Two enquiries can read the same to a sales desk: cold rolled coil, 1.0 millimetre, 1250 wide. The coils that arrive, however, can be annealed or full hard, skin passed or not, oiled or dry, and each of those words changes what the material can do on a press. Most mill enquiry forms carry fields for hardness, skin pass and oiling precisely because the mill needs the answers, and buyers who leave them blank get the mill's default rather than their process's requirement. It is worth knowing what each field actually commits the mill to before the order is placed rather than after the coil is slit.

Hardness bands are processing states, not grades

Coated and cold rolled coil is commonly offered across three hardness bands: soft at HRB 60 and below, medium hard from 60 to 85, and full hard coil at HRB 85 to 95. The band is set by how much cold reduction the strip takes after its last anneal. An annealed coil is soft because the reduction after annealing is only a light skin pass; a full hard coil has taken its reduction with no anneal behind it, so the grain stays work-hardened. The same base chemistry therefore ships with very different behaviour: full hard material is strong, flat and stiff, and it is the right choice for panels, ducting and partition profiles that are roll-formed once and never redrawn. It is also the wrong choice for anything that has to be bent twice, because a work-hardened strip has little ductility left to spend. The ASTM and ASI standard cold rolled mild steel coil listing states the point plainly: hardness full hard, skin pass yes, non-oiled, 0.2 to 3.0 millimetres, 700 to 2000 millimetres wide, with cutting and processing available. A buyer who needs to draw or stretch that coil should not be ordering it in that condition in the first place.

Reading the CQ and DQ mechanical lines

Where formability matters, the mechanical property table is the real specification. On the JIS standard 0.8 millimetre galvanized cold rolled coil page, commercial quality is listed at HRB 50 to 71 with tensile strength of 270 to 500 MPa, while drawing quality runs softer at HRB 45 to 55, tensile 270 to 420 MPa, yield 140 to 300 MPa, and elongation of 24 percent against 22 for commercial quality on thicknesses below 0.7 millimetres. Those two points of elongation look trivial on a datasheet and are decisive on a draw die. The designation systems encode the same ladder: SPCC, DC01 and ST12 for ordinary use, SPCD and DC03 for drawing quality, SPCE and DC04 and above for deep drawing, a mapping set out on the ST12 cold rolled steel sheet page along with the chemistry behind it, where drawing grades carry progressively lower carbon, phosphorus and sulphur limits. The practical rule is to specify by operation, not by number: name the forming operation and let the supplier map it to the grade and condition, as covered in the overview of the cold rolled steel family.

What skin pass actually does

Skin passing is a light final rolling pass, typically a reduction of well under one percent, and it does three jobs at once. It removes the yield-point elongation that causes stretcher strain, the parallel surface lines that appear on lightly formed panels when the sheet yields unevenly; it sets flatness, which matters on any coil destined for a levelled or cut-to-length line; and it fixes the surface texture the strip will carry into painting. The DC01 cold rolled steel sheet listing marks skin pass as yes and notes the trade-off that comes with any cold-worked product: internal stresses remain in the strip, and cutting, grinding or welding can release them as distortion. That is a shop-floor problem with a specification root, and it is why fabrication-heavy orders should say so at enquiry. A coil ordered without skin pass will be cheaper and will misbehave on exactly the parts that are formed the least.

Tolerance classes are not interchangeable

Thickness tolerance is quoted differently across the range and the difference is not a rounding error. The JIS-standard galvanized cold rolled coil is held to plus or minus 0.02 millimetres, several coated coil listings quote plus or minus 0.01 millimetres on thickness with 2 millimetres on width, and the broader cold rolled and coated programme quotes plus or minus one percent. On a 1.0 millimetre sheet one percent is ten microns, five times the tighter figure. An enquiry that says only "standard tolerance" lets the supplier pick. Gauge call-outs cause the same confusion in the other direction: a 26 gauge galvanized coil is a thickness statement in a different unit system, and the conversion should be pinned down in millimetres before the order is confirmed. Coil geometry completes the picture: weights of 3 to 5 tonnes on coated coil and 3 to 14 tonnes on DC01, with 508 or 610 millimetre inner diameters that must match the buyer's uncoiler.

Oiling is a transit decision, not a finish

Surface protection options on coil read like a menu: passivation, oiling, lacquer sealing, phosphating, anti-fingerprint, or untreated. Oiling is the one buyers most often waive and most often regret. An oiled surface arrives with a film that presses off with solvent; a non-oiled coil stored in a humid port can arrive with fingerprint rust that no solvent removes. The SECCN5 electro-galvanized coil is offered chromated and oiled or chromated and non-oiled, and the thinner electroplate makes that choice sharper than on hot dip material, since a 40 to 80 gram coating has less zinc to spend on early staining. The same menu appears on hot dip products, from the Z60 cold rolled industrial galvanized coil to the wider galvanized cold rolled series, and electro-galvanized SECC coil adds anti-fingerprint treatment for appliance and electronics work where the surface is seen. Anti-fingerprint is a functional layer rather than cosmetics; it is the difference between a panel that tolerates handling and one that shows every glove mark. Where the coil is destined for further work, the downstream surface treatment and shearing options should be costed together, since cutting and decoiling are priced separately from the coil itself.

A short checklist for the enquiry

Four fields settle most of it. State the forming operation, which fixes the hardness band and grade; confirm skin pass where panels will be lightly formed or painted; name the thickness tolerance in absolute terms; and choose the oiling and passivation package against the transit route rather than against the datasheet. The distinction between hot and cold processing, set out in how steel plate is divided into hot rolled and cold rolled, is the starting point, and custom processing closes the gap between a mill coil and a workable blank. Coil that arrives matched to the press is cheaper than coil that arrives at the right price.