Steel plate production is a complex process involving various stages where surface defects may arise. These defects not only impact the aesthetic appearance of the steel but can also compromise its mechanical properties, affecting its overall performance and usability. This article will explore the common surface defects in steel plates, analyze the underlying causes of these defects, and discuss potential solutions.

1. Roll Marks
Roll marks are one of the most commonly observed surface defects in steel plates. These marks usually appear as a series of irregular, periodic indentations or raised areas on the surface of the steel. They often have a repetitive, uniform pattern, but the shape and size of these marks can vary.

Causes:

Roller Wear: Over time, the rollers used in the steel manufacturing process can wear down or become improperly adjusted. This wear can create uneven contact with the steel surface, leading to indentations.
Contaminants on Rollers: When foreign materials such as dust or scale accumulate on the rollers, they can imprint these irregularities onto the steel plate during the rolling process.
Uneven Pressure Distribution: If the rollers are not calibrated properly or are operating under uneven pressure, this can cause distortion on the steel surface, resulting in roll marks.
2. Surface Inclusions
Surface inclusions refer to foreign particles embedded in the surface of the steel plate. These inclusions can vary in size and shape but are typically non-metallic. They often appear as spots or streaks of a different color, such as red, yellow, gray, or black.

Causes:

Improper Steel Ingot Quality: Inclusions can originate from the initial ingot or slab, where non-metallic particles like slag or oxides are not fully removed. These particles can then be pressed into the surface during the rolling process.
Contaminated Furnace Atmosphere: If the heating furnace contains impurities, such as refractory materials or slag, these particles can fall onto the steel surface during heating and become embedded in the material as inclusions.
Insufficient Cleaning During Rolling: If the steel surface is not adequately cleaned before rolling, residual contaminants can be pressed into the steel, creating surface inclusions.
3. Oxide Scale (Rust)
Oxide scale is a thin layer of oxidized iron that forms on the surface of steel during high-temperature processing. This scale is often dark or reddish-brown in color and can be either loosely attached or tightly bonded to the steel surface. Depending on the nature of the oxidation, oxide scale can vary from light, easily removed layers to thick, difficult-to-clean layers.

Causes:

Overheating of Steel: Prolonged exposure to excessive temperatures during heating can lead to the formation of oxide scale. The higher the temperature and the longer the exposure, the thicker and more persistent the scale becomes.
Inadequate Scale Removal: If the scale is not effectively removed during the cleaning or pickling processes, it can remain on the steel surface and be pressed in during rolling.
Excessive Oxygen in Furnace Atmosphere: A high concentration of oxygen in the furnace can accelerate the oxidation process, leading to thicker oxide layers on the steel plate.
4. Thickness Variability
Thickness variability refers to uneven distribution of thickness across the surface of a steel plate. In some areas, the steel may be thinner than the specified thickness, while in others, it may be excessively thick. Such variations can result in significant deviations from the required specifications.

Causes:

Uneven Roller Gaps: If the roller gaps are not adjusted correctly or are unevenly distributed, the steel will not be uniformly compressed during the rolling process, resulting in inconsistent thickness across the plate.
Roller Wear: As rollers wear down over time, their ability to maintain consistent pressure on the steel surface diminishes, causing variations in thickness.
Temperature Imbalance: Inadequate control of the heating temperature of the steel can lead to uneven heating, causing variations in thickness during the rolling process.
5. Pitting
Pitting is characterized by the appearance of small, localized indentations or holes on the surface of the steel plate. These pits can range in size and depth, with some being shallow and others penetrating deeper into the steel.

Causes:

Surface Oxidation: If the steel is exposed to atmospheric conditions for extended periods before rolling, oxidation can lead to the formation of small pits or holes on the surface.
Inconsistent Heating: Uneven heating of the steel slab can lead to the formation of surface imperfections, including pitting. Variations in temperature can cause certain areas to oxidize more than others, leading to deeper pits.
Contaminants in the Rolling Process: During rolling, contaminants such as debris or particles on the surface can cause localized surface damage, leading to pitting.
6. Bubbles (Gas Pockets)
Bubbles or gas pockets are surface defects that occur when air or gas becomes trapped within the steel as it solidifies. These bubbles may form small raised areas on the steel surface, which can range in size from minute pinholes to larger cavities.

Causes:

Poor Degassing Process: During the casting or cooling phase, if the steel is not properly degassed, residual gases may remain trapped within the metal, forming bubbles on the surface.
Inconsistent Cooling Rates: If cooling rates are not uniform across the steel plate, gas may become trapped in areas that cool more slowly, causing bubble formation.
Inadequate Surface Preparation: If the surface is not cleaned properly before rolling, impurities or gases that are present on the surface can become trapped within the steel.
7. Folding
Folding defects occur when portions of the steel plate overlap or fold over during the rolling process. This creates a visible raised area, often in the shape of a crease or fold, on the surface of the steel.

Causes:

Excessive Rolling Pressure: If the rollers are set to too high a pressure, it can cause the metal to fold over itself during the rolling process.
Inconsistent Rolling Speed: If the rollers are not rotating at consistent speeds or if there is too much fluctuation, it can lead to irregular pressure being applied to the steel, causing folding.
Uneven Heating: Uneven heating of the steel plate before rolling can cause areas of the steel to become softer and more malleable than others, leading to folding.
8. Edge Distortion (Wavy Edge)
Edge distortion, also known as a "wavy edge," occurs when the edges of the steel plate curl or distort unevenly. This can be due to various factors that affect how the steel is cooled or processed.

Causes:

Improper Roller Gap Adjustments: When the roller gaps are incorrectly calibrated, the edges of the steel plate may receive less pressure, causing them to curl or distort.
Uneven Cooling: If the steel is not cooled evenly across its entire surface, certain areas may contract more than others, leading to edge distortion.
Incorrect Roll Alignment: Misalignment of the rolls or improper control of the steel's entry into the rolls can lead to irregular edge formation.
9. Loose Coils
Loose coils refer to a situation where the steel plate or strip does not coil tightly during the winding process. This results in a loose, irregularly shaped coil that may be difficult to handle.

Causes:

Low Winding Tension: If the tension applied during the winding process is too low, the steel will not coil tightly and may form loose coils.
Inconsistent Rolling Process: Variations in the rolling process, such as inconsistent thickness or surface conditions, can cause the steel to coil loosely.
Incorrect Winding Speed: If the winding speed is too fast or too slow, it can result in loose coils that do not form a tight, uniform shape.
10. Flat Coils
Flat coils occur when the edges of the coil are not evenly wound, resulting in a coil with a flattened shape instead of the desired round shape. This defect can impact handling and further processing of the steel.

Causes:

Inconsistent Winding Tension: If the tension during the winding process is too high or too low, it can cause the edges of the coil to flatten or deform.
Inadequate Winding Equipment: The winding machinery itself may be faulty or improperly set up, leading to issues with coil formation.
Inconsistent Roll Pressure: Uneven pressure distribution during the rolling process can lead to variations in coil shape.
11. Edge Waves (Kite-shaped Curvature)
Edge waves or kite-shaped curvature refer to the appearance of a wavy or curved pattern along the edges of the steel plate. This defect can affect the aesthetic quality and the functional performance of the steel.

Causes:

Uneven Roller Pressure: When the rollers do not exert uniform pressure across the surface of the steel, it can lead to localized distortion, forming edge waves.
Temperature Variations: Significant differences in temperature during the rolling process can cause uneven contraction of the steel, leading to edge waves.
Roller Wear: As rollers wear down over time, they may exert uneven pressure on the steel, causing waves to form along the edges.
12. Buckle or Camelback
Buckle or camelback refers to a defect where the steel plate forms a distinct curvature along its length. This can result in an uneven surface that is difficult to process further.

Causes:

Incorrect Cooling Process: Rapid or uneven cooling during the manufacturing process can cause areas of the steel to contract more than others, creating a buckle or camelback shape.
Excessive Roll Pressure: Too much pressure during rolling can lead to distortion, causing the steel to curve along its length.
Roller Misalignment: If the rollers are misaligned or improperly adjusted, they can apply inconsistent pressure along the length of the steel, causing buckling.
Conclusion
The surface defects in steel plates are diverse and often complex, arising from a combination of factors in the production process. Identifying the root causes of these defects is essential to improving the quality of steel products. Through careful monitoring and adjustment of the manufacturing process, it is possible to reduce the occurrence of these defects and produce steel plates with superior surface quality.
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