Non-alloy structural steels

02Definition and Fundamental Characteristics of Non-Alloy Structural Steels

Non-alloy structural steels are iron-carbon alloys with an alloying-element content below the limits specified by the EN 10020 standard, designed specifically for structural applications where high mechanical properties, good weldability and workability are required.

The classification of non-alloy structural steels defines them, in accordance with EN 10020, as steels with maximum contents: Mn ≤1.65%, Si ≤0.60%, Cr ≤0.30%, Mo ≤0.08%, Ni ≤0.30%, Cu ≤0.40%, Al ≤0.30%, Nb ≤0.06%, V ≤0.12%, Ti ≤0.05%, and other elements within specified limits.

The distinctive feature of these materials lies in their ability to deliver reliable structural performance through optimised control of the basic chemical composition and of the production processes, without the need for costly alloying elements. This characteristic makes them the preferred solution for most conventional structural applications.

03Chemical Composition and Microalloying Elements

The chemical composition of non-alloy structural steels is optimised to ensure the mechanical properties of structural steels required by structural applications. The carbon content typically ranges from 0.17% to 0.24%, balanced to achieve adequate mechanical strength while maintaining good weldability and ductility.

Manganese, present in contents from 0.40% to 1.50%, plays a fundamental role in controlling the microstructure and improving the mechanical properties. Silicon, limited to 0.55% maximum, acts as a deoxidiser during production and contributes to the strengthening of the ferrite.

Microalloying elements such as niobium, vanadium and titanium, when present in limited quantities, help control the austenitic grain size and promote precipitation hardening, significantly improving the mechanical properties without compromising weldability.

04Fundamental Mechanical Properties

The mechanical properties of structural steels are defined by the fundamental parameters of yield strength (ReH), tensile strength (Rm), percentage elongation (A%) and impact toughness (KV). These parameters are closely related to the microstructure and vary as a function of the product thickness and of the delivery conditions.

The yield strength represents the main design parameter, ranging from 235 MPa for grade S235 up to 450 MPa for grade S450. The tensile strength maintains defined ratios with the yield strength, ensuring ductile behaviour of the material.

4.1Microstructure and Metallurgical Characteristics

The microstructure of non-alloy structural steels is predominantly ferritic-pearlitic, with grain size controlled through the rolling parameters and any heat treatments. The pearlite fraction, related to the carbon content, determines the mechanical strength, while the ferritic matrix ensures ductility and toughness.

Modern production technologies make it possible to obtain optimised microstructures through thermomechanical controlled rolling (TMCP), which allows superior mechanical properties to be achieved compared with conventional processes.

05Classification of Non-Alloy Structural Steels According to International Standards

5.1European Standard EN 10025

The ENstandard for structural steels consists of the EN 10025 series, divided into six parts that govern the technical delivery conditions for hot-rolled products of steels for structural applications. EN 10025-2 specifies non-alloy steels for structural applications, defining grades S235, S275, S355 and S450.

The standard establishes the requirements for chemical composition, mechanical properties, weldability (via maximum carbon equivalent values) and technological characteristics, ensuring uniform quality standards throughout Europe. The requirements are differentiated as a function of the product thickness, recognising the size effect on the mechanical properties.

5.2Designation System EN 10027

The designation system according to EN 10027-1 uses the letter “S” followed by the numerical value of the minimum yield strength in MPa for thicknesses up to 16 mm. The designation may be supplemented by additional symbols indicating specific properties or delivery conditions.

The system also provides symbols for the impact-toughness conditions (JR, J0, J2) that indicate the test temperature for the Charpy impact test: JR at +20 °C, J0 at 0 °C, J2 at -20 °C.

5.3Cross-references to International Standards (ASTM, JIS, GB)

Cross-references to international standards facilitate global trade and the interchangeability of materials. The ASTM A36 standard corresponds approximately to the European grade S235, while ASTM A572 covers a range of grades equivalent to the S275, S355 and S450 series.

5.4Comparative Table of Designations

The international harmonisation of designations is continuously evolving, with a trend towards greater standardisation of classification systems to facilitate global trade and reduce the risk of errors in material selection.

06Main Grades of Non-Alloy Structural Steels

6.1S235 Series (Fe360) - Characteristics and Applications

Grade S235, formerly designated Fe360, is the most widely used basic structural steel in steel construction. With a minimum yield strength of 235 MPa and a tensile strength of 360-510 MPa, it offers an optimal balance between mechanical properties and cost for general structural applications.

The typical chemical composition includes carbon ≤0.20%, manganese ≤1.40%, and limited phosphorus and sulphur content to ensure good weldability. The construction applications of structural steels for this grade include light steelwork, non-critical structures and secondary components.

6.2S275 Series (Fe430) - Properties and Uses

Grade S275 offers superior mechanical properties, with a yield strength of 275 MPa and tensile strength of 410-560 MPa (3-100 mm thickness). The increase in strength is achieved through tighter control of the chemical composition and of the process parameters.

This grade is particularly suited to medium-stress structures where a better strength-to-weight ratio is required, such as frames for industrial buildings, sheds and support structures for plant and equipment.

6.3S355 Series (Fe510) - High Performance

Grade S355 is the most widely used high-strength steel in Europe for critical structural applications. With a yield strength of 355 MPa and tensile strength of 470-630 MPa (3-100 mm thickness), it offers high performance while maintaining good weldability and workability.

The strength of non-alloy structural steels of this grade makes it ideal for bridges, offshore structures, skyscrapers and applications where reducing structural weight is critical. Its availability in versions with different impact-toughness levels (J0, J2) also allows use in severe climatic conditions.

6.4S460 Series - Special Applications

Grade S450 represents the upper limit of the non-alloy structural steels covered by EN 10025-2; higher-strength grades such as S460 belong to the fine-grain structural steels of EN 10025-3/-4/-6. Applications include heavily stressed structures where maximum performance is required without resorting to more costly alloy steels.

07Limitations of Non-Alloy Structural Steels

Non-alloy structural steels have limitations that it is important to be aware of before planning their use:

• Limited mechanical strength: a maximum of 460 MPa for standard structural applications
• Reduced hardenability: unsuitable for very thick sections (>100 mm)
• Corrosion resistance: requires surface protection in aggressive environments
• Fatigue behaviour: inferior to alloy steels for severe cyclic applications
• Service temperature: limited to ~350 °C in order to maintain mechanical properties
• Conditional weldability: dependent on the carbon equivalent and the thickness
• Limited formability: for complex forming operations

08Mechanical Properties and Performance Characteristics

8.1Yield Strength and Tensile Strength

The yield strength (ReH) is the fundamental parameter for structural design according to Eurocode 3. The values vary as a function of the product thickness, reflecting the size effect on the microstructure and on the mechanical properties.

For thicknesses up to 16 mm the nominal yield values apply, while for greater thicknesses progressive reductions are provided for. The tensile strength maintains defined ratios with the yield strength, ensuring ductile behaviour with an Rm/ReH ratio between 1.2 and 1.7.

8.2Impact Resistance and Low-Temperature Toughness

Charpy impact toughness is a critical parameter for applications in adverse climatic conditions. The S235 S275 S355 structural steel grades are available with different levels of guaranteed impact toughness:

• JR: 27 J at +20 °C for standard applications
• J0: 27 J at 0 °C for moderate conditions
• J2: 27 J at -20 °C for cold-climate applications

Low-temperature toughness is influenced by the microstructure, grain size and the presence of elements such as manganese and silicon, which shift the ductile-to-brittle transition temperature.

8.3Ductility and Elongation

The percentage elongation (A%) is a measure of the material's ductility, with minimum specified values that decrease as strength increases: approximately 26% for S235, 23% for S275, 22% for S355 and 17% for S450 (longitudinal specimens, thickness up to 40 mm). These values ensure adequate deformation capacity for structural behaviour under extreme loading conditions.

Ductility is essential for the redistribution of stresses in statically indeterminate structures and for resistance to progressive collapse under accidental conditions.

8.4Modulus of Elasticity and Fatigue Behaviour

The modulus of elasticity of structural steels is conventionally taken as 210,000 MPa for all grades, regardless of the mechanical strength. This value is used in structural calculations according to Eurocode 3.

Fatigue behaviour is influenced by the microstructure, surface conditions and the presence of notches. High-strength grades generally show better finite-life fatigue strength.

09Weldability and Workability

9.1Carbon Equivalent and Weldability

The weldability of non-alloy structural steels is assessed mainly through the carbon equivalent (CE) calculated according to the Dearden and O'Neill formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. For non-alloy structural steels, the CE is typically kept below 0.45% to ensure good weldability.

Controlling the carbon equivalent makes it possible to limit the formation of brittle microstructures in the heat-affected zone (HAZ) and to reduce the need for preheating during welding.

9.2Welding Precautions and Procedures

Welding procedures must be qualified according to EN ISO 15614 to ensure the integrity of the welded joints. For grades S235 and S275, welding without preheating is generally sufficient, whereas for S355 and S460 preheating at 100-150 °C may be required depending on the thickness.

Cooling rates must be controlled to prevent the formation of martensite in the HAZ, which is particularly critical for thick sections and severe thermal restraint conditions.

9.3Machining and Forming

Non-alloy structural steels offer good workability in cutting, drilling, bending and forming operations. The mechanical strength increasing from grade S235 to S460 requires greater forces for plastic-forming operations but does not significantly compromise machinability.

Cold-forming operations can induce localised work hardening that alters the mechanical properties in the deformed areas, an aspect to be considered in the design of formed components.

10Heat Treatments and Delivery Conditions

10.1As-Rolled and Normalised Conditions

Non-alloy structural steels are supplied mainly in the as-rolled (AR) or normalised (N) condition. The as-rolled condition involves air cooling after rolling, providing mechanical properties suitable for most applications.

The normalised condition, obtained through heating to 860-920 °C (temperatures vary depending on the specific composition and thickness) followed by air cooling, provides a more uniform microstructure and superior mechanical properties, which is particularly important for thick sections.

10.2Thermomechanical Treatments (TMCP)

Thermomechanical heat treatments for structural steels (TMCP - Thermo Mechanical Control Process) combine controlled rolling and accelerated cooling to achieve superior mechanical properties. This technology makes it possible to achieve higher-strength grades while maintaining good weldability.

The evolution of TMCP processes towards increasingly sophisticated controls will make it possible to obtain non-alloy steels with mechanical properties comparable to current micro-alloyed steels, reducing production costs.

10.3Effects of Treatments on Properties

Heat treatments significantly affect the microstructure and mechanical properties. Normalising produces a finer grain and a more uniform microstructure, improving toughness and weldability. TMCP treatments make it possible to obtain high yield strengths while maintaining good ductility.

11Industrial Applications of Non-Alloy Structural Steels

11.1Structural Steelwork and Civil Structures

The structural steel construction applications in structural steelwork include building frames, industrial sheds, commercial and residential structures. Grades S235 and S275 are used for standard structures, while S355 is preferred for tall buildings and structures with large spans.

The versatility of non-alloy structural steels makes them suitable for all types of connections: bolted, welded and mixed, ensuring design and construction flexibility.

11.2Shipbuilding and Offshore Construction

In the shipbuilding sector, non-alloy structural steels are used for merchant ship hulls, port structures and offshore platforms. Low-temperature impact toughness requirements are particularly critical for applications in marine environments.

Marine corrosion resistance generally requires additional protection through protective coatings or cathodic protection systems, as non-alloy steels are not intrinsically resistant to corrosion.

11.3Bridges and Infrastructure

Bridges represent a critical application where the strength of non-alloy structural steels is fully exploited. Grade S355 is widely used for medium- and large-span bridges, ensuring structural safety and long-term durability.

The cyclic loading typical of transport infrastructure requires particular attention to fatigue behaviour and structural durability checks.

11.4Tanks and Pressure Equipment

Non-alloy structural steels are used in tanks for storing liquids, silos for bulk materials and pressure equipment for the process industry. The applicable standards require more stringent quality controls to ensure in-service integrity.

12Quality Control and Certifications

12.1Standard Mechanical Tests

Quality control involves standard mechanical tests according to EN ISO 6892 for tensile testing, EN ISO 148 for Charpy impact toughness, and hardness checks. The tests are carried out on specimens taken from the finished product according to sampling schemes defined by the standards.

The frequency of testing varies depending on the steel grade, the thickness and the intended use, with stricter controls for critical applications.

12.2Non-Destructive Testing

Non-destructive testing includes ultrasonic examinations for detecting internal defects, magnetic-particle inspections for surface defects and radiography for welded joints. These inspections are mandatory for critical structural applications.

The evolution of non-destructive testing techniques towards automated and digitalised systems is improving reliability and reducing inspection times.

13Certifications and Quality Attestations

Inspection certificates according to EN 10204 attest to the material's conformity with the required specifications. Type 3.1 is generally required for structural applications, while type 3.2 is necessary for critical applications such as bridges and pressure equipment.

14Design and Calculation Considerations

14.1Safety Factors and Eurocode 3

Eurocode 3 defines partial safety factors: γM0 = 1.00 for cross-section resistance, γM1 = 1.00 for member buckling resistance, γM2 = 1.25 for net-section resistance in tension, with possible variations in the National Annexes.

The partial factor method makes it possible to manage uncertainties in materials and loads, ensuring adequate safety levels for all types of structures.

14.2Instability Phenomena and Buckling

Local and global instability phenomena are particularly critical for slender steel profiles. Eurocode 3 provides calculation methods for lateral-torsional buckling, local buckling of webs and flanges, and interaction between different buckling modes.

The choice of steel grade affects structural efficiency, with high-strength grades allowing more slender sections but requiring more accurate buckling checks.

14.3Connections and Joints

Connections are critical elements in steel structures, requiring particular attention in design and execution. Eurocode 3 defines calculation methods for welded, bolted and mixed connections.

The choice of connection type affects the overall structural behaviour, with rigid connections ensuring structural continuity and pinned connections allowing free rotations.

15Frequently Asked Questions about Non-Alloy Structural Steels

What is the main difference between grades S235, S275 and S355 in practical applications?

The main difference lies in the increasing yield strength (235, 275, 355 MPa), which allows lighter sections to be used for the same loads. Grade S235 is suitable for standard structures, S275 for medium loads, and S355 for high-performance applications such as bridges and skyscrapers.

How does thickness affect the mechanical properties of structural steels?

Mechanical properties decrease as thickness increases due to the size effect on the microstructure. The ENstandard for structural steels provides for progressive reductions in yield strength for thicknesses above 16 mm, down to reduced values for thicknesses over 100 mm.

What are the critical parameters for the weldability of structural steels?

The carbon equivalent (CE) is the main parameter, kept below 0.45% for good weldability. Other factors include thickness, cooling rate, any preheating and the composition of the filler metal. The weldability of non-alloy structural steels is generally excellent for all standard grades.

Is it possible to improve the mechanical properties through heat treatments?

Heat treatments for structural steels such as normalising can improve uniformity and toughness, but the increase in strength is limited. For significant improvements, alloyed or micro-alloyed steels are required. Thermomechanical treatments (TMCP) during production allow better performance.

How is fatigue strength assessed in structural applications?

Fatigue strength depends on the steel grade, surface conditions, the presence of notches and the type of loading. Eurocode 3 provides S-N curves for different categories of construction details, taking into account the effect of welds, holes and complex geometries.

What are the environmental considerations for structural steels?

Structural steels are highly recyclable materials, with a recycled-material content of up to 90%. Life cycle assessment (LCA) shows favourable environmental impacts thanks to their long structural service life and full recyclability at end of life.

This article provides a comprehensive technical overview of non-alloy structural steels, incorporating the latest European standards and engineering best practices for professionals in the metal construction sector.