Method for controlling pinholes and pits in surface of alloyed product

Abstract

The invention relates to a method for controlling pinholes and pits in the surface of an alloyed product, which comprises the following steps: S1, determining the morphology of a defect: observing the three-dimensional morphology of a defect area by using a laser confocal microscope, and determining whether a blocky foreign matter appears at the center of the defect area; s2, determining a defect source: if crystal grains below foreign matters in a defect area are equiaxial crystal grains or the crystal grains are finer than a normal area of the substrate, judging that the foreign matters are generated before entering a continuous annealing furnace; s3, checking key equipment: checking the key equipment of the cold rolling combined unit; and S4, the surface state of the process roller system is controlled, specifically, the corresponding process roller system is replaced, it is guaranteed that the surface state of the process roller is not damaged, and the strip steel surface quality change situation before and after the process roller system is replaced is observed. By controlling the black spot defect on the surface of the alloying substrate, the surface quality of the finished product after hot galvanizing alloying is improved, the surface defect of the alloying product is detected and analyzed, the defect generation source is defined, and the relation between the surface defect and the surface state of the roller system is verified.

Description

Technical Field

[0001] The invention relates to the technical field of hot-dip galvanizing alloying, and in particular to a method for controlling pinhole pits on the surface of an alloyed product. Background Art

[0002] During the hot-dip galvanizing and alloying process, the substrate often has some quality defects such as missed plating, over-thick coating, gray, light and dark stripes on the surface, particles on the coating surface, and bulges on the coating surface. There are many factors affecting the defects, and it is relatively difficult to control them. These defects seriously affect the surface quality of the coating.

[0003] The surface quality of alloyed hot-dip galvanized steel sheets is not only related to the process, but also to the surface condition of the substrate (such as substrate roughness, scratches, substrate surface elements, etc.), cleaning process, alloy elements in zinc liquid, etc. Poor surface condition of the steel substrate often causes surface defects such as dullness and stripes on the coating surface. Even if the alloyed galvanizing process is reasonable and stable, the surface condition of the steel substrate has a significant impact on the surface quality of the galvanized layer and the alloyed coating.

[0004] Compared with hot-dip pure zinc sheet, alloyed galvanized sheet needs to be alloyed after the strip is out of the zinc pot during production, so that the zinc and iron on the surface of the substrate diffuse with each other to form a zinc-iron alloy layer. The existence of the diffusion process makes the surface quality control of the alloyed sheet more complicated, and it is necessary to accurately control the heating temperature, strip speed and substrate quality to obtain a finished product with good zinc layer adhesion and surface quality.

[0005] Patent CN113430478A discloses a method for eliminating zinc corrugation defects of alloyed hot-dip galvanized steel sheets. The method eliminates zinc corrugation defects of alloyed hot-dip galvanized steel sheets by controlling the temperature of the zinc liquid in the zinc pot and the temperature of the strip entering the pot, controlling the mass percentage of aluminum in the zinc liquid and the running speed of the strip, adjusting the air knife parameters, and controlling the rolling force of the skin pass mill. The zinc corrugation defect control process of alloyed hot-dip galvanized steel sheets studied in this project can effectively solve the zinc corrugation quality problem by increasing the unit speed, increasing the air knife purge pressure, expanding the air knife gap, and increasing the unit skin pass rolling force. It is of great significance to eliminate zinc corrugation defects, improve the surface quality of alloyed products, enhance market competitiveness, and improve the yield rate of unit products. However, the project did not propose any relevant solutions to the black spot defect.

[0006] Patent CN115216607 discloses a method for controlling alloying spot defects of hot-dip galvanized iron alloy outer plates. The method for controlling alloying spot defects of hot-dip galvanized iron alloy outer plates of the invention comprises the steps of steelmaking, hot rolling, cold rolling acid rolling and cold rolling galvanizing. The conductivity in the cleaning section of the cold rolling galvanizing process is controlled to be 30-40ms / cm, the residual oil and residual iron on the surface of the steel plate after cleaning are ≤2000ppm, the oxygen content is controlled to be ≤10ppm in the annealing section, the hydrogen content is controlled to be 2%-6%, the dew point is controlled to be below -40°C, the Al content of the zinc pot is controlled to be 0.125-0.130% in the galvanizing section, the plate temperature entering the zinc pot is 460-480°C, the nitrogen humidification wet nitrogen flow valve is controlled to be below 3%, and the zinc pump flow is ≤300RPM. By adopting the method for controlling alloying spot defects of hot-dip galvanized iron alloy outer plates of the invention, the generation rate of alloying spot defects is reduced from the traditional 5% to 0.2%. However, the causes and microscopic morphology of spot defects and black spot defects are different, and the control methods used are also different. Summary of the invention

[0007] In view of the above problems, the purpose of the present invention is to provide a method for controlling pinhole pits on the surface of alloyed products, and to improve the surface quality of finished products after hot-dip galvanizing by controlling the black spot defects on the surface of alloyed substrates. The samples with surface defects of alloyed products are tested and analyzed to clarify the sources of defects. The relationship between surface defects and the surface state of the roller system is verified by checking the status of each process roller of the pickling mill.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A method for controlling pinhole pits on the surface of an alloyed product proposed by the present invention comprises the following steps:

[0010] S1. Determine the defect morphology: Use a laser confocal microscope to observe the three-dimensional morphology of the defect area and determine whether there is a block of foreign matter in the center of the defect area;

[0011] S2. Determine the defect source: If the grains below the foreign matter in the defect area are equiaxed grains or the grains are finer than those in the normal area of ​​the substrate, it is determined that the foreign matter was generated before entering the continuous annealing furnace;

[0012] S3. Key equipment inspection: inspect the key equipment of the cold rolling mill;

[0013] S4. Control the surface condition of the process roll system: replace the corresponding process roll system to ensure that the surface condition of the process roll is not damaged, and observe the changes in the surface quality of the strip before and after the process roll system is replaced.

[0014] Furthermore, in step S1, the surface morphology of the defective area shows that foreign matter is pressed into the interior of the substrate, and there are obvious extended cracks around the foreign matter; the cross-sectional morphology of the defective area is that there is an obvious coating on the surface of the foreign matter, and the surface layer of the foreign matter and other areas are on the same horizontal line; EDS energy spectrum analysis is performed on the defective area, and the main element composition of the defective area is determined based on the energy spectrum analysis results; the cross-section of the defective area is corroded, and if after corrosion, obvious grains appear in the substrate corrosion and the foreign matter is completely blackened, it can be determined that this defect is a foreign matter pressed in.

[0015] Furthermore, the step S3 specifically includes: checking the key equipment of the cold rolling mill, sampling the foreign matter on each process roller for EDS spectrum analysis, and if the element composition of the foreign matter in the spectrum is consistent with the element composition of the defective area, it is judged that the pit defect is caused by the foreign matter falling off onto the surface of the steel plate when the steel coil passes through the roller, and entering the interior of the steel plate after rolling by the rolling mill, and appearing as a tiny black spot defect after plating.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The invention improves the surface quality of the finished product after hot-dip galvanizing by controlling the black spot defects on the surface of the alloyed substrate for automobiles.

[0018] The present invention clarifies the causes of surface defects of hot-dip galvanized alloyed products through characterization and analysis of the microscopic morphology of the hot-dip galvanized alloyed products. That is, the surface coating of the defective area of ​​the hot-dip galvanized sheet is complete. After the foreign matter is pressed in, obviously refined equiaxed grains can be seen underneath it, indicating that it is subjected to greater strain before continuous annealing, resulting in accelerated nucleation. The source of this foreign matter is found on the surface of the process roller of the rolling mill process. The foreign matter residue falls off to the surface of the steel plate and enters the interior of the steel plate after being rolled by the rolling mill, and appears as pinhole pit defects after plating.

[0019] By adopting the defect control method of the present invention, pinhole pit defects no longer appear on the surface of the finished product produced by the galvanizing unit, stable production of hot-dip galvanized alloy products is achieved, stable operation of the unit is ensured, and surface quality meets the requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic flow chart of a method for controlling pinhole pits on the surface of an alloyed product proposed by the present invention;

[0021] Figure 2 This is a schematic diagram of the macroscopic morphology of black spot defects on the steel plate surface before the process roller system is replaced;

[0022] Figure 3 This is a schematic diagram of the microscopic morphology of black spot defects on the steel plate surface before the process roller system is replaced;

[0023] Figure 4 This is a schematic diagram of the macroscopic morphology of the steel plate surface after the process roller system is replaced in the present invention;

[0024] Figure 5 This is a schematic diagram of the microscopic morphology of the steel plate surface after the process roller system is replaced according to the present invention. DETAILED DESCRIPTION

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0026] A method for controlling pinhole pits on the surface of an alloyed product proposed by the present invention specifically comprises the following steps:

[0027] S1. Determine the defect morphology: Use a laser confocal microscope to observe the three-dimensional morphology of the defect area and determine whether there is a block of foreign matter in the center of the defect area;

[0028] The depth of the defect area is 0.1mm to 0.25mm.

[0029] The surface morphology of the defective area shows that the foreign matter is pressed into the interior of the substrate, and there are obvious extended cracks around the foreign matter; the cross-sectional morphology of the defective area shows that there is an obvious coating on the surface of the foreign matter, and the surface layer of the foreign matter and other areas are on the same horizontal line.

[0030] EDS energy spectrum analysis of the defective area showed that the foreign matter contained obvious Cr elements, with a content of 0.25-0.35%.

[0031] The cross section of the defective area is subjected to etching treatment. If after etching, obvious grains appear on the substrate and the foreign matter is completely blackened, it can be determined that the defect is caused by foreign matter intrusion.

[0032] S2. Determine the source of the defect;

[0033] If the grains under the foreign bodies in the defective area are equiaxed grains, it proves that they have undergone obvious annealing. Therefore, it can be judged that the foreign bodies have been produced before entering the continuous annealing furnace. Secondly, if the grains under the foreign bodies are obviously finer than those in the normal area of ​​the substrate, it means that the substrate under the foreign bodies has been subjected to a greater deformation than other areas before continuous annealing, which promotes nucleation and leads to grain refinement. Since the hot rolling final rolling temperature and curling temperature are high, the grains should be fully grown, so it can be judged that the source of foreign bodies may be more in the cold rolling process.

[0034] S3. Inspection of key equipment: Inspect the key equipment of the cold rolling mill and take samples of foreign matter on each process roller for EDS spectrum analysis. If the spectrum shows that the foreign matter contains a small amount of Cr, it is judged that the pit defect is caused by the foreign matter falling off onto the surface of the steel plate when the steel coil passes through the roller, and then entering the interior of the steel plate after rolling, and appearing as a tiny black spot defect after plating.

[0035] S4. Control the surface condition of the process roll system: replace the corresponding process roll system to ensure that the surface condition of the process roll is not damaged, and observe the changes in the surface quality of the strip before and after the process roll system is replaced.

[0036] In this embodiment, the specifications and surface conditions of the process rollers after replacement are shown in the following table:

[0037] Table 1 Specifications and surface conditions of the replaced process rollers

[0038]

[0039] For cold rolling mills, due to the harsh working conditions, foreign matter adhesion defects on the strip surface are a long-standing and common problem that affects the quality of cold rolled products. A notable feature of these foreign matter is that they are complex in composition and easily adhere to the surface of the strip. Since the working state of the rubber roller is closely related to the surface quality of the product, it is necessary to determine the defect generation mechanism and formulate a control method, which is of great significance to the high quality and stable production of alloyed products.

[0040] The material of the looper roller system in the process section is mainly polyurethane. The polyurethane rubber roller has excellent physical properties and can withstand high pressure, high speed and high humidity production environment. However, its temperature resistance is poor, and its resistance to water, alkali, aromatic hydrocarbons, alcohols and other solvents is not very good. Therefore, after long-term use, the aging of the rubber layer of the polyurethane rubber roller under the influence of air and various environments will cause its performance to decline and aggravate abnormal wear. Once it is found that the working conditions and measures are stable, and there is still intermittent foreign matter adhesion on the roller surface after repeated cleaning, it is necessary to focus on the use cycle, abnormal damage and other aspects of the rubber roller status of the unit, and verify it through targeted roller replacement.

[0041] When the polyurethane rubber roller is severely worn or aged, the wear debris generated by the friction between the roller system and the strip steel is carried into the next process and rolled by the rolling mill, which will form foreign matter indentation or spot defects on the surface of the steel plate. Through the characterization analysis of the defects and the EDS spectrum analysis of foreign matter on the surface of the roller system, it is determined that this defect is caused by foreign matter falling off the surface of the steel plate when the steel coil passes through the roller system, entering the interior of the steel plate after rolling by the rolling mill, and appearing as a pinhole pit defect after plating. The present invention effectively controls the pinhole pit defect by replacing the roller system and controlling the surface state of the roller surface.

[0042] The surface morphology of the steel plate before the process roller system is replaced is as follows Figure 1-2As shown in the figure, the surface morphology of the steel plate after replacement is as follows Figure 3-4 After the surface condition of the process rolls of the rolling mill unit was optimized, the surface quality of the alloyed high-grade surface substrate products for automobiles actually produced was significantly improved compared with the products before optimization, achieving stable production of hot-dip galvanized alloyed products and ensuring stable operation of the unit and surface quality that met the requirements.

[0043] Matters not described in detail in the present invention are all known technologies.

[0044] The embodiments described above are merely descriptions of preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for controlling pinhole pits on the surface of an alloyed product, characterized in that: The method comprises the following steps: S1. Determine the defect morphology: Use a laser confocal microscope to observe the three-dimensional morphology of the defect area and determine whether there is a block of foreign matter in the center of the defect area; S2. Determine the defect source: If the grains below the foreign matter in the defect area are equiaxed grains or the grains are finer than those in the normal area of ​​the substrate, it is determined that the foreign matter was generated before entering the continuous annealing furnace; S3. Key equipment inspection: inspect the key equipment of the cold rolling mill; S4. Control the surface condition of the process roll system: replace the corresponding process roll system to ensure that the surface condition of the process roll is not damaged, and observe the changes in the surface quality of the strip before and after the process roll system is replaced.

2. A method for controlling pinhole pits on the surface of an alloyed product according to claim 1, characterized in that: In step S1, the surface morphology of the defective area shows that the foreign matter is pressed into the interior of the substrate, and there are obvious extended cracks around the foreign matter; the cross-sectional morphology of the defective area shows that there is an obvious coating on the surface of the foreign matter, and the surface layer of the foreign matter and other areas are on the same horizontal line; Perform EDS energy spectrum analysis on the defect area, and determine the main element composition of the defect area based on the energy spectrum analysis results; perform corrosion treatment on the cross section of the defect area. If after corrosion, obvious grains appear in the substrate corrosion and the foreign matter is completely black, it can be determined that the defect is caused by foreign matter intrusion.

3. A method for controlling pinhole pits on the surface of an alloyed product according to claim 2, characterized in that: The step S3 specifically includes: checking the key equipment of the cold rolling mill, sampling the foreign matter on each process roller for EDS spectrum analysis, if the element composition of the foreign matter in the spectrum is consistent with the element composition of the defective area, it is judged that the pit defect is caused by the foreign matter falling off onto the surface of the steel plate when the steel coil passes through the roller, entering into the interior of the steel plate after being rolled by the rolling mill, and appearing as a tiny black spot defect after plating.

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