmaking, controlling the steelmaking and continuous casting processes leads to the formation of fine REO precipitates. This improves the morphology of manganese sulfide inclusions, reduces the formation of large inclusions, and effectively reduces crack initiation caused by inclusions during cold deformation of steel plates. Simultaneously, TiN adheres to rare earth oxides to form REO.TiN complexes, thereby modifying the shape of TiN to form spherical complexes and reducing the formation of large TiN particles. This improves the plasticity and toughness of steel. The rare earth element content in this invention ranges from 0.0005% to 0.0030%.

[0025] Magnesium: Trace amounts of Mg, when controlled during steelmaking and continuous casting processes, form fine MgO precipitates. This causes TiN to adhere to MgO, forming a composite precipitate MgO.TiN, thereby modifying the shape of cubic TiN, resulting in a near-spherical composite precipitate. Simultaneously, it controls the growth of harmful TiN particles, reducing the number of large TiN particles. This improves the toughness and plasticity of the steel. In this invention, the Mg content ranges from 0 to 0.003%.

[0026] Chromium: Cr content above 0.2% can improve the hardenability of steel and facilitate the formation of a full martensitic structure during quenching. Cr forms Cr carbides during tempering, which have the effect of resisting tempering softening. Cr content exceeding 1.20% will result in larger sparks during welding, affecting welding quality. The Cr content range of this invention is 0.20% to 1.20%.

[0027] Molybdenum (Mo): A certain amount of Mo can improve the hardenability of steel, which is beneficial for the formation of a full martensitic structure during quenching. During high-temperature tempering, Mo reacts with C to form carbide particles, which have the effect of resisting high-temperature tempering softening and weld joint softening. Too high a Mo content will lead to an increase in carbon equivalent, deteriorating weldability. At the same time, Mo is a precious metal, which will increase costs. The Mo content range of this invention is 0–0.40%.

[0028] Tungsten: W element can improve the hardenability of steel, forms carbide particles during tempering, and has a significant effect on resisting temper softening and temper brittleness. The W content in this invention ranges from 0% to 0.30%.

[0029] Nickel: A certain amount of Ni can refine the martensitic structure and improve the toughness of steel. However, too high a Ni content will lead to an increase in carbon equivalent, which will worsen weldability. Furthermore, Ni is a precious metal, which will increase costs. The Ni content in this invention ranges from 0% to 0.60%.

[0030] Copper: Cu can produce a certain precipitation strengthening effect during tempering. In addition, adding a certain amount of Cu can improve the corrosion resistance of ultra-high strength steel for engineering machinery. The Cu content in this invention ranges from 0% to 0.40%.

[0031] Boron: Trace amounts of B can improve the hardenability and strength of steel. However, B exceeding 0.0030% can easily cause segregation, forming carboboron compounds, which severely deteriorates the toughness of steel. The boron content range of this invention is 0.0005 to 0.0030%.

[0032] Aluminum: Al content of 0.02% or more acts as a deoxidizer on the one hand, and a small amount of Al2O3 can refine the grains during slab heating, thereby refining the microstructure of the rolled steel plate. Al content exceeding 0.06% easily leads to Al oxide inclusion defects. The Al content range of this invention is 0.02% to 0.06%.

[0033] Calcium: Trace amounts of Ca can purify molten steel during the steelmaking process, optimize the shape and size of inclusions such as MnS, and improve the toughness of steel. However, if the Ca content exceeds 0.004%, it is easy to form Ca compounds with larger sizes, which will worsen the toughness. The Ca content range of this invention is 0.0005 to 0.004%.

[0034] Nitrogen: Because this invention adds a relatively large amount of Ti, Ti easily combines with N to form large cubic TiN particles, which deteriorates the plasticity and toughness of the steel plate. This invention, on the one hand, strictly controls the N content to below 0.0050% through refining processes, and on the other hand, effectively reduces the size and quantity of TiN formation by using trace amounts of rare earth elements, steelmaking superheat, and the solidification process of molten steel.

[0035] Phosphorus, sulfur and oxygen: P, S and O as impurity elements affect the plasticity and toughness of steel. The control range of these elements in this invention is P≤0.020%, S≤0.0030% and O≤0.0025%, respectively.

[0036] In particular, the composition design of this invention also requires that it meet the following requirements:

[0037] ① The nanoprecipitate control index (NPI) is 5–26, and NPI = (Mo + W + 2.3 * Cr) / (Ti + V).

[0038] Mo, W, and Cr are all strong carbide-forming elements and can inhibit carbon diffusion. During long-term winding at 570–650℃, the precipitated TiC is relatively large, reaching 6–14 nm, with a weak precipitation strengthening effect of only 50–100 MPa. During heat treatment at 900℃ for 5–10 min, the 6–14 nm TiC partially dissolves, while VC completely dissolves. By adding a specific combination of Mo, W, and Cr, when 5 ≤ NPI ≤ 26, TiC growth can be inhibited, and the dissolution rate of TiC can be controlled within a suitable range, keeping the TiC size at the micro-nano scale of 3–6 nm. Quenching at 900℃ followed by tempering at 610–650℃ for 10–30 min causes further secondary precipitation of VC and TiC. The combined addition of Mo, W, and Cr can control the VC and TiC precipitates within the micro-nano size range of 3–6 nm. If the NPI is too low, VC and TiC become coarse; if the NPI is too high, VC and TiC precipitation is insufficient. Therefore, this invention controls the NPI range to be 5–26.

[0039] ②Ti+V≥0.11%.

[0040] This is to fully utilize the precipitation strengthening effect of TiC and VC during the heat treatment process. Only when Ti+V ≥ 0.11%, combined with NPI and the heat treatment process, can sufficient micro- and nano-precipitation strengthening effect be achieved. The micro- and nano-precipitation of TiC and VC in this invention can produce a precipitation strengthening effect of 180–250 MPa.

[0041] The method for producing ultra-high strength structural steel with excellent comprehensive performance according to the present invention includes the following steps:

[0042] 1) Smelting and casting

[0043] The above-mentioned components are used to produce steel billets through converter or electric furnace steelmaking, refining, and casting.

[0044] 2) Heating

[0045] The billet is heated in a furnace at 1220-1300℃. Once the core of the billet reaches the furnace temperature, it is held at that temperature for ≥30 minutes.

[0046] 3) Rolling

[0047] The billet is rolled to the target thickness using single-stand reciprocating rolling or multi-stand hot continuous rolling, with a final rolling temperature of 820–920℃.

[0048] 4) Cooling

[0049] After hot rolling, the steel plate is cooled to 570–650°C at a cooling rate of 10–30°C / s to obtain ferrite +

[0050] Pearlite + nano-precipitates, including TiC precipitation and VC precipitation;

[0051] 5) Quenching and tempering heat treatment

[0052] Quenching heat treatment involves heating the steel plate to Ac3+(20~50)℃ and holding it for 5~10 minutes.

[0053] Rapidly cool to room temperature at a cooling rate of ≥150℃ / s;

[0054] The tempering heat treatment involves heating the steel plate to 610–650℃, holding it at that temperature for 10–30 minutes, and then air-cooling it to room temperature; where Ac3 is the austenite transformation completion temperature.

[0055] Ac3=955-350C-25Mn+51Si+106Nb+100Ti+68Al-11Cr-33Ni-16Cu+67

[0056] Mo.

[0057] In the production method of nanoprecipitation-strengthened ultra-high-strength steel of the present invention:

[0058] The steelmaking process employs converter or circuit steelmaking followed by ladle refining (RH+LF). The molten steel can be cast into 200mm thick billets. The RH degassing time is controlled at 30-35 minutes; the LF deep desulfurization treatment lasts for at least 20 minutes to ensure N content is below 40ppm, O content below 25ppm, and S content below 30ppm; the ladle killing time is ≥15 minutes, and the superheat is controlled below 10℃. Refined iron (RE) and magnesium (Mg) elements are added during the steelmaking process, while the secondary cooling water volume in continuous casting is appropriately increased to accelerate the solidification of the molten steel. This results in the formation of rare earth and magnesium oxides. Due to the addition of a high amount of titanium (Ti), Ti readily reacts with nitrogen to form cubic TiN, with a maximum size of 10 micrometers. TiN significantly enhances the toughness and plasticity of ultra-high-strength steel; therefore, high levels of Ti are rarely added to ultra-high-strength steels above 700MPa, precisely to account for the influence of TiN. In this invention, the addition of RE and Mg, which are highly reactive, first forms fine spherical oxides. TiN adheres to REO and MgO to form complexes REO.TiN and MgO.TiN, thereby modifying cubic TiN into spherical shapes. Since rare earth oxides and magnesium oxides consume a significant amount of TiN, the amount of individually precipitated TiN decreases, and the maximum size is reduced to below 4 micrometers. Considering that excessive addition of either RE or Mg can easily coarsen the inclusions of rare earth oxides and magnesium oxides, the combined addition of both is more effective. Through the addition of rare earth and magnesium and the control of the continuous casting process, the rating of each non-metallic inclusion is kept below 1.0, and the sum of the ratings of all non-metallic inclusions is controlled below 3.0. The low-temperature toughness and plasticity of the ultra-high-strength steel are increased by more than 20% compared to conventional steel grades, and the cold bending performance is also significantly improved.

[0059] In the billet heating process, controlling the heating temperature at 1220-1300℃ and the core holding time >30min can ensure that the TiC precipitated during continuous casting is fully dissolved; when the heating temperature exceeds 1300℃, the austenite grains grow excessively, causing the intergranular bonding force to weaken, which makes it easy to generate cracks during rolling.

[0060] The finishing rolling temperature is 820–920℃, which refines the austenite grains through austenite recrystallization. After hot rolling, the temperature is cooled to 570–650℃ at a cooling rate of 10–30℃ / s to obtain a ferrite + pearlite + nanoprecipitate structure, at which the size of the precipitated TiC is 3–6 nm.

[0061] In the heat treatment process, the steel plate is heated to Ac3+(20~50)℃, and the holding time is controlled at 5~10min. By adding a certain combination of Mo, W and Cr to keep 5≤NPI≤26, the dissolution rate of TiC can be controlled within a suitable range, and the size of TiC can be controlled at the micro-nano level of 2~5mm. During this period, a large amount of nano-sized TiC strongly inhibits the growth of austenite, refining the austenite and the microstructure after quenching. During high-temperature tempering at 610-650℃ for 10~30min, martensite recrystallizes to obtain recrystallized martensite, which improves plasticity and toughness. VC and TiC precipitate secondary during tempering. Combined with the addition of Mo, W and Cr, the VC precipitates can be controlled at the micro-nano size range of 3~6nm during tempering at 610-650℃. The combined nano-precipitation of TiC and VC can produce a precipitation strengthening effect of 180~250MPa.

[0062] The beneficial effects of this invention are:

[0063] This invention achieves a martensitic recrystallized structure with nano-precipitates through controlled rolling, controlled cooling, and heat treatment processes. The extensive use of TiC and VC nano-precipitates ensures that the steel plate maintains a strength exceeding 1000 MPa after high-temperature tempering. The tempered sorbite obtained through high-temperature tempering enhances the plasticity of the steel plate. By purifying the molten steel with rare earth elements and controlling the size and shape of inclusions, the initiation of cracks caused by inclusions during deformation is reduced, further improving the plasticity of the ultra-high-strength steel.

[0064] This invention obtains a martensitic recrystallized structure with nano-precipitates through a process of quenching followed by high-temperature tempering at temperatures above 600℃. The large amount of nano-precipitates ensures that the strength can still reach 960MPa after high-temperature tempering. The purity of the molten steel is purified by rare earth and magnesium elements, and the size and shape of inclusions are controlled, reducing the crack initiation caused by inclusions during deformation and improving the plasticity of the steel plate.

[0065] Compared with existing technologies, this invention uses tempered sorbite + nano-precipitation structure, combined with rare earth purification of molten steel, and controls the size and shape of inclusions to obtain ultra-high strength steel with higher plasticity and toughness. Attached Figure Description

[0066] Figure 1 This is a typical optical microscope metallographic structure of steel after rapid heat treatment in Example 3 of the present invention.

[0067] Figure 2 This is a typical scanning electron microscope metallographic structure of steel after rapid heat treatment in Example 3 of the present invention. Detailed Implementation

[0068] The present invention will be further described below with reference to the embodiments.

[0069] The composition of the steel in the embodiments of the present invention is shown in Table 1. Table 2 shows the process parameters of the steel in the embodiments of the present invention. The performance of the samples in each embodiment is shown in Table 3.

[0070] The heat-treated steel plates were subjected to longitudinal tensile and longitudinal impact tests.

[0071] The inclusion levels of the high fatigue performance and high strength cold stamping steels in Examples 1-8 were rated according to GB / T 10561-2005 "Standard Rating Chart Microscopic Examination Method for Determination of Non-metallic Inclusion Content in Steel". The results are shown in Table 4.

[0072] Figure 1 Optical microscope images of the martensitic recrystallized structure obtained after heat treatment of the steel in Example 3 are provided. Figure 2 Scanning electron microscope (SEM) images of the microstructure of the steel after heat treatment in Example 3 are provided.

[0073] from Figure 1 , Figure 2 The metallographic images show that the original martensite lath morphology largely disappeared after high-temperature tempering, with the martensite recrystallizing into a ferrite matrix and carbides. The average grain size after heat treatment is approximately 7 micrometers. With the same composition and conventional heat treatment processes, the average grain size is 15-20 micrometers. Scanning electron microscopy reveals granular carbide precipitates, with over 90% of the TiC and TiV precipitates having a size of 3-6 nm.

[0074] In summary, this invention employs controlled rolling and cooling and offline heat treatment processes, controlling aspects such as chemical composition design, base material microstructure, heating rate, holding time, and cooling rate to ensure that while achieving ultra-high strength, it also possesses good elongation and low-temperature impact toughness.

[0075]

[0076]

[0077]

[0078]

Claims

1. A high-strength structural steel with excellent comprehensive performance, comprising the following composition by weight percentage: C: 0.15~0.21%, Si≤0.50%, Mn: 0.60~1.60%, Ti: 0.051~0.15%, V: 0.040~0.12%, RE: 0.0005~0.0030%, Cr: 0.20~1.20%, B: 0.0005~0.0030%, Al: 0.02~0.06%, Ca: 0.0005~0.004%, N≤0.004%, P≤0.020%, S≤0.0030%, O≤0.0025%, with the balance including Fe and other unavoidable impurities; and simultaneously satisfying the following: The nanoprecipitate control index (NPI) is 5~26, and NPI = (Mo + W + 2.3*Cr) / (Ti + V). Ti+V≥0.11%; The microstructure of the ultra-high strength structure is a martensitic recrystallized structure plus micro- and nano precipitates. The grain size of the martensitic recrystallized structure is 6-12 micrometers, and the size of the TiC and VC precipitates is 3-6 nm.

2. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable impurities.

3. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 1 or 2, characterized in that, It also contains one or more of the following elements: Nb: 0~0.060%, Mg: 0~0.003%, Mo: 0~0.40%, W: 0~0.30%, Ni: 0~0.60%, Cu: 0~0.40%.

4. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 1 or 2, characterized in that, The levels of all five types of inclusions (A, B, C, D, and DS) in the ultra-high strength structural steel are controlled below grade 1.0, and the total rating of all non-metallic inclusions is controlled below 3.

0.

5. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 3, characterized in that, The levels of all five types of inclusions (A, B, C, D, and DS) in the ultra-high strength structural steel are controlled below grade 1.0, and the total rating of all non-metallic inclusions is controlled below 3.

0.

6. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 1 or 2, characterized in that, The ultra-high strength structure has a yield strength ≥ 960 MPa, a tensile strength ≥ 1000 MPa, and cold bending performance meeting D = 3a, 90 degrees, and an elongation A. 50 ≥21%, impact energy at -60℃ ≥120J.

7. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 3, characterized in that, The ultra-high strength structure has a yield strength ≥ 960 MPa, a tensile strength ≥ 1000 MPa, and cold bending performance meeting D = 3a, 90 degrees, and an elongation A. 50 ≥21%, impact energy at -60℃ ≥120J.

8. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 4, characterized in that, The ultra-high strength structure has a yield strength ≥ 960 MPa, a tensile strength ≥ 1000 MPa, and cold bending performance meeting D = 3a, 90 degrees, and an elongation A. 50 ≥21%, impact energy at -60℃ ≥120J.

9. The ultra-high strength structural steel with excellent comprehensive performance as described in claim 5, characterized in that, The ultra-high strength structure has a yield strength ≥ 960 MPa, a tensile strength ≥ 1000 MPa, and cold bending performance meeting D = 3a, 90 degrees, and an elongation A. 50 ≥21%, impact energy at -60℃ ≥120J.

10. The method for producing ultra-high strength structural steel with excellent comprehensive performance as described in any one of claims 1 to 9, characterized in that, The steps include the following: 1) Smelting and casting The components described in claim 1, 2 or 3 are used to steelmaking, refining and casting in a converter or electric furnace to form a billet; 2) Heating The billet is heated in a furnace at 1220~1300℃. After the core of the billet reaches the furnace temperature, it is held at the temperature for ≥30 minutes. 3) Rolling The billet is rolled to the target thickness using single-stand reciprocating rolling or multi-stand hot continuous rolling, with a final rolling temperature of 820~920℃. 4) Cooling After hot rolling, the steel plate is cooled to 570-650℃ at a cooling rate of 10-30℃ / s to obtain a ferrite + pearlite + nano-precipitation structure. 5) Quenching and tempering heat treatment Quenching heat treatment involves heating the steel plate to Ac3+(20~50)℃, holding it at that temperature for 5~10 minutes, and then rapidly cooling it to room temperature at a cooling rate of ≥150℃ / s. Tempering heat treatment involves heating the steel plate to 610~650℃, holding it at that temperature for 10~30 minutes, and then air-cooling it to room temperature; among these steps, Ac3 is the austenite transformation end temperature; Ac3=955-350C-25Mn+51Si+106Nb+100Ti+68Al-11Cr-33Ni-16Cu+67Mo.

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