What Is Q235 Steel
Q235 is a carbon structural steel defined by GB/T 700, the Chinese national standard for general-purpose carbon structural steels. The letter Q stands for yield strength and 235 is the specified minimum yield strength in megapascals. It is the most widely used structural carbon steel in China for buildings, bridges, machinery and welded pipe, and it is the standard material of many ERW and spiral welded pipes supplied under GB/T 3091.
When a buyer asks about the compressive strength of Q235 pipe, the answer depends on which strength is meant: the yield strength, which governs design, or the ultimate strength under crushing load. The two are not the same, and confusing them leads to undersized or oversized pipe.
Compressive Strength vs Yield Strength
In compression, a short steel member fails by yielding or crushing, not by buckling. For design purposes, engineers compare the applied compressive stress against the yield strength divided by a safety factor. For Q235 this means the usable compressive stress starts from 235 MPa divided by the applicable factor of safety, before any buckling check is done for slender members.
The ultimate compressive strength of Q235 is close to its tensile strength range of 370 to 500 MPa, because ductile steel fails in compression at approximately the same true stress level as in tension. In practical terms:
Design basis in compression: yield strength, 235 MPa for thickness up to 16 mm
Ultimate crushing strength: in the 370 - 500 MPa range, close to the tensile range
So the common statement that the compressive strength of Q235 is 235 MPa is shorthand for the yield strength being 235 MPa, which is the correct basis for design.
Mechanical Properties of Q235 per GB/T 700
| Property | Value | Note |
|---|---|---|
| Density | 7.85 g/cm3 | Standard carbon steel density |
| Yield strength | At least 235 MPa | Thickness up to 16 mm |
| Yield strength | At least 225 MPa | 16 to 40 mm thickness |
| Yield strength | At least 215 MPa | 40 to 100 mm thickness |
| Tensile strength | 370 - 500 MPa | Thickness up to 100 mm |
| Elongation A | At least 26% | Depends on product form and thickness |
| Modulus of elasticity | About 206 GPa | Room temperature |
Note that the 235 MPa yield value applies to thickness up to 16 mm; as wall thickness increases, the specified yield drops to 225 MPa and then 215 MPa. A heavy-wall Q235 pipe does not carry the same unit stress as a thin one.
Q235A, Q235B, Q235C and Q235D: What the Letter Means
The suffix letter is a quality grade based on impact toughness and impurity control, not on strength:
| Grade | Impact test temperature | Typical use |
|---|---|---|
| Q235A | Not specified | General structures, frames, beams |
| Q235B | +20 C | Welded structures, pipes and tanks; the most common grade for welded pipe |
| Q235C | 0 C | Structures requiring better low-temperature toughness |
| Q235D | -20 C | Cold regions, dynamically loaded structures |
Q235B is the default grade for welded steel pipe because it guarantees impact energy at 20 C, which covers most indoor and temperate outdoor service.
Q235 vs Q355 vs SS400
| Grade | Standard | Yield, up to 16 mm | Tensile | Typical use |
|---|---|---|---|---|
| Q235 | GB/T 700 | 235 MPa | 370 - 500 MPa | General structure, welded pipe |
| Q355 | GB/T 1591 | 355 MPa | 470 - 630 MPa | Higher-load structures, cranes, bridges |
| SS400 | JIS G3101 | About 245 MPa | 400 - 510 MPa | Rolled steel for general structure |
Q355 costs more than Q235 because of higher alloy content and tighter control, and it is specified when the design stress would otherwise force a heavier wall in Q235. SS400 is specified by tensile strength rather than yield, and its typical yield of about 245 MPa is close to Q235, which is why the two are often substituted for each other in simple structures.
Temperature Limits for Q235 Pipe
Q235 properties are certified at room temperature. Continuous service above about 300 C progressively reduces yield strength and promotes creep, so Q235 is not recommended for hot-service piping; for elevated-temperature lines, use a carbon steel grade specified for high-temperature service, such as ASTM A106 Grade B or GB/T 5310 20G. At the low end, the grade letter decides the limit: Q235B is rated to +20 C impact testing, Q235C to 0 C and Q235D to -20 C. For pipeline service below -20 C, a low-temperature grade such as Q355D or a killed fine-grain steel should be selected.
Q235 in Welded Pipe: What to Specify
For welded steel pipe in Q235, the common purchasing standards are GB/T 3091, welded steel pipe for low-pressure fluid service, and GB/T 13793, longitudinally welded steel pipe. On the order, specify the standard, the grade with its suffix, for example Q235B, the outside diameter and wall thickness, the length, the end preparation, plain or beveled, and the test requirements: hydrostatic test, eddy current or ultrasonic testing, and the applicable tolerance class.
FAQ
Q1: Is the compressive strength of Q235 235 MPa?
235 MPa is the minimum yield strength for thickness up to 16 mm. Because compression design is limited by yield, this value is the design basis; the ultimate compressive strength of Q235 is in the 370 - 500 MPa range, close to its tensile strength.
Q2: What is the difference between Q235 and Q355?
Q355 has a minimum yield of 355 MPa, roughly 50% higher than Q235, with higher tensile strength and better toughness, at a higher price. Q235 is chosen where moderate strength and good weldability are enough.
Q3: What is the difference between Q235A and Q235B?
Q235B guarantees impact toughness at 20 C and has lower sulfur and phosphorus limits, making it the preferred grade for welded pipe and welded structures. Q235A has no specified impact requirement and is used for simple non-critical parts.
Q4: Can Q235 be welded?
Yes. Q235 has excellent weldability because of its low carbon equivalent, and it can be welded by SAW, MIG, TIG, MMA and resistance welding with no preheat under normal shop conditions.
Q5: What is the temperature range of Q235?
Rated properties apply at room temperature. Practical continuous service is usually limited to about 300 C on the high side and -20 C on the low side for grade D; below -20 C, specify a low-temperature steel.
Q6: What is the density of Q235?
7.85 g/cm3, the same as other plain carbon steels. It is used for calculating pipe weight: unit weight in kg/m is about (OD - wall) x wall x 0.02466.
