Z-Direction Steel Plate: Z15, Z25 and Z35 Explained
In short: Z-direction steel plate is plate tested for ductility through its thickness rather than along its length. The suffix — Z15, Z25 or Z35 — states the minimum average reduction of area measured in a through-thickness tensile test: 15%, 25% or 35%. It exists to prevent lamellar tearing in welded joints where shrinkage stress pulls across the plate surface.
What is Z-direction steel plate?
Z-direction steel plate is steel specified and tested for through-thickness properties. Normal plate specifications test strength and toughness along the rolling direction, because that is how the plate carries load in service. Z-direction plate adds a second requirement in the third axis — the thickness axis — because welded fabrication pulls on that axis too.
The “Z” refers to that axis. A Z-grade plate has been tested by pulling a specimen vertically through the plate thickness, and the result is expressed as reduction of area at fracture.
What do Z15, Z25 and Z35 actually measure?
The number is the minimum average through-thickness reduction of area, expressed as a percentage, under EN 10164.
| Designation | Minimum average reduction of area | Relative resistance to lamellar tearing |
|---|---|---|
| Z15 | 15% | Basic through-thickness quality |
| Z25 | 25% | Improved, for moderate restraint |
| Z35 | 35% | Highest commonly specified, for severe restraint |
Higher Z-numbers mean the steel can accommodate more through-thickness strain before separating. They do not describe strength, and they do not replace the base grade — a Z35 plate is still, for example, a Q460E or an S355J2 in every other respect. The full designation carries both: Q460E/Z35 is a Q460 grade with E-level impact toughness and Z35 through-thickness quality.
Why does steel fail through its thickness?
Thickness-direction failure is called lamellar tearing, and it starts at inclusions.
Rolling flattens non-metallic inclusions into planes parallel to the plate surface. When a welded joint shrinks in a restrained configuration, the resulting stress acts perpendicular to those planes — effectively trying to pull the plate apart layer by layer. If the inclusion content and shape are unfavourable, the plate separates along them.
The failure is not a strength problem. It is a cleanliness and inclusion-shape problem, which is why Z-direction quality is controlled at the steelmaking stage rather than at the rolling stage:
- Low sulphur — sulphur forms manganese sulphides, the inclusions most damaging to through-thickness ductility. Steel for Z-direction service is made to sulphur levels in the region of S ≤ 0.003%.
- Vacuum degassing — removes hydrogen and reduces oxygen content, cutting the oxide inclusions that also act as tear initiation sites. Lines serving this supply chain run a 95% vacuum degassing rate with O ≤ 20 ppm, H ≤ 2 ppm, P ≤ 0.010%.
- Inclusion shape control — treatment that stops sulphides from elongating into flat stringers during rolling.
- Soft reduction and electromagnetic stirring during continuous casting — reduces centreline segregation, which is the other common initiation point.
A supplier offering Z-grades without this metallurgy is offering a test, not a capability.
When should you specify Z-direction plate?
Specify Z-direction quality where a welded joint is thick, highly restrained, and loaded across the plate thickness. In practice that means:
- T-joints and corner joints in heavy sections — thick flange-to-web welds in structures and equipment
- Box sections and column-to-beam connections in high-rise and bridge steel structures
- Penstocks, spiral cases and embedded parts in hydropower stations, where thick plate meets severe restraint
- Offshore and marine structures with thick node connections
- Heavy machinery and press structures with thick welded assemblies
- Nuclear and pressure equipment where through-thickness properties are part of the design case
Where restraint is low and sections are thin, Z-quality adds cost without adding benefit. The specification should follow the joint design, not the habit of the buyer.
Which thicknesses are available?
Through this supply chain, Z-direction plate is available well beyond the thickness ranges where most mills stop:
| Reference case | Specification | Thickness |
|---|---|---|
| National Stadium “Bird’s Nest”, Beijing | Q460E/Z35 | 110 mm |
| Hydropower units, 700 MW class | Z-direction plate | 300 mm |
| Hydropower equipment, Baihetan | SXQ500D-Z35 | 260 mm |
The 300 mm Z-direction plate was certified by the State Council Three Gorges Project Construction Committee as equivalent to foreign product and suitable for 700 MW class turbine generator units — a thickness the supplying mill is the only enterprise in China able to produce.
Two reference projects that show why it matters
Beijing National Stadium (“Bird’s Nest”). The structure used 110 mm Q460E/Z35 plate — a world-first for this application at the time — with 3,700 t supplied. The combination is the point: 460 MPa yield strength, E-grade impact toughness, and Z35 through-thickness quality in a single 110 mm plate, for welded joints that could not tolerate lamellar tearing.
700 MW hydropower units. Thick Z-direction plate in penstock and embedded-part applications carries the highest restraint in the plant. Certification at 300 mm thickness is what allowed domestic plate to replace imported material in this duty.
What to put on the enquiry
To get a usable quotation for Z-direction plate, state:
- Base grade and standard — for example Q460E to GB/T 1591, or S355J2 to EN10025-2
- Z-designation — Z15, Z25 or Z35, and whether it is EN 10164 or a project specification
- Thickness, width and length, with the joint detail if restraint is unusual
- Delivery condition — as-rolled, normalized, or quenched and tempered
- Inspection requirement — including whether through-thickness testing must be witnessed
- Cutting requirement — plate as-is, or cut to drawing
Omitting the Z-designation is the single most common reason a Z-direction enquiry comes back quoted as ordinary plate.
Frequently asked questions
What does Z35 mean in steel? Z35 means the plate achieved a minimum average through-thickness reduction of area of 35% in a through-thickness tensile test, per EN 10164. It is the highest Z-designation commonly specified.
Is Z-direction the same as through-thickness testing? Yes. Z-direction properties are measured by a tensile test pulled through the plate thickness, and the result is reported as reduction of area.
Does a Z-grade plate have higher strength? No. The Z-designation describes through-thickness ductility only. Strength and impact toughness come from the base grade, such as Q460E or S355J2.
What thickness of Z-direction plate can be supplied? Through this supply chain, Z-direction plate is available to 300 mm, certified for 700 MW class hydropower applications, with reference cases at 110 mm and 260 mm.
Why is low sulphur important for Z-direction steel? Sulphur forms manganese sulphide inclusions that flatten into planes during rolling. Those planes are where lamellar tearing initiates, so low sulphur — around S ≤ 0.003% — is a precondition for through-thickness ductility.
Contact
Nanjing Tiange Metal Trading Co., Ltd. Z-direction and heavy plate supply, cut to drawing Email: postmaster@tigosteel.com Phone / WhatsApp / Telegram: +86 138 1389 7227 Web: www.tigosteel.com Nanjing, Jiangsu, China