Structural steels, essential for the construction and engineering industries, have undergone a profound evolution in their classification and designation. Until the advent of unified European standards, their identification was often linked to national standards: in Italy, for example, the S235 grade was commonly known as Fe360, S275 as Fe430, and S355 as Fe510.
The transition to a standardized system, established by the UNI EN 10025 standard, represents a significant turning point. The new designation moves away from referring to ultimate tensile strength to focus on the yield strength, an engineering parameter that is far more critical for structural design within the elastic range.
The Alphanumeric Designation of Steels
The current designation of these steels is a coded system that provides the designer and manufacturer with essential information about their properties. The complete code consists of several elements:
- S: indicates that the steel is intended for structural use;
- nnn: represents the minimum yield strength value, expressed in megapascals (MPa). The most common grades are S235 (235 MPa), S275 (275 MPa), and S355 (355 MPa). This value is guaranteed for nominal thicknesses up to 16 mm.
Toughness Symbols
These symbols indicate the material’s ability to absorb energy during an impact without fracturing (impact strength):
- JR: Minimum guaranteed impact energy of 27 J at +20 °C;
- J0: Minimum guaranteed impact energy of 27 J at 0 °C;
- J2: Minimum guaranteed impact energy of 27 J at -20 °C.
Technical Note: In the case of Zarri, the designation of the steels handled often includes the suffix +C, which declares the nature of the material as cold-worked due to the cold-drawing process.
Mechanical Properties and Chemical Composition
Chemical composition defines the physical and mechanical properties of steel. Non-alloy structural steels of the S series are characterized by a low carbon content, a feature that provides them with remarkable ductility and good weldability.
The most relevant mechanical parameters are as follows:
- Yield Strength (MPa): the maximum load before permanent plastic deformation occurs.
- Tensile Strength (MPa): the maximum stress the material can withstand before breaking.
- Elongation at Break (%): measures ductility, which is fundamental for safety in the event of seismic loads.
- Impact Strength (J): the capacity to absorb energy (measured via the Charpy V-Notch test).
Comparison between S235, S275, and S355 Grades
| Steel Grade | Characteristics | Typical Use |
|---|---|---|
| S235 | Cost-effective, excellent weldability and ductility. | Lightweight steelwork, secondary structures. |
| S275 | A compromise between cost and strength. | Applications with higher load-bearing capacity. Zarri handles the S275JR grade. |
| S355 | High strength, allows for lighter and more slender sections. | Cold environments and seismic zones. Zarri handles the S355J2 grade. |
The use of S355 steel, although more expensive per ton, can lead to overall savings in large projects by reducing the total weight of the structure, transport costs, and foundation loads.
In particular, the S355J2 steel handled by Zarri, with impact testing at -20°C, is ideal for ensuring maximum structural safety in critical environmental conditions.
Do you need more information about structural steels?
The Zarri technical team is at your disposal to support you in choosing the most suitable material for your project, ensuring maximum quality and compliance with current regulations.