Method for controlling surface white point defect of continuous annealing steel for automobile outer plate

Abstract

The invention discloses a method for controlling white spot defects on the surface of continuous annealing steel for an automobile outer plate, which comprises the following steps of: accurately identifying and classifying white spot defect characteristics by virtue of detection equipment; aiming at different types of white spot defects, carrying out verification tests around hot rolling, cold rolling and continuous annealing processes by adopting a one-time process separation method, and locking generation sources of different types of white spot defects; and quality improvement measures are formulated for quality improvement. According to the method, a continuous annealing production plan is reasonably optimized, the surface tumor situation of a furnace roller surface in a continuous annealing furnace is improved by changing steel types, and the hot coil surface quality is strictly controlled through process control such as quality classification and grading management of hot rolling working roller offline rollers, use of medium-pressure water according to the thickness of a hot rolled plate and increase of rack water flow; and finally, continuous annealing IF steel surface small white point defect improvement is achieved through continuous annealing IF steel surface quality management and process optimization of all related procedures, the effect of low-cost quality improvement is achieved, and the method is suitable for popularization and application of vast continuous annealing finished product quality control related units.

Description

Technical Field

[0001] The present invention relates to the technical field of automobile steel rolling, and in particular to a method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels. Background Art

[0002] Automotive manufacturers are increasingly demanding higher surface quality for vehicle exterior panels, often requiring steel manufacturers to provide "zero-defect" products. Automotive exterior panels are a product of the steel industry's long production process, characterized by a complex manufacturing process that integrates chemical, physical, heat treatment, and material plastic deformation and rolling processes. Achieving 100% product quality control across all steps is difficult. This is especially true for specific surface defects, requiring comprehensive surface defect analysis, accurate identification, source verification, and source-specific solution development to prevent their recurrence, mitigate quality risks during the automotive exterior panel manufacturing process, and enhance customer satisfaction.

[0003] The presence of small white spots in the continuously returned outer panels caused quality defects, which failed to meet the requirements of key strategic customers, causing serious disruption to customer use. Furthermore, the batch return and exchange process resulted in significant quality losses for the company. Therefore, it was urgent to investigate the source of these small white spots in the continuously returned outer panels, formulate measures to improve quality, and enhance the ability to secure outer panels for key strategic customers.

[0004] For example, patent CN108441613A discloses a method for preventing white spots in age-hardening plastic mold steel. The main technical solution is to perform solution treatment directly after forging, rather than dehydrogenation annealing. This direct solution treatment process prevents hydrogen in the steel from accumulating and allows it to be evenly fixed in the steel, preventing the formation of white spots. Furthermore, a suitable tempering process is used to fully remove residual stress in the steel, thus preventing the occurrence of white spots. Another example is patent CN117568557A, which discloses a coating-free, high-temperature, oxidation-resistant hot-forming steel and its smelting method. The main technical solution is to add Ce and / or Y during vacuum treatment in the second refining step. Through rare earth microalloying metallurgical treatment, this not only modifies the inclusions in the molten steel into rare earth inclusions, making the grain structure of the equiaxed crystal zone on the surface of the rolled steel plate finer and denser, but also significantly reduces quality defects such as hydrogen-induced white spots and cracks in the steel. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention provides a method for controlling white spot defects on the surface of continuous annealing steel for automobile outer panels, which can verify the source of the defects and improve the white spot defects through consistent quality management and process optimization of various related processes.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels comprises the following steps:

[0008] Step 1: Use detection equipment to accurately identify and classify white spot defect characteristics;

[0009] Step 2: For different types of white spot defects, a consistent process separation method is used to conduct verification tests around the hot rolling, cold rolling, and continuous annealing processes to identify the sources of different types of white spot defects;

[0010] Step 3: Develop quality improvement measures to improve quality.

[0011] Further:

[0012] In the step 1, two types of white spot defect samples of cold rolling and continuous annealing are respectively taken, and the defect positions are detected and analyzed through microscopic morphology and energy spectrum detection.

[0013] In step 1, the defects detected are divided into two categories, specifically, two types of white spot defects: irregularly distributed round dots without extension marks on the upper and lower surfaces, and irregularly distributed white stripes with extension marks at 1 / 4 to 3 / 4 of the width of the lower surface.

[0014] In the step 1, the strip defects appear to be long strips in appearance, with clear boundaries, the strip direction is consistent with the rolling direction, the length varies from 1 mm to 5 mm, the width is relatively fixed, 1 mm to 2 mm, some have a tactile feel, the depth is 20-30 μm, there are extension marks in the length direction as a whole, and no extension marks in the width direction, and the energy spectrum results show two component classifications of Fe or Fe + a small amount of O elements.

[0015] In the step 2, for defects such as irregularly distributed white dots on the upper and lower surfaces without any extension marks, a verification test plan for "adjusting the continuous annealing production plan" is formulated based on the changes in the overhaul intervals of the continuous annealing furnace, the experience of the furnace in the late stage of use, and the law of furnace roller accumulation during the operation of the furnace. The main method is to insert high-strength dual-phase steel and other steel types into the continuous annealing batch production IF steel plan at a certain frequency and in small batches to carry out mixed production. It is found that the severity of the white dot defects on the surface of the IF steel products is correlated with the plan adjustment. It is preliminarily inferred that this type of defect is related to the continuous annealing process itself, and the continuous annealing production plan is further optimized.

[0016] In the control method, in order to finally verify the conclusion, the furnace rollers that are most prone to nodules during the transition between the slow cooling section and the fast cooling section of the furnace are pulled out for inspection during a short maintenance opportunity of the continuous withdrawal production line. It was found that the furnace rollers had obvious nodules on the roller surface, and the irregular distribution of the nodules was similar to the distribution of the positions of the dot-shaped white spots. After the nodule-accumulating furnace rollers were replaced and production resumed, the dot-shaped white spot defects were significantly improved.

[0017] In the control method, defects such as white spots with extension marks that are irregularly distributed at 1 / 4 to 3 / 4 of the width direction of the lower surface are detected by reverse process correlation analysis of the white spots data of the continuously annealed outer plate, and the hot rolling feeding unit is locked. Through analysis of the production and feeding batches of the hot rolling unit, it is found that there is a certain correlation with the hot coil batch.

[0018] In the control method, the entire process of hot rolling, such as hot rolling, tempering, uncoiling, pickling, and continuous annealing, of the same hot rolling batch is selected for tracking. No obvious corresponding defects are found after hot rolling, tempering, and pickling. During the continuous annealing production, the test coils are checked for large-area white stripe-like white spot defects. After tracking and analyzing the entire process, the cause of the white spot defects is preliminarily determined to be from the hot rolling process.

[0019] In the control method, process separation is achieved, and hot coil flipping and pickling rolling tests are organized and carried out; the pickling rolling unit is organized to produce the flipped hot coil, and the production sequence is flipping, not flipping, flipping, and not flipping. The production results show that the flipped hot coil and the unflipped hot coil have white spot defects similar to continuous annealing on the corresponding lower surfaces of the hot rolling mill after pickling rolling. The continuous annealing unit is further organized to perform production tracking, and the production results are the same as those of pickling. The flipping hot coil and the unflipped hot coil have white spot defects on the corresponding lower surfaces of the hot rolling mill after the continuous annealing process. Through the flipping tests of the above processes, it is determined that the white spot defects in the continuous annealing process are generated in the hot rolling process.

[0020] In the control method, through the reverse investigation of small white strip and small white spot defects in the continuous annealing process, it was found that the defective coils were mostly in the middle and late stages of the hot rolling plan, and the roughness of the upper and lower plate surfaces of the hot rolling coils was quite different. After testing, it was determined that the roughness of the lower surface was much higher than that of the upper surface. In order to eliminate the difference in roughness between the lower and upper plate surfaces, the following improvement measures were mainly taken to improve quality: for the continuous annealing automobile plate plan, the mileage was strictly controlled to be ≤45KM; the F1-F7 grade standards and implementation requirements for the lower roll surface of the working rolls of the hot rolling finishing mill were formulated, and the quality of the lower roll surface of the working rolls of the F5-7 stands was monitored in particular; medium-pressure water was mandatory for hot-rolled coils with a thickness of h ≥ 3.0 mm, and the water flow between stands of automobile outer plate steel grades was increased.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels is reasonably designed. By rationally optimizing the production plan of the continuous annealing group, improving the surface nodules of the furnace rollers in the continuous annealing furnace by changing the steel type, and managing the quality of the hot-rolled working rollers by classifying and grading the rollers off the line, using medium-pressure water according to the thickness of the hot-rolled plate, increasing the water flow rate of the stand and other process controls, the surface quality of the hot-rolled coil is strictly controlled. Finally, through the consistent quality management and process optimization of all relevant processes, the small white spot defects on the surface of the continuously annealed IF steel are improved, achieving the effect of low-cost quality improvement. The method is suitable for promotion and application of the majority of units related to the quality control of continuously annealed finished products. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The following is a brief description of the contents and symbols in the drawings of this specification:

[0024] Figure 1 Schematic diagram of the actual object, microscopic morphology and energy spectrum composition of white spot defects in phosphating steel and IF steel of the present invention.

[0025] Figure 2 This is a schematic diagram of the levels indicated on the roller surface of the present invention.

[0026] Figure 3 This is a schematic diagram of information indicated on the roller surface of the present invention.

[0027] Figure 4 This is a schematic diagram showing an obvious downward trend in the sealing rate of quality defects of the present invention. DETAILED DESCRIPTION

[0028] The specific implementation of the present invention will be further explained in detail below through description of embodiments with reference to the accompanying drawings.

[0029] The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels comprises the following steps:

[0030] Step 1: Use scanning electron microscope, optical microscope and other testing equipment to accurately identify and classify white spot defect characteristics;

[0031] Step 2: For different types of white spot defects, a consistent process separation method is adopted (based on the characteristics of the production line equipment of each process and the personalized characteristics of the products of each process, focusing on white spot defects, conducting production tests of quality defects in each process, combining the test results of white spot defect samples and the analysis of the formation mechanism, to identify the process source of white spot quality defects). Verification tests are carried out around the hot rolling, cold rolling and continuous annealing processes to identify the sources of different types of white spot defects;

[0032] Step 3: Develop quality improvement measures to improve quality.

[0033] The present invention mainly realizes improvement of the surface nodules of furnace rollers in the continuous annealing furnace by reasonably optimizing the production plan of the continuous annealing group, improving the current situation of surface nodules of furnace rollers in the continuous annealing furnace by changing the steel type, and classifying and grading the quality of the hot-rolled working rollers off-line, using medium-pressure water according to the thickness of the hot-rolled plate, increasing the water flow rate of the frame and other process controls, strictly controlling the surface quality of the hot-rolled coil, and finally realizing improvement of small white spot defects on the surface of continuous annealing IF steel through consistent quality management and process optimization of various related processes, achieving the effect of low-cost quality improvement, and being suitable for promotion and application of a large number of units related to quality control of continuous annealing finished products.

[0034] The present invention is based on the subtle characteristic differences of small white spot quality defects on automobile outer panels. It uses a consistent method to identify and explore the sources of defects in different processes, quickly formulate effective improvement measures, ensure that the product meets user requirements, and reduce the risk of major quality losses. Specifically, by subdividing the microscopic morphological characteristics and energy spectrum analysis of small white spot defects on continuously detached outer panels, the process separation method in the consistent quality system is reversely adopted to verify the source of defects, quickly formulate targeted measures for quality improvement, and ultimately achieve the purpose of making such products suitable for mass production and maintaining stable quality.

[0035] Specific examples of the present invention are:

[0036] (1) Take two types of small white spot defect samples from cold rolling and continuous annealing respectively, and detect and analyze the defect positions. Through micromorphology and energy spectrum detection, they are divided into two types of small white spot defects: irregularly distributed round dots without extension marks on the upper and lower surfaces, and irregularly distributed small white strips with extension marks from 1 / 4 to 3 / 4 of the width direction of the lower surface. (2) Separate the processes of small white spot defects in the continuously annealed outer plate. According to the characteristics of these two types of defects, a consistent process separation method is adopted. Process tests are carried out around the relevant rolling processes to verify the sources of the two types of small white spot defects. (3) According to the specific sources of the two types of small white spot defects, quality improvement measures are formulated to improve them and improve the quality of continuously annealed outer plate products. The technologies for detecting small white spot defects in continuously annealed outer plates, separating processes and improving quality are as follows:

[0037] 1. Accurately identify and classify small white spot defect characteristics with the help of detection equipment;

[0038] (1) Two types of small white spot defect samples of cold rolling and continuous annealing were taken respectively, and the defect positions were detected and analyzed. Through micromorphology and energy spectrum detection, it was found that: the defects such as round white spots with no extension marks that were irregularly distributed on the upper and lower surfaces had unclear boundaries, and the energy spectrum results were basically Fe elements, and basically had no feel; the defects such as small white strip-shaped white spots with extension marks that were irregularly distributed at 1 / 4 to 3 / 4 of the width direction of the lower surface were long strips in appearance, with clear boundaries, the strip direction was consistent with the rolling direction, the length varied from 1mm to 5mm, the width was relatively fixed, about 1mm to 2mm, some had a feel, the depth was 20-30μm, there were extension marks in the overall length direction, and no extension marks in the width direction, and the energy spectrum results showed two component classifications of Fe or Fe+a small amount of O elements, such as Figure 1 shown.

[0039] (2) Based on the microscopic morphology and energy spectrum detection of the two types of small white spot defects, it is basically determined that the generation mechanisms of the two types of small white spot defects are different, and the generation sources need to be further clarified.

[0040] 2. For the two types of small white spot defects, a consistent process separation method was adopted. Verification tests were carried out around the hot rolling, cold rolling, and continuous annealing processes to identify the sources of the two types of small white spot defects and implement quality improvements.

[0041] (1) Aiming at the irregular distribution of small white spots on the upper and lower surfaces without extension marks, a verification test plan for "adjusting the continuous annealing production plan" was formulated based on the changes in the overhaul intervals of the continuous annealing furnace and the experience of the furnace in the late stage of use, as well as the law of furnace roller accumulation during the operation of the furnace. The plan was mainly to insert high-strength dual-phase steel and other steel types into the continuous annealing batch production plan of IF steel at a certain frequency in multiple batches and small batches for mixed production. It was found that the severity of the small white spots on the surface of IF steel products was correlated with the plan adjustment. It was preliminarily inferred that this type of defect was related to the continuous annealing process. By further optimizing the continuous annealing production plan, that is, reasonably inserting high-strength dual-phase steel, the small white spots on the surface of IF steel products can be significantly improved. In order to finally verify this conclusion, we took advantage of a short maintenance opportunity of the continuous withdrawal production line to pull out and inspect the furnace rollers that were most prone to nodules during the transition between the slow cooling section and the fast cooling section of the furnace. We found that the furnace rollers had obvious nodules on the roller surface, and the irregular distribution of the nodules was similar to the position distribution of the small white dots. After the nodule-accumulating furnace rollers were replaced and production resumed, the defects of the small white dots were significantly improved.

[0042] (2) For the irregularly distributed small white spots with extension marks at 1 / 4 to 3 / 4 of the width of the lower surface, the reverse process correlation analysis of the small white spots on the outer plate of the continuous annealing was conducted to identify the hot rolling feeding unit. The analysis of the production feeding batches of the hot rolling unit showed a certain correlation with the hot coil batches. At the same time, it was found that the proportion of small white spots in the continuous annealing of some hot coil batches was high. The hot rolling process of the same batch of hot rolling was selected for tracking. No obvious corresponding defects were found after hot rolling and pickling. The test coil had a large area of small white spots on the lower surface during the continuous annealing production. After full process tracking and analysis, the cause of the small white spots was initially identified as the hot rolling process. In order to further achieve process separation, hot coil flipping and pickling rolling tests were organized. The pickling mill was organized to produce the turned-over hot coils, and the production sequence was turning over, not turning over, turning over, and not turning over. The production results showed that the turned-over hot coils and the non-turned hot coils had small white strip-like white spot defects on the corresponding lower surfaces of the hot rolling mill after pickling rolling. The continuous annealing mill was further organized to track the production, and the production results were the same as those of the pickling rolling. The turned-over tests of the non-turned hot coils showed small white strip-like white spot defects on the corresponding lower surfaces of the hot rolling mill after the continuous annealing process. Through the turning-over tests of the above processes, it was determined that the small white strip-like white spot defects in the continuous annealing process were generated in the hot rolling process. Through the reverse investigation of small white strips and small white spots in the continuous annealing process, it was found that the defective coils were mostly in the middle and late stages of the hot rolling plan, and the roughness of the upper and lower plates of the hot rolling coils was quite different. After testing, it was determined that the roughness of the lower surface was much higher than that of the upper surface. In order to eliminate the difference in roughness between the lower and upper plates, the following improvement measures were mainly taken to improve the quality: for the continuous annealing of automobile plates, the mileage was strictly controlled to be ≤45KM; the F1-F7 grade standards and implementation requirements for the lower roll surface of the working rolls of the hot rolling finishing mill were formulated, and the quality of the lower roll surface of the working rolls of the F5-7 frames was monitored in particular; medium-pressure water was mandatory for hot-rolled coils with a thickness of h≥3.0mm, the water flow between the stands of automobile outer plate steel grades was increased, and the bottom water spray of the F5 inlet was increased, etc.

[0043] In this invention, by using detection equipment to fully identify defect characteristics, a consistent process separation method is adopted to find the source of defects and make quality improvements, thereby improving the quality of continuously annealed outer plate products. Significant results have been achieved in the following aspects:

[0044] 1. The closure rate of small white spot quality defects in continuously refurbished automotive exterior panels has dropped from approximately 10% before quality improvements to less than 1% after improvements, effectively improving the qualified rate of continuously refurbished automotive exterior panels and significantly reducing quality losses. 2. The disruption to supply to strategic customers caused by small white spot defects has been resolved, providing strong support for more stable supply in the future. 3. In the face of major and difficult quality defects, a consistent process separation technology has been implemented, and a small professional team with cross-process capabilities has been established to resolve quality issues. This provides effective solutions and ideas for resolving potential and difficult quality defects in the future.

[0045] In the embodiment of the present invention, the following are formulated: "The grade standards for the F1-F7 bottom roll surface of the work rolls of the hot rolling finishing mill, the implementation requirements, and the final implementation results of the continuous annealing outer plate finished product":

[0046] (1) Formulate and implement standards for hot rolling work roll oxide film classification and quality assessment:

[0047]

[0048] (2) Develop and implement a work roll grading diagram:

[0049]

[0050] (3) Formulate and implement the grading standards for the oxide film of hot-rolled work rolls corresponding to automobile outer plates:

[0051] <![CDATA[Hot rolling operation Roller Roll classification]]> Automobile outer panel (05 plate) Level I allow Level II allow Level III allow Level IV Not allowed V-level Not allowed Level VI Not allowed Tier VII Not allowed

[0052] (4) Formulate and implement the quality management rules for hot rolling work rolls (such as Figure 2 and Figure 3 shown):

[0053] a. Take photos of all rollers, write the rack number on the roller surface with a marker, and rate the upper and lower rollers separately, indicating the plan number. After taking photos, send them to a dedicated person for management.

[0054] b. For roller surfaces rated at IV to VII, partial photos must be taken, and the frame number, oxide film grade, distance from the transmission (operation) side, and peeling width must be noted on the roller surface with a marking pen. If both the upper and lower rollers have peeling, both must be photographed and sent to a dedicated person for management.

[0055] c. When taking photos, it is required to choose an angle with good lighting as much as possible, and the camera should remain still and not shake during the focusing and shooting delay to ensure that the photos are clear and the defects are clearly visible; in the hot rolling process, a dedicated person should be assigned to archive and manage the photos of the rollers for easy traceability and inquiry.

[0056] (5) After promoting and implementing a series of quality improvement measures such as the quality management of hot rolling work rolls, the sealing rate of small white spot quality defects in the outer plate of the continuously annealed automobile has shown a significant downward trend, such as Figure 4 shown.

[0057] The above achievements are remarkable, which well illustrates that quality is the foundation of an enterprise's survival. While meeting users' requirements and ensuring supply, it reduces manufacturing costs, enhances market competitiveness, and provides a solid foundation for better cooperation between automobile outer panel manufacturers and users, promoting the joint improvement of the quality of the upstream and downstream industrial chains of automobile outer panels.

[0058] The present invention is described above by way of example in conjunction with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. A method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels, characterized by: The control method comprises the following steps: Step 1: Use detection equipment to accurately identify and classify white spot defect characteristics; Step 2: For different types of white spot defects, a consistent process separation method is used to conduct verification tests around the hot rolling, cold rolling, and continuous annealing processes to identify the sources of different types of white spot defects; Step 3: Develop quality improvement measures to improve quality.

2. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 1, characterized in that: In the step 1, two types of white spot defect samples of cold-rolled and continuously annealed steel plates are respectively taken, and the defect positions are detected and analyzed through microscopic morphology and energy spectrum detection.

3. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 2, characterized in that: In step 1, the defects detected are divided into two categories, specifically, two types of white spot defects: irregularly distributed round dots without extension marks on the upper and lower surfaces, and irregularly distributed white stripes with extension marks at 1 / 4 to 3 / 4 of the width of the lower surface.

4. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 3, characterized in that: In the step 1, the strip defects appear to be long strips in appearance, with clear boundaries, the strip direction is consistent with the rolling direction, the length varies from 1 mm to 5 mm, the width is relatively fixed, 1 mm to 2 mm, some have a tactile feel, the depth is 20-30 μm, there are extension marks in the length direction as a whole, and no extension marks in the width direction, and the energy spectrum results show two component classifications of Fe or Fe + a small amount of O elements.

5. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 4, characterized in that: In step 2, for defects such as irregularly distributed round dot-shaped white spots with no extension marks on the upper and lower surfaces, a verification test plan for "adjusting the continuous annealing production plan" was formulated based on the changes in the overhaul intervals of the continuous annealing furnace, the experience of the furnace in the late stage of use, and the law of furnace roller accumulation during the operation of the furnace. This was mainly achieved by inserting high-strength dual-phase steel and other steel grades into the continuous annealing batch production plan of IF steel at a certain frequency in multiple small batches for mixed production. It was found that the severity of the round dot-shaped white spot defects on the surface of the IF steel products was correlated with the plan adjustment. It was preliminarily inferred that this type of defect was related to the continuous annealing process itself, and the continuous annealing production plan was further optimized.

6. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 5, characterized in that: In the control method, in order to finally verify the conclusion, the furnace rollers that are most prone to nodules during the transition between the slow cooling section and the fast cooling section of the furnace are pulled out for inspection during a short maintenance opportunity of the continuous withdrawal production line. It was found that the furnace rollers had obvious nodules on the roller surface, and the irregular distribution of the nodules was similar to the distribution of the positions of the dot-shaped white spots. After the nodule-accumulating furnace rollers were replaced and production resumed, the dot-shaped white spot defects were significantly improved.

7. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 5, characterized in that: In the control method, defects such as white spots with extension marks that are irregularly distributed at 1 / 4 to 3 / 4 of the width direction of the lower surface are detected by reverse process correlation analysis of the white spots data of the continuously annealed outer plate, and the hot rolling feeding unit is locked. Through analysis of the production and feeding batches of the hot rolling unit, it is found that there is a certain correlation with the hot coil batch.

8. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 7, characterized in that: In the control method, the entire process of hot rolling, such as hot rolling, tempering, uncoiling, pickling, and continuous annealing, of the same hot rolling batch is selected for tracking. No obvious corresponding defects are found after hot rolling, tempering, and pickling. During the continuous annealing production, the test coils are checked for large-area white stripe-like white spot defects. After tracking and analyzing the entire process, the cause of the white spot defects is preliminarily determined to be from the hot rolling process.

9. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 8, characterized in that: In the control method, process separation is achieved, and hot coil flipping and pickling rolling tests are organized and carried out; the pickling rolling unit is organized to produce the flipped hot coil, and the production sequence is flipping, not flipping, flipping, and not flipping. The production results show that the flipped hot coil and the unflipped hot coil have white spot defects similar to continuous annealing on the corresponding lower surfaces of the hot rolling mill after pickling rolling. The continuous annealing unit is further organized to perform production tracking, and the production results are the same as those of pickling. The flipping hot coil and the unflipped hot coil have white spot defects on the corresponding lower surfaces of the hot rolling mill after the continuous annealing process. Through the flipping tests of the above processes, it is determined that the white spot defects in the continuous annealing process are generated in the hot rolling process.

10. The method for controlling white spot defects on the surface of continuously annealed steel for automobile outer panels according to claim 9, characterized in that: In the control method, through the reverse investigation of small white strip and small white spot defects in the continuous annealing process, it was found that the defective coils were mostly in the middle and late stages of the hot rolling plan, and the roughness of the upper and lower plate surfaces of the hot rolling coils was quite different. After testing, it was determined that the roughness of the lower surface was much higher than that of the upper surface. In order to eliminate the difference in roughness between the lower and upper plate surfaces, the following improvement measures were mainly taken to improve quality: for the continuous annealing automobile plate plan, the mileage was strictly controlled to be ≤45KM; the F1-F7 grade standards and implementation requirements for the lower roll surface of the working rolls of the hot rolling finishing mill were formulated, and the quality of the lower roll surface of the working rolls of the F5-7 stands was monitored in particular; medium-pressure water was mandatory for hot-rolled coils with a thickness of h ≥ 3.0 mm, and the water flow between stands of automobile outer plate steel grades was increased.

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