A 635MPa grade passenger car steel and its production method

Abstract

This invention belongs to the field of metallic materials, and specifically relates to a 635MPa grade passenger car steel and its production method. The chemical composition of the steel is designed to contain C, Si, Mn, Al, Ti, V, Ta, Bi, Sb, Ni, Pt, and Y, with P ≤ 0.015%, S ≤ 0.005%, and N ≤ 0.006%, and the balance being Fe and unavoidable impurities. This invention employs a hot rolling + EPS + continuous annealing + finishing process, simplifying the production process and eliminating environmental pollution.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a 635MPa grade steel for passenger vehicles and its production method. Background Technology

[0002] The automotive industry is a major consumer of steel products, and the research 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 is moving towards lower cost, higher strength, greener and more environmentally friendly materials, and higher safety. To reduce the weight of automotive structural components and thus save energy and reduce environmental pollution, the application of high-performance, high-surface-quality automotive steel sheets has been promoted. Conventional automotive sheet production processes typically involve hot rolling + pickling + cold rolling + continuous annealing + finishing, for example:

[0003] Chinese patent application CN115584428A discloses a novel short-process, low-cost cold-rolled DH590 steel and its production method. It utilizes a common C-Mn composition system with added Mg and Ca to produce a hot-rolled + pickled + cold-rolled + continuous annealing + finishing steel sheet. The sheet exhibits a yield strength ≥330MPa, tensile strength ≥590MPa, and porosity ≥60%. However, the porosity fluctuates significantly, failing to meet the requirements for high-strength and highly formable passenger vehicle parts. Furthermore, it lacks sufficient corrosion resistance and oxidation resistance. The process involves hot rolling + pickling + cold rolling + continuous annealing + finishing, resulting in a large process flow, high production costs, and, in particular, the need for pickling. Pickling itself causes significant acid mist pollution, environmental pollution, and high waste acid treatment costs.

[0004] Chinese patent application CN115652207A discloses a short-process economical cold-rolled DH steel sheet with a yield strength of ≥440MPa and its production method. The steel sheet is produced by adding a certain amount of Ti, Mg and Ca to a common C-Mn composition system. The steel sheet has a yield strength of ≥440MPa, a hole expansion rate of ≥30%, and a small hole expansion rate, which does not meet the requirements of high-formability passenger car parts. It also does not have certain corrosion resistance and oxidation resistance. Furthermore, the process involves hot rolling, pickling, cold rolling, continuous annealing and finishing, which is a large process with high production costs. In particular, pickling is required, which causes problems such as large acid mist pollution, environmental pollution and high waste acid treatment costs.

[0005] Chinese patent application CN111979490A discloses a high-ductility, high-formability cold-rolled DH590 steel and its production method. It utilizes a common C-Mn composition system with added Nb, Cr, and Mo to produce a hot-rolled + pickled + cold-rolled + continuous annealing + finishing steel sheet. This sheet exhibits a yield strength of 350–430 MPa, a tensile strength of 590–700 MPa, and a porosity ≥50%. However, the yield strength and tensile strength fluctuate greatly, and the porosity is low, failing to meet the requirements for high-formability passenger vehicle parts. Furthermore, it lacks sufficient corrosion resistance and oxidation resistance. The process involves hot rolling + pickling + cold rolling + continuous annealing + finishing, resulting in a large process flow, high production costs, and, in particular, the need for pickling. Pickling itself causes significant acid mist pollution, environmental pollution, and high waste acid treatment costs. Summary of the Invention

[0006] The purpose of this invention is to provide a 635MPa grade passenger car steel and its production method. The steel plate has a yield strength ≥484MPa, tensile strength ≥635MPa, transverse elongation A ≥25%, hole expansion rate ≥85%, and passes the transverse cold bending test at 180° (D=a). It exhibits good surface quality, free of iron oxide scale streaks and color difference defects, with a surface roughness Ra of 1.40-1.90μm. Furthermore, it does not require the conventional hot rolling + pickling + cold rolling + continuous annealing + finishing process. Instead, it employs hot rolling + EPS (wet sandblasting descaling) + continuous annealing + finishing process, reducing the pickling and cold rolling steps, resulting in lower production costs and no environmental pollution.

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

[0008] A 635MPa grade passenger car steel has the following chemical composition by weight percentage: C: 0.055%–0.085%, Si: 0.25%–0.65%, Mn: 1.35%–1.65%, Al: 0.020%–0.050%, Ti: 0.025%–0.055%, V: 0.020%–0.035%, Ta: 0.010%–0.020%, Bi: 0.005%–0.014%, Sb: 0.010%–0.050%, Ni: 0.010%–0.030%, Pt: 0.001%–0.006%, Y: 0.004%–0.009%, with P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities.

[0009] The microstructure of the finished steel plate consists of 25%–45% ferrite, 50%–70% bainite, and 3%–9% retained austenite.

[0010] The steel plate has a yield strength ≥484MPa, tensile strength ≥635MPa, transverse elongation A ≥25%, hole expansion rate ≥85%, transverse cold bending 180° D=a is qualified, and surface roughness Ra is 1.40~1.90μm.

[0011] The thickness of the steel plate is 1.8 to 3.0 mm.

[0012] The main function of the 635MPa grade passenger car steel composition in this invention is as follows:

[0013] C: Carbon is a common strengthening element in steel. Interstitial carbon atoms cause lattice distortion in the matrix, playing a role in solid solution strengthening. In this invention, the addition of carbon in conjunction with manganese ensures an industrially achievable austenitizing temperature, promoting the stabilization of austenite in the critical region. Carbon also ensures the stability of residual austenite, thereby improving the formability and hole-expanding properties of the steel plate. Too low a carbon content will not yield the mechanical properties of the steel plate described in this invention, while too high a content will cause embrittlement, posing a risk of delayed fracture and hot-rolling edge cracking, and also negatively impacting the weldability, plasticity, and toughness of the steel plate. This invention requires the overall carbon content to be within a low range, which helps reduce the risk of delayed fracture and hot-rolling edge cracking, and also benefits the weldability of the steel plate. Therefore, the optimal carbon content in this invention is 0.055%–0.085%.

[0014] Si: Silicon is one of the key elements in this invention. Sufficient silicon addition to ferrite ensures the strength of the ferrite matrix, and silicon addition will improve the A of the steel plate. C3 The addition of silicon effectively regulates the annealing process window during continuous annealing, ensuring an appropriate ratio of ferrite and austenite in the critical zone at industrial continuous annealing temperatures. Sufficient silicon content also reduces inclusions in the steel, inhibits the decomposition of retained austenite and the formation of carbides, and prevents the steel sheet from experiencing reduced mechanical properties and hole-expanding performance due to the decomposition of retained austenite and the formation of carbides. However, if the silicon content is too low, it will not be sufficient to guarantee the strength of the ferrite matrix and inhibit the decomposition of retained austenite and the formation of carbides; if the content is too high, it will affect the surface quality of hot-rolled steel, resulting in a large amount of iron oxide scale and impaired weldability. Therefore, the silicon content in this invention is 0.25%–0.65%.

[0015] Mn: Manganese strengthens the solid solution in steel by inducing lattice distortion through substitution 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 is unstable, reducing the plasticity, toughness, and hole expansion performance of the steel plate. Furthermore, the added manganese content should not exceed the scope of this invention. The main consideration is that excessive manganese content will lead to C or Mn segregation, which will worsen the uniformity of the steel plate structure during hot rolling and easily cause severe banded defects in the structure. In addition, excessive manganese in the steel involved in this invention will increase hardenability, inhibit bainite formation, and is also not conducive to hole expansion performance. Moreover, excessive manganese content will lead to poor weldability of the steel plate. Therefore, considering all factors, this invention selects a manganese content of 1.35% to 1.65%.

[0016] P: Phosphorus is an impurity element in steel. It tends to agglomerate at grain boundaries. When the phosphorus content in steel is high, Fe2P particles are easily formed, which reduces the plasticity, toughness and porosity of the steel. Therefore, the lower its content, the better. In order to obtain a higher elongation, its upper limit is set at 0.015%.

[0017] 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. It seriously affects the plasticity, formability and hole expansion performance of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.005%.

[0018] Al: In traditional steelmaking processes, aluminum is a deoxidizer. Al can also combine with nitrogen (N) in steel to form AlN, refining grains, inhibiting the decomposition of residual austenite, and, together with silicon (Si), suppressing cementite formation, increasing the austenitizing temperature, facilitating better selection of the process window, and accelerating bainite transformation. Excessive Al content will cause nozzle blockage during continuous casting, affecting production efficiency and increasing production costs. Therefore, in this invention, the Al content is limited to 0.020%–0.050%.

[0019] Ti: Titanium 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 steel. Since free nitrogen atoms in steel deteriorate the toughness of the steel plate, Ti combines with impurity elements of nitrogen in the steel to form TiN, thus TiN formation has a nitrogen-fixing effect. Furthermore, Ti also combines with C and N to form Ti(C,N), playing a role in grain refinement and precipitation strengthening. It can also strengthen ferrite and bainite, but excessive Ti content will lead to excessively large TiN sizes, deteriorating the steel plate properties and reducing the toughness of the weld heat-affected zone. To obtain excellent mechanical properties and hole-expanding performance, the optimal range of Ti content in this invention is between 0.025% and 0.055%.

[0020] Vanadium (V) has significant precipitation strengthening and grain refinement effects. Its effect is mainly achieved through the formation of precipitates with carbon and nitrogen, especially the VN precipitation formed with nitrogen, which greatly improves the strength of the steel plate. In addition, the retention of a large amount of V precipitates within the steel plate microstructure acts as a hydrogen trap, reducing the risk of delayed cracking in the Sb-containing steel plate of this invention during use. Furthermore, through the combined addition of V and Ti, a large amount of V-Ti composite carbides is retained in the Sb-containing steel plate of this invention, which then acts as a hydrogen trap, significantly improving the resistance to hydrogen-induced cracking during service. This results in a steel plate with excellent mechanical properties, hole-expanding properties, and resistance to hydrogen-induced cracking. Higher V content also leads to a decrease in the toughness of the weld heat-affected zone; therefore, the V addition amount in this invention is 0.020%–0.035%.

[0021] Ta: Tantalum has a strong affinity for elements such as carbon and nitrogen in steel, and can form fine and dispersed carbides, nitrides or carbonitrides. These compounds hinder grain growth, refine the grains of steel, and thus improve the comprehensive mechanical properties of steel, such as strength, toughness, plasticity and porosity. In addition, 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 substrate, thereby improving the corrosion resistance of steel. Therefore, the optimal range of Ta content in this invention is between 0.010% and 0.020%.

[0022] Bismuth (Bi) is mainly distributed in steel at grain boundaries and within grains, improving the strength of the steel plate, reducing the diffusion rate of elements such as carbon and oxygen at grain boundaries, minimizing decarburization and oxidation, and improving the surface and mechanical properties of the steel plate. Therefore, this invention limits the Bi content to 0.005%–0.014%.

[0023] Sb: Antimony can densify corrosion products and inhibit the formation of H2O, O2, and Cl. - and SO4 2- It diffuses into the steel matrix and can accumulate near the steel matrix in an acidic environment, promoting the formation of a uniform and dense oxide film (rich in elements such as Sb) on the surface of the steel plate, resisting further corrosion of the steel matrix. However, Sb is a low-melting-point element, which easily accumulates at grain boundaries, causing grain boundary cracks. As the Sb content increases, the risk of steel plate cracking increases rapidly. Therefore, this invention limits the Sb content to 0.010% to 0.050%.

[0024] Ni: Nickel is a solid solution strengthening element that can improve the hardenability of materials, prevent temper brittleness, and improve the fatigue performance of materials. It also enhances the corrosion resistance of steel and has no adverse effects on the hardening properties and toughness of the weld heat-affected zone. Furthermore, nickel is an austenite stabilizing element that promotes the retention of retained austenite. However, Ni is a valuable element, and its content should not be too high. Therefore, the Ni content is limited to 0.010%–0.030%.

[0025] Pt: Platinum has excellent chemical stability and corrosion resistance. In steel, platinum 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. Therefore, the present invention limits the Pt content to 0.001% to 0.006%.

[0026] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of grain boundaries, improve the hole expansion performance, and also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures. Therefore, the Y content is limited to 0.004% to 0.009% in this invention.

[0027] N: For the N content in steel, the lower the N content, the better. However, too low a content will lead to production difficulties and increased costs. However, this invention requires the precipitation of VN formed with V to carry out precipitation strengthening and grain refinement strengthening, thereby improving the strength and pore-expanding performance of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0028] A method for producing 635MPa grade steel for passenger vehicles includes smelting, hot rolling, wet sandblasting descaling, continuous annealing, and finishing processes, as detailed below:

[0029] 1) Smelting: The RH+LF process is adopted, and the H and O contents are strictly controlled, with H≤0.0002% and 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 casting speed is ≤2.0m / min and the light reduction is 2.0~5.0mm. This reduces the center segregation of the continuous casting billet, which is beneficial to reducing the banded structure in the subsequent hot-rolled steel plate.

[0030] 2) Hot rolling: The continuously cast slab with a thickness of 110-210 mm and a width of 1050-2010 mm is directly hot-loaded into a walking beam furnace for heating at a temperature of 1200-1220℃ and a holding time of 122-153 min. The chemical composition of this invention contains Sb. Sb has a low melting point and is prone to enrichment at grain boundaries, which can cause cracks. Therefore, the heating temperature should not be too high. The roughing process adopts a 3+3 rolling mode (R1 is rolled in 3 passes, and R2 is rolled in 3 passes), for a total of 6 passes. The exit temperature of the roughing mill is 1020℃~1080℃, the thickness of the intermediate billet is 35.0~52.0mm, and the width is 1050~2010mm. The intermediate billet is insulated with a heat preservation cover before entering the hot rolling finishing mill to reduce the temperature drop of the intermediate billet on the delay roller table and the temperature difference in the head, tail and width directions. The finishing mill is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing milling. The entry temperature of the finishing mill is not higher than 1020℃, and the final rolling temperature is 860~900℃. After the final rolling, laminar flow cooling mode is adopted, with a laminar flow cooling rate of 20~25℃ / s. After cooling to 480~520℃, it is coiled and air-cooled to room temperature. The purpose of laminar cooling to 480–520℃ is to rapidly generate a large amount of bainite, which inhibits grain growth while ensuring the bainite content, thereby refining the bainite grains. Furthermore, the reduction in the finishing mill is as follows: F1 reduction is 16–25 mm, F2 reduction is 7–12 mm, F3 reduction is 4–7 mm, F4 reduction is 2.0–3.5 mm, F5 reduction is 1.0–2.2 mm, F6 reduction is 0.6–1.4 mm, and F7 reduction is 0.2–0.5 mm, with a rolling thickness of 1.8–3.0 mm. After rolling, the microstructure of the hot-rolled steel plate has a ferrite volume percentage of 25%–45%, a bainite volume percentage of 45%–65%, and a pearlite volume percentage of 4%–10%.

[0031] 3) Wet sandblasting descaling treatment (EPS): After cooling the 1.8-3.0mm thick steel coil to room temperature, wet sandblasting is performed to descale the iron oxide scale on the steel plate surface. The main processes are uncoiling, tension leveling, sandblasting descaling treatment, and coiling. The uncoiling tension of the steel plate is 25-30kN, the tension leveling elongation is 1.0%-1.2%, the travel speed is 20-55m / min, the hardness of the steel grit is HRC≥85, the sandblasting motor speed is 2600-3000rpm, and the coiling tension is 55-85kN.

[0032] 4) Continuous annealing: The steel coil with a thickness of 1.8 to 3.0 mm after wet sandblasting and descaling is continuously annealed. The belt speed is controlled at 50 to 100 m / min, the temperature of the soaking zone is 795 to 825℃, the soaking time is 7 to 12 min, the slow cooling outlet temperature is 625 to 655℃, the rapid cooling rate is greater than 30℃ / s, and the rapid cooling outlet temperature is 400 to 425℃. The soaking temperature is 795–825℃ to ensure a suitable ratio of ferrite to austenite in the critical zone. If the soaking temperature is higher than 825℃, the proportion of ferrite in the microstructure will decrease significantly, reducing the plasticity of the steel. If the soaking temperature is lower than 795℃, the austenite content in the critical zone will be significantly insufficient, affecting the subsequent bainite content and leading to insufficient strength. The soaking time is 7–12 minutes to ensure sufficient recrystallization of the steel plate grains during the heating and holding stages and to avoid excessive time leading to grain growth. 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 625–655℃, which aims to promote the rapid formation of ferrite. This suppresses grain growth while ensuring the ferrite content, thereby refining the ferrite grains. The fast cooling outlet temperature is 400–425℃, which aims to promote the rapid and abundant formation of bainite. This suppresses grain growth while ensuring the bainite content, thereby refining the bainite grains.

[0033] 5) Finishing: After rapid cooling, the product is directly air-cooled into the finishing mill. Finishing is controlled by rolling force, which is controlled at 1600-3200kN and the rolling tension is 800-1900kN.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. Tantalum (Ta) has a strong affinity for elements such as carbon and nitrogen in steel, forming fine and dispersed carbides, nitrides, or carbonitrides. These compounds hinder grain growth, refine the steel grains, and thus improve the comprehensive mechanical properties of steel, such as strength, toughness, and plasticity, as well as the porosity. 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 substrate, thereby improving the corrosion resistance of the steel.

[0036] 2. Bismuth is mainly distributed in steel at grain boundaries and within grains, which helps to improve the strength of steel plates, reduce the diffusion rate of elements such as carbon and oxygen at grain boundaries, reduce decarburization and oxidation, and improve the surface and mechanical properties of steel plates.

[0037] 3. The addition of Sb can densify the corrosion products and inhibit the production of H2O, O2, and Cl. - and SO4 2-Isotropic diffusion into the steel substrate allows it to accumulate near the steel substrate in an acidic environment, promoting the formation of a uniform and dense oxide film (rich in elements such as Sb) on the surface of the steel plate substrate, resisting further corrosion of the steel substrate.

[0038] 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 steel and promote the retention of retained austenite.

[0039] 5. The addition of Pt to steel can react with oxygen to form a dense oxide protective film. This protective film can isolate the steel from direct contact with corrosive media, 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 at high temperatures.

[0040] The addition of 6.Y can refine the grains in steel, enhance the strength and plasticity of grain boundaries, improve the hole expansion performance, and also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures.

[0041] 7. This invention does not require the conventional hot rolling + pickling + cold rolling + continuous annealing + finishing process. Instead, it uses hot rolling + EPS (wet sandblasting descaling process) + continuous annealing + finishing process, which reduces the pickling + cold rolling process, resulting in fewer process steps, lower production costs, and no environmental pollution.

[0042] 8. The microstructure of the steel of the present invention consists of ferrite, bainite and retained austenite, which can significantly improve the hole expansion performance of the steel plate during the forming process.

[0043] 9. The present invention has excellent mechanical properties, with yield strength ≥484MPa, tensile strength ≥635MPa, transverse elongation A ≥25%, hole expansion rate ≥85%, transverse cold bending 180° D=a qualified, and surface roughness Ra is 1.4-1.9μm. The steel plate of the present invention is mainly suitable for manufacturing passenger vehicle structural parts. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0045] The embodiments of the invention are as follows:

[0046] The smelting process employs RH+LF technology, strictly controlling the H and O content: H ≤ 0.0002%, O ≤ 0.0015%. Calcium treatment is performed during the refining process. Electromagnetic stirring and light reduction techniques are used in the continuous casting process, with a billet casting speed ≤ 2.0 m / min and a light reduction of 2.0 mm-5.0 mm. Continuously cast slabs (110-210 mm thick × (1050~2010 mm) wide are directly hot-charged into a walking beam furnace for heating at 1200~1220℃ for 122~153 min. Rough rolling uses a 3+3 rolling pattern (R1 uses 3 passes, R2 uses 3 passes), for a total of 6 passes. The roughing mill exit temperature is 1020℃~1080℃, and the intermediate slab thickness is 35.0~52.0 mm. The width is 1050-2010mm. The intermediate billet is insulated with a heat preservation cover before entering the hot rolling finishing mill to reduce the temperature drop of the intermediate billet on the delay roller table and the temperature difference in the head, tail and width directions. The finishing mill is a 7-stand continuous rolling process. High pressure water descaling is performed before finishing milling. The entry temperature of the finishing mill is not higher than 1020℃. The final rolling temperature is 860-900℃. After the final rolling, laminar flow cooling mode is adopted. The laminar flow cooling rate is about 20-25℃ / s. After cooling to 480-520℃, it is coiled and air-cooled to room temperature. The finishing mill's reduction is as follows: F1 reduction is 16–25 mm, F2 reduction is 7–12 mm, F3 reduction is 4–7 mm, F4 reduction is 2.0–3.5 mm, F5 reduction is 1.0–2.2 mm, F6 reduction is 0.6–1.4 mm, and F7 reduction is 0.2–0.5 mm, with a rolling thickness of 1.8–3.0 mm. After cooling the 1.8–3.0 mm thick steel coils to room temperature, wet sandblasting is used to descale the iron oxide scale on the steel plate surface. The main processes are uncoiling, tension leveling, sandblasting descaling, and coiling. The uncoiling tension is 25–30 kN, the tension leveling elongation is 1.0–1.2%, the travel speed is 20–55 m / min, the steel shot hardness is HRC≥85, the sandblasting motor speed is 2600–3000 rpm, and the coiling tension is 55–85 kN. Steel coils with a thickness of 1.8–3.0 mm, treated by wet sandblasting and descaling, are continuously annealed. The belt speed is controlled at 50–100 m / min, the soaking temperature is 795–825℃, the soaking time is 7–12 min, the slow cooling exit temperature is 625–655℃, the rapid cooling rate is greater than 30℃ / s, and the rapid cooling exit temperature is 400–425℃. After rapid cooling, the coils are directly air-cooled and then fed into a finishing mill. Finishing is performed using rolling force control, with the rolling force controlled at 1600–3200 kN and the rolling tension at 600–1900 kN. The finished product thickness is 1.8–3.0 mm.

[0047] The specific components, hot rolling process, EPS (wet sandblasting descaling process), continuous annealing process, steel plate properties and volume percentage of the six embodiments of the present invention are shown in Tables 1-6.

[0048] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0049]

[0050] Table 2 Hot rolling process of the present invention embodiment

[0051]

[0052] Table 3 EPS (Wet Sandblasting Descaling Process) Regulations of the Invention Embodiments

[0053]

[0054] Table 4 Continuous annealing process regime of the present invention embodiments

[0055]

[0056] Table 5 Mechanical performance parameters of embodiments of the present invention

[0057]

[0058] Table 6. Percentage of tissue volume in embodiments of the present invention

[0059] .

Claims

1. A 635MPa grade steel for passenger vehicles, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.055%–0.085%, Si: 0.25%–0.65%, Mn: 1.35%–1.65%, Al: 0.020%–0.050%, Ti: 0.025%–0.055%, V: 0.020%–0.035%, Ta: 0.010%–0.020%, Bi: 0.005%–0.014%, Sb: 0.010%–0.050%, Ni: 0.010%–0.030%, Pt: 0.001%–0.006%, Y: 0.004%–0.009%, with P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities. The production method of the 635MPa grade passenger car steel includes smelting, hot rolling, wet sandblasting descaling, continuous annealing and finishing processes, as detailed below: Hot rolling: heating temperature 1200~1220℃, holding time 122~153min, roughing mill exit temperature 1020℃~1080℃, intermediate billet is insulated with heat preservation cover before entering hot rolling finishing mill, finishing mill inlet temperature not higher than 1020℃, finishing rolling temperature 860~900℃, after finishing rolling, laminar flow cooling mode is adopted, laminar flow cooling rate is 20~25℃ / s, after cooling to 480~520℃, coiled, and air-cooled to room temperature; Continuous annealing: The steel coils after wet sandblasting and descaling are continuously annealed. The belt speed is controlled at 50-100 m / min, the temperature of the soaking zone is 795-825℃, the soaking time is 7-12 min, the slow cooling outlet temperature is 625-655℃, the rapid cooling rate is greater than 30℃ / s, and the rapid cooling outlet temperature is 400-425℃.

2. The 635MPa grade passenger car steel according to claim 1, characterized in that, The microstructure of the finished steel plate consists of 25%–45% ferrite, 50%–70% bainite, and 3%–9% retained austenite.

3. The 635MPa grade passenger car steel according to claim 1, characterized in that, The steel plate has a yield strength ≥484MPa, tensile strength ≥635MPa, transverse elongation A ≥25%, hole expansion rate ≥85%, transverse cold bending 180° D=a is qualified, and surface roughness Ra is 1.40~1.90μm.

4. The 635MPa grade passenger car steel according to claim 1, characterized in that, The thickness of the steel plate is 1.8 to 3.0 mm.

5. A method for producing 635MPa grade passenger car steel as described in any one of claims 1-4, characterized in that, The processes include smelting, hot rolling, wet sandblasting descaling, continuous annealing, and finishing, as detailed below: Hot rolling: heating temperature 1200~1220℃, holding time 122~153min, roughing mill exit temperature 1020℃~1080℃, intermediate billet is insulated with heat preservation cover before entering hot rolling finishing mill, finishing mill inlet temperature not higher than 1020℃, finishing rolling temperature 860~900℃, after finishing rolling, laminar flow cooling mode is adopted, laminar flow cooling rate is 20~25℃ / s, after cooling to 480~520℃, coiled, and air-cooled to room temperature; Continuous annealing: The steel coils after wet sandblasting and descaling are continuously annealed. The belt speed is controlled at 50-100 m / min, the temperature of the soaking zone is 795-825℃, the soaking time is 7-12 min, the slow cooling outlet temperature is 625-655℃, the rapid cooling rate is greater than 30℃ / s, and the rapid cooling outlet temperature is 400-425℃.

6. The method for producing 635MPa grade passenger car steel according to claim 5, characterized in that, The roughing mill adopts a 3+3 rolling mode, with an intermediate billet thickness of 35.0–52.0 mm and a width of 1050–2010 mm. The finishing mill adopts a 7-stand continuous rolling mode, with the following reductions for the finishing mill: F1 reduction is 16–25 mm, F2 reduction is 7–12 mm, F3 reduction is 4–7 mm, F4 reduction is 2.0–3.5 mm, F5 reduction is 1.0–2.2 mm, F6 reduction is 0.6–1.4 mm, and F7 reduction is 0.2–0.5 mm.

7. A method for producing 635MPa grade passenger car steel according to claim 5 or 6, characterized in that, The microstructure of hot-rolled steel plates after rolling has a ferrite volume percentage of 25%–45%, a bainite volume percentage of 45%–65%, and a pearlite volume percentage of 4%–10%.

8. The method for producing 635MPa grade passenger car steel according to claim 5, characterized in that, The smelting process adopts the RH+LF process, with a billet casting speed ≤2.0m / min and a light pressing reduction of 2.0~5.0mm; the continuous casting billet specifications are 110~210mm thick × 1050~2010mm wide.

9. The method for producing 635MPa grade passenger car steel according to claim 5, characterized in that, The wet sandblasting descaling process mainly includes uncoiling, tension leveling, sandblasting descaling, and coiling. The uncoiling tension of the steel plate is 25-30 kN, the tension leveling elongation is 1.0%-1.2%, the travel speed is 20-55 m / min, the hardness of the steel shot is HRC≥85, the sandblasting motor speed is 2600-3000 rpm, and the coiling tension is 55-85 kN.

10. A method for producing 635MPa grade passenger car steel according to claim 5, characterized in that, The finishing process involves rapid cooling followed by direct air cooling before entering the finishing mill. The finishing rolling force is controlled between 1600 and 3200 kN, and the rolling tension is between 800 and 1900 kN.

Images