DX51D+Z is a continuous hot-dip galvanized flat product defined in EN 10346. The designation is read in three parts: D means a flat product for cold forming, X means the rolling condition is not specified, and 51 marks the base steel series, while the suffix +Z denotes a pure zinc coating applied by continuous hot-dip galvanizing. In daily trade the grade is frequently specified for roofing profiles, cable trays, ducting, automotive inner parts, appliance bodies and general sheet metal work because it offers a good balance between formability, corrosion protection and cost.
Chemical composition of the DX51D base steel
The chemical requirements for the DX51D base metal are given in the informative annex of EN 10346. The maximum values commonly quoted are carbon 0.12%, manganese 0.60%, phosphorus 0.10%, sulphur 0.045% and titanium 0.30%. In practice most integrated mills run much tighter ladle analyses, typically carbon 0.05-0.08%, silicon 0.03% max, manganese 0.40-0.50%, phosphorus 0.025% max and sulphur 0.025% max, with the steel aluminium killed to at least 0.020%. Tighter carbon and manganese control improves bendability and keeps the yield spread narrow, which matters when the coil is later roll-formed or deep drawn.
| Element | EN 10346 max limits | Typical mill practice |
|---|---|---|
| C | 0.12% | 0.05-0.08% |
| Mn | 0.60% | 0.40-0.50% |
| P | 0.10% | 0.025% max |
| S | 0.045% | 0.025% max |
| Ti | 0.30% | not specified |
| Al | not specified | 0.020% min |
Mechanical properties guaranteed for DX51D
For a flat product thickness between 0.35 mm and 3.0 mm, EN 10346 requires a yield strength Rp0.2 of 140-300 MPa, a tensile strength Rm of 270-500 MPa and a minimum elongation A80 of 22%. No r-value or n-value is guaranteed for DX51D, which is why it suits bending, profiling and moderate forming rather than severe deep drawing; for demanding drawing work buyers step up to DX53D or DX54D.
Coating systems: Z, ZF and AZ
The letter after the plus sign describes the coating family. +Z is pure zinc applied by continuous hot-dip galvanizing. +ZF is a zinc-iron alloy coating produced by post-dip annealing, often called galvannealed; it welds and paints more easily and is preferred for automotive body panels. +AZ is an aluminium-zinc coating with roughly 55% aluminium, 43.4% zinc and 1.6% silicon, frequently described as aluminium-zinc type; it gives longer service in mildly corrosive and heat-reflective applications. Coating mass is expressed in grams per square metre measured on both surfaces, for example Z100, Z180 or Z275. A heavier coating extends service life but reduces formability at sharp bends, so the coating class should be matched to the exposure rather than maximised blindly.
DX51D versus DC51D and neighbouring grades
Both DX51D and DC51D appear in EN 10346. The D at the beginning means flat product for cold forming; the X in DX means the rolling condition is not fixed, while the C in DC means a cold-rolled substrate. In daily supply DX51D is the standard choice because it accepts both cold-rolled and hot-rolled feedstock, whereas DC51D is limited to a cold-rolled base. Within the DX family a higher number indicates better formability: DX52D, DX53D and DX54D offer progressively lower yield ranges and higher elongation at a slightly higher price. Structural buyers should instead look at the S-series, such as S250GD to S550GD, which guarantee higher yield strength for load-bearing profiles.
Buying checklist for DX51D+Z coils
When ordering, specify the governing standard and its edition, the base grade, the coating class and type, for example DX51D+Z275, the coating finish, whether regular spangle, minimised spangle or spangle-free, the surface quality class, and whether chromate passivation or oiling is required. Dimensional tolerances follow EN 10143. On delivery, check the mill test certificate for yield and tensile values, coating mass by triple-spot average and single-spot minimum, and the bend test result; the certificate should name the standard and the test methods used.
Frequently asked questions
Is DX51D+Z the same as SGCC?
Not exactly. DX51D is defined in EN 10346 with explicit mechanical ranges, while SGCC in JIS G3302 is a commercial-quality grade without guaranteed tensile values; the two are often treated as equivalents in trade, but the certificates differ.
What does Z275 mean on a DX51D coil?
Z275 means the total zinc coating mass on both surfaces is 275 g/m² by triple-spot measurement, with a single-spot minimum of about 235 g/m². It is a heavy coating used for outdoor structures and agricultural buildings.
Can DX51D+Z be deep drawn?
For moderate drawing it works, but where the drawing ratio is high, use DX53D or DX54D, which guarantee lower yield and better elongation, and consider the coating mass because thick zinc layers crack at tight radii.
How is the coating mass measured?
By weighing a specimen before and after dissolving the coating in inhibited acid, or by magnetic gauge on sheet samples; EN 10346 requires triple-spot and single-spot values reported in g/m².
Does +Z or +ZF weld differently?
Yes. Pure zinc vaporises around 907 °C during welding, which can create porosity and spatter; zinc-iron alloy coatings are easier to weld and are standard for automotive body panels. High coating masses above roughly 150 g/m² may need parameter adjustment.
What edge protection is needed after slitting?
Cut edges expose bare steel. For outdoor service, specify edge trimming in the slitter, apply a zinc-rich cold coating on cut edges, or select a heavier coating and design the profile so that edges are not directly exposed to rain.
