Galvanised steel for sheet metal parts: grade and formability

Galvanised steel for sheet metal parts is specified by two numbers that buyers routinely confuse: the coating mass and the steel grade. The coating mass is written as a designation like Z275, which is a mass in grams per square metre across both faces, not a thickness. The grade is written as DX51D, DX53D or S350GD, and it decides whether the part bends or splits.

Get either one wrong and the failure is quiet. A Z450 coating on a tight bend crazes and the part looks acceptable until white rust appears in the store. A DX51D blank on a draw that needed DX53D splits at the punch nose on the second stroke, after the tool is cut. Both failures are visible on the drawing before any steel is ordered, which is where the decision belongs.

The rest of this page converts coating designations into microns, maps the forming grades to draw ratio, gives the bend radius at which zinc starts to craze, and sets out what welding and laser cutting do to the coating. It closes with the drawing notes that keep a galvanised part from being quoted against the wrong coil.

Galvanised steel for sheet metal parts: which grade to call out?

Match the grade to the strain the part puts into the sheet, then check the coating mass against the exposure. The grade and the coating are independent choices, and a supplier who offers one designation without the other is not quoting the same part.

The DX series is the forming series: DX51D for bending and profiling, DX52D for drawing, DX53D for deep drawing and DX54D for the deepest draws. Their yield strength falls and their elongation rises as the number goes up, which is exactly the trade a draw needs. The S series is structural: S250GD, S280GD, S320GD and S350GD hold a minimum yield from 250 to 350 MPa at the cost of formability, and they are the right choice for a rack upright, a purlin or a bracket carrying load rather than a panel that has to stretch.

IS 277 uses letter grades that map onto the same ladder — O for commercial, D for drawing, DD for deep drawing and EDD for extra deep drawing — and ASTM A653 uses CS, FS, DDS, EDDS plus structural grades SS 33 to SS 40. All three systems describe the same physics, so a drawing that names the standard and the grade together removes the ambiguity.

Yield strength drives springback as well as formability. A S350GD panel at 350 MPa yield springs back roughly 2.5 times as far as a DX51D panel of the same thickness at 140 MPa, so a structural galvanised part needs its bend angles proved on a trial rather than assumed from the CRCA equivalent.

How much zinc is in each coating designation?

Divide the designation by two and by the density of zinc, 7.14 g/cm³. A Z275 sheet carries 137.5 g/m² on each face, which is 19.3 µm; a Z100 sheet carries 50 g/m² per face, or 7.0 µm. The arithmetic matters because corrosion life is a thickness argument, not a mass argument, and because a drawing that says "galvanised" without a mass has not specified the part.

The parenthesised figures are the ASTM equivalents, because G and Z designations use different units and a drawing that mixes them is a returns note waiting to happen. G90 is 0.90 oz/ft², which is 275 g/m², so G90 and Z275 describe the same coating.

The coating is a small part of the mass. One square metre of 0.8 mm steel weighs 6.28 kg and carries 0.275 kg of zinc at Z275, which is 4.4% of the sheet mass. At 2 mm the same coating is down to 1.75%, which is why the coating premium per tonne falls as the gauge rises.

How does galvanised sheet behave at a bend?

The coating fails before the steel does. Outer-fibre strain at a bend is t/(2R + t), with R the inside radius and t the thickness, so a 1 mm sheet bent over a 1t (1 mm) inside radius sees 1/(2 + 1) = 33% surface strain, and the same sheet over a 0.5t radius sees 50%.

That is the whole basis of the bend rule. Zinc starts to craze at around 20–25% surface strain on a continuous hot-dip coating, because the iron-zinc intermetallic layer at the interface is brittle even though the pure zinc on top is ductile. So:

• Z275 on DX51D: 1.0t inside radius minimum, which keeps strain at 33% and accepts micro-crazing that a paint system will bridge.
• Z100 or electro-galvanised: 0.5t inside radius minimum, because there is less zinc to craze and the substrate is usually a drawing grade.
• Z450: 1.5t minimum. The heavier coating has more intermetallic at the interface and cracks visibly below that radius.
• Bend line across the coil, not along it. A bend parallel to the rolling direction runs along the least ductile axis and cracks first, at any radius.

Two practical notes. First, the bend radius on a galvanised part is measured to the inside of the bend, and the drawing must say so, because an outside radius of 2t on a 1 mm sheet is an inside radius of 1t. Second, a hem or a flattening operation crushes the coating at the fold, and a hemmed galvanised edge will rust from inside the fold whatever the outside looks like. Design hems on galvanised parts out, or seal them.

Does galvanised sheet weld and laser cut the same way?

No, and the difference is zinc's boiling point. Zinc boils at 907 °C, roughly 600 °C below the melting point of the steel it protects, so any welding process drives the coating off before the steel becomes liquid.

In resistance spot welding, the zinc layer alloys with the copper electrode and the contact resistance changes. Expect 15–25% more current and a higher electrode force than the same stack in CRCA, and expect electrode life to fall sharply: tips that run thousands of welds on CRCA need dressing at a fraction of that count on galvanised sheet. A galvannealed coating, where the zinc is already alloyed with iron at the mill, welds closer to CRCA because the free zinc is gone, at the cost of less sacrificial protection.

In MIG welding, the boiling zinc blows through the arc, produces porosity in the bead, spatter on the surrounding coating, and zinc oxide fume that needs extraction. Grind the coating back 5–10 mm each side of the joint, weld, then restore the protection with a zinc-rich primer before topcoat. That bare strip is the reason a welded galvanised assembly needs a paint system rather than being left bare.

On the laser, the 12 kW fibre source cuts galvanised sheet cleanly, but the zinc vapour needs higher assist-gas pressure and good extraction, and the cut face is left uncoated. The zinc is depleted for 0.1–0.3 mm around the cut line, so the edge is bare steel with a thin transition. On a part that will be coated, that edge is where the film has to be thickest, and on a part left bare it is where red rust begins.

What thickness range is practical in galvanised sheet?

Zinc-coated coil runs from roughly 0.3 mm to 3.0 mm. The coating line sets that band: below 0.3 mm the strip is difficult to handle through the pot, and above 3 mm the coil coating economics and the mill's product range stop making sense for most buyers.

Above 3 mm, the route changes. The part is fabricated in hot-rolled or CRCA steel and hot-dip galvanised after fabrication to ISO 1461, where the minimum average coating is 55 µm for 3–6 mm sections and 70 µm above that. That is a far heavier coating than Z275 at 19.3 µm, it covers the cut edges and the welds, and it comes with a 445–460 °C bath that distorts thin sections and closes threads.

Between 1.0 and 3.0 mm, galvanised coil is the economical answer for an enclosure, a canopy panel, a cable tray or a rack upright. Below 1.0 mm the coating becomes a large fraction of the wall thickness, the bend radii get tight in absolute terms, and a CRCA part with a powder coat is often the better buy.

How is galvanised work run at Ganesh?

Zinc-coated sheet is bought to the drawing's coating designation and grade, and the forming routes are the same ones used for CRCA: mechanical presses from 20 T to 300 T with 250 T punching, a 200 T, 5-axis press brake on a 4,450 mm bed to 10 mm, and a 12 kW fibre laser on a 40 ft × 16 ft bed. Robotic MIG runs on three CLOOS cells with fixtures built in-house, and 30+ manual MIG, MAG, spot and SPM stations cover the assemblies that do not suit a robot.

The material range for custom parts is 0.8–40 mm in mild steel, CRCA, stainless, aluminium, brass and copper, with galvanised sheet inside the lower part of that band. Formed tolerance is ±0.5 mm typical, tighter on critical features by agreement, and PPAP or first article inspection is available. Ganesh has fabricated since 1992, holds ISO 9001:2015 and ISO 14001:2015, and runs five units in Mumbai (Vasai) and Nagpur, supplying customers including Mahindra, Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways.

The parts are not galvanised here. Where the drawing needs a post-fabrication zinc coating, the part is fabricated at Ganesh and galvanised at a specialist plant, then returned for inspection or for a duplex paint coat.

When galvanised sheet is the wrong substrate

Galvanised sheet loses to CRCA on three counts: a visible painted face, a tight bend, and a deep draw. A Class A panel needs a phosphate and a powder over a smooth substrate, and a zinc-coated face under a thin paint film shows spangle and reacts with the primer unless the system is designed for zinc. A bend below 0.5t crazes any coating. A draw ratio above about 1.6 in DX54D is a press problem, not a coating problem, and CRCA with a post-coating is the answer.

It is also wrong for a part that has to be welded into a leak-tight assembly, and for a part with a machined face, because the coating has to be removed before any machining and cannot be restored to the same thickness.

The bend radius guide covers the radius limits for the base materials, the degreasing guide for galvanised parts covers the cleaning window that protects the zinc, and the powder coating failure guide covers what happens when the coating system fights the substrate. The materials and grades hub holds the rest of the grade set.

What to put on the drawing

• The product standard and grade: IS 277 DD, ASTM A653 FS, or EN 10346 DX53D+Z, not the word galvanised alone.
• The coating designation and mass, with the units, and whether it is a Z, G, AZ or ZE type.
• Nominal thickness with its tolerance class, and the thickness the finished part needs after forming.
• Every bend radius to the inside, in mm, with the bend line orientation to the rolling direction noted where it matters.
• The draw ratio or the cup depth for any part that is formed rather than folded.
• Weld process, and whether the coating is ground back and re-primed at the joint.
• The edge condition: laser cut, deburred, sealed or left bare, and any edge that must be recoated.
• The finish system on top, since a duplex system and a bare part are different specifications.
• General tolerance class to ISO 2768-1, and any tighter geometric callout.

Send the drawing with the coating designation and the annual quantity to request a quote, and the grade, the coil width and the cut-edge protection are confirmed before the blank is nested. The laser cutting capability page shows the bed size and the thicknesses the 12 kW source handles on coated and uncoated steel.