Where the Two Grades Come From
Q235 is defined in GB/T 700, the Chinese standard for carbon structural steels, and is issued in four quality classes, A, B, C and D, which differ mainly in the impact test temperature. S355 is defined in EN 10025-2 for non-alloy structural steels and is issued in the quality classes JR, J0, J2 and K2, again distinguished by impact test temperature and energy. The two grades are the workhorses of their respective systems, and the direct Chinese counterpart of S355 is Q355 in GB/T 1591, which replaced the earlier Q345 designation in the 2018 edition of that standard. Q235 maps to S235 in the European system, so the comparison between Q235 and S355 is really a comparison between two strength classes one step apart.
Strength and Thickness Relationships
| Property | Q235B, GB/T 700 | S355J2, EN 10025-2 |
|---|---|---|
| Minimum yield strength, up to 16 mm | 235 MPa | 355 MPa |
| Minimum yield strength, 16 to 40 mm | 225 MPa | 345 MPa |
| Tensile strength | 370 to 500 MPa | 470 to 630 MPa |
| Minimum elongation | 26 % | 20 % |
| Typical density | 7850 kg/m3 | 7850 kg/m3 |
S355 delivers roughly 50 % more yield strength than Q235 in the same thickness, and its yield falls more gradually with thickness, so it retains 275 MPa even in plate beyond 200 mm. The counterpart Chinese grade Q355 has a minimum yield of 355 MPa up to 16 mm and a tensile range of 470 MPa to 630 MPa, which places it directly alongside S355. The practical effect of the strength difference is that a member designed in Q235 can often be produced in a lighter S355 section, which is why S355 is preferred for long-span and heavily loaded structures even though the material costs more per tonne.
Chemistry, Deoxidation and Weldability
| Element | Q235B | S355J2 |
|---|---|---|
| Carbon, max | 0.20 % | 0.20 % for thickness up to 30 mm |
| Silicon, max | 0.35 % | 0.55 % |
| Manganese, max | 1.40 % | 1.60 % |
| Phosphorus, max | 0.045 % | Restricted to a tighter limit than the JR class |
| Sulphur, max | 0.045 % | Restricted to a tighter limit than the JR class |
Both grades are readily welded with conventional processes, and neither normally requires preheat in moderate thickness under ordinary shop conditions. The higher manganese and the tighter residual limits of S355 give it a slightly higher carbon equivalent than Q235B, so the welding procedure should be qualified for the grade actually delivered rather than assumed from the Q235 procedure. Q235A is not intended for welded load-bearing structures in the same way as the higher classes because it carries no impact test requirement.
Impact Toughness and Temperature Range
The quality class is what determines the minimum service temperature of the material. Q235B is impact tested at plus 20 degrees Celsius, Q235C at 0 degrees Celsius and Q235D at minus 20 degrees Celsius, each against a minimum absorbed energy of 27 J in the longitudinal direction. S355JR has no low-temperature impact requirement, S355J0 is tested at 0 degrees Celsius, and S355J2 is tested at minus 20 degrees Celsius at 27 J, while S355K2 raises the energy requirement to 40 J at the same temperature. Where service temperatures fall below minus 20 degrees Celsius, the normalized or thermomechanically rolled low-temperature grades defined in the third and fourth parts of EN 10025 are used instead of the basic S355 classes.
Selection, Availability and Cost
Q235 is available as plate, bar, section, strip and tube from stock in most markets at a lower price per tonne, and it remains the economical choice for general fabrication, platforms, frames, supports and structures where the design stress is modest and low-temperature toughness is not critical. S355 is selected where the strength-to-weight ratio governs, where plate thickness must be limited, or where a defined impact performance is required by the design code. Availability differs by region and product form: heavy plate and large sections are more commonly held in the S355 and Q355 classes, while Q235 dominates small sections and light plate. In a design review the comparison should be made on the basis of the strength class that the code permits for the service temperature, not on the basis of the steel price alone.
Quality Control, Documentation and Substitution
Both grades are supplied with a mill test certificate at EN 10204 3.1 stating the heat number, chemical analysis, tensile results and, where the class requires it, the Charpy impact values with the test temperature. Dimensional verification follows the relevant plate or section tolerance standard, and ultrasonic examination is ordered where the project requires internal soundness. Substituting S355 for Q235 is generally feasible in strength terms but must be reviewed for the connections, for the welding procedure and for the effect of the higher yield strength on the intended mode of failure, while substituting Q235 for S355 is only acceptable where the design stress has been recalculated for the lower yield strength. Substitutions should always be documented and approved by the responsible engineer, and the certificate of the delivered material retained with the project quality record.
Frequently Asked Questions
Q: What is the main difference between Q235 and S355?
A: Yield strength. Q235 is specified at a minimum of 235 MPa and S355 at 355 MPa up to 16 mm thickness, so S355 is about 50 % stronger and allows lighter or thinner members for the same load.
Q: Is Q355 the same as S355?
A: They are the closest equivalents in their respective systems, with the same minimum yield strength and a similar tensile range. GB/T 1591 defines Q355, which replaced the former Q345 designation, and EN 10025-2 defines S355.
Q: Which grade is better for cold climates?
A: The relevant quality class matters more than the grade name. Q235D and S355J2 are both impact tested at minus 20 degrees Celsius, and for colder service the low-temperature grades of EN 10025-3 and EN 10025-4 are specified.
Q: Can Q235 and S355 be welded together?
A: Yes, with a procedure qualified for the combination. The controlling material is the higher-strength grade, and the consumable and heat input should be selected accordingly.
Q: Is Q235 cheaper than S355?
A: Per tonne, usually yes. The comparison on a finished structure basis can reverse, because the higher strength of S355 often permits a lighter section and a lower total weight of steel.
Q: What documents come with the two grades?
A: A mill test certificate at EN 10204 3.1 quoting the heat number, chemical analysis, tensile results and impact values for the relevant class, plus ultrasonic examination reports and third-party certificates where specified.
