Q355B steel, like other hot-rolled structural steels, can exhibit various surface defects originating from the continuous casting, rolling, or handling processes. Proper identification, inspection, and handling are crucial to ensure structural integrity and performance.
Here are the common surface defects, along with inspection methods and handling procedures.
I. Common Surface Defects of Q355B
| Defect Category | Specific Defects | Causes & Description |
|---|---|---|
| 1. Scaling/Oxide Defects | Mill Scale (Primary) | Thick, layered iron oxides formed during hot rolling. It is normal but must be removed before painting. |
| Secondary Rust | Red/brown rust (Fe₂O₃) due to improper storage or exposure to moisture. Can lead to pitting. | |
| 2. Rolling & Processing Defects | Roll Marks / Rolled-in Scale | Periodic indentations or impressions from damaged rolls; or scale pressed into the surface during rolling. |
| Scratches & Scuffs | Linear marks from mechanical handling, conveying, or stacking. | |
| Pitting | Small, shallow cavities often caused by localized corrosion under retained scale or from acidic contamination. | |
| 3. Metallurgical & Internal Defects | Sliver / Lamination | Elongated, thin separations parallel to the surface, caused by inclusions (e.g., sulfides) or gas pockets during casting. |
| Seams / Cracks | Tight, linear, often oxidized fissures from casting or rolling, running parallel to the rolling direction. | |
| Edge Cracks | Cracks at plate edges due to over-straining during rolling or low ductility at the edge. | |
| 4. Shape & Dimensional Defects | Warping / Camber | Deviation from flatness due to uneven cooling or residual stress. |
| Overfill / Underfill | Non-uniform width or thickness beyond tolerance. |
II. Inspection Methods
A combination of visual and instrumental methods is used.
1. Visual Inspection (VT) - Primary Method
Standard: Follow GB/T 2970 (for thick plates) or GB/T 14977 (for hot-rolled surface quality).
Procedure:
Conduct under adequate, consistent lighting.
Inspect 100% of the surface and edges.
Use wire brushes or grinders to clean suspicious areas for better assessment.
Tools: Magnifying glass, standard rust grade comparators (e.g., ISO 8501-1), gauges.
2. Non-Destructive Testing (NDT)
Ultrasonic Testing (UT):
Primary method for detecting internal/sub-surface defects like laminations, inclusions, and deep seams.
Standard: GB/T 2970 is the key Chinese standard for ultrasonic inspection of steel plates.
Typically performed as a grid scan over the entire plate, especially for critical applications.
Magnetic Particle Testing (MT): Effective for finding surface and near-surface cracks (seams) on ferromagnetic materials like Q355B.
Penetrant Testing (PT): Can be used for surface crack detection if the surface is smooth and non-porous.
3. Dimensional Inspection
Use calipers, micrometers, and straightedges to check for warpage, thickness, and width against GB/T 709 (dimensional tolerances for hot-rolled plates).
III. Handling & Acceptance Criteria
The action taken depends on the defect type, depth, extent, and the plate's intended use (as per project specifications or standards like GB 50205 for steel structures).
General Acceptance/Rejection Guidelines
| Defect Type | Typical Acceptance Criteria (General Construction) | Corrective Action / Handling |
|---|---|---|
| Light Rust / Mill Scale | Acceptable if within specified rust grade (e.g., A or B per ISO 8501-1). | Must be removed by abrasive blasting or power tool cleaning before painting. |
| Roll Marks, Scratches | Acceptable if depth ≤ 0.3mm or ≤ 3% of nominal thickness (per common specs). | For deeper marks: Grind smooth with a slope ≤ 1:4. Verify thickness after grinding is still above minimum design tolerance. |
| Pitting | Acceptable if not clustered and depth ≤ 1mm (subject to spec). | Isolated pits: Clean, dry, and fill with approved epoxy filler if needed for aesthetics/corrosion protection. Severe pitting: Reject section. |
| Seams, Cracks | Generally NOT acceptable on the surface. | Repair by grinding if depth is shallow (e.g., ≤ 3% of thickness). Full removal by grinding is required, and the groove must be smoothly contoured. Re-inspect with MT/PT after grinding. If defect penetrates deeply, reject the plate or cut out the affected portion. |
| Laminations / Slivers | NOT acceptable in shear load paths or at weld zones. May be acceptable in non-critical, non-welded interior areas if within limits of GB/T 2970. | Mark and avoid using laminated areas for welding or critical tensile zones. If found under a weld, excise and repair. For major laminations, reject the plate. |
| Edge Cracks | Not acceptable if depth > 5mm (typical). | Trim back the edge by cropping/cutting to sound metal. |
| Warping | Must comply with flatness tolerances in GB/T 709. | Can be corrected by cold straightening (pressing) if within the material's capability. Excessive warping may require rejection. |
Critical Procedure for Defect Repair:
Mark & Assess: Clearly mark the defect area and determine its extent via UT if subsurface.
Remove Defect: Use controlled grinding (with disk grinders) perpendicular to the surface. Maintain a smooth, shallow trough (slope ≤ 1:4).
Inspect: Perform VT and MT/PT on the ground area to ensure complete defect removal.
Verify Thickness: Ensure the remaining thickness is not less than the minimum allowable design thickness (considering corrosion allowance and negative thickness tolerance).
Document: Record the location, defect type, repair method, and inspection results.
IV. Preventive Measures
From Mill: Require proper implementation of GB/T 3274 (general technical delivery conditions) and specific ultrasonic testing (GB/T 2970 Class if required) in the purchase order.
During Storage: Store plates horizontally on flat, dry supports. Keep them covered and dry to prevent condensation and secondary rust.
During Handling: Use proper slings and avoid dragging plates.
Summary:
The most critical surface defects for Q355B are seams, cracks, and laminations, which require rigorous inspection (VT + UT) and strict repair protocols. Mill scale and light rust are process-inevitable but must be removed before fabrication. All handling decisions must be based on project specifications, applicable standards (GB/T 2970, GB 50205), and engineering judgment to ensure the final structure's safety and durability.