Q235A and Q235B in the Chinese Structural Steel System
Q235A and Q235B are two grades of the same carbon structural steel family defined by GB/T 700, the Chinese standard for carbon structural steels. The letter Q stands for yield and the number 235 is the minimum yield strength in MPa for the usual thickness range, so both grades are nominally 235 MPa steels used for general welded structures, bridges, machinery parts, building frames and pipework.
The trailing letter is the quality class and it is here that the two grades separate. Quality classes run from A to D in ascending order of toughness assurance. Class A requires no impact test, class B requires a Charpy V-notch impact test at room temperature, class C at 0 °C and class D at -20 °C. Q235A and Q235B therefore share a strength level but not a toughness guarantee.
The Essential Difference: Impact Testing
The decisive difference is that Q235B must pass a mandatory Charpy V-notch impact test at room temperature, taken longitudinally to the rolling direction with a minimum absorbed energy of 27 J, while Q235A is not impact tested at all. Q235B also carries slightly stricter limits on carbon and sulphur, which is what makes that toughness guarantee achievable in routine production.
| Item | Q235A | Q235B |
|---|---|---|
| Standard | GB/T 700 | GB/T 700 |
| Minimum yield strength | 235 MPa | 235 MPa |
| Impact test | Not required | Charpy V-notch at +20 °C, 27 J minimum longitudinal |
| Maximum carbon | 0.22 % | 0.20 % |
| Maximum sulphur | 0.050 % | 0.045 % |
| Typical use | General welded structures and parts | Structures needing toughness assurance |
Note the direction of the difference: the B grade is not stronger, it is tougher and cleaner. Where a design is governed by strength alone, Q235A performs identically to Q235B in tension, compression and bending within the same thickness range.
Chemical Composition
The two grades differ only marginally in composition, and both are plain carbon steels without deliberate alloying additions.
| Element | Q235A | Q235B |
|---|---|---|
| Carbon, C | 0.22 % max | 0.20 % max |
| Silicon, Si | 0.35 % max | 0.35 % max |
| Manganese, Mn | 1.40 % max | 1.40 % max |
| Phosphorus, P | 0.045 % max | 0.045 % max |
| Sulphur, S | 0.050 % max | 0.045 % max |
Because the manganese, phosphorus and silicon limits are the same, the two grades have essentially the same corrosion behaviour in a given environment; the slightly lower sulphur of Q235B only reduces the inclusion content. Neither grade is a weathering steel, so unprotected outdoor service requires painting or another barrier coating in both cases.
Mechanical Properties Compared
Strength values are identical for the two quality classes; only the low temperature behaviour and the guaranteed scatter band change.
| Property | Q235A | Q235B |
|---|---|---|
| Minimum yield strength, t up to 16 mm | 235 MPa | 235 MPa |
| Tensile strength | 370 - 500 MPa | 370 - 500 MPa |
| Minimum elongation | 26 % | 26 % |
| Impact energy | Not specified | 27 J minimum at +20 °C |
| Hardness | Not a specified property | Not a specified property |
Yield strength falls gradually with thickness in both grades, following the standard thickness steps, while the tensile band remains 370 to 500 MPa for the ordinary plate thicknesses. Hardness is not a controlled characteristic, because in hot rolled carbon steel plate it follows tensile strength and is not a basis for acceptance.
Weldability, Formability and Typical Applications
Weldability of both grades is good. The lower carbon and lower sulphur of Q235B give slightly better resistance to weld cracking and a cleaner heat affected zone, but Q235A is welded every day in general fabrication with ordinary electrodes and no special precautions.
Q235A: hot rolled sections, bars, plates and pipes for welded structures, bridges, bolts, tie rods, rivets and general machine parts.
Q235B: spiral and straight seam pipes for water supply, oil and gas transmission, urban construction and pressure pipework where toughness must be demonstrated.
Both grades are readily bent, sheared, punched and machined in the as-rolled condition.
For service at or below 0 °C, grade C or grade D should be selected instead of either A or B.
Formability is similarly close: the slightly lower carbon of Q235B gives a marginally better combination of strength and ductility, while Q235A is easier to bend in very tight radii. In practice the choice between them is made by the specification, not by the workshop.
How to Choose
Select Q235A for general structures and parts that operate at ambient temperature and above, where the design code does not demand impact testing. Select Q235B whenever a toughness test certificate is required, whenever the structure is dynamically loaded, whenever welding is heavy and restraint is high, or whenever the client specification calls for tested impact properties. If the minimum service temperature falls below 0 °C, move up to Q235C or Q235D rather than relying on the room temperature test of Q235B.
Frequently Asked Questions
Q: Is Q235B stronger than Q235A?
No. Both have a minimum yield strength of 235 MPa and the same 370 to 500 MPa tensile strength range for ordinary plate thicknesses.
Q: What is the main difference between Q235A and Q235B?
Q235B requires a mandatory Charpy V-notch impact test at +20 °C with a minimum energy of 27 J, while Q235A requires no impact test.
Q: At what temperature is Q235B impact tested?
The B quality class is tested at room temperature, taken as +20 °C; grade C is tested at 0 °C and grade D at -20 °C.
Q: Does Q235B resist corrosion better than Q235A?
Not meaningfully. Their alloying limits are the same, so both need a coating for outdoor or wet service.
Q: Can Q235A and Q235B be welded with the same consumables?
Yes, ordinary carbon steel electrodes and wires suit both, since both are low carbon structural steels with a similar carbon equivalent.
Q: Which grade should be used for cold climate structures?
Neither A nor B is adequate below 0 °C; Q235C or Q235D should be specified because they carry impact testing at 0 °C and -20 °C.
Q: Is hardness a specified property of these grades?
No. GB/T 700 controls yield strength, tensile strength, elongation and impact energy, and hardness varies with the as-rolled structure.
