What are common surface defects in extruded steel?

Surface defects on special-shaped steel (including non-rectangular/circular cross-section shapes produced by processes such as cold drawing, hot rolling, and extrusion, such as angle iron, channel iron, I-beam, special-shaped flat steel, and complex cross-section shapes) are strongly correlated with "forming process characteristics, cross-section shape complexity, and mold/roller contact status." Common types can be divided into general surface defects (common to most special-shaped steel) and process/cross-section specific defects (due to structure or processing methods), as follows:

General Surface Defects (All non-standard steels are prone to)

These defects are similar to those found in standard shapes (such as flat steel, round steel), but due to the irregular cross-section of the special steel, they tend to concentrate more in the corners, grooves, and transitions between varying cross-sections (areas of stress concentration or uneven contact).

Scratches (scrapes, nicks)

• Performance: Linear grooves along the length of the profile, varying in depth from 0.01-1mm, are more common at corners, inside grooves, and along flange edges (e.g., inside the groove of channel steel, the straight edge of angle steel).
• Cause:
• Surface oxidation scale and impurities on raw materials (steel billets) were not cleaned thoroughly, resulting in scratches during shaping.
• Mold / Roller surface roughness, wear marks, or adhered metal shavings (atypical steel mold/roller grooves and edges are prone to accumulate debris).
• During the conveying and straightening process, the guide device does not make uniform contact with the irregular surface of the profile, resulting in local friction and scuffing damage.
• Impact: Reduces surface smoothness, prone to rust and stress concentration at scratches, affecting fatigue strength (e.g., special-shaped steel for machining).

2. Oxide scale and rust

• Performance: The surface is covered with dark gray (oxide scale) and yellow-brown (rust) layers. Grooves and welds (such as welded non-standard steel) are prone to retaining oxide scales, which can develop into pinpoint rust in a humid environment.
• Cause:
• Hot rolled or hot extruded parts cooled too quickly, resulting in non-natural peeling of the oxide scale; incomplete pickling and phosphatizing before cold working.
• Finished products not timely treated for rust prevention (oiling, painting), stored in a humid environment with poor ventilation.
• Water and oil can easily accumulate in the grooves of complex cross-sections, accelerating rusting.
• Impact: Affects appearance; oxidation scale reduces the adhesion of subsequent coatings, corrosion can erode the substrate, and weaken strength.

3. Mottled (Mottled surface)

• Performance: Fine dimples (diameter 0.1-0.5mm) densely distributed on the surface, giving a rough texture, which may appear at the flat surfaces and corner transitions of the profile.
• Reason:
• Surface pores and inclusions in raw materials are exposed after forming.
• Mold surfaces may be rough or contaminants may be mixed into the lubricating fluid, leading to impurities being pressed into the surface during the forming process.
• Residual acid from pickling can lead to localized corrosion in grooves and hard-to-reach areas.
• Impact: Reduced surface accuracy, prone to accumulating dirt in pits, accelerates corrosion, and affects sealing and bonding effects (e.g., for non-standard steel used in assembly).

4. Peeling (flaking, scaling)

• Performance: The surface metal layer peels off in sheet or scale-like formations, revealing fresh metal surfaces, which commonly occurs at transition points with varying cross-sections (such as the connection between the flange and web of I-beams) and at angular edges.
• Reason:
• Raw materials (steel billets) have lamination defects (not fully welded during hot rolling), which lead to increased lamination when deformed into different shapes due to stress concentration.
• Overly large cold working deformation, without intermediate annealing, surface metal separates from internal structure.
• The mold/roll edges have excessively small radii, resulting in excessive shearing force on the surface metal during shaping.
• Impact: Severely affects strength and service life; the peeled areas are prone to cracking or corrosion spread.

5. Cracks (Surface Cracks)

• Performance: Linear irregular cracks, which can distribute along the length or horizontally, with a depth of more than 0.5mm, are prone to appear at sharp corners, bottom of grooves, and heat-affected zones of welds (such as焊接异型钢).
• Reason:
• Excessive amounts of harmful impurities like sulfur and phosphorus, or the presence of shrinkage holes and looseness.
• Inadequate temperature distribution during hot processing (e.g., rapid cooling at sharp edges) leads to thermal stress cracks; excessive deformation during cold processing, with residual stresses not relieved.
• The mold/cylinder groove design is unreasonable, resulting in localized stress concentration during shaping.
• Impact: Lethal flaw; cracks can propagate under load, leading to fracture failure (e.g., load-bearing specialty steel).

6. Imprint (Debossing)

• Performance: Local indentations (irregular shape), shallow in depth but large in area, commonly found on the flat surface and flange surface of the profile.
• Cause:
• Metal shavings and oxide scale are pressed into the surface during the molding process.
• Surface irregularities or adhered impurities on conveying and straightening rollers can cause indentation marks when in contact with irregular profiles.
• When stacked, finished products are compressed due to irregular cross-sections of the extruded steel, with concentrated pressure points at contact points during stacking.
• Impact: Affects appearance and surface flatness; reduces local load-bearing capacity when indentation is deep.

Section Specific Defects (exclusive to deformed steel or high incidence)

Due to the complex cross-sections of special steel (with multiple棱angles, grooves, and varying cross-sections), uneven stress during shaping often leads to the following unique defects:

Edge defects (missing corners, rounded corners exceeding standards)

• Performance:
• Notched: Local loss of material at the edges of profiles (such as the missing corner of a angle steel's right angle edge, or the chipped edge of a channel steel's slot).
• Rounded Corners Exceeding Standards: Corners with excessively large or excessively small rounding (nearing sharp angles, prone to cracking).
• Cause:
• Mold/roll edge wear and chipping (the edges of special steel molds are subject to concentrated stress, leading to easy wear).
• Metal flow is uneven during molding, resulting in insufficient filling at the corners (missing corners).
• Improper rounding parameters during mold design, or insufficient machining accuracy.
• Impact: Affects cross-sectional size accuracy and appearance; corners with missing angles are prone to stress concentration; excessively small radii can lead to cracking, while overly large radii affect assembly fitment.

Slot Defects (Inclusions, Dimpling, Underfill)

• Performance: Indentations observed within:
• Inclusions: Residual oxide scale, metal fragments.
• Dimple: Local inward depression.
• Insufficient Filling: Groove depth is not up to standard, or the inner wall is uneven.
• Cause:
• Difficult metal flow and insufficient filling at the indentation during molding.
• The mold groove inner side is rough and accumulates debris, leading to surface indentations or inclusions.
• During hot processing, the cooling rate at the grooves is too fast, resulting in uneven metal shrinkage.
• Impact: Reduces the effective cross-sectional area, affects uniform stress distribution, and slag and indentation within grooves are prone to cause corrosion.

Weld Seam Defects (exclusive to Welded Special Shape Steel)

Some sections of the special-shaped steel are welded into shape (such as welded channel steel, welded special-shaped beams), where weld seams and heat-affected zones are prone to appear:

• Performance: Weld bead undercut (edge not melted, showing grooves), porosity (small holes on the surface), slag inclusions, weld bead protrusion exceeding standards, cracks in the heat-affected zone.
• Cause:
• Improper welding parameters (excessive or insufficient current, too fast welding speed)
• Poor quality of welding materials (welding rods, welding wires) or incomplete bevel cleaning.
• Rapid cooling of weld seams results in thermal stress cracks.
• Effects: Insufficient weld strength, prone to cracking under stress, and the presence of gas pores and slag will accelerate corrosion.

4. Localized bulges (protrusions)

• Performance: Local bulging on the surface of the profile (commonly at transitions of varying cross-sections and in the middle of the flanges), resulting in an uneven surface around.
• Cause:
• Excessive metal flow in localized areas during molding (such as uneven mold design causing metal accumulation in certain regions).
• Uneven deformation distribution during cold working leads to local stress relief and bulging.
• The raw material has a locally thick thickness and was not uniformly stretched after molding.
• Impact: Affects appearance and assembly fit, prone to stress concentration when the bulging area is subjected to force.

Edge burrs (fly-offs)

• Performance: Sharp metal burrs appear at the edges of profiles (especially cold-rolled special-shaped steel), more pronounced at corners and cut edges.
• Cause:
• Die / Excessive gap between roller, causing metal overflow and creating burrs during shaping.
• Cold drawn/cold-pressed without edge trimming;
• Tool wear during cutting leads to excessive burrs on the edges.
• Impact: Prone to scratch operators, affecting assembly accuracy (e.g., burrs causing excessive fit gaps), and susceptible to rust at burr locations.

6. Striped Defects (High Incidence in Hot-Processed Special Steel)

• Performance: Light and dark alternating stripes along the length direction, or locally color-difference stripes, are commonly found on the profile plane and flange surface.
• Cause:
• During hot rolling or hot extrusion, uneven surface temperature of the roller/mold leads to varying degrees of oxidation on the metal surface.
• Inconsistent lubricant application, excessive friction between local metal and mold, resulting in heat streaks.
• Raw material surface composition segregates, becoming visible as stripes after molding.
• Impact: Primarily affects appearance; in severe cases, there may be flaking of the oxide scale or uneven hardness in the striped areas.

III. Summary of High Incidence Areas for Surface Defects on Special Section Steel

1. Edge / Transition Areas: Scratches, cracks, chipped corners, peeling (stress concentration + mold wear);
2. Grooves/voids: Oxide scale residue, rust, inclusions, insufficient filling (difficult metal flow + prone to dirt accumulation).
3. Weld Seam and Heat Affected Zone: Edge Cracking, Porosity, Cracks (Welding Process Fluctuations + Thermal Stress)
4. Flanges / Planes: Indentations, pockmarks, and bulges (due to frequent contact with equipment + metal accumulation/impurities pressed in).

Key Preventive Measures

1. Mold/Roll Optimization: Designs molds/rolls for irregular steel sections that ensure moderate radii at corners and grooves for even metal flow; regular polishing and maintenance to prevent wear and debris buildup.
2. Raw Material Pretreatment: Thoroughly remove oxidation scale and rust to ensure surface cleanliness; rigorously inspect the composition and surface quality of raw materials to avoid impurities and stratification.
3. Process Control: Ensure uniform temperature during hot processing, allocate deformation amounts reasonably during cold processing, and perform intermediate annealing when necessary; optimize welding parameters for special-shaped steel, and enhance bevel cleaning.
4. Post-processing Enhancement: Refine edge burrs after cold working and remove oxidation scale promptly after hot working; conduct anti-rust treatment on finished products, focusing on grooves and welds.
5. Targeted Inspections: Utilizing tools such as endoscopes (for detecting grooves), magnetic particle inspection (for detecting cracks), and high-precision projection systems (for detecting edges/sections), we focus on inspecting defects in high-occurrence areas.