Ultra-high-strength steel for passenger cars and production method thereof

Abstract

The present invention belongs to the field of metal materials, and particularly relates to an 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, Ta, Bi, Sb, Ni, Cr, Pt, Zr, Y: 0.006% to 0.010%, with a C / V ratio of 1.0 to 1.8, Ta + Pt: 0.014% to 0.026%, 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, resulting in low production costs and environmental friendliness.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and in particular relates to an 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 CN113416890A discloses a high-hole expansion and high-plasticity 980MPa grade cold-rolled continuously annealed steel plate and its preparation method. It uses a common C-Mn composition system to design and add a certain amount of Mo, Cu and B to produce hot-rolled + pickling + cold-rolled + continuous annealing + skin-passing steel plates. The yield strength is ≤850MPa, the tensile strength is ≥980MPa, and the hole expansion rate is ≥50%, which does not meet the requirements of ultra-high-strength passenger car parts. 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 CN116377334A discloses an ultra-high-plasticity isotropic 980MPa grade cold-rolled high-strength steel plate and its preparation method. The plate is produced by hot rolling, pickling, cold rolling, continuous annealing and skin-passing by adding a certain amount of Nb and Ti to a conventional C-Mn composition system. The plate has a yield strength of ≤850MPa, a tensile strength of ≥980MPa, and no hole expansion requirement. It does not meet the requirements of ultra-high-strength passenger car parts and does not have certain corrosion resistance and oxidation resistance. In addition, the process system 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.

[0005] 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, which does not meet the requirements of 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. Summary of the Invention

[0006] The purpose of the present invention is to provide an ultra-high-strength steel for passenger cars and a production method thereof, wherein the steel plate has a yield strength of ≥918MPa, a tensile strength of ≥1085MPa, a transverse elongation A ≥24%, a hole expansion rate ≥60%, a qualified transverse cold bending 180°D=a, good surface quality, no iron oxide stripes and color defects, and a surface roughness Ra of 1.25 to 1.50μm. 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 ≤0.43g / (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] An ultra-high strength steel for passenger cars, wherein the chemical composition of the steel is as follows by weight: C: 0.098%-0.156%, Si: 0.25%-0.65%, Mn: 1.99%-2.15%, Al: 0.020%-0.050%, Nb: 0.055%-0.066%, V: 0.082%-0.110%, Ta: 0.010%-0.020%, Bi: 0.008%-0.017%, Sb: 0.015%- 0.050%, Ni: 0.015% ~ 0.033%, Cr: 0.45% ~ 0.55%, Pt: 0.002% ~ 0.009%, Zr: 0.010% ~ 0.015%, Y: 0.006% ~ 0.010%, and C / V: 1.0 ~ 1.8, Ta + Pt: 0.014% ~ 0.026%, and limit P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.006%, the balance is Fe and unavoidable impurities.

[0009] The main functions of the composition of the 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 generally in the low carbon range, 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 0.098% to 0.156%.

[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 Point, effectively adjust the annealing process window in the continuous annealing stage, and ensure the appropriate ferrite and austenite ratio in the critical zone at the industrial continuous annealing temperature. At the same time, the role of silicon addition is that sufficient silicon addition can also reduce inclusions in the steel, inhibit the formation of inclusions, and avoid the steel plate from reducing the mechanical properties and hole expansion performance due to the formation of inclusions. However, if the silicon content is too low, it will not ensure the strength of the ferrite matrix and inhibit the formation of inclusions. If the content is too high, it will affect the hot rolling surface quality, resulting in a large amount of iron oxide scale and welding performance. Therefore, the silicon content in the present invention is 0.25-0.65%.

[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 1.99-2.15%.

[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 lead to nozzle blockage during continuous casting, impacting production efficiency and increasing costs. Therefore, in this invention, the Al content is limited to 0.020-0.050%.

[0016] Nb: Nb effectively delays the recrystallization of deformed austenite, prevents 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 0.055% and 0.066%.

[0017] V: Vanadium exhibits significant precipitation strengthening and grain refinement effects. Vanadium's 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 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, which act as hydrogen traps, significantly improving 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 (HAZ). Therefore, the V addition in this invention is 0.082-0.110%, maintaining a C / V ratio of 1.0-1.8.

[0018] 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 0.010 and 0.020%.

[0019] 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%.

[0020] 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 0.015-0.050%.

[0021] 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%.

[0022] 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 0.45% to 0.55%.

[0023] 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 0.002-0.009% and satisfies the Ta+Pt ratio of 0.014-0.026%.

[0024] Zr: Zirconium is a strong deoxidizer that can react with oxygen to form stable ZrO2, effectively reducing the oxygen content in steel, reducing oxide inclusions, and improving the purity of steel. It can also react with sulfur to form ZrS, reducing the sulfur content in steel. When the steel solidifies, it can act as a heterogeneous core to promote grain nucleation, inhibit grain growth, and refine the cast grains. During hot rolling and continuous annealing, it forms stable carbides, hindering the growth of austenite grains, obtaining a fine and uniform grain structure, and improving the strength, toughness, plasticity and hole expansion performance of the steel. Therefore, the present invention limits the Zr content to 0.010-0.015%.

[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 0.006-0.010%.

[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 10% to 20% by volume of ferrite, 15% to 25% by volume of bainite, 50% to 60% by volume of martensite, and 2% to 9% by volume of retained austenite.

[0028] The steel plate has a yield strength of ≥918MPa, a tensile strength of ≥1085MPa, a transverse elongation A of ≥24%, a hole expansion rate of ≥60%, a transverse cold bending of 180°D=a that is qualified, and a surface roughness Ra of 1.25~1.50μm.

[0029] The thickness of the steel plate is 1.8 to 3.0 mm. According to GB / T10125-2021 "Artificial atmosphere corrosion test - salt spray test", a neutral salt spray test is performed to simulate the marine atmosphere environment. A NaCl solution with a concentration of 50 g / L ± 5 g / L is used. During the 168 h test period, the corrosion rate of the steel plate is ≤ 0.43 g / (m 2 •h).

[0030] A method for producing 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 casting speed is ≤1.5m / min, and the light reduction is 1.5mm~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 105-205 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 1210-1240°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 1050-1095°C. The intermediate slab is 33.0-42.0 mm thick and 1000-1750 mm wide. The intermediate slab is kept warm by a heat preservation cover before entering the hot rolling finishing mill to reduce the temperature drop of the intermediate slab on the delayed roller and the temperature difference between the head and tail and the plate width direction. The finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is used before finishing rolling, the finishing rolling inlet temperature is not higher than 1050℃, the final rolling temperature is 885-940℃, and laminar cooling mode is adopted after final rolling. The laminar cooling rate is 45-55℃ / s. After cooling to 558-605℃, the steel is coiled and air-cooled to room temperature. The purpose of laminar cooling to 558-605℃ is to quickly generate bainite, while suppressing grain growth, also ensuring the bainite content, thereby refining the ferrite grains. In addition, the reduction of the finishing rolling unit F1 is 10-19 mm, the reduction of F2 is 6-11 mm, the reduction of F3 is 3-7 mm, the reduction of F4 is 2.0-3.5 mm, the reduction of F5 is 1.0-2.2 mm, the reduction of F6 is 0.7-1.4 mm, and the reduction of F7 is 0.2-0.5 mm. The rolling thickness is 1.8-3.0 mm. After rolling, the volume percentage of ferrite in the hot-rolled steel plate is 20%-25%, and the volume percentage of bainite is 75%-80%.

[0033] (3) EPS (wet sandblasting descaling process): After cooling the steel coil with a thickness of 1.8 to 3.0 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 50 to 55 kN, the straightening elongation is 0.85 to 1.05%, the travel speed is 20 to 30 m / min, the steel grit hardness is HRC ≥ 75, the sandblasting motor speed is 2600 to 3000 rpm, and the coiling tension is 90 to 100 kN.

[0034] (4) Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-75 m / min, the soaking section temperature at 799-835 °C, the soaking time at 8-14 min, the slow cooling outlet temperature at 690-725 °C, the rapid cooling rate at more than 45 °C / s, and the rapid cooling outlet temperature at 315-338 °C. The soaking section temperature is 799-835°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 835°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 799°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 690-725°C, the purpose of which is to enable the rapid generation of ferrite, while suppressing grain growth and ensuring the ferrite content, thereby refining the ferrite grains. The fast cooling outlet temperature is 315-338°C, the purpose of which is to enable the rapid generation of 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 1600~3200kN and the rolling tension is 800~1850kN.

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

[0037] 1. 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 such as strength, toughness, plasticity and pore expansion rate of steel. 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.

[0038] 2. 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.

[0039] 3. 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.

[0040] 4. 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.

[0041] 5. 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.

[0042] 6. 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.

[0043] 7. Zr: Effectively reduces the oxygen content in steel, reduces oxide inclusions, improves the purity of steel, inhibits grain growth, refines cast grains, and forms stable carbides during hot rolling and continuous annealing, hindering the growth of austenite grains, obtaining a fine and uniform grain structure, and improving the strength, toughness, plasticity and hole expansion performance of steel.

[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, martensite and retained austenite, 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 ≥918MPa, tensile strength ≥1085MPa, transverse elongation A ≥24%, hole expansion rate ≥60%, transverse cold bending 180° D=a qualified, surface roughness Ra of 1.25-1.50μm, 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 50g / L±5g / L, within a 168h test period, the steel plate corrosion rate is ≤0.43g / (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 105~205mm 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 1210~1240℃ and the holding time is 182~210min. The rough rolling adopts the 3+3 mode rolling process (R1 adopts 3 rolling passes and R2 adopts 3 rolling passes) for a total of 6 rolling passes. The rough rolling outlet temperature is 1050℃~1095℃. The intermediate billet thickness is 33.0~42.0mm 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 1050℃. The finishing rolling temperature is 885~940℃. After finishing rolling, the laminar cooling mode is adopted with a laminar cooling rate of 45~55℃ / s. After cooling to 558~605℃, it is coiled and air-cooled to room temperature. The finishing mill F1 has a reduction of 10-19mm, F2 has a reduction of 6-11mm, F3 has a reduction of 3-7mm, F4 has a reduction of 2.0-3.5mm, F5 has a reduction of 1.0-2.2mm, F6 has a reduction of 0.7-1.4mm, and F7 has a reduction of 0.2-0.5mm, with a rolling thickness of 1.8-3.0mm. After the 1.8-3.0mm 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, sandblasting, and coiling. The uncoiling tension is 50-55kN, the straightening elongation is 0.85-1.05%, the travel speed is 20-30m / min, the steel grit hardness is HRC ≥75, the sandblasting motor speed is 2600-3000rpm, and the coiling tension is 90-100kN. Continuous annealing is performed on wet sandblasted, descaled coils with a thickness of 1.8 to 3.0 mm. The strip speed is controlled between 50 and 75 m / min, the soaking zone temperature is 799 to 835°C, the soaking time is 8 to 14 minutes, the slow cooling outlet temperature is 690 to 725°C, the rapid cooling rate is greater than 45°C / s, and the rapid cooling outlet temperature is 305 to 348°C. After rapid cooling, the coils are directly air-cooled into the skin-pass mill. Skin-passing utilizes rolling force control, ranging from 1600 to 3200 kN, and the rolling tension is 800 to 1850 kN. The finished product thickness is 1.8 to 3.0 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. An ultra-high strength steel for passenger vehicles, characterized in that: The chemical composition of the steel is calculated by weight as follows: C: 0.098% ~ 0.156%, Si: 0.25% ~ 0.65%, Mn: 1.99% ~ 2.15%, Al: 0.020% ~ 0.050%, Nb: 0.055% ~ 0.066%, V: 0.082% ~ 0.110%, Ta: 0.010% ~ 0.020%, Bi: 0.008% ~ 0.017%, Sb: 0.015% ~ 0.050 %, Ni: 0.015% ~ 0.033%, Cr: 0.45% ~ 0.55%, Pt: 0.002% ~ 0.009%, Zr: 0.010% ~ 0.015%, Y: 0.006% ~ 0.010%, and C / V: 1.0 ~ 1.8, Ta + Pt: 0.014% ~ 0.026%, and limit P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.006%, the balance is Fe and unavoidable impurities; The production method of the 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 1210 ~ 1240 ℃, holding time 182 ~ 210min, rough rolling outlet temperature 1050 ~ 1095 ℃, the intermediate billet is kept warm by a heat preservation cover before entering the hot rolling and finishing mill, the finishing rolling inlet temperature is not higher than 1050 ℃, the finishing temperature is 885 ~ 940 ℃, and the laminar cooling mode is adopted after the final rolling, the laminar cooling rate is 45 ~ 55 ℃ / s, and the coiling is carried out after cooling to 558 ~ 605 ℃, and air cooling is carried out to room temperature; Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-75m / min, the soaking section temperature at 799-835°C, the soaking time at 8-14min, the slow cooling outlet temperature at 690-725°C, the rapid cooling rate at more than 45°C / s, and the rapid cooling outlet temperature at 315-338°C.

2. The ultra-high strength steel for passenger vehicles according to claim 1, characterized in that: The finished steel plate has a ferrite volume percentage of 10% to 20%, a bainite volume percentage of 15% to 25%, a martensite volume percentage of 50% to 60%, and a retained austenite volume percentage of 2% to 9%.

3. The ultra-high strength steel for passenger vehicles according to claim 1, characterized in that: The steel plate has a yield strength of ≥918MPa, a tensile strength of ≥1085MPa, a transverse elongation A of ≥24%, a hole expansion rate of ≥60%, a transverse cold bending of 180°D=a that is qualified, and a surface roughness Ra of 1.25~1.50μm.

4. The ultra-high strength steel for passenger vehicles according to claim 1, characterized in that: The thickness of the steel plate is 1.8 to 3.0 mm. According to GB / T10125-2021 "Artificial atmosphere corrosion test - salt spray test", a neutral salt spray test is performed to simulate the marine atmosphere environment. A NaCl solution with a concentration of 50 g / L ± 5 g / L is used. During the 168 h test period, the corrosion rate of the steel plate is ≤ 0.43 g / (m 2 •h).

5. A method for producing 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 1210 ~ 1240 ℃, holding time 182 ~ 210min, rough rolling outlet temperature 1050 ~ 1095 ℃, the intermediate billet is kept warm by a heat preservation cover before entering the hot rolling and finishing mill, the finishing rolling inlet temperature is not higher than 1050 ℃, the finishing temperature is 885 ~ 940 ℃, and the laminar cooling mode is adopted after the final rolling, the laminar cooling rate is 45 ~ 55 ℃ / s, and the coiling is carried out after cooling to 558 ~ 605 ℃, and air cooling is carried out to room temperature; Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-75m / min, the soaking section temperature at 799-835°C, the soaking time at 8-14min, the slow cooling outlet temperature at 690-725°C, the rapid cooling rate at more than 45°C / s, and the rapid cooling outlet temperature at 315-338°C.

6. The method for producing 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 33.0~42.0mm, the width is 1000~1750mm, the finishing rolling is 7-stand continuous rolling, the finishing rolling unit F1 reduction is 10~19mm, F2 reduction is 6~11mm, F3 reduction is 3~7mm, F4 reduction is 2.0~3.5mm, F5 reduction is 1.0~2.2mm, F6 reduction is 0.7~1.4mm, F7 reduction is 0.2~0.5mm.

7. The method for producing ultra-high strength steel for passenger vehicles according to claim 5 or 6, characterized in that: The volume percentage of ferrite in the hot-rolled steel plate after rolling is 20% to 25%, and the volume percentage of bainite is 75% to 80%.

8. The method for producing 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 pressing reduction is 1.5-2.5mm, and the continuous casting billet specification is 105-205mm thick×1000-1750mm wide.

9. The method for producing 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 50-55kN, the straightening elongation is 0.85%-1.05%, the travel speed is 20-30m / min, the steel sand hardness is HRC≥75, the sandblasting motor speed is 2600-3000rpm, and the coiling tension is 90-100kN.

10. The method for producing 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 1600-3200 kN, and the rolling tension is 800-1850 kN.

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