What are the defects of cold drawn flat steel?

Cold drawn flat steel is a shaped material processed through the cold stretching process. Its defects mainly originate from factors such as raw material quality, cold working process parameters, mold condition, and equipment accuracy. The types of defects can be divided into three major categories: surface defects, dimensional accuracy defects, and internal structure defects. Specifically, they are as follows:

Surface Defects (most common, directly affecting appearance and safety of use)

Scratches (dents, scratches)

• Performance: Linear grooves appear along the drawing direction on the surface, with depths ranging from several micrometers to millimeters, and in severe cases, they may penetrate the length of the local area.
• Cause:
• The raw material surface has oxidation scale, rust, and impurities that were not properly cleaned, which resulted in surface scratches during stretching.
• The mold (drawn mold, guide mold) inner wall has wear, scratches, or adhered metal debris; or insufficient mold lubrication.
• Equipment导向device, conveying roller surface is rough or contains foreign objects.
• Impact: Reduces surface smoothness, making scratch areas prone to stress concentration, which can lead to cracking during subsequent processing (such as bending, welding) or to premature rusting in corrosive environments.

2. Oxide Scale and Rust

• Performance: Surface covered with dark gray or yellow-brown oxide layer (scale), or pinpoint/spotty rusting; in severe cases, the scale may peel off to expose the base material.
• Reason:
• Raw materials (hot-rolled flat steel) exhibit incomplete pickling and phosphatizing treatments, resulting in residual oxide scale.
• Untreated for rust prevention (such as oiling, galvanizing) after cold drawing, or stored in a humid, poorly-ventilated environment.
• Lubricant failure during stretching resulted in localized high-temperature oxidation.
• Impact: Aesthetically, the oxidation scale reduces the adhesion of subsequent coatings (paint, electroplating), while rusting further corrodes the substrate.

3. Speckled (Mottled)

• Performance: Surface features with finely distributed pits (typically 0.1-1mm in diameter), appearing either densely packed or scattered, with a rough texture to the touch.
• Cause:
• Surface pores and inclusions in raw materials are exposed after cold drawing.
• The mold surface is rough, or impurities are mixed into the lubricating fluid, which gets pressed into the surface during stretching.
• Surface residue of acid wash leads to localized corrosion and the formation of pockmarks.
• Impact: Reduced surface accuracy; prone to accumulate oil and moisture in dents, accelerating corrosion.

4. Peeling (delamination, flaking)

• Performance: Local surface metal layer is separating from the substrate, appearing in sheet or scale-like form, with the exposed area showing fresh metal surface.
• Cause:
• Raw materials exhibit lamination defects (the billet was not fused during hot rolling).
• Excessive cold drawing deformation exceeds the material's yield limit, leading to separation of the surface metal from the internal structure.
• The corner radius at the mold entry is too small, resulting in excessive shearing force on the surface metal during stretching.
• Impact: Severely affects strength and service life; peeling areas are prone to cracking or corrosion.

Cracks (surface cracks)

• Performance: Irregular linear cracks appear on the surface, potentially distributed along the drawing direction or horizontally, with a significant depth (usually >0.5mm), and in severe cases, may penetrate through the cross-section.
• Reason:
• Excessive sulfur and phosphorus impurities in raw materials, or the presence of defects such as shrinkage holes and loose areas.
• Excessive cold drawing deformation without intermediate annealing to relieve stress.
• Excessive mold temperatures have led to overheating and embrittlement of the surface metal.
• Impact: Lethal flaw; cracks can propagate under stress, leading to failure by fracture.

6. Impressions (Debossing)

• Performance: Local indentation on the surface, typically in circular or irregular shapes, with shallow depth but large area.
• Cause:
• Foreign particles such as metal shavings and oxide scale are pressed into the surface during the stretching process.
• Surface irregularities or adherent impurities on the conveying roller and straightening roller.
• Products are being crushed against each other or in contact with hard objects during storage.
• Impact: Affects appearance and surface flatness; deep indentations may reduce local strength.

Dimensional Precision Defects (Affecting Assembly and Usage Effectiveness)

Size Exceeds Tolerance (either oversized or undersized)

• Performance: Actual cross-sectional dimensions (thickness, width) exceed the tolerance range specified by the standard (e.g., thickness is thicker, width is narrower).
• Cause:
• Mold design dimension discrepancies, or failure to promptly repair a mold after wear.
• Improper cold drawing process parameters (such as excessive stretching speed, inaccurate deformation control).
• Raw material cross-sectional dimensions are excessively variable; pre-adjustment was not conducted.
• Impact: Unable to meet assembly requirements, such as excessive width preventing insertion into the slot and insufficient thickness leading to inadequate load-bearing capacity.

2. Sectional deformation (elliptical, rhombic, warping)

• Performance: Irregular cross-sectional shape, such as uneven thickness, curvature in the width direction, and overall warping (exceeding flatness tolerance).
• Cause:
• Mold hole design is unreasonable, leading to uneven stress distribution.
• Misalignment of guide device during stretching, causing uneven force distribution on one side.
• Cold drawn without straightening or improper straightening parameters.
• Material internal stress distribution is uneven (such as raw material segregation).
• Impact: Affects installation accuracy, such as flatness deviation leading to loose fitting, and uneven cross-sections causing uneven force distribution.

Length variance (either too long or too short)

• Performance: Product length exceeds order requirements or is insufficient due to breakage during stretching.
• Reason:
• Stretching machine stroke control is inaccurate, or the speed is too fast, causing overstretching.
• Materials contain internal defects (such as cracks), which cause them to fracture during stretching.
• Raw material length fluctuations were not accounted for in advance.
• Impact: Leads to material waste (excessive length) or inability to meet usage requirements (insufficient length).

III. Internal Organizational Deficiencies (Strong Hiding Ability, Influential Learning Performance)

Large or uneven grain size

• Performance: Excessive internal grain size (exceeding standard requirements) or significant variation in grain size.
• Cause:
• The raw material was overheated during heating (overheated during hot rolling) and no normalizing treatment was performed.
• The cold drawing deformation is too small, failing to achieve the effect of refining grain structure.
• Overheating during intermediate annealing or excessive holding time can lead to grain growth.
• Impact: Decreased material strength, hardness, and toughness, prone to brittle fracture.

Stress Concentration (Excessive Residual Stress)

• Performance: There are residual tensile or compressive stresses that have not been eliminated internally, distributed unevenly.
• Cause:
• Over-deformation in cold drawing, without undergoing low-temperature annealing (stress relief).
• Overstretching at an excessive speed, with insufficient stress release.
• Sectional shape突变has resulted in local stress concentration.
• Impact: Can easily cause deformation during subsequent processing (such as cutting, bending) or cracking due to stress release during use.

3. Contaminated with segregates

• Performance: Non-metallic inclusions (such as oxides, sulfides) are present internally, or element distribution is uneven (such as segregation of carbon, manganese).
• Causes: Primarily due to defects in raw materials (defects during steel billet smelting and casting), which cannot be eliminated through cold working.
• Impact: Reduces the plasticity, toughness, and fatigue strength of materials, making inclusions prone to become sources of cracks.

4. Abnormal fiber organization

• Performance: Uneven distribution or chaotic arrangement of metal fiber (grain deformation direction) after cold drawing.
• Cause:
• Uneven stress during stretching (e.g., mold holes are asymmetric).
• Excessive deformation without intermediate annealing has led to overstretching of the fibers.
• Impact: Increased anisotropy in material mechanical properties, significant reduction in lateral toughness, prone to fracture along the fiber direction.

Section 4: Other Defects

1. Curved and Twisted

• Performance: The finished product exhibits overall curvature (longitudinal curvature) or distortion, exceeding the tolerance requirements for straightness.
• Reason:
• Uneven tension at both ends during stretching.
• Improper straightening techniques or worn straightening rolls.
• Improper stacking during storage led to deformation.
• Impact: Unable to install normally; requires additional adjustments, increasing processing costs.

Surface Contamination

• Performance: Surface residues of oil, pickling waste, dust, and other impurities.
• Cause:
• Cold drawn without proper cleaning or incomplete cleaning.
• Improper storage conditions or damaged packaging.
• Impact: Affects the effectiveness of subsequent surface treatments (such as painting, electroplating), leading to coating delamination.

Summary: Core Defect Prevention

1. Raw Material Control: Select high-quality hot-rolled billets, rigorously inspect surface quality and chemical composition to ensure the absence of scale, rust, inclusions, and other defects.
2. Mold Management: Design molds with proper hole patterns, regularly inspect mold wear, and promptly polish or replace as needed to ensure adequate lubrication.
3. Process Optimization: Control cold drawing deformation (avoid excessive), perform intermediate annealing to relieve stress when necessary, stabilize stretching speed and temperature.
4. Equipment Maintenance: Regularly calibrate stretching machines, straightening machines, and other equipment to ensure that guide devices and conveyor roller surfaces are smooth and free of defects.
5. Post-processing and Storage: Promptly clean and rust-proof after cold drawing, stack properly to avoid compression and contamination.

These measures significantly reduce the defect rate of cold-rolled flat steel, ensuring product quality meets usage requirements.