What the Q Grade Designation Means
In Chinese steel standards, the letter Q stands for yield strength, and the number that follows is the minimum yield strength in MPa. All four grades discussed here are low-carbon structural steels: Q195, Q215 and Q235 are covered by GB/T 700, while Q345 (now Q355 under GB/T 1591-2018) is a low-alloy high-strength structural steel. None of them are medium- or high-carbon steels, so they all remain weldable by normal shop procedures.
Grade-by-Grade Comparison
| Grade | Carbon (approx. max %) | Yield Strength (min, MPa) | Tensile Strength (MPa) | Typical Use |
|---|---|---|---|---|
| Q195 | 0.12 | 195 | 315-430 | Light tubes, wire, fence, galvanized conduit |
| Q215 | 0.15 | 215 | 335-450 | General tubing, scaffolding, light structures |
| Q235 | 0.22 | 235 | 370-500 | Building structures, water and gas piping, frames |
| Q345B | 0.20 | 345 | 470-630 | High load structures, bridges, machinery, pressure parts |
Note that Q345B is now formally replaced by Q355B in GB/T 1591-2018, which superseded the 2008 edition in 2019. Existing stock and many current projects still reference Q345, so the two names appear together in the market.
International Equivalents
| Chinese Grade | EN Equivalent | JIS Equivalent | ASTM Approximation |
|---|---|---|---|
| Q195 | S185 (EN 10025-2) | SS330 / SPHC | Grade B (low carbon) |
| Q215 | ~S195 / CS Type B | SS330 / SPHC | Grade C |
| Q235 | S235JR (EN 10025-2) | SS400 | A36 (close, not identical) |
| Q345B | S355JR / S355J2 (EN 10025-2) | SM490 | A572 Grade 50 (close) |
These are approximate mappings used for cross-specification buying. Exact substitution should always be confirmed against the project standard because impact testing rules, silicon limits and carbon equivalent differ between systems.
Manufacturing Routes
Q-grade carbon steel pipe is produced both as welded and seamless product:
Welded Pipe
ERW or HFW pipe is formed from hot rolled strip and welded longitudinally, covering small to medium diameters. LSAW pipe is formed from plate and welded with submerged arc welding for large diameters, and SSAW (spiral) pipe is used for the largest diameters in water and gas transmission.
Seamless Pipe
Q195-Q345 seamless pipe is made by rotary piercing and rolling, with cold drawing available for tight tolerances. Seamless is specified where pressure integrity and absence of a weld line are critical.
Dimensional Range and Surface Treatment
Typical supply ranges for these grades are 10-1,500 mm outside diameter with walls from 0.4 to 25 mm, and lengths from 1 to 12 m. Round, square and rectangular sections are all available. Surface options include bare with mill scale, oiled, painted, varnished, and hot-dip galvanized, with plastic end caps as standard protection for threaded or beveled ends.
Application Areas
Construction uses Q235 for structural support, handrails and framing, while Q345B is used for bridge members, crane booms and machine frames where higher load capacity saves weight. The oil and gas industry uses Q345B and equivalent line pipe for transmission, the chemical industry uses Q235 for general process lines, and manufacturing uses all four grades for shafts, cylinders and equipment frames.
Quality Checks Before Ordering
For each lot, the mill certificate should confirm heat number, chemistry, yield and tensile results, and hydrostatic test pressure. For welded pipe, ask whether the weld seam has been non-destructively tested and whether the lot includes bend and flattening samples. For galvanized product, verify coating thickness and adhesion test results.
FAQ
Is Q235 a medium carbon steel?
No. Q235 has a carbon limit around 0.22% and is classified as low-carbon structural steel. The confusion comes from marketing descriptions; the chemistry and weldability are those of mild steel.
What does the B in Q235B or Q345B mean?
The suffix letter indicates the impact test temperature: B means the Charpy test is performed at plus 20 degrees C with a minimum of 27 J for GB/T 700 grades, and 34 J for GB/T 1591 grades. Grades C, D and E are tested at 0, -20 and -40 degrees C respectively.
Can Q195 be used for structural purposes?
Yes, but only for light duty. Its yield strength of 195 MPa suits fencing, handrails, conduit and non-critical framing. For load-bearing structures, Q235 or higher is normally required.
Is Q345B the same as S355?
Roughly yes. Q345B has a minimum yield of 345 MPa and is often treated as the Chinese counterpart of S355JR, although the 2018 standard has moved the Chinese designation to Q355B with a 355 MPa minimum yield.
What is the difference between ERW, LSAW and SSAW pipe?
ERW pipe is welded by electric resistance from strip, usually up to 24 in. diameter. LSAW pipe is longitudinally welded from plate by submerged arc welding, and SSAW pipe is spirally welded from coil, with both covering large diameters above 400 mm.
Should I specify Q345 or Q355 in a new project?
For new design, specify Q355 per GB/T 1591-2018, since the 2008 edition has been withdrawn. If the project documents were written around Q345, most mills will still supply Q345 chemistry or dual-certified material on request.
