1. The Surface Finish Family
Surface finish is the result of the last processing steps the strip sees — which is why the same grade can be supplied in several finishes with different prices and different downstream behavior. For cold rolled strip the three names you will meet most are 2D, 2B and BA, and they form a logical ladder from dull to bright.
1.1 2D: dull, matte, substrate
2D is cold rolled, annealed and pickled. The pickling removes the annealing scale, leaving a dull, matte surface. Its roughness is a feature, not a flaw: it gives paint and coatings mechanical grip, and the soft annealed condition suits deep drawing. When the part is going to be painted, powder-coated or drawn hard, 2D is often the right call — and the cheapest of the three.
1.2 2B: the general-purpose workhorse
2B is 2D given a temper (skin pass) roll — a light cold rolling pass that smooths the surface and slightly hardens it. The result is a smoother, semi-bright surface with better flatness, and it is the default finish for general industrial work: stamping, forming, welding, and parts that need a decent surface without the cost of a bright finish.
1.3 BA: bright and seen
BA (bright annealed) is cold rolled, then annealed in a protective atmosphere — usually hydrogen or nitrogen based — so the surface never oxidizes and needs no pickling. A final temper roll refines the finish further. The result is a clean, bright, reflective surface for parts where appearance matters: appliance trims, visible brackets, mirror-adjacent components. BA costs more, and it is a poor paint substrate — its smoothness gives coatings nothing to grip.
1.4 No.1, No.4, No.8 and beyond
The broader family comes from the stainless steel finish nomenclature and appears in strip work too: No.1 is hot rolled, annealed and pickled (the thick, matte industrial surface); No.4 is a ground/brushed finish with a uniform short-grain line, used for architectural and food-contact parts; No.8 is a mirror polish. These are finishing operations applied after the mill finish, usually by the processor rather than the mill.
2. Production Routes Compared
| Finish | Production route | Notes |
|---|---|---|
| No.1 (HR) | Hot roll → anneal → pickle | Thick, matte; scale-free after pickling |
| 2D | Cold roll → anneal → pickle | Dull, matte; soft for deep drawing |
| 2B | 2D → temper (skin pass) roll | Smoother, semi-bright; better flatness |
| BA | Cold roll → bright anneal (protective atmosphere) → temper roll | Bright, reflective; no pickling step |
| No.4 | 2B/BA → abrasive belt brushing | Uniform brushed line |
| No.8 | Progressive polishing | Mirror reflectivity |
The term "temper rolling" (skin pass) here means the light final rolling pass that controls surface roughness and flatness without significant gauge reduction.
3. Roughness, Appearance, Application — The Table
The table below is the original data point of this guide: a practical orientation map for strip buyers. Ra values are typical ranges — confirm the agreed value on the inspection report, because mills can supply tighter or looser bands on request.
| Finish | Typical Ra (µm) | Appearance | Typical application |
|---|---|---|---|
| 2D | 0.8 – 1.6 | Dull, matte | Paint substrate, deep drawing, powder coat |
| 2B | 0.4 – 0.8 | Smooth, semi-bright | General stamping, forming, welding, enclosures |
| BA | 0.2 – 0.5 | Bright, reflective | Visible parts, appliance trim, decorative |
| No.4 | 0.4 – 1.0 | Brushed line | Architectural, food equipment |
| No.8 | ≤ 0.1 | Mirror | Decorative, reflectors, premium surfaces |
Typical orientation values. Ra is one number in a family of roughness parameters; for critical surfaces specify the parameter set and measurement standard with your supplier.
4. Which Finish for Which Part
Selection is driven by what happens to the surface after it leaves the supplier:
- Part will be painted or coated → 2D. The matte roughness is the adhesion mechanism; a bright finish would reject the coating.
- Part is stamped, formed or welded, surface not critical → 2B. Good balance of surface, flatness and cost.
- Part will be seen → BA, or No.4/No.8 where the design calls for a brushed or mirror look.
- Part is deep drawn → 2D in the annealed condition, for maximum ductility; bright finishes add cost without helping the draw.
- Part feeds a fine blanking or progressive die → the finish matters less than the condition and edge quality; our grade selection guide covers the priorities there.
5. Inspection Standards
Finish acceptance is a combination of instruments and eyes:
- Roughness tester: measures Ra (and Rt/Rz if agreed) along and across the rolling direction — the objective acceptance number.
- Gloss meter: quantifies reflectivity for BA and polished finishes, where Ra alone does not fully describe appearance.
- Visual inspection: under controlled, diffuse light, checking for scratches, roll marks, oxidation color, oil stains and handling damage. Visual acceptance limits should be agreed with reference samples — a written Ra alone never settles a scratch dispute.
- Sampling: check at several positions along the coil; surface can vary head to tail as rolls warm and cool.
6. Common Surface Defects
- Roll marks: a repeating pattern on the surface, transferred from a damaged or dirty mill roll. Prevention is mill-side; detection is visual and by roughness trace.
- Scratches: linear damage from handling, slitting or re-coiling with debris or misalignment. The edge story is related — see our edge burr guide for the slitting side.
- Oxidation color: yellow-blue discoloration from a failed atmosphere or poor storage — a bright-anneal quality failure on BA, or a storage problem on any finish.
- Oil stains: uneven process oil or contamination — a cleanliness issue that shows up as patchy wetness and can affect downstream coating.
7. Storage and Protection
Surface finish is earned at the mill and lost at the warehouse. Bright and smooth surfaces are the most vulnerable: BA shows fingerprints and storage marks that 2D would hide. Keep strip clean and dry, protect edges, respect the rust-prevention packaging agreed at order, and store coils upright on their edge with even support. If parts will be plated or painted after forming, tell the supplier the downstream process — the surface spec should be set for what the finish must accept, not just how it looks.
8. Frequently Asked Questions
Short answers here; the full schema FAQ follows below.
- Is BA always better than 2B? Better-looking, yes; better for the part, not necessarily. BA costs more and is a poor paint substrate — choose by function, not brightness.
- Can I get a specific Ra value? Yes, within the mill's capability — specify the Ra range and the parameter at RFQ, and confirm it on the inspection report.
- Does finish affect formability? Indirectly — the finish correlates with the delivery condition (2D is usually annealed/soft, 2B skin-passed). Specify both finish and condition explicitly.