③ 标准/对比页
Z15 / Z25 / Z35 Steel Plate: Through-Thickness Properties to GB/T 5313
Z15, Z25 and Z35 are through-thickness (Z-direction) performance classes defined in GB/T 5313 and aligned with EN 10164. Each class guarantees a minimum reduction of area in the thickness direction - 15%, 25% and 35% respectively - together with a tight sulfur ceiling, so that heavy welded joints resist lamellar tearing. This page compares the three classes and explains how to select the right one.
Why Lamellar Tearing HappensIn heavy welded joints, tensile stress can act through the plate thickness instead of along its rolling direction. Rolled plate is naturally anisotropic: non-metallic inclusions, especially sulfides, lie flat in the plane of the plate, so through-thickness (Z-direction) ductility is much lower than in-plane ductility. When thick flanges, node plates and cruciform joints shrink during welding, the resulting strain in the thickness direction can open up these weak planes and cause lamellar tearing below or beside a weld. Standards therefore define guaranteed Z-direction properties so designers can specify tested material for joints at real risk. GB/T 5313 is the Chinese standard for this purpose.
Z15, Z25 and Z35 Grades and Sulfur LimitsUnder GB/T 5313 (mirrored internationally by EN 10164), the Z-direction class is added to a base grade, giving designations such as Q355B-Z25 or Q345GJCZ25. The class number is the guaranteed minimum reduction of area through the thickness. Because sulfide inclusions are the main cause of low through-thickness ductility, each class also sets a tighter sulfur limit for the ladle analysis: Z15 requires S ≤ 0.010%, Z25 requires S ≤ 0.007%, and Z35 requires S ≤ 0.005%. Choosing a higher class means both a stricter sulfur ceiling and a higher guaranteed RA value in the mill test certificate.
Reduction of Area RequirementsThe key acceptance test is a tensile specimen taken in the thickness direction, and the measured reduction of area (RA) must meet the class minimum: RA ≥ 15% for Z15, RA ≥ 25% for Z25 and RA ≥ 35% for Z35, all per GB/T 5313. Results are reported on the mill test certificate, so fabricators and inspection bodies can verify each plate lot. If a project specification also calls for EN 10164 Z-classes, the numbers are aligned, but final acceptance always follows the purchase standard and the certificate. For unlisted combinations or class-to-grade pairings, ask us to confirm against the mill test certificate.
Where Z-Direction Plates Are UsedSpecify a Z-class when welded joints pull through the thickness: thick node and gusset plates in high-rise building columns, box-column diaphragms, heavy cross-beam connections, offshore and marine structures, and heavy plate butt welds in machinery bases. The Z requirement is an overlay on the base grade, so mechanical and chemical properties of Q355B-Z25 or Q345GJCZ25 follow the base standard, with the Z test added. Yunhao supplies Z-direction performance plate in thickness 8–100 mm and widths 1500–3200 mm, delivered in the base grade condition (hot-rolled, normalized or TMCP) as the project requires.
Quick FAQQ1: What does Z25 mean in steel? A: A Z-direction class to GB/T 5313 guaranteeing reduction of area ≥ 25% and sulfur ≤ 0.007% through the thickness. Q2: Can any grade get a Z-suffix? A: The class is overlaid on a base grade such as Q355B-Z25 or Q345GJCZ25; confirm the pairing with us. Q3: How is the Z test done? A: A through-thickness tensile specimen is tested and RA is reported on the mill test certificate.
Request a Z15/Z25/Z35 QuoteSend us your base grade, Z-class, thickness and length schedule, and we will reply with a mill test certificate sample, current lead time and an FOB/CIF offer. Regular stock is held in this category (status checked 2026-10-01); MOQ is 25 t and normal delivery is 15–30 days, loading at Tianjin or Qingdao, China. Availability varies by size and is confirmed in our quotation. For project quantities requiring third-party inspection or classification society witness testing, tell us in the inquiry so we can arrange it before rolling.
| Grade | Through-Thickness RA (min) | Sulfur S (max) |
|---|---|---|
| Z15 | ≥ 15 % | ≤ 0.010 % |
| Z25 | ≥ 25 % | ≤ 0.007 % |
| Z35 | ≥ 35 % | ≤ 0.005 % |