1.How to distinguish the "type" of pockmarks?
Iron oxide scale indentation type pitting:
Cause: Iron oxide scale on the surface of the hot-rolled coil was not thoroughly pickled and was pressed into the steel plate surface during cold rolling.
Characteristics: Pitting is usually shallow, may be unevenly distributed, and is blackish or dark gray in color.
Impact: Primarily affects appearance and coating quality. Deep pitting can lead to uneven plating/coating, blistering, or peeling.
Raw material inclusion/void type pitting:
Cause: Insufficient purity of molten steel, containing non-metallic inclusions (such as alumina), which detach during rolling, forming pits or micropores.
Characteristics: Pitting may be irregular in shape, rough at the bottom, and sometimes the inclusions are visible in cross-section.
Impact: This is a serious defect. It disrupts the continuity of the material, acts as a stress concentration point, and significantly reduces the material's fatigue strength, formability, and corrosion resistance. The greater the depth, the greater the damage.
Roll-printed/embossed pits (periodic):
Cause: Pitfalls or metal adhesion on the surface of the rolls or tension rolls periodically imprinted onto the plate surface.
Characteristics: Appears at equal intervals along the rolling direction, with consistent shape and depth.
Impact: Besides affecting appearance and coating, deep periodic pits may become crack initiation points during subsequent bending or stretching.
2.How does the depth of pitting affect high-end exterior components/home appliance panels?
The impact is significant and is generally unacceptable.
• "Orange peel" or "pitted" effects after spraying result in an unacceptable appearance.
• Cannot be covered after electroplating, becoming a noticeable defect.
3.How does the depth of pitting affect the usability of automotive outer panels?
The impact is extremely significant and unacceptable.
• It affects the smoothness and gloss of the paint surface (DOI value).
• Deep pitting creates weak points in paint adhesion, potentially leading to premature corrosion.
4.What are the testing and judgment criteria?
Depth Measurement Methods:
**Stylus Profilometer:** Most accurate, directly measures the maximum pit depth and contour of the pit.
**Metallurgical Microscope:** Allows for cross-sectional observation, distinguishing pit types (indentation or detachment).
**Roughness Tester Comparison:** Compares the Ra value difference between pitted and normal areas for indirect assessment.
**Experience and Touch:** Experienced quality inspectors can make a preliminary judgment by touching the pit with their fingernail or fingers.
**Judgment Basis:**
First, based on standards: National standards (such as GB/T), industry standards, or enterprise standards referenced in the contract. For example, many standards stipulate that "visible defects affecting use are not allowed on the surface," and specific limits for pits are often clearly defined in the technical agreement between the parties.
**Core Factor:** "Affecting Use": A comprehensive judgment must be made considering depth, diameter, and the number (density) per unit area. One deep pit is far more harmful than ten shallow pits.
5.What practical suggestions do you have?
**Define Application:** First, determine what product your cold-rolled coil will be used to manufacture. This is fundamental to determining acceptance.
**Identify Type and Depth:**
Observe the morphology of the pitting using a magnifying glass or microscope.
Measure the maximum depth using professional instruments if possible.
Determine whether it's a surface problem (oxide scale indentation, rust) or an internal problem (inclusions).
**Perform Process Validation:**
Take representative samples with pitting and simulate subsequent processing (e.g., spraying, electroplating, stamping, bending).
Check whether the pitting becomes visible after processing or causes product failure. This is the most reliable method of judgment.
**Communicate with Supplier:**
Provide clear photos of the defects, their location, depth data, and process validation results.
Handle quality objections according to the contract and technical agreement (requesting replacement, claims, or concessions and acceptance of discounted prices).