Compare Q345 and Q355 structural steel to understand the GB standard change, choose the right grade for modern projects, and learn the delivery and impact grade details to specify.
Q345 and Q355 are essentially the same structural steel separated by a standard revision: Q345 is the legacy grade name under the older Chinese GB standard, and Q355 is the renamed grade under the revised GB standard that aligns naming with European and international yield-strength conventions. For most steel the change is one of designation, but for buyers it affects certificates, project specifications, and inspection. This guide explains what the change means and how to order correctly under either name.
Why the Name Changed from Q345 to Q355
Under the older GB standard, the Q in Q345 referred to the yield strength in megapascals, nominally 345. The revised standard renamed the grade Q355 to reflect the more conservative, statistically defined minimum yield value of 355 megapascals, bringing the naming convention closer to that of European standards such as S355. In practice, the chemistry and production of the steel are largely continuous across the rename, so Q345 and Q355 are treated as equivalent material, but the certificate and designation must match the project standard.
When to Specify Q355
For new projects designed to the revised GB standard, and for any project whose drawings or customer specification name Q355, the order should specify Q355 with the correct impact-test grade such as Q355B, C, or D. Modern certificates, design software, and inspection will reference Q355, and supplying a Q345 certificate on a Q355 project can cause paperwork rejection even when the steel is compliant. For forward-looking and internationally documented projects, Q355 is the correct designation.
When Q345 Still Appears
Q345 remains in use on older drawings, legacy specifications, and existing stock, and many certificates still carry the Q345 name. For repair, maintenance, and match-to-existing work where the original structure was Q345, supplying Q345 or its Q355 equivalent is appropriate. The key for the buyer is to recognize that Q345 and Q355 are the same material under different names, so a Q345 certificate can support a Q355 project if the specification explicitly accepts the equivalence, which most revised standards do.
What to Specify on the RFQ
State the grade by the designation matching the project, Q345 or Q355, and the impact-test quality grade such as B, C, or D, since the letter governs the test temperature and toughness. Specify the product form, dimensions, and delivery condition, and request the certificate in the matching designation, or with an explicit equivalence statement if cross-acceptance is allowed. For export projects, also confirm the international equivalent such as S355, since many overseas customers specify by the European name. Stating grade, quality grade, form, and certificate designation together prevents both material and paperwork disputes.