Economical high-surface-quality high-forming automobile steel and production method thereof

Abstract

The invention belongs to the field of metal materials, and particularly relates to economical high-surface-quality high-forming steel for an automobile and a production method of the economical high-surface-quality high-forming steel. The steel comprises the following chemical components in percentage by weight: 0.83%-1.00% of Si, 0.66%-0.82% of Al and the balance of Fe and inevitable impurities, and comprises the following components in percentage by weight: 0.83%-1.00% of Si, 0.66%-0.82% of Al and the balance of Ce. The yield strength of the steel plate is larger than or equal to 225 MPa, the tensile strength is larger than or equal to 270 MPa, the longitudinal elongation A is larger than or equal to 50%, the hole expansion rate is larger than or equal to 140%, 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 is 1.15-1.68 micrometers.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to an economical high-surface-quality, high-formability automotive steel 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 for users while meeting their requirements.

[0003] Chinese patent application CN103509997A discloses a 440MPa grade cold-rolled high-strength automotive structural steel and its manufacturing method. Its main chemical composition (mass percentage) is: carbon: 0.06%~0.12%, silicon: 0.25%, manganese: 1.0%~1.4%, aluminum: 0.015%~0.060%, phosphorus: 0.02%, sulfur: 0.01%, with the balance being Fe and unavoidable impurities. The yield strength is 270MPa~370MPa, the tensile strength is 440MPa~510MPa, and the elongation is 31%~38%. By increasing the carbon content to replace expensive metals, production costs are reduced. Furthermore, the process allows carbon and Fe to form fine, dispersed carbides, uniformly distributed within the ferrite, resulting in a high-strength cold-rolled automotive structural steel. However, this invention has a high Si content, leading to surface defects such as oxide streaks and color differences, and a low elongation after fracture.

[0004] Chinese patent application CN113145642A discloses a pickled steel plate and its preparation method. The pickled steel plate prepared by this invention has a surface roughness of 0.84-0.91 μm, a yield strength of 246-270 MPa, a tensile strength of 369-376 MPa, and an elongation of 35.5-42%. It exhibits a bright metallic color and is of good quality, and can replace some low-end cold-rolled steel plates in the manufacture of slide rails. While this invention provides a certain level of surface quality assurance by reducing the Si content (0.02-0.04%), it overlooks the impact of Si content on the reduction of ferrite matrix strength and the increased risk of inclusion formation in the steel. Summary of the Invention

[0005] To address the development needs in the automotive steel industry, this invention provides an economical, high-surface-quality, and highly formable automotive steel and its production method. The steel sheet has a yield strength ≥225MPa, tensile strength ≥270MPa, longitudinal elongation A ≥50%, hole expansion rate ≥140%, 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 1.15~1.68μm.

[0006] To achieve the above objectives, the present invention employs the following technical solution: An economical high-surface-quality, high-formability automotive steel has the following chemical composition by weight percentage: C: 0.008%–0.013%, Si: 0.83%–1.00%, Mn: 0.23%–0.98%, Al: 0.66%–0.82%, Ce: 0.0030%–0.0060%, 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 is 89%–100% ferrite, 0%–7% pearlite, and 0%–4% bainite.

[0008] The steel plate has a yield strength ≥225MPa, tensile strength ≥270MPa, longitudinal elongation A ≥50%, hole expansion rate ≥140%, and longitudinal cold bending of 180° D=a 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 1.15–1.68 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 improving the overall stability of the structure.

[0010] The main function of the economical high surface quality and high formability automotive steel composition in this invention is as follows: 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, carbon ensures the strength, 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 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. 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 range for carbon in this invention is 0.008% to 0.013%.

[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 elongation, local elongation, and porosity 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 1.00%.

[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, delaying the transformation of austenite to pearlite, and lowering the transformation temperature of bainite. However, excessive Mn content leads to Mn segregation, which can reduce the plasticity of steel, worsen the uniformity of steel plate structure during hot rolling, and easily cause severe banded structural defects in the structure, which is not conducive to the expansion performance. Therefore, after comprehensive consideration, this invention selects a manganese content of 0.23% to 0.98%.

[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, 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 0.010%.

[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. 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 0.004%.

[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 will be generated before the oxide of Si, avoiding the formation of Fe2SiO4. This is beneficial for the absence of iron oxide scale streaks and color difference defects on the steel plate surface during subsequent pickling. Therefore, in this invention, the Al content is limited to 0.66% to 0.82%.

[0016] 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 0.0030%–0.0060%.

[0017] 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. In addition, it is necessary to ensure a certain amount of AlN formation to refine the grains and improve the strength and elongation of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0018] An economical method for producing high-surface-quality, high-formability automotive steel includes smelting and continuous casting, hot rolling, 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 0.008% to 0.013%. Argon gas is blown for more than 3 minutes when tapping the steel (the argon blowing and sedation 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 16 minutes. Calcium treatment is carried out in the refining LF process. After that, slab continuous casting is carried out. The superheat of continuous casting is 15 to 20℃, and the continuous casting drawing speed is 1.1 to 1.6 m / 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 100-200A, and the secondary cooling water volume to 0.85-1.20L / 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 2.8-4.2mm (to reduce the central porosity and segregation of the billet). Stack the billets after casting for more than 36 hours (to reduce the accumulation of residual H, suppress the generation of microcracks inside the billet, and ensure the toughness of the steel plate).

[0019] (2) Hot rolling process: The 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 1208~1260℃ and a holding time of 125~182 min. 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 ≥1030℃. The thickness of the intermediate slab is 36.0~55.0 mm and the width is 1060~2100 mm. The intermediate slab is subjected to a process before entering the hot rolling finishing mill. The insulation cover provides insulation to reduce the temperature drop of the intermediate billet on the delay roller table and the temperature difference between the head and tail and the width of the plate. The finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing rolling. The entry temperature of the finishing rolling is 1030-1090℃, and the final rolling temperature is 845-895℃. After the final rolling, a laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 15℃ / s. After cooling to 615-656℃, air cooling is performed. After air cooling for 13-18s, rapid cooling is performed with a rapid cooling rate greater than 20℃ / s. After rapid cooling to 505-555℃, the plate is coiled. After laminar flow cooling to 615–656℃, air cooling for 13–18 seconds is performed to rapidly generate ferrite, inhibiting grain growth while ensuring ferrite content and thus refining the ferrite grains. Rapid cooling to 505–555℃ aims to rapidly generate a small amount of bainite, inhibiting grain growth while ensuring bainite content and thus refining the bainite grains. The final microstructure of the rolled steel plate has a ferrite volume percentage of 89%–100%, a pearlite volume percentage of 0%–7%, and a bainite volume percentage of 0%–4%.

[0020] (3) After coiling, heat to 410-456℃ in a bell-type furnace, hold for 4.5-7.5 minutes, 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 410-456℃, 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.

[0021] (4) Pickling process: Hydrochloric acid is used to continuously clean the iron oxide scale of 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 0.5-1.5%. The pickling solution is hydrochloric acid. The pickling tank is divided into 4 tanks. The concentration of the pickling solution in tank 1 is 55-75 g / L and the temperature of the pickling solution in tank 1 is 75.0-85.0℃. The concentration of the pickling solution in tank 2 is 80-105 g / L and the temperature of the pickling solution in tank 2 is 70.0-75.0℃. The concentration of the pickling solution in tank 3 is 115-135 g / L and the temperature of the pickling solution in tank 3 is 60.0-70.0℃. The concentration of the pickling solution in tank 4 is 140-160 g / L and the temperature of the pickling solution in tank 4 is 50.0-60.0℃. During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.10% to 0.15% of the pickling solution by weight. The rinsing water temperature is 52 to 63°C, and the pickling and rinsing speeds are controlled at 75 to 125 m / min. The pickling process is carried out under tension, with a tension of 45 to 60 kN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50 to 6.00 mm and a surface roughness Ra of 1.15 to 1.68 μm.

[0022] Compared with the prior art, the beneficial effects of the present invention are: 1. The purpose of adding Al 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 the steel or in the 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 that of Si, and Al2O3 will be formed before the oxide of Si, avoiding the formation of Fe2SiO4. This is beneficial for the subsequent pickling process, which is to prevent iron oxide scale streaks and color differences on the steel plate surface.

[0023] 2. 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.

[0024] 3. The microstructure of the steel of this invention consists of ferrite, pearlite and bainite, which significantly improves the hole expansion performance of the steel plate during the forming process.

[0025] 4. This invention has excellent mechanical properties, with a yield strength ≥225MPa, tensile strength ≥270MPa, longitudinal elongation A ≥50%, hole expansion rate ≥140%, and longitudinal cold bending 180° D=a is qualified. It uses conventional high Si content components, so there is no need to specially reduce the Si content. It is easy to smelt, has low cost, and has good surface quality, with no iron oxide scale streaks and color difference defects. The surface roughness Ra is 1.15~1.68μm. Detailed Implementation

[0026] 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.

[0027] 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 0.008% and 0.013%. Argon gas is blown for at least 3 minutes during tapping, followed by the LF+RH process. The H and O contents are strictly controlled: H ≤0.0002%, O ≤0.0015%. RH vacuum degassing is maintained for at least 16 minutes. In the refining LF process, calcium treatment is carried out, followed by slab continuous casting. The superheat of continuous casting is 15-20℃, the casting speed is 1.1-1.6m / min, the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 100A-200A, the secondary cooling water volume is 0.85L / kg-1.20L / kg, and light pressure is applied in the horizontal section of the secondary cooling zone, i.e., at the end of solidification, the reduction of the continuous casting slab is 2.8-4.2mm, and the slabs are stacked for more than 36 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 1208–1260℃ and a holding time of 125–182 min. The roughing rolling process uses a 3+3 rolling mode (R1 is rolled in 3 passes, R2 in 3 passes), for a total of 6 passes. The roughing mill exit temperature is ≥1030℃. The intermediate slab has a thickness of 36.0–55.0 mm and a width of 1060–2100 mm. An insulation cover is used to protect the intermediate slab before it enters the hot finishing mill. Temperature control is implemented to reduce the temperature drop of intermediate billets on the delayed roller table and the temperature difference between the head and tail and in the width direction. Finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing rolling. The entry temperature of the finishing mill is 1030–1090℃, and the final rolling temperature is 845–895℃. After final rolling, a laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 15℃ / s. After cooling to 615–656℃, air cooling is performed. After 13–18 seconds of air cooling, rapid cooling is performed at a rate greater than 20℃ / s. After rapid cooling to 505–555℃, the billets are coiled. After hot rolling and coiling, the billets are heated to 410–456℃ in a bell-type furnace, held for 4.5–7.5 minutes, and then cooled in the furnace. After cooling, the steel plates are uncoiled on the pickling line and pickled with hydrochloric acid. Before pickling, the steel plates are tension-straightened with an elongation of 0.5-1.5%. The pickling solution is hydrochloric acid, and the pickling tanks are divided into four tanks: Tank 1 has a concentration of 55-75 g / L and a pickling solution temperature of 75.0-85.0℃; Tank 2 has a concentration of 80-105 g / L and a pickling solution temperature of 70.0-75.0℃; Tank 3 has a concentration of 115-135 g / L and a pickling solution temperature of 60.0-70.0℃; and Tank 4 has a concentration of 140-160 g / L and a pickling solution temperature of 50.0-60.0℃.During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.10–0.15% of the pickling solution by weight. The rinsing water temperature is 52–63℃, and the pickling and rinsing speeds are controlled at 75–125 m / min. The pickling process is carried out under tension, with a tension of 45–60 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 1.15–1.68 μm.

[0028] The specific components, smelting process, hot rolling process, and bell-type furnace heating process of the six embodiments of the present invention are as follows: The pickling process, steel plate properties, and volume percentage are shown in Table 1-6.

[0029] 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. An economical automotive steel with high surface quality and high formability, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.008%–0.013%, Si: 0.83%–1.00%, Mn: 0.23%–0.98%, Al: 0.66%–0.82%, Ce: 0.0030%–0.0060%, with P ≤ 0.010%, S ≤ 0.004%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities. The aforementioned production method for economical, high-surface-quality, high-formability automotive steel includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes. Specific details include: The entry temperature of the finishing mill is 1030-1090℃, and the final rolling temperature is 845-895℃. After the final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 15℃ / s. After cooling to 615-656℃, air cooling is performed. After air cooling for 13-18s, rapid cooling is performed with a rapid cooling rate greater than 20℃ / s. After rapid cooling to 505-555℃, the coil is then taken. After winding, the product is heated to 410–456°C in a bell-type furnace, held at that temperature for 4.5–7.5 minutes, and then cooled with the furnace.

2. The economical high surface quality and high formability automotive steel according to claim 1, characterized in that, The microstructure of the finished steel plate is 89%–100% ferrite, 0%–7% pearlite, and 0%–4% bainite.

3. The economical high surface quality and high formability automotive steel according to claim 1, characterized in that, The steel plate has a yield strength ≥225MPa, tensile strength ≥270MPa, longitudinal elongation A ≥50%, hole expansion rate ≥140%, and longitudinal cold bending 180° D=a is qualified.

4. The economical high surface quality and high formability automotive steel according to claim 1, characterized in that, The surface roughness Ra of the steel plate is 1.15 to 1.68 μm.

5. A method for producing economical, high-surface-quality, high-formability automotive steel 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 entry temperature of the finishing mill is 1030-1090℃, and the final rolling temperature is 845-895℃. After the final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 15℃ / s. After cooling to 615-656℃, air cooling is performed. After air cooling for 13-18s, rapid cooling is performed with a rapid cooling rate greater than 20℃ / s. After rapid cooling to 505-555℃, the coil is then taken. After winding, the product is heated to 410–456°C in a bell-type furnace, held at that temperature for 4.5–7.5 minutes, and then cooled with the furnace.

6. The method for producing economical, high-surface-quality, high-formability automotive steel according to claim 5, characterized in that, In the smelting and continuous casting process: argon gas is blown for more than 3 minutes when tapping steel from the converter, RH vacuum degassing is maintained for more than 16 minutes, the superheat of continuous casting is 15-20℃, the continuous casting billet pulling speed is 1.1-1.6m / min, the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage is controlled at 100-200A, the secondary cooling water volume is 0.85-1.20L / kg, and a light pressure is applied at the end of solidification, with a billet reduction of 2.8-4.2mm.

7. The method for producing economical, high-surface-quality, high-formability automotive steel according to claim 5, characterized in that, The hot rolling process also includes: heating temperature of 1208~1260℃, holding time of 125~182min, roughing mill exit temperature ≥1030℃, intermediate billet thickness of 36.0~55.0mm, width of 1060~2100mm, and the intermediate billet is insulated with a heat preservation cover before entering the hot rolling finishing mill.

8. The method for producing economical, high-surface-quality, high-formability automotive steel according to claim 5, characterized in that, The pickling process includes: pre-pickling straightening of the steel plate with an elongation rate of 0.5%–1.5%; the pickling tank is divided into four tanks: Tank 1 has a concentration of 55–75 g / L and a pickling solution temperature of 75.0–85.0℃; Tank 2 has a concentration of 80–105 g / L and a pickling solution temperature of 70.0–75.0℃; Tank 3 has a concentration of 115–135 g / L and a pickling solution temperature of 60℃. The pickling temperature in tank 4 is 50.0-60.0℃, with a concentration of 140-160 g / L. During pickling, a corrosion inhibitor is added to the acid solution, accounting for 0.10%-0.15% of the weight of the pickling solution. The rinsing water temperature is 52-63℃, and the pickling and rinsing speeds are controlled at 75-125 m / min. The pickling process is conducted under tension, with a tension of 45-60 kN.

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