Galvanized coil traded internationally is usually ordered to ASTM A653/A653M or to JIS G3302, and the two systems are not interchangeable. The coating designations look similar, the mechanical designations look similar, and the numbers inside them mean different physical quantities.
Where the Two Systems Diverge
ASTM A653/A653M is the North American standard for steel sheet, zinc-coated by the hot-dip process, and it is written around a combination of coating designation, surface finish, mechanical designation and coating mass tolerance. JIS G3302 is the Japanese standard for hot-dip zinc-coated steel sheet and coil, and it is organised around grade symbols that encode quality class and, for structural grades, a strength value. Both standards cover the same product form, so the differences lie in how chemistry, coating mass and mechanical properties are expressed and verified.
Substrate Chemical Composition
ASTM A653/A653M imposes requirements on the chemical composition of the base steel, referencing the appropriate base metal specification such as ASTM A1008/A1008M for cold-rolled carbon steel sheet. JIS G3302 specifies the coated product primarily through grade symbols and mechanical properties, and does not generally fix the full chemistry of the substrate in the same way. For a buyer, the practical consequence is that a purchase order referencing only a JIS grade gives the mill more freedom on analysis, so any additional chemistry restriction must be written into the order explicitly.
Coating Mass: Designations and Measurement
This is the largest source of confusion in trade enquiries. ASTM A653/A653M designates coating mass in ounces per square foot total on both surfaces, with the familiar codes G30 through G235, and the metric companion Z90 to Z600 expressed in grams per square metre; G90 and Z275 describe the same coating. Coating mass tolerances for the continuous process are defined in ASTM A924/A924M, and the gravimetric determination is carried out to ASTM A90/A90M.
JIS G3302 uses designations such as Z12, Z20, Z25 and Z27, where the number multiplied by ten gives the minimum coating mass on both surfaces in grams per square metre, so Z12 corresponds to 120 g/m2. Coatings applied to finished articles are covered separately by standards such as JIS H8641, which specifies minimum average coating mass after fabrication. Some measurement conventions differ as well: the older traditions emphasise minimum average coating mass by weighing stripped samples, while ISO and European practice increasingly relies on minimum local thickness measured by the magnetic method to EN ISO 2178. Two coils can therefore both be compliant on paper and still show different numbers on a gauge.
| Item | ASTM A653/A653M | JIS G3302 |
|---|---|---|
| Coating designation basis | oz/ft2 total both surfaces, G codes; metric Z codes | Code x 10 = minimum g/m2 total both surfaces |
| Common equivalent | G90 equals Z275 | Z27 equals 270 g/m2 |
| Coating tolerance source | ASTM A924/A924M | JIS G3302 tables; JIS H8641 for fabricated articles |
| Measurement reference | ASTM A90/A90M gravimetric | Gravimetric and magnetic methods as specified |
| Structural designations | SS Grade 230 to 550 (minimum yield point, MPa) | SGC340 to SGC570 (minimum tensile strength, MPa) |
| Commercial grade | Commercial steel, CS | SGCC |
Mechanical Property Designations
ASTM structural grades are named by minimum yield point, so SS Grade 230 requires 230 MPa and Grade 340 requires 340 MPa, and the older ksi-based labels such as Grade 33 and Grade 50 refer to the same scale. JIS G3302 structural grades are named by minimum tensile strength instead, so SGC340, SGC400, SGC440 and SGC490 refer to tensile values, not yield values. A specification written as "340" therefore means two different things across the two systems, which is why a direct grade-for-grade substitution fails.
The distinction has real procurement consequences. Matching a Chinese Q345 requirement, which is a yield strength basis, means selecting an ASTM grade with a minimum yield point at the required level, such as ASTM A653M Grade 340. On the JIS side the numeric label describes tensile strength, so the trade-off has to be worked out from the tensile-to-yield ratio rather than copied from the designation. Grade symbols also carry formability classes: SGCC denotes commercial quality, SGCD drawing quality and SGCH a higher strength hardness class.
Adhesion, Surface and Testing Methods
ASTM A653/A653M addresses coating adhesion through bend and forming requirements, with surface appearance controlled by the finish designations A, B and C for normal, minimised and spangle-free surfaces, and with visual acceptance criteria for spangle and surface defects. JIS G3302 sets comparable requirements but with different test geometry and sampling rules, and it does not mirror the ASTM approach in every detail. In practice this means that adhesion test results obtained under one standard should not be presented as evidence of compliance with the other without a referencing statement.
Choosing a Standard for a Given Market
Selection follows the end market rather than the mill's preference. Projects engineered in North America specify ASTM A653/A653M because the structural designations, tolerances and inspection records are written into local design codes. Projects in Japan, Korea and much of Southeast Asia specify JIS G3302, and the supply chain there is built around those grade symbols and the surface quality expectations that come with them. For export coils that will be converted locally, specifying both the coating mass in g/m2 and the mechanical property basis removes most of the ambiguity, and stating the measurement method avoids disputes at goods-in.
The safest purchase order states the standard and edition, the coating mass with its measurement method, the mechanical property basis with the numerical value and the property it refers to, and the surface finish. Where the customer's own drawing conflicts with the referenced standard, the order should resolve the conflict in writing before production.
Frequently Asked Questions
Q: Is a JIS G3302 Z27 coating the same as an ASTM G90 coating?
A: Not exactly. G90 is 0.90 oz/ft2, equivalent to about Z275 in grams per square metre, while JIS Z27 denotes 270 g/m2. They are close but not identical, so the g/m2 value should be the basis of comparison.
Q: Why do the structural grade numbers differ between the two standards?
A: ASTM names structural grades by minimum yield point, while JIS G3302 names them by minimum tensile strength. The same number therefore describes different properties, and direct substitution is not valid.
Q: Does JIS G3302 control substrate chemistry?
A: It is less prescriptive than ASTM A653/A653M, which references a base metal specification such as ASTM A1008/A1008M. Any chemistry limitation required by the application must be stated explicitly on the order when buying to a JIS grade.
Q: What is SGCC in JIS G3302?
A: SGCC is the commercial quality hot-dip zinc-coated steel sheet grade, intended for general forming where no structural strength is designed in. Drawing quality is designated SGCD.
Q: Which standard is used for coatings applied after fabrication?
A: Coatings on finished articles are covered separately, for example by JIS H8641 in Japan and by ISO 1461 or ASTM A123/A123M internationally. These are not part of the coil standard.
Q: Can adhesion test results be transferred between the two standards?
A: No. Test geometry, sampling and acceptance rules differ, so a result obtained to one standard should be reported against that standard and not presented as compliance with the other.