An ultra-high strength structural steel and a method for producing the same

Abstract

The application discloses a kind of super-high-strength structural steel and its production method, by designing steel grade component, and matching suitable production process, by smelting, refining, casting, rolling and heat treatment finally obtain the 1000MPa grade super-high-strength structural steel with excellent comprehensive performance, the surface layer of the super-high-strength structural steel has soft phase ferrite organization, the subsurface is ferrite and tempered sorbite mixed organization, the heart is tempered sorbite organization, it is also improved to a certain extent while having higher strength, cold bending performance and low temperature toughness.

Description

Technical Field

[0001] This invention belongs to the field of structural steel preparation technology, and relates to an ultra-high strength structural steel and its production method, particularly to a 1000MPa grade ultra-high strength structural steel and its production method. Background Technology

[0002] Hot-rolled ultra-high-strength structural steel is mainly used to manufacture structural components for engineering machinery and commercial vehicles, placing high demands on the steel plate's strength, plasticity, and low-temperature toughness. The traditional production process for hot-rolled ultra-high-strength structural steel is as follows: steelmaking – continuous casting – hot rolling – finishing – heat treatment, obtaining a uniform tempered sorbite structure through quenching and high-temperature tempering heat treatment. For ultra-high-strength structural steel with a tensile strength of 1000MPa, due to its high strength, its cold bending performance is poor, only meeting the requirements of D=5a~6a and 90-degree bending (D is the bending diameter, a is the steel plate thickness), thus limiting the application range of ultra-high-strength structural steel.

[0003] Forming a soft phase structure on the surface of steel plates can effectively improve their cold bending performance. Chinese Patent Publication No. CN110423953A improves the cold bending performance of a hot-formed steel with a tensile strength of 1800 MPa by controlling the decarburization process in an annealing furnace to form a 15-25 μm decarburized soft phase structure on the steel plate surface. Chinese Patent Publication No. CN110423945A improves the cold bending performance of steel plates by controlling process parameters such as heat treatment temperature and furnace dew point temperature to form a 15-25 μm decarburized layer on the steel plate surface.

[0004] When ultra-high strength structural steel is bent at a relatively small angle, the deformation on the outer surface of the steel plate is the greatest, and this is also where cracks first initiate and propagate. Ultra-high strength structural steel has a tempered sorbitic structure, which has limited plastic deformation capacity and is prone to cracking.

[0005] In view of the above, there is an urgent need to develop a 1000MPa grade ultra-high strength structural steel with excellent comprehensive performance and its production method, which can improve the cold bending performance and low temperature toughness of ultra-high strength structural steel while having high strength. Summary of the Invention

[0006] To address the aforementioned deficiencies in existing technologies, the present invention aims to provide an ultra-high strength structural steel and its production method. By designing the steel composition and matching it with a suitable production process, a 1000MPa grade ultra-high strength structural steel with excellent comprehensive performance is finally obtained through smelting, refining, casting, rolling, and heat treatment. This ultra-high strength structural steel has a soft phase ferrite structure on the surface, a mixed structure of ferrite and tempered sorbite in the subsurface, and a tempered sorbite structure in the core. While possessing high strength, its cold bending performance and low-temperature toughness are also improved to a certain extent.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] The first aspect of the present invention provides an ultra-high strength structural steel comprising the following components by weight percentage: C: 0.14-0.18%, Mn: 0.60-2.0%, Si: 0.05-1.0%, Al: 0.03-0.08%, Cr: 0.20-0.80%, Mo: 0.35-0.60%, W: 0-0.30%, B: 0.0005-0.0030%, P≤0.020%, S≤0.0050%, with the remainder being Fe and unavoidable impurities.

[0009] Preferably, the composition of the ultra-high strength structural steel satisfies the following formula:

[0010]

[0011] In the formula, [Si], [Al], [Cr], [Mo], and [W] represent the weight percentages of Si, Al, Cr, Mo, and W, respectively, in percent (%).

[0012] Preferably, the composition of the ultra-high strength structural steel further includes trace alloying elements, which are selected from one or more of Nb, Ti, and V, and the content of the trace alloying elements is <0.20 wt%; and / or

[0013] The composition of the ultra-high strength structural steel also includes Cu, with a content of 0–0.40 wt%; and / or

[0014] The ultra-high strength structural steel also includes Ni, with a content of 0 to 0.60 wt%.

[0015] Preferably, the surface layer of the ultra-high strength structural steel is a soft phase ferrite structure, the subsurface layer is a mixed structure of ferrite and tempered sorbite, and the core is a tempered sorbite structure.

[0016] Preferably, the thickness of the ultra-high strength structural steel surface layer is >50 μm, and the thickness of the subsurface layer is >50 μm; and / or

[0017] The thickness of the ultra-high strength structural steel is ≥4mm.

[0018] Preferably, the microhardness of the soft ferrite structure on the surface of the ultra-high strength structural steel is <230 HV, and the microhardness of the tempered sorbite structure in the core is 300–380 HV; and / or

[0019] The ultra-high strength structural steel has a yield strength ≥900MPa, tensile strength ≥1000MPa, and elongation ≥18%; and / or

[0020] The cold bending performance of the ultra-high strength structural steel meets the requirements of cold bending diameter D = 2 × a, 90° bending, where a is the thickness of the ultra-high strength structural steel, and the impact energy at -60℃ is >120J.

[0021] The second aspect of the present invention provides a method for producing ultra-high strength structural steel as described in the first aspect of the present invention, wherein raw materials are proportioned according to the composition of the ultra-high strength structural steel, and then ultra-high strength structural steel is obtained by smelting in a converter or electric furnace, refining in an external furnace, casting, rolling and heat treatment.

[0022] Preferably, during the rolling process, the cast billet formed by casting is heated in a gas furnace, and steel plates with a thickness of ≥4mm are obtained by single-stand reciprocating rolling or multi-stand hot continuous rolling, and then coiled or air-cooled to room temperature.

[0023] Preferably, the heat treatment includes quenching and tempering; in the quenching process, the set temperature of the quenching furnace is Ac3+10℃~Ac3+80℃, nitrogen or inert gas is used in the furnace, and the oxygen content in the furnace is controlled at 50~300ppm.

[0024] Preferably, during the quenching process, after the rolled steel plate is heated to the set temperature in the quenching furnace, it is held at that temperature for 15–30 minutes, and then immediately cooled to room temperature at a cooling rate of 50–200°C / s; and / or

[0025] During the tempering process, the quenched steel plate is heated to 460-580°C and held at that temperature for 20-45 minutes, and then air-cooled to room temperature.

[0026] In the composition design of the ultra-high strength structural steel of this invention:

[0027] Carbon: Solid solution strengthening, can adjust the strength and toughness of tempered sorbite. Experiments have shown that the tensile strength Rm of low-carbon tempered sorbite after heat quenching is related to the C content as follows: Rm = 2510 × C (%) + 790 (MPa). After quenching, the strength, plasticity and toughness can be 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. Therefore, the C content range of this invention is 0.14 to 0.18%.

[0028] Manganese: Mn content above 0.60 wt% can improve the hardenability of steel. Mn content exceeding 2.0 wt% is prone to segregation and inclusions such as MnS, which deteriorates the toughness of tempered sorbitic high-strength steel. Therefore, the Mn content range of this invention is 0.60 to 2.0%.

[0029] Silicon: Si is an oxygen-loving element. During heating, it can accelerate the expansion of oxygen atoms in the steel plate, causing carbon in the surface structure to react with oxygen and reducing the carbon content in the surface structure. During cooling, it can increase the energy of the austenite-to-ferrite transformation during heating, accelerating the ferrite phase transformation. Too high a Si content can easily produce red iron scale; therefore, the silicon content range of this invention is 0.05% to 1.0%.

[0030] Aluminum (Al): Al is an oxygen-loving element. During heating, it can accelerate the expansion of oxygen atoms in the steel plate, causing carbon in the surface structure to react with oxygen and reducing the carbon content in the surface structure. During cooling, it can increase the energy of the austenite-to-ferrite transformation during heating, accelerating the ferrite phase transformation. Too high an Al content affects welding quality; therefore, the Al content range of this invention is 0.03–0.08%.

[0031] Chromium: A chromium (Cr) content of 0.2% or higher can improve the hardenability of steel and also provide resistance to high-temperature oxidation, reducing surface burn-off during heating. A Cr content exceeding 0.80% will result in larger sparks during welding, affecting weld quality. Therefore, the Cr content range in this invention is 0.20%–0.80%.

[0032] Molybdenum: Ultra-high strength structural steel is typically tempered at high temperatures after quenching to obtain a sorbitic structure. During high-temperature tempering, Mo can inhibit the growth of carbide particles, exhibiting strong resistance to high-temperature tempering. Therefore, ultra-high strength structural steel usually contains a relatively high amount of Mo. However, excessively high Mo content can inhibit the diffusion of carbon from the near-surface structure of the steel plate to the surface during heating; moreover, excessively high hardenability will cause more austenite on the surface to transform into ferrite during quenching and cooling. Therefore, the Mo content should not be too high. Hence, the Mo content range of this invention is 0.35–0.60%.

[0033] Tungsten: Similar to Mo, W can inhibit the growth of carbide particles during high-temperature tempering, exhibiting strong resistance to high-temperature tempering. Therefore, a certain amount of Mo can be added to ultra-high-strength structural steel. However, excessive W content will inhibit the diffusion of carbon from the near-surface microstructure of the steel plate to the surface during heating; moreover, excessive hardenability will cause more austenite on the surface to transform into ferrite during quenching and cooling. Therefore, the W content should not be too high. Hence, the W content range of this invention is 0–0.30%.

[0034] Nickel: Ni (Ni) refines the tempered martensite structure, inhibits carbon diffusion, and improves the toughness of steel. However, excessive Ni content leads to an increase in carbon equivalent, which deteriorates weldability. Furthermore, Ni is a precious metal, increasing costs. Therefore, the Ni content in this invention ranges from 0% to 0.60%.

[0035] Copper: Cu can improve the corrosion resistance of steel. Therefore, the Cu content in this invention ranges from 0% to 0.40%.

[0036] Niobium, titanium and vanadium: At least one of the microalloying elements Nb, Ti and V can be added as needed, and Nb+Ti+V<0.20%. They form nanoscale precipitates with elements such as C and N, which inhibit the growth of austenite grains during heating, refine the grains, and improve the plasticity and toughness of the steel plate.

[0037] 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%.

[0038] Si, Al, Cr, Mo, and W are the controlling elements for surface ferrite. Si and Al are both oxygen-loving elements, accelerating the diffusion of oxygen atoms in the steel plate surface microstructure, promoting the reaction between carbon and oxygen atoms in the surface microstructure, and promoting the formation of a thicker carbon-depleted layer on the surface. They also increase the energy of austenite-to-ferrite transformation during quenching and cooling, promoting the ferrite phase transformation. Cr can reduce oxidation loss on the steel plate surface. Mo and W both inhibit the diffusion of carbon from the near-surface microstructure to the surface during heating, reducing the thickness of the carbon-depleted layer. Moreover, when Mo and W are too high, they increase hardenability, causing more austenite to transform into martensite during quenching and cooling, reducing the thickness of the ferrite layer. Comprehensive experimental verification shows that when the composition meets the requirements... At this time, a larger ferrite layer thickness can be obtained.

[0039] The ultra-high strength structural steel and its production method provided by this invention have the following beneficial effects:

[0040] 1. The ultra-high strength structural steel and its production method of the present invention, by designing the steel composition and matching it with a suitable production process, through smelting, refining, casting, rolling and heat treatment, finally obtains a 1000MPa grade ultra-high strength structural steel with excellent comprehensive performance. The surface layer of the ultra-high strength structural steel has a soft phase ferrite structure, the subsurface layer has a mixed structure of ferrite and tempered sorbite, and the core has a tempered sorbite structure. While having high strength, its cold bending performance and low temperature toughness are also improved to a certain extent.

[0041] 2. The ultra-high strength structural steel prepared by this invention has a microhardness of <230HV for the soft phase ferrite structure on the surface and a microhardness of 300-380HV for the tempered sorbite structure in the core. The ultra-high strength structural steel has a yield strength ≥900MPa, tensile strength ≥1000MPa, elongation ≥18%, and cold bending performance that meets the requirements of cold bending diameter D=2×a, 90° bending (a is the thickness of the ultra-high strength structural steel), and impact energy at -60℃ >120J. In contrast, the cold bending performance of ultra-high strength structural steel of the same grade manufactured by traditional processes can only meet the requirements of D=(5~6)×a, 90 degrees (a is the thickness of the ultra-high strength structural steel), and impact energy at -60℃ is 50-90J. It can be seen that the thicker soft phase ferrite structure on the surface of the ultra-high strength structural steel of this invention effectively improves the cold bending performance of the ultra-high strength structural steel, and at the same time, the low-temperature toughness is also improved to a certain extent. Attached Figure Description

[0042] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0043] Figure 1 This is a schematic diagram of the optical microscope metallographic structure of the ultra-high strength structural steel of Embodiment 3 of the present invention. Detailed Implementation

[0044] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with embodiments.

[0045] The ultra-high strength structural steel provided by this invention comprises the following components by weight percentage: C: 0.14–0.18%, Mn: 0.60–2.0%, Si: 0.05–1.0%, Al: 0.03–0.08%, Cr: 0.20–0.80%, Mo: 0.35–0.60%, W: 0–0.30%, B: 0.0005–0.0030%, P ≤ 0.020%, S ≤ 0.0050%, with the remainder being Fe and unavoidable impurities. In a specific embodiment, the composition satisfies the following formula:

[0046]

[0047] In the formula, [Si], [Al], [Cr], [Mo], and [W] represent the weight percentages of Si, Al, Cr, Mo, and W, respectively, in percent (%).

[0048] The ultra-high strength structural steel of this invention can also have trace alloying elements added to improve the plasticity and toughness of the steel plate. These trace alloying elements are selected from one or more of Nb, Ti, and V, and their content is <0.20 wt%.

[0049] In a specific embodiment, Cu can also be added to the composition of the ultra-high strength structural steel to improve the corrosion resistance of the steel, and its content is 0 to 0.40 wt%.

[0050] In a specific embodiment, Ni can be added to the composition of the ultra-high strength structural steel to improve the toughness of the steel, with a content of 0 to 0.60 wt%.

[0051] Based on simulation and experimental findings, this invention demonstrates that when the ferrite soft phase microstructure on the surface of ultra-high strength structural steel is sufficiently thick (>50 μm), the high uniform elongation of the ferrite microstructure effectively constrains the local necking of the tempered sorbite microstructure on the subsurface, limiting local deformation and thus significantly improving the cold bending performance of the steel plate. Conversely, when the ferrite on the surface of ultra-high strength structural steel is scarce, it only delays surface cracking and cannot coordinate the cold deformation of the tempered sorbite on the subsurface, resulting in limited improvement in cold bending performance. Therefore, the ultra-high strength structural steel of this invention features a soft ferrite microstructure on the surface, a mixed ferrite and tempered sorbite microstructure on the subsurface, and a tempered sorbite microstructure in the core. The thickness of both the surface and subsurface layers of the ultra-high strength structural steel is >50 μm, and the overall thickness of the ultra-high strength structural steel is ≥4 mm.

[0052] The microhardness of the soft ferrite structure on the surface of the ultra-high strength structural steel of this invention is <230 HV, and the microhardness of the tempered sorbite structure in the core is 300-380 HV. The ultra-high strength structural steel has a yield strength ≥900 MPa, tensile strength ≥1000 MPa, and elongation ≥18%. The cold bending performance of the ultra-high strength structural steel meets the requirements of a cold bending diameter D = 2 × a, 90° bending, where a is the thickness of the ultra-high strength structural steel, and an impact energy >120 J at -60℃.

[0053] The aforementioned ultra-high strength structural steel is produced by mixing raw materials according to the above-mentioned composition ratio, and then smelting in a converter or electric furnace, refining outside the ladle, casting, rolling, and heat treatment to obtain the ultra-high strength structural steel; the specific production method is as follows:

[0054] (1) Smelting and casting: Raw materials are proportioned according to the composition of ultra-high strength structural steel: C: 0.14-0.18%, Mn: 0.60-2.0%, Si: 0.05-1.0%, Al: 0.03-0.08%, Cr: 0.20-0.80%, Mo: 0.35-0.60%, W: 0-0.30%, B: 0.0005-0.0030%, P≤0.020%, S≤0.0050%, with the remainder being Fe and unavoidable impurities, and the composition meets the requirements. In the formula, [Si], [Al], [Cr], [Mo], and [W] represent the weight percentages of Si, Al, Cr, Mo, and W, respectively, in %. One or more trace alloying elements, Nb, Ti, and V, can be added as needed, with the content of these trace alloying elements <0.20 wt%, i.e., Nb + Ti + V < 0.20 wt%. Cu (0–0.40 wt%) and Ni (0–0.60 wt%) can also be added. The raw materials in the above proportions are smelted, refined, and cast in a converter or electric furnace to form a billet.

[0055] (2) Rolling: After the billet is heated in a gas furnace, it is rolled into a steel plate with a thickness of ≥4mm by single-stand reciprocating rolling or multi-stand hot continuous rolling, and then coiled or air-cooled to room temperature. Since a thick soft phase structure is formed on the surface of the steel plate during the subsequent heat treatment process, it has a significant impact on the tensile strength of the steel plate. When the steel plate is thin, the tensile effect cannot meet the application requirements.

[0056] (3) Heat treatment: Heat treatment is performed on the hot-rolled steel plate as the substrate, which includes quenching and tempering. The quenching is carried out in a quenching furnace with a set temperature of Ac3+10~80℃, where Ac3 is the critical temperature of hypoeutectoid steel, Ac3=955-350[C]-25[Mn]+51[Si]+106[Nb]+100[Ti]+68[Al]-11[Cr]-33[Ni]-16[Cu]+67[Mo], where the mass percentages of [C], [Mn], [Si], [Nb], [Ti], [Al], [Cr], [Ni], [Cu], and [Mo] are in %. The furnace atmosphere uses nitrogen or other inert gases, and the oxygen content in the furnace is controlled at 50-300 ppm. During the quenching process, after the rolled steel plate is heated to the set temperature in the quenching furnace, it is held for 15-30 minutes, and then immediately cooled to room temperature at a cooling rate of 50-200℃ / s. Then, tempering is performed. During the tempering process, the quenched steel plate is heated to 460-580℃ and held for 20-45 minutes, and then air-cooled to room temperature, finally obtaining ultra-high strength structural steel with a thickness ≥4mm.

[0057] The quenching temperature is controlled above Ac3 during the quenching process to promote austenitization. Nitrogen or other inert gases are used, and the oxygen content in the furnace is controlled between 50 and 300 ppm. Oxygen in the atmosphere diffuses from the steel plate surface inwards as oxygen atoms, especially rapidly along grain boundaries. These oxygen atoms react with carbon atoms in the steel plate: C + O = CO. CO then diffuses back to the steel plate surface and reacts: 2CO + O2 = 2CO2. This forms a carbon-depleted layer on the steel plate surface. Simultaneously, carbon from the subsurface layer diffuses further towards the surface, creating a carbon gradient. Si and Al are oxophiles, accelerating oxygen atom diffusion; Mo and W are carboxophiles, inhibiting carbon atom diffusion; Cr reduces oxidation loss on the steel plate surface. When the steel plate is heated for more than 8 minutes, a carbon-depleted layer of more than 150 micrometers will form on the steel plate surface. During quenching, the steel plate is rapidly cooled to room temperature at a rate of 50–200℃ / s. A suitable cooling rate promotes the formation of ferrite in the carbon-depleted layer on the surface, a mixed structure of ferrite and martensite in the subsurface layer, and martensite in the core. At a low cooling rate, martensite is not formed in the core; at a high rate, some of the carbon-depleted surface layer also quenches into martensite, resulting in insufficient ferrite on the surface and limited improvement in cold bending performance. The subsequent tempering treatment, heating the steel plate to 460–580℃ and tempering for 20–45 minutes, transforms the quenched martensite in the core into tempered sorbite, effectively improving the plasticity and toughness of the steel plate.

[0058] The microstructure of the ultra-high strength structural steel prepared above is as follows: the surface layer of the ultra-high strength structural steel is a soft phase ferrite structure with a thickness of >50μm, the subsurface layer is a mixed structure of ferrite and tempered sorbite with a thickness of >50μm, and the core is a tempered sorbite structure.

[0059] The mechanical properties of this ultra-high strength structural steel are as follows: the microhardness of the surface soft ferrite structure is <230 HV, and the microhardness of the tempered sorbite structure in the core is 300–380 HV. The yield strength of the ultra-high strength structural steel is ≥900 MPa, the tensile strength is ≥1000 MPa, and the elongation is ≥18%. The cold bending performance of the ultra-high strength structural steel meets the requirements of a cold bending diameter D = 2 × a, 90° bending (where a is the thickness of the ultra-high strength structural steel), and an impact energy at -60℃ >120 J.

[0060] The ultra-high strength structural steel and its production method of the present invention will be further introduced below with specific examples.

[0061] Example

[0062] Examples 1-10 utilize the production method of the ultra-high strength structural steel of this invention. The process flow is as follows: converter or electric furnace smelting → ladle refining → continuous casting → heating → rolling → cooling → heat treatment. The specific process is as follows:

[0063] (1) Smelting and casting: Smelting was carried out using a 500kg vacuum electric furnace. The molten steel was poured into 120mm thick billets and heated in the electric furnace. The composition of the ultra-high strength structural steels in Examples 1-10 is shown in Table 1, where FEE is the combination of elements that affect the thickness of ferrite formation:

[0064] 2) Rolling: The billet is rolled into a steel plate with a target thickness of 10mm in multiple passes and then slowly cooled to room temperature.

[0065] 3) Heat Treatment: Quenching: The quenching furnace temperature is set to Ac3+(10~80)℃ based on the chemical composition of the steel plate. Nitrogen gas is used to control the oxygen content to 50~300ppm. After reaching the quenching temperature, the steel plate is held at that temperature for 15~30min, and then rapidly cooled to room temperature at a cooling rate of 50~200℃ / s. Tempering: The steel plate is heated to 460~580℃ for tempering. After reaching the tempering temperature, the steel plate is held at that temperature for 20~45min, and then air-cooled to room temperature. The specific heat treatment process is shown in Table 2.

[0066] The properties of the ultra-high strength structural steel samples produced in Examples 1-10 after heat treatment are shown in Table 3. Figure 1 An optical microscope image of Example 3 is provided, showing that the surface layer of the steel plate is a ferrite layer with an average thickness of about 55 micrometers, the subsurface layer is a ferrite + tempered sorbite structure with an average thickness of about 57 micrometers, and the core of the steel plate is a tempered sorbite structure.

[0067] Compared to patents CN110423953A and CN110423945A, which improve the cold bending performance of steel plates by controlling process parameters such as heat treatment temperature and furnace dew point temperature to form a 15-25 micrometer decarburized layer on the steel plate surface, this invention uses a suitable combination of composition, heat treatment temperature, heating atmosphere, holding time, and cooling rate to obtain an ultra-high strength structural steel with a thick ferrite microstructure on the surface, a mixed microstructure of ferrite and tempered sorbite on the subsurface, and a tempered sorbite microstructure in the core. Compared with traditional ultra-high strength structural steel, this invention effectively improves the cold bending performance and also enhances the low-temperature toughness. Compared with the comparative patents, this invention shows significant differences in the method of obtaining the soft phase microstructure on the surface, the thickness of the soft phase microstructure, and the improvement effect on cold bending performance.

[0068] Table 1 shows the composition (wt%) of the ultra-high strength structural steel in the examples.

[0069] C Mn Si Al Cr Mo W Ni Example 1 0.17 1.24 0.1 0.053 0.2 0.46 0 0.15 Example 2 0.16 1.36 0.32 0.072 0.59 0.58 0.3 0.22 Example 3 0.14 2 0.75 0.036 0.39 0.35 0.16 0.42 Example 4 0.15 1.33 0.32 0.061 0.52 0.6 0.11 0.37 Example 5 0.18 0.95 0.05 0.055 0.72 0.36 0 0 Example 6 0.16 0.81 0.57 0.081 0.58 0.43 0.22 0.51 Example 7 0.14 0.6 0.55 0.068 0.36 0.39 0 0.6 Example 8 0.17 0.79 0.41 0.03 0.57 0.35 0.18 0.33 Example 9 0.16 1.62 0.11 0.038 0.75 0.58 0.23 0 Example 10 0.18 0.88 0.09 0.052 0.8 0.46 0.15 0 Cu Nb Ti V B P S FEE Example 1 0 0.022 0.022 0.012 0.0018 0.018 0.017 4 Example 2 0.4 0.019 0.032 0.022 0.003 0.017 0.0035 7.1 Example 3 0.12 0.012 0.039 0.013 0.0015 0.01 0.0027 12.4 Example 4 0.23 0.018 0.043 0.012 0.0005 0.013 0.0029 7.5 Example 5 0.14 0.011 0.011 0.023 0.0009 0.015 0.004 5.2 Example 6 0.39 0.017 0.051 0.011 0.0021 0.009 0.036 11 Example 7 0.12 0.021 0.026 0.014 0.0013 0.017 0.0033 11.2 Example 8 0.09 0.026 0.032 0.012 0.0023 0.011 0.0012 9.5 Example 9 0.36 0.011 0.016 0.016 0.0008 0.007 0.0031 4.3 Example 10 0.23 0.036 0.011 0.031 0.0022 0.012 0.0011 5.2

[0070] Note: FEE is the group of elements that affects the thickness of ferrite formation;

[0071] Table 2 shows the heat treatment process parameters in the ultra-high strength structural steel production method in the embodiments.

[0072]

[0073]

[0074] Table 3 shows the performance parameters of the ultra-high strength structural steel in the examples.

[0075]

[0076] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. An ultra-high strength structural steel, characterized in that, The composition includes the following components by weight percentage: C: 0.14–0.18%, Mn: 0.60–2.0%, Si: 0.05–1.0%, Al: 0.03–0.08%, Cr: 0.20–0.80%, Mo: 0.35–0.60%, W: 0–0.30%, B: 0.0005–0.0030%, P≤0.020%, S≤0.0050%, with the remainder being Fe and unavoidable impurities. The ultra-high strength structural steel has a soft phase ferrite structure on the surface, a mixed structure of ferrite and tempered sorbite on the subsurface, and a tempered sorbite structure in the core.

2. The ultra-high strength structural steel according to claim 1, characterized in that, The composition of the ultra-high strength structural steel satisfies the following formula: ； In the formula, [Si], [Al], [Cr], [Mo], and [W] represent the weight percentages of Si, Al, Cr, Mo, and W, respectively, in percent (%).

3. The ultra-high strength structural steel according to claim 2, characterized in that, The ultra-high strength structural steel also includes trace alloying elements, which are selected from one or more of Nb, Ti, and V, and the content of the trace alloying elements is <0.20 wt%; and / or The ultra-high strength structural steel also includes Cu, with a content of 0–0.40 wt%; and / or The ultra-high strength structural steel also includes Ni, with a content of 0 to 0.60 wt%.

4. The ultra-high strength structural steel according to claim 1, characterized in that, The thickness of the ultra-high strength structural steel surface layer is >50μm, the thickness of the subsurface layer is >50μm; and / or The thickness of the ultra-high strength structural steel is ≥4mm.

5. The ultra-high strength structural steel according to claim 4, characterized in that, The microhardness of the soft ferrite structure on the surface of the ultra-high strength structural steel is <230 HV, and the microhardness of the tempered sorbite structure in the core is 300–380 HV; and / or The ultra-high strength structural steel has a yield strength ≥900MPa, tensile strength ≥1000MPa, and elongation ≥18%; and / or The cold bending performance of the ultra-high strength structural steel meets the requirements of cold bending diameter D=2×a, 90° bending, where a is the thickness of the ultra-high strength structural steel, and the impact energy at -60℃ is >120J.

6. A method for producing ultra-high strength structural steel as described in any one of claims 1 to 5, characterized in that, The ultra-high strength structural steel is obtained by mixing raw materials according to the composition of the ultra-high strength structural steel, and then smelting, refining, casting, rolling and heat treatment in a converter or electric furnace.

7. The method for producing ultra-high strength structural steel according to claim 6, characterized in that, During the rolling process, the cast billet formed by casting is heated in a gas furnace and rolled by single-stand reciprocating rolling or multi-stand hot continuous rolling to obtain a steel plate with a thickness of ≥4mm, which is then coiled or air-cooled to room temperature.

8. The method for producing ultra-high strength structural steel according to claim 6, characterized in that, The heat treatment includes quenching and tempering; in the quenching process, the set temperature of the quenching furnace is Ac3+10℃~Ac3+80℃, nitrogen or inert gas is used in the furnace, and the oxygen content in the furnace is controlled at 50~300ppm.

9. The method for producing ultra-high strength structural steel according to claim 8, characterized in that, During the quenching process, after the rolled steel plate is heated to the set temperature in the quenching furnace, it is held at that temperature for 15–30 minutes, and then immediately cooled to room temperature at a cooling rate of 50–200°C / s; and / or During the tempering process, the quenched steel plate is heated to 460-580°C and held at that temperature for 20-45 minutes, and then air-cooled to room temperature.

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