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. This article describes the common surface defects of Q355B plates, the inspection methods used to find them, and the acceptance and repair criteria applied during fabrication.
Scaling, Rolling and Processing Defects
Mill scale is a thick, layered iron oxide that forms during hot rolling. It is a normal product of the process but must be removed before painting because it prevents coating adhesion and can cause localized corrosion beneath the scale. Secondary rust is red or brown rust that develops because of improper storage or exposure to moisture and can lead to pitting. Both defect types are evaluated against rust grades such as A or B of ISO 8501-1, and surfaces are cleaned by abrasive blasting or power tool cleaning before coating.
Roll marks are periodic indentations or impressions caused by damaged rolls, and rolled-in scale is scale pressed into the surface during rolling. Scratches and scuffs are linear marks from mechanical handling, conveying, or stacking. Pitting appears as small, shallow cavities often caused by localized corrosion under retained scale or by acidic contamination. These defects are generally acceptable when their depth is within limits such as 0.3 mm or 3% of the nominal thickness, and deeper marks are ground smooth with a slope of 1:4 or less, provided the remaining thickness is still above the minimum design tolerance.
Metallurgical and Internal Defects
Slivers and laminations are elongated, thin separations parallel to the surface, caused by inclusions such as sulfides or gas pockets during casting. Seams and cracks are tight, linear, often oxidized fissures from casting or rolling that run parallel to the rolling direction, and edge cracks occur at plate edges because of over-straining during rolling or low edge ductility. Seams and cracks are generally not acceptable on the surface; shallow defects may be removed by grinding and the area re-inspected by magnetic particle or penetrant testing, while deep defects require rejection or cutting out of the affected portion. Laminations are not acceptable in shear load paths or at weld zones and are evaluated against the acceptance classes of GB/T 2970.
Shape and Dimensional Defects
Warping and camber are deviations from flatness or straightness caused by uneven cooling or residual stress, and overfill or underfill are non-uniform width or thickness beyond tolerance. Flatness and dimensional tolerances are checked with calipers, micrometers, and straightedges against GB/T 709. Warping within the tolerance can be corrected by cold straightening by pressing when the material is capable, while excessive warping may require rejection of the plate.
Inspection Methods
Visual inspection is the primary method and is performed under adequate, consistent lighting over 100% of the surface and edges, following the guidance of GB/T 14977 for hot-rolled surface quality. Wire brushes or grinders are used to clean suspicious areas for better assessment, with magnifying glasses and rust grade comparators as aids. Ultrasonic testing is the primary non-destructive method for detecting internal and sub-surface defects such as laminations, inclusions, and deep seams, typically performed as a grid scan over the entire plate in accordance with GB/T 2970. Magnetic particle testing is effective for surface and near-surface cracks on ferromagnetic material, and penetrant testing can be used on smooth, non-porous surfaces. Dimensional inspection verifies thickness, width, and flatness against GB/T 709.
Handling, Repair and Prevention
The action taken depends on the defect type, depth, extent, and the intended use of the plate, per project specifications or standards such as GB 50205 for steel structures. The repair procedure is to mark and assess the defect, remove it by controlled grinding perpendicular to the surface with a smooth shallow trough at a slope of 1:4 or less, inspect the ground area by visual and magnetic particle or penetrant testing, verify that the remaining thickness is not less than the minimum allowable design thickness, and document the location, defect type, repair method, and inspection results. Preventive measures include requiring GB/T 3274 general technical delivery conditions and specific ultrasonic testing classes in the purchase order, storing plates horizontally on flat dry supports, and avoiding dragging during handling. The most critical defects for Q355B are seams, cracks, and laminations, which require rigorous inspection and strict repair protocols.
Frequently Asked Questions
What are the most critical surface defects of Q355B?
Seams, cracks, and laminations are the most critical defects because they affect structural integrity. They require rigorous inspection by visual and ultrasonic testing and strict repair or rejection protocols.
What standard is used for ultrasonic testing of Q355B plates?
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.
Can mill scale be painted over?
No. Mill scale must be removed by abrasive blasting or power tool cleaning before painting, otherwise it prevents coating adhesion and can cause corrosion beneath the scale.
When should a Q355B plate be rejected?
A plate should be rejected when defects penetrate deeply, such as deep seams or major laminations, when pitting is severe or clustered, when edge cracks exceed typical limits such as 5 mm, or when the remaining thickness after repair would fall below the minimum design tolerance.
How are seams and cracks repaired?
Shallow seams and cracks are removed by controlled grinding with a smooth trough at a slope of 1:4 or less, followed by magnetic particle or penetrant re-inspection. Deep defects require rejection or cutting out of the affected portion.
What standards govern surface quality and dimensional tolerances of Q355B plates?
Surface quality follows GB/T 14977 for hot-rolled surface quality, ultrasonic acceptance follows GB/T 2970, dimensional tolerances follow GB/T 709, and general technical delivery conditions follow GB/T 3274.