Analysis and Explanation of Galvanized Defects

Causes and Characteristics of Friction Stain Formation in Hot-Dip Galvanizing

Friction stain on the surface of cold-rolled thin sheets is a typical product defect named using the naming convention of “cause of occurrence + morphological features.”

“Friction” refers to the relative displacement between layers of thin sheet products packaged in coil or sheet form during storage and transportation, under the influence of forces such as the gravity of the steel plate or pressure between layers. This relative displacement changes the surface morphology and color of the sheet. In simple terms, “black spots” are the visual results of surface changes caused by relative displacement between layers of steel plates. Friction stains are defects that occur during the storage and transportation process and fall within the scope of storage and transportation, including lifting and transportation inside warehouses.

Although friction stains are formed during transportation, low tension during the production process, loose winding of the outer circle, and other factors contribute to the occurrence of friction stains. Overall, the impact of production process control and transportation reasons generally follows the “20-30” or “30-70” principle, with production reasons accounting for 20-30% and transportation reasons accounting for 70-80%.

  1. Macroscopic Morphological Features of Friction Stains

For steel coils, as shown in Figure 1, friction stains typically occur at the 3 o’clock, 5 o’clock, 7 o’clock, and 9 o’clock directions of the steel coil. The 5 o’clock and 7 o’clock positions are usually the force-bearing positions during storage and transportation, where the steel coil experiences a normal force F as shown in the figure. Special attention should be paid to the contact and force situation between the bundling and the support frame, as the probability of friction stains occurring at the contact point between bundling and support frame is highest. If multiple steel coils are placed side by side during transportation, the coils in the front and back will squeeze each other, subjected to the pressure F in the vertical cross-section, and it is important to note that the force F is generally not in the center of the steel coil but mostly at the edges.

Figure 1 A schematic diagram of the steel coil cross section and the locations where friction black spots usually appear.

In general, it is difficult to completely eliminate friction stains. If they are controlled within the outermost 1-2 coils of the steel coil, the cut loss within this range is acceptable to customers. However, if serious friction stains occur with 5-10 coils for light coils or 10-20 coils for heavy coils, the cut loss will be significant, and in severe cases, the cut loss length can exceed 100 meters, which is unacceptable to customers. Therefore, it is necessary to effectively control the key points that lead to the formation of friction stains and minimize their occurrence.

Various types of typical morphological features of friction stains are presented below:

(1) Imprint-type friction stains at the contact between steel coils and wooden support frames.

Figure 2: Black spots on the upper and lower surfaces of the strip where the steel coil contacts the wooden bracket

As shown in Figure 2, this type of friction stain occurs at the corresponding positions where the steel coil contacts the wooden support frame after unwrapping. These locations are typically at the 5 o’clock and 7 o’clock directions mentioned earlier. This type of friction stain is similar to the imprint caused by the force on the steel coil but is not an actual imprint. The friction stain occurs after relative sliding between the steel strip and the support frame at the contact point, making it a typical friction stain.

(2) Friction stains generated by mutual compression of steel coils at the edges during transportation.

As shown in Figure 3, when steel coils are placed one after another during transportation, and all steel coil edges have protective rings, the areas where mutual contact occurs are mainly at the edges when the vehicle experiences front-to-back oscillation. After unwrapping, continuous friction stains are found at a distance of 40 cm from the edge of the steel coil, corresponding to the areas where the steel coils come into contact.

Figure 3: The appearance of friction black spots on the edge of the coil and the corresponding placement of the steel coil during transportation

(3) Friction stains corresponding to the force-bearing positions of steel coil bundling.

Figure 4 Friction black spots caused by relative sliding between the strip steel and the strap

As shown in Figure 4, friction stains occur at the contact points between the steel strip and the bundling due to relative sliding. The position and width of the friction stain completely coincide with the position of the bundling. The friction stain occurs at the main force-bearing position between the steel coil and the bundling.

(4) Irregular-shaped friction stains.

Figure 5 Irregular shape friction black spots

As shown in Figure 5, this category represents irregular-shaped friction stains with a large area and significant impact. These extensive friction stains correspond to defects on the entire length of the steel strip. If friction stains occur in bundled materials, they are often similar to the large-area stains shown in Figure 5.

  1. Microscopic Morphological Features of Friction Stains

As mentioned earlier, the microscopic organizational analysis of friction stains presented in this article cannot be the primary basis for analyzing the root causes of this defect. Unlike other defect analyses that focus on microscopic structure analysis and aim to infer process anomalies from microscopic structural changes to analyze the fundamental causes of defects, the analysis of the causes of friction stains requires starting from the process, especially factors such as winding tension. Careful analysis of every detail in packaging, lifting, transportation, etc., is necessary to determine the causes. Microscopic organizational detection serves only to showcase the microscopic features of friction stains, deepening the understanding of this defect.

Comprehensive analysis of multiple microscopic organizational results of friction stains can be summarized into four main features:

  1. Light friction stains, as shown in Figure 6, exhibit slight abrasion on the surface of the zinc layer, with minimal changes in the original texture of the zinc layer surface.
Figure 6 Slight wear morphology of zinc layer

2. Moderate friction stains, as shown in Figure 7, have noticeable scratches on the surface of the zinc layer formed after significant relative displacement. Many areas of the zinc layer surface have been flattened under pressure and relative friction, losing their original morphology.

Figure 7 Moderate wear morphology of zinc layer

3. After mutual abrasion of the zinc layer, the morphology of the material transferred after friction and shedding is shown in Figure 8. This also belongs to the category of moderate abrasion.

Figure 8 Material transfer after relative displacement and wear on the surface of the zinc layer

4. Severe abrasion of the steel strip surface, even with severe damage to the substrate, is the most serious type. As shown in Figure 9, the surface of the substrate has been severely damaged. The occurrence of such severe damage to the galvanized sheet surface indicates significant problems in the storage and transportation process.

Figure 9 Contusion on the surface of the substrate after severe wear of the zinc layer

Microscopic organizational analysis results indicate that the zinc layer or substrate at the friction stain location has experienced varying degrees of damage, with light, moderate, and severe abrasion. These features are directly related to the magnitude of the frictional force and relative sliding displacement, providing a microscopic basis for judging the severity of friction stains.

  1. Causes of Friction Stains

As mentioned earlier, the causes of friction stains are complex and mainly involve factors such as the winding tension during the production process, the tightness of the winding of the outer circle, and the bundling and transportation processes. Based on the analysis of the causes of friction stains in recent years, the following main points are summarized:

  1. Influence of the winding tension during the production process:
    • Loose winding: When the winding tension is insufficient, the thin sheet material is easily displaced relative to the inner layer during the transportation process, leading to the formation of friction stains.
    • Uneven winding: The uneven winding of thin sheets during the production process can cause the edges of adjacent coils to rub against each other, resulting in friction stains.
  2. Impact of the bundling process:
    • Force concentration: During the bundling process, if the force is concentrated on a small area, it can lead to localized friction stains at the bundling contact points.
  3. Influence of the transportation process:
    • Bundling pressure: If the bundling pressure is too high during transportation, it can cause excessive friction between the steel coil and the bundling material, leading to friction stains.
    • Wooden support frame: The use of wooden support frames without proper protection or excessive protrusions can result in friction stains at the contact points with the steel coils.
  4. Impact of storage conditions:
    • Temperature and humidity: Extreme temperature and humidity conditions during storage can contribute to the formation of friction stains.
    • Storage time: Extended storage time can increase the likelihood of friction stains due to the prolonged exposure of steel coils to environmental factors.
  5. Influence of external factors during transportation:
    • Vehicle conditions: The condition of the transportation vehicle, including the presence of shock absorbers, can impact the likelihood of friction stains.
    • Handling during loading and unloading: Rough handling or improper loading and unloading procedures can lead to friction stains.
  6. Other factors:
    • Protective measures: The effectiveness of protective measures, such as the use of edge protectors and proper bundling materials, plays a crucial role in preventing friction stains.
  7. Prevention and Control Measures

To effectively prevent and control friction stains, a combination of measures related to the production process, bundling, transportation, and storage conditions is required. Key preventive measures include:

  1. Optimizing winding tension during production:
    • Ensure proper winding tension to prevent relative displacement of thin sheet layers during transportation.
  2. Improving bundling process:
    • Distribute bundling force evenly to avoid concentrated pressure points.
    • Use appropriate bundling materials and protective measures to minimize friction between the steel coil and bundling material.
  3. Enhancing transportation conditions:
    • Adjust bundling pressure to an optimal level to avoid excessive friction.
    • Ensure proper conditions inside the transportation vehicle, including shock absorption measures.
  4. Implementing effective protective measures:
    • Use edge protectors to minimize contact between adjacent steel coils.
    • Ensure proper packaging and protection of steel coils, especially at the edges and bundling points.
  5. Monitoring storage conditions:
    • Maintain suitable temperature and humidity levels in storage areas.
    • Implement timely inspections to identify and address potential issues.
  6. Training and awareness:
    • Provide training to personnel involved in handling and transportation to ensure proper procedures are followed.
    • Raise awareness among employees about the importance of preventing friction stains and the impact of their actions on product quality.
  7. Regular quality checks:
    • Conduct regular inspections of finished products to identify any signs of friction stains early in the process.
    • Establish a robust quality control system to monitor and address friction stain issues promptly.
  8. Collaboration with transportation partners:
    • Work closely with transportation partners to communicate specific handling requirements and ensure the use of suitable transportation conditions.

By implementing these preventive measures and addressing the various factors contributing to friction stains, manufacturers can significantly reduce the occurrence of this defect and enhance the overall quality of galvanized products. Continuous monitoring, improvement of processes, and collaboration across the production and transportation chain are essential for effective friction stain prevention and control.

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