A36, SS400, Q235 and Q355 are the four most commonly traded carbon steel plate grades in international structural projects. They are written to different standard systems - ASTM A36, JIS G3101, GB/T 700 and GB/T 1591 - but serve overlapping duties in buildings, bridges, machinery frames, storage tanks, transmission towers and general fabrication. This guide compares their chemical limits and mechanical guarantees, explains how to read the designations, and gives practical selection and welding advice for buyers who need to substitute one grade for another.
Standard systems and what the designations mean
A36 is defined in ASTM A36/A36M as a structural carbon steel with a minimum yield strength of 250 MPa for plates up to 200 mm thick. SS400 in JIS G3101 is a rolled carbon steel for general structure, where the number 400 refers to the minimum tensile strength in N/mm2 rather than yield. In the Chinese GB system the letter Q stands for yield: Q235 in GB/T 700 means a minimum yield strength of 235 MPa, and Q355 in GB/T 1591 means 355 MPa minimum yield with a micro-alloyed low-alloy composition.
| Grade | Standard | Min yield (t ≤ 16 mm) | Tensile strength | Typical use |
|---|---|---|---|---|
| A36 | ASTM A36/A36M | 250 MPa | 400-550 MPa | General structural, machinery |
| SS400 | JIS G3101 | 245 MPa | 400-510 MPa | General structure, ship equipment |
| Q235 | GB/T 700 | 235 MPa | 370-500 MPa | Buildings, frames, tanks |
| Q355 | GB/T 1591 | 355 MPa | 470-630 MPa | High-strength structures |
Chemical composition
The table lists the guaranteed maximum values that each standard publishes; mills normally run considerably tighter ladle analyses. For A36 the manganese requirement applies to plates over 20 mm thickness, and the full thickness-dependent table is given in ASTM A36.
| Grade | C max % | Mn % | Si max % | P max % | S max % |
|---|---|---|---|---|---|
| A36 | 0.26 | 0.80-1.20 (t > 20 mm) | 0.40 | 0.040 | 0.050 |
| SS400 | 0.25 | not specified | not specified | 0.050 | 0.050 |
| Q235A | 0.14-0.22 | 0.30-0.65 | 0.30 | 0.045 | 0.050 |
| Q235B | 0.12-0.20 | 0.30-0.65 | 0.30 | 0.045 | 0.045 |
| Q355B | 0.24 | 1.60 | 0.55 | 0.035 | 0.035 |
The Q235A/Q235B split shows the effect of sulphur and phosphorus control: Q235B limits both to 0.045% for better weldability, while Q355B goes further to 0.035% because it is used in more demanding welded structures.
Cross-standard equivalence
A36, SS400 and Q235 sit at roughly the same strength level and are frequently treated as substitutes in non-critical structures; many mills certify one plate against two or even three of these standards. Q355 has no ASTM A36 counterpart: it is better compared with S355JR (EN 10025-2) or ASTM A572 Grade 50. Equivalence is a property match, not a certificate match. Before substitution, always verify impact energy, carbon equivalent, thickness-dependent yield values and the inspection regime required by the project specification.
Welding and fabrication notes
All four grades weld well with SMAW, GMAW and SAW. For Q355 plates above roughly 25-30 mm, preheating to 100-150 C is common practice, with matching consumables such as E7018 electrodes or ER70S-6 wire. A36 and Q235 rarely need preheating below 40 mm. Where Charpy impact energy is required for bridges, cold regions or dynamic loading, specify the grade suffix instead of the base grade: Q235C/D or Q355C/D, or A36 with supplementary impact testing as agreed in the purchase order.
Frequently Asked Questions
Is A36 the same as SS400? Not exactly. Both deliver roughly 250 MPa yield and 400-550 MPa tensile range, but they are published by different standard bodies with different testing, tolerances and certification rules. Treat them as comparable in strength, not identical in documentation.
What does the Q in Q235 stand for? Q is the pinyin initial of qu fu, meaning yield. The number after Q is the minimum yield strength in MPa at the reference thickness, so Q235 guarantees 235 MPa yield and Q355 guarantees 355 MPa.
Can Q235 be replaced by A36? In general structural work the two are interchangeable on strength, provided the project does not demand Charpy impact energy or a specific standard. Confirm carbon equivalent and the acceptance standard before substitution.
What is the difference between Q235 and Q355? Q355 has a much higher minimum yield (355 vs 235 MPa) and is micro-alloyed with niobium, vanadium or titanium, which raises strength and toughness while keeping carbon relatively low. It costs more but allows lighter sections in the same design.
Which plate grade should be used for ship hulls? Hull structural members must follow classification rules and use hull grades such as GB 712 or ASTM A131 series (A, B, D, E), not ordinary Q235 or Q355. The four grades in this article are suitable for ship equipment, ladders, foundations and non-class parts.
Does Q355 need preheating before welding? It depends on thickness and carbon equivalent. Above about 25-30 mm, preheating of 100-150 C is common practice to avoid hydrogen cracking in the heat-affected zone; below that thickness Q355 normally welds without preheat.