635MPa-grade passenger car steel and production method thereof

Abstract

The invention belongs to the field of metal materials, and particularly relates to steel for a 635MPa-grade passenger car and a production method of the steel, the steel comprises the following chemical components: C, Si, Mn, Al, Ti, V, Ta, Bi, Sb, Ni, Pt and Y, P is limited to be less than or equal to 0.015%, S is limited to be less than or equal to 0.005%, N is limited to be less than or equal to 0.006%, and the balance is Fe and inevitable impurities. The steel plate is obtained by adopting the processes of hot rolling, EPS, continuous annealing and finishing, the production process is simplified, and environmental pollution is avoided.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and in particular relates to a 635MPa grade passenger car steel and a production method thereof. 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: 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 + pickled + 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-strength and high-formability 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.

[0003] 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 hole expansion rate is ≥30%, and the hole expansion rate is small, which does not meet the requirements of high-formability 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] 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 is produced by adding a certain amount of Nb, Cr, and Mo to a conventional C-Mn composition system to produce a hot-rolled, pickled, cold-rolled, continuously annealed, and skin-passed steel plate. The steel plate has a yield strength of 350 to 430 MPa, a tensile strength of 590 to 700 MPa, and a hole expansion ratio of 50%. The yield strength and tensile strength fluctuate greatly, and the hole expansion ratio is small, which does not meet the requirements for high-formability passenger vehicle parts and does not have certain corrosion resistance and oxidation resistance. Furthermore, the steel is prepared through a hot-rolled, pickled, cold-rolled, continuous annealed, and skin-passed process, which has a large process flow and high production costs. In particular, pickling is required, and the pickling process has problems such as large acid mist pollution, environmental pollution, and high waste acid treatment costs. Summary of the Invention

[0005] The present invention aims to provide a 635MPa-grade passenger vehicle steel and its production method. The steel plate exhibits a yield strength of 484MPa or greater, a tensile strength of 635MPa or greater, a transverse elongation A of 25% or greater, a hole expansion ratio of 85% or greater, and a qualified transverse cold bend (D=a) at 180°. The steel also exhibits excellent surface quality, free of scale and color defects, and a surface roughness Ra of 1.40-1.90μm. Furthermore, the steel is produced through a hot rolling, EPS (wet sandblasting descaling) treatment, continuous annealing, and skin-passing process, eliminating the conventional hot rolling, pickling, cold rolling, continuous annealing, and skin-passing process. This eliminates the pickling and cold rolling steps, resulting in lower production costs and a pollution-free environment.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A 635MPa grade steel for passenger vehicles, wherein the chemical composition of the steel is as follows, 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 being limited to 0.015%, S being 0.005%, and N being 0.006%, with the balance being Fe and unavoidable impurities.

[0007] The microstructure of the finished steel plate is 25% to 45% by volume of ferrite, 50% to 70% by volume of bainite, and 3% to 9% by volume of retained austenite.

[0008] The steel plate has a yield strength of ≥484MPa, a tensile strength of ≥635MPa, a transverse elongation A ≥25%, a hole expansion rate ≥85%, a transverse cold bending 180°D=a that is qualified, and a surface roughness Ra of 1.40~1.90μm.

[0009] The thickness of the steel plate is 1.8~3.0mm.

[0010] The main functions of the composition of the 635MPa grade passenger car steel in the present invention are: 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. In the present invention, the addition of carbon and manganese elements ensures an industrially applicable austenitizing temperature, which promotes the stabilization of austenite in the critical zone. The role of carbon in the present invention can also ensure the stability of the residual austenite, 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 0.055% to 0.085%.

[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 decomposition of retained austenite and the formation of carbides, and avoid the steel plate from reducing the mechanical properties and hole expansion performance due to the decomposition of retained austenite and the formation of carbides. However, if the silicon content is too low, it will not ensure 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 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% 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 1.35% to 1.65%.

[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: Aluminum is a traditional deoxidizer in steelmaking. It also combines with nitrogen in steel to form AlN, which refines grains and inhibits the decomposition of retained austenite. It also works with silicon to inhibit 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] Titanium: Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine the grains, and improve the strength and toughness of the steel. Because free nitrogen atoms in the steel deteriorate the toughness of the steel plate, Ti combines with the impurity element nitrogen in the steel to form TiN. The formation of TiN has a nitrogen-fixing effect. Furthermore, Ti forms Ti(C, N) with carbon and nitrogen, which provides grain refinement and precipitation strengthening. It can also strengthen ferrite and bainite. However, excessive addition of Ti will result in excessively large TiN, deteriorating the performance of the steel plate and reducing the toughness of the weld heat-affected zone. To achieve excellent mechanical properties and hole expansion performance, the optimal range of Ti content in the present invention is between 0.025% and 0.055%.

[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 Ti allows for the retention of a large number of V and Ti composite carbides within the Sb-containing steel plate, 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 (HAZ), so the V addition in this invention is 0.020% to 0.035%.

[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 0.005% to 0.014%.

[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. Therefore, the present invention limits the Sb content to 0.010% to 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 0.010% to 0.030%.

[0022] 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 0.001% to 0.006%.

[0023] 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 0.004% to 0.009%.

[0024] 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%.

[0025] A production method for 635MPa grade passenger car steel includes smelting, hot rolling, wet sandblasting descaling, continuous annealing, and skin-passing steps, as follows: 1) Smelting: RH+LF process is adopted, and the 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 soft reduction technology are used in the continuous casting process. The casting speed is ≤2.0m / min, and the soft 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 rolling of hot-rolled steel plates.

[0026] 2) Hot rolling: The continuous casting slabs with a thickness of 110 to 210 mm and a width of 1050 to 2010 mm are directly hot-transferred and hot-charged into a walking beam heating furnace for heating at a temperature of 1200 to 1220° C. for a holding time of 122 to 153 minutes. The chemical composition of the present invention contains Sb, which has a low melting point and is easily enriched at grain boundaries to generate cracks. Therefore, the heating temperature should not be too high. The rough rolling adopts a 3+3 rolling process (R1 adopts 3 rolling passes, R2 adopts 3 rolling passes), with a total of 6 rolling passes. The rough rolling outlet temperature is 1020℃~1080℃, the intermediate billet thickness 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 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 1020℃. The final rolling temperature is 860~900℃. After final rolling, the laminar cooling mode is adopted, the laminar cooling rate is 20~25℃ / s, and it is coiled after cooling to 480~520℃ and air-cooled to room temperature. The purpose of laminar cooling to 480-520°C is to rapidly generate a large amount of bainite, while suppressing grain growth and ensuring the bainite content, thereby refining the bainite grains. Furthermore, the reduction of the finishing mill F1 is 16-25mm, the reduction of F2 is 7-12mm, 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 0.6-1.4mm, and the reduction of F7 is 0.2-0.5mm. The rolling thickness is 1.8-3.0mm. After rolling, the microstructure of the hot-rolled steel plate has a volume percentage of ferrite of 25%-45%, a volume percentage of bainite of 45%-65%, and a volume percentage of pearlite of 4%-10%.

[0027] 3) Wet sandblasting descaling (EPS): 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 25 to 30 kN, the straightening elongation is 1.0% to 1.2%, the travel speed is 20 to 55 m / min, the steel grit hardness is HRC ≥ 85, the sandblasting motor speed is 2600 to 3000 rpm, and the coiling tension is 55 to 85 kN.

[0028] 4) Continuous annealing: The steel coils with a thickness of 1.8 to 3.0 mm after wet sandblasting descaling are continuously annealed. The belt speed is controlled at 50 to 100 m / min, the soaking section temperature is 795 to 825°C, the soaking time is 7 to 12 minutes, the slow cooling outlet temperature is 625 to 655°C, the rapid cooling rate is greater than 30°C / s, and the rapid cooling outlet temperature is 400 to 425°C. The soaking section temperature is 795-825℃, the purpose is to ensure the appropriate phase ratio of ferrite and austenite in the critical zone. If the soaking section temperature is greater than 825℃, the ferrite ratio in the organization will be greatly reduced, reducing the plasticity of the steel. If the soaking section temperature is less than 795℃, the austenite content in the critical zone temperature stage is obviously insufficient, affecting the subsequent bainite content, which in turn leads to insufficient strength. The soaking time is 7-12min, the purpose is to ensure that the steel plate grains are fully recrystallized 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 time for continuous annealing and recrystallization process, resulting in a decrease in the elongation of the steel plate. The slow cooling outlet temperature is 625-655°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 400-425°C, the purpose of which is to quickly generate bainite in large quantities, while suppressing grain growth, and ensuring the bainite content, thereby refining the bainite grains.

[0029] 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 800~1900kN.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 5. 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.

[0035] 6. 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.

[0036] 7. 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.

[0037] 8. The microstructure of the steel of the present invention is ferrite, bainite and retained austenite, which can significantly improve the hole expansion performance of the steel plate during the forming process.

[0038] 9. The present invention has excellent mechanical properties, 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 1.4-1.9μm. The steel plate of the present invention is mainly suitable for manufacturing passenger car structural parts. DETAILED DESCRIPTION

[0039] 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.

[0040] The embodiments of the invention are as follows: The smelting process adopts the RH+LF process, strictly controlling the H and O contents, H≤0.0002% and O≤0.0015%. Calcium treatment is carried out during the refining process. Electromagnetic stirring and soft reduction technology are used in the continuous casting process. The casting speed is ≤2.0m / min, and the soft reduction is 2.0mm-5.0mm. The continuous casting slabs with a thickness of (110-210)mm and a width of (1050-2010mm) are directly hot-charged into a walking beam heating furnace for heating at a temperature of 1200-1220℃ and a holding time of 122-153min. The rough rolling adopts a 3+3 rolling process (R1 uses 3 rolling passes and R2 uses 3 rolling passes) for a total of 6 rolling passes. The rough rolling outlet temperature is 1020℃-1080℃, and the intermediate billet thickness is 35.0-52.0mm. The width is 1050~2010mm. The intermediate billet is kept warm by an insulation cover before entering 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 7-stand continuous rolling. High-pressure water descaling is used before finishing rolling. The finishing rolling entrance temperature is not higher than 1020℃. The finishing rolling temperature is 860~900℃. After finishing rolling, the laminar cooling mode is adopted. The laminar cooling rate is about 20~25℃ / s. After cooling to 480~520℃, it is coiled and air-cooled to room temperature. The finishing mill F1 has a reduction of 16-25mm, F2 has a reduction of 7-12mm, 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 0.6-1.4mm, 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 plate surface. The main processes include uncoiling, straightening, sandblasting, and coiling. The uncoiling tension is 25-30kN, the straightening elongation is 1.0-1.2%, the travel speed is 20-55m / min, the steel grit hardness is HRC ≥85, the sandblasting motor speed is 2600-3000rpm, and the coiling tension is 55-85kN. Continuous annealing is performed on wet-blast descaling coils with a thickness of 1.8 to 3.0 mm. The coil speed is controlled at 50 to 100 m / min, the soaking zone temperature is 795 to 825°C, the soaking time is 7 to 12 minutes, the slow cooling outlet temperature is 625 to 655°C, the rapid cooling rate is greater than 30°C / s, and the rapid cooling outlet temperature is 400 to 425°C. After rapid cooling, the coil is directly air-cooled before entering the skin-pass mill. Skin-passing utilizes rolling force control, ranging from 1600 to 3200 kN, and the rolling tension is 600 to 1900 kN. The finished product thickness is 1.8 to 3.0 mm.

[0041] 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.

[0042] Table 1 Chemical composition of the present invention (wt, %)

[0043] Table 2 Hot rolling process system of the embodiment of the present invention

[0044] Table 3 EPS (wet sandblasting descaling process) system of the embodiment of the present invention

[0045] Table 4 Continuous annealing process system of the embodiment of the present invention

[0046] Table 5 Mechanical properties parameters of the embodiments of the present invention

[0047] Table 6 Tissue volume percentage in the examples of the present invention .

Claims

1. A 635MPa grade passenger car steel, characterized in that: The chemical composition of the steel by weight percentage is: 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%, and P is limited to ≤0.015%, S≤0.005%, N≤0.006%, and the balance is Fe and unavoidable impurities.

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

3. The 635MPa grade passenger car steel according to claim 1, characterized in that: The steel plate has a yield strength of ≥484MPa, a tensile strength of ≥635MPa, a transverse elongation A ≥25%, a hole expansion rate ≥85%, a transverse cold bending 180°D=a that is qualified, and a surface roughness Ra of 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 1.8~3.0mm.

5. A method for producing 635 MPa grade passenger car steel 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 1200 ~ 1220 ℃, holding time 122 ~ 153min, rough rolling outlet temperature 1020 ℃ ~ 1080 ℃, 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 1020 ℃, the finishing temperature is 860 ~ 900 ℃, and the laminar cooling mode is adopted after the final rolling, the laminar cooling rate is 20 ~ 25 ℃ / s, and the coil is taken up after cooling to 480 ~ 520 ℃ and air-cooled to room temperature; Continuous annealing: The steel coils after wet sandblasting descaling are continuously annealed, with the belt speed controlled at 50-100 m / min, the soaking section temperature at 795-825°C, the soaking time at 7-12 min, the slow cooling outlet temperature at 625-655°C, the rapid cooling rate at more than 30°C / s, and the rapid cooling outlet temperature at 400-425°C.

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

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

8. The method for producing 635MPa grade passenger car steel according to claim 5, characterized in that: The smelting adopts RH+LF process, the casting billet pulling speed is ≤2.0m / min, the soft pressure reduction is 2.0-5.0mm; the specifications of the continuous casting billet 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 treatment: the main processes include uncoiling, straightening, sandblasting descaling and coiling, the steel plate uncoiling tension is 25 to 30 kN, the straightening elongation is 1.0% to 1.2%, the travel speed is 20 to 55 m / min, the steel sand hardness is HRC ≥ 85, the sandblasting motor speed is 2600 to 3000 rpm, and the coiling tension is 55 to 85 kN.

10. The method for producing 635MPa grade passenger car steel according to claim 5, characterized in that: The skin-passing process: after rapid cooling, the steel is directly air-cooled and enters the skin-passing mill. The skin-passing rolling force is controlled at 1600-3200 kN, and the rolling tension is 800-1900 kN.

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