Why Designations Follow Standards Rather Than Trade Names
A galvanized steel sheet designation is not a marketing label; it is a coded description of what the product is and what it will do. The codes are set by national and regional standardization bodies - GB in China, EN in Europe, ASTM in the United States, JIS in Japan - and each body builds its system around the same four dimensions: coating process, coating composition, mechanical or forming grade, and application. Because the underlying attributes are universal, the same steel can appear under three or four different codes, and the first task of anyone reading a designation is to identify which standard system the code belongs to.
The logic is consistent in every system: letters classify qualitative characteristics such as coating type and forming grade, while numbers carry measurable values such as minimum yield strength or coating mass. Once that split is understood, most designations can be read directly from the code without consulting the full standard text.
How Coating Process Is Encoded in the Letters
Hot dip coated product is identified by the structure of the designation. In the European system, EN 10346 uses S for structural steels followed by a strength figure and D for hot dip coating, so S250GD+Z is a structural grade with 250 MPa minimum yield strength and a zinc coating. The commercial forming series uses the DX prefix, as in DX51D+Z. The Chinese system for continuously hot dip coated sheet follows a closely aligned format, so DX51D+Z, DX52D+Z and DX53D+Z appear in much the same way. In the Japanese system, JIS G3302 designates hot dip galvanized sheet with the SGC series, such as SGC400 or SGC490, where the number is a strength indicator. ASTM A653 takes a different route again: it identifies the coating by a letter and a coating mass figure, so G60 or G90 means a zinc coating of 0.60 or 0.90 oz/ft2 total on both sides, and the steel grade is stated separately as a commercial, forming or structural quality.
Electrolytically coated product is marked by a different family of codes because the process is different. In the European and Chinese systems the letter E indicates zinc deposited electrolytically, as in DC01+ZE or the SECC family used for commercial electrogalvanized sheet. The E in the code is the flag that tells the reader the coating was applied by current rather than by immersion in molten zinc, and it implies a thin, smooth, very uniform coating rather than a heavy one.
Numbers: Strength, Coating Mass and Drawing Grade
Numbers in a designation carry two different kinds of information, and confusing them is the most common reading error. In grades such as S250GD or S350GD the number is a mechanical property: the minimum yield strength in megapascals. In grades such as DX51D, on the other hand, the two digits are a formability code from the standard's grade table rather than a strength value, so DX51D is not a 51 MPa steel. In the JIS SGC series the number indicates the minimum tensile strength band for the grade.
Coating mass is written either inside the coating suffix or as a separate figure. GB/T 2518 and GB/T 12754 express hot dip zinc coatings as Z followed by grams per square metre, so Z100 is a total coating mass of 100 g/m2 on both sides combined, Z180 is 180 g/m2 and Z275 is 275 g/m2. The value is a total for both sides, and the standard applies both an average and a minimum requirement, which is why the figure cannot be divided by two and treated as a guaranteed per-side coating. Where a per-side minimum exists, it is stated separately.
Coating Suffixes: Z, ZF, ZA, AZ and AS
| Suffix | Coating | Character |
|---|---|---|
| +Z | zinc | standard hot dip galvanized, best sacrificial protection |
| +ZF | zinc-iron alloy | galvannealed, matte surface, excellent for painting and welding |
| +ZA | zinc-aluminium | higher aluminium content than plain zinc, improved heat resistance |
| +AZ | aluminium-zinc | aluminium-rich coating for long-term atmospheric exposure |
| +AS | aluminium-silicon | aluminium-based coating for high temperature service |
The suffix is therefore not decoration. A +ZF coating behaves differently from a +Z coating in the press shop, in the welding cell and under a paint system, and the two are not interchangeable even when the coating mass figure is identical.
Stamping and Forming Letters
The trailing letter in forming grades states the forming severity the steel is intended for. In the European and Chinese coated series the sequence from DX51D to DX54D moves from commercial quality through drawing to deep drawing quality, and the higher the forming index the lower the carbon and the freer the steel is from nitrogen, so the r-value rises and the yield point elongation disappears. In the uncoated cold rolled series covered by EN 10130, the same progression runs DC01 to DC06. Chinese domestic practice additionally uses a letter suffix on some cold rolled and coated grades to state normal drawing, deep drawing and extra deep drawing suitability, with later letters denoting better ductility for more complex pressings.
Frequently Asked Questions
Q: Does the same steel have the same designation under GB, EN, ASTM and JIS?
A: No, but the designations can be correlated. The underlying attributes - coating type, coating mass, strength level and forming grade - can be matched across systems, but the codes themselves differ in structure, and a correlation table must be used rather than a direct reading of the numbers.
Q: What does the D at the end of DX51D mean?
A: It marks the product as drawing quality within the coated sheet grade table. It is not a coating code. The coating information sits in the suffix that follows, for example +Z or +ZF.
Q: Is Z100 the same on the front and the back of a coil?
A: No. Z100 states a total coating mass for both sides combined. The distribution between the two faces is not equal for most coated products, and the governing standard sets separate requirements for the average and the minimum coating mass.
Q: Can a +ZF designation be used where a +Z coating was specified?
A: Not without re-qualification. The zinc-iron coating is harder and less ductile than plain zinc. It offers better paint adhesion and better welding behaviour, but it sacrifices some of the sacrificial protection and changes the friction conditions in the press.
Q: Why does the European system use S for some grades and DX for others?
A: S grades are structural steels ordered for a guaranteed minimum yield strength, while DX grades are forming steels ordered for forming quality. The designations are kept separate because the two families are qualified by different tests.
Q: Where is the coating mass checked against the designation?
A: On the finished coil, by non-destructive magnetic measurement with gravimetric stripping used as the arbitration method when a result is disputed. Coating mass is a release property, so it is verified per coil rather than per heat.