A galvanized coil quotation can read like alphabet soup: SGCC, DX51D+Z, G60, G90, Z100, Z275, AZ150, SPCC. The letters are not interchangeable decorations; each belongs to a standard system, and a buyer who mixes them can end up with a coating far lighter or heavier than intended. This guide explains the three naming systems and how to compare them.
SPCC and SGCC: The JIS Building Blocks
SPCC is the commercial-quality cold-rolled steel of JIS G3141, the base material for many coated products. Its typical specification is sheet from about 0.5 mm to 3.0 mm in widths up to 2000 mm, with strip and coil from 0.3 mm to 2.5 mm in widths of 355-1500 mm. SGCC is the hot-dip galvanized commercial grade of JIS G3302, made by galvanizing a cold-rolled base; SECC is the electro-galvanized counterpart of JIS G3313 with a thinner, smoother coating. The practical hierarchy for corrosion resistance is SGCC (thick hot-dip zinc) above SECC (thin electro zinc) above SPCC (bare, requires additional protection).
Why Is It Called Cold-Rolled Steel?
Cold rolling deforms the steel at or near room temperature after hot rolling and pickling. It improves surface finish, thickness tolerance and mechanical strength through work hardening. These process characteristics are different from the formal grade designations, which define chemistry and guaranteed properties, so a grade name and a process description should never be confused.
Coating Designation Systems Compared
| System | Example | Meaning |
|---|---|---|
| EN 10346 | Z100, Z200, Z275 | g/m2 total both surfaces, triple-spot method |
| JIS G3302 | Z12, Z27, Z35 | Class number x 10 = g/m2 (Z27 = 270 g/m2) |
| ASTM A653 | G60, G90 | oz/ft2 total; G60 = 183 g/m2, G90 = 275 g/m2 |
| Aluminium-zinc | AZ150 | 150 g/m2 of 55% Al-43.4% Zn-1.6% Si coating |
To compare offers, convert everything to grams per square metre. G90 and Z275 are the same physical coating mass; Z27 in JIS notation is also 270 g/m2. Never compare Z60 (EN, 60 g/m2) with a JIS Z60 class (600 g/m2) without checking the system, because the numbers diverge by a factor of ten.
Thickness Ranges and Tolerances
Cold-rolled coil is typically supplied from 0.20 mm to 2.0 mm in the classic definition, while cold-rolled sheet can range up to about 4.5 mm at most mills. Galvanized coil in common building grades runs from 0.30 mm to 3.0 mm. Thickness tolerance follows the governing standard and should be stated on the order; using a tighter tolerance than the standard costs extra but saves downstream adjustment.
SGCC versus DX51D: Same Product, Different Paperwork
SGCC (JIS G3302) and DX51D+Z (EN 10346) describe the same type of product: a hot-dip galvanized low-carbon coil for forming. The chemistry, coating and testing rules differ slightly, and the certificate is issued to the governing standard. When a destination market specifies one system, supply to that system and provide the corresponding mill certificate rather than mixing designations.
Frequently Asked Questions
1. What does AFP mean in coil offers?
AFP stands for anti-fingerprint, a thin organic passivation layer applied to coated steel (often electro-galvanized SECC or aluminium-zinc GL) that prevents fingerprint marks and light corrosion during handling of appliance and electronics parts.
2. Is GI the same as GL?
No. GI is galvanized iron, a pure-zinc hot-dip coating (EN Z class); GL is aluminium-zinc, an aluminium-zinc coating of about 55% aluminium. GL resists heat and corrosion better in many conditions but welds and forms differently.
3. Which coating is better, G90 or Z275?
They are the same mass (275 g/m2) expressed in two systems. Choose the designation your contract standard uses; the performance is equivalent.
4. Why is SPCC corrosion resistance lower than SGCC?
SPCC is bare cold-rolled steel with no metallic coating. It protects itself only by oiling or painting, whereas SGCC carries a sacrificial zinc layer, so in the same environment SGCC lasts far longer without maintenance.
5. What does hard galvanized coil mean?
Hard (full-hard) temper means the base steel is not annealed after cold rolling, so the coil is stiff and strong but not formable. It is used for flat products such as decking and profiles that are roll-formed without severe deformation.
6. Can AZ150 be substituted for Z275?
For many roofing applications yes, but they are different coating systems: AZ150 is 150 g/m2 of aluminium-zinc, which behaves better at cut edges and high temperature, while Z275 is heavier pure zinc. Confirm the design life and environment before substituting.