Background: One Standard Change, Two Names
Q345 was the standard low-alloy high-strength structural steel in China under GB/T 1591-2008. When the standard was revised to GB/T 1591-2018, the grade system was updated and Q345 was replaced by Q355. The 2008 edition has been withdrawn, so new designs in China are written around Q355, but Q345 remains common in existing structures, stock lists and international project documents.
The Q in both names stands for yield strength, and the number is the minimum yield strength in MPa at the base thickness. The letter suffix identifies the Charpy impact test temperature: B tests at plus 20 degrees C, C at 0 degrees C, D at minus 20 degrees C, and E at minus 40 degrees C. This lettering is the same in both editions, which is a common source of confusion.
Yield Strength Comparison
| Thickness (mm) | Q345B Yield (MPa) | Q355B Yield (MPa) |
|---|---|---|
| Up to 16 | 345 | 355 |
| Over 16 to 35 / 40 | 325 | 345 |
| Over 35 to 50 / 63 | 295 | 335 |
| Over 50 to 100 | 275 | 315-325 depending on tier |
Two points matter here. First, the yield gain is largest in the mid-thickness ranges. Second, GB/T 1591-2018 redefined the thickness tiers, so the step boundaries in the two editions are not identical; any comparison table must state which edition is being quoted.
Tensile and Impact Requirements
The tensile strength range is effectively unchanged: 470-630 MPa for both Q345B and Q355B in the common thickness range. The Charpy requirement is 34 J minimum for grade B in both editions, tested at plus 20 degrees C. A widely repeated claim that Q355B tests at minus 20 degrees C is incorrect; the minus 20 degrees C condition belongs to grade D. If a project needs low-temperature toughness, it must specify Q355D, not Q355B.
Chemistry Changes
| Element | Q345B (GB/T 1591-2008) | Q355B (GB/T 1591-2018) |
|---|---|---|
| Phosphorus (max) | 0.030% | 0.030% |
| Sulfur (max) | 0.030% | 0.025% |
| Carbon equivalent CEV (max) | Not specified | 0.44% (grade B) |
The lower sulfur limit and the new carbon equivalent ceiling improve weldability, which matters for thick plates and restrained joints. The 2018 edition also allows finer control of niobium, vanadium and titanium microalloying.
Weldability and Fabrication
Both grades weld without preheat in most thicknesses below about 30 mm using common filler metals such as ER50-6 or E5015. Above that thickness, or in cold weather, preheat around 100-150 degrees C is good practice. The CEV limit in Q355 reduces the risk of hydrogen cracking in heavier sections, which is one practical reason to prefer Q355 in new work.
Availability and Substitution
Chinese mills now produce Q355 as the standard high-strength structural grade, while Q345 remains available mainly for legacy projects. Q355 is generally treated as a direct substitute for Q345 at equal or better properties, but designs certified around Q345 chemistry should be rechecked where the thickness tier boundary falls differently. For export orders, both grades are often mapped to EN S355JR or ASTM A572 Grade 50, although exact equivalence must be confirmed per project.
FAQ
Is Q355 the same steel as Q345?
Not exactly. Q355 is the successor grade in GB/T 1591-2018 with a slightly higher yield minimum and tighter chemistry limits. It replaces Q345 for new design but the two designations are not identical chemistries.
Does Q355B require impact testing at minus 20 degrees C?
No. Grade B is tested at plus 20 degrees C in both editions. Minus 20 degrees C testing is the grade D condition, so specify Q355D when low-temperature toughness is needed.
What is the carbon equivalent limit for Q355B?
GB/T 1591-2018 sets a maximum CEV of 0.44% for grade B, which improves weldability compared with the 2008 edition where no limit was specified for Q345.
Can Q345 be substituted with Q355 on an existing project?
Usually yes, since Q355 has equal or higher strength at each thickness tier. Confirm the thickness tier boundaries and any certified chemistry requirements with the design engineer before substitution.
What are the EN and ASTM equivalents of Q355?
Q355 is commonly compared with S355JR (EN 10025-2) and A572 Grade 50 (ASTM), with the caveat that impact temperature rules differ between systems. For precise mapping, compare the actual Charpy and chemistry tables.
Why did the standard move from Q345 to Q355?
The 2018 revision aligned Chinese structural steel with international practice, raised the nominal yield, tightened impurity limits and added weldability controls. The yield increase also reflects modern steelmaking that consistently achieves higher strengths.
