Galvanised sheet DX51D+Z: how to read the grade and the zinc class Z120, Z275

The designation of galvanised sheet has two halves, and buyers most often lose the second. DX51D is the grade of the steel substrate, +Z275 the type and mass of the coating. A DX51D sheet without a zinc class is a sheet with an unknown service life; “0.5 mm galvanised” without a grade is a sheet with an unknown ability to bend. The full designation under EN 10346 reads DX51D+Z275-M-A-C, and every part of it is checked against the certificate.

First half: the steel

D is flat product for cold forming; X means the rolling condition is not specified, the substrate may be hot- or cold-rolled; 51 is the grade number; the final D means hot-dip coated. DX51D is a bending and profiling quality: only tensile strength 270–500 N/mm² and elongation not below 22 % are specified. DX52D and DX53D are drawing qualities with yield strength not above 300 and 260 N/mm² and elongation not below 26 and 30 %. For load-bearing profiled sheet and purlins the structural grades are used: S250GD with yield strength not below 250 N/mm², S350GD not below 350. GB/T 2518-2019 repeats the EN system with the same designations; GOST 14918-2020 describes the substrate by steel grade, 08ps, and coating class.

Second half: the zinc

The number after Z is the mass of zinc in grams per square metre of sheet, both sides together, minimum by the triple-spot test. Z275 is 275 g/m² over two sides, 137.5 on each. The density of zinc, 7.14 g/cm³, turns grams into microns: 137.5 g/m² is about 19 µm per side. Coating thickness is service life: zinc is consumed protecting the steel at a constant rate for a given environment, and a Z275 sheet lasts twice as long in the same environment as a Z140 sheet.

Z120 is a class from GB/T 2518: Chinese mills readily offer it as “standard galvanising” because it is cheaper than Z140 and Z200. EN 10346 has no such class, nor does GOST 14918-2020; a project written for Z140 or Z200 is not satisfied by a Z120 sheet. The previous edition, GOST 14918-80, counted the coating by thickness rather than mass: class 2 was 10–18 µm, class 1 18–40 µm, class P 40–60 µm; on that scale Z275 falls into class 1 and Z120 into class 2.

Surface and treatment

After the zinc class in the EN designation come three letters. The first is the spangle: N normal, M minimised, for painting. The second is surface quality: A allows minor defects, B is improved, C is for high-quality paint finishes. The third is treatment: C passivated, O oiled, CO both, P phosphated, U untreated. For profiled sheet in the open, N-A-C is usually enough; for sheet to be painted afterwards, M-B-CO. GB/T 2518 uses the same letters.

What to write into the specification

• The substrate grade and zinc class together: DX51D+Z275 to EN 10346 or GB/T 2518; for a delivery under GOST, 08ps, coating class 275 to GOST 14918-2020.
• Thickness, stating whether it includes the coating, and the thickness tolerance category.
• Spangle, surface quality and treatment as three letters, otherwise the mill chooses for you.
• The coating mass record in the mill certificate: a figure in g/m² from the triple-spot test, not the word “Z275” in the header.

Zinc has weight: Z275 adds 0.275 kg to every square metre, and for a 0.5 mm sheet that is seven percent of the mass. With shipment by actual weight the coating is paid for as steel, one more reason to check the class against the certificate rather than the price list. Pipes are galvanised differently: the finished pipe is dipped whole into the melt, and its zinc thickness is specified in microns under GOST 9.307 and EN 10240, not by a Z class.

Grade equivalents

Equivalents are the closest match, not identical: chemistry and tolerances differ between standards. The exact grade and standard are written into the contract.