High-surface-quality steel plate for automobile and production method of high-surface-quality steel plate

Abstract

The invention belongs to the field of metal materials, and particularly relates to a high-surface-quality steel plate for an automobile and a production method of the high-surface-quality steel plate. The steel comprises the following chemical components in percentage by weight: 0.65%-0.75% of Si, 0.76%-0.83% of Al, 1.01%-1.17% of Al / Si and the balance of Fe and inevitable impurities, and further comprises the following components in percentage by weight: 0.65%-0.75% of Si, 0.76%-0.83% of Al, 1.01%-1.17% of Ce and the balance of Fe. The yield strength of the steel plate ranges from 235 MPa to 315 MPa, the tensile strength ranges from 370 MPa to 450 MPa, the longitudinal elongation A ranges from 33% to 50%, 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 1.70 micrometers to 1.91 micrometers.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a 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 CN102703808B discloses a 300MPa grade steel for automotive structural components and a method for producing the same. Its main components and their weight percentage content are as follows: C: 0.06~0.08%, Si: 0.05~0.09%, Mn: 0.30~0.45%, Al: 0.025~0.055%, P: 0.015~0.025%, S≤0.006%, Nb: 0.010~0.019%, N≤0.006%. Under the premise of meeting the requirements of yield strength of 300~340MPa and tensile strength of 390~445MPa, the elongation is not less than 31%, the plastic strain ratio r: 1.45~1.80, the strain hardening index n: 0.16~0.20, the bending test is 180°, the bending mandrel diameter d=0, and in the metallographic structure, equiaxed ferrite is 85~90% and granular pearlite is 10~15%. The invention adds a high-cost Nb alloy with extremely low Si content, which provides a certain guarantee for the surface quality of the steel plate, but ignores the risks of reduced strength of the ferrite matrix and the formation of inclusions in the steel.

[0004] Chinese patent application CN115595503A discloses a hot-rolled pickled steel sheet for high-hardness axial grids. Its alloy composition, by weight percentage, is: 0.05–0.08% C, ≤0.05% Si, 0.2–0.4% Mn, 0.02–0.07% Als, P≤0.025%, S≤0.025%, N≤0.007%, with the balance being Fe and unavoidable inclusions. The preparation method includes hot rolling, cold rolling, and pickling processes. This hot-rolled pickled steel sheet has a yield strength of 200–280 MPa, a tensile strength of 300–360 MPa, an elongation of 40–50%, and a hardness value (HRB) of 100–125. However, the Si content in this invention is 0.2–0.4%, requiring special reduction, which makes smelting difficult, increases smelting costs, and easily results in oxidation streaks and color differences on the surface. Summary of the Invention

[0005] To address the development needs in the automotive steel industry, this invention provides a high surface quality steel sheet for automobiles and its production method. The steel sheet has a yield strength of 235–315 MPa, a tensile strength of 370–450 MPa, a longitudinal elongation A of 33%–50%, and meets the acceptable requirements for longitudinal cold bending at 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 with good surface quality, free of iron oxide scale streaks and color difference defects. The surface roughness Ra is 1.70–1.91 μm.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A high surface quality steel sheet for automobiles, wherein the chemical composition of the steel, by weight percentage, is: C: 0.060%–0.095%, Si: 0.65%–0.75%, Mn: 0.55%–1.00%, Al: 0.76%–0.83%, Ce: 0.0040%–0.0090%, and Al / Si: 1.01–1.17, and P ≤ 0.010%, S ≤ 0.004%, N ≤ 0.006%, with the balance being Fe and unavoidable impurities.

[0007] The microstructure of the finished steel plate is 95%–100% ferrite by volume and 0%–5% pearlite by volume.

[0008] The steel plate has a yield strength of 235-315 MPa, a tensile strength of 370-450 MPa, a longitudinal elongation A of 33%-50%, 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 1.70–1.91 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 high surface quality steel sheet composition for automobiles in this invention is as follows: C: Carbon is a common strengthening element in steel. Interstitial solid-solution 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 0.060% to 0.095%.

[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 0.65% to 0.75%.

[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 the formability. Therefore, after comprehensive consideration, the present invention selects a manganese content of 0.55% to 1.00%.

[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 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 0.76% to 0.83%, and the Al / Si ratio is satisfied as 1.01 to 1.17.

[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 0.0040%–0.0090%.

[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] A method for producing high surface quality steel sheets for automobiles includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes, specifically including: 1) The raw material undergoes KR hot metal pretreatment to control the sulfur content below 0.004%. After slag removal, it 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 0.060% and 0.095%. Argon gas is blown for at least 4 minutes during tapping (argon blowing and calming before continuous casting promotes the removal of inclusions in the molten steel and improves the uniformity of the steel composition). Next, the LF+RH process is carried out, strictly controlling the H and O contents: H ≤0.0002%, O ≤0.0015%. RH vacuum degassing is maintained for at least 17 minutes. Calcium treatment is performed in the refining LF process, followed by slab continuous casting. The superheat during continuous casting is ≤20℃, and the casting speed is 0.8–1.3 m / min (to reduce...). Overheating and reducing the continuous casting speed can improve the macroscopic segregation of the continuously cast 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 180-280A, and the secondary cooling water volume to 0.90-1.15L / 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, a light pressure is applied, and the billet reduction is 4.5-5.0mm (to reduce the central porosity and segregation of the billet). The billets are stacked for more than 48 hours after being removed from the line (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: A continuously cast slab (170–230 mm thick × 1060–2100 mm wide) is loaded into a walking beam furnace and heated to 1182–1222℃ for 120–180 min. The roughing process uses a 3+3 rolling pattern (R1 is rolled in 3 passes, R2 in 3 passes), for a total of 6 passes. The roughing mill exit temperature is ≥1060℃, and the intermediate slab thickness is 33.0–5 mm. The intermediate billet has a diameter of 0.0 mm and a width of 1060–2100 mm. Before entering the hot finishing mill, an insulation cover is used 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 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 1040–1060℃, and the final rolling temperature is 865–925℃. The rapid cooling rate is greater than 25℃ / s. After rapid cooling to 608–658℃, the billet is coiled. Rapid cooling to 608–658℃ aims to rapidly generate ferrite, inhibiting grain growth while ensuring the ferrite content, thus refining the ferrite grains. The final microstructure of the rolled steel plate has a ferrite volume percentage of 95%–100% and a pearlite volume percentage of 0%–5%.

[0020] (3) After coiling, heat to 286-388℃ in a bell-type furnace, hold for 4.5-7.0 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 286-388℃, 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 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 0.8-1.5%. The pickling solution is hydrochloric acid. The pickling tank is divided into 3 tanks. The concentration of the pickling solution in tank 1 is 85-120 g / L, and the temperature of the pickling solution in tank 1 is 77.0-85.0℃. The concentration of the pickling solution in tank 2 is 110-130 g / L, and the temperature of the pickling solution in tank 2 is 70.0-79.0℃. The concentration of the pickling solution in tank 3 is 135-155 g / L. The pickling solution temperature in tank 3 is 60.0–65.0℃. During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.08–0.13% of the weight of the pickling solution. The rinsing water temperature is 50–60℃, and the pickling and rinsing speeds are controlled at 95–160 m / min. The pickling process is 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 1.70–1.91 μm.

[0022] 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 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 even stronger at high temperatures. It will preferentially react with oxygen in the steel or in the air to first generate Al2O3. Si, on the other hand, has relatively low oxidation 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 generated before the oxide of Si, avoiding the formation of Fe2SiO4. This is beneficial for the steel plate surface to be free of iron oxide scale streaks and color difference defects during subsequent pickling.

[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 in this invention is ferrite and pearlite, which significantly improves the mechanical properties of the steel plate during the forming process.

[0025] 4) This invention possesses excellent mechanical properties, with a yield strength of 235–315 MPa, a tensile strength of 370–450 MPa, a longitudinal elongation A of 33%–50%, and a qualified longitudinal cold bending 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 producing a steel plate with good surface quality, free of oxide scale streaks and color difference defects, and a surface roughness Ra of 1.70–1.91 μm. 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 0.060% and 0.095%. Argon gas is blown for at least 4 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 17 minutes. The above process involves calcium treatment in the refining LF process, followed by slab continuous casting. The superheat of the continuous casting is ≤20℃, the casting speed is 0.8~1.3m / min, the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 180A~280A, the secondary cooling water volume is 0.90L / kg~1.15L / 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 4.5~5.0mm, and the slabs are stacked for more than 48 hours after being taken off 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 1182-1222℃ for a holding time of 120-180 min. The roughing rolling process uses 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 ≥1060℃, and the intermediate slab thickness is 33.0-50.0 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 1040-1060℃, and the final rolling temperature is 865-925℃. The rapid cooling rate is greater than 25℃ / s. After rapid cooling to 608-658℃, the billets are coiled. After hot rolling, the coils are heated to 286-388℃ in a bell-type furnace, held for 4.5-7.0 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-straightened with an elongation of 0.8–1.5%. The pickling solution is hydrochloric acid, and the pickling tanks are divided into three tanks: Tank 1 has a concentration of 85–120 g / L and a pickling solution temperature of 77.0–85.0℃; Tank 2 has a concentration of 110–130 g / L and a pickling solution temperature of 70.0–79.0℃; and Tank 3 has a concentration of 135–155 g / L and a pickling solution temperature of 60℃. During pickling at 0–65.0℃, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.08–0.13% of the pickling solution by weight. The rinsing water temperature is 50–60℃, and the pickling and rinsing speeds are controlled at 95–160 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 1.70–1.91 μm.

[0028] 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.

[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. A high surface quality steel sheet for automobiles, characterized in that the chemical composition of the steel, by weight percentage, is: C: 0.060%–0.095%, Si: 0.65%–0.75%, Mn: 0.55%–1.00%, Al: 0.76%–0.83%, Ce: 0.0040%–0.0090%, and Al / Si: 1.01~1.17, with P≤0.010%, S≤0.004%, N≤0.006%, and the balance being Fe and unavoidable impurities.

2. The high surface quality steel sheet for an automobile according to claim 1, characterized by, The microstructure of the finished steel plate is 95%–100% ferrite by volume and 0%–5% pearlite by volume.

3. The high surface quality steel sheet for an automobile according to claim 1, characterized by, The steel plate has a yield strength of 235-315 MPa, a tensile strength of 370-450 MPa, a longitudinal elongation A of 33%-50%, and a longitudinal cold bending of 180° D=a is qualified.

4. The high surface quality steel sheet for an automobile according to claim 1, characterized by, The surface roughness Ra of the steel plate is 1.70 to 1.91 μm.

5. A method of producing a high surface quality steel sheet for an automobile as claimed in any one of claims 1 to 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 1040-1060℃, a finishing mill temperature of 865-925℃, a rapid cooling rate of more than 25℃ / s, and rapid cooling to 608-658℃ before coiling. After winding, the coils are heated to 286–388°C in a bell-type furnace, held for 4.5–7.0 minutes, and then cooled in the furnace.

6. The method for producing 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 4 minutes when tapping steel from the converter, RH vacuum degassing is maintained for more than 17 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 180~280A, the secondary cooling water volume is 0.90~1.15L / kg, a light pressure is applied at the end of solidification, the reduction of the continuous casting billet is 4.5~5.0mm, and the billets are stacked for more than 48 hours after leaving the line.

7. The method for producing high surface quality steel sheet for automobiles according to claim 5, characterized in that, The hot rolling process also includes: a heating temperature of 1182~1222℃, a holding time of 120~180min, a roughing mill exit temperature ≥1060℃, an intermediate billet thickness of 33.0~50.0mm, 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 high surface quality steel sheet for automobiles according to claim 5, characterized in that, The pickling process includes: straightening the steel plate before pickling, with a straightening elongation of 0.8% to 1.5%; the pickling tank is divided into three tanks: tank 1 has a concentration of 85 to 120 g / L and a pickling solution temperature of 77.0 to 85.0℃; tank 2 has a concentration of 110 to 130 g / L and a pickling solution temperature of 70.0 to 79.0℃; tank 3 has a concentration of 135 to 155 g / L and a pickling solution temperature of 60.0 to 65.0℃; during pickling, a corrosion inhibitor is added to the acid solution, with the corrosion inhibitor accounting for 0.08% to 0.13% of the weight of the pickling solution; the rinsing water temperature is 50 to 60℃; and the pickling and rinsing speeds are controlled at 95 to 160 m / min.

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