Analysis and Measures for Color Difference Defects in Hot-Dip Galvanized Steel Strips

Hot-dip galvanized steel strips are high-performance, high-value products produced from hot-rolled coil materials through the processes of cold rolling, annealing, and galvanizing. These products play an immeasurable and irreplaceable role in reducing corrosion, extending lifespan, and conserving energy and materials for steel. Currently, they are widely used in industries such as automotive and home appliances.

Color difference defects are a collective term for surface defects in galvanized steel strips, directly impacting the appearance and coating effect of the strips. More severely, they can have adverse effects on the mechanical properties of the strips.

From an optical perspective, color difference defects result from abnormal surface roughness at the defect location, causing differences in light reflectivity in that area. Based on the current domestic and international research, factors related to color difference defects in steel strips include uneven transverse roughness on the steel strip surface, foreign objects on the zinc layer surface, and uneven zinc grain size.

Edge Color Difference

The macroscopic appearance of edge color difference defects is shown in Figure (a), where the color difference is located around 5cm from the edge of the galvanized coil. It continuously exists throughout the coil, appearing slightly brighter compared to normal surface positions. When measured with a roughness gauge, the roughness (Ra) at the edge color difference position is 0.56μm, while the normal surface roughness is 0.72μm.

Figure (b) depicts edge color difference defects in a cold-rolled coil, also located around 5cm from the edge, continuously existing throughout the coil, appearing slightly whitish compared to the normal surface. Due to the thicker surface iron oxide scale on hot-rolled coils, color difference defects are not visible to the naked eye. Considering the correlation between edge color difference defects and the original material coil, it is suspected that the edge color difference in galvanized coils is caused by issues in the original material.

Subsequently, samples were taken from the galvanized coils with edge color difference and the same batch of original material coils. The microscopic morphology of their surfaces and cross-sections was observed. Figure 2 shows the cross-sectional microscopic morphology of the edge color difference position and the normal position under an electron microscope. As seen in Figure 2 (a), the surface of the galvanized base plate is relatively smooth at the edge color difference position, with a zinc layer thickness of approximately 7-10μm. Figure 2 (b), representing the normal position, shows surface undulations of 3-4μm and a zinc layer thickness of approximately 10-12μm.

At the edge color difference position, the zinc layer thickness is thinner, and the roughness is lower. This is because the cold-rolled coil used as galvanizing material has lower roughness within the range of 5cm from the edge. During the cooling process in the hot rolling stage, the edge of the steel strip cools faster, resulting in smaller grain size, higher hardness and strength, and a thin but dense surface iron oxide scale.

This type of hot-rolled coil, after pickling, retains a dense layer of surface iron oxide scale. After cold continuous rolling, due to the combined effects of fine grain strengthening and lubrication from the surface iron oxide scale, the roughness at the edge of the cold-rolled coil is lower. To control edge color difference defects in galvanized finished products, a method of edge shielding during the cooling process was adopted. This improved the unevenness in lateral temperature, structure, and stress distribution during the cooling process of hot-rolled coils, resulting in a significant improvement in the overall surface quality of cold-rolled and galvanized coils.

Color Difference After Grinding

The macroscopic appearance of color difference defects after sanding the surface of hot-dip galvanized steel strips is shown in Figure (a), presenting dark circular spots. In contrast, the appearance after sanding the surface without color difference is shown in Figure (b).

Color difference defects after sanding were found in galvanized steel strips at various positions after zinc pot exit, indicating that the defect originated from the gas knife position. Under the same gas knife process parameters, the oscillation of the steel strip was observed: when the steel strip speed was 60m/min, the steel strip ran smoothly with minimal oscillation, resulting in a consistent surface after grinding, and color difference defects were not prominent. However, at a steel strip speed of 100m/min, the steel strip exhibited slight vibrations, and color difference defects on the surface were more noticeable after grinding.

This indicates a significant correlation between color difference after grinding and production line speed and the amplitude of steel strip vibration after exiting the zinc pot. During the vibration process, the distance between the upper and lower gas knives changes. Under equal air pressure conditions, this change in distance results in variations in zinc layer thickness.

The reasons for steel strip vibration after exiting the zinc pot are mainly:

  1. Poor accuracy in the installation of sinking rolls or severe wear of the shaft head and sleeve in the later stages of sinking roll use.
  2. Poor incoming strip shape.
  3. Inconsistent blowing pressure on the upper and lower surfaces of the fast cooling air box in the post-galvanized cooling section.

To control steel strip vibration in the hot-dip galvanizing process, the following measures were implemented:

  1. After processing the sinking roll, and before installation, check its surface roughness and whether there are chamfers, burrs, or scratches that may affect installation accuracy. During sinking roll installation, check if the centerline of the steel strip between the sinking roll and the top roll of the cooling tower is consistent.
  2. Adjust the airflow on the upper and lower surfaces of the moving air box in the post-galvanized cooling section to ensure that the opening and pressure of the baffles on the upper and lower surfaces of the air box are consistent.

Color Difference After Stamping

Color difference defects after stamping appear on the stamped parts of hot-dip galvanized steel plates, as shown in the macroscopic appearance in the figure, presenting irregular, patchy shapes.

Color difference defects after stamping are caused by damage to the zinc layer on the surface of galvanized steel plates during the stamping process, resulting in cracks and internal oxidation of the zinc layer. This leads to color difference defects on the surface of stamped parts. Various factors contribute to the surface damage of galvanized steel plates during stamping, such as the galvanizing process, mold material, and lubrication effects.

To prevent surface damage during stamping, measures were taken, including:

  1. Nitriding treatment of the stamping mold surface.
  2. Conducting stretching and extrusion tests with different lubricants. The study showed that the surface film generated by the lubricant on the friction pair surface can reduce surface damage to stamped parts.

Conclusion

  1. Edge color difference in galvanized coils is caused by lower roughness within 5cm from the edge of the cold-rolled coil used as galvanizing material. During the cooling process in the hot rolling stage, the fast cooling of the edge of the steel strip results in small grain size, high hardness and strength, and a thin but dense surface iron oxide scale.After pickling, the dense surface iron oxide scale remains at the edge. During cold continuous rolling, the combination of fine grain strengthening and lubrication from the surface iron oxide scale leads to lower roughness at the edge of the cold-rolled coil. To control edge color difference defects in galvanized finished products, a method of edge shielding during the cooling process was adopted, improving the unevenness in lateral temperature, structure, and stress distribution during the cooling process of hot-rolled coils. This significantly improved the overall surface quality of cold-rolled and galvanized coils.
  2. Color difference defects after grinding on the surface of hot-dip galvanized steel strips are significantly correlated with production line speed and the amplitude of steel strip vibration after exiting the zinc pot. Controlling steel strip vibration, improving sinking roll installation accuracy, and adjusting the airflow in the post-galvanized cooling section were implemented to address this issue.
  3. Color difference defects after stamping on hot-dip galvanized steel plates are caused by damage to the zinc layer during the stamping process. Nitriding treatment of the stamping mold surface and the use of appropriate lubricants were implemented to prevent surface damage during stamping.

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