What G550 Steel Is
G550 is a hot dip zinc coated structural steel supplied as coil with a spangled surface and a guaranteed minimum yield strength of 550 MPa. It is produced for roll formed sections and light gauge steel framing, where a high strength to weight ratio lets thinner sections carry the same load. The grade is defined as G550 in AS 1397 and is commonly compared with ASTM A653 SS Grade 80 and the European grade S550GD.
Cold formed members made from this coil are used in frames, purlins, girts, studs, trusses and racking, and the coating supplies the corrosion resistance that plain carbon steel cannot offer in wet or coastal service.
Chemical Composition Requirements
AS 1397 sets maximum values for the main residual elements so that the high strength level is reached without losing formability or coating adhesion.
| Element, cast analysis | Maximum |
|---|---|
| Carbon | 0.20 % |
| Manganese | 1.20 % |
| Phosphorus | 0.04 % |
| Sulphur | 0.03 % |
Keeping carbon and manganese inside these limits protects bendability, because the low elongation margin of a 550 MPa grade leaves little room for hard microstructures.
Mechanical Property Requirements
| Property | Requirement for G550 |
|---|---|
| Minimum yield strength | 550 MPa |
| Minimum tensile strength | 550 MPa |
| Minimum elongation, Lo = 50 mm | 2 % |
| Minimum elongation, Lo = 80 mm | 2 % |
| Transverse bend | Mandrel diameter set by coating class |
Because minimum elongation is only 2 %, G550 is not intended for severe drawing. It is designed for bending, roll forming and light profiling, where tight bend radii and small thickness reductions must be planned rather than assumed.
Coating Classes and 180 Degree Bend Adhesion
Coating adhesion is verified with a 180 degree bend test, and the permitted mandrel diameter is expressed as a multiple of product thickness t. The lightest coating classes such as Z100, ZA90, ZF80 and ZF100 pass with a mandrel of 1t. Mid range classes in the Z, ZA and ZM families, and the aluminium zinc and aluminium magnesium classes AZ150, AM100, AM125 and AM150, are tested at 2t. The heaviest zinc class, Z600, is tested at 3t, reflecting the thicker and less ductile coating layer.
Zinc coatings: Z100 through Z600
Zinc aluminium coatings: ZA90 through ZA450, AZ150 through AZ200
Zinc iron coatings: ZF80 and ZF100
Zinc magnesium and aluminium magnesium coatings: ZM90 through ZM450, AM100 through AM225
Equivalent Grades Across Standards
| Standard | Grade designation |
|---|---|
| AS 1397 | G550 |
| EN 10346 | S550GD |
| ASTM A653 | SS Grade 80, minimum yield 550 MPa |
| JIS G 3302 | SGC550 |
Buyers comparing quotations should confirm that the yield strength, coating mass and coating class are all quoted, because two coils can share the same base grade and still differ substantially in durability.
Fabrication and Service Notes
Roll forming is the natural process for G550: the material work hardens further during forming, so roll passes should be arranged to spread deformation rather than concentrate it in a single stand. Bending should respect the minimum inside radius compatible with 2 % elongation, and sheared edges should be dressed or protected, since a cut edge is the only place where the coating cannot protect the steel sacrificially. In sheltered interior framing, thin coating classes are usually adequate; for coastal, industrial or humid exposure, heavier zinc or zinc aluminium coatings are selected to extend service life.
FAQ
Q: What does G550 mean in galvanized steel?
G550 is an AS 1397 designation for hot dip zinc coated structural steel with a guaranteed minimum yield strength of 550 MPa and a spangled surface.
Q: What is the minimum yield strength of G550 coil?
The guaranteed minimum yield strength is 550 MPa, and the minimum tensile strength is also specified as 550 MPa.
Q: Which standards cover G550 material?
AS 1397 defines G550 directly. EN 10346 designates the equivalent as S550GD, JIS G 3302 uses SGC550, and ASTM A653 describes a comparable product as SS Grade 80.
Q: What coating classes can be supplied on G550?
Zinc, zinc aluminium, zinc iron, zinc magnesium and aluminium magnesium classes are available, ranging from Z100 up to Z600 and including ZA, AZ, ZF, ZM and AM designations.
Q: Why is elongation limited to 2 %?
The high strength level leaves a narrow ductility margin, so the grade is intended for bending and roll forming rather than severe drawing, and tight radii must be planned carefully.
Q: How is coating adhesion tested?
A 180 degree bend test is used, with the mandrel diameter specified as 1t, 2t or 3t depending on the coating class of the product.
Q: Where is G550 typically used?
It is used in roll formed sections for light gauge steel framing, including frames, purlins, girts, studs and racking where lightness and strength both matter.