③ 标准/对比页
Q355B vs Q345B: Same Family, Different Standard Editions
Q355B and Q345B belong to the same low-alloy high-strength family, but they come from different editions of GB/T 1591. The 2018 revision merged Q345 into the Q355 series, so new drawings usually specify Q355B while Q345B remains common on legacy projects. The nominal yield gap is 10 MPa; tensile and Charpy requirements carry over.
Why Two Names ExistGB/T 1591-2008 defined Q345B as the workhorse low-alloy grade. The 2018 revision restructured the grade family and merged Q345 into the Q355 series, aligning Chinese naming with the yield value in MPa. Q355B is therefore not a new steel but the successor designation: legacy drawings keep Q345B, new designs specify Q355B, and mills produce both against the respective edition.
Mechanical Values Side by SideFor the thinnest product range (up to 16 mm), Q355B guarantees a minimum yield of 355 MPa while Q345B guarantees 345 MPa — a nominal 10 MPa step. The tensile band is identical at 470–630 MPa, and both B qualities require Charpy KV2 of at least 34 J tested at +20 ℃. Above 16 mm, the yield requirement steps down for both grades per the standard tables.
Chemistry: One Limit MovedThe visible change sits in the carbon cap: Q345B allowed up to 0.20 % C, while Q355B allows up to 0.24 %. P and S limits remain at 0.035 % in both editions. Weldability stays in the same practical window, and consumables qualified for Q345B structures are normally re-qualified without drama when drawings move to Q355B.
Substitution PracticeBecause mechanical values carry over, Q355B is generally accepted where Q345B was specified — but the decision belongs to the designer of record. State both designations on the RFQ, require the mill test certificate to declare the actual grade and standard edition, and keep welding procedure records linked to the original qualification.
Ordering From YunhaoWe supply both designations under GB/T 1591 in plate thickness 8–100 mm and widths 1500–3200 mm, MOQ 25 t, shipping in 15–30 days from Tianjin or Qingdao. Every shipment leaves with a mill test certificate stating grade, edition and heat chemistry, so a Q345B legacy line and a Q355B new-build line can run side by side.
| Item | Q355B | Q345B |
|---|---|---|
| Standard edition | GB/T 1591-2018 | GB/T 1591-2008 |
| Yield ReH (≤16 mm) | ≥355 MPa | ≥345 MPa |
| Tensile Rm | 470–630 MPa | 470–630 MPa |
| Charpy KV2 (B, +20 ℃) | ≥34 J | ≥34 J |
| Carbon cap | ≤0.24 % | ≤0.20 % |