Method for reducing short-strip shadow on surface of alloying hot galvanized plate

Abstract

The invention discloses a method for reducing short strip shadows on the surface of an alloyed hot-dip galvanized plate, and belongs to the technical field of hot-dip galvanizing alloying. The method comprises the following steps: firstly, detecting and analyzing a defect area, wherein an analysis result shows that defects come from a galvanization process; then, by controlling the content of Al and Fe in a zinc pot in the galvanization process, controlling the strip steel surface cleaning quality and controlling the alloying process, the defects of short strips and shadows on the surface of the alloyed hot galvanized plate are reduced, the defects of short strips and shadows on the surface of a finished product produced by a galvanization unit do not occur any more, and the surface quality of an alloyed coating is improved; and the yield is remarkably improved, stable production of hot-dip galvanizing alloying products is achieved, it is guaranteed that a unit operates stably, the surface quality meets the requirement, the customer satisfaction degree is improved, and the market competitiveness is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-dip galvanizing, and in particular relates to a method for reducing short strip shadows on the surface of an alloyed hot-dip galvanized sheet. Background Art

[0002] In recent years, with the continuous expansion of the application of high-grade alloyed hot-dip galvanized steel sheets in automobiles and home appliances, people's requirements for the surface quality of alloyed steel sheets have become higher and higher. In order to improve the product quality of hot-dip galvanized steel sheets, steel companies have also paid more and more attention to the surface quality of galvanized steel sheets. The high surface quality requirements of alloyed hot-dip galvanized steel sheets generally refer to the strict control of the surface roughness and surface state of the steel sheets through post-processing technology, that is, the plate shape is flat, the dimensional accuracy is high, the tolerance is small, and there are no surface defects. As a high-value-added coating product, the surface quality of alloyed hot-dip galvanized steel sheets has always been the focus and difficulty in production control, especially hot-dip galvanized steel sheets for automobiles. The appearance quality of the paint film after the car is painted is an important indicator to determine its quality. There are many factors that affect the appearance quality of hot-dip galvanized sheets for cars after painting, among which the influence of the surface morphology of hot-dip galvanized steel sheets is particularly important. The research on the surface defects of alloyed coatings has become one of the key issues in the development of the industry.

[0003] The invention patent with application number 201680023058.5 discloses a method for suppressing the generation of patterned surface defects, thereby providing a galvanized steel sheet with good surface properties; the composition of the alloyed hot-dip galvanized original sheet is adjusted to contain C: less than 0.005%, Si: less than 0.03%, Mn: more than 0.5% and less than 1.0%, Nb: more than 0.005% and less than 0.015%, P: more than 0.01% and less than 0.05%, S: less than 0.03%, Al: more than 0.01% and less than 0.08% and N: less than 0.005%, and the balance is Fe and unavoidable impurities, so that the intensity of Mn in the surface layer of the steel sheet based on glow discharge emission spectrometry (GDS) is less than 3.5 (V). This patent mainly eliminates pattern defects by adjusting the chemical composition of the hot-dip galvanized original plate, and does not involve the study of short strip defects. It is of great significance to study the short strip defects that affect the surface quality of the alloyed coating and thus improve the surface quality of the product. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a method for reducing short strips and dark shadows on the surface of alloyed hot-dip galvanized sheet. The present invention detects and analyzes the short strips and dark shadow defects on the surface of alloyed hot-dip galvanized sheet, determines the source of the defects, and takes corresponding control measures to improve the surface quality of the alloyed coating. The yield rate is significantly improved, customer satisfaction is improved, and market competitiveness is improved.

[0005] The object of the present invention is achieved by the following methods:

[0006] The present invention provides a method for reducing short strip shadows on the surface of alloyed hot-dip galvanized sheet, comprising the following steps:

[0007] (1) The defective area was inspected and analyzed, and the analysis results showed that the defect originated from the galvanizing process;

[0008] (2) By controlling the Al and Fe content of the zinc pot during the galvanizing process, the quality control of the strip surface cleaning, and the alloying process control, the short strip shadow defects on the surface of the alloyed hot-dip galvanized sheet can be reduced.

[0009] Based on the above technical solution, further, the detection and analysis method described in step (1) includes optical microscopy inspection, EDS spectrum analysis of the defect area, stripping treatment of the defect area, corrosion treatment of the defect area and cross-sectional morphology inspection of the defect area.

[0010] Based on the above technical solution, further, the defective area is corroded using a nitric acid solution with a mass percentage of 1-4%.

[0011] Based on the above technical solution, further, in step (2), the Al content of the zinc pot is controlled within the range of 0.128% to 0.135%, and the Fe content is controlled within the range of 0 to 0.045%.

[0012] Based on the above technical solution, further, the residual oil content on the surface of the strip after surface cleaning in step (2) is ≤10mg / m 2 , residual iron content ≤10mg / m 2 .

[0013] Based on the above technical solution, further, in step (2), the alloying process is controlled by adopting different alloying processes for different steel strips.

[0014] Based on the above technical scheme, further, IF steel: the insulation temperature of the moving section is controlled at not less than 290°C, the insulation temperature of the fixed section is controlled at not less than 310°C, and the strip speed is not less than 110m / min; bake hardening steel: the insulation temperature of the moving section is controlled at not less than 360°C, the insulation temperature of the fixed section is controlled at not less than 390°C, and the strip speed is not less than 100m / min.

[0015] The beneficial effects of the present invention compared with the prior art are as follows:

[0016] The present invention detects and analyzes the short strip dark shadow defects on the surface of alloyed hot-dip galvanized steel plates to determine the sources of the defects. Starting from controlling the galvanizing process, by controlling key process parameters such as the content of Al and Fe in the zinc pot, the quality control of the strip surface cleaning, and the alloying process control, the short strip dark shadow defects no longer appear on the surface of the finished products produced by the galvanizing unit, the surface quality of the alloyed coating is improved, the yield rate is significantly improved, the stable production of hot-dip galvanized alloyed products is achieved, the stable operation of the unit is guaranteed and the surface quality meets the requirements, the customer satisfaction is improved, and the market competitiveness is enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention, the drawings related to the embodiments are briefly introduced below.

[0018] Figure 1 This is a diagram showing the optical microscope detection results of the defect area of ​​the alloyed hot-dip galvanized sheet in Example 1.

[0019] Figure 2 It is a schematic diagram of the galvanizing and alloying process of hot-dip galvanized alloyed steel plate. DETAILED DESCRIPTION

[0020] The present invention is described in detail below with reference to the embodiments, but the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only partial embodiments of the present invention. For those skilled in the art, other similar embodiments obtained without creative labor all fall within the protection scope of the present invention.

[0021] Example 1

[0022] This embodiment provides a method for reducing short dark strips on the surface of alloyed hot-dip galvanized steel sheets, by detecting and analyzing short dark strips on the surface of alloyed hot-dip galvanized steel sheets, determining the source of the defects, and taking corresponding control measures to improve the surface quality of the alloyed coating.

[0023] The steps include:

[0024] 1. Detect and analyze the defective area:

[0025] Under an optical microscope, a large number of black spots were found in the defect area, indicating that there were many black block crystals in the defect area ( Figure 1 ), the defective areas are polished and are all pits. Inside the pits is a typical crystalline Zn-Fe phase, which falls off as a whole after polishing, resulting in pits.

[0026] EDS energy spectrum analysis was performed on the defect area, and the Fe content in the defect area reached 20.1% to 39.1% (the Fe content in the defect-free area was 12.4%). The Fe content in the pits was higher and the Zn-Fe phase was more brittle.

[0027] The defective area is stripped and a short black mark can be seen on the surface of the substrate after stripping. Laser confocal observation shows that there is no obvious depth on the surface of the defective area, and there is no obvious difference in morphology from the defect-free area, indicating that there is no problem with the substrate in the defective area.

[0028] Corrosion treatment of defective areas: The defective areas are corroded with 4% nitric acid solution. After corrosion, it can be clearly seen that the grains in the defective areas are more fully corroded, while the grains in the normal areas are not very obvious after corrosion, and the defective areas appear brighter than the normal areas. The main reason is that the iron content in the defective areas is relatively high, the zinc and iron diffuse faster in the defective areas, and the coating is relatively thick. When the steel plate passes through the finishing roller, the finishing is more sufficient, so the defective areas are smoother, and the smooth and sufficient areas have a stronger ability to reflect light. Therefore, the macroscopic appearance of the defective areas is brighter and the grains are corroded more fully.

[0029] Cross-sectional morphology of defective area: The γ phase in the cross section of the coating and the substrate is very obvious, and the Fe content inside the coating is relatively high.

[0030] 2. Determine the source of the defect:

[0031] After the defective sample was polished, a brittle phase fell off at the defect, causing a pit defect in the coating. The Fe content at the defect was significantly higher than that in the normal area. A small amount of material suspected to be zinc slag was found in the defect. It was difficult for the slag to form an alloy layer with the substrate, resulting in insufficient diffusion. The defect appeared as a short strip on the steel plate. It can be judged that the source of the defect was generated during the galvanizing process.

[0032] 3. Confirmation of factors affecting zinc slag:

[0033] The schematic diagram of the galvanizing and alloying process of hot-dip galvanized alloyed steel sheet is as follows Figure 2 As shown, the galvanized strip is led out of the zinc pot by the sinking roller and the correcting roller, and the residual zinc liquid on the surface of the strip is blown off by the air knife, and then immediately enters the induction heating section of the alloying furnace, and then the strip temperature is maintained in the insulation section to allow alloying reaction to occur between the galvanized layer and the strip substrate, and an alloyed coating is formed. Finally, in the cooling section, the outside air is blown in by the fan and sprayed through the slit nozzle on the bellows. The formed gap jet impacts the surface of the strip and quickly cools the strip, so that the strip at the outlet of the furnace top roller can be cooled. The control of the zinc pot composition, the control of the strip surface cleaning quality and the alloying temperature may lead to the formation of zinc slag.

[0034] (1) Zinc pot composition control: The Al and Fe content in the zinc pot used for hot-dip galvanizing alloy has a great influence on the Fe-Al-Zn reaction. The balance moves to the right, and the zinc pot is mainly composed of δ phase bottom slag; when the Al content in the zinc pot is low and the Fe content is high, the amount of δ phase generated in the zinc pot increases; in order to avoid the large-scale generation of zinc slag, the Al and Fe element concentrations in the zinc pot should be reasonably set, and the element content in the zinc pot should be kept stable. The content control of Al and Fe in the zinc pot is shown in Table 1.

[0035] (2) Strip surface cleaning quality control: Hot-dip galvanized strip is a cold hardened steel coil after pickling and cold rolling. There will be some residues on the surface of the strip, mainly grease, iron powder particles and dust from the surrounding environment. Once these residues enter the zinc pot, they will affect the cleanliness of the zinc liquid, increase the iron content of the zinc liquid, and increase the zinc slag. Therefore, the better the cleaning effect of the cleaning section at the front of hot-dip galvanizing, the less surface residues there will be when the strip enters the zinc pot, and the less zinc slag will be produced. The cleaning process control is shown in Table 2.

[0036] (3) Alloying process control: Alloying temperature and time have a great influence on the surface quality of the alloyed coating, so the alloying temperature is controlled. Different alloying processes are used for different strip steels (IF steel and bake-hardened steel). The specific alloying process control is shown in Table 3.

[0037] Table 1 Control of Al and Fe content in zinc pot

[0038]

[0039] Table 2 Cleaning process control

[0040]

[0041] Table 3 Alloying process control

[0042]

[0043] After using this method, the defect degradation rate of the alloyed product surface caused by short dark shadows is 0.

[0044] Comparative Example 1

[0045] The experimental process of Comparative Example 1 is the same as that of Example 1, with the main difference being that the contents of Al and Fe in the zinc pot are controlled as shown in Table 4, the cleaning process is controlled as shown in Table 5, and the alloying process is controlled as shown in Table 6.

[0046] Table 4 Control of Al and Fe content in zinc pot

[0047]

[0048] Table 5 Cleaning process control

[0049]

[0050] Table 6 Alloying process control

[0051]

[0052] Using this method, the defect degradation rate of the alloyed product surface due to short dark shadows is 1.8%.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for reducing short strip shadows on the surface of alloyed hot-dip galvanized sheet, characterized in that: The steps include: (1) The defective area was inspected and analyzed, and the analysis results showed that the defect originated from the galvanizing process; (2) By controlling the Al and Fe content of the zinc pot during the galvanizing process, the quality control of the strip surface cleaning, and the alloying process control, the short strip shadow defects on the surface of the alloyed hot-dip galvanized sheet can be reduced.

2. The method according to claim 1, characterized in that The detection and analysis methods described in step (1) include optical microscopy, EDS spectrum analysis of the defect area, stripping treatment of the defect area, corrosion treatment of the defect area and cross-sectional morphology inspection of the defect area.

3. The method according to claim 2, characterized in that The defective area is corroded using a nitric acid solution with a mass percentage of 1-4%.

4. The method according to claim 1, characterized in that: In step (2), the Al content of the zinc pot is controlled within the range of 0.128% to 0.135%, and the Fe content is controlled within the range of 0 to 0.045%.

5. The method according to claim 1, characterized in that The residual oil content on the surface of the strip after surface cleaning in step (2) is ≤10mg / m 2 , residual iron content ≤10mg / m 2 .

6. The method according to claim 1, characterized in that In step (2), the alloying process is controlled by adopting different alloying processes for different strip steels; for IF steel, the insulation temperature of the moving section is controlled at not less than 290°C, the insulation temperature of the fixed section is controlled at not less than 310°C, and the strip speed is not less than 110 m / min; for bake-hardened steel, the insulation temperature of the moving section is controlled at not less than 360°C, the insulation temperature of the fixed section is controlled at not less than 390°C, and the strip speed is not less than 100 m / min.

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