A method for reducing short line shadow on the surface of an alloyed hot-dip galvanized sheet

Abstract

The application discloses a method for reducing short strip shadow on the surface of an alloyed hot galvanizing plate and belongs to the technical field of hot galvanizing alloying. The method comprises the following steps: firstly, detecting and analyzing a defect area, and the analysis result shows that the defect is derived from a galvanizing process; then, controlling the content of Al and Fe in a zinc pot, the surface cleaning quality control of a strip steel and the alloying process control in the galvanizing process, thereby reducing the short strip shadow defect on the surface of the alloyed hot galvanizing plate, and the surface of a finished product produced by a galvanizing unit is free of the short strip shadow defect, the surface quality of the alloyed coating is improved, the material yield is remarkably improved, the stable production of the hot galvanizing alloyed product is realized, the stable operation of the unit is ensured, the surface quality meets the requirements, the customer satisfaction is improved, and the market competitiveness is improved.

Description

Technical Field

[0001] This invention belongs to the field of hot-dip galvanizing alloying technology, specifically relating to a method for reducing short shadows on the surface of alloyed hot-dip galvanized sheets. Background Technology

[0002] In recent years, with the increasing application of high-grade alloyed hot-dip galvanized steel sheets in automobiles and home appliances, the requirements for the surface quality of alloyed steel sheets have become increasingly stringent. Steel companies are also paying more and more attention to the surface quality of galvanized steel sheets in order to improve product quality. The high surface quality requirements for alloyed hot-dip galvanized steel sheets generally refer to the strict control of the surface roughness and surface condition of the steel sheet through subsequent processing, requiring flatness, high dimensional accuracy, small tolerances, and no surface defects. As a high-value-added coating product, the surface quality of alloyed hot-dip galvanized steel sheets has always been a focus and challenge in production control, especially for automotive hot-dip galvanized steel sheets. The appearance quality of the paint film after a car is painted is an important indicator for judging its quality. Many factors affect the appearance quality of hot-dip galvanized steel sheets after painting, among which the surface morphology of the hot-dip galvanized steel sheet is particularly important. Research on surface defects in alloyed coatings has become one of the key issues of concern in the industry.

[0003] Patent application number 201680023058.5 discloses a method for suppressing the generation of patterned surface defects, thereby providing galvanized steel sheets with good surface properties; the composition of the alloyed hot-dip galvanized base plate 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%, with the balance being Fe and unavoidable impurities, so that the Mn intensity of the steel sheet surface layer based on glow discharge emission spectroscopy (GDS) is less than 3.5 (V). This patent mainly eliminates patterned defects by adjusting the chemical composition of the hot-dip galvanized base plate, but does not involve the study of short strip defects. Researching short strip defects that affect the surface quality of alloyed coatings and thus improving product surface quality is of great significance. Summary of the Invention

[0004] Therefore, the purpose of this invention is to provide a method for reducing short shadows on the surface of alloyed hot-dip galvanized steel sheets. This invention detects and analyzes the short shadow defects on the surface of alloyed hot-dip galvanized steel sheets to determine the source of the defects, thereby taking corresponding control measures to improve the surface quality of the alloyed coating, significantly improve the yield, enhance customer satisfaction, and improve market competitiveness.

[0005] The objective of this invention is achieved through the following means:

[0006] This invention provides a method for reducing short shadows on the surface of alloyed hot-dip galvanized steel sheets, comprising the following steps:

[0007] (1) The defect area was detected and analyzed. The analysis results showed that the defect originated from the galvanizing process.

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

[0009] Based on the above technical solution, the detection and analysis method in step (1) further includes optical microscopy inspection, EDS energy dispersive spectroscopy analysis of the defect area, deplating 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, furthermore, the defect 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 content of Al in the zinc pot is controlled at 0.128% to 0.135%, and the content of Fe is controlled at 0 to 0.045%.

[0012] Based on the above technical solution, further, in step (2), the residual oil content on the strip surface after surface cleaning is ≤10mg / m. 2 Residual iron content ≤10mg / m³ 2 .

[0013] Based on the above technical solution, further, in step (2), the alloying process control adopts different alloying processes for different strip steels.

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

[0015] The advantages of this invention over the prior art are as follows:

[0016] This invention detects and analyzes short-strip shadow defects on the surface of alloyed hot-dip galvanized steel sheets to determine the source of the defects. Starting from controlling the galvanizing process, by controlling key process parameters such as the Al and Fe content in the zinc pot, the quality control of strip surface cleaning, and the alloying process, the surface of the finished products produced by the galvanizing unit no longer exhibits short-strip shadow defects. This improves the surface quality of the alloyed coating, significantly increases the yield, achieves stable production of hot-dip galvanized alloyed products, ensures stable unit operation and surface quality that meets requirements, improves customer satisfaction, and enhances market competitiveness. Attached Figure Description

[0017] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0018] Figure 1 This is an image showing the results of optical microscopy examination of the defect area of ​​the alloyed hot-dip galvanized sheet in Example 1.

[0019] Figure 2 This is a schematic diagram of the galvanizing and alloying process for hot-dip galvanized alloyed steel sheets. Detailed Implementation

[0020] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0021] Example 1

[0022] This embodiment provides a method for reducing short shadows on the surface of alloyed hot-dip galvanized steel sheets. By detecting and analyzing the short shadow defects on the surface of alloyed hot-dip galvanized steel sheets, the source of the defects is determined, and corresponding control measures are taken to improve the surface quality of the alloyed coating.

[0023] Includes the following steps:

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

[0025] Examination under an optical microscope revealed numerous black spots in the defect area, indicating the presence of many black blocky crystals within the defect region. Figure 1 The defective areas were polished, and all of them were pits. The interior of the pits contained typical crystalline Zn-Fe phases, which were completely detached after polishing, resulting in the pits.

[0026] EDS energy dispersive spectroscopy analysis of the defect area showed that the Fe content in the defect area reached 20.1% to 39.1% (the Fe content in the defect-free area was 12.4%), and the Fe content in the pits was even higher, and the Zn-Fe phase was more brittle.

[0027] After the defective area was stripped of plating, short black marks could be seen on the substrate surface. Laser confocal microscopy showed no obvious depth on the surface of the defective area and no significant difference in morphology from the non-defective area, indicating that the substrate in the defective area was not problematic.

[0028] Defect area corrosion treatment: The defect area is treated with 4% nitric acid solution. After corrosion, it can be clearly seen that the grain corrosion in the defect area is more complete, while the grain corrosion in the normal area is not very obvious. The defect area is also brighter than the normal area. The main reason is that the iron content in the defect area is higher, and the zinc-iron diffusion is faster in the defect area, resulting in a thicker coating. When the steel plate passes through the finishing roller, the finishing is more complete, so the defect area is flatter. The flatter area has a stronger ability to reflect light, so the defect area appears brighter and the grain corrosion is more complete.

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

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

[0031] After polishing, the defective sample showed brittle phase detachment at the defect site, resulting in plating pits. The Fe content at the defect site was significantly higher than that in the normal area. A small amount of material suspected to be zinc slag was found in the defect. The slag was difficult to form an alloy layer with the substrate, resulting in insufficient diffusion. The defect appeared as short strips on the steel plate, indicating that the defect originated from the zinc plating process.

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

[0033] A schematic diagram of the galvanizing and alloying process for hot-dip galvanized alloyed steel sheets is shown below. Figure 2 As shown, after galvanizing, the strip steel is drawn from the zinc pot by the submerged roller and the straightening roller. The residual zinc liquid on the surface of the strip steel is blown off by the air knife. Then, it immediately enters the induction heating section of the alloying furnace. After that, it passes through the heat preservation section to maintain the temperature of the strip steel so that the galvanized layer and the strip steel substrate can undergo an alloying reaction and form an alloy coating. Finally, in the cooling section, the outside air is blown in by the fan and sprayed through the slit nozzles on the air box. The slit jet formed impacts the surface of the strip steel and cools the strip steel rapidly. This allows the strip steel at the furnace top roller exit to be cooled. The control of zinc pot composition, the quality control of strip steel surface cleaning, and the alloying temperature may lead to the formation of zinc dross.

[0034] (1) Zinc pot composition control: The content of Al and Fe elements in the zinc pot used for hot-dip galvanizing alloys has a significant impact on the Fe-Al-Zn reaction. In low-aluminum zinc pots, The equilibrium shifts to the right, and the zinc pot mainly contains 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. To avoid the generation of a large amount of zinc slag, the concentrations of Al and Fe in the zinc pot should be set reasonably, and the element content in the zinc pot should be kept stable. The control of Al and Fe content in the zinc pot is shown in Table 1.

[0035] (2) Quality control of strip surface cleaning: Hot-dip galvanized strip is a cold-hardened steel coil that has undergone pickling and cold rolling. Some residues will remain 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 amount of zinc dross. Therefore, the better the cleaning effect of the cleaning section before hot-dip galvanizing, the less residue will remain on the surface of the strip when it enters the zinc pot, and the less zinc dross will be produced. The cleaning process control is shown in Table 2.

[0036] (3) Alloying process control: Alloying temperature and time have a great impact on the surface quality of alloyed coatings. Therefore, the alloying temperature is controlled. Different alloying processes are adopted for different strip steels (IF steel and bake hardening steel). The specific alloying process control is shown in Table 3.

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

[0038]

[0039] Table 2 Cleaning Process Control

[0040]

[0041] Table 3 Alloying process control

[0042]

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

[0044] Comparative Example 1

[0045] The experimental procedures for Comparative Example 1 and Example 1 are the same. The main difference is that the Al and Fe content in the zinc pot is 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 pots

[0047]

[0048] Table 5 Cleaning Process Control

[0049]

[0050] Table 6 Alloying Process Control

[0051]

[0052] Using this method, the defect degradation rate of alloyed products due to short shadows on the surface 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, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions 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 shadows on the surface of alloyed hot-dip galvanized steel sheets, characterized in that, Includes the following steps: (1) The defective area was inspected and analyzed. The results showed that the defect originated from the galvanizing process. (2) By controlling the content of Al and Fe in the zinc pot during the galvanizing process, controlling the quality of strip surface cleaning, and controlling the alloying process, the short strip shadow defect on the surface of alloyed hot-dip galvanized sheet can be reduced. The detection and analysis methods described in step (1) include optical microscopy inspection, EDS energy dispersive spectroscopy analysis of the defect area, decoating treatment of the defect area, corrosion treatment of the defect area, and inspection of the cross-sectional morphology of the defect area. For corrosion treatment of defective areas, a nitric acid solution with a mass percentage of 1-4% is used. In step (2), the Al content of the zinc pot is controlled at 0.128%~0.135%, and the Fe content is controlled at 0~0.045%. In step (2), the residual oil content on the strip surface after surface cleaning is ≤10mg / m. 2 Residual iron content ≤10mg / m³ 2 ; In step (2), the alloying process is controlled by using different alloying processes for different strip steels. For IF steel, the insulation temperature of the moving section is controlled at not less than 290℃, the insulation temperature of the fixed section is controlled at not less than 310℃, and the strip speed is not less than 110m / min. For bake-hardening steel, the insulation temperature of the moving section is controlled at not less than 360℃, the insulation temperature of the fixed section is controlled at not less than 390℃, and the strip speed is not less than 100m / min.

Images