High-formability ultra-high-strength passenger car steel and production method thereof

Abstract

The present invention belongs to the field of metal materials, and particularly relates to a high-formability, ultra-high-strength steel for passenger vehicles and its production method. The steel plate of the present invention is primarily suitable for manufacturing passenger vehicle structural components. The chemical composition of the steel is designed to be: C, Si, Mn, Al, Nb, V, W, Ta, Bi, Sb, Ni, Cr, Pt, and Y, with P ≤ 0.015%, S ≤ 0.005%, and N ≤ 0.006%, with the remainder being Fe and unavoidable impurities. The steel is produced using a hot rolling + EPS + continuous annealing + skin-passing process, eliminating the pickling and cold rolling steps, simplifying the process, and ensuring environmental safety.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and particularly relates to a high-formability, ultra-high-strength steel for passenger vehicles and a production method thereof. The steel plate of the present invention is mainly suitable for manufacturing passenger vehicle structural parts. Background Art

[0002] The automotive industry is a major user of steel products, and the research, development, and application of steel products are closely related to the development trends of the automotive industry. In today's society, with the rapid development of my country's automotive industry, automotive steel has begun to develop in the direction of low cost, high strength, green environmental protection, and high safety. In order to reduce the weight of automotive structural components and reduce their weight, thereby saving energy and reducing environmental pollution, the application of high-performance and high-surface quality automotive steel plates has been promoted. Conventional technology for the production of automotive thin plates is obtained by hot rolling + pickling + cold rolling + continuous annealing + skin-passing process, for example:

[0003] Chinese patent application publication number CN115584428A discloses a new short-process, low-cost cold-rolled DH590 steel and its production method. It uses a common C-Mn composition system to design and add a certain amount of Mg and Ca to produce hot-rolled + pickling + cold-rolled + continuous annealing + skin-passing steel plates with a yield strength of ≥330MPa, a tensile strength of ≥590MPa, and a hole expansion rate of ≥60%. The hole expansion rate fluctuates greatly, which does not meet the requirements of high-formability and ultra-high-strength passenger car parts, and does not have certain corrosion resistance and oxidation resistance. In addition, the process system is prepared through hot rolling + pickling + cold rolling + continuous annealing + skin-passing, which has a large process flow and high production costs. In particular, pickling is required. The pickling process has problems such as large acid mist pollution, environmental pollution, and high waste acid treatment costs.

[0004] Chinese patent application publication number CN115652207A discloses a 780MPa-grade short-process economical cold-rolled DH steel plate and its production method. It uses a common C-Mn composition system to design and add a certain amount of Ti, Mg and Ca to produce hot-rolled + pickling + cold-rolled + continuous annealing + skin-passing steel plates. The yield strength is ≥440MPa, the tensile strength is ≥780MPa, the hole expansion rate is ≥30%, and the hole expansion rate is small. It does not meet the requirements of high-formability and ultra-high-strength passenger car parts, and does not have certain corrosion resistance and oxidation resistance. In addition, the process system is prepared through hot rolling + pickling + cold rolling + continuous annealing + skin-passing. The process flow is large and the production cost is high. In particular, pickling is required. The pickling process has problems such as large acid mist pollution, environmental pollution, and high waste acid treatment costs.

[0005] A Chinese patent application with publication number CN111979490A discloses a high-ductility, high-formability cold-rolled DH590 steel and a production method thereof. The steel plate is produced by adding a certain amount of Nb, Cr and Mo to a conventional C-Mn composition system and undergoing hot rolling, pickling, cold rolling, continuous annealing and skin-passing. The steel plate has a yield strength of 350-430 MPa, a tensile strength of 590-700 MPa, a hole expansion rate of 50% or more, large fluctuations in yield strength and tensile strength, and a small hole expansion rate. The steel plate does not meet the requirements of high-formability and ultra-high-strength passenger car parts and does not have certain corrosion resistance and oxidation resistance. The steel plate is prepared by hot rolling, pickling, cold rolling, continuous annealing and skin-passing, which has a large process flow and high production costs. In particular, pickling is required. The pickling process has problems such as large acid mist pollution, environmental pollution, and high waste acid treatment costs. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-formability ultra-high-strength passenger car steel and a production method thereof, wherein the steel plate has a yield strength of ≥775MPa, a tensile strength of ≥880MPa, a transverse elongation A ≥27%, a hole expansion rate ≥66%, a transverse cold bending 180°D=a that is qualified, a good surface quality, no iron oxide stripes and color defects, and a surface roughness Ra of 1.20 to 1.43μm. According to GB / T10125-2021 "Artificial Atmosphere Corrosion Test-Salt Spray Test", a neutral salt spray test is performed to simulate the marine atmospheric environment, using a NaCl solution with a concentration of 50g / L±5g / L. Within a 168h test period, the steel plate corrosion rate is ≤0.48g / (m 2 •h). Furthermore, the conventional hot rolling + pickling + cold rolling + continuous annealing + skin-passing process is not required. Instead, the hot rolling + EPS (wet sandblasting descaling process) + continuous annealing + skin-passing process is adopted, which reduces the pickling + cold rolling process, resulting in low production costs and no pollution to the environment.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A high-formability, ultra-high-strength steel for passenger cars. The chemical composition of the steel is as follows by weight: C: 0.085%-0.130%, Si: 0.25%-0.65%, Mn: 1.78%-1.98%, Al: 0.020%-0.050%, Nb: 0.035%-0.046%, V: 0.045%-0.058%, W: 0.010%-0.036%, Ta: 0.015% ~0.023%, Bi: 0.008%~0.017%, Sb: 0.015%~0.050%, Ni: 0.015%~0.033%, Cr: 0.40%~0.50%, Pt: 0.002%~0.009%, Y: 0.006%~0.010%, and limit P≤0.015%, S≤0.005%, N≤0.006%, the balance is Fe and unavoidable impurities.

[0009] The main functions of the components of the high-formability and ultra-high-strength passenger car steel in the present invention are:

[0010] C: Carbon is a common strengthening element in steel. The interstitial solid solution atomic carbon causes a certain lattice distortion in the matrix, which plays a role in solid solution strengthening. The role of carbon in the present invention can ensure the bainite content, thereby improving the forming performance and hole expansion performance of the steel plate. If the carbon content is too low, the mechanical properties of the steel plate in the present invention cannot be obtained. If the content is too high, the steel plate will become brittle, and there will be a risk of delayed fracture and hot-rolled edge cracking, which is also detrimental to the welding performance, plasticity and toughness of the steel plate. The carbon requirement in the present invention is in the low carbon range as a whole, which is beneficial to reducing the risk of delayed fracture and hot-rolled edge cracking, and is also beneficial to the welding performance of the steel plate. Therefore, the optimal range of carbon in the present invention is 0.085% to 0.130%.

[0011] Si: Silicon is one of the important elements of the present invention. Sufficient silicon addition to ferrite can ensure the strength of the ferrite matrix, and silicon addition will increase the A C3 The silicon content in the steel sheet is 0.25% to 0.65%, which is a significant improvement over the conventional annealing process. The addition of silicon to the steel sheet effectively regulates the annealing process window during the continuous annealing phase, ensuring an appropriate ferrite-austenite ratio in the critical zone at industrial continuous annealing temperatures. Furthermore, sufficient silicon addition can reduce inclusions in the steel sheet, inhibiting their formation and preventing the mechanical and hole expansion properties of the steel sheet from being compromised by inclusion formation. However, if the silicon content is too low, it will not ensure the strength of the ferrite matrix and inhibit inclusion formation. If the silicon content is too high, it will affect the hot-rolled surface quality, resulting in the formation of large amounts of iron oxide scale and poor weldability. Therefore, the silicon content in the present invention is 0.25% to 0.65%.

[0012] Mn: Manganese strengthens the solid solution in steel by inducing lattice distortion through substitutional solid solution. It is also an austenite stabilizing element in steel, expanding the austenite region, reducing the critical quenching rate of steel, and delaying the transformation of austenite to pearlite. However, if the manganese content is too low, the supercooled austenite will be unstable, reducing the plasticity, toughness and hole expansion performance of the steel plate. In addition, the added content of manganese should not exceed the scope of the present invention. The main consideration is the problem of C or Mn segregation caused by excessive manganese content, which deteriorates the uniformity of the steel plate structure during hot rolling and easily causes serious banded structure defects in the structure. In addition, the excessive manganese involved in the steel of the present invention leads to increased hardenability, suppressed bainite formation, and is not conducive to hole expansion performance. In addition, excessive manganese content will lead to poor welding performance of the steel plate. Therefore, based on comprehensive considerations, the present invention selects a manganese content of 1.79% to 1.98%.

[0013] P: Phosphorus is an impurity element in steel and is easily segregated at grain boundaries. When the phosphorus content in steel is high, Fe2P particles are easily formed, which reduces the plasticity, toughness and hole expansion performance of the steel. Therefore, the lower the content, the better. In order to obtain a higher elongation, the upper limit is set at 0.015%.

[0014] S: Sulfur is an impurity element in steel. It easily combines with Mn to form MnS inclusions, which become the starting point of cracks and deteriorate the processing performance, seriously affecting the plasticity, formability and hole expansion performance of the steel plate. Therefore, the lower the content, the better. The upper limit is set at 0.005%.

[0015] Al: Al is a deoxidizer in traditional steelmaking processes. It also combines with nitrogen in steel to form AlN, which refines grains and inhibits the decomposition of retained austenite. Together with silicon, it inhibits cementite formation, raising the austenitization temperature, facilitating optimal process window selection and accelerating bainite transformation. Excessive Al content can cause nozzle blockage during continuous casting, impacting production efficiency and increasing costs. Therefore, in this invention, the Al content is limited to 0.020% to 0.050%.

[0016] Nb: Nb effectively delays the recrystallization of deformed austenite, inhibits austenite grain growth, increases the austenite recrystallization temperature, refines grains, and improves the strength and toughness of the steel. Because free nitrogen atoms in the steel degrade the toughness of the steel plate, Nb combines with impurity nitrogen in the steel to form NbN. This NbN formation acts as a nitrogen fixation agent. Furthermore, Nb combines with carbon and nitrogen to form Nb(C, N), which contributes to grain refinement and precipitation strengthening. It also strengthens ferrite and bainite, resulting in excellent mechanical properties and hole expansion performance. Therefore, the optimal range of Nb content in the present invention is between 0.035% and 0.046%.

[0017] V: Vanadium has significant precipitation strengthening and grain refinement effects. Its effects are primarily achieved by forming precipitates with carbon and nitrogen. In particular, the VN precipitation formed with nitrogen significantly enhances the strength of the steel plate. Furthermore, a large number of V precipitates remain within the steel plate structure, acting as hydrogen traps and reducing the risk of delayed cracking in the Sb-containing steel plate of the present invention during service. The combined addition of V and Nb allows for the retention of a large number of V and Nb composite carbides within the Sb-containing steel plate of the present invention, which act as hydrogen traps and significantly enhance the Sb-containing steel plate's resistance to hydrogen-induced cracking during service, resulting in excellent mechanical properties, hole expansion performance, and resistance to hydrogen-induced cracking. Higher V contents degrade the toughness of the weld heat-affected zone, so the V addition in the present invention is 0.045% to 0.058%.

[0018] W: Tungsten has the highest melting point of any metal, and the WC it forms is highly hard. Its effect in steel is similar to that of molybdenum, enhancing wear resistance more effectively than molybdenum. It also improves the hardenability of steel and effectively inhibits grain growth. When its content is less than 0.010%, the effect is minimal, while exceeding 0.036% increases brittleness. Therefore, the optimal range for the W content in this invention is between 0.010% and 0.036%.

[0019] Ta: Tantalum has a strong affinity with elements such as carbon and nitrogen in steel, and can form fine and dispersed carbides, nitrides or carbonitrides. These compounds hinder grain growth and refine the grains of the steel, thereby improving the comprehensive mechanical properties of the steel, such as strength, toughness, and plasticity, and the pore expansion rate. In addition, tantalum forms a dense and stable oxide film (Ta2O5) on the surface of the steel. This oxide film can prevent external corrosive media from contacting the steel matrix, thereby improving the corrosion resistance of the steel. Therefore, the optimal range of the Ta content in the present invention is between 0.015% and 0.023%.

[0020] Bi: Bismuth (Bi) is primarily distributed within grain boundaries and grain interiors in steel, increasing steel sheet strength, reducing the diffusion rate of elements like carbon and oxygen at grain boundaries, minimizing decarburization and oxidation, and improving the surface and mechanical properties of the steel sheet. Therefore, the present invention limits the Bi content to 0.008% to 0.017%.

[0021] Sb: Antimony can make the corrosion products dense and inhibit H2O, O2, Cl - and SO4 2- Sb diffuses into the steel matrix and accumulates near the steel matrix in an acidic environment, forming a uniform, dense oxide film (rich in elements such as Sb) on the steel plate surface, resisting further corrosion. However, Sb is a low-melting-point element and tends to accumulate at grain boundaries, causing grain boundary cracks. The risk of steel plate cracking increases rapidly with increasing Sb content, so the present invention limits the Sb content to 0.015% to 0.050%.

[0022] Ni: Nickel is a solid solution strengthening element that can improve the material's hardenability, prevent temper brittleness, and improve the material's fatigue properties. It can also improve the steel's corrosion resistance and has no adverse effects on the hardenability and toughness of the steel's weld heat-affected zone. Furthermore, nickel is an austenite stabilizing element that can promote the retention of retained austenite. However, Ni is a precious element and its content should not be too high. Therefore, the Ni content is limited to 0.015% to 0.033%.

[0023] Cr: Chromium delays pearlite transformation and improves the hardenability of steel. This facilitates the formation of bainite and refines the structure, resulting in a strengthening effect. Chromium forms a dense oxide film (primarily Cr2O3) on the steel surface. This film prevents further contact between oxygen and water and the steel matrix, improving the steel's corrosion resistance. Therefore, the Cr content is limited to 0.40% to 0.50%.

[0024] Pt: Platinum has excellent chemical stability and corrosion resistance. Platinum can react with oxygen in steel to form a dense oxide protective film. This protective film can isolate the steel from direct contact with the corrosive medium, thereby slowing the corrosion process, improving the corrosion resistance and oxidation resistance of the steel, and increasing the service life of the steel in highly corrosive environments and high temperatures. Therefore, the present invention limits the Pt content to 0.002% to 0.009%.

[0025] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of the grain boundaries, and is beneficial to improving the hole expansion performance. It can also improve the welding performance and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures. Therefore, the present invention limits the Y content to 0.006% to 0.010%.

[0026] N: Regarding the N content in steel, the lower the N content, the better. However, too low a content will lead to production difficulties and increase costs. However, the present invention requires the precipitation of VN formed with V to perform precipitation strengthening and fine grain strengthening to improve the strength and hole expansion performance of the steel plate. Therefore, the N content in the present invention is ≤0.006%.

[0027] The microstructure of the finished steel plate is 15% to 20% by volume of ferrite, 65% to 75% by volume of bainite, and 10% to 20% by volume of martensite.

[0028] The steel plate has a yield strength of ≥775MPa, a tensile strength of ≥880MPa, a transverse elongation A of ≥27%, a hole expansion rate of ≥66%, a transverse cold bending of 180°D=a that is qualified, and a surface roughness Ra of 1.20~1.43μm.

[0029] The thickness of the steel plate is 2.22~3.20mm; according to GB / T10125-2021 "Artificial atmosphere corrosion test - salt spray test", a neutral salt spray test is used to simulate the marine atmospheric environment. A NaCl solution with a concentration of 50±5g / L is used. During the 168h test period, the corrosion rate of the steel plate is ≤0.48g / (m 2 •h).

[0030] A method for producing high-formability and ultra-high-strength steel for passenger vehicles, comprising smelting, hot rolling, wet sandblasting descaling, continuous annealing, and skin-passing steps, specifically as follows:

[0031] (1) Smelting process: RH+LF process is adopted, H and O contents are strictly controlled, H≤0.0002%, O≤0.0015%, calcium treatment is carried out in the refining process, electromagnetic stirring and light reduction technology are used in the continuous casting process, the billet pulling speed is ≤1.5m / min, and the light reduction amount is 1.5~2.5mm, which reduces the center segregation of the continuous casting billet, which is beneficial to reduce the banded structure in the subsequent rolling of hot rolled steel plates.

[0032] (2) Hot rolling process: The continuous casting slab with a thickness of 155-225 mm and a width of 1000-1750 mm is directly hot-transferred and hot-charged into a step-beam heating furnace for heating at a temperature of 1185-1195°C and a holding time of 182-210 min. The chemical composition of the present invention contains Sb, which has a low melting point and is easily enriched at the grain boundary to produce cracks, so the heating temperature should not be too high. The rough rolling adopts a 3+3 mode rolling process (R1 adopts 3 passes and R2 adopts 3 passes) for a total of 6 passes. The rough rolling outlet temperature is 1050-1072°C. The intermediate billet is 35.0-52.0 mm thick and 1000-1750 mm wide. An insulation cover is used to keep the intermediate billet warm before it enters the hot rolling finishing mill to reduce the temperature drop of the intermediate billet on the delayed roller and the temperature difference between the head and tail and the plate width. The finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is used before rolling, the finishing rolling inlet temperature is not higher than 1050℃, the final rolling temperature is 870-930℃, and the laminar cooling mode is adopted after final rolling. The laminar cooling rate is 35-46℃ / s. After cooling to 598-635℃, the steel is coiled and air-cooled to room temperature. The purpose of laminar cooling to 598-635℃ is to quickly generate ferrite and bainite, while suppressing grain growth, the ferrite content is also guaranteed, thereby refining the ferrite grains. In addition, the reduction of the finishing rolling unit F1 is 17-25mm, the reduction of F2 is 8-12mm, the reduction of F3 is 4-7mm, the reduction of F4 is 2.0-3.5mm, the reduction of F5 is 1.0-2.2mm, the reduction of F6 is 0.8-1.4mm, and the reduction of F7 is 0.2-0.5mm. The rolling thickness is 2.22-3.20mm. After rolling, the volume percentage of ferrite in the hot-rolled steel plate is 15%-20%, the volume percentage of pearlite is 10%-20%, the volume percentage of bainite is 55%-65%, and the volume percentage of cementite is 0-6%.

[0033] (3) EPS (wet sandblasting descaling process): After cooling the steel coil with a thickness of 2.22-3.20 mm to room temperature, wet sandblasting is performed to remove the iron oxide scale on the steel plate surface. The main processes are uncoiling, straightening, sandblasting descaling, and coiling. The uncoiling tension of the steel plate is 35-40 kN, the straightening elongation is 0.85-1.05%, the travel speed is 20-30 m / min, the steel grit hardness is HRC>85, the sandblasting motor speed is 2600-3000 rpm, and the coiling tension is 75-85 kN.

[0034] (4) Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-80 m / min, the soaking section temperature at 765-785 °C, the soaking time at 8-14 min, the slow cooling outlet temperature at 715-728 °C, the rapid cooling rate at more than 40 °C / s, and the rapid cooling outlet temperature at 305-348 °C. The soaking section temperature is 765-785°C, the purpose of which is to ensure a suitable phase ratio of ferrite and austenite in the critical zone. If the soaking section temperature is greater than 785°C, the ferrite ratio in the organization will be greatly reduced, reducing the plasticity of the steel. If the soaking section temperature is less than 765°C, the austenite content in the critical zone temperature stage is obviously insufficient, affecting the subsequent bainite and martensite content, which in turn leads to insufficient strength. The soaking time is 8-14 minutes, the purpose of which is to ensure sufficient recrystallization of the steel plate grains during the heating and holding stages and to avoid excessive time leading to grain growth of the steel plate. If the soaking time is too short, the steel plate will not have enough time for continuous annealing and recrystallization, resulting in a decrease in the elongation of the steel plate. The slow cooling outlet temperature is 715-728°C, the purpose of which is to quickly generate ferrite, while suppressing grain growth, and ensuring the ferrite content, thereby refining the ferrite grains. The fast cooling outlet temperature is 305-348°C, the purpose of which is to quickly generate bainite and martensite, while suppressing grain growth, and ensuring the bainite and martensite content, thereby refining the bainite grains.

[0035] (5) Skin pass: After rapid cooling, the steel is directly air-cooled and enters the skin pass mill. The skin pass adopts rolling force control, which is controlled at 1650~3250kN and the rolling tension is 750~1850kN.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. The WC formed by W has high hardness, enhances wear resistance, improves the hardenability of steel, and effectively inhibits grain growth.

[0038] 2. Tantalum has a strong affinity with elements such as carbon and nitrogen in steel, and can form fine and dispersed carbides, nitrides or carbonitrides. These compounds hinder grain growth and refine the grains of steel, thereby improving the comprehensive mechanical properties of steel such as strength, toughness, plasticity and pore expansion rate. Tantalum forms a dense and stable oxide film (Ta2O5) on the surface of steel. This oxide film can prevent external corrosive media from contacting the steel matrix, thereby improving the corrosion resistance of steel.

[0039] 3. Bismuth is mainly distributed in the grain boundaries and inside the grains in steel, which improves the strength of the steel plate, reduces the diffusion rate of elements such as carbon and oxygen on the grain boundaries, reduces decarburization and oxidation, and improves the surface and mechanical properties of the steel plate.

[0040] 4. The addition of Sb can make the corrosion products dense and inhibit the formation of H2O, O2, and Cl- and SO4 2- It diffuses isotropically into the steel matrix and can be enriched near the steel matrix in an acidic environment, prompting the formation of a uniform and dense oxide film (rich in elements such as Sb) on the surface of the steel plate matrix to resist further erosion of the steel matrix.

[0041] 5. The addition of Ni can improve the hardenability of the material, prevent temper brittleness, improve the fatigue performance of the material, and at the same time improve the corrosion resistance of the steel and promote the retention of retained austenite.

[0042] 6. Cr can delay pearlite transformation and improve the hardenability of steel. This facilitates the formation of bainite and refines the structure, thus strengthening it. Chromium forms a dense oxide film (primarily Cr2O3) on the steel surface. This film prevents further contact between oxygen and water and the steel matrix, improving the steel's corrosion resistance.

[0043] 7. The addition of Pt in steel can react with oxygen to form a dense oxide protective film. This protective film can isolate the steel from direct contact with the corrosive medium, thereby slowing down the corrosion process, improving the corrosion resistance and oxidation resistance of the steel, and increasing the service life of the steel in strong corrosive environments and high temperatures.

[0044] 8. The addition of Y can refine the grains in the steel, enhance the strength and plasticity of the grain boundaries, which is beneficial to the improvement of the hole expansion performance. It can also improve the welding performance and oxidation resistance of the steel, thereby increasing the service life of the steel at high temperatures.

[0045] 9. The present invention does not require the conventional hot rolling + pickling + cold rolling + continuous annealing + skin-passing process, but adopts hot rolling + EPS (wet sandblasting descaling process) + continuous annealing + skin-passing process, which reduces the pickling + cold rolling process, has fewer process flows, low production costs, and does not pollute the environment.

[0046] 10. The microstructure of the steel of the present invention is ferrite, bainite and martensite, which can significantly improve the hole expansion performance of the steel plate during the forming process.

[0047] 11. The present invention has excellent mechanical properties, yield strength ≥775MPa, tensile strength ≥880MPa, transverse elongation A ≥27%, hole expansion rate ≥66%, transverse cold bending 180° D=a qualified, surface roughness Ra is 1.20-1.43μm, according to GB / T10125-2021 "Artificial atmosphere corrosion test - salt spray test", a neutral salt spray test is used to simulate the marine atmospheric environment, using a NaCl solution with a concentration of 50±5g / L, within a 168h test period, the corrosion rate is ≤0.48g / (m 2 •h). DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with examples. The following examples are used to specifically illustrate the contents of the present invention. These examples are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0049] The embodiments of the invention are as follows:

[0050] The smelting adopts RH+LF process, strictly controlling the H and O contents, H≤0.0002%, O≤0.0015%, and calcium treatment is carried out in the refining process. Electromagnetic stirring and soft reduction technology are used in the continuous casting process. The casting speed is ≤1.5m / min, and the soft reduction is 1.5~2.5mm. The continuous casting slabs with a thickness of 155~225mm and a width of 1000~1750mm are directly hot-transferred and hot-charged into the walking beam heating furnace for heating. The heating temperature is 1185~1195℃, and the holding time is 182~210min. The rough rolling adopts the 3+3 mode rolling process (R1 adopts 3 rolling passes, R2 adopts 3 rolling passes) for a total of 6 rolling passes. The rough rolling outlet temperature is 1050℃~1072℃, the intermediate billet thickness is 35.0~52.0mm, and the width is 1000~1750mm. The intermediate billet is 1000~1750mm in thickness. A heat preservation cover is used before the intermediate billet enters the hot rolling and finishing rolling unit to reduce the temperature drop of the intermediate billet on the delay roller and the temperature difference between the head, tail and plate width directions. The finishing rolling is a 7-stand continuous rolling. High-pressure water descaling is used before finishing rolling. The finishing rolling inlet temperature is not higher than 1060℃. The finishing rolling temperature is 870~930℃. After finishing rolling, the laminar cooling mode is adopted with a laminar cooling rate of 35~46℃ / s. After cooling to 598~635℃, it is coiled and air-cooled to room temperature. The finishing mill F1 has a reduction of 17-25mm, F2 has a reduction of 8-12mm, F3 has a reduction of 4-7mm, F4 has a reduction of 2.0-3.5mm, F5 has a reduction of 1.0-2.2mm, F6 has a reduction of 0.8-1.4mm, and F7 has a reduction of 0.2-0.5mm, resulting in a rolling thickness of 2.22-3.20mm. After the 2.22-3.20mm thick steel coils are cooled to room temperature, they are wet-sandblasted to remove scale from the steel surface. The main processes include uncoiling, straightening, descaling, and coiling. Uncoiling tension is 25-40kN, straightening elongation is 0.85-1.05%, travel speed is 20-30m / min, grit hardness is HRC>85, blasting motor speed is 2600-3000rpm, and coiling tension is 75-85kN. Continuous annealing is performed on wet-sandblast-descaled coils with a thickness of 2.22 to 3.20 mm. The strip speed is controlled at 50 to 80 m / min, the soaking zone temperature is 765 to 785°C, the soaking time is 8 to 14 minutes, the slow cooling outlet temperature is 715 to 728°C, the rapid cooling rate is greater than 40°C / s, and the rapid cooling outlet temperature is 305 to 348°C. After rapid cooling, the coils are directly air-cooled before entering the skin-pass mill. Skin-passing utilizes rolling force control, ranging from 1650 to 3250 kN, and the rolling tension is 750 to 1850 kN. The finished product thickness is 2.22 to 3.20 mm.

[0051] The specific composition, hot rolling process, EPS (wet sandblasting descaling process), continuous annealing process, steel plate properties and microstructure volume percentages of the six embodiments of the present invention are shown in Tables 1-6.

[0052] Table 1 Chemical composition of the present invention (wt, %)

[0053]

[0054] Table 2 Hot rolling process system of the embodiment of the present invention

[0055]

[0056] Table 3 EPS (wet sandblasting descaling process) system of the embodiment of the present invention

[0057]

[0058] Table 4 Continuous annealing process system of the embodiment of the present invention

[0059]

[0060] Table 5 Mechanical properties parameters of the embodiments of the present invention

[0061]

[0062] Table 6 Tissue volume percentage in the examples of the present invention

[0063] .

Claims

1. A high-formability and ultra-high-strength steel for passenger vehicles, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.085% ~ 0.130%, Si: 0.25% ~ 0.65%, Mn: 1.79% ~ 1.98%, Al: 0.020% ~ 0.050%, Nb: 0.035% ~ 0.046%, V: 0.045% ~ 0.058%, W: 0.010% ~ 0.036%, Ta: 0.015% ~ 0.023% , Bi: 0.008% ~ 0.017%, Sb: 0.015% ~ 0.050%, Ni: 0.015% ~ 0.033%, Cr: 0.40% ~ 0.50%, Pt: 0.002% ~ 0.009%, Y: 0.006% ~ 0.010%, and limit P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.006%, the balance is Fe and unavoidable impurities; The production method of the high-formability and ultra-high-strength passenger car steel includes smelting, hot rolling, wet sandblasting descaling, continuous annealing and skin-passing processes, as follows: Hot rolling: heating temperature 1185-1195℃, holding time 182-210min, rough rolling outlet temperature 1050-1072℃, use insulation cover to keep the intermediate billet warm before entering the hot rolling and finishing mill, finishing rolling inlet temperature not higher than 1050℃, finishing temperature 870-930℃, laminar cooling mode is adopted after finishing rolling, laminar cooling rate is 35-46℃ / s, coiling is done after cooling to 598-635℃, and air cooling is done to room temperature; Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-80 m / min, the soaking section temperature at 765-785°C, the soaking time at 8-14 min, the slow cooling outlet temperature at 715-728°C, the rapid cooling rate at more than 40°C / s, and the rapid cooling outlet temperature at 305-348°C.

2. The high-formability and ultra-high-strength steel for passenger vehicles according to claim 1, characterized in that: The microstructure of the finished steel plate is 15% to 20% by volume of ferrite, 65% to 75% by volume of bainite, and 10% to 20% by volume of martensite.

3. The high formability and ultra-high strength steel for passenger vehicles according to claim 1, characterized in that: The steel plate has a yield strength of ≥775MPa, a tensile strength of ≥880MPa, a transverse elongation A of ≥27%, a hole expansion rate of ≥66%, a transverse cold bending of 180°D=a that is qualified, and a surface roughness Ra of 1.20~1.43μm.

4. The high formability and ultra-high strength steel for passenger vehicles according to claim 1, characterized in that: The thickness of the steel plate is 2.22~3.20mm; according to GB / T10125-2021 "Artificial atmosphere corrosion test - salt spray test", a neutral salt spray test is used to simulate the marine atmospheric environment. A NaCl solution with a concentration of 50±5g / L is used. During the 168h test period, the corrosion rate of the steel plate is ≤0.48g / (m 2 •h).

5. A method for producing high-formability and ultra-high-strength steel for passenger vehicles according to any one of claims 1 to 4, characterized in that: It includes smelting, hot rolling, wet sandblasting descaling, continuous annealing and skin-passing processes, as follows: Hot rolling: heating temperature 1185-1195℃, holding time 182-210min, rough rolling outlet temperature 1050-1072℃, use insulation cover to keep the intermediate billet warm before entering the hot rolling and finishing mill, finishing rolling inlet temperature not higher than 1050℃, finishing temperature 870-930℃, laminar cooling mode is adopted after finishing rolling, laminar cooling rate is 35-46℃ / s, coiling is done after cooling to 598-635℃, and air cooling is done to room temperature; Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-80 m / min, the soaking section temperature at 765-785°C, the soaking time at 8-14 min, the slow cooling outlet temperature at 715-728°C, the rapid cooling rate at more than 40°C / s, and the rapid cooling outlet temperature at 305-348°C.

6. The method for producing high-formability and ultra-high-strength steel for passenger vehicles according to claim 5, characterized in that: The rough rolling adopts the 3+3 mode rolling process, the intermediate billet thickness is 35.0~52.0mm, the width is 1000~1750mm, the finishing rolling unit F1 reduction is 17~25mm, F2 reduction is 8~12mm, F3 reduction is 4~7mm, F4 reduction is 2.0~3.5mm, F5 reduction is 1.0~2.2mm, F6 reduction is 0.8~1.4mm, F7 reduction is 0.2~0.5mm.

7. The method for producing high-formability and ultra-high-strength steel for passenger vehicles according to claim 5 or 6, characterized in that: The microstructure of the hot rolled steel plate after rolling is as follows: ferrite volume percentage 15% to 20%, pearlite volume percentage 10% to 20%, bainite volume percentage 55% to 65%, and cementite volume percentage 0 to 6%.

8. The method for producing high-formability and ultra-high-strength steel for passenger vehicles according to claim 5, characterized in that: The smelting adopts RH+LF process, the casting billet pulling speed is ≤1.5m / min, the soft pressure reduction is 1.5-2.5mm, and the continuous casting billet specifications are 155-225mm thick×1000-1750mm wide.

9. The method for producing high-formability and ultra-high-strength steel for passenger vehicles according to claim 5, characterized in that: The wet sandblasting descaling treatment: the main processes include uncoiling, straightening, sandblasting descaling and coiling, the steel plate uncoiling tension is 35 to 40 kN, the straightening elongation is 0.85% to 1.05%, the travel speed is 20 to 30 m / min, the steel sand hardness is HRC>85, the sandblasting motor speed is 2600 to 3000 rpm, and the coiling tension is 75 to 85 kN.

10. The method for producing high-formability and ultra-high-strength steel for passenger vehicles according to claim 5, characterized in that: The skin-passing process is as follows: after rapid cooling, the steel is directly air-cooled and then enters the skin-passing mill; the skin-passing rolling force is controlled at 1650-3250 kN, and the rolling tension is 750-1850 kN.

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