The designation of a carbon steel plate determines its guaranteed strength, its behaviour under welding and forming, and its price. This guide compares seven designations that appear together in many inquiry lists - ASTM A36, Q195, Q235, Q345, SS400, A283 and A387 - so that buyers can identify which grade actually matches the application. The first five are structural and general-purpose carbon steels; the last two serve very different duties and are frequently misunderstood.
The Seven Grades at a Glance
The grades come from three standard families. Q195 and Q235 are defined in GB/T 700, the Chinese standard for carbon structural steels. Q345 was defined in GB/T 1591 for low-alloy high-strength structural steels and has been renamed Q355 in the current GB/T 1591:2018 edition. SS400 is the general structural steel of JIS G3101 in Japan. ASTM A36/A36M is the best-known structural carbon steel plate standard in the United States. ASTM A283/A283M covers low and intermediate tensile strength carbon steel plates, and ASTM A387/A387M covers chromium-molybdenum alloy plates for elevated-temperature pressure vessel service.
Mechanical Property Comparison
Yield strength is the design value used for structural sizing, while tensile strength and elongation define the material behaviour during forming and overload. The table below gives the guaranteed minimum values for the most common thickness range (up to 16 mm) so the grades can be compared on the same basis.
| Grade | Standard | Yield strength min (MPa) | Tensile strength (MPa) | Elongation |
|---|---|---|---|---|
| Q195 | GB/T 700 | 195 | 315-430 | 33% min |
| Q235 | GB/T 700 | 235 | 370-500 | 26% min |
| Q345 (Q355) | GB/T 1591 | 345 (355) | 470-630 | 22% min (t up to 16 mm) |
| SS400 | JIS G3101 | 245 | 400-510 | 17-21% by thickness |
| A36 | ASTM A36/A36M | 250 | 400-550 | 20% min (200 mm gauge) |
| A283 Gr C | ASTM A283/A283M | 205 | 380-515 | 27% min (200 mm gauge) |
Values apply to the common structural thickness range; A387 is deliberately left out of this table because it is a different product family.
Chemical Composition and Weldability
All five general-purpose grades are low-carbon steels. Q195 has a maximum carbon of about 0.12%, Q235 about 0.20-0.22% depending on quality class, Q345 about 0.20% with manganese up to 1.70%, SS400 limits phosphorus and sulphur to 0.050%, and A36 allows carbon up to 0.26% with manganese of 0.80-1.20%. All are readily weldable by SMAW, GMAW, FCAW and SAW processes, and thin sections need no preheat. The higher manganese of Q345 increases strength with only a minor effect on welding. A387 is different: it is a Cr-Mo alloy steel (for example 2.25% Cr-1% Mo for Grade 22) made for elevated-temperature pressure vessels. It requires controlled welding with preheat and often post-weld heat treatment, and it must never be substituted for structural plate.
Focus: Q195 Low-Carbon Steel
Q195 has the lowest carbon content in the GB/T 700 family, which gives it distinctive behaviour.
Ductility and forming
Its low carbon content gives excellent cold bending, stamping and tensile forming behaviour, with a low tendency to crack, which makes it a first choice for bending and forming processes.
Weldability
It has excellent weldability. The low carbon content and low impurity levels result in a low tendency for weld cracking, and both arc welding and resistance welding produce reliable joint strength.
Heat treatment
Quenching is generally not recommended because the hardening effect is poor. Strength can be increased by cold rolling and cold drawing, but this reduces ductility.
Corrosion resistance
It has moderate atmospheric corrosion resistance and rusts in humid environments without protection. Surface treatments such as hot-dip galvanizing per ISO 1461 or painting are normally required for outdoor use.
Cold working
Cold drawing and cold rolling raise strength while decreasing ductility; typical products made this way include steel wire and cold-rolled strip.
How to Choose the Right Grade
Bending, stamping, wire and cold-rolled strip: Q195 or Q235.
General structures, frames and brackets: Q235 or SS400 - often treated as equivalents in trade, but confirm against the governing project standard.
Higher-strength members at moderate cost: Q345/Q355 or A36.
Export to Japan or the United States: SS400 per JIS G3101 or A36 per ASTM A36/A36M, supplied with a mill certificate.
Elevated-temperature pressure service: A387 Cr-Mo grades only, selected per the design code.
FAQ
What is the difference between Q235 and SS400?
They are similar in strength level. Q235 per GB/T 700 guarantees a yield strength of at least 235 MPa, while SS400 per JIS G3101 guarantees a tensile strength of 400-510 MPa with a yield strength of at least 245 MPa for thin plates. They are often used interchangeably in trade, but the governing standard should always be stated in the order.
Is Q345 still an active grade?
GB/T 1591:2018 renamed Q345 as Q355 with a slightly higher yield requirement (355 MPa minimum for thin sections). Existing stock and older certificates still show Q345; treat them as the same product family but specify per the current edition.
Can A36 plates be galvanized?
Yes. Hot-dip galvanizing per ISO 1461 or ASTM A123 is commonly applied after fabrication. Silicon content influences coating thickness and appearance, so the galvanizer should be given the steel chemistry.
What is A387 used for?
A387 is a Cr-Mo alloy plate steel for pressure vessels and boilers operating at elevated temperature, for example Grade 22 (2.25Cr-1Mo). It requires controlled welding, typically with preheat and post-weld heat treatment, and is not interchangeable with structural grades.
Which grade is easiest to bend and stamp?
Q195 and Q235, together with their cold-rolled equivalents such as DC01 and SPCC, are the easiest to form because of their low carbon content and high elongation.
How do I verify the material on delivery?
Request the mill test certificate showing the ladle analysis, mechanical test results and heat number against the applicable standard, and carry out random spectral and tensile checks for critical orders.