Non-prime steel sheet defects - classification and impact on processing

Material traded as non-prime or second choice steel sheet has no normative definition, and the catalogue of deviations that lead to downgrading follows mill and trading practice rather than a standard. In technological terms these deviations fall into four groups: coating or substrate surface defects, shape defects, dimensional deviations and property changes caused by material ageing. Chemical composition and mechanical properties usually remain unchanged, because downgrading most often results from visual or dimensional reasons or from the age of the stock¹.

Offers describing non-prime material are usually limited to a formula about minor surface defects with no effect on use. For engineering departments this is insufficient, because each type of deviation affects a different stage of the manufacturing process and some of them only become apparent on the press or in the roll forming line. In periods of weak demand mills move part of their output from first to second choice, and suitability of a batch is decided above all by whether the finished product has a visible surface².

Reference frame: what the standard permits in prime material

Assessing a defect requires a point of reference. For continuously hot-dip coated products this is provided by EN 10346, which defines three surface quality classes and explicitly lists the imperfections permitted in each of them³. A significant part of the imperfections found in non-prime material falls within the description of class A, that is the surface in the as-coated condition. Downgrading occurs when the severity of a deviation exceeds the limit of the class, or when the batch was produced against an order requiring a higher class.

The surface class is completed in the product designation by the surface finish, that is normal or minimised spangle, and by the surface treatment: chemical passivation, oiling, passivation with oiling, phosphating or sealing with an organic film.

The second part of the reference frame consists of tolerances on dimensions and shape. For hot-dip coated products these are set by EN 10143, covering thicknesses from 0.20 mm to 6.50 mm and providing normal tolerances as well as restricted tolerances designated S⁴. For hot-rolled material cut from coil the relevant standard is EN 10051⁵. A discrepancy between the declared tolerance and the actual condition is an independent reason for downgrading, unrelated to surface condition.

Storage stain

White or grey stain on a galvanized surface forms when sheets in close contact inside a coil or a bundle become wet with restricted access of air. Zinc reacts with moisture to form zinc hydroxide, which under a shortage of carbon dioxide does not convert into the compact zinc carbonate film responsible for the corrosion resistance of the coating⁶. It is a product of oxidation of the coating, not corrosion of the steel substrate.

The technological consequences depend on severity. Light stain usually does not shorten product life, medium stain calls for assessment and treatment, and heavy stain may require the coating to be restored⁷. Colour has diagnostic value: white corresponds to oxidation products of zinc alone, while grey or black appears after prolonged wetting, once a significant part of the coating has been consumed and iron from the substrate takes part in the reaction⁶. Skin passed product shows darker staining at an early stage because of the higher proportion of aluminium oxides on the surface.

Removing the stain restores the uniformity of the surface image but does not remove its etched condition, so the area remains matt and visible⁶. This matters only in exposed parts. In painted products what matters is that both zinc corrosion products and passivation residues change the conditions of paint adhesion, so surface preparation has to be matched to the actual condition of the batch rather than to its nominal condition.

The risk of staining further down the supply chain is limited by storage in climate controlled premises, keeping the product temperature above the dew point, transport in covered and watertight conveyances, storage on dunnage with air circulation and by limiting the storage time of freshly coated material⁶.

Scratches and discontinuities of the coating

Scratches arise during transport, coil rewinding and warehouse handling. The decisive question is whether the scratch breaks the continuity of the metallic coating. A scratch that does not break the coating is a purely visual deviation and falls within the description of class A.

Where the substrate is exposed, durability is decided by the cathodic protection provided by the adjacent zinc. The range of this protection depends on the conductivity and thickness of the electrolyte layer on the surface and on the thickness of the sheet. In atmospheric exposure tests a 1 mm sheet remained free of red rust on the cut edge after six months, while a 2.3 mm sample showed rust after two weeks; for thin sheet below 1 mm a cut edge can remain free of rust for the entire service life of the component in most environments⁸. The practical conclusion is that the significance of a scratch grows with material thickness and with the aggressiveness of the service environment.

Scratched material is used in enclosed components, in internal structures and in products finished with an opaque paint system.

Roll marks, indentations and rolled-in imperfections

Roll marks occur cyclically along the length of the strip, with a pitch corresponding to the circumference of the roll. This feature has practical value, because it allows the real scale of material loss to be estimated from a control length instead of assessing the whole batch. Single indentations, caused by foreign bodies in the line, are random in nature and require assessment over the full length.

In bending and roll forming this group of deviations is usually of no consequence. In deep drawing they disturb the distribution of stresses and may initiate cracking in the area of greatest deformation. In products finished with high gloss paint they remain visible after finishing, because thin organic coatings reproduce the geometry of the substrate instead of masking it.

Coil breaks and stretcher strain

The group of deviations most often omitted from commercial descriptions, and the most troublesome in series production, is related to strain ageing. In low carbon steels carbon and nitrogen atoms diffuse to dislocations and restore a pronounced upper yield point, which during forming appears as Lüders bands, that is irregular strain markings on the surface of the product⁹.

Prevention works in two ways. Skin passing with a deformation of at least 0.5% eliminates the upper yield point, and an extension of the order of 1.25-2.0% provides freedom from stretcher strain for approximately six months. Complete freedom is offered only by steels with very low carbon content, down to 0.002%, free of interstitial atoms and designated as IF steels. It is also relevant that skin passing or levelling carried out above 38 degrees Celsius can accelerate the process to two weeks⁹.

For non-prime material the conclusions are direct. The age of a batch, one of the more frequent reasons for downgrading stock¹, translates into a risk of stretcher strain in components with high deformation and a visible surface. When buying material older than six months this risk is limited by roller levelling immediately before the forming operation⁹ and by assigning such a batch to products with a lower degree of deformation. The same group includes coil breaks, that is transverse markings formed when a fully annealed low carbon coil is uncoiled; the standard lists marks of this type among the imperfections permitted in surface class B³.

Strip edge damage

Crushed edges, laminations and edge waviness translate directly into material yield. Where longitudinal slitting into narrower strips is planned, part of this area is removed together with the edge trim, so damage contained within the width of the trim causes no loss. Where material is bought in the final width, edge damage is a real loss and should be reflected in the commercial terms.

The type of edge is a separate question. Hot-rolled material delivered with a mill edge has a width tolerance from 0 to plus 20 mm, whereas a trimmed edge falls within a range from 0 to plus 3 mm for widths below 1200 mm, up to plus 5 mm for widths from 1200 mm to 1500 mm and up to plus 6 mm above 1500 mm⁵. Comparing offers without taking the type of edge into account therefore leads to apparent rather than real conclusions.

Discolouration and local corrosion

On uncoated material surface rust is a natural phenomenon and is removed in pickling or during processing. On galvanized material spot corrosion of iron indicates a discontinuity of the coating and requires individual assessment against the service conditions of the product, above all the corrosivity category of the environment and the required durability.

In painted products the behaviour of discolouration after surface preparation is verified on a trial batch, because that behaviour depends on the surface preparation system and the type of coating rather than on the appearance of the incoming material.

Dimensional and shape deviations

This group covers deviations of thickness and width as well as shape deviations: lack of flatness, edge waviness, centre buckle and camber. For hot-dip coated products the thickness tolerances depend on nominal thickness, width and the yield strength of the steel; for example, for thicknesses from 0.80 mm to 1.00 mm, widths up to 1200 mm and steel with a yield strength below 260 MPa the normal tolerance is 0.06 mm and the restricted tolerance 0.045 mm, while for high coating masses, Z450 and Z600, these values increase by 0.01 mm⁴. Flatness in that standard is described by normal and restricted tolerance classes.

For hot-rolled material cut from coil, flatness deviations depend on width and product category, and camber should not exceed 0.5% of the actual length of the plate for lengths below 5000 mm; for longer products the limit is 20 mm per 5000 mm of length, and 15 mm for a trimmed edge⁵.

The technological significance of this group is clearly concentrated. Camber and edge waviness have the greatest influence in roll forming and in automatic feeding of material, where they determine the repeatability of strip position and the twist of the profile. Thickness deviation affects products designed for stiffness, welding and resistance welding parameters and the settlement of the batch by mass.

Summary table

The table is indicative. Acceptance of material is decided by the assessment of a specific batch against a specific product and process.

Qualifying a batch before purchase

Assessment of non-prime material rests on three questions, ordered according to the technological weight of the deviations.

Non-prime material is usually accompanied by a mill certificate issued on the basis of non-specific inspection, because downgrading does not result from chemical composition or mechanical properties but from manufacturing deviations. The scope in which law or a product standard requires an inspection certificate is covered by a separate article on inspection documents under EN 10204. The distinction between downgraded material and prime surplus is presented in the article on excess prime steel and mill overruns, and the characteristics of the category itself in the article on non-prime steel sheet.

Available steel grades, coating types and formats of coils, strips and sheets are presented in the HEN-STOL steel sheet offer.

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Frequently asked questions

Sources

1. Metal Center News, Secondary report, K. Zajac-Frazee, July 2026
2. Kallanish, Non-prime coil gaining favour in Europe, Ch. Koehl, February 2026
3. EN 10346, Continuously hot-dip coated steel flat products, technical delivery conditions
4. EN 10143, Continuously hot-dip coated steel sheet and strip, tolerances on dimensions and shape
5. EN 10051, Continuously hot-rolled strip and plate/sheet cut from wide strip, tolerances on dimensions and shape
6. GalvInfo Center, GalvInfoNote 3.2, Protecting galvanized steel sheet products from storage stain
7. American Galvanizers Association, Wet storage stain
8. GalvInfo Center, GalvInfoNote 3.6, Galvanic and cut edge corrosion of galvanized sheet steel
9. GalvInfo Center, GalvInfoNote 2.8.1, Strain ageing in galvanized low carbon steel sheet