V-Cr composite reinforced high-surface-quality steel plate for automobile and production method of V-Cr composite reinforced high-surface-quality steel plate

Abstract

The invention belongs to the field of metal materials, and particularly relates to a V-Cr composite reinforced high-surface-quality steel plate for an automobile and a production method of the V-Cr composite reinforced high-surface-quality steel plate. The steel comprises the following chemical components in percentage by weight: 0.83%-0.97% of Si, 0.77%-0.97% of Al, 0.93%-1.00% of Al / Si, and the balance of Fe and inevitable impurities, and further comprises the following components in percentage by weight: 0.83%-0.97% of Si, 0.77%-0.97% of Al, 0.93%-1.00% of Cr and the balance of Ce. The yield strength of the steel plate ranges from 433 MPa to 505 MPa, the tensile strength is larger than or equal to 560 MPa, the longitudinal elongation A ranges from 28% to 38%, longitudinal cold bending is conducted by 180 degrees, D = a, the steel plate is qualified, a conventional component with the high Si content is adopted, the Si content does not need to be specially reduced, smelting is easy, the cost is low, the surface quality is good, oxide scale stripes and color difference defects do not exist, and the surface roughness Ra ranges from 1.48 micrometers to 1.66 micrometers.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a V-Cr composite reinforced high surface quality steel sheet for automobiles and its production method. Background Technology

[0002] With the rapid development of the automotive industry, the demands for lightweighting, safety, and cost reduction are increasing. Hot-rolled pickled steel sheet is made from high-quality hot-rolled coils through processes such as uncoiling, pickling, rinsing, drying, edge trimming, and oiling. Hot-rolled pickled steel sheet offers a high cost-performance ratio, falling between cold-rolled and hot-rolled steel sheets. Its surface quality and application requirements are between those of hot-rolled and cold-rolled steel sheets, making it an ideal substitute for some types of both. Hot-rolled pickled steel sheet is widely used in automotive chassis, wheels, and body structural parts. Currently, given the challenging steel market conditions, it is one of the most profitable products for steel companies, effectively reducing procurement costs while meeting user requirements.

[0003] Chinese patent application CN106086634A discloses a method for producing structural grade galvanized steel strip with a yield strength ≥450MPa, belonging to the field of galvanized steel strip production technology. This invention employs a continuous galvanizing process to produce high-elongation structural grade galvanized steel strip with a yield strength ≥450MPa. Its chemical composition contains relatively expensive Nb, Ti, and Mo, while controlling the Si content to be extremely low, resulting in high smelting costs. Although it provides a certain guarantee for the surface quality of the steel sheet, it neglects the risks of reduced ferrite matrix strength and the formation of inclusions in the steel.

[0004] Chinese patent application CN117187667A discloses a high-yield-strength (780MPa) dual-phase steel for continuous annealing and its preparation method, belonging to the field of cold-rolled sheet and strip production. Its yield strength is 505–565MPa, tensile strength is 790–850MPa, elongation (A80) is 15.0–19.5%, yield-to-tensile ratio is 0.60–0.71, and porosity is 50%–75%. Its chemical composition contains high-cost Ti and B, and Si: 0.30%–0.45%, requiring special reduction of Si content, making it difficult to smelt, costly, and prone to surface defects such as oxide streaks and color differences, and also exhibiting low elongation after fracture. Summary of the Invention

[0005] To address the development needs in the automotive steel industry, this invention provides a V-Cr composite reinforced high surface quality steel sheet for automobiles and its production method. The steel sheet has a yield strength of 433–505 MPa, a tensile strength ≥560 MPa, a longitudinal elongation A of 28%–38%, and meets the acceptable longitudinal cold bending requirement of 180° (D=a). It uses a conventional composition with a high Si content, eliminating the need for special Si reduction, making it easy to smelt, low in cost, and exhibiting excellent surface quality with no oxide scale streaks or color difference defects. The surface roughness Ra is 1.48–1.66 μm.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A V-Cr composite reinforced high surface quality steel sheet for automobiles, wherein the chemical composition of the steel by weight percentage is: C: 0.090%~0.220%, Si: 0.83%~0.97%, Mn: 0.70%~1.10%, Al: 0.77%~0.97%, V: 0.030%~0.066%, Cr: 0.15%~0.45%, Ce: 0.0040%~0.0070%, and Al / Si: 0.93~1.00, with P≤0.010%, S≤0.004%, N≤0.006%, and the balance being Fe and unavoidable impurities.

[0007] The microstructure of the finished steel plate consists of 30%–40% ferrite, 55%–65% pearlite, and 1%–6% bainite.

[0008] The steel plate has a yield strength of 433-505 MPa, a tensile strength of ≥560 MPa, a longitudinal elongation A of 28%-38%, and a longitudinal cold bend of 180° D=a that is qualified (the diameter of the bending mandrel is equal to the thickness of the steel plate).

[0009] The surface roughness Ra of the steel plate is 1.48–1.66 μm. The roughness of the steel plate in this invention, while meeting the surface quality requirements for steel plates, can increase the frictional resistance during welding, which is beneficial for heat transfer and fusion during welding, improving weld quality. During spot welding, it can keep the contact resistance within a reasonable range, avoiding incomplete welds or burn-through due to poor contact, and improving weld strength and stability. It can also increase the friction coefficient of the friction surface, improving the anti-slip ability of the connection, thereby enhancing the overall stability of the structure.

[0010] The main function of the V-Cr composite reinforced high surface quality steel sheet composition for automobiles in this invention is as follows: C: Carbon is a common strengthening element in steel. Interstitial carbon atoms cause certain lattice distortions in the matrix, playing a role in solid solution strengthening. In this invention, carbon ensures the strength and formability 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 the steel plate to become brittle, posing a risk of delayed fracture and hot-rolling edge cracking, and also negatively impacting the weldability, plasticity, and toughness of the steel plate. In this invention, the overall carbon content is required to be within a low range, which helps reduce the risk of delayed fracture and hot-rolling edge cracking, and is also beneficial to the weldability of the steel plate. Therefore, the optimal range for carbon in this invention is 0.090% to 0.220%.

[0011] Si: Silicon is one of the key elements in this invention. Silicon dissolves in the crystal lattice, producing a solid solution strengthening effect, increasing the strength and yield point of the steel sheet, while maintaining good ductility, which is particularly important for high-strength passenger vehicle chassis components. It can also refine the grains, improving the uniform and local elongation of the steel sheet, making it less prone to cracking and wrinkling during stamping, which is beneficial for manufacturing complex passenger vehicle chassis components. Therefore, the silicon content in this invention is 0.83% to 0.97%.

[0012] 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, excessive Mn content leads to Mn segregation, which can reduce the plasticity of steel, worsen the uniformity of steel sheet structure during hot rolling, and easily cause severe banded structural defects in the structure, which is not conducive to formability. Therefore, considering all factors, this invention selects a manganese content of 0.70% to 1.10%.

[0013] 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 and toughness of the steel. Therefore, the lower its content, the better. In order to obtain a higher elongation, its upper limit is set at 0.010%.

[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 and formability of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.004%.

[0015] Al: When the Si content in steel is high, a red iron oxide scale, mainly composed of Fe2O3, will form on the surface of the steel plate during rolling. This is because Si promotes the formation of Fe-Si-O composite oxides, such as Fe2SiO4, at the interface between the iron oxide scale and the matrix. Fe2SiO4 will penetrate into the space between FeO and the matrix in a liquid state, and after solidification, it will exhibit an anchor-like morphology, firmly anchoring the FeO layer and making the iron oxide scale difficult to remove. The unremoved FeO layer will be crushed during subsequent hot rolling, increasing the contact area with air and accelerating the transformation process of FeO→Fe3O4→Fe2O3, ultimately forming a red iron oxide scale mainly composed of Fe2O3. Furthermore, during subsequent pickling, the steel plate surface will have iron oxide scale streaks and color difference defects. The role of adding Al in this invention is that Al's oxidation activity (i.e., its affinity for oxygen) is much higher than that of Si. Al is a strong deoxidizing element, and its ability to combine with oxygen is stronger at high temperatures. It will preferentially react with oxygen in the steel or in the air to first generate Al2O3. Si has relatively low oxidizing activity. Only after the oxidation reaction of Al is basically completed (or the oxygen supply is sufficient) will Si gradually combine with oxygen to form iron-silicon composite oxides (such as Fe2SiO4). Therefore, in the process of iron oxide scale formation, the oxidation reaction of Al occurs before Si, and Al2O3 is generated before the oxide of Si, avoiding the formation of Fe2SiO4. Furthermore, this invention contains Cr, and in the process of iron oxide scale formation, the oxidation reaction of Al occurs before Cr, and Al2O3 is generated before the oxide of Cr, avoiding the formation of FeCr2O4. This is beneficial for the absence of iron oxide scale streaks and color difference defects on the steel plate surface during subsequent pickling. Therefore, the Al content in this invention is limited to 0.77% to 0.97%, and the Al / Si ratio is satisfied as 0.93 to 1.00.

[0016] 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 during use and resulting in steel plates with excellent mechanical properties and resistance to hydrogen-induced cracking. Higher V content leads to poorer toughness in the weld heat-affected zone; therefore, the V addition amount in this invention is 0.030%–0.066%.

[0017] Cr: Chromium can effectively improve the hardenability and strength of steel. In addition, chromium can form a relatively dense protective layer on the surface of steel in corrosive environments, protecting the substrate and effectively improving the corrosion resistance of the steel. However, excessive chromium content in steel can deteriorate the toughness of the steel plate. Therefore, the optimal range of Cr content in this invention is between 0.15% and 0.45%.

[0018] Ce: Cerium has a strong affinity for harmful elements such as oxygen and sulfur in steel, forming stable oxides (e.g., Ce₂O₃) and sulfides (e.g., CeS). These compounds have high melting points and are easily removed from molten steel by flotation, reducing non-metallic inclusions and improving steel purity. For inclusions that cannot be completely removed, cerium can alter their morphology and distribution, transforming brittle, elongated sulfides into spherical or dot-shaped cerium sulfides, reducing the adverse effects of inclusions on the mechanical properties of steel, especially improving toughness and fatigue strength. The addition of cerium can inhibit grain growth during heating or cooling, refining austenite or ferrite grains, thereby improving the strength and toughness of the steel. Through purification and modification, it enhances the plasticity and toughness of steel and reduces the risk of cracking during hot and cold working. It can also improve the weldability and oxidation resistance of steel, thus increasing its service life at high temperatures. Therefore, this invention limits the Ce content to 0.0040%–0.0070%.

[0019] 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 needs to ensure a certain amount of AlN formation to refine the grains, improve the strength and elongation of the steel plate, and also needs VN precipitation to form with V for precipitation strengthening and grain refinement strengthening to improve the strength of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0020] A method for producing V-Cr composite reinforced high surface quality steel sheet for automobiles includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes. Specific details include: (1) Smelting process: The raw materials are pretreated with KR hot metal to control the S content to be less than 0.004%. After slag removal, they enter the converter. In the converter smelting, the double slag method is used to remove P, and the P content is controlled to be ≤0.010%. At the end of the converter smelting, the C content is controlled to be 0.090~0.220%. Argon gas is blown for more than 3.5 minutes when tapping the steel (the argon blowing and killing before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition). Then, the LF+RH process is carried out, and the H and O contents are strictly controlled, H≤0.0002% and O≤0.0015%. The RH vacuum degassing is maintained for more than 15 minutes. Calcium treatment is carried out in the refining LF process. After that, the slab is continuously cast. The superheat of continuous casting is ≤20℃ and the continuous casting drawing speed is 0.8~1.3m / min. (Reducing superheat and casting speed can improve macroscopic segregation of the billet, reduce the spacing of secondary dendrite arms in the solidification structure of the billet, and help reduce billet segregation and internal structural defects.) Control the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage to 160-260A, and the secondary cooling water volume to 0.90-1.30L / kg (to reduce the average carbon segregation index, suppress segregation, and limit the intensity of secondary cooling to suppress the tendency of central cracks in the billet to worsen). In the horizontal section of the secondary cooling zone, i.e., at the end of solidification, apply light pressure, and reduce the billet by 3.0-4.0mm (to reduce the central porosity and segregation of the billet). Stack the billets after casting for more than 48 hours (to reduce the accumulation of residual H, suppress the generation of microcracks inside the billet, and ensure the toughness of the steel plate).

[0021] The hot rolling process includes: (2) Hot rolling process: The continuously cast slab with a thickness of (170~230) mm × a width of (1060~2100) mm is loaded into a walking beam furnace for heating. The heating temperature is 1212~1272℃, and the holding time is 120~180 min. The rough rolling adopts a 3+3 rolling process (R1 is rolled in 3 passes, and R2 is rolled in 3 passes) for a total of 6 passes. The exit temperature of the rough rolling is ≥1070℃, and the thickness of the intermediate slab is 3 mm. The intermediate billets are 6.0–45.0 mm thick and 1060–2100 mm wide. Before entering the hot finishing mill, the intermediate billets are insulated with a heat-insulating cover to reduce temperature drop on the delay roller table and temperature differences at the head and tail and in the width direction. Finishing is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing. The entry temperature is 1030–1090℃, and the final rolling temperature is 870–930℃. A rapid cooling rate greater than 30℃ / s is used, and the billets are rapidly cooled to 601–642℃ before coiling. Rapid cooling to 601–642℃ aims to rapidly generate pearlite, inhibiting grain growth while ensuring sufficient pearlite content, thus refining the pearlite grains. The final microstructure of the rolled steel plate has a ferrite volume percentage of 30%–40%, a pearlite volume percentage of 55%–65%, and a bainite volume percentage of 1%–6%.

[0022] (3) After coiling, heat to 425-466℃ in a bell-type furnace, hold for 4.0-7.0 min, and then cool with the furnace. The purpose is to release residual stress. During hot rolling, uneven temperature and plastic deformation will cause residual stress inside the steel plate. At 425-466℃, atoms gain a small amount of energy and can release stress through dislocation movement, avoiding warping due to stress release when the steel plate is uncoiled and cut, thus optimizing the plate shape. It can also reduce the wear of shearing tools during processing, indirectly improving processing efficiency and reducing production material costs. In addition, the reaction rate of iron and oxygen is extremely slow in this temperature range. Combined with the nitrogen protective atmosphere commonly used in bell-type furnaces, it can minimize the formation of iron oxide scale, preserve the original surface state of the steel plate, and reduce the difficulty and cost of subsequent pickling surface treatment.

[0023] (4) Pickling process: Hydrochloric acid is used for continuous cleaning of iron oxide scale on the steel plate through the coil. After hot rolling, the steel plate is uncoiled and pickled on the pickling line. Before pickling, the steel plate is tension leveled with an elongation of 0.8% to 1.2%. The pickling solution is hydrochloric acid. The pickling tank is divided into 3 tanks. The concentration of the pickling solution in tank 1 is 80 to 125 g / L, and the temperature of the pickling solution in tank 1 is 78.0 to 88.0℃. The concentration of the pickling solution in tank 2 is 115 to 135 g / L, and the temperature of the pickling solution in tank 2 is 75.0 to 80.0℃. The concentration of the pickling solution in tank 3 is 140 to 150 g / L. The pickling solution temperature in tank 3 is 65.0–70.0℃. During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.09%–0.14% of the weight of the pickling solution. The rinsing water temperature is 50–60℃, and the pickling and rinsing speeds are controlled at 80–120 m / min. The pickling process is carried out under tension, with a tension of 33–50 kN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50–6.00 mm and a surface roughness Ra of 1.48–1.66 μm.

[0024] Compared with the prior art, the beneficial effects of the present invention are: 1. The purpose of adding Al in this invention is that Al's oxidizing activity (i.e., its affinity for oxygen) is much higher than that of Si. Al is a strong deoxidizing element, and its ability to combine with oxygen is even stronger at high temperatures. It will preferentially react with oxygen in steel or air to first form Al2O3. Si, on the other hand, has relatively low oxidizing activity. Only after the oxidation reaction of Al is basically complete (or the oxygen supply is sufficient) will it gradually combine with oxygen to form iron-silicon composite oxides (such as Fe2SiO4). Therefore, in the process of iron oxide scale formation, the oxidation reaction of Al occurs before Si, and Al2O3 will be formed before the oxide of Si, avoiding the formation of Fe2SiO4. Furthermore, this invention contains Cr. In the process of iron oxide scale formation, the oxidation reaction of Al occurs before Cr, and Al2O3 will be formed before the oxide of Cr, avoiding the formation of FeCr2O4. This is beneficial for the steel plate surface to be free of iron oxide scale streaks and color difference defects during subsequent pickling.

[0025] 2. The addition of vanadium mainly enhances the strength of the steel plate by forming VN precipitation with nitrogen. It can also retain a large amount of V precipitation phase inside the steel plate structure as a hydrogen trap, reducing the risk of delayed cracking during use. This results in a steel plate with excellent mechanical properties and resistance to hydrogen-induced cracking.

[0026] 3. Chromium can effectively improve the hardenability and strength of steel. In corrosive environments, chromium can form a relatively dense protective layer on the surface of steel, protecting the substrate and effectively improving the corrosion resistance of the steel.

[0027] 4. Cerium has a strong affinity for harmful elements such as oxygen and sulfur in steel, forming stable oxides (e.g., Ce₂O₃) and sulfides (e.g., CeS). These compounds have high melting points and are easily removed from molten steel by flotation, reducing non-metallic inclusions and improving steel purity. For inclusions that cannot be completely removed, cerium can alter their morphology and distribution, transforming brittle, elongated sulfides into spherical or dot-shaped cerium sulfides, reducing the adverse effects of inclusions on the mechanical properties of steel, especially improving toughness and fatigue strength. The addition of cerium can inhibit grain growth during heating or cooling, refining austenite or ferrite grains, thereby improving the strength and toughness of the steel. Through purification and modification, it enhances the plasticity and toughness of steel and reduces the risk of cracking during hot and cold working. It can also improve the weldability and oxidation resistance of steel, thus extending its service life at high temperatures.

[0028] 5. The microstructure of the steel of this invention consists of ferrite, pearlite and bainite, which significantly improves the comprehensive mechanical properties of the steel plate during the forming process.

[0029] 6. This invention has excellent mechanical properties. The yield strength of the steel plate is 433-505 MPa, the tensile strength is ≥560 MPa, the longitudinal elongation A is 28%-38%, the longitudinal cold bending 180° D=a is qualified, it uses conventional high Si content composition, no special Si content reduction is required, it is easy to smelt, the cost is low, and the surface quality is good, without iron oxide scale streaks and color difference defects. The surface roughness Ra of the steel plate is 1.48-1.66 μm. Detailed Implementation

[0030] 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.

[0031] The specific embodiments of the present invention are as follows: The smelting raw materials undergo KR hot metal pretreatment to control the sulfur content below 0.004%. After slag removal, the material enters the converter. During converter smelting, a double-slag method is used to remove phosphorus (P), controlling the P content to ≤0.010%. At the end of converter smelting, the carbon content is controlled between 0.090% and 0.220%. Argon gas is blown for at least 3.5 minutes during tapping, followed by the LF+RH process. H and O contents are strictly controlled: H ≤0.0002%, O ≤0.0015%. RH vacuum degassing is maintained for 15 minutes. For n and above, calcium treatment is carried out in the refining LF process, followed by slab continuous casting. The superheat of continuous casting is ≤20℃, the continuous casting drawing speed is 0.8~1.3m / min, the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage is controlled at 160A~260A, the secondary cooling water volume is 0.90L / kg~1.30L / kg, and light pressure is applied in the horizontal section of the secondary cooling zone, that is, at the end of solidification. The reduction of the continuous casting billet is 3.0~4.0mm, and the billets are stacked for more than 48 hours after leaving the line. A continuously cast slab with a thickness of (170-230) mm and a width of (1060-2100) mm is loaded into a walking beam furnace for heating at a temperature of 1212-1272℃ for a holding time of 120-180 min. The roughing process employs a 3+3 rolling pattern (R1 is rolled in 3 passes, and R2 is rolled in 3 passes), for a total of 6 passes. The roughing mill exit temperature is ≥1070℃, and the intermediate slab thickness is 36.0-45.0 mm. The intermediate billets are 1060-2100 mm in diameter and 1060-2100 mm in width. Before entering the hot finishing mill, an insulation cover is used to reduce the temperature drop of the intermediate billets on the delay roller table and the temperature difference between the head and tail and in the width direction. Finishing is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing. The entry temperature of the finishing mill is 1030-1090℃, and the final rolling temperature is 870-930℃. The rapid cooling rate is greater than 30℃ / s. After rapid cooling to 601-642℃, the billets are coiled. After hot rolling and coiling, the billets are heated to 425-466℃ in a bell-type furnace, held for 4.0-7.0 min, and then cooled in the furnace. After cooling, the steel plate is uncoiled and pickled on the pickling line. Before pickling, the steel plate is tension-leveled with an elongation of 0.8–1.2%. The pickling solution is hydrochloric acid, and the pickling tanks are divided into three tanks: Tank 1 has a concentration of 80–125 g / L and a temperature of 78.0–88.0℃; Tank 2 has a concentration of 115–135 g / L and a temperature of 75.0–80.0℃; and Tank 3 has a concentration of 140–150 g / L and a temperature of 65℃. During pickling at 0.0–70.0℃, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.09%–0.14% of the weight of the pickling solution. The rinsing water temperature is 50–60℃, and the pickling and rinsing speeds are controlled at 80–120 m / min. The pickling process is carried out under tension, with a tension of 33–50 kN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50–6.00 mm and a surface roughness Ra of 1.48–1.66 μm.

[0032] The specific components, smelting process, hot rolling process, bell-type furnace heating process, pickling process, steel plate properties, and volume percentage of the six embodiments of the present invention are shown in Tables 1-6.

[0033] Table 1 Chemical composition (wt, %) of the embodiments of the present invention: Table 2 Smelting process of the present invention embodiment: Table 3 Hot rolling process and bell-type furnace heating process of the present invention: Table 4. Pickling process parameters of the present invention embodiments: Table 5 Mechanical performance parameters of embodiments of the present invention: Table 6. Tissue volume percentage in the embodiments of the present invention: .

Claims

1. A V-Cr composite reinforced high surface quality steel sheet for automobiles, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.090%–0.220%, Si: 0.83%–0.97%, Mn: 0.70%–1.10%, Al: 0.77%–0.97%, V: 0.030%–0.066%, Cr: 0.15%–0.45%, Ce: 0.0040%–0.0070%, and Al / Si: 0.93–1.00, with P ≤ 0.010%, S ≤ 0.004%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities. The production method of the aforementioned V-Cr composite reinforced high surface quality steel sheet for automobiles includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes, specifically including: The hot rolling process includes: a finishing mill inlet temperature of 1030-1090℃, a finishing mill temperature of 870-930℃, a rapid cooling rate of more than 30℃ / s, and coiling after rapid cooling to 601-642℃. After winding, the product is heated to 425–466°C in a bell-type furnace, held for 4.0–7.0 minutes, and then cooled with the furnace.

2. The V-Cr composite reinforced high surface quality steel sheet for automobiles according to claim 1, characterized in that, The microstructure of the finished steel plate consists of 30%–40% ferrite, 55%–65% pearlite, and 1%–6% bainite.

3. The V-Cr composite reinforced high surface quality steel sheet for automobiles according to claim 1, characterized in that, The steel plate has a yield strength of 433-505 MPa, a tensile strength of ≥560 MPa, a longitudinal elongation A of 28%-38%, and a longitudinal cold bending of 180° D=a is qualified.

4. The V-Cr composite reinforced high surface quality steel sheet for automobiles according to claim 1, characterized in that, The surface roughness Ra of the steel plate is 1.48 to 1.66 μm.

5. A method for producing V-Cr composite reinforced high surface quality steel sheet for automobiles as described in any one of claims 1-4, characterized in that, This includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes. Specific details include: The hot rolling process includes: a finishing mill inlet temperature of 1030-1090℃, a finishing mill temperature of 870-930℃, a rapid cooling rate of more than 30℃ / s, and coiling after rapid cooling to 601-642℃. After winding, the product is heated to 425–466°C in a bell-type furnace, held for 4.0–7.0 minutes, and then cooled with the furnace.

6. The method for producing V-Cr composite reinforced high surface quality steel sheet for automobiles according to claim 5, characterized in that, In the smelting and continuous casting process: argon gas is blown for more than 3.5 minutes when tapping steel from the converter, RH vacuum degassing is maintained for more than 15 minutes, the superheat of continuous casting is ≤20℃, the continuous casting billet pulling speed is 0.8~1.3m / min, the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage is controlled at 160~260A, the secondary cooling water volume is 0.90~1.30L / kg, a light pressure is applied at the end of solidification, the continuous casting billet reduction is 3.0~4.0mm, and the billets are stacked for more than 48 hours after leaving the line.

7. The method for producing V-Cr composite reinforced high surface quality steel sheet for automobiles according to claim 5, characterized in that, The hot rolling process also includes: a heating temperature of 1212~1272℃, a holding time of 120~180min, a roughing mill exit temperature ≥1070℃, an intermediate billet thickness of 36.0~45.0mm, a width of 1060~2100mm, and an insulation cover for the intermediate billet before it enters the hot rolling finishing mill.

8. The method for producing V-Cr composite reinforced high surface quality steel sheet for automobiles according to claim 5, characterized in that, The pickling process includes: a tensile elongation rate of 0.8%–1.2%; the pickling tank is divided into three tanks: tank 1 has a concentration of 80–125 g / L and a pickling solution temperature of 78.0–88.0℃; tank 2 has a concentration of 115–135 g / L and a pickling solution temperature of 75.0–80.0℃; tank 3 has a concentration of 140–150 g / L and a pickling solution temperature of 65.0–70.0℃; during pickling, a corrosion inhibitor is added to the acid solution, with the corrosion inhibitor accounting for 0.09%–0.14% of the weight of the pickling solution; the rinsing water temperature is 50–60℃; the pickling and rinsing speeds are controlled at 80–120 m / min; the pickling process is under tension, with a tension of 33–50 kN.

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