Common Defects and Control Measures in Pickled Plates

Overview of Pickled Products

Pickled plates are intermediate products produced from hot-rolled steel coils. After pickling, they exhibit surface quality and usage characteristics between hot-rolled and cold-rolled plates. Compared to hot-rolled plates, pickled plates offer advantages such as improved surface quality, higher dimensional accuracy, enhanced surface smoothness, improved aesthetics, and reduced environmental pollution caused by dispersed pickling. Additionally, pickled products, in contrast to hot-rolled ones, facilitate welding due to the removal of surface iron oxide, making them suitable for surface treatments like oiling and painting.

The surface quality grades for general hot-rolled products are classified as FA, while pickled products fall under FB, and cold-rolled products under FB/FC/FD. Pickled products can replace cold-rolled products for certain structural components, effectively substituting hot for cold.

Pickled Plates
Pickled Plates

Common Defects in Pickled Plates

Common defects in the production process of pickled plates include: embedding of iron oxide scales, oxide spots (surface landscape defects), waist bends (transverse folds), scratches, yellow spots, under-pickling, over-pickling, etc.
(Note: Defects are linked to standards or agreements; those not meeting the requirements are considered defects. For convenience, the term “defects” is used to represent a certain type of appearance.)

2.1 Embedding of Iron Oxide Scales

The embedding of iron oxide scales is a surface defect formed during the hot rolling process. After pickling, it often appears as black, dot-like or streak-like impressions with a rough surface, noticeable to the touch, and sporadically or densely distributed.

Causes: Various factors contribute to the formation of iron oxide scales, including heating furnace conditions, scale removal processes, rolling processes, roll material and condition, roller table condition, and rolling schedules.

Control Measures: Optimize heating processes, increase scale removal passes, and regularly inspect and maintain roller tables and rolls to ensure a well-maintained rolling line.

2.2 Oxide Spots (Surface Landscape Defects)

Oxide spot defects refer to dot-like, linear, or pitted patterns left on the surface after removing iron oxide scales from hot-rolled coils. Visually, they present irregular color variations resembling a landscape painting, hence also called landscape defects. These appear as dark, mountainous patterns with an overall or partial distribution on the strip surface. Essentially, they are traces of iron oxide scales that were not completely removed and got pressed into the substrate during subsequent rolling, becoming visible after pickling.

Causes: The defect is related to the incomplete removal of iron oxide scales on the surface of hot-rolled steel strips, accentuated after pickling.

Control Measures: Lower the exit steel temperature from the heating furnace, increase roughing descaling passes, and optimize finishing rolling cooling water processes.

2.3 Waist Bends

Waist bends are transverse wrinkles, folds, or deformations perpendicular to the rolling direction. Visible to the naked eye during uncoiling, severe instances can be felt by touch.

Causes: Low-carbon aluminum-killed steels have an inherent yield plateau. When uncoiling under bending stress, the uniform bend turns into an uneven one, resulting in waist bends.

Control Measures: 1) Adjust the tension of the uncoiling mandrel appropriately; 2) Regularly inspect the surface condition of the mandrel and replace it when abnormal; 3) Perform rework on coils with poor incoming strip shapes and loose inner wraps.

2.4 Yellow Spots

Yellow spots are localized or entire surface areas with yellowish marks that cannot be covered after oiling, affecting product quality appearance.

Causes: The high reactivity of the freshly pickled steel strip surface leads to inadequate rinsing, causing surface oxidation and yellowing. Additionally, issues like clogged spray beams and improper angles contribute.

Control Measures: Regularly inspect the condition of spray beams and nozzles, and clean nozzles periodically. Ensure proper water pressure for rinsing.

2.5 Scratches

Scratches manifest as surface abrasions with irregular depths, negatively impacting product surface quality.

Causes: Inappropriate tension of the entry looper, wear of nylon lining plates, incoming strip shape variations, and loose coiling inside the hot coil are contributing factors.

Control Measures: 1) Adjust the tension of the entry looper appropriately; 2) Periodically inspect the surface condition of lining plates and replace those with abnormal conditions; 3) Rework coils with poor incoming strip shapes and loose inner wraps.

2.6 Under-Pickling

Under-pickling refers to incomplete or insufficient removal of localized iron oxide scales, resulting in a gray-black appearance with scale-like or wavy patterns on the plate surface.

Causes: Related to the pickling acid process and the condition of the steel strip surface. Factors include inadequate acid concentration, low temperature, high strip running speed, insufficient immersion in acid, uneven thickness of the iron oxide scale on the hot-rolled coil, and waviness of the steel coil.

Control Measures: Adjust pickling processes, optimize hot-rolling processes, control strip shapes, and establish a rational process system.

2.7 Over-Pickling

Over-pickling occurs when the pickling process is excessive, resulting in a dark black or brown-black appearance with blocky or patchy black or yellow spots. The surface is generally rough.

Causes: Opposite to under-pickling, over-pickling may occur with higher acid concentration, higher temperature, and slower strip speed, especially in the central and wider portions of the strip.

Control Measures: Adjust and optimize pickling processes, establish appropriate process systems, and conduct quality training to enhance quality management.

Understanding Quality Management of Pickled Plates

In comparison to hot-rolled plates, it is commonly believed that pickled plates, with only an additional pickling step, should be relatively easy to produce with qualified quality. However, practical experience shows that ensuring the quality of pickled products requires not only maintaining a well-functioning pickling line but also stabilizing the production and operation of preceding processes (steelmaking and hot rolling). This ensures the quality of incoming hot-rolled material. Therefore, a consistent quality management approach must be adhered to, ensuring that each process maintains normal quality conditions to guarantee the final product’s quality.