Q355B vs A572 Gr.50
These two grades are listed as equivalents of each other: their chemistry windows overlap and they serve the same applications. They are not always identical — check the differences below before substituting, especially in certified designs.
Q355B
Structural & HSLA Steel
Q355B is the current GB/T 1591 designation that replaced Q345B in the 2018 revision, with the yield class raised from 345 to 355 MPa to align with international S355 practice. It is now the standard Chinese HSLA for structures, machinery and pipe.
Standards: DIN S355J2 (near) · AISI A572 Gr.50 (near) · JIS SM490B (near) · GB Q355B (GB/T 1591)
A572 Gr.50
Structural & HSLA Steel
ASTM A572 Grade 50 is the standard American HSLA structural steel — the 50 ksi yield grade that replaced A36 in most US bridge and heavy construction work. Strength comes from Nb/V microalloying and grain refinement rather than carbon, so weldability remains good.
Standards: DIN S355J2 (near) · AISI A572 Gr.50 · JIS SM520B (near) · GB Q355B (near)
Chemical composition compared (%)
| Element | Q355B | A572 Gr.50 |
|---|---|---|
| C | ≤0.24 | ≤0.23 |
| Si | ≤0.55 | ≤0.40 |
| Mn | ≤1.60 | ≤1.35 |
| P | ≤0.035 | ≤0.035 |
| S | ≤0.035 | ≤0.045 |
| V | — | 0.01–0.15 (Type 2) |
Properties compared
| Property | Q355B | A572 Gr.50 |
|---|---|---|
| Yield strength | ≥355 MPa (≤16 mm) | ≥345 MPa |
| Tensile strength | 470–630 MPa | ≥450 MPa |
| Impact | B grade: 34 J at 20 °C | — |
| Density | 7.85 g/cm³ | 7.85 g/cm³ |
Sourcing either grade?
Nasteel stocks both families from Shenzhen with EN 10204 3.1 mill certificates by heat number.
Ask for a quotationSee full data sheets: Q355B · A572 Gr.50. Substitution decisions for certified parts stay with the design authority.