S235 and S355 are the two most widely specified non-alloy structural steel grades in the EN 10025-2 system. Both are hot-rolled, weldable and available as plate, yet they are not interchangeable. The grade number states the minimum yield strength in MPa measured on material up to 16 mm thick, so S355 carries roughly 50% more load before permanent deformation than S235. This comparison sets out the designations, tensile and yield properties, toughness subgrades, chemistry, international equivalents and application rules that govern which grade is the economical choice for a given structure.
What the S235 and S355 Designations Tell You
EN 10025-2 is the European technical delivery standard for hot-rolled products of non-alloy structural steels. In a designation such as S355J2, the letter S marks structural steel, the number is the minimum yield strength in MPa for thickness up to 16 mm, and the trailing letters define the Charpy V-notch impact test condition. JR guarantees 27 J at +20 C, J0 guarantees 27 J at 0 C, J2 guarantees 27 J at -20 C, and K2 raises the minimum absorbed energy at -20 C to 40 J. A suffix never changes the strength class; it only fixes the temperature at which toughness is guaranteed.
Both grades are rolled as plate, strip, sections and bars and are supplied as-rolled (+AR), normalized (+N) or thermomechanically rolled (+M) according to thickness and property requirements. Neither grade is a quenched and tempered product.
Yield Strength and Tensile Strength Compared
Yield strength is specified per thickness range because heavy plate cools more slowly and develops a coarser microstructure. EN 10025-2 therefore steps the minimum yield strength down as section thickness increases.
| Thickness range (mm) | S235 minimum yield (MPa) | S355 minimum yield (MPa) |
|---|---|---|
| up to 16 | 235 | 355 |
| over 16 to 40 | 225 | 345 |
| over 40 to 63 | 215 | 335 |
| over 63 to 80 | 215 | 325 |
| over 80 to 100 | 215 | 315 |
| over 100 to 150 | 195 | 295 |
| over 150 to 200 | 185 | 285 |
| over 200 to 250 | 175 | 275 |
Tensile strength follows the same pattern: S235 is supplied at 360-510 MPa for thicknesses up to 100 mm, while S355 is supplied at 470-630 MPa over the same range. In practical terms, an S355 member can replace a geometrically identical S235 member and carry approximately half again as much load, or keep the same capacity with a thinner and lighter section. That weight saving is what justifies the higher price per tonne of the stronger grade.
Chemical Composition
Both grades are low-carbon, low-alloy steels, so the chemistry difference is modest. Strength in the S355 range comes mainly from a slightly higher carbon and manganese ceiling combined with controlled rolling and cooling practice rather than from deliberate alloying. Maximum ladle analysis values under EN 10025-2 are listed below.
| Grade (steel number) | C max % | Mn max % | P max % | S max % | Cu max % |
|---|---|---|---|---|---|
| S235JR (1.0038) | 0.17 | 1.40 | 0.035 | 0.035 | 0.55 |
| S235J0 (1.0114) | 0.17 | 1.40 | 0.030 | 0.030 | 0.55 |
| S235J2 (1.0117) | 0.17 | 1.40 | 0.025 | 0.025 | 0.55 |
| S355J0, up to 16 mm | 0.20 | 1.60 | 0.030 | 0.030 | 0.55 |
Carbon and manganese rise by only a few hundredths of a percent between the two families, and sulfur and phosphorus are tightly restricted because both elements damage toughness and weldability. The lowest carbon content in the S235 range makes that grade the easier of the two to weld without preheat and the easier to cold form.
Toughness Subgrades and Low-Temperature Service
Choosing a strength level is only half of the specification. The subgrade determines whether the plate is suitable for cold climates, dynamic loading or fatigue-sensitive details. For service temperatures around freezing or below, JR material should not be used even if its strength is adequate; J0 or J2 subgrades exist precisely for that duty. Charpy testing is carried out on the specified test piece orientation and, for plate over 12 mm, the test location and orientation should be stated on the mill certificate so that the results can be compared across suppliers.
International Equivalent Grades
Designers working to other national systems frequently need a like-for-like substitution. The table below lists the most common equivalents.
| System | Equivalent to S235 | Equivalent to S355 |
|---|---|---|
| ASTM / ASME | A36; A283 Grade C | A572 Grade 50; A709 Grade 50 for bridges |
| JIS | SS400 | SM490A |
| GB/T | Q235B | Q355B |
| DIN (historic) | St37-2 | St52-3 |
| BIS | IS 2062 Grade B | IS 2062 E350 |
Equivalence is never automatic. ASTM A36 and A572 Grade 50 do not require Charpy impact testing unless the purchaser or the governing code demands it, whereas S235 and S355 always carry a guaranteed impact energy in the JR, J0, J2 or K2 condition. Substituting an ASTM plate into an EN design therefore needs an explicit toughness requirement on the purchase order.
Applications and Selection Rules
S235 is used for light to medium duty structures, general fabrication, frames, brackets, handrails, stair stringers, temporary works and low-load platforms where cost and ease of fabrication dominate.
S355 is used for bridges, high-rise building frames, crane booms, heavy machinery bases, offshore structures and other details where high load capacity or weight saving matters.
Thickness drives cost in both directions: thicker S235 plate increases weight, welding volume and handling cost, while S355 plate is more expensive per tonne, so the crossover point should be checked on a case-by-case basis.
Neither grade can be strengthened by hardening. Where higher strength is required, the correct route is to step up the grade, not to heat treat these steels.
S355 is often loosely described as a high-strength steel because S235 was the reference grade for decades, but in EN terms high-strength structural steels begin at S690 and are produced with different chemistry and stricter welding controls.
Frequently Asked Questions
Q: How much stronger is S355 than S235?
For plate up to 16 mm the difference is 355 MPa against 235 MPa, so S355 is about 50% stronger in yield, not twice as strong.
Q: What is the American equivalent of S235 steel?
ASTM A36 is the closest general-purpose equivalent, with ASTM A283 Grade C as a lower-yield alternative at 205 MPa minimum.
Q: What is the American equivalent of S355 steel?
ASTM A572 Grade 50 is the standard substitution, and ASTM A709 Grade 50 is used for bridge components.
Q: What is the Chinese equivalent of S235JR?
Q235B under GB/T 700 is the nearest equivalent, with very similar mechanical properties.
Q: Can S355 steel be hardened by heat treatment?
No. It is a non-alloy low-carbon steel, so case hardening gives only a shallow skin and through hardening is not applicable.
Q: Which grade should be used for cold climates?
Specify J0 or J2 subgrades rather than JR, because those are the grades with guaranteed Charpy impact energy at 0 C and -20 C.
