A steel plate can meet every strength and impact requirement on the drawing and still tear apart during welding. The failure is not in the weld and not in the heat-affected zone: it is in the parent plate, a few millimetres below the fusion line, and it runs parallel to the surface. That is lamellar tearing, and Z-quality plate is the defense against it.
The cause sits in the rolling direction. When a slab is rolled, non-metallic inclusions—sulphides and oxides—are flattened into thin planes that lie parallel to the plate surface. The plate is strong in-plane and weak through its thickness. Weld a heavy T-joint or a cross-joint into that plate and the shrinking weld pulls in the through-thickness direction; if the plate cannot stretch, it separates layer by layer.
Z grades measure that ductility. EN 10164 defines the through-thickness reduction of area from a tensile test taken in the Z direction: Z15 requires at least 15 percent, Z25 at least 25 percent, Z35 at least 35 percent, measured as the average of three specimens. Nothing about yield strength changes. The grade only describes how much the plate can deform before it tears.
Making Z35 is a steelmaking job, not a rolling trick. The mills control sulphur—often to 0.005 percent or below—and treat the steel with calcium so the remaining inclusions are globular instead of flat. That is why Z-quality plate costs more and takes longer: it is a different heat, not a sorted plate.
When the requirement is real: thick plate welded into T- or cross-joints, heavy columns to base plates, crane and offshore node connections, bridge diaphragms, and any joint where the weld shrinks against a stiff restraint in the through-thickness direction. Z15 covers most ordinary heavy fabrication; Z25 and Z35 belong on highly restrained, thick, dynamically loaded joints.
On the purchase order, write the standard as well as the grade—EN 10164 Z35, sulphur limit, and ultrasonic testing class, because Z-quality plate is normally supplied with UT as well. Design changes are usually cheaper than the steel: smaller fillets, balanced welding, preheat, and lower restraint often remove the need for Z35 altogether.
We supply structural plate to EN 10164 Z15, Z25 and Z35, with UT and full mill certification, quoted against your joint details.
