DX51D+Z in the EN 10346 Family
DX51D is the base commercial quality grade of the continuously hot-dip zinc-coated flat product family standardised in EN 10346. The designation is systematic: D stands for flat products for cold forming, X for the non-defined yield strength series, 51 for the lowest formability class of that series, D for hot-dip coating, and +Z for a pure zinc coating. The grade is a low-carbon, fully annealed steel designed to be formed and coated rather than to carry structural load, and it is among the most widely ordered substrates for building panels, appliance shells and general fabrication sheet.
Because the grade is soft, its properties are usually specified as ranges rather than minimum values, and what the designer really buys is forming behaviour.
Core Mechanical Properties to EN 10346
| Property | Typical range | Test standard | What it means in practice |
|---|---|---|---|
| Tensile strength, Rm | 270-350 MPa | ISO 6892-1 | Moderate strength; formability takes priority |
| Yield strength, ReL | 140-260 MPa | ISO 6892-1 | The lower yield point drives the design check |
| Elongation, A80 | ≥30 % | ISO 6892-1 | 80 mm gauge length; plastic deformation capacity |
| Strain hardening exponent, n | 0.18-0.22 | ISO 10275 | Higher n distributes strain and delays local necking |
| Plastic strain ratio, r | 1.4-2.0 | ISO 10113 | Higher r resists thickness reduction in deep drawing |
Reading the Numbers: Low Strength, High Ductility
The low-carbon chemistry, with C at or below 0.12 percent, combined with full recrystallisation annealing produces a soft ductile sheet that draws and stretches well but carries little load. Compared with a micro-alloyed high-strength grade such as HX340LAD+Z, it offers roughly 30 percent lower tensile strength while gaining more than 50 percent in elongation. That is the trade-off which suits skin panels, appliance housings and deeply formed parts rather than stressed members.
Anisotropy is the second lever. A high r value means the sheet resists thinning through the thickness, so deep-drawn parts such as fuel tanks, sinks and complex housings can be produced with fewer drawing stages. The n value describes how the material distributes strain: a higher n spreads deformation over a wider area instead of concentrating it in one band, which lowers the risk of localised cracking.
Grade Equivalents Across Standards
| Standard | Grade | Tensile strength (MPa) | Yield strength (MPa) | Elongation A80 (%) |
|---|---|---|---|---|
| EN 10346 | DX51D+Z | 270-350 | 140-260 | ≥30 |
| JIS G 3302 | SGCC | 270-380 | 120-260 | ≥30 |
| GB/T 2518 | DX51D+Z | 270-350 | 140-260 | ≥30 |
The JIS G 3302 window for SGCC is deliberately wider than the EN 10346 window for DX51D+Z. In practice mills hold production towards the lower or middle part of the band according to the customer forming requirement, so two coils that both satisfy the standard can behave quite differently on a press line.
What Shifts the Property Window
Carbon and sulphur: C at or below 0.12 percent and S at or below 0.025 percent keep the sheet plastic and limit inclusion content.
Manganese: a small addition in the 0.2 to 0.5 percent range raises strength, but excess manganese reduces ductility.
Annealing: full recrystallisation annealing between 700 °C and 800 °C restores ductility after cold reduction.
Cold reduction: usually held between 50 and 70 percent; beyond that range strength rises and elongation falls.
Coating: the zinc bath runs at about 450 °C, far below the annealing temperature, so it barely affects substrate properties; spangle size however changes the surface friction coefficient and therefore press behaviour.
Forming, Welding and Coating Balance in Service
For stamping, a die clearance of 1.1 to 1.2 times the sheet thickness is a reliable starting point, and the blank should be oriented so that the direction of greatest strain runs transverse to the rolling direction. Where the coil is welded, the zinc layer must first be removed locally by grinding, because zinc boils at only 907 °C and otherwise forms porosity. A low-carbon filler wire classified as ER70S-6 under AWS A5.18 is commonly used, with welding current reduced by 10 to 15 percent compared with uncoated steel of the same thickness.
Corrosion resistance and press performance pull in opposite directions. A heavier coating such as Z275 gives a much longer service life outdoors than Z100, but the thicker zinc layer raises friction in the die and may call for a drawing lubricant formulated for coated stock.
Frequently Asked Questions
Q: Can DX51D+Z be strengthened by heat treatment?
No. Heat treatment strong enough to change the substrate would damage the zinc layer; where higher strength is needed, step up to a micro-alloyed grade such as HX340LAD+Z.
Q: Which direction should tensile samples be taken from?
For stamped parts, samples are normally taken transverse to the rolling direction, because the dominant strain in service and in the press is usually transverse, and the resulting data are more conservative.
Q: Is an A80 of 30 percent enough for deep drawing?
Not on elongation alone. The n and r values decide the outcome, and a fuel tank or a deep housing normally calls for an A80 of at least 32 percent together with an r value of about 1.8 or higher.
Q: Does hot-dip galvanizing change the mechanical properties of the base steel?
Only marginally. The bath temperature of about 450 °C is well below the annealing temperature, so the substrate keeps the properties set on the annealing line.
Q: Which coating mass should be specified?
It follows from the corrosivity of the site: light interior use is served by Z100 to Z140, general exterior cladding by Z180, and coastal or industrial exposure by Z275 or a zinc-aluminium coating.
Q: Can DX51D+Z be used for load-bearing members?
No. It is a forming grade with no guaranteed structural yield strength; load-bearing work should use a structural grade from the structural series of EN 10346 or from EN 10025.