Structural Steel - S235, S275, S355 Chemical Composition, Mechanical Properties and Common Applications

Structural steel is a standard construction material made from specific grades of steel and formed in a range of industry-standard cross-sectional shapes (or ‘Sections’). Structural steel grades are designed with specific chemical compositions and mechanical properties formulated for particular applications.

Image Credits: Thanate Rooprasert/shutterstock.com

In Europe, structural steel must comply with the European Standard EN 10025, which is governed by the European Committee for Iron and Steel Standardization (ECISS), a subset of the European Committee for Standardization (CEN).

There are many examples of European grades of structural steel – for example, S195, S235, S275, S355, S420, and S460. For the purposes of this article, we will focus on the chemical composition, mechanical properties, and applications of S235, S275, and S355, which are three common structural steel grades used in all manner of construction projects across the EU.

In line with the European Standard classifications, structural steels must be referenced using standard symbols including but not limited to S, 235, J2, K2, C, Z, W, JR, and JO, where:

• ‘S’ denotes the fact that it is structural steel;
• ‘235’ which relates to the minimum yield strength of the steel (tested at a thickness of 16mm);
• ‘J2’, ‘K2’, ‘JR’, and ‘JO’ all demonstrate the material toughness in relation to the Charpy impact or ‘V’ notch test methodology;
• ‘W’ is weathering steel (atmospheric corrosion-resistant);
• ‘Z’ represents structural steel with improved strength perpendicular to the surface, and
• ‘C’ is cold-formed.

Depending on the manufacturing process, chemical composition and relevant application, further letters and classifications might be used to reference particular grades or products of structural steel.

The EU standard classifications are not a global standard and therefore a number of corresponding grades with the same chemical and mechanical properties may be used in other parts of the world. For example, structural steels fabricated for the US market must be specified in accordance with the American Society for Testing and Materials (ASTM). International guidelines are referenced with an ‘A’ and then the relevant grade, for example, A36 or A53.

EU and US Equivalent Grades

EU

US

S235

A283C

S275

A570Gr40

S355

A572Gr50

In most countries, structural steel is regulated and must meet a minimum specific criterion for shape, size, chemical composition and strength.

The chemical composition of structural steel is extremely important and highly regulated. It is a fundamental factor which defines the mechanical properties of the steel. In the following table, you can see the maximum percentage levels of certain regulated elements present in European structural steel grades S235, S275, and S355.

Chemical Composition of Structural Steels - S235, S275 and S355

The Chemical composition of Structural Steel is extremely important and highly regulated. It is a fundamental factor which defines the Mechanical properties of the steel material. In the following table you can see the Max % levels of certain regulated elements present in European Structural steel grades S235, S275 and S355.

EU Grade

C%

Mn%

P%

S%

Si%

S235

0.22 max

1.60 max

0.05 max

0.05 max

0.05 max

S275

0.25 max

1.60 max

0.04 max

0.05 max

0.05 max

S355

0.23 max

1.60 max

0.05 max

0.05 max

0.05 max

The chemical composition of structural steel is incredibly important to the engineer and will change with specific grades depending on their intended use. For example, S355K2W is a structural steel that has been hardened, denoted by K2, and has been designed with a chemical composition to withstand increased weathering - W. Therefore, this grade of structural steel will have a slightly different chemical composition to the standard S355 grade.

Mechanical Properties of Structural Steel - S235, S275, S355

The mechanical properties of structural steel are fundamental to its classification and application. Even though chemical composition is a dominant factor in determining the mechanical properties of steel, it is also very important to understand the minimum standards for the mechanical properties or performance characteristics, such as yield strength and tensile strength, which are described in more detail below.

Yield Strength

The yield strength of structural steel measures the minimum force required to create a permanent deformation in the steel. The naming convention used in European Standard EN10025 refers to the minimum yield strength of the steel grade tested at 16mm thick.

Structural Steel Grade at 16mm

Minimum Yield Strength at nominal thickness 16mm

psi

N/mm2 (MPa)

S235

33 000

235 N/mm2

S275

36 000

275 N/mm2

S355

50 000

355 N/mm2

Tensile Strength

The Tensile Strength of structural steel relates to the point at which permanent deformation occurs when the material is pulled or stretched laterally along its length.

Structural Steel Grade

Tensile Strength MPa at Nom thickness between 3mm and 16mm

S235

360 – 510 MPa

S275

370 – 530 MPa

S355

470 – 630 MPa

Typical Structural Steel ‘Sections’ / Cross-Sectional Shapes

Structural steel comes in many grades but is normally sold pre-formed with a defined cross-sectional shape, designed for specific applications. For example, it is common to find structural steel sold in I-beams, Z-beams, box lintels, hollow structural section (HSS), L-shaped and steel plate.

Depending on the desired application, an engineer will specify a grade of steel -usually to meet minimum strength, maximum weight and possibly weathering requirements, as well as the sectional shape - relative to the desired location and expected load to be carried or job to be performed.

Applications of Structural Steel

Structural steels are used in many ways and their application can be diverse. They are particularly useful because they offer the unique combination of good welding properties with guaranteed strengths. Structural steel is an extremely adaptable product and is often favored by engineers trying to maximize strength or ‘S’ structure while minimizing its weight.

It will come as no surprise that the construction industry is the biggest consumer of structural steel, where it is used for a number of purposes. Whether a small box lintel is used to carry the load of a structural wall in a residential property or a vast I-beam is bolted in place to hold the road surface on a bridge, structural steel can be specified, designed and fabricated for any type of job.

This information has been sourced, reviewed and adapted from materials provided by Masteel UK Ltd.

For more information on this source, please visit Masteel UK Ltd.

This article was updated on 7th August, 2019.

In today’s world, we see and use steel structures everywhere. The existence of the railways we use every day from home to work, or one of the world’s most visited tourist attraction points Eiffel Tower, is made possible by the use of steel as a structural material. Thanks to its unique properties, all types of steel form a basis for our modern society. But what exactly is structural steel? What is the difference between mild and structural steel? Why and how do people use it? All of these questions and more will be thoroughly explained in this post.

What is structural steel?

Steel, one of the most essential materials of the modern world, comes in various different grades and shapes. It is simply an alloy of iron and a very low amount of carbon (up to 2.1% in weight). The versatile nature of this material enables designers and engineers to use steel in many different kinds of areas. From the automotive industry to the construction of structures, steel is widely used. Structural steel is a type of steel that is used as a construction material. They are designed to have a good strength/weight ratio (which is also called specific strength) and to be cost-effective in order to be benefited as a structural component in buildings, roads, bridges, etc.

A brief history…

The very first introduction of a metal as a structural material was done in England, Shropshire in 1779. Construction of Coalbrookdale Arch Bridge over River Severn was done by using cast iron (having a carbon content higher than 2.1%).

It was a material considered four times stronger than stone and thirty times stronger than wood (1). This kind of cast-iron structure has been continuously constructed in the following years until the 1840s when malleable wrought iron (carbon content less than 0.15%) began to replace heavy cast iron. After that, different grades of steel with more suitable and superior properties for structural applications have been continuously developed until today. Technological advancements such as the development of the Bessemer process and such made steel production easier and more profitable (2). Therefore, the usage of structural steel led to huge developments in architecture and civil engineering in the past century.

Types and grades

There is not just one type of structural steel. There exist various different shapes and grades, depending on the needs for that specific application. Structural steels are classified by the shape of their cross-sections, such as the most frequently used I, T, C shapes (2). Besides their shape, the grade of steel directly affects mechanical properties. So, different grades of structural steel must be chosen according to different design requirements.

Several types of steel can be shaped and used as a beam, rod, plate, bar, or profile. Here are the standard structural steel materials:

Carbon Steels:
Steel can be defined as carbon steel when the addition of any other alloying element (such as tungsten, zirconium, cobalt, nickel, etc.) is not required, copper content does not exceed 0.4% or the following elements does not exceed indicated percentages in weight (Mn: 1.6%, Si: 0.6%, Cu: 0.6%) (3). Carbon steels are generally categorized by their carbon content as low-carbon (< 0.3%), medium-carbon (0.3-0.6%), high carbon (0.6-1%) and ultrahigh carbon (1.25-2%) steels. It is mostly used in structural pipe and tubing.

High Strength Low Alloy Steels:
This kind of steels are designed to have better mechanical properties and be more resistant against atmospheric corrosion than carbon steels. They contain manganese up to 2.0%. Small portions of other alloying elements such as chromium, nickel, molybdenum, nitrogen, vanadium, niobium, and titanium can be used in different combinations to alter the properties (3). Weathering steels which are a sub-type of high strength low alloy steels have high resistance against atmospheric corrosion by forming a passive, rust-like layer on the surface, being one of the important structural steels. Mostly used in structural shapes and plates.

Forged Steels:
Forging is simply the process of shaping the metal while it is in the solid state. It is done by applying mechanical and thermal energy to steel ingots or billets. This process introduces a uniform grain structure to the material, which decreases discontinuity in the matrix by removing voids, gas bubbles and increases overall strength (4).
Quenched and Tempered Alloy Steels:
A type of structural steel (for example A514) mostly used in building constructions. As can be understood from its name, this type of steel has undergone quenching and tempering heat treatment steps.

Why use steel as a structural material? What are the advantages?

Higher strength/weight ratio:
When compared, steel dominates every other conventional structural material such as stone, cement, or wood in terms of strength/weight ratio. It means that the tolerance against poor foundations is higher.

Good ductility:
Ductility is the ability of the material to withstand loads without failure. Thanks to the elastic nature of steel, it can return to its original shape after bending. Yielding up to some point prevents premature failures. Hard and brittle materials can fail suddenly, thus are not favorable.

High Toughness:
The ability of a material to absorb energy is called toughness. Structural steels have high toughness values; thus they are very suitable for construction applications. They are both strong and ductile. Also, it should be mentioned that the main difference between mild and structural steel is that structural steels are stiffer in order to carry higher loads without needless sagging.

Architectural variety:
Steel structures make so various different architectural designs possible. All around the world, astonishing steel buildings, towers, and bridges can be witnessed. One would be surprised to find out that this material was not economically reasonable to use as a structural component, just one hundred years ago.

Saving space:
In a comparison with reinforced concrete, a 40×40 cm2 steel carrier can do the same job as a 100×100 cm2 reinforced concrete carrier. This example shows that a net saving in terms of a usable area can be done (5).

There exist some drawbacks which can be solved with additional costs like corrosion and fire susceptibility, or the risk of buckling under heavy loads. Yet, major advantages may still outweigh these drawbacks in particular design applications.

How is it made?

Steelmaking is a rather long and complicated process to explain all in detail. So, for now, we will exclude all the initial steps of the process. After hot steel is produced, it can be cast either directly into the desired shape or as slab. Slabs can be formed into different shapes by cold working or forging. Heat treatment can be also done, depending on the application. After fabrication, chemical and mechanical tests should be performed according to standards. Without proper quality testing, catastrophic accidents may occur due to flaws in the material.

How to choose your manufacturer?

It can be troubling to decide on a such important matter. A wrong decision in the manufacturer can be costly. Our advice given below can be useful when choosing your manufacturer (6) (7).

Know what you are looking for.
Determine your priorities and expectations from your manufacturer before the interview with the candidates.

Take your time.
There is no point in rushing into a relationship with a manufacturer. Test multiple companies if necessary and choose the best one for you.

Ask for the background.
A company with a good history is less likely to let you down, best to keep that in mind.

Think at the margin.
Things may not go so bright for you all the time. On the days like these, the number of your orders would fall. Therefore, it is important to negotiate and come to terms on defining minimums of order before establishing a collaboration.

(1) M.H. Sawyer. World’s First Iron Bridge. Civil Engineering (New York: ASCE, December, 1979) pp.46-49
(2) McCormac, J. C. (1971). Structural steel design. Intext Educational Publishers.
(3) “Classification of Carbon and Low Alloy Steels” Retrieved in 15-07-2019 from https://www.totalmateria.com/page.aspx?ID=CheckArticle&LN=TR&site=kts&NM=62
(4) “How does forging strengthen metal?” Retrieved in 15-07-2019 from http://www.dropforging.net/how-does-forging-strengthen-metal.html
(5) ”What is structural steel?” Retrieved in 16-07-2019 from
https://apec.com.tr/en/what-is-structural-steel/
(6) “How to find the right manufacturer for your products?” Retrieved in 16-07-2019 from
https://www.sourcify.com/how-to-find-the-right-manufacturer-for-your-products/
(7) ”How to find a factory to manufacture your product?” Retrieved in 16-07-2019 from
https://www.businessnewsdaily.com/8820-how-to-find-factory.html

You May Be Interested