A method and system for controlling surface defects in automotive outer panels

Abstract

This invention discloses a method and system for controlling surface defects in automotive outer panels. The method includes: performing a calming operation before ladle pouring, controlling the pouring speed at 0.9-1.5 m / min; during ladle pouring, controlling the immersion pouring of the intermediate ladle and adding a covering agent after pouring, adjusting the three argon gas supply based on liquid level fluctuations, replacing the immersion nozzle based on the rising trend of the stopper rod or the liquid level fluctuations, and controlling the intermediate ladle to maintain a set tonnage during the pouring and ladle changing processes; controlling the ladle to perform a steel retention operation until the ladle pouring is completed; and controlling the machine cleaning equipment to perform machine cleaning treatment on the qualified slabs obtained from the pouring.

Description

Technical Field

[0001] This application relates to the field of steelmaking process technology, and in particular to a method and system for controlling surface defects in automotive outer panels. Background Technology

[0002] Currently, with the continuous improvement of road regulations, automakers are becoming increasingly strict in their control over vehicle quality. For example, automotive outer panels are extremely sensitive to surface defects such as inclusions, zinc slag, and "caterpillars" in the no-primer coating process, requiring steel mills to have the capability to manufacture high-end outer panels with "zero defects," such as the Boyue Facial Cover, thereby improving vehicle quality.

[0003] Against the backdrop of fierce competition in the automotive manufacturing market, the cost of producing cars is strictly limited. Therefore, more and more automakers are using methods such as process optimization and material substitution to reduce costs and increase efficiency. For example, they are controlling the cost of automotive outer panels by thinning and replacing materials.

[0004] However, this thinning method will affect the surface quality of the slab. Inclusions and slag defects on or inside the material are prone to cracking under high forming conditions, which will lead to defects on the surface or subsurface of the slab.

[0005] Therefore, how to eliminate defects on the surface or subsurface of slabs is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method and system for controlling surface defects in automotive outer panels. By optimizing various continuous casting processes, the occurrence rate of steelmaking inclusions and slag contamination defects on the surface of high-end automotive panels is reduced, thereby eliminating surface or subsurface defects in the slab and ensuring the slab's pass rate.

[0007] To address the aforementioned technical problems, a first aspect of the present invention discloses a method for controlling surface defects in automotive outer panels, the method comprising:

[0008] Before pouring the steel ladle, a calming operation should be performed, and the casting speed should be controlled at 0.9-1.5 m / min.

[0009] During the steel ladle casting process, the intermediate ladle is controlled to start the immersion pouring and the intermediate ladle covering agent is added after the start of pouring. The three argon gas lines are adjusted according to the liquid level fluctuation. The immersion nozzle is replaced according to the upward trend of the stopper rod or the liquid level fluctuation. The intermediate ladle is controlled to maintain the set tonnage during the pouring and ladle changing processes.

[0010] Control the ladle to perform the steel retention operation until the ladle pouring is completed;

[0011] The control cleaning equipment performs mechanical cleaning on the qualified slabs obtained from casting.

[0012] Preferably, the calming operation performed before ladle pouring specifically includes:

[0013] The sedation operation is performed before the steel ladle is poured, and the sedation time is controlled between 25 min and 40 min.

[0014] Preferably, the pulling speed is controlled at 0.9-1.5 m / min, specifically including:

[0015] Different casting speeds are controlled according to different casting section specifications; among which:

[0016] 1600≤cast section≤1800, casting speed controlled at 0.9-1.1m / min;

[0017] For casting sections with a diameter of 1300 ≤ casting cross-section < 1600, the casting speed should be controlled at 1.2-1.3 m / min.

[0018] For casting sections with a diameter of 1000 ≤ casting section < 1300, the casting speed should be controlled at 1.4-1.5 m / min.

[0019] Preferably, the control of the intermediate tundish immersion pouring and the addition of an intermediate tundish covering agent after pouring specifically includes:

[0020] The immersion depth of the tundish long nozzle should be greater than 100mm.

[0021] The amount of covering agent added to the intermediate ladle: 3.75 kg / t of steel is added to the middle of the baffle wall during the start-up furnace, and 32.5 kg / t of steel is added to each side of the baffle wall. For the intermediate continuous casting furnace, 0.625 kg / t of steel is added to the middle part of the baffle wall for each furnace.

[0022] Preferably, the three argon gas channels are: argon gas at the top water inlet, argon gas at the stopper rod, and argon gas between the plates;

[0023] The adjustment of the three argon gas streams based on liquid level fluctuations specifically includes:

[0024] When the first furnace is started, the argon gas flow rates at the top water inlet, stopper rod, and between plates are controlled at 4L / min, 4L / min, and 10L / min, respectively.

[0025] For each slab whose liquid level fluctuates and the fluctuation range is outside of [-5mm, +5mm], the flow rates of argon gas at the inlet and stopper rod will be reduced by [0.5L / min, 3L / min] respectively.

[0026] If the argon gas level of the slab fluctuates again after the argon gas at the top of the water inlet and the stopper rod is adjusted to the lowest level, and the fluctuation range is outside [-5mm, +5mm], then the argon gas flow rate between the slabs will be reduced once for each slab with a fluctuation frequency of [2L / min, 6L / min].

[0027] Preferably, replacing the submersible nozzle based on the upward trend of the stopper rod or the fluctuation of the liquid level specifically includes:

[0028] Using the bar position at the constant casting speed stage as the reference bar position height, if the stopper bar rises by more than 15mm during casting; or if the slab liquid level fluctuates outside the range of [-5mm, +5mm] and continuously reaches 2 slabs, replace the submerged nozzle on the intermediate slab of the next furnace.

[0029] Preferably, controlling the tundish to maintain a set tonnage during the casting and ladle changing processes specifically includes:

[0030] During normal casting, the tundish tonnage is greater than 85% of the nominal tonnage, while during ladle changing, the tundish tonnage is at least 80% of the nominal tonnage.

[0031] Preferably, the control of the ladle to perform the steel retention operation until the ladle pouring is completed specifically includes:

[0032] The amount of steel left in the ladle should be controlled to be more than 8% of the ladle weight until the ladle is completely poured.

[0033] Preferably, after the controlled cleaning equipment performs mechanical cleaning on the qualified slab obtained from casting, the method further includes:

[0034] Check whether the machine-cleaned slab meets the following requirements:

[0035] The surface of the machine-cleaned slab is guaranteed to be bright, and there are no abnormal defects such as melt grooves, molten steel, or slag.

[0036] The height of the raised ribs on the surface of the machine-cleaned slab is less than 2mm;

[0037] The cleaning depth of the edge area within 50mm of the edge of the machine-cleaned slab reaches more than 2% of the total thickness of the slab;

[0038] The middle part, which is more than 50mm away from the edge of the machine-cleaned slab, reaches more than 3.4% of the total thickness of the slab.

[0039] A second aspect of the present invention discloses a surface defect control system for an automotive outer panel, the system comprising:

[0040] The calming control unit is used to perform calming operations before the ladle is poured, and the casting speed is controlled at 0.9-1.5 m / min;

[0041] The casting control unit is used to control the immersion casting of the tundish and add tundish covering agent after casting, adjust the three argon gas according to the liquid level fluctuation, replace the immersion nozzle according to the stopper rod rising trend and the liquid level fluctuation, and control the tundish to maintain the set tonnage during the casting and ladle changing processes.

[0042] The steel retention control unit is used to control the ladle to perform the steel retention operation until the ladle is completely poured;

[0043] The machine cleaning equipment unit is used to perform machine cleaning on qualified slabs obtained from casting.

[0044] Through one or more technical solutions of the present invention, the present invention has the following beneficial effects or advantages:

[0045] This invention discloses a method and system for controlling surface defects in automotive outer panels. The method involves performing a calming operation before ladle casting, controlling the casting speed at 0.9-1.5 m / min; during ladle casting, controlling the tundish immersion start-up and adding tundish covering agent after start-up; adjusting the three-way argon gas supply based on liquid level fluctuations; replacing the immersion nozzle based on the stopper rod's upward trend or the liquid level fluctuations; maintaining the set tonnage of the tundish during casting and ladle changing; controlling the ladle to perform a steel retention operation until ladle casting is complete; and controlling the machine cleaning equipment to perform machine cleaning treatment on the qualified slabs obtained from casting. The technical solution of this invention optimizes each process in continuous casting, reducing the incidence of steelmaking inclusions and slag contamination defects on the surface of high-end automotive panels, eliminating surface or subsurface defects in slabs, and thus ensuring the slab qualification rate.

[0046] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0047] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0048] In the attached diagram:

[0049] Figure 1 A schematic flowchart of a method for controlling surface defects in an automotive outer panel according to an embodiment of the present invention is shown.

[0050] Figure 2 A schematic diagram of a surface defect control system for an automotive outer panel according to an embodiment of the present invention is shown. Detailed Implementation

[0051] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0052] This invention discloses a method for controlling surface defects in automotive outer panels, see below. Figure 1 The method includes:

[0053] Step 101: Perform a calming operation before pouring the steel ladle, and control the casting speed at 0.9-1.5 m / min.

[0054] In this embodiment, if the sedation time is too short, inclusions in the steel cannot float sufficiently to the surface; if the time is too long, the top slag of the ladle reacts with the molten steel, leading to secondary oxidation and affecting the cleanliness of the molten steel. Considering the above factors, the sedation time is controlled between 25 and 40 minutes.

[0055] In this embodiment, the casting speed is controlled at 0.9-1.5 m / min. If the casting speed is too low, the temperature in the crystallizer is low, and inclusions are easily captured by the solidified billet shell. If the casting speed is too high, the inclusions do not have enough time to float to the billet.

[0056] Furthermore, the casting speed control can be formulated with reference to the specifications of the casting section, with different casting section specifications controlling different casting speeds. Specifically, for casting sections of 1600 ≤ casting section ≤ 1800, the casting speed should be controlled at 0.9-1.1 m / min; for casting sections of 1300 ≤ casting section < 1600, the casting speed should be controlled at 1.2-1.3 m / min; and for casting sections of 1000 ≤ casting section < 1300, the casting speed should be controlled at 1.4-1.5 m / min.

[0057] Step 102: During the ladle casting process, control the immersion-type start-up of the tundish and add tundish covering agent after start-up. Adjust the three argon gas supply according to the liquid level fluctuation. Replace the immersion nozzle according to the rising trend of the stopper rod or the liquid level fluctuation. Control the tundish to maintain the set tonnage during the casting and ladle changing processes.

[0058] In the casting process of this embodiment, the above-mentioned measures were strictly implemented to prevent defects from occurring on the surface or subsurface of the slab.

[0059] The following is a detailed introduction to each measure.

[0060] During the immersion pouring process in the tundish, the immersion depth of the long nozzle of the tundish should be controlled to be >100mm.

[0061] The amount of covering agent added to the intermediate ladle: 3.75 kg / t of steel is added to the middle of the baffle wall during the start-up furnace, and 32.5 kg / t of steel is added to each side of the baffle wall. For the intermediate continuous casting furnace, 0.625 kg / t of steel is added to the middle part of the baffle wall for each furnace.

[0062] The three argon gas sources in this embodiment are: argon gas at the top water inlet, argon gas at the stopper rod, and argon gas between the plates.

[0063] During the adjustment of the three argon gas flows based on liquid level fluctuations, if the first casting furnace is started, the argon flow rates at the top water inlet, stopper rod, and inter-plate should be controlled at 4 L / min, 4 L / min, and 10 L / min, respectively. Furthermore, for each slab liquid level fluctuation outside the range of [-5 mm, +5 mm], the flow rates of the top water inlet and stopper rod argon should be reduced by [0.5 L / min, 3 L / min] each time. If the slab liquid level fluctuates again after the top water inlet and stopper rod argon are adjusted to the lowest level, and the fluctuation range is outside the range of [-5 mm, +5 mm], the flow rate of the inter-plate argon should be reduced by [2 L / min, 6 L / min] for each slab liquid level fluctuation.

[0064] In the process of replacing the submersible nozzle in conjunction with the upward trend of the stopper rod or the fluctuation of the liquid level, the upward trend of the stopper rod and the fluctuation of the liquid level are considered as parallel conditions, and the submersible nozzle needs to be replaced if either of them is triggered.

[0065] Specifically, when replacing the submerged nozzle based on the upward trend of the stopper rod, the nozzle position is taken as the reference position height when the casting reaches the constant casting speed stage. If the stopper rod rises more than 15mm during casting, the submerged nozzle is replaced on the intermediate billet of the next heat.

[0066] When replacing the submerged nozzle based on the liquid level fluctuation, if the slab liquid level fluctuation range is outside [-5mm, +5mm] and continuously reaches 2 slabs, replace the submerged nozzle on the intermediate slab of the next furnace.

[0067] When controlling the tundish to maintain the set tonnage during the casting and ladle changing processes, the tundish adopts high-tonnage casting. The tonnage of the tundish during normal casting is greater than 85% of the nominal tonnage, and the tonnage of the tundish during the ladle changing process is at least 80% of the nominal tonnage.

[0068] By optimizing the various operational procedures and parameters of the intermediate batch, cracking defects caused by inclusions can be reduced.

[0069] Step 103: Control the ladle to perform the steel retention operation until the ladle pouring is completed.

[0070] In this embodiment, the amount of steel left in the ladle is controlled to be more than 8% of the ladle weight until the ladle is completely poured.

[0071] Step 104: Control the cleaning equipment to perform mechanical cleaning on the qualified slab obtained from casting.

[0072] In this embodiment, all slabs shall undergo machine cleaning.

[0073] Before cleaning, it is necessary to confirm the parameters of the cleaning equipment and its operating condition.

[0074] Before cleaning, the mechanical cleaning equipment must meet the following parameters: the power air source pressure of the mechanical cleaning equipment is 6.5 kg / cm². 2 The above ensures smooth burner closure; the burner purification and purging air pressure is 0.1 kg / cm². 2 The above, with a maximum not exceeding 0.2 kg / cm². 2 .

[0075] Before cleaning, the machine cleaning equipment must meet the following conditions: the gap between the upper and lower blocks of the online burner is less than 1mm; if it exceeds the usage range, use copper pads to make it less than 1mm; the online burner shoe block has ≥3 wear-resistant strips; replace shoe blocks that do not meet the requirements; the gap between adjacent shoe blocks is ≤3mm; if it is greater than 3mm, use copper sheets to fill it; the number of online burner blocks used is less than 800; cleaning the slab is prohibited if more than 800 blocks are used; the online burner must not have water leakage or flame leakage.

[0076] The parameters of the cleaning equipment during cleaning need to meet the following conditions: preheating time control: for slabs with a temperature above 500℃, the preheating time should not exceed 25s; for slabs with a temperature between 400℃ and 500℃, the preheating time should not exceed 35s; for slabs with a temperature below 400℃, the preheating time should not exceed 50s. If the preheating time requirement is exceeded, check the status of the burners at the corresponding positions and the supply status of oxygen and natural gas pressure.

[0077] Furthermore, during the cleaning process, the difference between the actual pressure and the set pressure is monitored in real time. When the difference exceeds 0.2 g / cm³, the pressure is immediately checked. 2 If the valve is 100% open, cleaning should be stopped immediately and the cause investigated. If two consecutive valves experience leakage at the same location, cleaning should be stopped immediately and the cause investigated.

[0078] After the machine cleaning process, check whether the machine-cleaned slab meets the following requirements: the surface of the machine-cleaned slab is guaranteed to be bright and free of abnormal defects such as molten grooves, molten steel, and slag; the height of the protrusions on the surface of the machine-cleaned slab is less than 2mm; the machine cleaning depth in the edge area within 50mm of the edge of the machine-cleaned slab reaches more than 2% of the total slab thickness; and the middle part beyond 50mm of the edge of the machine-cleaned slab reaches more than 3.4% of the total slab thickness.

[0079] If there are any leaks in the slab, manual cleaning and polishing are necessary. The surface must be free of residual slag and peeling. The pits that need to be manually cleaned must be polished smooth.

[0080] Deep cleaning of qualified slabs can reduce cracking defects caused by inclusions.

[0081] To further illustrate and explain the technical solution of the present invention, the following comparative examples and embodiments are used for comparison.

[0082] Example 1

[0083] For heat No. 1, the pre-casting calming time was 28 minutes, the cross-section was 1100mm, and the casting speed was 1.5m / min. The immersion depth of the tundish nozzle was 110mm, and the tundish covering agent was added as required after casting began. During production, the three argon gas supply was controlled at 3.5 / 3.5 / 10L / min respectively. During the third heat casting, the slab liquid level fluctuated by ≥±5mm for two consecutive slabs, and the immersion nozzle was replaced. During normal casting, the tundish tonnage was 86% of the nominal capacity, and during ladle replacement, the tundish weight was 66% of the nominal capacity. The steel retention before the ladle was finished was 9%. After cleaning, the slab surface was free of slag and had a bright finish. The edge cleaning depth was 2.1% of the total slab thickness, and the center cleaning depth was 3.5% of the total slab thickness.

[0084] Example 2

[0085] For heat No. 2, the pre-casting calming time was 30 minutes, the cross-section was 1800mm, and the casting speed was 1.0m / min. The immersion depth of the long nozzle in the tundish was 115mm. After casting began, the tundish covering agent was added as required. During production, the three argon gas flows were controlled at 3 / 3 / 6L / min respectively. During the casting of the fourth heat, the stopper rod position rose by 16mm, and the submerged nozzle was replaced. During normal casting, the tundish tonnage was 88% of the nominal capacity, and during ladle replacement, the tundish weight was 69% of the nominal capacity. The steel retention before the ladle was finished was 10%. After cleaning, the slab surface was free of slag and had a bright finish. The edge cleaning depth was 2.2% of the total slab thickness, and the center cleaning depth was 3.55% of the total slab thickness.

[0086] Comparative Example 1

[0087] For heat No. 1, the pre-casting calming time for the ladle was 45 minutes, the cross-section was 1400 mm, and the casting speed was 1.35 m / min. The immersion depth of the long nozzle in the tundish was 105 mm. After casting began, the tundish covering agent was added as required. During production, the three argon gas flows were 4 / 4 / 10 L / min respectively. When casting the 5th heat, the liquid level of a slab fluctuated by ≥±5 mm, and the three argon gas flows were reduced to 2.5 / 2.5 / 5 L / min respectively. During normal casting, the tundish tonnage was 86% of the nominal capacity, and during ladle changes, the tundish weight was 66% of the nominal capacity. The steel retention before the ladle was finished was 9%. After cleaning, the slab surface was free of slag and had a bright finish. The edge cleaning depth was 2.1% of the total slab thickness, and the center cleaning depth was 3.5% of the total slab thickness.

[0088] Comparative Example 2

[0089] For heat No. 2, the pre-casting calming time for the ladle was 35 minutes, the cross-section was 1600 mm, and the casting speed was 1.1 m / min. The immersion depth of the long nozzle in the tundish was 115 mm. After casting began, the tundish covering agent was added as required. During production, the three argon gas flows were controlled at 3 / 3 / 6 L / min respectively. During the casting of the fourth heat, the stopper rod position rose by 10 mm. During normal casting, the tundish tonnage was 88% of the nominal capacity, and during ladle changing, the tundish weight was 69% of the nominal capacity. The steel left in the ladle before casting was 10%. The burner gap reached 2 mm. After cleaning, there were melt groove defects on the slab surface. The edge cleaning depth was 2.1% of the total slab thickness, and the middle cleaning depth was 4% of the total slab thickness.

[0090] The effects of the examples and comparative examples are shown in Table 1.

[0091] Table 1

[0092]

[0093] Through process optimization of the above technical solutions, the overall success rate of outer panel production can be improved, quality degradation can be reduced, and economic benefits of 20 million yuan can be achieved; the occurrence rate of small defects can be reduced, customer satisfaction can be improved, and the market share of high-end automotive products can be increased.

[0094] The technical solution of this invention optimizes each process in continuous casting, which can reduce the occurrence rate of steelmaking inclusions and slag defects on the surface of high-end automotive steel plates, eliminate defects on the surface or subsurface of the slab, and thus ensure the qualification rate of the slab.

[0095] Based on the same inventive concept as one or more of the foregoing embodiments, the following embodiments disclose a surface defect control system for automotive outer panels, see below. Figure 2 The system includes:

[0096] The calming control unit 201 is used to perform calming operations before steel ladle pouring, and the casting speed is controlled at 0.9-1.5 m / min;

[0097] The casting control unit 202 is used to control the immersion casting of the intermediate ladle and add the intermediate ladle covering agent after the casting process, adjust the three argon gas according to the liquid level fluctuation, replace the immersion nozzle according to the rising trend of the stopper rod and the liquid level fluctuation, and control the intermediate ladle to maintain the set tonnage during the casting and ladle changing processes.

[0098] The steel retention control unit 203 is used to control the ladle to perform the steel retention operation until the ladle is completely poured.

[0099] The machine cleaning equipment unit 204 is used to perform machine cleaning on qualified slabs obtained from casting.

[0100] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0101] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for controlling surface defects in automotive outer panels, characterized in that, The method includes: Before pouring the steel ladle, a calming operation should be performed, and the casting speed should be controlled at 0.9-1.5 m / min. During the ladle casting process, the submerged tundish is controlled for initial pouring, and a covering agent is added after pouring. The three argon gas lines are adjusted based on liquid level fluctuations. The submerged nozzle is replaced based on the rising trend of the stopper rod or the liquid level fluctuations. The tundish is maintained at a set tonnage during the pouring and ladle changing processes. The three argon gas lines are: argon gas at the top nozzle, argon gas at the stopper rod, and argon gas between the plates. Adjusting the three argon gas lines based on liquid level fluctuations specifically includes: during the initial pouring of the first furnace, the argon gas at the top nozzle, the stopper rod, and the argon gas between the plates are supplied at rates of 4 L / min, 4 L / min, and 10 L / min, respectively. L / min control; for each slab level fluctuation outside the range of [-5mm, +5mm], the flow rates of argon gas at the inlet and stopper rod are reduced by [0.5L / min, 3L / min] each time; if the slab level fluctuates again after the argon gas at the inlet and stopper rod are adjusted to the minimum, and the fluctuation range is outside the range of [-5mm, +5mm], the flow rate of argon gas between slabs is reduced by [2L / min, 6L / min] each time the slab level fluctuates. Control the ladle to perform the steel retention operation until the ladle pouring is completed; The machine cleaning equipment is used to clean the qualified slabs obtained from casting. The machine-cleaned slabs are checked to ensure that they meet the following requirements: the surface of the machine-cleaned slabs is in a bright state and there are no abnormal defects such as molten grooves, molten steel, or molten slag; the height of the protrusions on the surface of the machine-cleaned slabs is less than 2mm; the machine cleaning depth in the edge area within 50mm of the edge of the machine-cleaned slabs reaches more than 2% of the total thickness of the slabs; and the middle part more than 50mm from the edge of the machine-cleaned slabs reaches more than 3.4% of the total thickness of the slabs.

2. The method as described in claim 1, characterized in that, The aforementioned calming operation performed before ladle pouring specifically includes: The sedation operation is performed before the steel ladle is poured, and the sedation time is controlled between 25 min and 40 min.

3. The method as described in claim 1, characterized in that, The pulling speed is controlled at 0.9-1.5 m / min, specifically including: Different casting speeds are controlled according to different casting section specifications; among which: 1600≤cast section≤1800, casting speed controlled at 0.9-1.1m / min; For casting sections with a diameter of 1300 ≤ casting cross-section < 1600, the casting speed should be controlled at 1.2-1.3 m / min. For casting sections with a diameter of 1000 ≤ casting section < 1300, the casting speed should be controlled at 1.4-1.5 m / min.

4. The method as described in claim 1, characterized in that, The control of the intermediate tundish immersion pouring and the addition of an intermediate tundish covering agent after pouring specifically includes: The immersion depth of the tundish long nozzle should be greater than 100mm. The amount of covering agent added to the intermediate ladle: 3.75 kg per ton of steel is added to the middle of the baffle wall during the start-up furnace, and 32.5 kg per ton of steel is added to each side of the baffle wall. For the intermediate continuous casting furnace, an additional 0.625 kg per ton of steel is added to the middle part of the baffle wall for each furnace.

5. The method as described in claim 1, characterized in that, The replacement of the submersible nozzle based on the upward trend of the stopper rod or the fluctuation of the liquid level specifically includes: Using the bar position at the constant casting speed stage as the reference bar position height, if the stopper bar rises by more than 15mm during casting; or if the slab liquid level fluctuates outside the range of [-5mm, +5mm] and continuously reaches 2 slabs, replace the submerged nozzle on the intermediate slab of the next furnace.

6. The method as described in claim 1, characterized in that, Maintaining the set tonnage in the tundish during the pouring and changing processes includes: During normal casting, the tundish tonnage is greater than 85% of the nominal tonnage, while during ladle changing, the tundish tonnage is at least 80% of the nominal tonnage.

7. The method as described in claim 1, characterized in that, The control of the ladle to perform the steel retention operation until the ladle pouring is completed specifically includes: The amount of steel left in the ladle should be controlled to be more than 8% of the ladle weight until the ladle is completely poured.

8. A surface defect control system for an automotive outer panel, said system being used in the surface defect control method for an automotive outer panel as described in any one of claims 1-7, characterized in that, The system includes: The calming control unit is used to perform calming operations before the ladle is poured, and the casting speed is controlled at 0.9-1.5 m / min; The casting control unit is used to control the immersion casting of the tundish and add tundish covering agent after casting, adjust the three argon gas according to the liquid level fluctuation, replace the immersion nozzle according to the stopper rod rising trend and the liquid level fluctuation, and control the tundish to maintain the set tonnage during the casting and ladle changing processes. The steel retention control unit is used to control the ladle to perform the steel retention operation until the ladle is completely poured; The machine cleaning equipment unit is used to machine clean qualified slabs obtained from casting; it checks whether the machine-cleaned slabs meet the following requirements: the surface of the machine-cleaned slab is guaranteed to be bright, and there are no abnormal defects such as molten grooves, molten steel, or molten slag; the height of the protrusions on the surface of the machine-cleaned slab is less than 2mm; the machine cleaning depth in the edge area within 50mm of the edge of the machine-cleaned slab reaches more than 2% of the total thickness of the slab; and the middle part more than 50mm from the edge of the machine-cleaned slab reaches more than 3.4% of the total thickness of the slab.

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